[ { "text": "Directional association of TeV to PeV astrophysical neutrinos with radio\n blazars: Recently we have shown that high-energy neutrinos above 200 TeV detected by\nIceCube are produced within several parsecs in the central regions of\nradio-bright blazars, that is active galactic nuclei with jets pointing towards\nus. To independently test this result and extend the analysis to a wider energy\nrange, we use public data for all neutrino energies from seven years of IceCube\nobservations. The IceCube point-source likelihood map is analyzed against the\npositions of blazars from a statistically complete sample selected by their\ncompact radio flux density. The latter analysis delivers a 3.0 sigma\nsignificance with the combined post-trial significance of both studies being\n4.1 sigma. The correlation is driven by a large number of blazars. Together\nwith fainter but physically similar sources not included in the sample, they\nmay explain the entire IceCube astrophysical neutrino flux as derived from\nmuon-track analyses. The neutrinos can be produced in interactions of\nrelativistic protons with X-ray self-Compton photons in parsec-scale blazar\njets.", "category": "astro-ph_HE" }, { "text": "Lunar gamma ray emission seen during the first year by Fermi: We report the detection of the lunar gamma-ray emission during the first year\nof Fermi-LAT observations. Such emission is produced by cosmic ray nuclei\ninteracting with the lunar surface. Thanks to the solar minimum conditions and\nthe reduced effects of heliospheric modulation, the lunar flux was at its\nmaximum due to the increased flux of Galactic cosmic rays hitting the lunar\nsurface. Fermi-LAT instrument has a superior sensitivity, angular resolution,\nand observes the whole sky every two orbits. It is the only gamma-ray mission\ncapable of detecting the lunar emission with high confidence and to monitor it\nover the full 24th solar cycle. We also report the status of a search of the\ngamma-ray emission from major planets and asteroid populations in the ecliptic\nplane.", "category": "astro-ph_HE" }, { "text": "XMM-Newton Observes the Intrabinary Shock of PSR J1959+2048: In a multi-orbit (100 ks) $\\mathrm{\\it XMM-Newton}$ exposure of the original\nblack widow pulsar, PSR J1959+2048, we measure the strong orbital modulation\ncaused by intrabinary shock (IBS) emission. The IBS light curve peak appears\nasymmetric, which we attribute to sweep-back effects in the companion wind. We\nalso see evidence for an X-ray eclipse by the companion and its wind. Together\nwith the IBS fit, this supports an edge-on $i\\sim 90^\\circ$ view of the system\nand a modest $\\sim 1.8M_\\odot$ mass for the recycled pulsar. Our IBS fit\nparameters imply a wind flux that, if persistent, would evaporated the\ncompanion within a few Gyr.", "category": "astro-ph_HE" }, { "text": "Optical quasi-periodic oscillation and color behavior of blazar PKS\n 2155-304: PKS 2155-304 is a well studied BL Lac object in the southern sky. The\nhistorical optical data during different period have been collected and\ncompiled. Light curves with a time span of 35 years have been constructed. The\nR-band light curve has been analyzed by means of three methods: epoch folding\nmethod, Jurkevich method and discrete correlation function (DCF) method. It is\nderived that there is an evident periodic component of 317 days (i.e. 0.87 yr)\nsuperposed on a long-term trend with large-amplitude variation in the light\ncurve. The variability of this source is accompanied by a slight color\nvariation, and the brightness and color index are correlated with each other.\nOn the long time-scale, PKS 2155-304 exhibits a tendency of\nbluer-when-brighter, which means the spectrum becomes flatter when the source\nbrightens.", "category": "astro-ph_HE" }, { "text": "No X-Ray Excess from the HESS J1741-302 Region except a New Intermediate\n Polar Candidate: With the Suzaku satellite, we observed an unidentified TeV gamma-ray source\nHESS J1741$-$302 and its surroundings. No diffuse or point-like X-ray sources\nare detected from the bright southern emission peak of HESS J1741$-$302. From\nits neighborhood, we found a new intermediate polar candidate at the position\nof $(\\alpha, \\delta)_{\\rm J2000.0} = (\\timeform{17h40m35.6s},\n\\timeform{-30D14m16s})$, which is designated as Suzaku J174035.6$-$301416. The\nspectrum of Suzaku J174035.6$-$301416 exhibits emission lines at the energy of\n6.4, 6.7 and 7.0 keV, which can be assigned as the K$\\alpha$ lines from\nneutral, He-like and H-like iron, respectively. A coherent pulsation is found\nat a period of 432.1 $\\pm$ 0.1 s. The pulse profile is quasi-sinusoidal in the\nhard X-ray band (4$-$8 keV), but is more complicated in the soft X-ray band\n(1$-$3 keV). The moderate period of pulsation, the energy flux, and the\npresence of the iron K$\\alpha$ lines indicate that Suzaku J174035.6$-$301416 is\nlikely an intermediate polar, a subclass of magnetized white dwarf binaries\n(cataclysmic variables).\n Based on these discoveries, we give some implications on the origin of GCDX\nand brief comments on HESS J1741$-$302 and PSR B1737$-$30.", "category": "astro-ph_HE" }, { "text": "MC Simulations of the Broadband Spectra of Sagittarius A* through the\n use of GRMHD: We present results of simulations of the spectrum of the accretion flow onto\nthe supermassive black hole in our Galactic Centre, Sagittarius A*, generated\nwith a coupling of Monte-Carlo (MC) radiation and general relativistic\nmagnetohydrodynamic (GRMHD) codes. In our modeling, we use the 2D HARM GRMHD\ncode to first model the physical parameters of the disk, then feed its results\ninto our 2D MC photon transport code. We will discuss results obtained which\nfit radio, IR, and Chandra-obtained flaring or quiescent x-ray data points, as\nwell as the validity of the amount of scaling of input parameters (density,\ntemperature, and magnetic field) required to fit these points. HARM output will\nbe used to suggest whether the scaling is within reasonable limits.", "category": "astro-ph_HE" }, { "text": "Radio Constraints on $r$-process Nucleosynthesis by Collapsars: The heaviest elements in the Universe are synthesized through rapid neutron\ncapture ($r$-process) in extremely neutron rich outflows. Neutron star mergers\nwere established as an important $r$-process source through the multi-messenger\nobservation of GW170817. Collapsars were also proposed as a potentially major\nsource of heavy elements; however, this is difficult to probe through optical\nobservations due to contamination by other emission mechanisms. Here we present\nobservational constraints on $r$-process nucleosynthesis by collapsars based on\nradio follow-up observations of nearby long gamma-ray bursts. We make the\nhypothesis that late-time radio emission arises from the collapsar wind ejecta\nresponsible for forging $r$-process elements, and consider the constraints that\ncan be set on this scenario using radio observations of a sample of Swift/BAT\nGRBs located within 2 Gpc. No radio counterpart was identified in excess of the\nradio afterglow of the GRBs in our sample, limiting the collapsar $r$-process\ncontribution to $\\lesssim0.2$ M$_\\odot$ under the models we considered, with\nconstant circum-merger densities giving more stringent constraints. While our\nresults are in tension with collapsars being the majority $r$-process\nproduction sites, the ejecta mass and velocity profile of collapsar winds is\nnot yet well modeled. As such, our results are currently subject to large\nuncertainties, but further theoretical work could greatly improve them.", "category": "astro-ph_HE" }, { "text": "TeV gamma-ray survey of the northern sky using the ARGO-YBJ experiment: The ARGO-YBJ experiment is an extensive air shower array with full coverage\nRPC detectors located at Yangbajing (4300 m asl, Tibet, China). It is operated\nwith high duty cycle (>86%) and a large field of view ($\\sim$ 2sr). It\ncontinuously monitors the entire overhead sky at $\\gamma$-ray energies above\n0.1 TeV. In the talk, we will present the result of the northern sky survey\n(between declinations of -10$^{\\circ}$ and 70$^{\\circ}$) from an analysis of ~4\nyears of the ARGO-YBJ data (between July 2006 and February 2011). There are\nfour known TeV sources observed with significance greater than 5 S.D.. The\nsignificance from Crab Nebula is more than 16 S.D.. 90% confidence level upper\nlimits to the flux from all directions in the sky are also presented, which\nvary from 0.09 to 0.53 Crab unit for Crab-like point sources.", "category": "astro-ph_HE" }, { "text": "Constraining properties of neutron star merger outflows with radio\n observations: The jet opening angle and inclination of GW170817 -- the first detected\nbinary neutron star merger -- were vital to understand its energetics, relation\nto short gamma-ray bursts, and refinement of the standard siren-based\ndetermination of the Hubble constant, $H_0$. These basic quantities were\ndetermined through a combination of the radio lightcurve and Very Long Baseline\nInterferometry (VLBI) measurements of proper motion. In this paper we discuss\nand quantify the prospects for the use of radio VLBI observations and\nobservations of scintillation-induced variability to measure the source size\nand proper motion of merger afterglows, and thereby infer properties of the\nmerger including inclination angle, opening angle and energetics. We show that\nthese techniques are complementary as they probe different parts of the\ncircum-merger density/inclination angle parameter space and different periods\nof the temporal evolution of the afterglow. We also find that while VLBI\nobservations will be limited to the very closest events it will be possible to\ndetect scintillation for a large fraction of events beyond the range of current\ngravitational wave detectors. Scintillation will also be detectable with next\ngeneration telescopes such as the Square Kilometre Array, 2000 antenna Deep\nSynoptic Array and the next generation Very Large Array, for a large fraction\nof events detected with third generation gravitational wave detectors. Finally,\nwe discuss prospects for the measurement of the $H_0$ with VLBI observations of\nneutron star mergers and compare this technique to other standard siren\nmethods.", "category": "astro-ph_HE" }, { "text": "An accurate and efficient numerical calculation of detonation waves in\n multidimensional supernova simulations using a burning limiter and adaptive\n quasi-statistical equilibrium: Resolving the small length-scale of thermonuclear detonation waves (TNDWs) in\nsupernovae is currently not possible in multidimensional full-star simulations.\nAdditionally, multidimensional simulations usually use small, oversimplistic\nreaction networks and adopt an ad hoc transition criterion to nuclear\nstatistical equilibrium (NSE). The errors due to the applied approximations are\nnot well understood. We present here a new accurate and efficient numerical\nscheme that accelerates the calculations by orders of magnitudes and allows the\nstructure of TNDWs to be resolved. The numerical scheme has two important\ningredients: (1) a burning limiter that broadens the width of the TNDW while\naccurately preserving its internal structure, and (2) an adaptive separation of\nisotopes into groups that are in nuclear statistical quasi-equilibrium, which\nresolves the time-consuming burning calculation of reactions that are nearly\nbalanced out. Burning is calculated in situ employing the required large\nnetworks without the use of post-processing or pre-describing the conditions\nbehind the TNDW. In particular, the approach to and deviation from NSE are\ncalculated self-consistently. The scheme can be easily implemented in\nmultidimensional codes. We test our scheme against accurate solutions of the\nstructure of TNDWs and against homogeneous expansion from NSE. We show that\nwith resolutions that are typical for multidimensional full-star simulations,\nwe reproduce the accurate thermodynamic trajectory (density, temperature, etc.)\nto an accuracy that is better than a percent for the resolved scales (where the\nburning limiter is not applied), while keeping the error for unresolved scales\n(broadened by the burning limiter) within a few percent.", "category": "astro-ph_HE" }, { "text": "Hydrodynamics of Young Supernova Remnants and the Implications for their\n Gamma-ray emission: Supernovae (SNe) are generally classified into Type I and Type II. Most SNe\n(~ 80%), including all the subtypes of Type II, and Type Ib/c, arise from the\ncore-collapse of massive stars. During their lifetime, mass-loss from these\nstars considerably modifies the medium around the stars. When the stars explode\nas SNe, the resulting shock wave will expand in this wind-modified medium. In\ncontrast, Type Ia SNe will expand in a relatively uniform medium, but the\ndynamics are different from those of core-collapse SNe. For young supernova\nremnants, the properties of the ejecta as well as the surrounding medium are\nimportant in determining the subsequent evolution of the SN shock wave, and the\ndynamics and kinematics of the remnant. This will influence the acceleration of\nparticles at the SN shocks, and consequently affect the gamma-ray emission from\nthe remnant.\n Herein we discuss the expected properties, especially the density structure,\nof the medium around various types and sub-types of SNe, as suggested by\ncurrent stellar evolution models. Using analytic and semi-analytic models and\nnumerical simulations, we investigate how these affect the kinematics of the SN\nshock waves, assess the impact this would have on the production of cosmic\nrays, and show how it influences the time-evolution of the hadronic gamma-ray\nemission from the remnant. In the case of SNRs evolving in a wind medium, the\nemission should reach a maximum early on, and thereafter decrease with time.\nFor SNe in a constant density medium, the emission would be expected to\nincrease with time upto the advent of the Sedov stage.", "category": "astro-ph_HE" }, { "text": "Echo-Mapping of Swift J1753.5-0127: We present two epochs of coordinated X-ray-optical timing observations of the\nblack hole candidate Swift J1753.5-0127 during its 2005 outburst. The first\nepoch in July occurred at outburst peak. Two consecutive nights of observations\nusing the McDonald Observatory Argos camera with the Rossi X-ray Timing\nExplorer show a consistent correlation with an immediate response and an\nextended tail lasting ~5s. The properties of the variability and the\ncorrelation are consistent with thermal reprocessing in an accretion disk. The\nshortness of the lag suggests a short orbital period consistent with that\nrecently claimed. The second epoch in August used the VLT FORS2 HIT mode again\nin conjunction with RXTE. Again a repeatable correlation is seen between two\nindependent subsets of the data. In this case, though, the cross-correlation\nfunction has an unusual structure comprising a dip followed by a double-peak.\nWe suggest that this may be equivalent to the dip plus single peak structure\nseen by Kanbach et al. (2001) in XTE J1118+480 and attributed there to\nsynchrotron emission; a similar structure was seen during later activity of\nSwift J1753.5-0127 by Durant et al. (2008).", "category": "astro-ph_HE" }, { "text": "Rates of Stellar Tidal Disruption as Probes of the Supermassive Black\n Hole Mass Function: Rates of stellar tidal disruption events (TDEs) by supermassive black holes\n(SMBHs) due to two-body relaxation are calculated using a large galaxy sample\n(N=146) in order to explore the sensitivity of the TDE rates to observational\nuncertainties, such as the parametrization of galaxy light profiles and the\nstellar mass function. The largest uncertainty arises due to the poorly\nconstrained occupation fraction of SMBHs in low-mass galaxies, which otherwise\ndominate the total TDE rate. The detection rate of TDE flares by optical\nsurveys is calculated as a function of SMBH mass and other observables for\nseveral physically-motivated models of TDE emission. We also quantify the\nfraction of galaxies that produce deeply penetrating disruption events. If the\nmajority of the detected events are characterized by super-Eddington\nluminosities (such as disk winds, or synchrotron radiation from an off-axis\nrelativistic jet), then the measured SMBH mass distribution will tightly\nconstrain the low-end SMBH occupation fraction. If Eddington-limited emission\nchannels dominate, however, then the occupation fraction sensitivity is much\nless pronounced in a flux-limited survey (although still present in a\nvolume-complete event sample). The SMBH mass distribution of the current sample\nof TDEs, though highly inhomogeneous and encumbered by selection effects,\nalready suggests that Eddington-limited emission channels dominate. Even our\nmost conservative rate estimates appear to be in tension with much lower\nobservationally inferred TDE rates, and we discuss several possible resolutions\nto this discrepancy.", "category": "astro-ph_HE" }, { "text": "Mergers of binary neutron star systems: a multi-messenger revolution: On 17 August 2017, less than two years after the direct detection of\ngravitational radiation from the merger of two ~30 Msun black holes, a binary\nneutron star merger was identified as the source of a gravitational wave signal\nof ~100 s duration that occurred at less than 50 Mpc from Earth. A short GRB\nwas independently identified in the same sky area by the Fermi and INTEGRAL\nsatellites for high energy astrophysics, which turned out to be associated with\nthe gravitational event. Prompt follow-up observations at all wavelengths led\nfirst to the detection of an optical and infrared source located in the\nspheroidal galaxy NGC4993 and, with a delay of ~10 days, to the detection of\nradio and X-ray signals. This paper revisits these observations and focusses on\nthe early optical/infrared source, which was thermal in nature and powered by\nthe radioactive decay of the unstable isotopes of elements synthesized via\nrapid neutron capture during the merger and in the phases immediately following\nit. The far-reaching consequences of this event for cosmic nucleosynthesis and\nfor the history of heavy elements formation in the Universe are also\nillustrated.", "category": "astro-ph_HE" }, { "text": "Late-time radio observations of the short GRB200522A: constraints on the\n magnetar model: GRB200522A is a short duration gamma-ray burst (GRB) at redshift $z$=0.554\ncharacterized by a bright infrared counterpart. A possible, although not\nunambiguous, interpretation of the observed emission is the onset of a luminous\nkilonova powered by a rapidly rotating and highly-magnetized neutron star,\nknown as magnetar. A bright radio flare, arising from the interaction of the\nkilonova ejecta with the surrounding medium, is a prediction of this model.\nWhereas the available dataset remains open to multiple interpretations (e.g.\nafterglow, r-process kilonova, magnetar-powered kilonova), long-term radio\nmonitoring of this burst may be key to discriminate between models. We present\nour late-time upper limit on the radio emission of GRB200522A, carried out with\nthe Karl G. Jansky Very Large Array at 288 days after the burst. For kilonova\nejecta with energy $E_{\\rm ej} \\approx 10^{53} \\rm erg$, as expected for a\nlong-lived magnetar remnant, we can already rule out ejecta masses $M_{\\rm ej}\n\\lesssim0.03 \\mathrm{M}_\\odot$ for the most likely range of circumburst\ndensities $n\\gtrsim 10^{-3}$ cm$^{-3}$. Observations on timescales of\n$\\approx$3-10 yr after the merger will probe larger ejecta masses up to $M_{\\rm\nej} \\sim 0.1 \\mathrm{M}_\\odot$, providing a robust test to the magnetar\nscenario.", "category": "astro-ph_HE" }, { "text": "Associating Fast Radio Bursts with Extragalactic Radio Sources: General\n Methodology and a Search for a Counterpart to FRB 170107: The discovery of a repeating fast radio burst has led to the first precise\nlocalization, an association with a dwarf galaxy, and the identification of a\ncoincident persistent radio source. However, further localizations are required\nto determine the nature of FRBs, the sources powering them, and the possibility\nof multiple populations. Here we investigate the use of associated persistent\nradio sources to establish FRB counterparts, taking into account the\nlocalization area and the persistent source flux density. Due to the lower\nareal number density of radio sources compared to faint optical sources, robust\nassociations can be achieved for less precise localizations as compared to\ndirect optical host galaxy associations. For generally larger localizations\nwhich preclude robust associations, the number of candidate hosts can be\nreduced based on the ratio of radio-to-optical brightness. We find that\nconfident associations with $\\sim 0.01-$1 mJy sources, comparable to the\nluminosity of the persistent source associated with FRB 121102 over the\nredshift range $z \\approx 0.1 - 1$, require FRB localizations of $\\lesssim\n20''$. In the absence of a robust association, constraints can be placed on the\nluminosity of an associated radio source as a function of localization and DM.\nFor DM $\\approx 1000 \\rm \\ pc \\ cm^{-3}$, an upper limit comparable to the\nluminosity of the FRB 121102 persistent source can be placed if the\nlocalization is $\\lesssim 10''$. We apply our analysis to the case of the ASKAP\nFRB 170107, using optical and radio observations of the localization region. We\nidentify two candidate hosts based on a ratio of radio-to-optical brightness of\n$\\gtrsim 100$. We find that if one of these is associated with FRB 170107, the\nresulting radio luminosity ($1 \\times 10^{29} - 4 \\times 10^{30} \\ \\rm erg \\\ns^{-1} \\ Hz^{-1}$) is comparable to the luminosity of the FRB 121102 persistent\nsource.", "category": "astro-ph_HE" }, { "text": "The TANAMI Multiwavelength Program: Dynamic SEDs of Southern Blazars: Simultaneous broadband spectral and temporal studies of blazars are an\nimportant tool for investigating active galactic nuclei (AGN) jet physics. We\nstudy the spectral evolution between quiescent and flaring periods of 22\nradio-loud AGN through multi-epoch, quasi-simultaneous broadband spectra. For\nmany of these sources these are the first broadband studies. We use a Bayesian\nblock analysis of \\Fermi/LAT light curves in order to determine time ranges of\nconstant flux for constructing quasi-simultaneous SEDs. The shapes of the\nresulting 81 SEDs are described by two logarithmic parabolas and a blackbody\nspectrum where needed. For low states the peak frequencies and luminosities\nagree well with the blazar sequence, higher luminosity implying lower peak\nfrequencies. This is not true for sources in a high state. The $\\gamma$-ray\nphoton index in Fermi/LAT correlates with the synchrotron peak frequency in low\nand intermediate states. No correlation is present in high states. The black\nhole mass cannot be determined from the SEDs. Surprisingly, the thermal excess\noften found in FSRQs at optical/UV wavelengths can be described by blackbody\nemission and not an accretion disk spectrum. The \"harder-when-brighter\" trend,\ntypically seen in X-ray spectra of flaring blazars, is visible in the blazar\nsequence. Our results for low and intermediate states, as well as the Compton\ndominance, are in agreement with previous results. Black hole mass estimates\nusing the parameters from Bonchi (2013) are in agreement with some of the more\ndirect measurements. For two sources, estimates disagree by more than four\norders of magnitude, possibly due to boosting effects. The shapes of the\nthermal excess seen predominantly in flat spectrum radio quasars are\ninconsistent with a direct accretion disk origin.", "category": "astro-ph_HE" }, { "text": "From Supernova to Remnant: Tracking the Evolution of the Oldest Known\n X-ray Supernovae: Core-collapse supernovae (SNe) expand into a medium created by winds from the\npre-SN progenitor. The SN explosion and resulting shock wave(s) heat up the\nsurrounding plasma, giving rise to thermal X-ray emission, which depends on the\ndensity of the emitting material. Tracking the variation of the X-ray\nluminosity over long periods of time thus allows for investigation of the\nkinematics of the SN shock waves, the structure of the surrounding medium, and\nthe nature of the progenitor star. In this paper X-ray observations of five of\nthe oldest known X-ray supernovae - SN 1970G, SN 1968D, SN 1959D, SN 1957D and\nSN 1941C - are analyzed, with the aim of reconstructing their light curves over\nseveral decades. For those supernovae for which we can extract multi-epoch\ndata, the X-ray luminosity appears to decline with time, although with large\nerror bars. No increase in the X-ray emission from SN 1970G is found at later\nepochs, contrary to previous reports. All five SNe show X-ray luminosities that\nare of comparable magnitude. We compare the late-time X-ray luminosities of\nthese SNe to those of supernova remnants (SNRs) in the Galaxy which are a few\nhundred years old, and find that when the tentative decline is taken into\naccount, the luminosity of the old SNe studied herein could fall below the\nluminosity of some of the younger SNRs within a few hundred years. However, the\nX-ray luminosity should begin to increase as the SNe expand in the Sedov phase,\nthus reaching that of the observed SNRs.", "category": "astro-ph_HE" }, { "text": "An Exploration of X-ray Supernova Remnants in the Milky Way and Nearby\n Galaxies: We probe the environmental properties of X-ray supernova remnants (SNRs) at\nvarious points along their evolutionary journey, especially the S-T phase, and\ntheir conformance with theoretically derived models of SNR evolution. The\nremnant size is used as a proxy for the age of the remnant. Our data set\nincludes 34 Milky Way, 59 Large Magellanic Cloud (LMC), and 5 Small Magellanic\nCloud (SMC) SNRs. We select remnants that have been definitively typed as\neither core-collapse (CC) or Type Ia supernovae, with well-defined size\nestimates, and a thermal X-ray flux measured over the entire remnant. A catalog\nof SNR size and X-ray luminosity is presented and plotted, with ambient density\nand age estimates from the literature. Model remnants with a given density, in\nthe Sedov-Taylor (S-T) phase, are overplotted on the diameter-vs-luminosity\nplot, allowing the evolutionary state and physical properties of SNRs to be\ncompared to each other, and to theoretical models. We find that small, young\nremnants are predominantly Type Ia remnants or high luminosity CCs, suggesting\nthat many CC SNRs are not detected until after they have emerged from the\nprogenitor's wind-blown bubble. An examination of the distribution of SNR\ndiameters in the Milky Way and LMC reveals that LMC SNRs must be evolving in an\nambient medium which is 30% as dense as that in the Milky Way. This is\nconsistent with ambient density estimates for the Galaxy and LMC.", "category": "astro-ph_HE" }, { "text": "Unusual Emission Variations Near the Eclipse of A Black Widow PSR\n J1720$-$0533: We report on an {unusually} bright observation of PSR J1720$-$0533 using the\nFive-hundred-meter Aperture Spherical radio Telescope (FAST). The pulsar is in\na black widow system that {was discovered by the Commensal Radio Astronomy FAST\nSurvey (CRAFTS). By coincidence, a bright scintillation maximum was\nsimultaneous with the eclipse in our observation which allowed for precise\nmeasurements of flux density variations, as well as dispersion measure (DM) and\npolarization.} We found that there are quasi-periodic pulse emission variations\nwith a modulation period of $\\sim$ {22\\,s} during the ingress of the eclipse,\nwhich could be caused by plasma lensing. {No such periodic modulation was found\nduring the egress of the eclipse. } {The linear polarization of the pulsar\ndisappears before the eclipse, even before there is a visually obvious change\nin DM. We also found that the pulse scattering maybe play an important role in\nthe eclipse of PSR J1720$-$0533.}", "category": "astro-ph_HE" }, { "text": "A Survey of Fermi Catalog Sources using data from the Milagro Gamma-Ray\n Observatory: The Fermi LAT has released a list of the most significant 205 sources with\nthree months of Fermi data (Bright Source List). The Milagro Gamma-Ray\nObservatory is sensitive to gamma rays above 100 GeV with a peak sensitivity\nbetween 10 and 30 TeV, overlapping and extending the energy range of Fermi. Of\nthe 34 Galactic LAT sources in the field of view of Milagro, 6 are observed\nwith significance greater than 5 sigma and 14 are observed at greater than 3\nsigma. Of these 14 sources, 9 are pulsars. Since the VHE emission detected by\nMilagro is often found to be extended and likely un-pulsed, the VHE component\npresumably arises from the pulsar winds. Six of the 14 sources have not been\npreviously detected at TeV energies. The details of the Milagro survey will be\npresented. We will also present the energy spectra of the high-significance\ndetections. Should the full 1-year source list be available prior to the\nsymposium, we will expand our analysis to include the larger Fermi catalog.", "category": "astro-ph_HE" }, { "text": "The first 48: Discovery and progenitor constraints on the Type Ia\n supernova 2013gy: We present an early-phase $g$-band light curve and visual-wavelength spectra\nof the normal Type Ia supernova (SN) 2013gy. The light curve is constructed by\ndetermining the appropriate S-corrections to transform KAIT natural-system $B$-\nand $V$-band photometry and Carnegie Supernova Project natural-system $g$-band\nphotometry to the Pan-STARRS1 $g$-band natural photometric system. A Markov\nChain Monte Carlo calculation provides a best-fit single power-law function to\nthe first ten epochs of photometry described by an exponent of\n$2.16^{+0.06}_{-0.06}$ and a time of first light of MJD\n56629.4$^{+0.1}_{-0.1}$, which is $1.93^{+0.12}_{-0.13}$ days (i.e., $<48$~hr)\nbefore the discovery date (2013 December 4.84 UT) and $-19.10^{+0.12}_{-0.13}$\ndays before the time of $B$-band maximum (MJD 56648.5$\\pm0.1$). The estimate of\nthe time of first light is consistent with the explosion time inferred from the\nevolution of the Si II $\\lambda$6355 Doppler velocity. Furthermore, discovery\nphotometry and previous nondetection limits enable us to constrain the\ncompanion radius down to $R_c \\leq 4\\,R_{\\odot}$. In addition to our early-time\nconstraints, we use a deep +235 day nebular-phase spectrum from Magellan/IMACS\nto place a stripped H-mass limit of $< 0.018\\,M_{\\odot}$. Combined, these\nlimits effectively rule out H-rich nondegenerate companions.", "category": "astro-ph_HE" }, { "text": "X-ray Emission from SN 2012ca: A Type Ia-CSM Supernova Explosion in a\n Dense Surrounding Medium: X-ray emission is one of the signposts of circumstellar interaction in\nsupernovae (SNe), but until now, it has been observed only in core-collapse\nSNe. The level of thermal X-ray emission is a direct measure of the density of\nthe circumstellar medium (CSM), and the absence of X-ray emission from Type Ia\nSNe has been interpreted as a sign of a very low density CSM. In this paper, we\nreport late-time (500--800 days after discovery) X-ray detections of SN 2012ca\nin {\\it Chandra} data. The presence of hydrogen in the initial spectrum led to\na classification of Type Ia-CSM, ostensibly making it the first SN~Ia detected\nwith X-rays. Our analysis of the X-ray data favors an asymmetric medium, with a\nhigh-density component which supplies the X-ray emission. The data suggest a\nnumber density $> 10^8$ cm$^{-3}$ in the higher-density medium, which is\nconsistent with the large observed Balmer decrement if it arises from\ncollisional excitation. This is high compared to most core-collapse SNe, but it\nmay be consistent with densities suggested for some Type IIn or superluminous\nSNe. If SN 2012ca is a thermonuclear SN, the large CSM density could imply\nclumps in the wind, or a dense torus or disk, consistent with the\nsingle-degenerate channel. A remote possibility for a core-degenerate channel\ninvolves a white dwarf merging with the degenerate core of an asymptotic giant\nbranch star shortly before the explosion, leading to a common envelope around\nthe SN.", "category": "astro-ph_HE" }, { "text": "Semi-implicit scheme for treating radiation under M1 closure in general\n relativistic conservative fluid dynamics codes: A numerical scheme is described for including radiation in multi-dimensional\ngeneral-relativistic conservative fluid dynamics codes. In this method, a\ncovariant form of the M1 closure scheme is used to close the radiation moments,\nand the radiative source terms are treated semi-implicitly in order to handle\nboth optically thin and optically thick regimes. The scheme has been\nimplemented in a conservative general relativistic radiation hydrodynamics code\nKORAL. The robustness of the code is demonstrated on a number of test problems,\nincluding radiative relativistic shock tubes, static radiation pressure\nsupported atmosphere, shadows, beams of light in curved spacetime, and\nradiative Bondi accretion. The advantages of M1 closure relative to other\napproaches such as Eddington closure and flux-limited diffusion are discussed,\nand its limitations are also highlighted.", "category": "astro-ph_HE" }, { "text": "Gas perturbations in cool cores of galaxy clusters: effective equation\n of state, velocity power spectra and turbulent heating: We present the statistical analysis of X-ray surface brightness and gas\ndensity fluctuations in cool cores of ten, nearby and bright galaxy clusters\nthat have deep Chandra observations and show observational indications of\nradio-mechanical AGN feedback. Within the central parts of cool cores the total\nvariance of fluctuations is dominated by isobaric and/or isothermal\nfluctuations on spatial scales ~ 10-60 kpc, which are likely associated with\nslow gas motions and bubbles of relativistic plasma. Adiabatic fluctuations\nassociated with weak shocks constitute less than 10 per cent of the total\nvariance in all clusters. The typical amplitude of density fluctuations is\nsmall, ~ 10 per cent or less on scales of ~ 10-15 kpc. Subdominant contribution\nof adiabatic fluctuations and small amplitude of density fluctuations support a\nmodel of gentle AGN feedback as opposed to periodically explosive scenarios\nwhich are implemented in some numerical simulations. Measured one-component\nvelocities of gas motions are typically below 100-150 km/s on scales < 50 kpc,\nand can be up to ~ 300 km/s on ~ 100 kpc scales. The non-thermal energy is < 12\nper cent of the thermal energy. Regardless of the source that drives these\nmotions the dissipation of the energy in such motions provides heat that is\nsufficient to balance radiative cooling on average, albeit the uncertainties\nare large. Presented results here support previous conclusions based on the\nanalysis of the Virgo and Perseus Clusters, and agree with the Hitomi\nmeasurements. With next generation observatories like Athena and Lynx, these\ntechniques will be yet more powerful.", "category": "astro-ph_HE" }, { "text": "Variability of the X-ray Broad Iron Spectral Features in Active Galactic\n Nuclei and Black-hole Binaries: The \"broad iron spectral features\" are often seen in X-ray spectra of Active\nGalactic Nuclei (AGN) and black-hole binaries (BHB). These features may be\nexplained either by the \"relativistic disc reflection\" scenario or the \"partial\ncovering\" scenario: It is hardly possible to determine which model is valid\nfrom time-averaged spectral analysis. Thus, X-ray spectral variability has been\ninvestigated to constrain spectral models. To that end, it is crucial to study\niron structure of BHBs in detail at short time-scales, which is, for the first\ntime, made possible with the Parallel-sum clocking (P-sum) mode of XIS\ndetectors on board Suzaku. This observational mode has a time-resolution of\n7.8~ms as well as a CCD energy-resolution. We have carried out systematic\ncalibration of the P-sum mode, and investigated spectral variability of the BHB\nGRS 1915+105. Consequently, we found that the spectral variability of GRS\n1915+105 does not show iron features at sub-seconds. This is totally different\nfrom variability of AGN such as 1H0707--495, where the variation amplitude\nsignificantly drops at the iron K-energy band. This difference can be naturally\nexplained in the framework of the \"partial covering\" scenario.", "category": "astro-ph_HE" }, { "text": "Upper Limit on Correlated Current Variations in the Crab Pulsar: The high energy emission of rotation powered pulsars is supposed to be\nproduced in \"gaps\" in the pulsar magnetosphere where charges are accelerated\nand currents are produced. The rest of the magnetosphere is supposed to be\nmostly a \"force-free\" plasma without any currents. Two important currents are\nthe main current that flows away from the pulsar, that produces the observed\nradiation, and the current that returns to the pulsar to maintain charge\nneutrality. This work attempts to study the return current in the Crab pulsar\nusing the soft X-ray data from the {\\it{NICER}} observatory. It is assumed that\nthe two currents vary as a function of time. This would modulate the electric\nfields in the \"gaps\", which would affect the observed X-ray flux. These flux\nvariations will show up only in the on-pulse phases, while those caused by the\nCrab Nebula, instrumental effects, etc. will be present in the off-pulse phases\nalso. This work obtains the correlation coefficient of the flux variations in\nthe two peaks of the Crab pulsar, after removing the off-pulse flux variations.\nNo correlation was observed; its error of $0.000012$ sets an upper limit of\n$0.036\\%$ on the rms variation of correlated X-ray flux in the Crab pulsar.\nReasons exist for the return current variations to be correlated, while the\nmain current variations are probably uncorrelated. So the above number is\nconsidered an upper limit on correlated return current variations, which may be\nan important constraint for pulsar magnetospheric structure.", "category": "astro-ph_HE" }, { "text": "A multi-wavelength study of the hard and soft states of MAXI J1820+070\n during its 2018 outburst: We present a comprehensive multi-wavelength spectral analysis of the black\nhole X-ray binary MAXI J1820+070 during its 2018 outburst, utilizing AstroSat\nfar UV, soft and hard X-ray data, along with (quasi-)simultaneous optical and\nX-ray data from Las Cumbres Observatory and NICER, respectively. In the soft\nstate, we detect soft X-ray and UV/optical excess components over and above the\nintrinsic accretion disk emission ($kT_{\\rm in}\\sim 0.58$ keV) and a steep\nX-ray power-law component. The soft X-ray excess is consistent with a\nhigh-temperature blackbody ($kT\\sim 0.79$ keV), while the UV/optical excess is\ndescribed by UV emission lines and two low-temperature blackbody components\n($kT\\sim 3.87$ eV and $\\sim 0.75$ eV). Employing continuum spectral fitting, we\ndetermine the black hole spin parameter ($a=0.77\\pm0.21$), using the jet\ninclination angle of $64^{\\circ}\\pm5^{\\circ}$ and a mass spanning\n$5-10M_{\\odot}$. In the hard state, we observe a significantly enhanced\noptical/UV excess component, indicating a stronger reprocessed emission in the\nouter disk. Broad-band X-ray spectroscopy in the hard state reveals a\ntwo-component corona, each associated with its reflection component, in\naddition to the disk emission ($kT_{\\rm in}\\sim 0.19$ keV). The softer coronal\ncomponent dominates the bolometric X-ray luminosity and produces broader\nrelativistic reflection features, while the harder component gets reflected far\nfrom the inner disk, yielding narrow reflection features. Furthermore, our\nanalysis in the hard state suggests a substantial truncation of the inner disk\n($\\gtrsim 51$ gravitational radii) and a high disk density ($\\sim 10^{20}\\ \\rm\ncm^{-3}$).", "category": "astro-ph_HE" }, { "text": "The Effect of a Cosmic Ray Precursor in SN 1006?: Like many young supernova remnants, SN 1006 exhibits what appear to be clumps\nof ejecta close to or protruding beyond the main blast wave. In this paper we\nexamine 3 such protrusions along the east rim. They are semi-aligned with\nejecta fingers behind the shock-front, and exhibit emission lines from O VII\nand O VIII. We first interpret them in the context of an upstream medium\nmodified by the saturated nonresonant Bell instability which enhances the\ngrowth of Rayleigh-Taylor instabilities when advected postshock. We discuss\ntheir apparent periodicity if the spacing is determined by properties of the\nremnant or by a preferred size scale in the cosmic ray precursor. We also\nbriefly discuss the alternative that these structures have an origin in the\nejecta structure of the explosion itself. In this case the young evolutionary\nage of SN 1006 would imply density structure within the outermost layers of the\nexplosion with potentially important implications for deflagration and\ndetonation in thermonuclear supernova explosion models.", "category": "astro-ph_HE" }, { "text": "Bose-Einstein condensate stars in combined Rastall-Rainbow gravity: We study zero and finite temperature static Bose-Einstein condensate (BEC)\nstars in the combined Rastall-Rainbow (RR) theory of gravity by considering\ndifferent BEC equation of states (EoSs). We obtain the global properties of BEC\nstars by solving the modified Tolman-Oppenheimer-Volkoff equations with values\nof Rastall parameter $\\kappa$ and Rainbow function $\\Sigma$ chosen accordingly\nto get the results in theories of Rastall, Rainbow and RR. We observe that the\nparameter $\\kappa$ has negligible effect on the maximum mass of the stars\nconsidered, whereas $\\Sigma$ alters it significantly, and increasing the value\nof $\\kappa$ beyond a certain limit results in unstable solutions for any value\nof $\\Sigma$. We report that the inclusion of temperature in our analysis\nexpands the parameter space by including more values of $\\kappa$. However,\ntemperature has negligible effect on the maximum mass of the stellar profiles\nin all the three theories. We find that the maximum masses and radii of the\nstars within RR theory can have good agreement with the observational data on\npulsars for all the EoSs considered and in particular, the\nColpi-Wasserman-Shapiro EoS, which was ruled out in General Relativity (GR). We\nalso find that, in contrast to the results of GR, BEC stars consistent with\nobservations can be realised in the RR theory with smaller bosonic\nself-interaction strength.", "category": "astro-ph_HE" }, { "text": "Spectral and polarimetric signatures of X-ray eclipses in AGN: X-ray observations of active galactic nuclei (AGN) show variability on\ntimescales ranging from a few hours up to a few days. Some of this variability\nmay be associated with occultation events by clouds in the broad line region.\nIn this work, we aim to model the spectral and polarization variability arising\nfrom X-ray obscuration events, serving as probes of the relativistic effects\nthat dominate the emission from the innermost regions. We show that asymmetries\ncan be clearly detected in the AGN spectra as the cloud is shading different\nparts of the accretion disc. We also show that these effects can be detected in\nthe temporal evolution of the polarization degree ($P$) and the polarization\nposition angle ($\\Psi$). The variations in $P$ and $\\Psi$ are highly dependent\non the inclination of the system, the position of the primary source and its\nintrinsic polarization. Considering the disc-corona system only, for an\ninclination $\\theta = 30^\\circ$ (60$^\\circ$), $P$ increases up to $\\sim 20$%\n(30)%, in the 4-8 keV band, when the unpolarized primary source is obscured.\nHowever, after accounting for the contribution of parsec-scale material\nscattering the light in our line of sight (narrow-line region and molecular\ntorus), the variability is smoothed out and the polarization degree can be\nreduced down to $\\sim 1$% (2%). Our results suggest that the study of eclipses\nin AGN with the next generation of X-ray spectral and polarimetric missions\ncould provide unique information on the physics and structure of the innermost\nregions as well as of the parsec-scale material.", "category": "astro-ph_HE" }, { "text": "Too small to fail: characterizing sub-solar mass black hole mergers with\n gravitational waves: The detection of a sub-solar mass black hole could yield dramatic new\ninsights into the nature of dark matter and early-Universe physics, as such\nobjects lack a traditional astrophysical formation mechanism. Gravitational\nwaves allow for the direct measurement of compact object masses during binary\nmergers, and we expect the gravitational-wave signal from a low-mass\ncoalescence to remain within the LIGO frequency band for thousands of seconds.\nHowever, it is unclear whether one can confidently measure the properties of a\nsub-solar mass compact object and distinguish between a sub-solar mass black\nhole or other exotic objects. To this end, we perform Bayesian parameter\nestimation on simulated gravitational-wave signals from sub-solar mass black\nhole mergers to explore the measurability of their source properties. We find\nthat the LIGO/Virgo detectors during the O4 observing run would be able to\nconfidently identify sub-solar component masses at the threshold of\ndetectability; these events would also be well-localized on the sky and may\nreveal some information on their binary spin geometry. Further, next-generation\ndetectors such as Cosmic Explorer and the Einstein Telescope will allow for\nprecision measurement of the properties of sub-solar mass mergers and tighter\nconstraints on their compact-object nature.", "category": "astro-ph_HE" }, { "text": "A Model of White Dwarf Pulsar AR Scorpii: A 3.56-hour white dwarf (WD) - M dwarf (MD) close binary system, AR Scorpii,\nwas recently reported to show pulsating emission in radio, IR, optical, and UV,\nwith a 1.97-minute period, which suggests the existence of a WD with a rotation\nperiod of 1.95 minutes. We propose a model to explain the temporal and spectral\ncharacteristics of the system. The WD is a nearly perpendicular rotator, with\nboth open field line beams sweeping the MD stellar wind periodically. A bow\nshock propagating into the stellar wind accelerates electrons in the wind.\nSynchrotron radiation of these shocked electrons can naturally account for the\nbroad-band (from radio to X-rays) spectral energy distribution of the system.", "category": "astro-ph_HE" }, { "text": "Detailed Analysis of Filamentary Structure in the Weibel Instability: We present results of a 2D3V kinetic Vlasov simulation of the Weibel\ninstability. The kinetic Vlasov simulation allows us to investigate the\nvelocity distribution of dilute plasmas, in which the effect of collisions\nbetween particles is negligible, and has the advantage that the accuracy of the\ncalculated velocity distribution does not depend on the density of plasmas at\neach point in the physical space. We succeed in reproducing some features of\nthe Weibel instability shown by other simulations, for example, the\nexponentially growing phase, the saturation of the magnetic field strength, the\nformation of filamentary structure, and the coalescence of the filaments.\nEspecially, we concentrate on the behavior of the filaments after the\nsaturation of the magnetic field strength and find that there is a kind of\nquasi-equilibrium states before the coalescence occurs. Furthermore, it is\nfound that an analytical solution for stationary states of the 2D3V\nVlasov-Maxwell system can reproduce some dominant features of the\nquasi-equilibrium, e.g, the configuration of the magnetic field and the\nvelocity distribution at each point. The analytical expression could give a\nplausible model for the transition layer of a collisionless shock where a\nstrong magnetic field generated by the Weibel instability provides an effective\ndissipation process instead of collisions between particles.", "category": "astro-ph_HE" }, { "text": "On The Evolution of The Radio Pulsar PSR J1734-3333: Recent measurements showed that the period derivative of the 'high-B' radio\npulsar PSR J1734-3333 is increasing with time. For neutron stars evolving with\nfallback disks, this rotational behavior is expected in certain phases of the\nlong-term evolution. Using the same model as employed earlier to explain the\nevolution of anomalous X-ray pulsars and soft gamma-ray repeaters, we show that\nthe period, the first and second period derivatives and the X-ray luminosity of\nthis source can simultaneously acquire the observed values for a neutron star\nevolving with a fallback disk. We find that the required strength of the dipole\nfield that can produce the source properties is in the range of 10^{12} -\n10^{13} G on the pole of the neutron star. When the model source reaches the\ncurrent state properties of PSR J1734-3333, accretion onto the star has not\nstarted yet, allowing the source to operate as a regular radio pulsar. Our\nresults imply that PSR J1734-3333 is at an age of ~ 3 x 10^4 - 2 x 10^5 years.\nSuch sources will have properties like the X-ray dim isolated neutron stars or\ntransient AXPs at a later epoch of weak accretion from the diminished fallback\ndisk.", "category": "astro-ph_HE" }, { "text": "Stellar oscillations in Eddington-inspired Born-Infeld gravity: We consider the stellar oscillations of relativistic stars in the\nEddington-inspired Born-Infeld gravity (EiBI). In order to examine the specific\nfrequencies, we derive the perturbation equations governing the stellar\noscillations in EiBI by linearizing the field equations, and numerically\ndetermine the oscillation frequencies as changing the coupling parameter in\nEiBI, $\\kappa$, and stellar models. As a result, we find that the frequencies\ndepend strongly on the value of $\\kappa$, where the frequencies in EiBI with\nnegative $\\kappa$ become higher and those with positive $\\kappa$ become lower\nthan the expectations in general relativity. We also find that, via the\nobservation of the fundamental frequency, one could distinguish EiBI with\n$8\\pi\\epsilon_0|\\kappa|\\gtrsim 0.03$ from general relativity, independently of\nthe equation of state (EOS) for neutron star matter, where $\\epsilon_0$ denotes\nthe nuclear saturation density and $\\epsilon_0\\kappa$ become dimensionless\nparameter. With the further constraints on EOS, one might distinguish EiBI even\nwith $8\\pi\\epsilon_0|\\kappa|\\lesssim 0.03$ from general relativity.", "category": "astro-ph_HE" }, { "text": "Detections of simultaneous brightening of gamma-ray and optical\n emissions of a distant blazar GB 1508+5714 at redshift 4.3: GB 1508+5714 is a high-redshift blazar ($z$ = 4.3) and a spectrally soft\n$\\gamma$-ray source has been detected in its direction. By analyzing 11.4-yr\n{\\it Fermi}-LAT data, significant long-term variability of the $\\gamma$-ray\nsource is confirmed. More importantly, a $\\gamma$-ray emission flare appeared\nin an epoch of several tens of days in year 2018, when the flux is about four\ntimes of the value from the global fit. Meanwhile, optical flares displayed in\nboth $r$- and $i$-bands from the {\\it Zwicky} Transient Facility light curves.\nDetections of the simultaneous $\\gamma$-ray and optical brightening provide a\ndecisive evidence to pin down the association between the $\\gamma$-ray source\nand GB 1508+5714, which makes it the {\\it first} identified $\\gamma$-ray blazar\nbeyond redshift 4. A broadband spectral energy distribution in the high flux\nstate is constructed and the origin of the multiwavelength brightening is also\nbriefly discussed. Upcoming wide-deep-fast optical telescopes together with the\n$\\gamma$-ray sky surveyors will shed lights on the role that the AGN jets play\nin the early cosmic time.", "category": "astro-ph_HE" }, { "text": "Tests of Lorentz Invariance Violation with Gamma Rays to probe Quantum\n Gravity: We discuss the prospects of doing tests of Lorentz invariance with gamma-rays\nobserved with present and future ground based gamma-ray observatories.", "category": "astro-ph_HE" }, { "text": "Probing the origin of cosmic rays in Cygnus Cocoon using\n ultrahigh-energy gamma-ray and neutrino observations: Recent ultrahigh energy gamma-ray observations by the HAWC up to 100 TeV and\nLHAASO observatories up to 1.4 PeV energies from the direction of Fermi-LAT\n4FGL source 4FGL J2028.6+4110e (Cygnus Cocoon), are indicative of a hadronic\norigin over a leptonic process for their creation. The IceCube Neutrino\nObservatory has reported IceCube-201120A, a neutrino event coming from the same\ndirection, suggesting that the Cygnus Cocoon may correspond to one of the most\nplausible sources of high-energy cosmic rays. The apparent relationship of the\nneutrino event with the observed ultra high energy gamma-rays from Cygnus\nCocoon is investigated in this work to study if it can be explained\nconsistently in hadronic interactions of accelerated cosmic rays with ambient\nmatter. Our findings reveal that leptonic mechanisms, together with pure\nhadronic mechanisms, make a considerable contribution to the understanding of\nthe total electromagnetic spectrum as well as the observed neutrino event. The\nestimate of expected muon neutrino events from the Cygnus cocoon agrees with\nthe one muon neutrino event detected so far in IceCube multi-year observations.\nThus, our results are indicative of the potential of the Cygnus Cocoon to be a\ngalactic cosmic ray source capable of accelerating at least up to PeV energies.", "category": "astro-ph_HE" }, { "text": "Understanding the cosmic ray positron flux: Recent precision measurements of the flux of cosmic ray positrons by the\nAlpha Magnetic Spectrometer show that the spectrum has a marked softening\nfeature for energies close to one TeV. A possible interpretation of this result\nis that the observed feature measures the maximum energy of a new hard source\nof positrons perhaps associated to dark matter self--annihilation or decay, or\nto positron accelerators. A gradual hardening of the positron flux centered at\n$E \\simeq 25$~GeV can also be understood as the signature of the transition\nwhere the new source overtakes the conventional component due to secondary\nproduction. This interpretation is simple and attractive, but it is not unique.\nThe alternative possibility, that the positron flux is entirely of secondary\norigin, remains viable. In such a scenario the spectral softening observed by\nAMS for positrons is generated by energy loss effects, and a feature of\nsimilar, but not identical structure should be also visible in the $e^-$\nspectrum. Spectral features similar to both the hardening and softening of the\npositron flux are in fact observed for electrons and call for a consistent\nexplanation. Precision measurements of the $e^+$ and $e^-$ spectra in the TeV\nand multi--TeV energy range are crucial to clarify the problem.", "category": "astro-ph_HE" }, { "text": "X-ray and Optical Study of Low Core Density Globular Clusters NGC6144\n and E3: We report on the Chandra X-ray Observatory and Hubble Space Telescope\nobservation of two low core density globular clusters, NGC6144 and E3. By\ncomparing the number of X-ray sources inside the half-mass radius to those\noutside, we found 6 X-ray sources within the half-mass radius of NGC6144, among\nwhich 4 are expected to be background sources; 3 X-ray sources are also found\nwithin the half-mass radius of E3, of which 3 is expected to be background\nsource. Therefore, we cannot exclude that all our sources are background\nsources. However, combining the results from X-ray and optical observations, we\nfound that 1-2 sources in NGC6144 and 1 source in E3 are likely to be\ncataclysmic variables and that 1 source in NGC6144 is an active binary, based\non the X-ray and optical properties. The number of faint X-ray sources in\nNGC6144 and E3 found with Chandra and HST is higher than a prediction based on\ncollision frequency, but is closer to that based on mass. Our observations\nstrongly suggest that the compact binary systems in NGC6144 and E3 are\nprimordial in origin.", "category": "astro-ph_HE" }, { "text": "SN 2010jl: Optical to hard X-ray observations reveal an explosion\n embedded in a ten solar mass cocoon: (Abridged) Some supernovae (SNe) may be powered by the interaction of the SN\nejecta with a large amount of circumstellar matter (CSM). Here we outline a\nmethod to measure the mass of the optically thick CSM around such SNe. We\npresent observations of SN2010jl, including the first detection of a SN using\nNuSTAR. The total radiated luminosity of SN2010jl is extreme, at least 9e50\nerg. By modeling the visible-light data, we robustly show that the mass of the\ncircumstellar material within ~1e16 cm of the progenitor was in excess of 10\nsolar masses, likely ejected tens of years prior to the SN explosion. Our\nmodeling suggests that the shock velocity during shock breakout was ~6000 km/s,\ndecelerating to ~2600 km/s about two years after maximum light. Our late-time\nNuSTAR+XMM spectra of the SN presumably provide the first direct measurement of\nSN shock velocity two years after the SN maximum light -- measured to be in the\nrange of 2000 to 4500 km/s if the ions and electrons are in equilibrium, and\n>~2000 km/s if they are not in equilibrium. This measurement is in agreement\nwith the shock velocity predicted by our modeling of the optical data. We also\nshow that the mean radial density distribution of the CSM roughly follows an\nr^-2 law. A possible explanation for the massive CSM with a wind-like profile\nis that they are the result of multiple pulsational pair instability events\nprior to the SN explosion, separated from each other by years.", "category": "astro-ph_HE" }, { "text": "Hadronic vs leptonic models for $\u03b3$-ray emission from VER J2227+608: Recent observations of VER J2227+608 reveal a broken power $\\gamma$-ray\nspectrum with the spectral index increasing from $\\sim 1.8$ in the GeV energy\nrange to $\\sim 2.3$ in the TeV range. Such a spectral break can be attributed\nto radiative energy loss of energetic electrons in the leptonic scenario for\nthe $\\gamma$-ray emission, which, in combination with characteristic age of the\nnearby pulsar, can be used to constrain magnetic field in the emission region.\nWe show that the radio and X-ray observations can also be explained in such a\nscenario. In the hadronic scenario, the spectral break can be attributed to\ndiffusion of energetic ions in a turbulent medium and detailed spectral\nmeasurement can be used to constrain the diffusion coefficient. These two\nmodels, however, predict drastically different spectra above 100 TeV, which\nwill be uncovered with future high-resolution observations, such as LHAASO\nand/or CTA.", "category": "astro-ph_HE" }, { "text": "Glitches detected in southern radio pulsars: Parkes pulse arrival-time data for 165 radio pulsars spanning from 1990 to\n2011 have been searched for period glitches. Forty-six events out of the\ndetected 107 glitches were found to be new contributions to the entire glitch\npopulation which currently contains approximately 400 events.", "category": "astro-ph_HE" }, { "text": "Unwrapping the X-ray Spectra of Active Galactic Nuclei: Active galactic nuclei (AGN) are complex phenomena. At the heart of an AGN is\na relativistic accretion disk around a spinning supermassive black hole (SMBH)\nwith an X-ray emitting corona and, sometimes, a relativistic jet. On larger\nscales, the outer accretion disk and molecular torus act as the reservoirs of\ngas for the continuing AGN activity. And on all scales from the black hole\noutwards, powerful winds are seen that probably affect the evolution of the\nhost galaxy as well as regulate the feeding of the AGN itself. In this review\narticle, we discuss how X-ray spectroscopy can be used to study each of these\ncomponents. We highlight how recent measurements of the high-energy cutoff in\nthe X-ray continuum by NuSTAR are pushing us to conclude that X-ray coronae are\nradiatively-compact and have electron temperatures regulated by\nelectron-positron pair production. We show that the predominance of\nrapidly-rotating objects in current surveys of SMBH spin is entirely\nunsurprising once one accounts for the observational selection bias resulting\nfrom the spin-dependence of the radiative efficiency. We review recent progress\nin our understanding of fast (v~0.1-0.3c), highly-ionized (mainly visible in\nFeXXV and FeXXVI lines), high-column density winds that may dominate\nquasar-mode galactic feedback. Finally, we end with a brief look forward to the\npromise of Astro-H and future X-ray spectropolarimeters.", "category": "astro-ph_HE" }, { "text": "Theoretically motivated search and detection of non-thermal pulsations\n from PSRs J1747-2958, J2021+3651, and J1826-1256: Based on a theoretical selection of pulsars as candidates for detection at\nX-ray energies, we present an analysis of archival X-ray observations done with\nChandra and XMM-Newton of PSR J1747-2958 (the pulsar in the \"Mouse\" nebula),\nPSR J2021+3651 (the pulsar in the \"Dragonfly\" nebula), and PSR J1826-1256.\nX-ray pulsations from PSR J1747-2958 and PSR J1826-1256 are detected for the\nfirst time, and a previously reported hint of an X-ray pulsation from PSR\nJ2021+3651 is confirmed with a higher significance. We analyze these pulsars'\nspectra in regards to the theoretically predicted energy distribution, finding\na remarkable agreement, and provide here a refined calculation of the model\nparameters taking into account the newly derived X-ray spectral data.", "category": "astro-ph_HE" }, { "text": "Diffuse PeV neutrinos from EeV cosmic ray sources: semi-relativistic\n hypernova remnants in star-forming galaxies: We argue that the excess of sub-PeV/PeV neutrinos recently reported by\nIceCube could plausibly originate through pion-production processes in the same\nsources responsible for cosmic rays (CRs) with energy above the second knee\naround $10^{18}\\,$eV. The pion production efficiency for escaping CRs that\nproduce PeV neutrinos is required to be $\\gtrsim 0.1$ in such sources. On the\nbasis of current data, we identify semi-relativistic hypernova remants as\npossible sources that satisfy the requirements. By virtue of their fast ejecta,\nsuch objects can accelerate protons to EeV energies, which in turn can interact\nwith the dense surrounding medium during propagation in their host galaxies to\nproduce sufficient high-energy neutrinos via proton--proton ($pp$) collisions.\nTheir accompanying gamma ray flux can remain below the diffuse isotropic gamma\nray background observed by the {\\it Fermi} Large Area Telescope (LAT). In order\nto test this scenario and discriminate from alternatives, the density of target\nprotons/nuclei and the residence time of CRs in the interacting region are\ncrucial uncertainties that need to be clarified. As long as the neutrinos and\nEeV CRs originate from the same source class, detection of $\\gtrsim 10\\,$PeV\nneutrinos may be expected within 5-10 years' operation of IceCube. Together\nwith further observations in the PeV range, the neutrinos can help in revealing\nthe currently unknown sources of EeV CRs.", "category": "astro-ph_HE" }, { "text": "Signatures of Hierarchical Mergers in Black Hole Spin and Mass\n distribution: Recent gravitational wave (GW) observations by LIGO/Virgo show evidence for\nhierarchical mergers, where the merging BHs are the remnants of previous BH\nmerger events. These events may carry important clues about the astrophysical\nhost environments of the GW sources. In this paper, we present the\ndistributions of the effective spin parameter ($\\chi_\\mathrm{eff}$), the\nprecession spin parameter ($\\chi_\\mathrm{p}$), and the chirp mass\n($m_\\mathrm{chirp}$) expected in hierarchical mergers. Under a wide range of\nassumptions, hierarchical mergers produce (i) a monotonic increase of the\naverage of the typical total spin for merging binaries, which we characterize\nwith ${\\bar \\chi}_\\mathrm{typ}\\equiv\n\\overline{(\\chi_\\mathrm{eff}^2+\\chi_\\mathrm{p}^2)^{1/2}}$, up to roughly the\nmaximum $m_\\mathrm{chirp}$ among first-generation (1g) BHs, and (ii) a plateau\nat ${\\bar \\chi}_\\mathrm{typ}\\sim 0.6$ at higher $m_\\mathrm{chirp}$. We suggest\nthat the maximum mass and typical spin magnitudes for 1g BHs can be estimated\nfrom ${\\bar \\chi}_\\mathrm{typ}$ as a function of $m_\\mathrm{chirp}$. The GW\ndata observed in LIGO/Virgo O1--O3a prefers an increase in ${\\bar\n\\chi}_\\mathrm{typ}$ at low $m_\\mathrm{chirp}$, which is consistent with the\ngrowth of the BH spin magnitude by hierarchical mergers, at $\\sim 2 \\sigma$\nconfidence. A Bayesian analysis suggests that 1g BHs have the maximum mass of\n$\\sim 15$--$30\\,M_\\odot$ if the majority of mergers are of high-generation BHs\n(not among 1g-1g BHs), which is consistent with mergers in active galactic\nnucleus disks and/or nuclear star clusters, while if mergers mainly originate\nfrom globular clusters, 1g BHs are favored to have non-zero spin magnitudes of\n$\\sim 0.3$. We also forecast that signatures for hierarchical mergers in the\n${\\bar \\chi}_\\mathrm{typ}$ distribution can be confidently recovered once the\nnumber of GW events increases to $\\gtrsim O(100)$.", "category": "astro-ph_HE" }, { "text": "Accelerated binary black holes in globular clusters: forecasts and\n detectability in the era of space-based gravitational-wave detectors: The motion of the center of mass of a coalescing binary black hole (BBH) in a\ngravitational potential imprints a line-of-sight acceleration (LOSA) onto the\nemitted gravitational wave (GW) signal. The acceleration could be sufficiently\nlarge in dense stellar environments, such as globular clusters (GCs), to be\ndetectable with next-generation space-based detectors. In this work, we use\noutputs of the \\textsc{cluster monte carlo (cmc)} simulations of dense star\nclusters to forecast the distribution of detectable LOSAs in DECIGO and LISA\neras. We study the effect of cluster properties -- metallicity, virial and\ngalactocentric radii -- on the distribution of detectable accelerations,\naccount for cosmologically-motivated distributions of cluster formation times,\nmasses, and metallicities, and also incorporate the delay time between the\nformation of BBHs and their merger in our analysis. We find that larger\nmetallicities provide a larger fraction of detectable accelerations by virtue\nof a greater abundance of relatively lighter BBHs, which allow a higher number\nof GW cycles in the detectable frequency band. Conversely, smaller\nmetallicities result in fewer detections, most of which come from relatively\nmore massive BBHs with fewer cycles but larger LOSAs. We similarly find\ncorrelations between the virial radii of the clusters and the fractions of\ndetectable accelerations. Our work, therefore, provides an important science\ncase for space-based GW detectors in the context of probing GC properties via\nthe detection of LOSAs of merging BBHs.", "category": "astro-ph_HE" }, { "text": "Interpreting the large amplitude X-ray variation of GRS 1915+105 and IGR\n J17091-3624 as modulations of an accretion disc: Using the flux resolved spectroscopy for the first time, we analyse the\nRXTE/PCA data of the black hole X-ray binaries GRS 1915+105 and IGR\nJ17091-3624, when both sources show large amplitude, quasi-regular oscillations\nin 2.0-60.0 keV X-ray light curves. For different observations, we extract\nspectra during the peak (spectrally soft) and dip (spectrally hard) intervals\nof the oscillation, and find that their spectra are phenomenologically complex,\nrequiring at least two distinct spectral components. Besides a thermal\nComptonization component, we find that the disc emission is better modelled by\nan index-free multicolour disc blackbody component (p-free disc model) rather\nthan that from a standard accretion disc. While the peak and dip spectra are\ncomplex, remarkably, their difference spectra constructed by treating dip\nspectra as the background spectra of the peak spectra, can be modelled as a\nsingle p-free disc component. Moreover, the variability at different\ntime-scales and energy bands of the peak flux level is always greater than or\nequal to the variability of the dip flux level, which strengthens the\npossibility that the peak flux level may be due to an independent spectral\ncomponent added to the dip one. We also verify it using joint spectral analysis\nof peak and dip spectra with a variable emission component. Our result\nsubstantiates the standard hypothesis that the oscillations are due to the\nlimit cycle behaviour of an unstable radiation pressure dominated inner disc.\nHowever, in this interpretation, the flux variation of the unstable disc can be\nseveral order of magnitudes as expected from some theoretical simulations and\nneed not be fine tuned to match the factor ~10 variation seen between the peak\nand dip levels.", "category": "astro-ph_HE" }, { "text": "Neutron star dynamics under time dependent external torques: The two component model describes neutron star dynamics incorporating the\nresponse of the superfluid interior. Conventional solutions and applications\ninvolve constant external torques, as appropriate for radio pulsars on\ndynamical timescales. We present the general solution of two component dynamics\nunder arbitrary time dependent external torques, with internal torques that are\nlinear in the rotation rates, or with the extremely non-linear internal torques\ndue to vortex creep. The two-component model incorporating the response of\nlinear or nonlinear internal torques can now be applied not only to radio\npulsars but also to magnetars and to neutron stars in binary systems, with\nstrong observed variability and noise in the spin-down or spin-up rates. Our\nresults allow the extraction of the time dependent external torques from the\nobserved spin-down (or spin-up) time series, $\\dot{\\Omega}(t)$. Applications\nare discussed.", "category": "astro-ph_HE" }, { "text": "Supermassive Black Hole Winds in X-rays -- SUBWAYS. I. Ultra-fast\n outflows in QSOs beyond the local Universe: We present a new X-ray spectroscopic study of $22$ luminous\n($2\\times10^{45}\\lesssim L_{\\rm bol}\\rm /erg\\,s^{-1} \\lesssim 2\\times10^{46}$)\nactive galactic nuclei (AGNs) at intermediate-redshift ($0.1 \\lesssim z\n\\lesssim 0.4$), as part of the SUpermassive Black hole Winds in the x-rAYS\n(SUBWAYS) sample, mostly composed of quasars (QSOs) and type\\,1 AGN. Here, 17\ntargets were observed with \\textit{XMM-Newton} between 2019--2020 and the\nremaining 5 are from previous observations. The aim of this large campaign\n($1.45\\,\\rm Ms$ duration) is to characterise the various manifestations of\nwinds in the X-rays driven from supermassive black holes in AGN. In this paper\nwe focus on the search and characterization of ultra-fast outflows (UFOs),\nwhich are typically detected through blueshifted absorption troughs in the\nFe\\,K band ($E>7\\,\\rm keV$). By following Monte Carlo procedures, we confirm\nthe detection of absorption lines corresponding to highly ionised iron (e.g.,\nFe\\,\\textsc{xxv}\\,H$\\alpha$, Fe\\,\\textsc{xxvi}\\,Ly$\\alpha$) in 7/22 sources at\nthe $\\gtrsim95\\%$ confidence level (for each individual line). The global\ncombined probability of such absorption features in the sample is $>99.9\\%$.\nThe SUBWAYS campaign extends at higher luminosity and redshifts than previous\nlocal studies on Seyferts, obtained using \\xmm and \\suzaku observations. We\nfind a UFO detection fraction of $\\sim30\\%$ on the total sample that is in\nagreement with the previous findings. This work independently provides further\nsupport for the existence of highly-ionised matter propagating at mildly\nrelativistic speed ($\\gtrsim0.1c$) in a considerable fraction of AGN over a\nbroad range of luminosities, which is expected to play a key role in the\nself-regulated AGN feeding-feedback cycle, as also supported by hydrodynamical\nmultiphase simulations.", "category": "astro-ph_HE" }, { "text": "Effects of a non-universal IMF and binary parameter correlations on\n compact binary mergers: Binary population synthesis provides a direct way of studying the effects of\ndifferent choices of binary evolution models and initial parameter\ndistributions on present-day binary compact merger populations, which can then\nbe compared to empirical properties such as observed merger rates. Samples of\nzero-age main sequence binaries to be evolved by such codes are typically\ngenerated from an universal IMF and simple, uniform, distributions for orbital\nperiod $P$, mass ratio $q$ and eccentricity $e$. More recently, however,\nmounting observational evidence has suggested the non-universality of the IMF\nand the existence of correlations between binary parameters. In this study, we\nimplement a metallicity- and redshift-dependent IMF alongside correlated\ndistributions for $P$, $q$ and $e$ in order to generate representative\npopulations of binaries at varying redshifts, which are then evolved with the\nCOMPAS code in order to study the variations in merger rates and overall\npopulation properties.", "category": "astro-ph_HE" }, { "text": "The link between broad emission line fluctuations and non-thermal\n emission from the inner AGN jet: AGN reverberate when the broad emission lines respond to changes of the\nionizing thermal continuum emission. Reverberation measurements have been\ncommonly used to estimate the size of the broad-line region (BLR) and the mass\nof the central black hole. However, reverberation mapping studies have been\nmostly performed on radio-quiet sources where the contribution of the jet can\nbe neglected. In radio-loud AGN, jets and outflows may affect substantially the\nrelation observed between the ionizing continuum and the line emission. To\ninvestigate this relation, we have conducted a series of multiwavelength\nstudies of radio-loud AGN, combining optical spectral line monitoring with\nregular VLBI observations. Our results suggest that at least a fraction of the\nbroad-line emitting material can be located in a sub-relativistic outflow\nionized by non-thermal continuum emission generated in the jet at large\ndistances (> 1pc) from the central engine of AGN. This finding may have a\nstrong impact on black hole mass estimates based on measured widths of the\nbroad emission lines and on the gamma-ray emission mechanisms.", "category": "astro-ph_HE" }, { "text": "Dark matter annihilation and jet quenching phenomena in the early\n universe: Dark-matter particles like neutralinos should decouple from the hot cosmic\nplasma at temperatures of about 40 GeV. Later they can annihilate each other\ninto standard-model particles, which are injected into the dense primordial\nplasma and quickly loose energy. This process is similar to jet quenching in\nultrarelativistic heavy-ion collisions, actively studied in RHIC and LHC\nexperiments. Using empirical information from heavy-ion experiments I show that\nthe cosmological (anti)quark and gluon jets are damped very quickly until the\nplasma remains in the deconfined phase. The charged hadron and lepton jets are\nstrongly damped until the recombination of electrons and protons. The\nconsequences of energy transfer by the annihilation products to the cosmic\nmatter are discussed.", "category": "astro-ph_HE" }, { "text": "Radiation Pressure and Mass Ejection in Rho-like States of GRS 1915+105: We present a unifying scenario to address the physical origin of the\ndiversity of X-ray lightcurves within the rho variability class of the\nmicroquasar GRS 1915+105. This 'heartbeat' state is characterized by a bright\nflare that recurs every ~50-100 seconds, but the profile and duration of the\nflares varies significantly from observation to observation. Based on a\ncomprehensive, phase-resolved study of heartbeats in the RXTE archive, we\ndemonstrate that very different X-ray lightcurves do not require origins in\ndifferent accretion processes. Indeed, our detailed comparison of the\nphase-resolved spectra of a double-peaked oscillation and a single-peaked\noscillation shows that different cycles can have basically similar X-ray\nspectral evolution. We argue that all heartbeat oscillations can be understood\nas the result of a combination of a thermal-viscous radiation pressure\ninstability, a local Eddington limit in the disk, and a sudden,\nradiation-pressure-driven evaporation or ejection event in the inner accretion\ndisk. This ejection appears to be a universal, fundamental part of the rho\nstate, and is largely responsible for a hard X-ray pulse seen in the lightcurve\nof all cycles. We suggest that the detailed shape of oscillations in the mass\naccretion rate through the disk is responsible for the phenomenological\ndifferences between different rho-type lightcurves, and we discuss how future\ntime-dependent simulations of disk instabilities may provide new insights into\nthe role of radiation pressure in the accretion flow.", "category": "astro-ph_HE" }, { "text": "Radioactive heating rate of r-process elements and macronova light curve: We study the heating rate of r-process nuclei and thermalization of decay\nproducts in neutron star merger ejecta and macronova (kilonova) light curves.\nThermalization of charged decay products, i.e., electrons, $\\alpha$-particles,\nand fission fragments is calculated according to their injection energy. The\n$\\gamma$-ray thermalization processes are also properly calculated by taking\nthe $\\gamma$-ray spectrum of each decay into account. We show that the\n$\\beta$-decay heating rate at later times approaches a power-law decline as\n$\\propto t^{-2.8}$, which agrees with the result of Waxman et al. (2019). We\npresent a new analytic model to calculate macronova light curves, in which the\ndensity structure of the ejecta is accounted for. We demonstrate that the\nobserved bolometric light curve and temperature evolution of the macronova\nassociated with GW170817 are reproduced well by the $\\beta$-decay heating rate\nwith the solar r-process abundance pattern. We interpret the break in the\nobserved bolometric light curve around a week as a result of the diffusion wave\ncrossing a significant part of the ejecta rather than a thermalization break.\nWe also show that the time-weighted integral of the bolometric light curve\n(Katz integral) is useful to provide an estimate of the total r-process mass\nfrom the observed data, which is independent of the highly uncertain radiative\ntransfer. For the macronova in GW170817, the ejecta mass is robustly estimated\nas $\\approx 0.05M_{\\odot}$ for $A_{\\rm min}\\leq 72$ and $85\\leq A_{\\rm min}\\leq\n130$ with the solar r-process abundance pattern. The code for computation of\nthe heating rate and light curve for given initial nuclear abundances is\npublicly available.", "category": "astro-ph_HE" }, { "text": "Search for Individual Ultrahigh Energy Cosmic Ray Sources in the Future\n Data: We propose a new way to detect individual bright Ultra-High Energy Cosmic Ray\n(UHECR) sources above background if the Galactic Magnetic Field (GMF) gives the\nmain contribution to UHECR deflections. This method can be directly applied to\nmaps given by experiments. It consists in starting from at least two high\nenergy events above 6x10^19 eV, and looking at lower energy tails. We test the\nefficiency of the method and investigate its dependence on different\nparameters. In case of detection, the source position and the local GMF\ndeflection power are reconstructed. Both reconstructions are strongly affected\nby the turbulent GMF. With the parameters adopted in this study, for 68 % of\nreconstructed sources, the angular position is less than one degree from the\nreal one. For typical turbulent field strengths of 4 \\mu G at the Earth\nposition and 1.5 kpc extension in the halo, one can reconstruct the deflection\npower with 25 % precision in 68 % of cases.", "category": "astro-ph_HE" }, { "text": "On the equivalent width of the Fe K$\u03b1$ line produced by a dusty\n absorber in active galactic nuclei: Obscured AGNs provide an opportunity to study the material surrounding the\ncentral engine. Geometric and physical constraints on the absorber can be\ndeduced from the reprocessed AGN emission. In particular, the obscuring gas may\nreprocess the nuclear X-ray emission producing a narrow Fe K$\\alpha$ line and a\nCompton reflection hump. In recent years, models of the X-ray reflection from\nan obscuring torus have been computed; however, although the reflecting gas may\nbe dusty, the models do not yet take into account the effects of dust on the\npredicted spectrum. We study this problem by analyzing two sets of models, with\nand without the presence of dust, using the one dimensional photo-ionization\ncode Cloudy. The calculations are performed for a range of column densities\n($22 <{\\rm log}[N_H(\\rm cm^{-2})]< 24.5$ ) and hydrogen densities ( $6 <{\\rm\nlog}[n_H(\\rm cm^{-3})]< 8$). The calculations show the presence of dust can\nenhance the Fe K$\\alpha$ equivalent width (EW) in the reflected spectrum by\nfactors up to $\\approx$ 8 for Compton thick (CT) gas and a typical ISM grain\nsize distribution. The enhancement in EW with respect to the reflection\ncontinuum is due to the reduction in the reflected continuum intensity caused\nby the anisotropic scattering behaviour of dust grains. This effect will be\nmost relevant for reflection from distant, predominately neutral gas, and is a\npossible explanation for AGNs which show a strong Fe K$\\alpha$ EW and a\nrelatively weak reflection continuum. Our results show it is an important to\ntake into account dust while modeling the X-ray reflection spectrum, and that\ninferring a CT column density from an observed Fe K$\\alpha$ EW may not always\nbe valid. Multi-dimensional models are needed to fully explore the magnitude of\nthe effect.", "category": "astro-ph_HE" }, { "text": "ALMA CO Observations of the Gamma-Ray Supernova Remnant RX J1713.7-3946:\n Discovery of Shocked Molecular Cloudlets and Filaments at 0.01 pc scales: RX J1713.7-3946 is a unique core-collapse SNR that emits bright TeV\ngamma-rays and synchrotron X-rays caused by cosmic rays, in addition to\ninteractions with interstellar gas clouds. We report here on results of ALMA\n$^{12}$CO($J$ = 1-0) observations toward the northwestern shell of the SNR. We\nnewly found three molecular complexes consisting of dozens of shocked molecular\ncloudlets and filaments with typical radii of $\\sim$0.03-0.05 pc and densities\nof $\\sim$$10^4$ cm$^{-3}$. These cloudlets and filaments are located not only\nalong synchrotron X-ray filaments, but also in the vicinity of X-ray hotspots\nwith month or year-scale time variations. We argue that X-ray hotspots were\ngenerated by shock-cloudlet interactions through magnetic-field amplification\nup to mG. The ISM density contrast of $\\sim$$10^5$, coexistence of molecular\ncloudlets and low-density diffuse medium of $\\sim$0.1 cm$^{-3}$, is consistent\nwith such a magnetic field amplification as well as a wind-bubble scenario. The\nsmall-scale cloud structures also affect hadronic gamma-ray spectra considering\nthe magnetic field amplification on surface and inside clouds.", "category": "astro-ph_HE" }, { "text": "Correlated variability of the reflection fraction with the X-ray flux\n and spectral index for Mkn 478: The X-ray spectrum of Mkn 478 is known to be dominated by a strong soft\nexcess which can be described using relativistic blurred reflection. Using\nobservations from {\\it XMM-Newton}, {\\it AstroSat} and {\\it Swift}, we show\nthat for the long-term ($\\sim$ years) and intermediate-term (days to months)\nvariability, the reflection fraction is anti-correlated with the flux and\nspectral index, which implies that the variability is due to the hard X-ray\nproducing corona moving closer to and further from the black hole. Using\nflux-resolved spectroscopy of the {\\it XMM-Newton} data, we show that the\nreflection fraction has the same behaviour with flux and index on short\ntime-scales of hours. The results indicate that both the long-term and\nshort-term variability of the source is determined by the same physical\nmechanism of strong gravitational light bending causing enhanced reflection and\nlow flux as the corona moves closer to the black hole.", "category": "astro-ph_HE" }, { "text": "Fermi LAT Measurements of the Diffuse Gamma-Ray Emission at Intermediate\n Galactic Latitudes: The diffuse Galactic gamma-ray emission is produced by cosmic rays (CRs)\ninteracting with the interstellar gas and radiation field. Measurements by the\nEnergetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton\nGamma-Ray Observatory indicated excess gamma-ray emission > 1 GeV relative to\ndiffuse Galactic gamma-ray emission models consistent with directly measured CR\nspectra (the so-called ``EGRET GeV excess''). The excess emission was observed\nin all directions on the sky, and a variety of explanations have been proposed,\nincluding beyond-the-Standard-Model scenarios like annihilating or decaying\ndark matter. The Large Area Telescope (LAT) instrument on the Fermi Gamma-ray\nSpace Telescope has measured the diffuse gamma-ray emission with unprecedented\nsensitivity and resolution. We report on LAT measurements of the diffuse\ngamma-ray emission for energies 100 MeV to 10 GeV and Galactic latitudes 10\ndeg. <= |b| <= 20 deg. The LAT spectrum for this region of the sky is well\nreproduced by the diffuse Galactic gamma-ray emission models mentioned above\nand inconsistent with the EGRET GeV excess.", "category": "astro-ph_HE" }, { "text": "Effects of shock and turbulence properties on electron acceleration: Using test particle simulations we study electron acceleration at\ncollisionless shocks with a two-component model turbulent magnetic field with\nslab component including dissipation range. We investigate the importance of\nshock normal angle $\\theta_{Bn}$, magnetic turbulence level\n$\\left(b/B_0\\right)^2$, and shock thickness on the acceleration efficiency of\nelectrons. It is shown that at perpendicular shocks the electron acceleration\nefficiency is enhanced with the decreasing of $\\left(b/B_0\\right)^2$, and at\n$\\left(b/B_0\\right)^2=0.01$ the acceleration becomes significant due to strong\ndrift electric field with long time particles staying near the shock front for\nshock drift acceleration (SDA). In addition, at parallel shocks the electron\nacceleration efficiency is increasing with the increasing of\n$\\left(b/B_0\\right)^2$, and at $\\left(b/B_0\\right)^2=10.0$ the acceleration is\nvery strong due to sufficient pitch-angle scattering for first-order Fermi\nacceleration, as well as due to large local component of magnetic field\nperpendicular to shock normal angle for SDA. On the other hand, the high\nperpendicular shock acceleration with $\\left(b/B_0\\right)^2=0.01$ is stronger\nthan the high parallel shock acceleration with ($\\left(b/B_0\\right)^2=10.0$),\nthe reason might be the assumption that SDA is more efficient than first-order\nFermi acceleration. Furthermore, for oblique shocks, the acceleration\nefficiency is small no matter the turbulence level is low or high. Moreover,\nfor the effect of shock thickness on electron acceleration at perpendicular\nshocks, we show that there exists the bend-over thickness, $L_{\\text{diff,b}}$.\nThe acceleration efficiency does not change evidently if the shock thickness is\nmuch smaller than $L_{\\text{diff,b}}$. However, if the shock thickness is much\nlarger than $L_{\\text{diff,b}}$, the acceleration efficiency starts to drop\nabruptly.", "category": "astro-ph_HE" }, { "text": "TESS Shines Light on the Origin of the Ambiguous Nuclear Transient\n ASASSN-18el: We analyze high-cadence data from the Transiting Exoplanet Survey Satellite\n(TESS) of the ambiguous nuclear transient (ANT) ASASSN-18el. The optical\nchanging-look phenomenon in ASASSN-18el has been argued to be due to either a\ndrastic change in the accretion rate of the existing active galactic nucleus\n(AGN) or the result of a tidal disruption event (TDE). Throughout the TESS\nobservations, short-timescale stochastic variability is seen, consistent with\nan AGN. We are able to fit the TESS light curve with a damped-random-walk (DRW)\nmodel and recover a rest-frame variability amplitude of $\\hat{\\sigma} = 0.93\n\\pm 0.02$ mJy and a rest-frame timescale of $\\tau_{DRW} = 20^{+15}_{-6}$ days.\nWe find that the estimated $\\tau_{DRW}$ for ASASSN-18el is broadly consistent\nwith an apparent relationship between the DRW timescale and central\nsupermassive black hole mass. The large-amplitude stochastic variability of\nASASSN-18el, particularly during late stages of the flare, suggests that the\norigin of this ANT is likely due to extreme AGN activity rather than a TDE.", "category": "astro-ph_HE" }, { "text": "Log-normal flux distribution of bright Fermi blazars: We present the results of the $\\gamma$-ray flux distribution study on the\nbrightest blazars which are observed by the \\emph{Fermi}-LAT. We selected 50\nbrightest blazars based on the maximum number of detection reported in the LAT\nthird AGN catalog. We performed standard unbinned maximum likelihood analysis\non the LAT data during the period between August 2008 and December 2016, in\norder to obtain the average monthly flux. After quality cuts, blazars for which\nat least 90\\% of the total flux was survived were selected for the further\nstudy, and this includes 19 FSRQs and 19 BL Lacs. The Anderson-Darling and\n$\\chi^2$ tests suggest that the integrated monthly flux follow a log-normal\ndistribution for all sources, except for three FSRQs for which neither a normal\nnor a log-normal distribution was preferred. A double log-normal flux\ndistribution tendency were observed in these sources, though it has to be\nconfirmed with improved statistics. We also found that, the standard deviation\nof the log-normal flux distribution increases with the mean spectral index of\nthe blazar, and can be fitted with a line of slope 0.24$\\pm$0.04. We repeat our\nstudy on three additional brightest unclassified blazars to identify their flux\ndistribution properties. Based on the features of their log-normal flux\ndistribution, we infer these unclassified blazars may be closely associated\nwith FSRQs. We also highlight that considering the log-normal behavior of the\nflux distribution of blazars, averaging their long term flux in linear scale\ncan largely under estimate the nominal flux and this discrepancy can propagate\ndown to the estimation of source parameters through spectral modeling.", "category": "astro-ph_HE" }, { "text": "The Role of Superluminal Electromagnetic Waves in Pulsar Wind\n Termination Shocks: The dynamics of a standing shock front in a Poynting-flux dominated\nrelativistic flow is investigated by using a one-dimensional, relativistic,\ntwo-fluid simulation. An upstream flow containing a circularly polarized,\nsinusoidal magnetic shear wave is considered, mimicking a wave driven by an\nobliquely rotating pulsar. It is demonstrated that this wave is converted into\nlarge amplitude electromagnetic waves with superluminal phase speeds by\ninteracting with the shock when the shock-frame frequency of the wave exceeds\nthe proper plasma frequency. The superluminal waves propagate in the upstream,\nmodify the shock structure substantially, and form a well-developed precursor\nregion ahead of a subshock. Dissipation of Poynting flux occurs in the\nprecursor as well as in the downstream region through a parametric instability\ndriven by the superluminal waves. The Poynting flux remaining in the downstream\nregion is carried entirely by the superluminal waves. The downstream plasma is\ntherefore an essentially unmagnetized, relativistically hot plasma with a\nnon-relativistic flow speed, as suggested by observations of pulsar wind\nnebulae.", "category": "astro-ph_HE" }, { "text": "The NANOGrav 11-year Data Set: High-precision timing of 45 Millisecond\n Pulsars: We present high-precision timing data over time spans of up to 11 years for\n45 millisecond pulsars observed as part of the North American Nanohertz\nObservatory for Gravitational Waves (NANOGrav) project, aimed at detecting and\ncharacterizing low-frequency gravitational waves. The pulsars were observed\nwith the Arecibo Observatory and/or the Green Bank Telescope at frequencies\nranging from 327 MHz to 2.3 GHz. Most pulsars were observed with approximately\nmonthly cadence, with six high--timing-precision pulsars observed weekly, and\nall were observed at widely separated frequencies at each observing epoch in\norder to fit for time-variable dispersion delays. We describe our methods for\ndata processing, time-of-arrival (TOA) calculation, and the implementation of a\nnew, automated method for removing outlier TOAs. We fit a timing model for each\npulsar that includes spin, astrometric, and, if necessary, binary parameters,\nin addition to time-variable dispersion delays and parameters that quantify\npulse-profile evolution with frequency. The new timing solutions provide three\nnew parallax measurements, two new Shapiro delay measurements, and two new\nmeasurements of large orbital-period variations. We fit models that\ncharacterize sources of noise for each pulsar. We find that 11 pulsars show\nsignificant red noise, with generally smaller spectral indices than typically\nmeasured for non-recycled pulsars, possibly suggesting a different origin.\nFuture papers will use these data to constrain or detect the signatures of\ngravitational-wave signals.", "category": "astro-ph_HE" }, { "text": "Exploding neutron stars in close binaries: The discovery of GW signal from merging neutron stars by LIGO on 17th August\n2017 was followed by a short GRB170817A discovered by FERMI and INTEGRAL 1.7\nseconds after the loss of the GW signal when it just reached its maximum. Here\nwe present a reproduction of the first paper (published by us in 1984)\npredicting a short GRB after GW signal of merging neutron stars. Our paper\nfollowed the scenario by Clark and Eardley (1977) who predicted a catastrophic\ndisruption of a neutron star in a binary 1.7 seconds after the peak of GW\nsignal. Our next paper in 1990 predicted all the main properties of the short\nGRB with quite a reasonable accuracy. Typos in English translation are\ncorrected and a few comments are added in the current publication as numbered\nfootnotes (the only footnote from the original paper is marked by an asterisk).", "category": "astro-ph_HE" }, { "text": "Characterising the rotational irregularities of the Vela pulsar from 21\n yr of phase-coherent timing: Pulsars show two classes of rotational irregularities that can be used to\nunderstand neutron-star interiors and magnetospheres: glitches and timing\nnoise. Here we present an analysis of the Vela pulsar spanning nearly 21 yr of\nobservation and including 8 glitches. We identify the relative pulse number of\nall of the observations between glitches, with the only pulse-number\nambiguities existing over glitch events. We use the phase coherence of the\ntiming solution to simultaneously model the timing noise and glitches in a\nBayesian framework, allowing us to select preferred models for both. We find\nthe glitches can be described using only permanent and transient changes in\nspin frequency, i.e., no step changes in frequency derivative. For all of the\nglitches, we only need two exponentially decaying changes in spin frequency to\nmodel the transient components. In contrast to previous studies, we find that\nthe dominant transient components decay on a common $\\approx$ 1300 d time\nscale, and that a larger fraction ( $\\gtrsim 25\\%$) of glitch amplitudes are\nassociated with these transient components. We also detect shorter-duration\ntransient components of $\\approx$ 25 d, as previously observed, but are limited\nin sensitivity to events with shorter durations by the cadence of our\nobservations. The timing noise is well described by a steep power-law process\nthat is independent of the glitches and subdominant to the glitch recovery. The\nbraking index is constrained to be $<$ 8 with 95% confidence. This methodology\ncan be used to robustly measure the properties of glitches and timing noise in\nother pulsars.", "category": "astro-ph_HE" }, { "text": "Stability of interlinked neutron vortex and proton flux-tube arrays in a\n neutron star -- III. Proton feedback: The coupled, time-dependent Gross-Pitaevskii and Ginzburg-Landau equations\nare solved simultaneously in three dimensions to investigate the equilibrium\nstate and far-from-equilibrium, spin-down dynamics of an interpenetrating\nneutron superfluid and proton type-II superconductor, as an idealized\ndescription of the outer core of a neutron star. The simulations generalize\nprevious calculations without the time-dependent Ginzburg-Landau equation,\nwhere proton feedback is absent. If the angle $\\theta$ between the rotation and\nmagnetic axes does not equal zero, the equilibrium state consists of\ngeometrically complicated neutron vortex and proton flux-tube tangles, as the\ntopological defects pin to one another locally but align with different axes\nglobally. During spin-down, new types of motion are observed. For $\\theta = 0$,\nentire vortices pair rectilinearly with flux tubes and move together while\npinned. For $\\theta \\neq 0$, vortex segments pair with segments from one or\nmore flux tubes, and the paired segments move together while pinned. The degree\nto which proton feedback impedes the deceleration of the crust is evaluated as\na function of $\\theta$ and the pinning strength, $\\eta$. Key geometric\nproperties of vortex-flux-tube tangles, such as filament length, mean\ncurvature, and polarity are analysed. It is found that proton feedback smooths\nthe deceleration of the crust, reduces the rotational glitch sizes, and\nstabilizes the vortex tangle dynamics. The dimensionless control parameters in\nthe simulations are mutually ordered to match what is expected in a real\nneutron star, but their central values and dynamics ranges differ from reality\nby many orders of magnitude due to computational limitations.", "category": "astro-ph_HE" }, { "text": "The drifting subpulses of PSR B0031-07 and its synchronously modulated\n radio polarization: We establish that for PSR B0031-07 the orthogonal polarization modes switch\nat a single pulse level synchronously with the periodic drifting subpulses seen\nin total intensity. There are only four other pulsars known for which this\nphenomenon is observed. PSR B0031-07 is unique as it is the only source in this\ngroup which has multiple stable drift modes. For both drift modes visible at\nour observing frequency centered at 1369 MHz, the modulation of polarization\nmodes is synchronous with the drifting subpulses. In one of the drift modes, a\ndiscontinuity in the modulation pattern of polarization properties occurs\nhalfway through the pulse, coinciding with a slight change in the slope of the\nintensity drift band. In contrast to what has been suggested for this pulsar in\nthe past, this, plus other differences in the polarization of the modulated\nemission observed for the two drift modes, suggests that a drift mode change is\nmore than a change in the underlying carousel radius and that magnetospheric\npropagation effects play an important role. The ellipticity evolves\nasymmetrically in time during the modulation cycle, which in the framework of a\ncarousel model implies that the polarized sub-beams are asymmetric with respect\nto the sense of circulation, something which is not observed for other pulsars.\nBirefringence in the magnetosphere, resulting in the orthogonal polarization\nmodes to spatially separate, is not enough to explain these results. It is\nargued that more complex magnetospheric processes, which possibly allow\nconversion between orthogonal polarization modes, play a role.", "category": "astro-ph_HE" }, { "text": "Indirect evidence of GeV Dark Matter: Recently, an excess of GeV gamma ray near the Galactic Centre has\nbeenvreported. The spectrum obtained can be best fitted with the\nannihilationvof $30-40$ GeV dark matter particles through $b \\bar{b}$ channel.\nIn this letter, I show that this annihilation model can also solve the\nmysteries of heating source in x-ray plasma and the unexpected high gamma-ray\nluminosity. The cross section constrained by these observations give excellent\nagreements with both the predicted range by using Fermi-LAT data and the\ncanonical thermal relic abundance cross section.", "category": "astro-ph_HE" }, { "text": "Relativistic Viscous Radiation Hydrodynamic Simulations of Geometrically\n Thin Disks: I. Thermal and Other Instabilities: We present results from two-dimensional, general relativistic, viscous,\nradiation hydrodynamic numerical simulations of Shakura-Sunyaev thin disks\naccreting onto stellar mass Schwarzschild black holes. We consider cases on\nboth the gas- and radiation-pressure-dominated branches of the thermal\nequilibrium curve, with mass accretion rates spanning the range from $\\dot{M} =\n0.01 L_\\mathrm{Edd}/c^2$ to $10 L_\\mathrm{Edd}/c^2$. The simulations directly\ntest the stability of this standard disk model on the different branches. We\nfind clear evidence of thermal instability for all radiation-pressure-dominated\ndisks, resulting universally in the vertical collapse of the disks, which in\nsome cases then settle onto the stable, gas-pressure-dominated branch. Although\nthese results are consistent with decades-old theoretical predictions, they\nappear to be in conflict with available observational data from black hole\nX-ray binaries. We also find evidence for a radiation-pressure-driven\ninstability that breaks the unstable disks up into alternating rings of high\nand low surface density on a timescale comparable to the thermal collapse.\nSince radiation is included self-consistently in the simulations, we are able\nto calculate lightcurves and power density spectra (PDS). For the most part, we\nmeasure radiative efficiencies (ratio of luminosity to mass accretion rate)\nclose to 6%, as expected for a non-rotating black hole. The PDS appear as\nbroken power laws, with a break typically around 100 Hz. There is no evidence\nof significant excess power at any frequencies, i.e. no quasi-periodic\noscillations are observed.", "category": "astro-ph_HE" }, { "text": "Properties of strongly magnetized ultradense matter and their imprints\n on magnetar pulsations: We investigate the effect of strong magnetic fields on the adiabatic radial\noscillations of hadronic stars. We describe magnetized hadronic matter within\nthe framework of the relativistic nonlinear Walecka model and integrate the\nequations of relativistic radial oscillations to determine the fundamental\npulsation mode. We consider that the magnetic field increases, in a density\ndependent way, from the surface, where it has a typical magnetar value of\n$10^{15}$ G, to the interior of the star where it can be as large as $3 \\times\n10^{18}$ G. We show that magnetic fields of the order of $10^{18}$ G at the\nstellar core produce a significant change in the frequency of neutron star\npulsations with respect to unmagnetized objects. If radial pulsations are\nexcited in magnetar flares, they can leave an imprint in the flare lightcurves\nand open a new window for the study of highly magnetized ultradense matter.", "category": "astro-ph_HE" }, { "text": "A simplified view of blazars: why BL Lacertae is actually a quasar in\n disguise: We put forward a scenario where blazars are classified as flat-spectrum radio\nquasars, BL Lacs, low synchrotron, or high synchrotron peaked objects according\nto a varying combination of Doppler boosted radiation from the jet, emission\nfrom the accretion disk, the broad line region, and light from the host galaxy.\nWe thoroughly test this new approach, which builds upon unified schemes, using\nMonte Carlo simulations and show that it can provide simple answers to a number\nof long-standing open issues. We also demonstrate that selection effects play a\nvery important role in the diversity observed in radio and X-ray samples and in\nthe correlation between luminosity and peak frequency of the synchrotron power\n(the so-called \"blazar sequence\"). It turns out that sources so far classified\nas BL Lacs on the basis of their observed weak, or undetectable, emission lines\nare of two physically different classes: intrinsically weak-lined objects, more\ncommon in X-ray selected samples, and heavily diluted broad-lined sources, more\nfrequent in radio selected samples, which explains some of the confusion in the\nliterature.", "category": "astro-ph_HE" }, { "text": "From Supernova to Remnant: Tracking the Evolution of the Oldest Known\n X-ray Supernovae: Core-collapse supernovae (SNe) expand into a medium created by winds from the\npre-SN progenitor. The SN explosion and resulting shock wave(s) heat up the\nsurrounding plasma, giving rise to thermal X-ray emission, which depends on the\ndensity of the emitting material. Tracking the variation of the X-ray\nluminosity over long periods of time thus allows for investigation of the\nkinematics of the SN shock waves, the structure of the surrounding medium, and\nthe nature of the progenitor star. In this paper X-ray observations of five of\nthe oldest known X-ray supernovae - SN 1970G, SN 1968D, SN 1959D, SN 1957D and\nSN 1941C - are analyzed, with the aim of reconstructing their light curves over\nseveral decades. For those supernovae for which we can extract multi-epoch\ndata, the X-ray luminosity appears to decline with time, although with large\nerror bars. No increase in the X-ray emission from SN 1970G is found at later\nepochs, contrary to previous reports. All five SNe show X-ray luminosities that\nare of comparable magnitude. We compare the late-time X-ray luminosities of\nthese SNe to those of supernova remnants (SNRs) in the Galaxy which are a few\nhundred years old, and find that when the tentative decline is taken into\naccount, the luminosity of the old SNe studied herein could fall below the\nluminosity of some of the younger SNRs within a few hundred years. However, the\nX-ray luminosity should begin to increase as the SNe expand in the Sedov phase,\nthus reaching that of the observed SNRs.", "category": "astro-ph_HE" }, { "text": "3 to 12 millimetre studies of dense gas towards the western rim of\n supernova remnant RX J1713.7-3946: The young X-ray and gamma-ray-bright supernova remnant RXJ1713.7-3946 (SNR\nG347.3-0.5) is believed to be associated with molecular cores that lie within\nregions of the most intense TeV emission. Using the Mopra telescope, four of\nthe densest cores were observed using high-critical density tracers such as\nCS(J=1-0,J=2-1) and its isotopologue counterparts, NH3(1,1) and (2,2) inversion\ntransitions and N2H+(J=1-0) emission, confirming the presence of dense gas\n>10^4cm^-3 in the region. The mass estimates for Core C range from 40M_{\\odot}\n(from CS(J=1-0)) to 80M_{\\odot} (from NH3 and N2H+), an order of magnitude\nsmaller than published mass estimates from CO(J=1-0) observations. We also\nmodelled the energy-dependent diffusion of cosmic-ray protons accelerated by\nRXJ1713.7-3946 into Core C, approximating the core with average density and\nmagnetic field values. We find that for considerably suppressed diffusion\ncoefficients (factors \\chi=10^{-3} down to 10^{-5} the galactic average), low\nenergy cosmic-rays can be prevented from entering the inner core region. Such\nan effect could lead to characteristic spectral behaviour in the GeV to TeV\ngamma-ray and multi-keV X-ray fluxes across the core. These features may be\nmeasurable with future gamma-ray and multi-keV telescopes offering arcminute or\nbetter angular resolution, and can be a novel way to understand the level of\ncosmic-ray acceleration in RXJ1713.7-3946 and the transport properties of\ncosmic-rays in the dense molecular cores.", "category": "astro-ph_HE" }, { "text": "On the Origin of Diffuse Radio Emission in Abell 85 -- Insights from new\n GMRT Observations: Extended, steep, and ultra-steep spectrum radio emission in a galaxy cluster\nis usually associated with recent mergers. Simulations show that radio\nphoenixes are aged radio galaxy lobes whose emission reactivates when a low\nMach shock compresses it. A85 hosts a textbook example of a radio phoenix at\nabout 320 kpc southwest of the cluster center. We present a new high resolution\n325 MHz GMRT radio map illustrating this radio phoenix's complex and\nfilamentary structure. The full extent of the radio structure is revealed for\nthe first time from these radio images of A85. Using archival \\textit{Chandra}\nX-ray observations, we applied an automated 2-D shock finder to the X-ray\nsurface brightness and Adaptive Circular Binning (ACB) temperature maps which\nconfirmed a bow shock at the location of the radio phoenix. We also compared\nthe Mach number from the X-ray data with the radio-derived Mach number in the\nsame region using multi-frequency radio observations and find that they are\nconsistent within the 1$\\sigma$ error level.", "category": "astro-ph_HE" }, { "text": "Supernova Explosions inside Carbon-Oxygen Circumstellar Shells: Motivated by a recent discovery of Supernova 2010gx and numerical results of\nFryer et al.(2010), we simulate light curves for several type I supernova\nmodels, enshrouded by dense circumstellar shells, or \"super-wind\", rich in\ncarbon and oxygen and having no hydrogen. We demonstrate that the most luminous\nevents like SN2010gx can be explained by those models at moderate explosion\nenergies (2-3) foe if the total mass of SN ejecta and a shell is (3-5) Msun and\nthe radius of the shell is ~10^{16} cm.", "category": "astro-ph_HE" }, { "text": "The time-dependent one-zone hadronic model - First principles: We present a time-dependent approach to the one-zone hadronic model in the\ncase where the photon spectrum is produced by ultrarelativistic protons\ninteracting with soft photons that are produced from protons and low magnetic\nfields. Assuming that protons are injected at a certain rate in a homogeneous\nspherical volume containing a magnetic field, the evolution of the system can\nbe described by five coupled kinetic equations, for protons, electrons,\nphotons, neutrons, and neutrinos. Photopair and photopion interactions are\nmodelled using the results of Monte-Carlo simulations and, in particular from\nthe SOPHIA code for the latter. The coupling of energy losses and injection\nintroduces a self-consistency in our approach and allows the study of the\ncomparative relevancy of processes at various conditions, the efficiency of the\nconversion of proton luminosity to radiation, the resulting neutrino spectra,\nand the effects of time variability on proton injection, among other topics. We\npresent some characteristic examples of the temporal behaviour of the system\nand show that this can be very different from the one exhibited by leptonic\nmodels. Furthermore, we argue that, contrary to the wide-held belief, there are\nparameter regimes where the hadronic models can become quite efficient.\nHowever, to keep the free parameters at a minimum and facilitate an in-depth\nstudy of the system, we have only concentrated on the case where protons are\ninjected; i.e., we did not consider the effects of a co-accelerated leptonic\ncomponent.", "category": "astro-ph_HE" }, { "text": "A generic method to constrain the dark matter model parameters from\n Fermi observations of dwarf spheroids: Observation of gamma-rays from dwarf galaxies is an effective way to search\nfor particle dark matter. Using 4-year data of Fermi-LAT observations on a\nseries of Milky Way satellites, we develop a general way to search for the\nsignals from dark matter annihilation in such objects. Instead of giving prior\ninformation about the energy spectrum of dark matter annihilation, we bin the\nFermi-LAT data into several energy bins and build a likelihood map in the\n\"energy bin - flux\" plane. The final likelihood of any spectrum can be easily\nderived through combining the likelihood of all the energy bins. It gives\nconsistent result with that directly calculated using the Fermi Scientific\nTool. This method is very efficient for the study of any specific dark matter\nmodels with gamma-rays. We use the new likelihood map with Fermi-LAT 4 year\ndata to fit the parameter space in three representative dark matter models: i)\ntoy dark matter model, ii) effective dark matter operators, and iii)\nsupersymmetric neutralino dark matter.", "category": "astro-ph_HE" }, { "text": "The spin and orbit of the newly discovered pulsar IGR J17480-2446: We present an analysis of the spin and orbital properties of the newly\ndiscovered accreting pulsar IGR J17480-2446, located in the globular cluster\nTerzan 5. Considering the pulses detected by the Rossi X-ray Timing Explorer at\na period of 90.539645(2) ms, we derive a solution for the 21.27454(8) hr binary\nsystem. The binary mass function is estimated to be 0.021275(5) Msun,\nindicating a companion star with a mass larger than 0.4 Msun. The X-ray pulsar\nspins up while accreting at a rate of between 1.2 and 1.7E-12 Hz/s, in\nagreement with the accretion of disc matter angular momentum given the observed\nluminosity.\n We also report the detection of pulsations at the spin period of the source\nduring a Swift observation performed ~2 d before the beginning of the RXTE\ncoverage. Assuming that the inner disc radius lies in between the neutron star\nradius and the corotation radius while the source shows pulsations, we estimate\nthe magnetic field of the neutron star to be within ~2E8 G and ~2.4E10 G. From\nthis estimate, the value of the spin period and of the observed spin-up rate,\nwe associate this source with the still poorly sampled population of slow,\nmildly recycled, accreting pulsars.", "category": "astro-ph_HE" }, { "text": "TeV gamma-ray survey of the Northern sky using the ARGO-YBJ detector: The ARGO-YBJ detector is an extensive air shower array that has been used to\nmonitor the northern $\\gamma$-ray sky at energies above 0.3 TeV from 2007\nNovember to 2013 January. In this paper, we present the results of a sky survey\nin the declination band from $-10^{\\circ}$ to $70^{\\circ}$, using data recorded\nover the past five years. With an integrated sensitivity ranging from 0.24 to\n$\\sim$1 Crab units depending on the declination, six sources have been detected\nwith a statistical significance greater than 5 standard deviations. Several\nexcesses are also reported as potential $\\gamma$-ray emitters. The features of\neach source are presented and discussed. Additionally, $95\\%$ confidence level\nupper limits of the flux from the investigated sky region are shown. Specific\nupper limits for 663 GeV $\\gamma$-ray AGNs inside the ARGO-YBJ field of view\nare reported. The effect of the absorption of $\\gamma$-rays due to the\ninteraction with extragalactic background light is estimated.", "category": "astro-ph_HE" }, { "text": "Suzaku Discovery of a Slowly Varying Hard X-ray Continuum from the Type\n I Seyfert Galaxy NGC 3516: The bright type I Seyfert galaxy NGC 3516 was observed by {\\it Suzaku} twice,\nin 2005 October 12--15 and 2009 October 28--November 2, for a gross time\ncoverage of 242 and 544 ksec and a net exposure of 134 and 255 ksec,\nrespectively. The 2--10 keV luminosity was $2.8 \\times 10^{41}$ erg s$^{-1}$ in\n2005, and $1.6 \\times 10^{41}$ erg s$^{-1}$ in 2009. The 1.4--1.7 keV and 2--10\nkeV count rates both exhibited peak-to-peak variations by a factor of $\\sim2$\nin 2005, while $\\sim 4$ in 2009. In either observation, the 15--45 keV count\nrate was less variable. The 2--10 keV spectrum in 2005 was significantly more\nconvex than that in 2009. Through a count-count-plot technique, the 2--45 keV\nsignals in both data were successfully decomposed in a model-independent way\ninto two distinct broadband components. One is a variable emission with a\nfeatureless spectral shape, and the other is a non-varying hard component\naccompanied by a prominent Fe-K emission line at 6.33 keV (6.40 keV in the rest\nframe). The former was fitted successfully by an absorbed power-law model,\nwhile the latter requires a new hard continuum in addition to a reflection\ncomponent from distant materials. The spectral and variability differences\nbetween the two observations are mainly attributed to long-term changes of this\nnew hard continuum, which was stable on time scales of several hundreds ksec.", "category": "astro-ph_HE" }, { "text": "Full Monte-Carlo description of the Moscow State University Extensive\n Air Shower experiment: The Moscow State University Extensive Air Shower (EAS-MSU) array studied\nhigh-energy cosmic rays with primary energies ~(1-500) PeV in the Northern\nhemisphere. The EAS-MSU data are being revisited following recently found\nindications to an excess of muonless showers, which may be interpreted as the\nfirst observation of cosmic gamma rays at ~100 PeV. In this paper, we present a\ncomplete Monte-Carlo model of the surface detector which results in a good\nagreement between data and simulations. The model allows us to study the\nperformance of the detector and will be used to obtain physical results in\nfurther studies.", "category": "astro-ph_HE" }, { "text": "Cosmic-ray acceleration and escape from post-adiabatic Supernova\n remnants: Supernova remnants are known to accelerate cosmic rays on account of their\nnon-thermal emission of radio waves, X-rays, and gamma rays. Although there are\nmany models for the acceleration of cosmic rays in Supernova remnants, the\nescape of cosmic rays from these sources is yet understudied.\n We use our time-dependent acceleration code RATPaC to study the acceleration\nof cosmic rays and their escape in post-adiabatic Supernova remnants and\ncalculate the subsequent gamma-ray emission from inverse-Compton scattering and\nPion decay.\n We performed spherically symmetric 1-D simulations in which we simultaneously\nsolve the transport equations for CRs, magnetic turbulence, and the\nhydrodynamical flow of the thermal plasma in a volume large enough to keep all\nCRs in the simulation. The transport equations for cosmic-rays and magnetic\nturbulence are coupled via the cosmic-ray gradient and the spatial diffusion\ncoefficient of the cosmic rays, while the cosmic-ray feedback onto the shock\nstructure can be ignored. Our simulations span 100kyrs, thus covering the\nremnants evolution until the beginning of the post-adiabatic phase.\n At later stages of the evolution cosmic rays over a wide range of energy can\nreside outside of the remnant, creating spectra that are softer than predicted\nby standard DSA and feature breaks in the 10-100 GeV-range. The total spectrum\nof cosmic rays released into the interstellar medium has a spectral index of\ns~2.4 above roughly 10 GeV which is close to that required by Galactic\npropagation models. We further find the gamma-ray luminosity to peak around an\nage of 4,000 years for inverse-Compton-dominated high-energy emission. Remnants\nexpanding in low-density media emit generally more inverse-Compton radiation\nmatching the fact that the brightest known supernova remnants - RCW86, Vela Jr,\nHESSJ1731-347 and RXJ1713.7-3946 - are all expanding in low density\nenvironments.", "category": "astro-ph_HE" }, { "text": "Cosmic ray driven Galactic winds: The escape of cosmic rays from the Galaxy leads to a gradient in the cosmic\nray pressure that acts as a force on the background plasma, in the direction\nopposite to the gravitational pull. If this force is large enough to win\nagainst gravity, a wind can be launched that removes gas from the Galaxy,\nthereby regulating several physical processes, including star formation. The\ndynamics of these cosmic ray driven winds is intrinsically non-linear in that\nthe spectrum of cosmic rays determines the characteristics of the wind\n(velocity, pressure, magnetic field) and in turn the wind dynamics affects the\ncosmic ray spectrum. Moreover, the gradient of the cosmic ray distribution\nfunction causes excitation of Alfven waves, that in turn determine the\nscattering properties of cosmic rays, namely their diffusive transport. These\neffects all feed into each other so that what we see at the Earth is the result\nof these non-linear effects. Here we investigate the launch and evolution of\nsuch winds, and we determine the implications for the spectrum of cosmic rays\nby solving together the hydrodynamical equations for the wind and the transport\nequation for cosmic rays under the action of self-generated diffusion and\nadvection with the wind and the self-excited Alfven waves.", "category": "astro-ph_HE" }, { "text": "Environmental dependence of Type IIn supernova properties: Type IIn supernovae occur when stellar explosions are surrounded by dense\nhydrogen-rich circumstellar matter. The dense circumstellar matter is likely\nformed by extreme mass loss from their progenitors shortly before they explode.\nThe nature of Type IIn supernova progenitors and the mass-loss mechanism\nforming the dense circumstellar matter are still unknown. In this work, we\ninvestigate if there are any correlations between Type IIn supernova properties\nand their local environments. We use Type IIn supernovae with well-observed\nlight-curves and host-galaxy integral field spectroscopic data so that we can\nestimate both supernova and environmental properties. We find that Type IIn\nsupernovae with a higher peak luminosity tend to occur in environments with\nlower metallicity and/or younger stellar populations. The circumstellar matter\ndensity around Type IIn supernovae is not significantly correlated with\nmetallicity, so the mass-loss mechanism forming the dense circumstellar matter\naround Type IIn supernovae might be insensitive to metallicity.", "category": "astro-ph_HE" }, { "text": "X-Ray Emission from the Galactic Supernova Remnant G12.0-0.1: We present results of the Suzaku/XIS observation around the radio supernova\nremnant (SNR) G12.0-0.1. No significant diffuse emission extending in or along\nthe radio shell was observed. Instead two compact X-ray sources, Suzaku\nJ181205-1835 and Suzaku J181210-1842, were found in or near G12.0-0.1. Suzaku\nJ181205-1835 is located at the northwest of the radio shell of G12.0-0.1. The\nX-ray profile is slightly extended over the point spread function of the Suzaku\ntelescope. The X-ray spectrum has no line-like structure and is well\nrepresented by a power-law model with a photon index of 2.2 and an absorption\ncolumn of N_{H}=4.9x10^{22} cm^{-2}. The distances of Suzaku J181205-1835 and\nG12.0-0.1 are estimated from the absorption column and the Sigma-D relation,\nrespectively, and are nearly the same with each other. These results suggest\nthat Suzaku J181205-1835 is a candidate of a pulsar wind nebula associated with\nG12.0-0.1. From its location, Suzaku J181210-1842 would be unrelated object to\nG12.0-0.1. The X-ray profile is point-like and the spectrum is thin thermal\nemission with Fe K-lines at 6.4, 6.7, and 6.97 keV, similar to those of\ncataclysmic variables.", "category": "astro-ph_HE" }, { "text": "GRBs on probation: testing the UHE CR paradigm with IceCube: Gamma ray burst (GRB) fireballs provide one of very few astrophysical\nenvironments where one can contemplate the acceleration of cosmic rays to\nenergies that exceed 10^20 eV. The assumption that GRBs are the sources of the\nobserved cosmic rays generates a calculable flux of neutrinos produced when the\nprotons interact with fireball photons. With data taken during construction\nIceCube has already reached a sensitivity to observe neutrinos produced in\ntemporal coincidence with individual GRBs provided that they are the sources of\nthe observed extragalactic cosmic rays. We here point out that the GRB origin\nof cosmic rays is also challenged by the IceCube upper limit on a possible\ndiffuse flux of cosmic neutrinos which should not be exceeded by the flux\nproduced by all GRB over Hubble time. Our alternative approach has the\nadvantage of directly relating the diffuse flux produced by all GRBs to\nmeasurements of the cosmic ray flux. It also generates both the neutrino flux\nproduced by the sources and the associated cosmogenic neutrino flux in a\nsynergetic way.", "category": "astro-ph_HE" }, { "text": "An up-scattered cocoon emission model of Gamma-Ray Burst high-energy\n lags: The Fermi Gamma-ray Space Telescope recently detected the most energetic\ngamma-ray burst so far, GRB 080916C, and reported its detailed temporal\nproperties in an extremely broad spectral range: (i) the time-resolved spectra\nare well described by broken power-law forms over the energy range of $10 {\\rm\nkeV}-10$ GeV, (ii) the high-energy emission (at $\\epsilon > 100$ MeV) is\ndelayed by $\\approx 5$s with respect to the $\\epsilon \\lesssim 1$ MeV emission,\nand (iii) the emission onset times shift towards later times in the higher\nenergy bands. We show that this behavior of the high-energy emission can be\nexplained by a model in which the prompt emission consists of two components:\none is the emission component peaking at $\\epsilon \\sim 1$ MeV due to the\nsynchrotron-self-Compton radiation of electrons accelerated in the internal\nshock of the jet and the other is the component peaking at $\\epsilon \\sim 100$\nMeV due to up-scattering of the photospheric X-ray emission of the expanding\ncocoon (i.e., the hot bubble produced by dissipation of the jet energy inside\nthe progenitor star) off the same electrons in the jet. Based on this model, we\nderive some constraints on the radius of the progenitor star and the total\nenergy and mass of the cocoon of this GRB, which may provide information on the\nstructure of the progenitor star and the physical conditions of the jet\npropagating in the star. The up-scattered cocoon emission could be important\nfor other Fermi-GRBs as well. We discuss some predictions of this model,\nincluding a prompt bright optical emission and a soft X-ray excess.", "category": "astro-ph_HE" }, { "text": "Extragalactic cosmic ray sources with very small contribution in the\n particle flux on the Earth and their study: Possible existence of extragalactic ultra-high energy cosmic ray sources\ngiving a very small particle flux on the Earth is considered. Accretion discs\naround supermassive black holes where particles are accelerated in electric\nfields are discussed as an illustration of such sources. Due to acceleration\nmechanism particle injection spectra are assumed to be hard. In this case\nparticle flux on the Earth is too low for detection. But particles produce in\nspace a noticeable flux of diffuse gamma-ray emission via electromagnetic\ncascades. It should be accounted for when analyzing other source models and\ndark matter models. Also neutrinos are produced in cascades, and at energies E\n>10^19 eV cascade neutrino spectra depends on injection spectra. Therefore it\nis proposed to study cosmic ray sources under consideration using data on\ngamma-ray and neutrino emission.", "category": "astro-ph_HE" }, { "text": "Strong late-time circumstellar interaction in the peculiar supernova\n iPTF14hls: We present a moderate-resolution spectrum of the peculiar Type II supernova\niPTF14hls taken on day 1153 after discovery. This spectrum reveals the clear\nsignature of shock interaction with dense circumstellar material (CSM). We\nsuggest that this CSM interaction may be an important clue for understanding\nthe extremely unusual photometric and spectroscopic evolution seen over the\nfirst 600 days of iPTF14hls. The late-time spectrum shows a double-peaked\nintermediate-width H-alpha line indicative of expansion speeds around 1000\nkm/s, with the double-peaked shape hinting at a disc-like geometry in the CSM.\nIf the CSM was highly asymmetric, perhaps in a disc or torus that was ejected\nfrom the star 3-6 years prior to explosion, then the CSM interaction could have\nbeen overrun and hidden below the SN ejecta photosphere from a wide range of\nviewing angles. In that case, CSM interaction luminosity would have been\nthermalized well below the photosphere, possibly sustaining the high luminosity\nwithout exhibiting the traditional observational signatures of strong CSM\ninteraction (narrow H-alpha emission and X-rays). Variations in density\nstructure of the CSM could account for the multiple rebrightenings of the\nlightcurve. We propose that enveloped CSM interaction as seen in some recent\nSNe, rather than an entirely new explosion mechanism, may be adequate to\nexplain the peculiar evolution of iPTF14hls.", "category": "astro-ph_HE" }, { "text": "The origin of the first neutron star -- neutron star merger: The first neutron star-neutron star (NS-NS) merger was discovered on August\n17, 2017 through gravitational waves (GW170817) and followed with\nelectromagnetic observations. This merger was detected in an old elliptical\ngalaxy with no recent star formation. We perform a suite of numerical\ncalculations to understand the formation mechanism of this merger. We probe\nthree leading formation mechanisms of double compact objects: classical\nisolated binary star evolution, dynamical evolution in globular clusters and\nnuclear cluster formation to test whether they are likely to produce NS-NS\nmergers in old host galaxies. Our simulations with optimistic assumptions show\ncurrent NS-NS merger rates at the level of 10^-2 yr^-1 from binary stars, 5 x\n10^-5 yr^-1 from globular clusters and 10^-5 yr^-1 from nuclear clusters for\nall local elliptical galaxies (within 100 Mpc^3). These models are thus in\ntension with the detection of GW170817 with an observed rate 1.5 yr^-1 (per 100\nMpc^3; LIGO/Virgo estimate). Our results imply that either (i) the detection of\nGW170817 by LIGO/Virgo at their current sensitivity in an elliptical galaxy is\na statistical coincidence; or that (ii) physics in at least one of our three\nmodels is incomplete in the context of the evolution of stars that can form\nNS-NS mergers; or that (iii) another very efficient (unknown) formation channel\nwith a long delay time between star formation and merger is at play.", "category": "astro-ph_HE" }, { "text": "Characteristics of the diffuse astrophysical electron and tau neutrino\n flux with six years of IceCube high energy cascade data: We report on the first measurement of the astrophysical neutrino flux using\nparticle showers (cascades) in IceCube data from 2010 -- 2015. Assuming\nstandard oscillations, the astrophysical neutrinos in this dedicated cascade\nsample are dominated ($\\sim 90 \\%$) by electron and tau flavors. The flux,\nobserved in the sensitive energy range from $16\\,\\mathrm{TeV}$ to\n$2.6\\,\\mathrm{PeV}$, is consistent with a single power-law model as expected\nfrom Fermi-type acceleration of high energy particles at astrophysical sources.\nWe find the flux spectral index to be $\\gamma=2.53\\pm0.07$ and a flux\nnormalization for each neutrino flavor of $\\phi_{astro} = 1.66^{+0.25}_{-0.27}$\nat $E_{0} = 100\\, \\mathrm{TeV}$, in agreement with IceCube's complementary muon\nneutrino results and with all-neutrino flavor fit results. In the measured\nenergy range we reject spectral indices $\\gamma\\leq2.28$ at $\\ge3\\sigma$\nsignificance level. Due to high neutrino energy resolution and low atmospheric\nneutrino backgrounds, this analysis provides the most detailed characterization\nof the neutrino flux at energies below $\\sim100\\,{\\rm{TeV}}$ compared to\nprevious IceCube results. Results from fits assuming more complex neutrino flux\nmodels suggest a flux softening at high energies and a flux hardening at low\nenergies (p-value $\\ge 0.06$). The sizable and smooth flux measured below $\\sim\n100\\,{\\rm{TeV}}$ remains a puzzle. In order to not violate the isotropic\ndiffuse gamma-ray background as measured by the Fermi-LAT, it suggests the\nexistence of astrophysical neutrino sources characterized by dense environments\nwhich are opaque to gamma-rays.", "category": "astro-ph_HE" }, { "text": "Modeling Magnetic Disk-Wind State Transitions in Black Hole X-ray\n Binaries: We analyze three prototypical black hole (BH) X-ray binaries (XRBs), \\4u1630,\n\\gro1655\\ and \\h1743, in an effort to systematically understand the intrinsic\nstate transition of the observed accretion-disk winds between \\windon\\ and\n\\windoff\\ states by utilizing state-of-the-art {\\it Chandra}/HETGS archival\ndata from multi-epoch observations. We apply our magnetically-driven wind\nmodels in the context of magnetohydrodynamic (MHD) calculations to constrain\ntheir (1) global density slope ($p$), (2) their density ($n_{17}$) at the foot\npoint of the innermost launching radius and (3) the abundances of heavier\nelements ($A_{\\rm Fe,S,Si}$). Incorporating the MHD winds into {\\tt xstar}\nphotoionization calculations in a self-consistent manner, we create a library\nof synthetic absorption spectra given the observed X-ray continua. Our analysis\nclearly indicates a characteristic bi-modal transition of multi-ion X-ray\nwinds; i.e. the wind density gradient is found to steepen (from $p \\sim\n1.2-1.4$ to $\\sim 1.4-1.5$) while its density normalization declines as the\nsource transitions from \\windon\\ to \\windoff\\ state. The model implies that the\nionized wind {\\it remains physically present} even in \\windoff\\ state, despite\nits absent appearance in the observed spectra. Super-solar abundances for\nheavier elements are also favored. Our global multi-ion wind models, taking\ninto account soft X-ray ions as well as Fe K absorbers, show that the internal\nwind condition plays an important role in wind transitions besides\nphotoionization changes. % Simulated {\\it XRISM}/Resolve and {\\it Athena}/X-IFU\nspectra are presented to demonstrate a high fidelity of the multi-ion wind\nmodel for better understanding of these powerful ionized winds in the coming\ndecades.", "category": "astro-ph_HE" }, { "text": "Calculating transient rates from surveys: We have developed a method to determine the transient surface density and\ntransient rate for any given survey, using Monte-Carlo simulations. This method\nallows us to determine the transient rate as a function of both the flux and\nthe duration of the transients in the whole flux-duration plane rather than one\nor a few points as currently available methods do. It is applicable to every\nsurvey strategy that is monitoring the same part of the sky, regardless the\ninstrument or wavelength of the survey, or the target sources. We have\nsimulated both top-hat and Fast Rise Exponential Decay light curves,\nhighlighting how the shape of the light curve might affect the detectability of\ntransients. Another application for this method is to estimate the number of\ntransients of a given kind that are expected to be detected by a survey,\nprovided that their rate is known.", "category": "astro-ph_HE" }, { "text": "A search for lines in the bright X-ray afterglow of GRB120711A: GRB120711A, discovered and rapidly localized by the INTEGRAL satellite,\nattracted particular interest due to its high gamma-ray fluence, very bright\nX-ray afterglow, and the detection of a prompt optical transient and of\nlong-lasting emission at GeV energies. A follow-up observation carried out with\nthe XMM-Newton satellite has provided an X-ray spectrum in the 0.3-10 keV with\nunprecedented statistics for a GRB afterglow 20 hours after the burst. The\nspectrum is well fit by a power-law with photon index 1.87+-0.01, modified by\nabsorption in our Galaxy and in the GRB host at z=1.4. A Galactic absorption\nconsistent with that estimated from neutral hydrogen observations is obtained\nonly with host metallicity lower than 0.05 of the Solar value. We report the\nresults of a sensitive search for emission and absorption lines using the\nmatched filter smoothing method (Rutledge and Sako 2003). No statistically\nsignificant lines were found. The upper limits on the equivalent width of\nemission lines, derived through Monte Carlo simulations, are few tens of eV, a\nfactor about 10 lower than that of the possible lines reported in the\nliterature for other bursts.", "category": "astro-ph_HE" }, { "text": "Long duration radio transients lacking optical counterparts are possibly\n Galactic Neutron Stars: (abridged) Recently, a new class of radio transients in the 5-GHz band was\ndetected by Bower et al. We present new deep near-Infrared (IR) observations of\nthe field containing these transients, and find no counterparts down to a\nlimiting magnitude of K=20.4 mag. We argue that the bright (>1 Jy) radio\ntransients recently reported by Kida et al. are consistent with being\nadditional examples of the Bower et al. transients. We refer to these groups of\nevents as \"long-duration radio transients\". The main characteristics of this\npopulation are: time scales longer than 30 minute but shorter than several\ndays; rate, ~10^3 deg^-2 yr^-1; progenitors sky surface density of >60 deg^-2\n(95% C.L.) at Galactic latitude ~40 deg; 1.4-5 GHz spectral slopes, f_\\nu ~\n\\nu^alpha, with alpha>0; and most notably the lack of any counterparts in\nquiescence in any wavelength. We rule out an association with many types of\nobjects. Galactic brown-dwarfs or some sort of exotic explosions remain\nplausible options. We argue that an attractive progenitor candidate for these\nradio transients is the class of Galactic isolated old neutron stars (NS). We\nconfront this hypothesis with Monte-Carlo simulations of the space distribution\nof old NSs, and find satisfactory agreement for the large areal density.\nFurthermore, the lack of quiescent counterparts is explained quite naturally.\nIn this framework we find: the mean distance to events in the Bower et al.\nsample is of order kpc; the typical distance to the Kida et al. transients are\nconstrained to be between 30 pc and 900 pc (95% C.L.); these events should\nrepeat with a time scale of order several months; and sub-mJy level bursts\nshould exhibit Galactic latitude dependence. We discuss possible mechanisms\ngiving rise to the observed radio emission.", "category": "astro-ph_HE" }, { "text": "A pulsar in a binary with a compact object in the mass gap between\n neutron stars and black holes: Among the compact objects observed in gravitational wave merger events a few\nhave masses in the gap between the most massive neutron stars (NSs) and least\nmassive black holes (BHs) known. Their nature and the formation of their\nmerging binaries are not well understood. We report on pulsar timing\nobservations using the Karoo Array Telescope (MeerKAT) of PSR J0514-4002E, an\neccentric binary millisecond pulsar in the globular cluster NGC 1851 with a\ntotal binary mass of $3.887 \\pm 0.004$ solar masses. The companion to the\npulsar is a compact object and its mass (between $2.09$ and $2.71$ solar\nmasses, 95% confidence interval) is in the mass gap, so it either is a very\nmassive NS or a low-mass BH. We propose the companion was formed by a merger\nbetween two earlier NSs.", "category": "astro-ph_HE" }, { "text": "Phase shift of gravitational waves induced by aberration: The velocity of a gravitational wave (GW) source provides crucial information\nabout its formation and evolution processes. Previous studies considered the\nDoppler effect on the phase of GWs as a potential signature of a time-dependent\nvelocity of the source. However, the Doppler shift only accounts for the time\ncomponent of the wave vector, and in principle motion also affects the spatial\ncomponents. The latter effect, known as ``aberration'' for light, is analyzed\nin this paper for GWs and applied to the waveform modeling of an accelerating\nsource. We show that the additional aberrational phase shift could be\ndetectable in two astrophysical scenarios, namely, a recoiling binary black\nhole (BBH) due to GW radiation and a BBH in a triple system. Our results\nsuggest that adding the aberrational phase shift in the waveform templates\ncould significantly enhance the detectability of moving sources.", "category": "astro-ph_HE" }, { "text": "Early Spectral Evolution of the Rapidly Expanding Type Ia SN 2006X: We present optical spectroscopic and photometric observations of Type Ia\nsupernova (SN) 2006X from --10 to +91 days after the $B$-band maximum. This SN\nexhibits one of the highest expansion velocity ever published for SNe Ia. At\npremaximum phases, the spectra show strong and broad features of\nintermediate-mass elements such as Si, S, Ca, and Mg, while the O{\\sc\ni}$\\lambda$7773 line is weak. The extremely high velocities of Si{\\sc ii} and\nS{\\sc ii} lines and the weak O{\\sc i} line suggest that an intense\nnucleosynthesis might take place in the outer layers, favoring a delayed\ndetonation model. Interestingly, Si{\\sc ii}$\\lambda$5972 feature is quite\nshallow, resulting in an unusually low depth ratio of Si{\\sc ii}$\\lambda$5972\nto $\\lambda$6355, $\\cal R$(Si{\\sc ii}). The low $\\cal R$(Si{\\sc ii}) is usually\ninterpreted as a high photospheric temperature. However, the weak Si{\\sc\niii}$\\lambda$4560 line suggests a low temperature, in contradiction to the low\n$\\cal R$(Si{\\sc ii}). This could imply that the Si{\\sc ii}$\\lambda$5972 line\nmight be contaminated by underlying emission. We propose that $\\cal R$(Si{\\sc\nii}) may not be a good temperature indicator for rapidly expanding SNe Ia at\npremaximum phases.", "category": "astro-ph_HE" }, { "text": "Constraining Low-luminosity Gamma-Ray Bursts as Ultra-high-energy Cosmic\n Ray Sources Using GRB 060218 as a Proxy: We study the connection between low-luminosity gamma-ray bursts (llGRBs) and\nultra-high-energy cosmic rays (UHECRs) using the canonical low-luminosity GRB\n060218 as a proxy. We focus on the consequential synchrotron emission from\nelectrons that are coaccelerated in the UHECR acceleration region, comparing\nthis emission to observations. Both the prompt and afterglow phases are\nconsidered. For the prompt phase, we assume the coaccelerated electrons are\ninjected with a power law distribution instantaneously (without additional\nheating or reacceleration), which results in bright optical-UV emission in\ntension with observations. For the afterglow phase, we constrain the total\nkinetic energy of the blast wave by comparing electron thermal synchrotron\nradiation to available radio data at $\\sim~3$ days. Considering mildly\nrelativistic outflows with bulk Lorentz factor $\\Gamma \\gtrsim 2$ (slower\ntrans-relativistic outflows are not treated), we find that the limited\navailable energy does not allow for GRB 060218-like afterglows to be the main\norigin of UHECRs. This analysis independently constrains the prompt phase as a\nmajor UHECR source as well, given that the prompt energy budget is comparable\nto that of the afterglow kinetic energy. More generally, our study demonstrates\nthat synchrotron emission from thermal electrons is a powerful diagnostic of\nthe physics of mildly relativistic shocks.", "category": "astro-ph_HE" }, { "text": "Megaparsec-scale Radio Structure Associated with a Hybrid Blazar SBS\n B1646+499: Episodic Jet Activity with Precessing Axis: Here we report on the total-intensity 610 MHz GMRT observations of the\npeculiar hybrid blazar SBS B1646+499, which merges the properties of BL\nLacertae objects and flat-spectrum radio quasars. The complex radio structure\nof SBS B1646+499, emerging from the archival radio data and our new GMRT\nobservations, consists of the Mpc-scale, elongated halo, the unilateral\nkpc-scale jet, and the nuclear jet extending up to 20pc from the compact core.\nThe giant halo is characterized by a steep radio spectrum, indicative of the\nadvanced ageing of the electron population within the lobes. For the\nlarge-scale jet, we detected a spectral gradient along and across the outflow,\nand in particular spectral flattening of the radio continuum toward the jet\nedges, suggestive of the spine--boundary shear layer morphology. The nuclear\njet displays superluminal knots emerging from the self-absorbed and variable\nradio core. We interpret all these findings in the framework of the model of an\nepisodic jet activity with a precessing jet axis.", "category": "astro-ph_HE" }, { "text": "On the nature of the 'hostless' short GRBs: A significant proportion ($\\sim30\\%$) of the short-duration gamma-ray bursts\n(SGRBs) localised by Swift have no detected host galaxy coincident with the\nburst location to deep limits, and also no high-likelihood association with\nproximate galaxies on the sky. These SGRBs may represent a population at\nmoderately high redshifts ($z\\gtrsim1$), for which the hosts are faint, or a\npopulation where the progenitor has been kicked far from its host or is sited\nin an outlying globular cluster. We consider the afterglow and host\nobservations of three 'hostless' bursts (GRBs 090305A, 091109B and 111020A),\ncoupled with a new observational diagnostic to aid the association of SGRBs\nwith putative host galaxies to investigate this issue. Considering the well\nlocalised SGRB sample, 7/25 SGRBs can be classified as 'hostless' by our\ndiagnostic. Statistically, however, the proximity of these seven SGRBs to\nnearby galaxies is higher than is seen for random positions on the sky. This\nsuggests that the majority of 'hostless' SGRBs have likely been kicked from\nproximate galaxies at moderate redshift. Though this result still suggests only\na small proportion of SGRBs will be within the AdLIGO horizon for NS-NS or\nNS-BH inspiral detection ($z\\sim0.1$), in the particular case of GRB 111020A a\nplausible host candidate is at $z=0.02$.", "category": "astro-ph_HE" }, { "text": "Hints of a second explosion (a quark nova) in Cassiopeia A Supernova: We show that the explosive transition of the neutron star (NS) to a quark\nstar (QS) (a Quark Nova) in Cassiopeia A (Cas A) a few days following the SN\nproper can account for several of the puzzling kinematic and nucleosynthetic\nfeatures observed. The observed decoupling between Fe and 44Ti and the lack of\nFe emission within 44Ti regions is expected in the QN model owing to the\nspallation of the inner SN ejecta by the relativistic QN neutrons. Our model\npredicts the 44Ti to be more prominent to the NW of the central compact object\n(CCO) than in the SE and little of it along the NE-SW jets, in agreement with\nNuStar observations. Other intriguing features of Cas A such as the lack of a\npulsar wind nebula (PWN) and the reported a few percent drop of the CCO\ntemperature over a period of 10 years are also addressed.", "category": "astro-ph_HE" }, { "text": "A Study of the Long-term Spectral Variations of 3C 66A Observed with the\n Fermi and Kanata Telescopes: 3C 66A is an intermediate-frequency-peaked BL Lac object detected by the\nLarge Area Telescope onboard the Fermi Gamma-ray Space Telescope.\n We present a study of the long-term variations of this blazar seen over 2\nyears at GeV energies with Fermi and in the optical (flux and polarization) and\nnear infrared with the Kanata telescope.\n In 2008, the first year of the study, we find a correlation between the\ngamma-ray flux and the measurements taken with the Kanata telescope.\n This is in contrast to the later measurements performed during 2009--2010\nwhich show only a weak correlation along with a gradual increase of the optical\nflux. We calculate an external seed photon energy density assuming that the\ngamma-ray emission is due to external Compton scattering. The energy density of\nthe external photons is found to be higher by a factor of two in 2008 compared\nto 2009--2010. We conclude that the different behaviors observed between the\nfirst year and the later years might be explained by postulating two different\nemission components.", "category": "astro-ph_HE" }, { "text": "Searching for Dark Matter Annihilation in M87: Clusters of galaxies, such as the Virgo cluster, host enormous quantities of\ndark matter, making them prime targets for efforts in indirect dark matter\ndetection via potential radiative signatures from annihilation of dark matter\nparticles and subsequent radiative losses of annihilation products. However, a\ncareful study of ubiquitous astrophysical backgrounds is mandatory to single\nout potential evidence for dark matter annihilation. Here, we construct a\nmultiwavelength spectral energy distribution for the central radio galaxy in\nthe Virgo cluster, M87, using a state-of-the-art numerical Synchrotron Self\nCompton approach. Fitting recent Chandra, Fermi-LAT and Cherenkov observations,\nwe probe different dark matter annihilation scenarios including a full\ntreatment of the inverse Compton losses from electrons and positrons produced\nin the annihilation. It is shown that such a template can substantially improve\nupon existing dark matter detection limits.", "category": "astro-ph_HE" }, { "text": "Calibrating X-ray binary luminosity functions via optical reconnaissance\n II. The high-mass XLF and globular cluster population of X-ray binaries in\n the low star-forming spiral M81: We characterize the optical counterparts to the compact X-ray source\npopulation within the nearby spiral galaxy M81 using multi-band Hubble Space\nTelescope (HST) imaging data. By comparing the optical luminosities and colors\nmeasured for candidate donor stars and host clusters to stellar and cluster\nevolutionary models, respectively, we estimate the likely masses and upper age\nlimits of the field and cluster X-ray binaries. We identify 15 low-mass X-ray\nbinaries (i.e. donor star mass $\\leq$ 3 solar masses) within ancient globular\nclusters, as well as 42 candidate high-mass X-ray binaries (i.e. donor star\nmass $\\geq$ 8 solar masses). To estimate the likelihood of misclassifications,\nwe inject 4,000 artificial sources into the HST mosaic image and conclude that\nour classifications of globular clusters and high-mass X-ray binaries are\nreliable at the >90% level. We find that globular clusters that host X-ray\nbinaries are on average more massive and more compact than globular clusters\nthat do not. However, there is no apparent correlation between the X-ray\nbrightness of the clusters and their masses or densities, nor are X-ray binary\nhosts more X-ray luminous than the general field population of low-mass X-ray\nbinaries. This work represents one of the first in-depth analyses of the\npopulation of X-ray binaries within globular clusters in a spiral galaxy.", "category": "astro-ph_HE" }, { "text": "Cosmic searches for Lorentz invariance violation: Cosmic messengers (gamma rays, cosmic rays, neutrinos and gravitational\nwaves) provide a powerful complementary way to search for Lorentz invariance\nviolating effects to laboratory-based experiments. The long baselines and high\nenergies involved make Cherenkov telescopes, air-shower arrays, neutrino\ntelescopes and gravitational wave detectors unique tools to probe the expected\ntiny effects that the breaking of Lorentz invariance would cause in the\npropagation of these messengers, in comparison with the standard scenario. In\nthis chapter we explain the expected effects that the mentioned detectors can\nmeasure and summarize current results of searches for Lorentz violation.", "category": "astro-ph_HE" }, { "text": "A variable ULX and possible IMBH candidate in M51a: ULX-7, in the northern spiral arm of M51, demonstrates unusual behaviour for\nan ultraluminous X-ray source, with a hard X-ray spectrum but very high\nshort-term variability. This suggests that it is not in a typical ultraluminous\nstate. We analyse the source using archival data from XMM-Newton, Chandra and\nNuSTAR, and by examining optical and radio data from HST and VLA. Our X-ray\nspectral analysis shows that the source has a hard power-law spectral shape\nwith a photon index Gamma~1.5, which persists despite the source's X-ray\nluminosity varying by over an order of magnitude. The power spectrum of the\nsource features a break at 6.5^{+0.5}_{-1.1}x10^-3 Hz, from a low-frequency\nspectral index of alpha_1=-0.1^{+0.5}_{-0.2} to a high-frequency spectral index\nof alpha_2=0.65^{+0.05}_{-0.14}, making it analogous to the low-frequency break\nfound in the power spectra of low/hard state black holes (BHs). We can take a\nlower frequency limit for a corresponding high-frequency break to calculate a\nBH mass upper limit of 1.6x10^3 solar masses. Using the X-ray/radio fundamental\nplane we calculate another upper limit to the BH mass of 3.5x10^4 solar masses\nfor a BH in the low/hard state. The hard spectrum, high rms variability and\nmass limits are consistent with ULX-7 being an intermediate-mass BH; however we\ncannot exclude other interpretations of this source's interesting behaviour,\nmost notably a neutron star with an extreme accretion rate.", "category": "astro-ph_HE" }, { "text": "The Mergers in Abell 2256: Displaced Gas and its Connection to the\n Radio-emitting Plasma: We present the results of deep Chandra and XMM-Newton X-ray imaging and\nspatially-resolved spectroscopy of Abell 2256, a nearby (z=0.058) galaxy\ncluster experiencing multiple mergers and displaying a rich radio morphology\ndominated by a large relic. The X-ray data reveals three subclusters: (i) the\n`main cluster'; (ii) the remnant of an older merger in the east of the cluster\nwith a ~ 600 kpc long tail; (iii) a bright, bullet-like, low-entropy infalling\nsystem, with a large line-of-sight velocity component. The low-entropy system\ndisplays a 250 kpc long cold front with a break and an intriguing surface\nbrightness decrement. Interestingly, the infalling gas is not co-spatial with\nbright galaxies and the radio loud brightest cluster galaxy of the infalling\ngroup appears dissociated from the low entropy plasma by 50 kpc in projection,\nto the south of the eastern edge of the cold front. Assuming that the dark\nmatter follows the galaxy distribution, we predict that it is also\nsignificantly offset from the low-entropy gas. Part of the low frequency radio\nemission near the cold front might be revived by magnetic field amplification\ndue to differential gas motions. Using analytical models and numerical\nsimulations, we investigate the possibility that the supersonic infall of the\nsubcluster generates a large scale shock along our line-of-sight, which can be\ndetected in the X-ray temperature map but is not associated with any clear\nfeatures in the surface brightness distribution.", "category": "astro-ph_HE" }, { "text": "The polarization of the boundary layer around weakly magnetized neutron\n stars in X-ray binaries: X-ray binaries hosting a compact object have been among the main targets of\nthe Imaging X-ray Polarimetry Explorer (IXPE) since its launch, due to their\nhigh brightness in the 2-8 keV energy band. The spectropolarimetric analysis\nperformed so far has proved to be of great importance in providing constraints\non the accretion geometry of these systems. However, the data statistics is not\nenough to unambiguously disentangle the contribution of the single components\nto the net observed polarimetric signal. In this work, we aim to present a\nmodel for computing the polarization degree and polarization angle of the\nboundary layer around weakly magnetized neutron stars in low-mass X-ray\nbinaries in the soft state. The main motivation is to provide strong\ntheoretical support to data interpretation of observations performed by IXPE or\nfuture satellites for X-ray polarimetry. The results were obtained by modeling\nthe boundary layer as an equatorial belt around the compact object and locally\napproximating it as a plane-parallel scattering atmosphere, for which the\nassociated radiative transfer equation for polarized radiation in the Thomson\nlimit was solved. The polarimetric quantities were then transformed from the\ncomoving frame to the observer frame using the numerical methods formerly\ndeveloped for X-ray pulsars. For typical values of the optical depth and\nelectron temperature of the boundary layer of these systems in a soft state,\nthe polarization degree was less then 0.5\\%, while the polarization angle was\nrotated by $\\protect \\la 5^{\\circ}$ with respect to the neutron star spin axis\ndue to special and general relativistic effects for fast rotation, the amount\nprogressively decreasing for lower spin frequencies. The derived quantities can\nbe used to remove degeneracy when multicomponent spectropolarimetry is\nperformed.", "category": "astro-ph_HE" }, { "text": "Status And Highlights Of VERITAS: VERITAS (Very Energetic Radiation Imaging Telescope Array System) is an array\nof atmospheric Cherenkov telescopes sensitive to very high energy (VHE)\ngamma-rays above 100 GeV. Located at the Fred Lawrence Whipple Observatory in\nsouthern Arizona, USA, the VERITAS array of four 12m-diameter telescopes began\nfull operation in September 2007. Two major upgrades, the relocation of\ntelescope 1 in Summer 2009 and the upgrade of the level-2 trigger in Fall 2011,\nmade VERITAS the most sensitive VHE instrument in the northern hemisphere. The\nVERITAS Collaboration consists of scientists from institutions in the USA,\nCanada, Germany and Ireland. VERITAS is performing observations that cover a\nbroad range of science topics, including the study of galactic and\nextragalactic astrophysical sources of VHE radiation and the study of particle\nastrophysics, such as the indirect search for dark matter in astrophysical\nenvironments. The VERITAS observational campaigns resulted in the detection of\n40 VHE sources, including the discovery of 20 new VHE gamma-ray emitting\nsources. Here we summarize the current status of the observatory, describe the\nrecent scientific highlights and outline plans for the future.", "category": "astro-ph_HE" }, { "text": "Importance of resolving the spectral support of beam-plasma\n instabilities in simulations: Many astrophysical plasmas are prone to beam-plasma instabilities. For\nrelativistic and dilute beams, the {\\it spectral} support of the beam-plasma\ninstabilities is narrow, i.e., the linearly unstable modes that grow with rates\ncomparable to the maximum growth rate occupy a narrow range of wave numbers.\nThis places stringent requirements on the box-sizes when simulating the\nevolution of the instabilities. We identify the implied lower limits on the box\nsize imposed by the longitudinal beam plasma instability, i.e., typically the\nmost stringent condition required to correctly capture the linear evolution of\nthe instabilities in multidimensional simulations. We find that sizes many\norders of magnitude larger than the resonant wavelength are typically required.\nUsing one-dimensional particle-in-cell simulations, we show that the failure to\nsufficiently resolve the spectral support of the longitudinal instability\nyields slower growth and lower levels of saturation, potentially leading to\nerroneous physical conclusion.", "category": "astro-ph_HE" }, { "text": "Search for Decaying Dark Matter in the Virgo Cluster of Galaxies with\n HAWC: The decay or annihilation of dark matter particles may produce a steady flux\nof very-high-energy gamma rays detectable above the diffuse background. Nearby\nclusters of galaxies provide excellent targets to search for the signatures of\nparticle dark matter interactions. In particular, the Virgo cluster spans\nseveral degrees across the sky and can be efficiently probed with a wide\nfield-of-view instrument. The High Altitude Water Cherenkov (HAWC) observatory,\ndue to its wide field of view and sensitivity to gamma rays at an energy scale\nof 300 GeV--100 TeV is well-suited for this search. Using 2141 days of data, we\nsearch for gamma-ray emission from the Virgo cluster, assuming well-motivated\ndark matter sub-structure models. Our results provide some of the strongest\nconstraints on the decay lifetime of dark matter for masses above 10 TeV.", "category": "astro-ph_HE" }, { "text": "Hints of the jet composition in gamma-ray bursts from dissipative\n photosphere models: We present a model for gamma-ray bursts where a dissipative photosphere\nprovides the usual spectral peak around MeV energies accompanied by a\nsubdominant thermal component. We treat the initial acceleration of the jet in\na general way, allowing for magnetic field- and baryon dominated outflows. In\nthis model, the GeV emission associated with GRBs observed by Fermi LAT, arises\nas the interaction of photospheric radiation and the shocked electrons at the\ndeceleration radius. Through recently discovered correlations between the\nthermal and non-thermal peaks within individual bursts, we are able to infer\nwhether the jet was Poynting flux or baryon dominated.", "category": "astro-ph_HE" }, { "text": "Indirect dark matter search in the Galactic Centre with IceCube: It is assumed that dark matter can annihilate or decay into Standard Model\nparticles which then can produce a neutrino flux detectable at IceCube. Such a\nsignal can be emitted from the Galactic Center thanks to the high density of\ndark matter abundance being gravitationally captured. This analysis aims at\nsearching for neutrino signals from dark matter annihilation and decay in the\nGalactic Center using $\\sim$9 years of IceCube-DeepCore data with an optimized\nselection for low energy. In this contribution, we present the sensitivities on\nthe thermally averaged dark matter self-annihilation cross-section for dark\nmatter masses ranging from 5 GeV up to 8 TeV.", "category": "astro-ph_HE" }, { "text": "The interstellar environment in the outer Galaxy as seen in gamma rays\n by Fermi: Gamma-ray emission produced by interactions between cosmic rays (CRs) and\ninterstellar gas traces the product of their densities throughout the Milky\nWay. The outer Galaxy is a privileged target of investigation to separate\ninterstellar structures seen along the line of sight. Recent observations by\nthe Fermi Large Area Telescope (LAT) shed light on open questions of the EGRET\nera about the distribution of CR densities and the census of the interstellar\nmedium. The gradient of gamma-ray emissivities measured in the outer Galaxy is\nsignificantly flatter than predictions from widely used CR propagation models\ngiven the rapid decline of putative CR sources beyond the solar circle. Large\npropagation volumes, with halo heights up to 20 kpc, or a flat CR source\ndistribution are required to match the data. Other viable possibilities include\nnon-uniform CR diffusion properties or more gas than accounted for by the\nradio/mm-wave data. Gamma-ray data constrain the evolution of the\nXco=N(H2)/W(CO) ratio within a few kpc from the Sun. There is a significant\nincrease by a factor 2 from nearby clouds in the Gould Belt to the local spur.\nNo further significant variations are measured from the local spur to the\nPerseus spiral arm. At the level of statistical accuracy provided by the LAT\ndata, the most important source of uncertainty, often overlooked so far, is due\nto the optical depth correction applied to derive the column densities of H I.\nReliable determinations of the amount of atomic gas in the plane are key to\nbetter probe the properties of CRs in the Galaxy.", "category": "astro-ph_HE" }, { "text": "Blueshifted absorption lines from X-ray reflection in IRAS 13224-3809: We explore a disc origin for the highly-blueshifted, variable absorption\nlines seen in the X-ray spectrum of the Narrow Line Seyfert 1 galaxy\nIRAS13224-3809. The blueshift corresponds to a velocity of about 0.25c. Such\nfeatures in other Active Galactic Nuclei are often interpreted as UltraFast\nOutflows (UFOs). The velocity is of course present in the orbital motions of\nthe inner disk. The absorption lines in IRAS13224-3809 are best seen when the\nflux is low and the reflection component of the disk is strong relative to the\npower-law continuum. The spectra are consistent with a model in which the\nreflection component passes through a thin, highly-ionized absorbing layer at\nthe surface of the inner disc, the blue-shifted side of which dominates the\nflux due to relativistic aberration (the disc inclination is about 70 deg). No\nfast outflow need occur beyond the disc.", "category": "astro-ph_HE" }, { "text": "Lorentz Factor-Isotropic Luminosity/Energy Correlations of GRBS and\n Their Interpretation: The bulk Lorentz factor of the gamma-ray burst (GRB) ejecta (Gamma_0) is a\nkey parameter to understand the GRB physics. Liang et al. have discovered a\ncorrelation between Gamma_0 and isotropic gamma-ray energy:\nGamma_0\\proptoE_{gamma,iso,52}^{0.25}. By including more GRBs with updated data\nand more methods to derive Gamma_0, we confirm this correlation and obtain\nGamma_0~91E_{gamma,iso,52}^{0.29}. Evaluating the mean isotropic gamma-ray\nluminosities L_{gamma,iso} of the GRBs in the same sample, we discover an even\ntighter correlation Gamma_0~249 L_{gamma,iso,52}^{0.30}. We propose an\ninterpretation to this later correlation. Invoking a neutrino-cooled\nhyperaccretion disk around a stellar mass black hole as the central engine of\nGRBs, we derive jet luminosity powered by neutrino annihilation and baryon\nloading from a neutrino-driven wind. Applying beaming correction, we finally\nderive Gamma_0\\proptoL_{gamma,iso}^{0.22}, which is well consistent with the\ndata. This suggests that the central engine of long GRBs is likely a stellar\nmass black hole surrounded by a hyper-accreting disk.", "category": "astro-ph_HE" }, { "text": "Waves in pulsar winds: The radio, optical, X-ray and gamma-ray nebulae that surround many pulsars\nare thought to arise from synchrotron and inverse Compton emission. The energy\npowering this emission, as well as the magnetic fields and relativistic\nparticles, are supplied by a \"wind\" driven by the central object. The inner\nparts of the wind can be described using the equations of MHD, but these break\ndown in the outer parts, when the density of charge carriers drops below a\ncritical value. This paper reviews the wave properties of the inner part\n(striped wind), and uses a relativistic two-fluid model (cold electrons and\npositrons) to re-examine the nonlinear electromagnetic modes that propagate in\nthe outer parts. It is shown that in a radial wind, two solutions exist for\ncircularly polarised electromagnetic modes. At large distances one of them\nturns into a freely expanding flow containing a vacuum wave, whereas the other\ndecelerates, corresponding to a confined flow.", "category": "astro-ph_HE" }, { "text": "Ultra High Energy Neutrinos: Absorption, Thermal Effects and Signatures: We study absorption of ultra high energy neutrinos by the cosmic neutrino\nbackground, with full inclusion of the effect of the thermal distribution of\nthe background on the resonant annihilation channel. For a hierarchical\nneutrino mass spectrum (with at least one neutrino with mass below $\\sim\n10^{-2}$ eV), thermal effects are important for ultra high energy neutrino\nsources at $z \\gtrsim 16$. The neutrino transmission probability shows no more\nthan two separate suppression dips since the two lightest mass eigenstates\ncontribute as a single species when thermal effects are included. Results are\napplied to a number of models of ultra high energy neutrino emission.\nSuppression effects are strong for sources that extend beyond $z \\sim 10$,\nwhich can be realized for certain top down scenarios, such as superheavy dark\nmatter decays, cosmic strings and cosmic necklaces. For these, a broad\nsuppression valley should affect the neutrino spectrum at least in the energy\ninterval $10^{12} - 10^{13}$ GeV -- which therefore is disfavored for ultra\nhigh energy neutrino searches -- with only a mild dependence on the neutrino\nmass spectrum and hierarchy. The observation of absorption effects would\nindicate a population of sources beyond $z \\sim 10$, and favor top-down\nmechanisms; it would also be an interesting probe of the physics of the relic\nneutrino background in the unexplored redshift interval $z \\sim 10 -100$.", "category": "astro-ph_HE" }, { "text": "A Photosphere-Internal Shock Model of Gamma-Ray Bursts: Case Studies of\n Fermi/LAT Bursts: Radially inhomogeneous gamma-ray burst (GRB) jets release variable\nphotospheric emission and can have internal shocks occurring above the\nphotosphere. We generically formulate a photospheric emission model of GRBs\nincluding Compton up-scattered photospheric (UP) emission off the electrons\n(and positrons) in the internal shocks, and find that the photospheric emission\nmay correspond to the traditional (Band) component at <~1 MeV and the UP\nemission to the high-energy emission observed by Fermi/LAT for some GRBs at >~\n10 MeV. The two components can be separate in the spectrum in some cases or can\nmimic a smooth broad Band spectrum in other cases. We apply our formulation to\nthe well-studied long and short LAT GRBs, GRB 080916C, GRB 090902B, and GRB\n090510, and typically find reasonable parameters for fitting the time-binned\nspectra, although fine tuning of several parameters is required. The observed\ndelays of the high-energy emission with respect to the MeV emission which are\nlarge compared to the variability times are unlikely to be due to simple\nkinematic effects of a non-evolving jet. These delays may be attributed to the\ntemporal evolution of the physical parameters of the jet, and thus the delay\ntimescales could provide a potential tool for investigating the structures of\nGRB jets themselves and their progenitors. The difference of the delay\ntimescales of long and short GRBs inferred from the Fermi data might be due to\nthe differences in the progenitors of long and short GRBs. Some other\nproperties and consequences of this model are discussed, including temporal\ncorrelations among the prompt optical, the soft X-ray, and the distinct\nhigh-energy component as well as the Band component.", "category": "astro-ph_HE" }, { "text": "Equations of state in the Hartle-Thorne model of neutron stars selecting\n acceptable variants of the resonant switch model of twin HF QPOs in the atoll\n source 4U 1636-53: The Resonant Switch (RS) model of twin high-frequency quasi-periodic\noscillations (HF QPOs) observed in neutron star binary systems, based on switch\nof the twin oscillations at a resonant point, has been applied to the atoll\nsource 4U 1636-53 under assumption that the neutron star exterior can be\napproximated by the Kerr geometry. Strong restrictions of the neutron star\nparameters M (mass) and a (spin) arise due to fitting the frequency pairs\nadmitted by the RS model to the observed data in the regions related to the\nresonant points. The most precise variants of the RS model are those combining\nthe relativistic precession frequency relations with their modifications. Here,\nthe neutron star mass and spin estimates given by the RS model are confronted\nwith a variety of equations of state (EoS) governing structure of neutron stars\nin the framework of the Hartle-Thorne theory of rotating neutron stars applied\nfor the observationally given rotation frequency f_rot~580 Hz (or alternatively\nf_rot~290 Hz) of the neutron star at 4U 1636-53. It is shown that only two\nvariants of the RS model based on the Kerr approximation are compatible with\ntwo EoS applied in the Hartle-Thorne theory for f_rot~580 Hz, while no variant\nof the RS model is compatible for f_rot~290 Hz. The two compatible variants of\nthe RS model are those giving the best fits of the observational data. However,\na self-consistency test by fitting the observational data to the RS model with\noscillation frequencies governed by the Hartle-Thorne geometry described by\nthree spacetime parameters M, a and (quadrupole moment) q related by the two\navailable EoS puts strong restrictions. The test admits only one variant of the\nRS model of twin HF QPOs for the Hartle-Thorne theory with the Gandolfi et al.\n(2010) EoS predicting the parameters of the neutron star $M \\sim 2.10\n\\mathrm{M}_{\\odot}$, $a \\sim 0.208$, and $q/a^2 \\sim 1.77$.", "category": "astro-ph_HE" }, { "text": "The XMM-Newton survey of the Small Magellanic Cloud: Although numerous archival XMM-Newton observations existed towards the Small\nMagellanic Cloud (SMC) before 2009, only a fraction of the whole galaxy was\ncovered. Between May 2009 and March 2010 we carried out an XMM-Newton survey of\nthe SMC, in order to obtain a complete overage of both its bar and wing.\nThirty-three observations of 30 different fields with a total exposure of about\nne Ms filled the missing parts. We systematically processed all available SMC\ndata from the European Photon Imaging Camera. After rejecting observations with\nvery high background we included 53 archival and the 33 survey observations. We\nproduced images in five different energy bands. We applied astrometric\nboresight corrections using secure identifications of X-ray sources and combine\nall the images to produce a mosaic, which covers the main body of the SMC. We\npresent an overview of the XMM-Newton observations, describe their analysis and\nsummarise first results which will be presented in follow-up papers in detail.\nHere, we mainly focus on extended X-ray sources like supernova remnants (SNRs)\nand clusters of galaxies which are seen in our X-ray images. The XMM-Newton\nsurvey represents the deepest complete survey of the SMC in the 0.15-12.0 keV\nX-ray band. We propose three new SNRs with low surface brightness of a few\n10^-14 erg s^-1 cm^-2 arcmin^-2 and large extent. Also several known remnants\nappear larger than previously measured from X-rays or other wavelengths\nextending the size distribution of SMC SNRs to larger values.", "category": "astro-ph_HE" }, { "text": "The destruction and recreation of the X-ray corona in a changing-look\n Active Galactic Nucleus: We present the drastic transformation of the X-ray properties of the active\ngalactic nucleus 1ES 1927+654, following a changing-look event. After the\noptical/UV outburst the power-law component, produced in the X-ray corona,\ndisappeared, and the spectrum of 1ES 1927+65 instead became dominated by a\nblackbody component ($kT\\sim 80-120$ eV). This implies that the X-ray corona,\nubiquitously found in AGN, was destroyed in the event. Our dense $\\sim 450$ day\nlong X-ray monitoring shows that the source is extremely variable in the X-ray\nband. On long time scales the source varies up to $\\sim 4$ dex in $\\sim 100$\ndays, while on short timescales up to $\\sim2$ dex in $\\sim 8$ hours. The\nluminosity of the source is found to first show a strong dip down to $\\sim\n10^{40}\\rm\\,erg\\,s^{-1}$, and then a constant increase in luminosity to levels\nexceeding the pre-outburst level $\\gtrsim $300 days after the optical event\ndetection, rising up asymptotically to $\\sim 2\\times10^{44}\\rm\\,erg\\,s^{-1}$.\nAs the X-ray luminosity of the source increases, the X-ray corona is recreated,\nand a very steep power-law component ($\\Gamma\\simeq 3$) reappears, and\ndominates the emission for 0.3-2 keV luminosities $\\gtrsim\n10^{43.7}\\rm\\,erg\\,s^{-1}$, $\\sim 300$ days after the beginning of the event.\nWe discuss possible origins of this event, and speculate that our observations\ncould be explained by the interaction between the accretion flow and debris\nfrom a tidally disrupted star. Our results show that changing-look events can\nbe associated with dramatic and rapid transformations of the innermost regions\nof accreting SMBHs.", "category": "astro-ph_HE" }, { "text": "Lorentz invariance violation from GRB221009A: The Large High Altitude Air Shower Observatory~(LHAASO) reported observation\nof photons with energies above 10~TeV from gamma ray burst GRB221009A. A\nsuggestion was proposed that this result may contradict with our knowledge of\nspecial relativity~(SR) and the standard model~(SM), according to which photons\nof about 10~TeV from such a distant object should be severely suppressed\nbecause of the absorption by extragalactic background light. As a result, a\nnumber of mechanisms have been proposed to solve this potential puzzle,\nincluding Lorentz invariance violation~(LIV). In this work, we perform a\ndetailed numerical calculation and show the feasibility to constrain LIV of\nphotons from the LHAASO observation of GRB221009A quantitatively.", "category": "astro-ph_HE" }, { "text": "In-medium enhancement of the modified Urca neutrino reaction rates: We calculate modified Urca neutrino emission rates in the dense nuclear\nmatter in neutron star cores. We find that these rates are strongly enhanced in\nthe beta-stable matter in regions of the core close to the direct Urca process\nthreshold. This enhancement can be tracked to the use of the in-medium nucleon\nspectrum in the virtual nucleon propagator. We describe the in-medium nucleon\nscattering in the non-relativistic Bruckner-Hartree-Fock framework taking into\naccount two-body as well as the effective three-body forces, although the\nproposed enhancement does not rely on a particular way of the nucleon\ninteraction treatment. Finally we suggest a simple approximate expression for\nthe emissivity of the n-branch of the modified Urca process that can be used in\nthe neutron stars cooling simulations with any nucleon equation of state of\ndense matter.", "category": "astro-ph_HE" }, { "text": "Revealing the variation mechanism of ON 231 via the two-components\n shock-in-jet model: The variation mechanism of blazars is a long-standing unresolved problem. In\nthis work, we present a scenario to explain diverse variation phenomena for ON\n231, where the jet emissions are composed of the flaring and the less variable\ncomponents (most probably from the post-flaring blobs), and the variation is\ndominated by shock-in-jet instead of the Doppler effect. We perform correlation\nanalysis for the multiwavelength light curves and find no significant\ncorrelations. For optical band, ON 231 exhibits a harder when brighter (HWB)\ntrend, and the trend seems to shift at different periods. Correspondingly, the\ncorrelation between polarization degree and flux exhibits a V-shaped behavior,\nand a similar translation relation during different periods is also found.\nThese phenomena could be understood via the superposition of the flaring\ncomponent and slowly varying background component. We also find that the slopes\nof HWB trend become smaller at higher flux levels, which indicates the\nenergy-dependent acceleration processes of the radiative particles. For X-ray,\nwe discover a trend transition from HWB to softer when brighter (SWB) to HWB.\nWe consider that the X-ray emission is composed of both the synchrotron tail\nand the Synchrotron Self-Compton components, which could be described by two\nlog-parabolic functions. By varying the peak frequency, we reproduce the\nobserved trend transition in a quantitative manner. For $\\gamma$-ray, we find\nthe SWB trend, which could be explained naturally if a very-high-energy\n$\\gamma$-ray background component exists. Our study elucidates the variation\nmechanism of intermediate synchrotron-peaked BL Lac objects.", "category": "astro-ph_HE" }, { "text": "Exact analytical expression for the synchrotron radiation spectrum in\n the Gaussian turbulent magnetic field: We demonstrate that the exact solution for the spectrum of synchrotron\nradiation from an isotropic population of mono-energetic electrons in turbulent\nmagnetic field with Gaussian distribution of local field strengths can be\nexpressed in the simple analytic form: $\\left( \\frac{{\\rm d} \\dot{N}}{{\\rm d}\n\\omega} \\right)_t\n = \\frac{\\alpha}{3} \\frac{1}{\\gamma^2} \\left( 1 + \\frac{1}{x^{2/3}} \\right)\n\\exp \\left( - 2 x^{2/3} \\right)$, where $x = \\frac{\\omega}{\\omega_0}\\, ;\n\\omega_0 = \\frac{4}{3} \\gamma^2 \\frac{eB_0}{m_e c}\\, .$ We use this expression\nto find approximate synchrotron spectra for power-law electron distributions\nwith $\\propto \\exp\\left( -\\left[ \\gamma/\\gamma_0 \\right]^\\beta\\right)$ type\nhigh-energy cut-off; the resulting synchrotron spectrum has the exponential\ncut-off factor with frequency raised to $2\\beta/(3\\beta+4)$ power in the\nexponent. For the power-law electron distribution without high-energy cut-off,\nwe find the coefficient $a_m$ as a function of the power-law index, which\nresults in exact expression for the synchrotron spectrum when using\nmonochromatic (i.e., each electron radiates at frequency $\\omega_m = a_m\n\\gamma^2 \\, \\frac{e B_0}{m_e c}$) approximation.", "category": "astro-ph_HE" }, { "text": "Magnetically Confined Mountains on Accreting Neutron Stars in General\n Relativity: The general relativistic formulation of the problem of magnetically confined\nmountains on neutron stars is presented, and the resulting equations are solved\nnumerically, generalising previous Newtonian calculations. The hydromagnetic\nstructure of the accreted matter and the subsequent magnetic burial of the\nstar's magnetic dipole moment are computed. Overall, it is observed that\nrelativistic corrections reduce the hydromagnetic deformation associated with\nthe mountain. The magnetic field lines are curved more gently than in previous\ncalculations, and the screening of the dipole moment is reduced.\nQuantitatively, it is found that the dimensionless dipole moment ($m_{\\rm d}$)\ndepends on the accreted mass ($M_{\\rm a}$) as $m_{\\rm d} =\n-3.2\\times10^{3}M_{\\rm a}/M_\\odot + 1.0$, implying approximately three times\nless screening compared to the Newtonian theory. Additionally, the\ncharacteristic scale height of the mountain, governing the gradients of\nquantities like pressure, density, and magnetic field strength, reduces by\napproximately $40\\%$ for an isothermal equation of state.", "category": "astro-ph_HE" }, { "text": "Very high energy gamma-ray follow-up observations of novae and dwarf\n novae with the MAGIC telescopes: In the last few years the Fermi-LAT instrument has detected GeV gamma-ray\nemission from several novae. Such GeV emission can be interpreted in terms of\ninverse Compton emission from electrons accelerated in the shock or in terms of\nemission from hadrons accelerated in the same conditions. The latter might\nreach much higher energies and could produce a second component in the\ngamma-ray spectrum at TeV energies. We perform follow-up observations of\nselected novae and dwarf novae in search of the second component in TeV energy\ngamma rays. This can shed light on the acceleration process of leptons and\nhadrons in nova explosions. We have performed observations with the MAGIC\ntelescopes of 3 sources, a symbiotic nova YY Her, a dwarf nova ASASSN-13ax and\na classical nova V339 Del, shortly after their outbursts. We did not detect TeV\ngamma-ray emission from any of the objects observed. The TeV upper limits from\nMAGIC observations and the GeV detection by Fermi constrain the acceleration\nparameters for electrons and hadrons.", "category": "astro-ph_HE" }, { "text": "Test of the string loop oscillation model using kHz quasiperiodic\n oscillations in a neutron star binary: The model of current-carrying string loop oscillations is tested to explain\nthe special set of frequencies related to the high-frequency quasiperiodic\noscillations (HF QPOs) observed recently in the low-mass X-ray binary XTE\nJ1701-407 containing a neutron star. The external geometry of the neutron star\nis approximated by the Kerr geometry, introducing errors not exceeding $10~\\%$\nfor slowly rotating massive neutron stars. The frequencies of the radial and\nvertical string loop oscillations are then governed by the mass $M$ and\ndimensionless spin $a$ of the neutron star, and by the dimensionless parameter\n$\\omega$ describing combined effects of the string loop tension and its angular\nmomentum. It is explicitly demonstrated that the string-loop oscillation model\ncan explain the observed kHz frequencies for the neutron star parameters\nrestricted to the intervals ${0.2}{\\sim} 10^{38}$ erg s$^{-1}$. Above ~20 keV, hard\nX-ray emission in the central 10 pc region around Sgr A* consists of the\ncandidate PWN G359.95-0.04 and the CHXE, possibly resulting from an unresolved\npopulation of massive CVs with white dwarf masses $M_{\\rm WD} \\sim 0.9\nM_{\\odot}$. Spectral energy distribution analysis suggests that G359.95-0.04 is\nlikely the hard X-ray counterpart of the ultra-high gamma-ray source HESS\nJ1745-290, strongly favoring a leptonic origin of the GC TeV emission.", "category": "astro-ph_HE" }, { "text": "No evidence for a low-mass black hole in Swift J1753.5-0127: We present high-resolution, time-resolved optical spectroscopy of the black\nhole X-ray transient Swift J1753.5-0127. Our optical spectra do not show\nfeatures that we can associate with the companion star. However we do observe\nbroad, double-peaked emission lines, typical of an accretion disc. We show that\nthe mass of the compact object is likely $>7.4\\pm1.2M_{\\odot}$, much higher\nthan previous suggestions of a low-mass ($<5M_{\\odot}$) black hole.", "category": "astro-ph_HE" }, { "text": "Variable mass accretion and failed wind explain changing look phenomena\n in NGC 1365: Changing look active galactic nuclei (CLAGNs) show complex nature in their\nX-ray spectral shape and line of sight column density variation. The physical\nmechanisms responsible for these variations are unclear. Here, we study the\nspectral properties of a CLAGN, NGC\\,1365 using combined {\\it XMM-Newton} and\n{\\it NuSTAR} observations to understand the CL behavior. The model fitted mass\naccretion rate varied between $0.003\\pm 0.001$ and $0.009\\pm0.002$ $\\dot M_{\\rm\nEdd}$ and the dynamic corona changed from $28\\pm 3$ to $10\\pm1$ $r_g$. We found\nthat the variable absorption column density correlates with the mass accretion\nrate and the geometry of the corona. The derived wind velocity was sufficiently\nlow compared to the escape velocity to drive the wind away from the disc for\nthe epochs when column densities were high. This suggests that the high and\nvariable absorption can be due to failed winds from the disc. Our estimated\nratio of mass outflow to inflow rate from the inner region of the disc lies\nbetween $0.019\\pm0.006$ and $0.12\\pm0.04$. From spectral fitting of the\ncombined data, we found the mass of the central black hole to be constant\n$4.38\\pm0.34 - 4.51\\pm0.29 \\times10^{6} M_\\odot$, consistent with earlier\nfindings. The confidence contours of $N_H$ with other model parameters show\nthat the model fitted parameters are robust and non-degenerate. Our study\nconstrued that the changing accretion rate, which is a fundamental physical\nquantity and the geometry of the corona driving the CL phenomena in NGC\\,1365.\nThe physical picture considered in this work connects both variable continuum\nand variable absorbing medium scenarios.", "category": "astro-ph_HE" }, { "text": "Probing dark matter annihilation in the Galaxy with antiprotons and\n gamma rays: A possible hint of dark matter annihilation has been found in Cuoco,\nKorsmeier and Kr\\\"amer (2017) from an analysis of recent cosmic-ray antiproton\ndata from AMS-02 and taking into account cosmic-ray propagation uncertainties\nby fitting at the same time dark matter and propagation parameters. Here, we\nextend this analysis to a wider class of annihilation channels. We find\nconsistent hints of a dark matter signal with an annihilation cross-section\nclose to the thermal value and with masses in range between 40 and 130 GeV\ndepending on the annihilation channel. Furthermore, we investigate in how far\nthe possible signal is compatible with the Galactic center gamma-ray excess and\nrecent observation of dwarf satellite galaxies by performing a joint global fit\nincluding uncertainties in the dark matter density profile. As an example, we\ninterpret our results in the framework of the Higgs portal model.", "category": "astro-ph_HE" }, { "text": "High-energy neutrinos from Galactic sources: Even 100 years after the discovery of cosmic rays their origin remains a\nmystery. In recent years, TeV gamma-ray detectors have discovered and\ninvestigated many Galactic sources where particles are accelerated up to\nenergies of 100 TeV. However, it has not been possible up to now to identify\nthese sites unambiguously as sources of hadronic acceleration. The observation\nof cosmic high-energy neutrinos from these or other sources will be a\nsmoking-gun evidence for the sites of the acceleration of cosmic rays.", "category": "astro-ph_HE" }, { "text": "Update on the GRB universal scaling\n E$_{\\rm{X,iso}}$-E$_{\\rm{\u03b3,iso}}$-E$_{\\rm{pk}}$ with ten years of\n $Swift$ data: From a comprehensive statistical analysis of $Swift$ X-ray light-curves of\ngamma-ray bursts (GRBs) collected from December 2004 to the end of 2010, we\nfound a three-parameter correlation between the isotropic energy emitted in the\nrest frame 1-10$^4$ keV energy band during the prompt emission\n(E$_{\\rm{\\gamma,iso}}$), the rest frame peak of the prompt emission energy\nspectrum (E$_{\\rm{pk}}$), and the X-ray energy emitted in the rest frame 0.3-30\nkeV observed energy band (E$_{\\rm{X,iso}}$), computed excluding the\ncontribution of the flares. In this paper, we update this correlation with the\ndata collected until June 2014, expanding the sample size with $\\sim$35% more\nobjects, where the number of short GRBs doubled. With this larger sample we\nconfirm the existence of a universal correlation that connects the prompt and\nafterglow properties of long and short GRBs. We show that this correlation does\nnot depend on the X-ray light-curve morphology and that further analysis is\nnecessary to firmly exclude possible biases derived by redshift measurements.\nIn addition we discuss about the behavior of the peculiar objects as ultra-long\nGRBs and we propose the existence of an intermediate group between long and\nshort GRBs. Interestingly, two GRBs with uncertain classification fall into\nthis category. Finally, we discuss the physics underlying this correlation, in\nthe contest of the efficiency of conversion of the prompt $\\gamma$-ray emission\nenergy into the kinetic energy of the afterglow, the photosferic model, and the\ncannonball model.", "category": "astro-ph_HE" }, { "text": "Modelling of blazar SEDs with the nonlinear SSC cooling process: Observations of blazar flaring states reveal remarkably different variability\ntime scales. Especially rapid flares with flux doubling time scales of the\norder of minutes have been puzzling for quite some time. Many modeling attempts\nuse the well known linear relations for the cooling and emission processes in\nthe jet in a steady-state scenario, albeit the obvious strongly time-dependent\nnature of flares. Due to the feedback of self-produced radiation with\nadditional scattering by relativistic electrons, the synchrotron-self Compton\n(SSC) effect is inherently time-dependent. Although this feedback is usually\nimplemented in numerical treatments, only recently an analytical analysis of\nthe effects of this nonlinear behaviour has been performed. Here, we report our\nresults concerning the effect of the time-dependent SSC on the spectral energy\ndistribution (SED) of blazars. We calculated analytically the synchrotron and\nthe SSC component, giving remarkably different spectral features compared to\nthe standard linear approach. Adding an external photon field to the original\nsetting, we could implement quite easily the effect of an additional external\nCompton (EC) cooling, since such strong external photon fields are observed in\nflat spectrum radio quasars (FSRQ), a subclass of blazars. Calculating the\nresulting flux due to the EC cooling, we were able to show that the resulting\ninverse Compton component strongly depends on the free parameters, and that SSC\ncould potentially have a strong effect in FSRQs, contrary to what is usually\nassumed.", "category": "astro-ph_HE" }, { "text": "The Soft State of the Black Hole Transient Source MAXI J1820+070:\n Emission from the Edge of the Plunge Region?: The Galactic black hole X-ray binary MAXI J1820+070 had a bright outburst in\n2018 when it became the second brightest X-ray source in the Sky. It was too\nbright for X-ray CCD instruments such as XMM-Newton and Chandra, but was well\nobserved by photon-counting instruments such as NICER and NuSTAR. We report\nhere on the discovery of an excess emission component during the soft state. It\nis best modelled with a blackbody spectrum in addition to the regular disk\nemission, modelled either as diskbb or kerrbb. Its temperature varies from\nabout 0.9 to 1.1 keV which is about 30 to 80 per cent higher than the inner\ndisc temperature of diskbb. Its flux varies between 4 and 12 percent of the\ndisc flux. Simulations of magnetised accretion discs have predicted the\npossibility of excess emission associated with a non-zero torque at the\nInnermost Stable Circular Orbit (ISCO) about the black hole, which from other\nNuSTAR studies lies at about 5 gravitational radii or about 60 km (for a black\nhole mass is 8 M). In this case the emitting region at the ISCO has a width\nvarying between 1.3 and 4.6 km and would encompass the start of the plunge\nregion where matter begins to fall freely into the black hole.", "category": "astro-ph_HE" }, { "text": "New Measurement of Muon Neutrino Disappearance from the IceCube\n Experiment: The IceCube Neutrino Observatory is a Cherenkov detector located at the South\nPole. Its main component consists of an in-ice array of optical modules\ninstrumenting one cubic kilometer of deep Glacial ice. The DeepCore\nsub-detector is a denser in-fill array with a lower energy threshold, allowing\nus to study atmospheric neutrinos oscillations with energy below 100 GeV\narriving through the Earth. We present preliminary results of an atmospheric\nmuon neutrino disappearance analysis using data from 2012 to 2021 and employing\nconvolutional neural networks (CNNs) for precise and fast event\nreconstructions.", "category": "astro-ph_HE" }, { "text": "The Origin of the Prompt Emission for Short GRB 170817A: Photosphere\n Emission or Synchrotron Emission?: The first gravitational-wave event from the merger of a binary neutron star\nsystem (GW170817) was detected recently. The associated short gamma-ray burst\n(GRB 170817A) has a low isotropic luminosity ($\\sim 10^{47}$ erg s$ ^{-1}$) and\na peak energy $E_{p}\\sim 145$ keV during the initial main emission between -0.3\nand 0.4 s. The origin of this short GRB is still under debate, but a plausible\ninterpretation is that it is due to the off-axis emission from a structured\njet. We consider two possibilities. First, since the best-fit spectral model\nfor the main pulse of GRB 170817A is a cutoff power law with a hard low-energy\nphoton index ($\\alpha =-0.62_{-0.54}^{+0.49} $), we consider an off-axis\nphotosphere model. We develop a theory of photosphere emission in a structured\njet and find that such a model can reproduce a low-energy photon index that is\nsofter than a blackbody through enhancing high-latitude emission. The model can\nnaturally account for the observed spectrum. The best-fit Lorentz factor along\nthe line of sight is $\\sim 20$, which demands that there is a significant delay\nbetween the merger and jet launching. Alternatively, we consider that the\nemission is produced via synchrotron radiation in an optically thin region in\nan expanding jet with decreasing magnetic fields. This model does not require a\ndelay of jet launching but demands a larger bulk Lorentz factor along the line\nof sight. We perform Markov Chain Monte Carlo fitting to the data within the\nframework of both models and obtain good fitting results in both cases.", "category": "astro-ph_HE" }, { "text": "First search for GeV neutrinos from bright gamma-ray solar flares using\n the IceCube Neutrino Observatory: In response to a reported increase in the total neutrino flux in the\nHomestake experiment in coincidence with solar flares at the end of the\neighties, solar neutrino detectors have searched for solar flare signals. Solar\nflares convert magnetic energy into thermal energy of plasma and kinetic energy\nof charged particles such as protons. As a consequence of magnetic\nreconnection, protons are injected downwards from the coronal acceleration\nregion and can interact with dense plasma in the lower solar atmosphere,\nproducing mesons that will subsequently decay into gamma rays and neutrinos at\nO(MeV-GeV) energies. The main motivation to search for solar flare neutrinos\ncomes from their hadronic origin. As inherent products of high-energy proton\ncollisions with the chromosphere, they are a direct probe of the proton\naccelerated towards the chromosphere. Using a multi-messenger approach, it is\ntherefore possible to constrain the proton acceleration taking place in the\nsolar flares, including the spectral index of the accelerated flux and its\nshape. We present the results of the first search for GeV neutrinos emitted\nduring solar flares carried out with the IceCube Neutrino Observatory. We\npresent a new approach which allows us to strongly lower the energy threshold\nof IceCube, originally designed to detect 10 GeV - PeV neutrinos. We compare\nthe results with theoretical estimates of the corresponding flux.", "category": "astro-ph_HE" }, { "text": "SN 2023emq: a flash-ionised Ibn supernova with possible CIII emissio: SN 2023emq is a fast-evolving transient initially classified as a rare Type\nIcn supernova (SN), interacting with a H- and He-free circumstellar medium\n(CSM) around maximum light. Subsequent spectroscopy revealed the unambiguous\nemergence of narrow He lines, confidently placing SN 2023emq in the more common\nType Ibn class. Photometrically SN 2023emq has several uncommon properties\nregardless of its class, including its extreme initial decay (faster than > 90%\nof Ibn/Icn SNe) and sharp transition in the decline rate from 0.20 mag/d to\n0.07 mag/d at +20 d. The bolometric light curve can be modelled as CSM\ninteraction with 0.32M_Sun of ejecta and 0.12M_Sun of CSM, with 0.006M_Sun of\nnickel, as expected of fast interacting SNe. Furthermore, broad-band\npolarimetry at +8.7 days (P = 0.55 +/- 0.30%) is consistent with spherical\nsymmetry. A discovery of a transitional Icn/Ibn SN would be unprecedented and\nwould give valuable insights into the nature of mass loss suffered by the\nprogenitor just before death, but we favour an interpretation that SN 2023emq\nis a type Ibn SN that exhibited flash-ionised features in the earliest\nspectrum, as the features are not an exact match with other SNe Icn to date.\nHowever, the feature at 5700{\\AA}, in the region of C III and N II emission, is\nsignificantly stronger in SN 2023emq than in the few other flash-ionised Type\nIbn SNe, and if it is related to C III, it possibly implies a continuum of\nproperties between the two classes.", "category": "astro-ph_HE" }, { "text": "Testing the rotating hot spot model using X-ray burst oscillations from\n 4U 1636-536: Precise and accurate measurements of neutron star masses and radii would\nprovide valuable information about the still uncertain properties of cold\nmatter at supranuclear densities. One promising approach to making such\nmeasurements involves analysis of the X-ray flux oscillations often seen during\nthermonuclear (type 1) X-ray bursts. These oscillations are almost certainly\nproduced by emission from hotter regions on the stellar surface modulated by\nthe rotation of the star. One consequence of the rotation is that the\noscillation should appear earlier at higher photon energies than at lower\nenergies. Ford (1999) found compelling evidence for such a hard lead in the\ntail oscillations of one type 1 burst from Aql X-1. We have therefore analyzed\nindividually the oscillations observed in the tails of the four type 1 bursts\nfrom 4U 1636-536 that, when averaged, provided the strongest evidence for a\nsoft lead in the analysis by Muno et al. (2003). We have also analyzed the\noscillation observed during the superburst from this star. We find that the\ndata from these five bursts, treated both individually and jointly, are fully\nconsistent with a rotating hot spot model. Unfortunately, the uncertainties in\nthese data are too large to provide interesting constraints on the mass and\nradius of this star.", "category": "astro-ph_HE" }, { "text": "Supernova remnants in the very-high-energy sky: prospects for the\n Cherenkov Telescope Array: The Cherenkov Telescope Array is expected to lead to the detection of many\nnew supernova remnants in the TeV and multi-TeV range. In addition to the\nindividual study of each SNR, the study of these objects as a population can\nhelp constraining the parameters describing the acceleration of particles and\nincreasing our understanding of the mechanisms involved. We present Monte Carlo\nsimulations of the population of Galactic SNRs emitting TeV gamma rays. We also\ndiscuss how the simulated population can be confronted with future observations\nto provide a novel test for the SNR hypothesis of cosmic ray origins.", "category": "astro-ph_HE" }, { "text": "Neutron stars as sources of gravitational waves: The global network of ground-based gravitational-wave detectors (the Advanced\nLIGO and the Advanced Virgo) is sensitive at the frequency range corresponding\nto relativistic stellar-mass compact objects. Among the promising types of\ngravitational-wave sources are binary systems and rotating, deformed neutron\nstars. I will describe these sources and present predictions of how their\nobservations will contribute to modern astrophysics in the near future.", "category": "astro-ph_HE" }, { "text": "Gamma-ray burst spectra and spectral correlations from sub-photospheric\n Comptonization: One of the most important unresolved issues in gamma-ray burst physics is the\norigin of the prompt gamma-ray spectrum. Its general non-thermal character and\nthe softness in the X-ray band remain unexplained. We tackle these issues by\nperforming Monte Carlo simulations of radiation-matter interactions in a\nscattering dominated photon-lepton plasma. The plasma -- initially in\nequilibrium -- is driven to non-equilibrium conditions by a sudden energy\ninjection in the lepton population, mimicking the effect of a shock wave or the\ndissipation of magnetic energy. Equilibrium restoration occurs due to energy\nexchange between the photons and leptons. While the initial and final\nequilibrium spectra are thermal, the transitional photon spectra are\ncharacterized by non-thermal features such as power-law tails, high energy\nbumps, and multiple components. Such non-thermal features are observed at\ninfinity if the dissipation occurs at small to moderate optical depths, and the\nspectrum is released before thermalization is complete. We model the synthetic\nspectra with a Band function and show that the resulting spectral parameters\nare similar to observations for a frequency range of 2-3 orders of magnitude\naround the peak. In addition, our model predicts correlations between the\nlow-frequency photon index and the peak frequency as well as between the low-\nand high-frequency indices. We explore baryon and pair dominated fireballs and\nreach the conclusion that baryonic fireballs are a better model for explaining\nthe observed features of gamma-ray burst spectra.", "category": "astro-ph_HE" }, { "text": "Search for fingerprints of disoriented chiral condensates in cosmic ray\n showers: Although the generation of disoriented chiral condensates (DCCs), where the\norder parameter for chiral symmetry breaking is misaligned with respect to the\nvacuum direction in isospin state, is quite natural in the theory of strong\ninteractions, they have so far eluded experiments in accelerators and cosmic\nrays. If DCCs are formed in high-energy nuclear collisions, the relevant\noutcome are very large event-by-event fluctuations in the neutral-to-charged\npion fraction. In this note we search for fingerprints of DCC formation in\nobservables of ultra-high energy cosmic ray showers. We present simulation\nresults for the depth of the maximum ($X_{max}$) and number of muons on the\nground, evaluating their sensitivity to the neutral-to-charged pion fraction\nasymmetry produced in the primary interaction.", "category": "astro-ph_HE" }, { "text": "Astrophysical signatures of leptonium: More than 10^43 positrons annihilate every second in the centre of our Galaxy\nyet, despite four decades of observations, their origin is still unknown. Many\ncandidates have been proposed, such as supernovae and low mass X-ray binaries.\nHowever, these models are difficult to reconcile with the distribution of\npositrons, which are highly concentrated in the Galactic bulge, and therefore\nrequire specific propagation of the positrons through the interstellar medium.\nAlternative sources include dark matter decay, or the supermassive black hole,\nboth of which would have a naturally high bulge-to-disc ratio.\n The chief difficulty in reconciling models with the observations is the\nintrinsically poor angular resolution of gamma-ray observations, which cannot\nresolve point sources. Essentially all of the positrons annihilate via the\nformation of positronium. This gives rise to the possibility of observing\nrecombination lines of positronium emitted before the atom annihilates. These\nemission lines would be in the UV and the NIR, giving an increase in angular\nresolution of a factor of 10^4 compared to gamma ray observations, and allowing\nthe discrimination between point sources and truly diffuse emission.\n Analogously to the formation of positronium, it is possible to form atoms of\ntrue muonium and true tauonium. Since muons and tauons are intrinsically\nunstable, the formation of such leptonium atoms will be localised to their\nplaces of origin. Thus observations of true muonium or true tauonium can\nprovide another way to distinguish between truly diffuse sources such as dark\nmatter decay, and an unresolved distribution of point sources.", "category": "astro-ph_HE" }, { "text": "Study of Asymptotic Velocity in the Bondi-Hoyle Accretion Flows in the\n Domain of Kerr and 4-D Einstein-Gauss-Bonnet Gravities: Understanding the physical structures of the accreated matter very close to\nthe black hole in quasars and active galactic nucleus (AGNs) is an important\nmilestone to constrain the activities occurring in their centers. In this\npaper, we numerically investigate the effects of the asymptotic velocities on\nthe physical structures of the accretion disk around the Kerr and\nEinstein-Gauss-Bonnet (EGB) rapidly rotating black holes. The Bondi-Hoyle\naccretion is considered with a falling gas towards the black hole in upstream\nregion of the computational domain. The shock cones are naturally produced in\nthe downstream part of the flow around both black holes. It is found that the\nstructure of the cones and the amount of the accreated matter depend on\nasymptotic velocity $V_{\\infty}$ (Mach number) and the types of the gravities\n(Kerr or EGB). Increasing the Mach number of the inflowing matter in the\nsupersonic region causes the shock opening angle and accretion rates getting\nsmaller because of the rapidly falling gas towards the black hole. The EGB\ngravity leads to an increase in the shock opening angle of the shock cones\nwhile the mass accretion rates $\\dot{M}$ are decreasing in EGB gravity with a\nGauss-Bonnet (GB) coupling constant $\\alpha$. It is also confirmed that\naccretion rates and drag forces are significantly altered in the EGB gravity.\nOur numerical simulation results could be used to identify the accreation\nmechanism and physical properties of the accretion disk and black hole in the\nobserved $X-$ rays such as NGC $1313$ $X-1$ and $1313$ $X-2$ and MAXI\n$J1803-298$.", "category": "astro-ph_HE" }, { "text": "Neutrino cooling and spin-down of rapidly rotating compact stars: The gravitational-wave instability of r-modes in rapidly rotating compact\nstars is believed to spin them down to angular frequencies of about a tenth of\nthe Kepler frequency soon after their birth in a Supernova. We point out that\nthe r-mode perturbation also impacts the neutrino cooling and viscosity in hot\ncompact stars via processes that restore weak equilibrium. We illustrate this\nfact with a simple model of spin-down due to gravitational wave emission in\ncompact stars composed entirely of three-flavor degenerate quark matter (a\nstrange quark star). Non-equilibrium neutrino cooling of this oscillating fluid\nmatter is quantified. Our results imply that a consistent treatment of thermal\nand spin-frequency evolution of a young and hot compact star is a requisite in\nestimating the persistence of gravitational waves from such a source.", "category": "astro-ph_HE" }, { "text": "Efficiency of Thin Magnetically-Arrested Disks Around Black Holes: The radiative and jet efficiencies of thin magnetized accretion disks around\nblack holes (BHs) are affected by BH spin and the presence of a magnetic field\nthat, when strong, could lead to large deviations from Novikov-Thorne (NT) thin\ndisk theory. To seek the maximum deviations, we perform general relativistic\nmagnetohydrodynamic (GRMHD) simulations of radiatively efficient thin\n(half-height $H$ to radius $R$ of $H/R\\approx 0.10$) disks around moderately\nrotating BHs with $a/M=0.5$. First, our simulations, each evolved for more than\n$70,000r_g/c$ (gravitational radius $r_g$ and speed of light $c$), show that\nlarge-scale magnetic field readily accretes inward even through our thin disk\nand builds-up to the magnetically-arrested disk (MAD) state. Second, our\nsimulations of thin MADs show the disk achieves a radiative efficiency of\n$\\eta_{\\rm r}\\approx 15\\%$ (after estimating photon capture), which is about\ntwice the NT value of $\\eta_{\\rm r}\\sim 8\\%$ for $a/M=0.5$ and gives the same\nluminosity as a NT disk with $a/M\\approx 0.9$. Compared to prior simulations\nwith $\\lesssim 10\\%$ deviations, our result of an $\\approx 80\\%$ deviation sets\na new benchmark. Building on prior work, we are now able to complete an\nimportant scaling law which suggest that observed jet quenching in the\nhigh-soft state in BH X-ray binaries is consistent with an ever-present MAD\nstate with a weak yet sustained jet.", "category": "astro-ph_HE" }, { "text": "Radio and X-ray observations of Five TeV SNRs: We briefly summarize recent results of five TeV SNRs from radio and X-ray\nobservations. We focus on remeasuring kinematic distances of 5 TeV SNRs, i.e.\nHESS J1732-347/SNR G353.6-0.7 (3.2 kpc), HESS J1834-087/G23.3-0.3 (also W41,\n4.0 kpc), HESS J1833-105/G21.5-0.9 (4.8 kpc), HESS J1846-029/G29.7-0.3 (Kes 75,\n6.3 kpc) and TeV SNR G54.1-0.3 (6.5 kpc), and studying non-thermal X-ray\nemissions from two old SNRs (G353.6-0.7 and W41). These not only allow\nconstraining the TeV SNR basic physical properties, but also help reveal\nacceleration mechanisms of TeV Gamma-rays in the SNRs which are either related\nwith the SNRs or the pulsar wind nebulae.", "category": "astro-ph_HE" }, { "text": "Evidence for quark-matter cores in massive neutron stars: The theory governing the strong nuclear force, Quantum Chromodynamics,\npredicts that at sufficiently high energy densities hadronic nuclear matter\nundergoes a deconfinement transition to a new phase of quarks and gluons.\nAlthough this has been observed in ultrarelativistic heavy-ion collisions, it\nis currently an open question whether quark matter exists inside neutron stars.\nBy combining astrophysical observations and theoretical ab-initio calculations\nin a model-independent way, we find that the inferred properties of matter in\nthe cores of neutron stars with mass corresponding to 1.4 solar masses are\ncompatible with nuclear model calculations. However, the matter in the interior\nof maximally massive, stable neutron stars exhibits characteristics of the\ndeconfined phase, which we interpret as evidence for the presence of\nquark-matter cores. For the heaviest reliably observed neutron stars with\nmasses of about two solar masses, the presence of quark matter is found to be\nlinked to the behaviour of the speed of sound c_s in strongly interacting\nmatter. If the conformal bound (c_s)^2 < 1/3 is not strongly violated, massive\nneutron stars are predicted to have sizable quark-matter cores. This finding\nhas important implications for the phenomenology of neutron stars, and affects\nthe dynamics of neutron star mergers with at least one sufficiently massive\nparticipant.", "category": "astro-ph_HE" }, { "text": "The Correlation between Dispersion Measure and X-ray Column Density from\n Radio Pulsars: Pulsars are remarkable objects that emit across the entire electromagnetic\nspectrum, providing a powerful probe of the interstellar medium. In this study,\nwe investigate the relation between dispersion measure (DM) and X-ray\nabsorption column density NH using 68 radio pulsars detected at X-ray energies\nwith the Chandra X-ray Observatory or XMM-Newton. We find a best-fit empirical\nlinear relation of NH (10^20 cm^-2)= 0.30+0.13-0.09 DM (pc cm^-3), which\ncorresponds to an average ionization of 10+4-3%, confirming the ratio of one\nfree electron per ten neutral hydrogen atoms commonly assumed in the\nliterature. We also compare different NH estimates and note that some NH values\nobtained from X-ray observations are higher than the total Galactic HI column\ndensity along the same line of sight, while the optical extinction generally\ngives the best NH predictions.", "category": "astro-ph_HE" }, { "text": "On the simultaneous generation of radio and soft X-ray emission by AXP\n 4U 0142+61: In the present paper we study the possibility of a simultaneous generation of\nradio waves and soft $X$-rays by means of the quasi-linear diffusion (QLD) in\nthe anomalous pulsar AXP 4U 0142+61. Considering the magnetosphere composed of\nthe so-called beam component and the plasma component respectively, we argue\nthat the frozen-in condition will inevitably lead to the generation of the\nunstable cyclotron waves. These waves, via the QLD, will in turn influence the\nparticle distribution function, leading to certain values of the pitch angles,\nthus to an efficient synchrotron mechanism, producing soft $X$-ray photons. We\nshow that for physically reasonable parameters of magnetospheric plasmas, the\nQLD can provide generation of radio waves in the following interval $40$\nMHz-$111$ MHz connected to soft $X$-rays for the domain $0.3$keV-$1.4$keV.", "category": "astro-ph_HE" }, { "text": "GN-z11-flash in the context of Gamma-Ray Burst Afterglows: The recently discovered rapid transient GN-z11-flash has been suggested to be\nthe prompt-emission ultraviolet flash associated with a gamma-ray burst\nserendipitously exploding in the ultra-high-$z$ galaxy GN-z11. We here place\nthe flash into the context of the early ultraviolet emission of gamma-ray\nbursts, and find it is in agreement with the luminosity distribution of these\nevents.", "category": "astro-ph_HE" }, { "text": "Extreme photometric and polarimetric variability of blazar S4 0954+65 at\n its maximum optical and $\u03b3$-ray brightness levels: In 2022 the BL Lac object S4 0954+65 underwent a major variability phase,\nreaching its historical maximum brightness in the optical and $\\gamma$-ray\nbands. We present optical photometric and polarimetric data acquired by the\nWhole Earth Blazar Telescope (WEBT) Collaboration from 2022 April 6 to July 6.\nMany episodes of unprecedented fast variability were detected, implying an\nupper limit to the size of the emitting region as low as $10^{-4}$ parsec. The\nWEBT data show rapid variability in both the degree and angle of polarization.\nWe analyse different models to explain the polarization behaviour in the\nframework of a twisting jet model, which assumes that the long-term trend of\nthe flux is produced by variations in the emitting region viewing angle. All\nthe models can reproduce the average trend of the polarization degree, and can\naccount for its general anticorrelation with the flux, but the dispersion of\nthe data requires the presence of intrinsic mechanisms, such as turbulence,\nshocks, or magnetic reconnection. The WEBT optical data are compared to\n$\\gamma$-ray data from the Fermi satellite. These are analysed with both fixed\nand adaptive binning procedures. We show that the strong correlation between\noptical and $\\gamma$-ray data without measurable delay assumes different slopes\nin faint and high brightness states, and this is compatible with a scenario\nwhere in faint states we mainly see the imprint of the geometrical effects,\nwhile in bright states the synchrotron self-Compton process dominates.", "category": "astro-ph_HE" }, { "text": "Swift: the science across the rainbow. Mission Overview and Highlights\n of Results: I present an overview of the Swift mission, which was launched on November\n20, 2004 to discover and observe the most energetic of astrophysical phenomena,\ngamma-ray bursts (GRBs). After almost 6 years in space the Observatory is in\nexcellent shape, with all systems and instruments performing nominally and in\nburst chasing mode for an average of 97% of the time. Swift is also a\nmulti-purpose multi-frequency mission with the observing time evolving from\nmostly GRB targets, to mainly secondary science ones such as supernovae,\ncataclysmic variables and novae, active galactic nuclei, Galactic transients,\nactive stars and comets. I present the most recent science highlights.", "category": "astro-ph_HE" }, { "text": "Highly Ionized Fe-K Absorption Line from Cygnus X-1 in the High/Soft\n State Observed with Suzaku: We present observations of a transient He-like Fe K alpha absorption line in\nSuzaku observations of the black hole binary Cygnus X-1 on 2011 October 5 near\nsuperior conjunction during the high/soft state, which enable us to map the\nfull evolution from the start and the end of the episodic accretion phenomena\nor dips for the first time. We model the X-ray spectra during the event and\ntrace their evolution. The absorption line is rather weak in the first half of\nthe observation, but instantly deepens for ~10 ks, and weakens thereafter. The\noverall change in equivalent width is a factor of ~3, peaking at an orbital\nphase of ~0.08. This is evidence that the companion stellar wind feeding the\nblack hole is clumpy. By analyzing the line with a Voigt profile, it is found\nto be consistent with a slightly redshifted Fe XXV transition, or possibly a\nmixture of several species less ionized than Fe XXV. The data may be explained\nby a clump located at a distance of ~10^(10-12) cm with a density of\n~10^((-13)-(-11)) g cm^-3, which accretes onto and/or transits the\nline-of-sight to the black hole, causing an instant decrease in the observed\ndegree of the ionization and/or an increase in density of the accreting matter.\nContinued monitoring for individual events with future X-ray calorimeter\nmissions such as ASTRO-H and AXSIO will allow us to map out the accretion\nenvironment in detail and how it changes between the various accretion states.", "category": "astro-ph_HE" }, { "text": "Broad band polarimetric follow-up of Type IIP SN 2012aw: We present the results based on R-band polarimetric follow-up observations of\nthe nearby (~10 Mpc) Type II-plateau SN 2012aw. Starting from ~10 days after\nthe SN explosion, these polarimetric observations cover ~90 days (during the\nplateau phase) and are distributed over 9 epochs. To characterize the Milky Way\ninterstellar polarization (ISP_MW ), we have observed 14 field stars lying in a\nradius of 10 degree around the SN. We have also tried to subtract the host\ngalaxy dust polarization component assuming that the dust properties in the\nhost galaxy are similar to that observed for Galactic dust and the general\nmagnetic field follow the large scale structure of the spiral arms of a galaxy.\nAfter correcting the IS_PMW , our analysis infer that SN 2012aw has maximum\npolarization of 0.85% +- 0.08% but polarization angle does not show much\nvariation with a weighted mean value of ~138 degree. However, if both ISP_MW\nand host galaxy polarization (ISP_HG ) components are subtracted from the\nobserved polarization values of SN, maximum polarization of the SN becomes\n0.68% +- 0.08%. The distribution of Q and U parameters appears to follow a loop\nlike structure. The evolution of polarimetric light curve (PLC) properties of\nthis event is also compared with other well studied core-collapse supernovae of\nsimilar type.", "category": "astro-ph_HE" }, { "text": "Astrophysical Implications of the Binary Black-Hole Merger GW150914: The discovery of the gravitational-wave source GW150914 with the Advanced\nLIGO detectors provides the first observational evidence for the existence of\nbinary black-hole systems that inspiral and merge within the age of the\nUniverse. Such black-hole mergers have been predicted in two main types of\nformation models, involving isolated binaries in galactic fields or dynamical\ninteractions in young and old dense stellar environments. The measured masses\nrobustly demonstrate that relatively \"heavy\" black holes ($\\gtrsim 25\\,\nM_\\odot$) can form in nature. This discovery implies relatively weak\nmassive-star winds and thus the formation of GW150914 in an environment with\nmetallicity lower than $\\sim 1/2$ of the solar value. The rate of binary\nblack-hole mergers inferred from the observation of GW150914 is consistent with\nthe higher end of rate predictions ($\\gtrsim 1 \\, \\mathrm{Gpc}^{-3} \\,\n\\mathrm{yr}^{-1}$) from both types of formation models. The low measured\nredshift ($z \\sim 0.1$) of GW150914 and the low inferred metallicity of the\nstellar progenitor imply either binary black-hole formation in a low-mass\ngalaxy in the local Universe and a prompt merger, or formation at high redshift\nwith a time delay between formation and merger of several Gyr. This discovery\nmotivates further studies of binary-black-hole formation astrophysics. It also\nhas implications for future detections and studies by Advanced LIGO and\nAdvanced Virgo, and gravitational-wave detectors in space.", "category": "astro-ph_HE" }, { "text": "Massive protostars as gamma-ray sources: Massive protostars have associated bipolar outflows with velocities of\nhundreds of km s$^{-1}$. Such outflows can produce strong shocks when interact\nwith the ambient medium leading to regions of non-thermal radio emission. We\naim at exploring under which conditions relativistic particles are accelerated\nat the terminal shocks of the protostellar jets and can produce significant\ngamma-ray emission. We estimate the conditions necessary for particle\nacceleration up to very high energies and gamma-ray production in the\nnon-thermal hot spots of jets associated with massive protostars embedded in\ndense molecular clouds. We show that relativistic Bremsstrahlung and\nproton-proton collisions can make molecular clouds with massive young stellar\nobjects detectable by the {\\it Fermi}{} satellite at MeV-GeV energies and by\nCherenkov telescope arrays in the GeV-TeV range. Gamma-ray astronomy can be\nused to probe the physical conditions in star forming regions and particle\nacceleration processes in the complex environment of massive molecular clouds.", "category": "astro-ph_HE" }, { "text": "Impact of Massive Binary Star and Cosmic Evolution on Gravitational Wave\n Observations II: Double Compact Object Rates and Properties: Making the most of the rapidly increasing population of gravitational-wave\ndetections of black hole (BH) and neutron star (NS) mergers requires comparing\nobservations with population synthesis predictions. In this work we investigate\nthe combined impact from the key uncertainties in population synthesis\nmodelling of the isolated binary evolution channel: the physical processes in\nmassive binary-star evolution and the star formation history as a function of\nmetallicity, $Z$, and redshift $z, \\mathcal{S}(Z,z)$. Considering these\nuncertainties we create 560 different publicly available model realizations and\ncalculate the rate and distribution characteristics of detectable BHBH, BHNS,\nand NSNS mergers. We find that our stellar evolution and $\\mathcal{S}(Z,z)$\nvariations can impact the predicted intrinsic and detectable merger rates by\nfactors $10^2$-$10^4$. We find that BHBH rates are dominantly impacted by\n$\\mathcal{S}(Z,z)$ variations, NSNS rates by stellar evolution variations and\nBHNS rates by both. We then consider the combined impact from all uncertainties\nconsidered in this work on the detectable mass distribution shapes (chirp mass,\nindividual masses and mass ratio). We find that the BHNS mass distributions are\npredominantly impacted by massive binary-star evolution changes. For BHBH and\nNSNS we find that both uncertainties are important. We also find that the shape\nof the delay time and birth metallicity distributions are typically dominated\nby the choice of $\\mathcal{S}(Z,z)$ for BHBH, BHNS and NSNS. We identify\nseveral examples of robust features in the mass distributions predicted by all\n560 models, such that we expect more than 95% of BHBH detections to contain a\nBH $\\gtrsim 8\\,\\rm{M}_{\\odot}$ and have mass ratios $\\lesssim 4$. Our work\ndemonstrates that it is essential to consider a wide range of allowed models to\nstudy double compact object merger rates and properties.", "category": "astro-ph_HE" }, { "text": "Reverse Shock Emission Driven By Post-Merger Millisecond Magnetar Winds:\n Effects of the Magnetization Parameter: The study of short-duration gamma-ray bursts provides growing evidence that a\ngood fraction of double neutron star mergers lead to the formation of stable\nmillisecond magnetars. The launch of Poynting flux by the millisecond magnetars\ncould leave distinct electromagnetic signatures that reveal the energy\ndissipation processes in the magnetar wind. In previous studies (Wang & Dai\n2013b; Wang et al. 2015), we assume that the magnetar wind becomes completely\nlepton-dominated so that electrons/positrons in the magnetar wind are\naccelerated by a diffusive shock. However, theoretical modeling of pulsar wind\nnebulae shows that in many cases the magnetic field energy in the pulsar wind\nmay be strong enough to suppress diffusive shock acceleration. In this paper,\nwe investigate the reverse shock emission as well as the forward shock emission\nwith an arbitrary magnetization parameter $\\sigma$ of a magnetar wind. We find\nthat the reverse shock emission strongly depends on $\\sigma$, and in\nparticular, $\\sigma \\sim 0.3$ leads to the strongest reverse shock emission.\nFuture observations would be helpful to diagnose the composition of the\nmagnetar wind.", "category": "astro-ph_HE" }, { "text": "Connections Between Local and Global Turbulence in Accretion Disks: We analyze a suite of global magnetohydrodynamic (MHD) accretion disk\nsimulations in order to determine whether scaling laws for turbulence driven by\nthe magnetorotational instability, discovered via local shearing box studies,\nare globally robust. The simulations model geometrically-thin disks with zero\nnet magnetic flux and no explicit resistivity or viscosity. We show that the\nlocal Maxwell stress is correlated with the self-generated local vertical\nmagnetic field in a manner that is similar to that found in local simulations.\nMoreover, local patches of vertical field are strong enough to stimulate and\ncontrol the strength of angular momentum transport across much of the disk. We\ndemonstrate the importance of magnetic linkages (through the low-density\ncorona) between different regions of the disk in determining the local field,\nand suggest a new convergence requirement for global simulations -- the\nvertical extent of the corona must be fully captured and resolved. Finally, we\nexamine the temporal convergence of the average stress, and show that an\ninitial long-term secular drift in the local flux-stress relation dies away on\na time scale that is consistent with turbulent mixing of the initial magnetic\nfield.", "category": "astro-ph_HE" }, { "text": "Dynamical Modeling of CXOGBS J175553.2-281633: A 10 Hour Long Orbital\n Period Cataclysmic Variable: We present modeling of the long-term optical light curve and radial velocity\ncurve of the binary stellar system CXOGBS J175553.2-281633, first detected in\nX-rays in the \\textit{Chandra} Galactic Bulge Survey. We analyzed 7 years of\noptical I-band photometry from OGLE and found long-term variations from year to\nyear. These long-term variations can most likely be explained with by either\nvariations in the luminosity of the accretion disk or a spotted secondary star.\nThe phased light curve has a sinusoidal shape, which we interpret as being due\nto ellipsoidal modulations. We improve the orbital period to be $P = 10.34488\n\\pm 0.00006$ h with a time of inferior conjunction of the secondary star $T_0 =\n{\\rm HJD\\ } 2455260.8204 \\pm 0.0008$. Moreover, we collected 37 spectra over 6\nnon-consecutive nights. The spectra show evidence for an evolved K7 secondary\ndonor star, from which we obtain a semi-amplitude for the radial velocity curve\nof $K_2 = 161 \\pm 6 $ km s$^{-1}$. Using the light curve synthesis code {\\tt\nXRbinary}, we derive the most likely orbital inclination for the binary of $i =\n63.0\\pm0.7$ deg, a primary mass of $M_1 = 0.83 \\pm 0.06$ M$_\\odot$, consistent\nwith a white dwarf accretor, and a secondary donor mass of $M_2 = 0.65 \\pm\n0.07$ M$_\\odot$, consistent with the spectral classification. Therefore, we\nidentify the source as a long orbital period cataclysmic variable star.", "category": "astro-ph_HE" }, { "text": "Interesting Clues to Detect Hidden Tidal Disruption Events in Active\n Galactic Nuclei: In the manuscript, effects of Tidal Disruption Events (TDEs) are estimated on\nlong-term AGN variability, to provide interesting clues to detect probable\nhidden TDEs in normal broad line AGN with apparent intrinsic variability which\noverwhelm the TDEs expected variability features, after considering the unique\nTDEs expected variability patterns. Based on theoretical TDEs expected\nvariability plus AGN intrinsic variability randomly simulated by Continuous\nAutoRegressive process, long-term variability properties with and without TDEs\ncontributions are well analyzed in AGN. Then, interesting effects of TDEs can\nbe determined on long-term observed variability of AGN. First, more massive\nBHs, especially masses larger than $10^7{\\rm M_\\odot}$, can lead to more\nsensitive and positive dependence of $\\tau_{TN}$ on $R_{TN}$, with $\\tau_{TN}$\nas variability timescale ratio of light curves with TDEs contributions to\nintrinsic light curves without TDEs contributions, and $R_{TN}$ as ratio of\npeak intensity of TDEs expected variability to the mean intensity of intrinsic\nAGN variability without TDEs contributions. Second, stronger TDEs contributions\n$R_{TN}$ can lead to $\\tau_{TN}$ quite larger than 5. Third, for intrinsic AGN\nvariability having longer variability timescales, TDEs contributions will lead\n$\\tau_{TN}$ to be increased more slowly. The results actually provide an\ninteresting forward-looking method to detect probable hidden TDEs in normal\nbroad line AGN, due to quite different variability properties, especially\ndifferent DRW/CAR process expected variability timescales, in different epochs,\nespecially in normal broad line AGN with shorter intrinsic variability\ntimescales and with BH masses larger than $10^7{\\rm M_\\odot}$.", "category": "astro-ph_HE" }, { "text": "Gamma Ray Burst Prompt correlations: The mechanism responsible for the prompt emission of gamma-ray bursts (GRBs)\nis still a debated issue. The prompt phase-related GRB correlations can allow\nto discriminate among the most plausible theoretical models explaining this\nemission. We present an overview of the observational two-parameter\ncorrelations, their physical interpretations, their use as redshift estimators\nand possibly as cosmological tools. The nowadays challenge is to make GRBs, the\nfarthest stellar-scaled objects observed (up to redshift $z=9.4$), standard\ncandles through well established and robust correlations. However, GRBs\nspanning several orders of magnitude in their energetics are far from being\nstandard candles. We describe the advances in the prompt correlation research\nin the past decades, with particular focus paid to the discoveries in the last\n20 years.", "category": "astro-ph_HE" }, { "text": "Compact Object Modeling in the Globular Cluster 47 Tucanae: The globular cluster 47~Tucanae (47~Tuc) is one of the most massive star\nclusters in the Milky Way and is exceptionally rich in exotic stellar\npopulations. For several decades it has been a favorite target of observers,\nand yet it is computationally very challenging to model because of its large\nnumber of stars ($N\\gtrsim 10^6$) and high density. Here we present detailed\nand self-consistent 47~Tuc models computed with the \\texttt{Cluster Monte\nCarlo} code (\\texttt{CMC}). The models include all relevant dynamical\ninteractions coupled to stellar and binary evolution, and reproduce various\nobservations, including the surface brightness and velocity dispersion\nprofiles, pulsar accelerations, and numbers of compact objects. We show that\nthe present properties of 47~Tuc are best reproduced by adopting an initial\nstellar mass function that is both bottom-heavy and top-light relative to\nstandard assumptions \\citep[as in, e.g.,][]{Kroupa2001}, and an initial Elson\nprofile \\citep{Elson1987} that is overfilling the cluster's tidal radius. We\ninclude new prescriptions in \\texttt{CMC} for the formation of binaries through\ngiant star collisions and tidal captures, and we show that these mechanisms\nplay a crucial role in the formation of neutron star binaries and millisecond\npulsars in 47~Tuc; our best-fit model contains $\\sim 50$ millisecond pulsars,\n$70\\%$ of which are formed through giant collisions and tidal captures. Our\nmodels also suggest that 47~Tuc presently contains up to $\\sim 200$\nstellar-mass black holes, $\\sim 5$ binary black holes, $\\sim 15$ low-mass X-ray\nbinaries, and $\\sim 300$ cataclysmic variables.", "category": "astro-ph_HE" }, { "text": "Neutron stars - thermal emitters: Confronting theoretical models with observations of thermal radiation emitted\nby neutron stars is one of the most important ways to understand the properties\nof both, superdense matter in the interiors of the neutron stars and dense\nmagnetized plasmas in their outer layers. Here we review the theory of thermal\nemission from the surface layers of strongly magnetized neutron stars, and the\nmain properties of the observational data. In particular, we focus on the\nnearby sources for which a clear thermal component has been detected, without\nbeing contaminated by other emission processes (magnetosphere, accretion,\nnebulae). We also discuss the applications of the modern theoretical models of\nthe formation of spectra of strongly magnetized neutron stars to the observed\nthermally emitting objects.", "category": "astro-ph_HE" }, { "text": "Approximating new ice models with B-splines for improved IceCube event\n reconstruction: application to cascades and tracks: Event signatures in IceCube are complex, modulated by both particle physics\nand properties of the ice and detector. Event reconstruction thus requires\naccurate modeling of ice properties and detector effects to fit for physics\nparameters, such as energy and direction. Here, we highlight how improvements\nin calibration can translate into substantially improving the angular\nresolution of electromagnetic showers. Since showers are also used to model\nstochastic energy losses of tracks, we further show how improved ice modeling,\nalong with other track-specific optimizations, leads to more meaningful\ndirectional likelihood spaces for high-energy muons. The median angular\nresolution for showers is improved by a factor of two over an older B-spline\nmodel, and accurate directional contours for tracks can be obtained with Wilks'\ntheorem.", "category": "astro-ph_HE" }, { "text": "Inferring spin tilts at formation from gravitational wave observations\n of binary black holes: Interfacing precession-averaged and orbit-averaged\n spin evolution: Two important parameters inferred from the gravitational wave signals of\nbinaries of precessing black holes are the spin tilt angles, i.e., the angles\nat which the black holes' spin axes are inclined with respect to the binary's\norbital angular momentum. The LIGO-Virgo parameter estimation analyses\ncurrently provide spin tilts at a fiducial reference frequency, often the\nlowest frequency used in the data analysis. However, the most astrophysically\ninteresting quantities are the spin tilts when the binary was formed, which can\nbe significantly different from those at the reference frequency for strongly\nprecessing binaries. The spin tilts at formally infinite separation are a good\napproximation to the tilts at formation in many formation channels and can be\ncomputed efficiently for binary black holes using precession-averaged\nevolution. Here, we present a new code for computing the tilts at infinity that\ncombines the precession-averaged evolution with orbit-averaged evolution at\nhigh frequencies and illustrate its application to GW190521 and other binary\nblack hole detections from O3a. We have empirically determined the transition\nfrequency between the orbit-averaged and precession-averaged evolution to\nproduce tilts at infinity with a given accuracy. We also have regularized the\nprecession-averaged equations in order to obtain good accuracy for the very\nclose-to-equal-mass binary parameters encountered in practice. This\nadditionally allows us to investigate the singular equal-mass limit of the\nprecession-averaged expressions, where we find an approximate scaling of $1/(1\n- q)$ with the mass ratio $q$.", "category": "astro-ph_HE" }, { "text": "Discovery of a 26.2 day period in the long-term X-ray light curve of SXP\n 1323: a very short orbital period for a long spin period pulsar: Context. About 120 Be/X-ray binaries (BeXBs) are known in the Small\nMagellanic Cloud (SMC); about half of them are pulsating with periods from a\nfew to hundreds of seconds. SXP 1323 is one of the longest-period pulsars known\nin this galaxy. Aims. SXP 1323 is in the field of view of a large set of\ncalibration observations that we analyse systematically, focusing on the time\nanalysis, in search of periodic signals. Methods. We analyse all available\nX-ray observations of SXP 1323 from Suzaku, XMM-Newton, and Chandra, in the\ntime range from 1999 to the end of 2016. We perform a Lomb-Scargle periodogram\nsearch in the band 2.5-10 keV on all observations to detect the neutron star\nspin period and constrain its long-term evolution. We also perform an orbital\nperiod search on the long-term light curve, merging all datasets. Results. We\nreport the discovery of a 26.188+-0.045 d period analysing data from Suzaku,\nXMM-Newton, and Chandra, which confirms the optical period derived from the\nOptical Gravitational Lensing Experiment (OGLE) data. If this corresponds to\nthe orbital period, this would be very short with respect to what is expected\nfrom the spin/orbital period relationship. We furthermore report on the spin\nperiod evolution in the last years. The source is spinning-up with an average\nrate of Pdot/P of 0.018 yr-1, decreasing from 1340 to 1100 s, in the period\nfrom 2006 to the end of 2016, which is also extreme with respect to the other\nBe/X-ray pulsars. From 2010 to the end of 2014, the pulse period is not clearly\ndetectable, although the source was still bright. Conclusions. SXP 1323 is a\npeculiar BeXB due to its long pulse period, rapid spin-up for several years,\nand short orbital period. A continuous monitoring of the source in the next\nyears is necessary to establish the long-term behaviour of the spin period.", "category": "astro-ph_HE" }, { "text": "Magnetic Deflections of Ultra-High Energy Cosmic Rays from Centaurus A: We present the results of a study that simulates trajectories of ultra-high\nenergy cosmic rays from Centaurus A to Earth, for particle rigidities from $E/Z\n= 2$ EV to 100 EV, i.e., covering the possibility of primary particles as heavy\nas Fe nuclei with energies exceeding 50 EeV. The Galactic magnetic field is\nmodeled using the recent work of Jansson and Farrar (JF12) which fitted its\nparameters to match extragalactic Faraday rotation measures and WMAP7\nsynchrotron emission maps. We include the random component of the GMF using the\nJF12 3D model for $B_{\\rm rand}(\\vec{r})$ and explore the impact of different\nrandom realizations, coherence length and other features on cosmic ray\ndeflections. Gross aspects of the arrival direction distribution such as mean\ndeflection and the RMS dispersion depend mainly on rigidity and differ\nrelatively little from one realization to another. However different\nrealizations exhibit non-trivial substructure whose specific features vary\nconsiderably from one realization to another, especially for lower rigidities.\nAt the lowest rigidity of 2 EV, the distribution is broad enough that it might\nbe compatible with a scenario in which Cen A is the principle source of all\nUHECRs. No attempt is made here to formulate a robust test of this possibility,\nalthough some challenges to such a scenario are noted.", "category": "astro-ph_HE" }, { "text": "The pulsar synchrotron: coherent radio emission: We propose a simple physical picture for the generation of coherent radio\nemission in the axisymmetric pulsar magnetosphere that is quite different from\nthe canonical paradigm of radio emission coming from the magnetic polar caps.\nIn this first paper we consider only the axisymmetric case of an aligned\nrotator. Our picture capitalizes on an important element of the MHD\nrepresentation of the magnetosphere, namely the separatrix between the\ncorotating closed-line region (the `dead zone') and the open field lines that\noriginate in the polar caps. Along the separatrix flows the return current that\ncorresponds to the main magnetospheric electric current emanating from the\npolar caps. Across the separatrix, both the toroidal and poloidal components of\nthe magnetic field change discontinuously. The poloidal component discontinuity\nrequires the presence of a significant annular electric current which has up to\nnow been unaccounted for. We estimate the position and thickness of this\nannular current at the tip of the closed line region, and show that it consists\nof electrons (positrons) corotating with Lorentz factors on the order of 10^5,\nemitting incoherent synchrotron radiation that peaks in the hard X-rays. These\nparticles stay in the region of highest annular current close to the equator\nfor a path-length of the order of one meter. We propose that, at wavelengths\ncomparable to that path-length, the particles emit coherent radiation, with\nradiated power proportional to N^2, where N is the population of particles in\nthe above path-length. We calculate the total radio power in this wavelength\nregime and its scaling with pulsar period and stellar magnetic field and show\nthat it is consistent with estimates of radio luminosity based on observations.", "category": "astro-ph_HE" }, { "text": "GRB 090510: a short burst from a massive star ?: GRB afterglow 090510 is (so far) the best-monitored afterglow in the optical,\nX-ray, and above 100 MeV, measurements covering 2-3 decades in time at each\nfrequency. Owing to its power-law temporal decay and power-law spectrum, it\nseems very likely that the highest energy emission is from the forward-shock\nenergizing the ambient medium (the standard blast-wave model for GRB\nafterglows), the GeV flux and its decay rate being consistent with that model's\nexpectations. However, the synchrotron emission from a collimated outflow (the\nstandard jet model) has difficulties in accounting for the lower-energy\nafterglow emission, where a simultaneous break occurs at 2 ks in the optical\nand X-ray light-curves, but with the optical flux decay (before and after the\nbreak) being much slower than in the X-rays (at same time). The measured X-ray\nand GeV fluxes are incompatible with the higher-energy afterglow emission being\nfrom same spectral component as the lower-energy afterglow emission, which\nsuggests a synchrotron self-Compton model for this afterglow. Cessation of\nenergy injection in the blast-wave and an ambient medium with a wind-like n ~\nr^{-2} density can explain all features of the optical and X-ray light-curves\nof GRB afterglow 090510. Such an ambient medium radial structure is\nincompatible with this short-GRB originating from the merger of two compact\nstars.", "category": "astro-ph_HE" }, { "text": "A joint spectro-imaging analysis of the XMM-Newton and HESS observations\n of the supernova remnant RX J1713.7-3946: The supernova remnant (SNR) RX J1713.7-3946 (also known as G347.3-0.5) is\npart of the class of remnants dominated by synchrotron emission in X-rays. It\nis also one of the few shell-type SNRs observed at TeV energies allowing to\ninvestigate particle acceleration at SNRs shock.\n Our goal is to compare spatial and spectral properties of the remnant in X-\nand gamma-rays to understand the nature of the TeV emission. This requires to\nstudy the remnant at the same spatial scale at both energies. To complement the\nnon-thermal spectrum of the remnant, we attempt to provide a reliable estimate\nfor the radio flux density. In radio, we revisited ATCA data and used HI and\nmid-infrared observations to disentangle the thermal from the non-thermal\nemission. In X-rays, we produced a new mosaic of the remnant and degraded the\nspatial resolution of the X-ray data to the resolution of the HESS instrument\nto perform spatially resolved spectroscopy at the same spatial scale in X- and\ngamma-rays. Radial profiles were obtained to investigate the extension of the\nemission at both energies.\n We found that part of the radio emission within the SNR contours is thermal\nin nature. Taking this into account, we provide new lower and upper limits for\nthe integrated synchrotron flux of the remnant at 1.4 GHz of 22 Jy and 26 Jy\nrespectively. In X-rays, we obtained the first full coverage of RX J1713.7-3946\nwith XMM-Newton. The spatial variation of the photon index seen at small scale\nin X-rays is smeared out at HESS resolution. A non-linear correlation between\nthe X- and gamma-ray fluxes of the type Fx \\propto Fg^2.41 is found.", "category": "astro-ph_HE" }, { "text": "Spectral and morphological analysis of the remnant of Supernova 1987A\n with ALMA & ATCA: We present a comprehensive spectral and morphological analysis of the remnant\nof Supernova (SN) 1987A with the Australia Telescope Compact Array (ATCA) and\nthe Atacama Large Millimeter/submillimeter Array (ALMA). The non-thermal and\nthermal components of the radio emission are investigated in images from 94 to\n672 GHz ($\\lambda$ 3.2 mm to 450 $\\mu$m), with the assistance of a\nhigh-resolution 44 GHz synchrotron template from the ATCA, and a dust template\nfrom ALMA observations at 672 GHz. An analysis of the emission distribution\nover the equatorial ring in images from 44 to 345 GHz highlights a gradual\ndecrease of the east-to-west asymmetry ratio with frequency. We attribute this\nto the shorter synchrotron lifetime at high frequencies. Across the transition\nfrom radio to far infrared, both the synchrotron/dust-subtracted images and the\nspectral energy distribution (SED) suggest additional emission beside the main\nsynchrotron component ($S_{\\nu}\\propto\\nu^{-0.73}$) and the thermal component\noriginating from dust grains at $T\\sim22$ K. This excess could be due to\nfree-free flux or emission from grains of colder dust. However, a second\nflat-spectrum synchrotron component appears to better fit the SED, implying\nthat the emission could be attributed to a pulsar wind nebula (PWN). The\nresidual emission is mainly localised west of the SN site, as the spectral\nanalysis yields $-0.4\\lesssim\\alpha\\lesssim-0.1$ across the western regions,\nwith $\\alpha\\sim0$ around the central region. If there is a PWN in the remnant\ninterior, these data suggest that the pulsar may be offset westward from the SN\nposition.", "category": "astro-ph_HE" }, { "text": "Gravitational wave astronomy of single sources with a pulsar timing\n array: Abbreviated:\n We investigate the potential of detecting the gravitational wave from\nindividual binary black hole systems using pulsar timing arrays (PTAs) and\ncalculate the accuracy for determining the GW properties. This is done in a\nconsistent analysis, which at the same time accounts for the measurement of the\npulsar distances via the timing parallax.\n We find that, at low redshift, a PTA is able to detect the nano-Hertz GW from\nsuper massive black hole binary systems with masses of $\\sim10^8 -\n10^{10}\\,M_{\\sun}$ less than $\\sim10^5$\\,years before the final merger, and\nthose with less than $\\sim10^3 - 10^4$ years before merger may allow us to\ndetect the evolution of binaries.\n We derive an analytical expression to describe the accuracy of a pulsar\ndistance measurement via timing parallax. We consider five years of bi-weekly\nobservations at a precision of 15\\,ns for close-by ($\\sim 0.5 - 1$\\,kpc)\npulsars. Timing twenty pulsars would allow us to detect a GW source with an\namplitude larger than $5\\times 10^{-17}$. We calculate the corresponding GW and\nbinary orbital parameters and their measurement precision. The accuracy of\nmeasuring the binary orbital inclination angle, the sky position, and the GW\nfrequency are calculated as functions of the GW amplitude. We note that the\n\"pulsar term\", which is commonly regarded as noise, is essential for obtaining\nan accurate measurement for the GW source location.\n We also show that utilizing the information encoded in the GW signal passing\nthe Earth also increases the accuracy of pulsar distance measurements. If the\ngravitational wave is strong enough, one can achieve sub-parsec distance\nmeasurements for nearby pulsars with distance less than $\\sim 0.5 - 1$\\,kpc.", "category": "astro-ph_HE" }, { "text": "Optical and Radio Variability of the Blazar S4 0954+658: We present an optical-to-radio study of the BL Lac object S4 0954+658\nobservations during 1998-2023. The measurements were obtained with the SAO RAS\nZeiss-1000 1-m and AS-500/2 0.5-m telescopes in 2003-2023, with the RATAN-600\nradio telescope at 1.25 (0.96, 1.1), 2.3, 4.7 (3.7, 3.9), 8.2 (7.7), 11.2, 22.3\n(21.7) GHz in 1998-2023, with the IAA RAS RT-32 Zelenchukskaya and Badary\ntelescopes at 5.05 and 8.63 GHz in 2020--2023, and with the RT-22 single-dish\ntelescope of CrAO RAS at 36.8 GHz in 2009-2023. In this period the blazar had\nbeen showing extremely high broadband activity with the variability amplitude\nof flux densities up to 70-100% both in the optical and radio domains. In the\nperiod of 2014-2023 the blazar had been showing the historically highest\nactivity in the radio wavelengths, and we detected multiple radio flares of\nvarying amplitude and duration. The large flares last on average from 0.3 to 1\nyear at 22-36.8 GHz and slightly longer at 5-11.2 GHz. The optical flares are\nshorter and last 7-50 days. In the most active epoch of 2018-2023 the\ncharacteristic time scale $\\tau$ of variation at 5-22 GHz is about 100 days and\nabout 1000 days for the state with lower activity in 2009-2014. We found a\ngeneral correlation between the optical, radio, and $\\gamma$-ray flux\nvariations, which suggests that we observe the same photon population from\ndifferent emission regions. We estimated linear size of this region as 0.5-2 pc\nfor different epochs. A broadband two components radio spectrum of S4 0954+658\njet was modelled by using both electrons and protons as emitting particles. It\nis shown that the synchrotron radio waves in this AGN may be generated by\nrelativistic protons.", "category": "astro-ph_HE" }, { "text": "Discovery of the Optical Counterparts to Four Energetic Fermi\n Millisecond Pulsars: In the last few years, over 43 millisecond radio pulsars have been discovered\nby targeted searches of unidentified gamma-ray sources found by the Fermi\nGamma-Ray Space Telescope. A large fraction of these millisecond pulsars are in\ncompact binaries with low-mass companions. These systems often show eclipses of\nthe pulsar signal and are commonly known as black widows and redbacks because\nthe pulsar is gradually destroying its companion. In this paper, we report on\nthe optical discovery of four strongly irradiated millisecond pulsar\ncompanions. All four sources show modulations of their color and luminosity at\nthe known orbital periods from radio timing. Light curve modelling of our\nexploratory data shows that the equilibrium temperature reached on the\ncompanion's dayside with respect to their nightside is consistent with about\n10-30% of the available spin-down energy from the pulsar being reprocessed to\nincrease the companion's dayside temperature. This value compares well with the\nrange observed in other irradiated pulsar binaries and offers insights about\nthe energetics of the pulsar wind and the production of gamma-ray emission. In\naddition, this provides a simple way of estimating the brightness of irradiated\npulsar companions given the pulsar spin-down luminosity. Our analysis also\nsuggests that two of the four new irradiated pulsar companions are only\npartially filling their Roche lobe. Some of these sources are relatively bright\nand represent good targets for spectroscopic follow-up. These measurements\ncould enable, among other things, mass determination of the neutron stars in\nthese systems.", "category": "astro-ph_HE" }, { "text": "Search for Gravitational Waves Associated with Fast Radio Bursts\n Detected by CHIME/FRB During the LIGO--Virgo Observing Run O3a: We search for gravitational-wave transients associated with fast radio bursts\n(FRBs) detected by the Canadian Hydrogen Intensity Mapping Experiment Fast\nRadio Burst Project (CHIME/FRB), during the first part of the third observing\nrun of Advanced LIGO and Advanced Virgo (1 April 2019 15:00 UTC-1 Oct 2019\n15:00 UTC). Triggers from 22 FRBs were analyzed with a search that targets\ncompact binary coalescences with at least one neutron star component. A\ntargeted search for generic gravitational-wave transients was conducted on 40\nFRBs. We find no significant evidence for a gravitational-wave association in\neither search. Given the large uncertainties in the distances of the FRBs\ninferred from the dispersion measures in our sample, however, this does not\nconclusively exclude any progenitor models that include emission of a\ngravitational wave of the types searched for from any of these FRB events. We\nreport $90\\%$ confidence lower bounds on the distance to each FRB for a range\nof gravitational-wave progenitor models. By combining the inferred maximum\ndistance information for each FRB with the sensitivity of the\ngravitational-wave searches, we set upper limits on the energy emitted through\ngravitational waves for a range of emission scenarios. We find values of order\n$10^{51}$-$10^{57}$ erg for a range of different emission models with central\ngravitational wave frequencies in the range 70-3560 Hz. Finally, we also found\nno significant coincident detection of gravitational waves with the repeater,\nFRB 20200120E, which is the closest known extragalactic FRB.", "category": "astro-ph_HE" }, { "text": "The nova-like nebular optical spectrum of V404 Cygni at the beginning of\n the 2015 outburst decay: We report on FORS2 optical spectroscopy of the black hole X-ray binary V404\nCygni, performed at the very beginning of its 2015 outburst decay, complemented\nby quasi-simultaneous $Swift$ X-ray and ultra-violet as well as REM\nnear-infrared observations. Its peculiar spectrum is dominated by a wealth of\nemission signatures of HI, HeI, and higher ionisation species, in particular\nFeII. The spectral features are divided between broad red-shifted and narrow\nstationary varieties, the latter being emitted in the outer regions. Continuum\nand line variability at short time scale is high and we find Baldwin\neffect-like anti-correlations between the full-widths at half-maximum and\nequivalent widths of the broad lines with their local continua. The Balmer\ndecrement H{\\alpha}/H{\\beta} is also abnormally large at $4.61\\pm0.62$. We\nargue that these properties hint at the broad lines being optically thick and\narising within a circumbinary component in which shocks between faster\noptically thick and slower optically thin regions may occur. We associate it to\na nova-like nebula formed by the cooling remnant of strong accretion disc winds\nthat turned off when the mass-accretion rate dropped following the last major\nflare. The FeII lines likely arise from the overlap region between this nebula\nand the companion star winds, whereas we favour the shocks within the nebula as\nresponsible for the optical continuum via self-absorbed optically thin\nbremsstrahlung. The presence of a near-infrared excess also points towards the\ncontribution of a strongly variable compact jet or a dusty component.", "category": "astro-ph_HE" }, { "text": "Time-dependent neutrino emission from Mrk 421 during flares and\n predictions for IceCube: Blazars are prime candidate sources for the high energy neutrinos recently\ndetected by IceCube. Being one of the brightest sources in the extragalactic\nX-ray and $\\gamma$-ray sky as well as one of the nearest blazars to Earth, Mrk\n421 is an excellent source for testing the scenario of the blazar-neutrino\nconnection. Here, we model the spectral energy distribution of Mrk 421 during a\n13-day flare in 2010 with unprecedented multi-wavelength coverage, and\ncalculate the respective neutrino flux. We find a correlation between the $>1$\nPeV neutrino and photon fluxes, in all energy bands. Using typical IceCube\nthrough-going muon event samples with good angular resolution and high\nstatistics, we derive the mean event rate above 100 TeV ($\\sim0.57$ evt/yr) and\nshow that it is comparable to that expected from a four-month quiescent period\nin 2009. Due to the short duration of the flare, an accumulation of similar\nflares over several years would be necessary to produce a meaningful signal for\nIceCube. To better assess this, we apply the correlation between the neutrino\nand $\\gamma$-ray fluxes to the 6.9 yr Fermi-LAT light curve of Mrk 421. We find\nthat the mean event count above 1 PeV for the full IceCube detector livetime is\n$3.59\\pm0.60$ ($2.73\\pm0.38$) $\\nu_\\mu+\\bar{\\nu}_\\mu$ with (without) major\nflares included in our analysis. This estimate exceeds, within the\nuncertainties, the $95\\%$ ($90\\%$) threshold value for the detection of one or\nmore muon (anti-)neutrinos. Meanwhile, the most conservative scenario, where no\ncorrelation of $\\gamma$-rays and neutrinos is assumed, predicts $1.60\\pm0.16$\n$\\nu_\\mu+\\bar{\\nu}_\\mu$ events. We conclude that a non-detection of high-energy\nneutrinos by IceCube would probe the neutrino/$\\gamma$-ray flux correlation\nduring major flares or/and the hadronic contribution to the blazar emission.", "category": "astro-ph_HE" }, { "text": "QPO-jet relation in X-ray binaries: In the past years, a clear picture of the evolution of outbursts of\nblack-hole X-ray binaries has emerged. While the X-ray properties can be\nclassified into our distinct states, based on spectral and timing properties,\nthe observations in the radio band have shown strong links between accretion\nand ejection properties. Here I briefly outline the association between X-ray\ntiming and jet properties.", "category": "astro-ph_HE" }, { "text": "Detection of diffuse gamma-ray emission near the young massive cluster\n NGC 3603: We report the Fermi Large Area Telescope's detection of extended gamma-ray\nemission towards the direction of the young massive star cluster NGC 3603. The\nemission shows a hard spectrum with a photon index of 2.3 from 1 GeV to 250\nGeV. The large size and high luminosity of this structure make it unlikely a\npulsar wind nebular. On the other hand the spatial correlation with the ionised\ngas indicate a hadronic origin. The total cosmic ray (CR) protons energy are\nestimated to be of the order $10^{50} ~\\rm erg$ assuming the gamma-ray are\nproduced in the interaction of CRs with ambient gas . The environment and\nspectral features show significant similarity with the Cygnus cocoon. It\nreveals that the young star clusters may be a gamma-ray source population and\nthey can potentially accelerate a significant fraction of the Galactic cosmic\nrays.", "category": "astro-ph_HE" }, { "text": "Insight-HXMT, NuSTAR and INTEGRAL Data Show Disk Truncation in the Hard\n State of the Black-Hole X-Ray Binary MAXI J1820+070: We study X-ray and soft gamma-ray spectra from the hard state of the\naccreting black-hole binary MAXI J1820+070. We perform analysis of joint\nspectra from HXMT, NuSTAR and INTEGRAL. We find an overall agreement between\nthe spectra from all three satellites. Satisfactory fits to the data require\nsubstantial spectral complexity, with our models including two Comptonization\nregions and their associated disk reflection, a disk blackbody and a narrow Fe\nK$\\alpha$ line. Our fits confirm the presence of the truncation of the\nreflecting optically-thick disk at least at $>$10 gravitational radii. However,\nwe find that the HXMT data alone cannot significantly constrain the disk inner\nradii.", "category": "astro-ph_HE" }, { "text": "Constraining cosmic-ray acceleration in the magnetospheric gaps of Sgr\n A*: Sagittarius A* (Sgr A*) is a potential VHE gamma-ray and cosmic-ray source.\nWe examine limits to gap-type particle acceleration in the magnetosphere of Sgr\nA*, showing that in the current phase of activity proton acceleration to PeV\nenergies is possible, with injection powers into the environment usually\nlimited to several 10^{36} erg/s. Compton upscattering of ambient soft photons\nby gap-accelerated electrons could yield TeV emission compatible with the\ndetected VHE points source. We explore the dependency of the results on changes\nin the accretion rate showing that higher stages in the past are unlikely to\nincrease the power output unless the inner accretion flows itself changed its\nconfiguration.", "category": "astro-ph_HE" }, { "text": "Is the energy generation rate of nuclear reactions in hot accretion\n flows important?: The temperature of hot accretion flows around black holes is sufficiently\nhigh for the ignition of nuclear reactions. This is potentially an important\nnucleosynthesis mechanism in the universe. As the first step in studying this\nproblem, we need to measure physical quantities such as density and temperature\nof the accretion flow. In usual studies of the hot accretion flow, viscous\ndissipation is considered to be the only heating mechanism, while the heating\ncaused by nuclear reactions is not considered. In this paper, we investigate\nwhether the energy generation rate of nuclear reaction is important compared to\nthe viscous heating. Our calculation indicates that the former is at most one\npercent of the latter and thus is not important. The dynamics of accretion flow\ncan be therefore calculated in the usual way, without the need to consider\nheating due to nuclear reactions.", "category": "astro-ph_HE" }, { "text": "High-energy electromagnetic offline follow-up of LIGO-Virgo\n gravitational-wave binary coalescence candidate events: We present two different search methods for electromagnetic counterparts to\ngravitational-wave (GW) events from ground-based detectors using archival NASA\nhigh-energy data from the Fermi-GBM and RXTE-ASM instruments. To demonstrate\nthe methods, we use a limited number of representative GW background noise\nevents produced by a search for binary neutron star coalescence over the last\ntwo months of the LIGO-Virgo S6/VSR3 joint science run. Time and sky location\nprovided by the GW data trigger a targeted search in the high-energy photon\ndata. We use two custom pipelines: one to search for prompt gamma-ray\ncounterparts in GBM, and the other to search for a variety of X-ray afterglow\nmodel signals in ASM. We measure the efficiency of the joint pipelines to weak\ngamma-ray burst counterparts, and a family of model X-ray afterglows. By\nrequiring a detectable signal in either electromagnetic instrument coincident\nwith a GW event, we are able to reject a large majority of GW candidates. This\nreduces the signal-to-noise of the loudest surviving GW background event by\naround 15-20%.", "category": "astro-ph_HE" }, { "text": "The power-law component of the X-ray emissions from pulsar wind nebulae\n and their pulsars: To look for possible phenomenological connections between pulsar's timing\nproperties and emissions from pulsar wind nebulae and their pulsars, we studied\nthe power-law component of the X-ray emissions from 35 pulsar wind nebulae\nwhich have a detected pulsar in X-rays. Our major results are in the following:\n(1) The power-law component of the X-ray luminosities, in the energy range from\n0.5 keV to 8 keV, of the nebulae and of the pulsar both show a strong\ncorrelation with the pulsar spin-down power ($\\dot{E}$), consistent with\nearlier studies. However, equally significant correlations with the magnetic\nfield strength at the light cylinder ($B_{\\rm lc}$) are also found. The similar\nsignificance level of the correlations with $\\dot{E}$ and with $B_{\\rm lc}$\nsuggests that not only $\\dot{E}$ but also $B_{\\rm lc}$ plays an important role\nin understanding these power-law emissions. (2) Thermal X-ray emissions are\ndetected in 12 pulsars among the 35 samples. With derived temperature as one\nadditional variable, we found that the photon indices of pulsar's non-thermal\nX-ray power-law spectra can be well described by a linear function of $\\log P$,\n$\\log\\dot{P}$ and temperature logarithm $\\log T$. It indicates that the surface\ntemperature of neutron stars plays an important role in determining the energy\ndistribution of the radiating pair plasma in pulsar's magnetospheres.", "category": "astro-ph_HE" }, { "text": "Evidence for a change in the radiation mechanism in the hard state of\n GRO J1655-40. Hysteresis in the broad-band noise components: We have analysed RXTE archival data to study the aperiodic variability of the\nblack-hole binary GRO J1655-40 during the hard state of the 2005 outburst. We\ncomputed the 0.008-64 Hz power spectral density during the rise and decay of\nthe 2005 outburst, and we found that they were reasonably well modelled by the\nsum of two, broad Lorenztian functions in most cases (plus a narrow QPO) which\ncorrespond to three different variability components. Our aim is to study the\nevolution of the timing properties of the source during the outburst, by\nstudying the correlation between the characteristics of the broad-band noise\ncomponents in the power spectra and the source luminosity. Our results suggest\nthat the whole power spectrum shifts to high (low) frequencies as the source\nluminosity increases (decreases), in agreement with previous studies of other\nblack hole binaries. However, we also detect a strong \"hysteresis\" pattern in\nthe \"frequency-luminosity\" plots, and show that the \"critical\" luminosity\nlimit, above which the timing properties of the source change, is different\nduring the rise and the decay phase of the outburst. We discuss the general\nimplications of these results in the context of the truncated disc model.", "category": "astro-ph_HE" }, { "text": "Constraints on the X-ray Luminosity Function of AGN at z=5.7-6.4 with\n the Extragalactic Serendipitous Swift Survey: X-ray luminosity functions (XLFs) of Active Galactic Nuclei (AGN) trace the\ngrowth and evolution of supermassive black hole populations across cosmic time,\nhowever, current XLF models are poorly constrained at redshifts of z>6, with a\nlack of spectroscopic constraints at these high redshifts. In this work we\n\\redit{place limits} on the bright-end of the XLF at z=5.7-6.4 using\nhigh-redshift AGN identified within the Extragalactic Serendipitous Swift\nSurvey (ExSeSS) catalogue. Within ExSeSS we find one serendipitously X-ray\ndetected z>6 AGN, ATLAS J025.6821-33.4627, with an X-ray luminosity of\n$L_\\mathrm{X}=8.47^{+3.40}_{-3.13}\\times10^{44}\\mathrm{erg.s^{-1}}$ and\n$z=6.31\\pm0.03$ making it the highest redshift, spectroscopically confirmed,\nserendipitously X-ray detected quasar known to date. We also calculate an upper\nlimit on the space density at higher luminosities where no additional sources\nare found, enabling us to place constraints on the shape of the XLF. Our\nresults are consistent with the rapid decline in the space densities of\nhigh-luminosity AGN toward high redshift as predicted by extrapolations of\nexisting parametric models of the XLF. We also find that our X-ray based\nmeasurements are consistent with estimates of the bolometric quasar luminosity\nfunction based on UV measurements at $z\\gtrsim6$, although they require a large\nX-ray to bolometric correction factor at these high luminosities.", "category": "astro-ph_HE" }, { "text": "A Luminous Gamma-ray Binary in the Large Magellanic Cloud: Gamma-ray binaries consist of a neutron star or a black hole interacting with\na normal star to produce gamma-ray emission that dominates the radiative output\nof the system. Only a handful of such systems have been previously discovered,\nall within our Galaxy. Here we report the discovery with the Fermi Large Area\nTelescope (LAT) of a luminous gamma-ray binary in the Large Magellanic Cloud\nfrom a search for periodic modulation in all sources in the third Fermi LAT\ncatalog. This is the first such system to be found outside the Milky Way. The\nsystem has an orbital period of 10.3 days and is associated with a massive\nO5III star located in the supernova remnant DEM L241, previously identified as\nthe candidate high-mass X-ray binary (HMXB) CXOU J053600.0-673507. X-ray and\nradio emission are also modulated on the 10.3 day period, but are in anti-phase\nwith the gamma-ray modulation. Optical radial velocity measurements suggest\nthat the system contains a neutron star. The source is significantly more\nluminous than similar sources in the Milky Way at radio, optical, X-ray and\ngamma-ray wavelengths. The detection of this extra-galactic system, but no new\nGalactic systems raises the possibility that the predicted number of gamma-ray\nbinaries in our Galaxy has been overestimated, and that HMXBs may be born\ncontaining relatively slowly rotating neutron stars.", "category": "astro-ph_HE" }, { "text": "Search for transitions between states in redbacks and black widows using\n seven years of Fermi-LAT observations: Considering about seven years of Fermi-Large Area Telescope (LAT) data, we\npresent a systematic search for variability possibly related to transitions\nbetween states in redbacks and black widow systems. Transitions are\ncharacterized by sudden and significant changes in the gamma-ray flux that\npersist on a timescale much larger than the orbital period. This phenomenology\nwas already detected in the case of two redback systems, PSR J1023+0038 and PSR\nJ1227-4853, for which we present here a dedicated study. We show the existence\nof only one transition for each of these systems over the past seven years. We\ndetermine their spectra, establishing high-energy cutoffs at a few GeV for the\nhigh gamma-ray state of PSR J1023+0038 and for both states of PSR J1227-4853.\nThe surveying capability of the Fermi-LAT allows studying whether similar\nphenomenology has occurred in other sources. Although we have not found any\nhint for a state transition for most of the studied pulsars, we note two\nblack-widow systems, PSR J2234+0944 and PSR J1446-4701, whose apparent\nvariability is reminiscent of the transitions in PSR J1023+0038 and PSR\nJ1227-4853. For the other systems we set limits on potential transitions in\ntheir measured gamma-ray light curves.", "category": "astro-ph_HE" }, { "text": "Bhjet: a public multi-zone, steady state jet + thermal corona spectral\n model: Accreting black holes are sources of major interest in astronomy, particular\nthose launching jets because of their ability to accelerate particles, and\ndramatically affect their surrounding environment up to very large distances.\nThe spatial, energy and time scales at which a central active black hole\nradiates and impacts its environment depend on its mass. The implied\nscale-invariance of accretion/ejection physics between black hole systems of\ndifferent central masses has been confirmed by several studies. Therefore,\ndesigning a self-consistent theoretical model that can describe such systems,\nregardless of their mass, is of crucial importance to tackle a variety of\nastrophysical sources. We present here a new and significantly improved version\nof a scale invariant, steady-state, multi-zone jet model, which we rename\nbhjet, resulting from the efforts of our group to advance the modelling of\nblack hole systems. We summarise the model assumptions and basic equations, how\nthey have evolved over time, and the additional features that we have recently\nintroduced. These include additional input electron populations, the extension\nto cyclotron emission in near-relativistic regime, an improved multiple inverse\nCompton scattering method, external photon seeds typical of AGN and a\nmagnetically-dominated jet dynamical model as opposed to the pressure-driven\njet configuration present in older versions. In this paper, we publicly release\nthe code on Github and, in order to facilitate the user's approach to its many\npossibilities, showcase a few applications as a tutorial.", "category": "astro-ph_HE" }, { "text": "The Long-term Monitoring Results of Insight-HXMT in the First 4 Yr\n Galactic Plane Scanning Survey: The first X-ray source catalog of Insight-HXMT Galactic Plane (|b|<10deg)\nScanning Survey (GPSS) is presented based on the data accumulated from June\n2017 to August 2021. The 4 yr limit sensitivities at main energy bands can\nreach 8.2x10^(-12) erg/s/cm^2} (2-6 keV), 4.21x10^(-11) erg/s/cm^2 (7-40 keV)\nand 2.78x10^(-11) erg/s/cm^2 (25-100 keV). More than 1300 sources have been\nmonitored at a wide band (1$-$100\\,keV), of which 223 sources have a\nsignal-to-noise ratio greater than 5. We combined the GPSS data of Insight-HXMT\nand MAXI and found it is feasible to obtain more complete long-term light\ncurves from their scanning results. The flux variabilities at different energy\nbands of the 223 bright sources are analyzed based on the excess variances. It\nis found that the fluxes of X-ray binaries are more active than those of\nsupernova remnants and isolated pulsars. Different types of binaries, e.g.,\nlow-mass X-ray binaries (LMXBs), high-mass X-ray binaries (HMXBs), neutron star\nbinaries, and black hole binaries, also distinctively show different\nregularities. In addition, the relations between the hardness ratio (HR) and\nexcess variances, and HR and source types are analyzed. It is obvious that the\nHRs of HMXBs tend to be harder than those of LMXBs and HMXBs tend to be more\nactive than those of LMXBs.", "category": "astro-ph_HE" }, { "text": "Enhancing gravitational wave astronomy with galaxy catalogues: Joint gravitational wave (GW) and electromagnetic (EM) observations, as a key\nresearch direction in multi-messenger astronomy, will provide deep insight into\nthe astrophysics of a vast range of astronomical phenomena. Uncertainties in\nthe source sky location estimate from gravitational wave observations mean\nfollow-up observatories must scan large portions of the sky for a potential\ncompanion signal. A general frame of joint GW-EM observations is presented by a\nmulti-messenger observational triangle. Using a Bayesian approach to\nmulti-messenger astronomy, we investigate the use of galaxy catalogue and host\ngalaxy information to reduce the sky region over which follow-up observatories\nmust scan, as well as study its use for improving the inclination angle\nestimates for coalescing binary compact objects. We demonstrate our method\nusing a simulated neutron stars inspiral signal injected into simulated\nAdvanced detectors noise and estimate the injected signal sky location and\ninclination angle using the Gravitational Wave Galaxy Catalogue. In this case\nstudy, the top three candidates in rank have $72\\%$, $15\\%$ and $8\\%$ posterior\nprobability of being the host galaxy, receptively. The standard deviation of\ncosine inclination angle (0.001) of the neutron stars binary using\ngravitational wave-galaxy information is much smaller than that (0.02) using\nonly gravitational wave posterior samples.", "category": "astro-ph_HE" }, { "text": "Massive White Dwarfs in the Galactic Center: A Chandra X-ray\n Spectroscopy of Cataclysmic Variables: Previous X-ray observations toward the Nuclear Star Cluster (NSC) at the\nGalactic center have discovered thousands of point sources, most of which were\nbelieved to be cataclysmic variables (CVs), i.e., a white dwarf (WD) accreting\nfrom a low-mass companion. However, the population properties of these CVs\nremain unclear, which otherwise contain important information about the\nevolutionary history of the NSC. In this work we utilize ultradeep archival\n\\textit{Chandra} observations to study the spectral properties of the NSC CVs,\nin close comparison with those in the Solar vicinity. We find that the NSC CVs\nhave strong Fe XXV and Fe XXVI lines (both of which show equivalent widths\n$\\sim200-300$ eV), indicating metal-rich companions. Moreover, their Fe XXVI to\nFe XXV line flux ratio is used to diagnose the characteristic white dwarf mass\n($M_{\\rm WD}$) of NSC CVs. The results show that the CVs with $L_{\\rm 2-10\nkeV}>6\\times10^{31}$ erg s$^{-1}$ have a mean $M_{\\rm WD}$ of\n$\\sim0.6/1.0\\,M_{\\odot}$ if they are magnetic/non-magnetic CVs; while those\nwith $L_{\\rm 2-10 keV}$ between $1-6\\times10^{31}$ erg s$^{-1}$ have a mean\n$M_{\\rm WD}$ of $\\sim0.8/1.2\\,M_{\\odot}$ if they are magnetic/non-magnetic CVs.\nAll these \\textit{Chandra}-detected CVs collectively contribute $\\sim$30-50\\%\nof the unresolved 20-40 keV X-ray emission from the NSC. The CV population with\nmassive (i.e., $M_{\\rm WD}\\sim1.2M_{\\odot}$) WDs have not been observed in the\nSolar vicinity or the Galactic bulge, and they might have been formed via\ndynamical encounters in the NSC.", "category": "astro-ph_HE" }, { "text": "Nitrogen K-shell photoabsorption: Reliable atomic data have been computed for the spectral modeling of the\nnitrogen K lines, which may lead to useful astrophysical diagnostics. Data sets\ncomprise valence and K-vacancy level energies, wavelengths, Einstein\n$A$-coefficients, radiative and Auger widths and K-edge photoionization cross\nsections. An important issue is the lack of measurements which are usually\nemployed to fine-tune calculations so as to attain spectroscopic accuracy. In\norder to estimate data quality, several atomic structure codes are used and\nextensive comparisons with previous theoretical data have been carried out. In\nthe calculation of K photoabsorption with the Breit--Pauli $R$-matrix method,\nboth radiation and Auger damping, which cause the smearing of the K edge, are\ntaken into account. This work is part of a wider project to compute atomic data\nin the X-ray regime to be included in the database of the popular {\\sc xstar}\nmodeling code.", "category": "astro-ph_HE" }, { "text": "Exploring binary-neutron-star-merger scenario of short-gamma-ray bursts\n by gravitational-wave observation: We elucidate the feature of gravitational waves (GWs) from binary neutron\nstar merger collapsing to a black hole by general relativistic simulation. We\nshow that GW spectrum imprints the coalescence dynamics, formation process of\ndisk, equation of state for neutron stars, total masses, and mass ratio. A\nformation mechanism of the central engine of short $\\gamma$-ray bursts, which\nare likely to be composed of a black hole and surrounding disk, therefore could\nbe constrained by GW observation.", "category": "astro-ph_HE" }, { "text": "Outflow-Driven Transients from the Birth of Binary Black Holes I:\n Tidally-Locked Secondary Supernovae: We propose a new type of electromagnetic transients associated with the birth\nof binary black holes (BBHs), which may lead to merger events accompanied by\ngravitational waves in $\\sim0.1-1$ Gyr. We consider the newborn BBHs formed\nthrough the evolution of isolated massive stellar binaries. For a close massive\nbinary, consisting of a primary black hole (BH) and a secondary Wolf-Rayet (WR)\nstar that are orbiting around each other, the spin period of the WR star can be\ntidally synchronized to its orbital period. Then, the angular momentum of the\nouter material of the WR star is large enough to form an accretion disk around\na newborn, secondary BH, following its core-collapse. This disk may produce an\nenergetic outflow with a kinetic energy of $\\sim10^{50}-10^{52}$ erg and an\noutflow velocity of $\\sim10^{10}\\rm~cm~s^{-1}$, resulting in an optical\ntransient with an absolute magnitude from $\\sim -14$ to $\\sim-17$ with a\nduration of around a day. This type of transient also produces detectable radio\nsignals $\\sim1-10$ years after the birth of BBHs, via synchrotron emission from\nnon-thermal electrons accelerated at external shocks. The predicted optical\ntransients have a shorter duration than ordinary core-collapse supernovae.\nDedicated optical transient surveys can detect them, and distinguish it from\nordinary SNe using the different features of its light curve and late-time\nspectrum. In the paper I, we investigate disk-driven outflows from the\nsecondary BH, whereas possible signals from the primary BH will be discussed in\nthe paper II.", "category": "astro-ph_HE" }, { "text": "A NICER view of PSR J0030+0451: evidence for a global-scale multipolar\n magnetic field: Recent modeling of NICER observations of thermal X-ray pulsations from the\nsurface of the isolated millisecond pulsar PSR J0030+0451 suggests that the hot\nemitting regions on the pulsar's surface are far from antipodal, which is at\nodds with the classical assumption that the magnetic field in the pulsar\nmagnetosphere is predominantly that of a centered dipole. Here, we review these\nresults and examine previous attempts to constrain the magnetospheric\nconfiguration of PSR J0030+0451. To the best of our knowledge, there is in fact\nno direct observational evidence that PSR J0030+0451's magnetic field is a\ncentered dipole. Developing models of physically motivated, non-canonical\nmagnetic field configurations and the currents that they can support poses a\nchallenging task. However, such models may have profound implications for many\naspects of pulsar research, including pulsar braking, estimates of birth\nvelocities, and interpretations of multi-wavelength magnetospheric emission.", "category": "astro-ph_HE" }, { "text": "Optical Spectroscopy of Bright Fermi LAT Blazars: We report on HET and Palomar 5 m spectroscopy of recently identified\n$\\gamma$-ray blazars in the {\\it Fermi} LAT Bright Source List. These data\nprovide identifications for 10 newly discovered $\\gamma$-ray flat spectrum\nradio quasars (FSRQ) and six new BL Lacs plus improved spectroscopy for six\nadditional BL Lacs. We substantially improve the identification completeness of\nthe bright LAT blazars and give new redshifts and $z$ constraints, new\nestimates of the black hole masses and new measurements of the optical SED.", "category": "astro-ph_HE" }, { "text": "A multi-wavelength study of the gamma-ray binary candidate HESS\n J1832-093: We investigate the nature of the unidentified very-high-energy (VHE)\ngamma-ray object, HESS J1832-093, in a multi-wavelength context. Based on X-ray\nvariability and spectral index ($\\Gamma_X\\sim\\,1.5$), and its broad-band\nspectrum (which was remarkably similar to HESS J0632+057, a confirmed gamma-ray\nbinary), HESS J1832-093 has been considered to be a strong gamma-ray binary\ncandidate in previous works. In this work, we provide further evidence for this\nscenario. We obtained a spectrum of its IR counterpart using Gemini/Flamingo,\nfinding absorption lines that are usually seen in massive stars, in particular\nO stars. We also obtained a rather steep ATCA spectrum\n($\\alpha=-1.18^{+1.04}_{-0.88}$) which prefers a gamma-ray binary over an AGN\nscenario. Based on spatial-spectral analysis and variability search, we found\nthat 4FGL J1832.9-0913 is possible to be associated with SNR G22.7-0.2 rather\nthan with HESS J1832-093 only.", "category": "astro-ph_HE" }, { "text": "Diverse Properties of Interstellar Medium Embedding Gamma-Ray Bursts at\n the Epoch of Reionization: Analysis is performed on ultra-high resolution large-scale cosmological\nradiation-hydrodynamic simulations to, for the first time, quantify the\nphysical environment of long-duration gamma-ray bursts (GRBs) at the epoch of\nreionization. We find that, on parsec scales, 13% of GRBs remain in high\ndensity ($\\ge 10^4$cm$^{-3}$) low-temperature star-forming regions, whereas 87%\nof GRBs occur in low-density ($\\sim 10^{-2.5}$cm$^{-3}$) high temperature\nregions heated by supernovae. More importantly, the spectral properties of GRB\nafterglows, such as the neutral hydrogen column density, total hydrogen column\ndensity, dust column density, gas temperature and metallicity of intervening\nabsorbers, vary strongly from sightline to sightline. Although our model\nexplains extant limited observationally inferred values with respect to\ncircumburst density, metallicity, column density and dust properties, a\nsubstantially larger sample of high-z GRB afterglows would be required to\nfacilitate a statistically solid test of the model. Our findings indicate that\nany attempt to infer the physical properties (such as metallicity) of the\ninterstellar medium of the host galaxy based on a very small number of (usually\none) sightlines would be precarious. Utilizing high-z GRBs to probe\ninterstellar medium and intergalactic medium should be undertaken properly\ntaking into consideration the physical diversities of the interstellar medium.", "category": "astro-ph_HE" }, { "text": "Multi-wavelength observations of Isolated Neutron Stars: Almost 30 Isolated Neutron Stars (INSs) of different flavours have been\nidentified at optical, ultraviolet, or infrared (UVOIR) wavelengths. Here, I\npresent a short review of the historical background and describe the scientific\nimpact of INS observations in the UVOIR. Then, I focus on UVOIR observations of\nrotation-powered pulsars, so far the most numerous class of INSs identified at\nthese wavelengths, and their observational properties. Finally, I present the\nresults of new UVOIR observations and an update of the follow-ups of gamma-ray\npulsars detected by Fermi.", "category": "astro-ph_HE" }, { "text": "Hot disk of the Swift J0243.6+6124 revealed by Insight-HXMT: We report on analysis of observations of the bright transient X-ray pulsar\n\\src obtained during its 2017-2018 giant outburst with Insight-HXMT,\n\\emph{NuSTAR}, and \\textit{Swift} observatories. We focus on the discovery of a\nsharp state transition of the timing and spectral properties of the source at\nsuper-Eddington accretion rates, which we associate with the transition of the\naccretion disk to a radiation pressure dominated (RPD) state, the first ever\ndirectly observed for magnetized neutron star. This transition occurs at\nslightly higher luminosity compared to already reported transition of the\nsource from sub- to super-critical accretion regime associate with onset of an\naccretion column. We argue that this scenario can only be realized for\ncomparatively weakly magnetized neutron star, not dissimilar to other\nultra-luminous X-ray pulsars (ULPs), which accrete at similar rates. Further\nevidence for this conclusion is provided by the non-detection of the transition\nto the propeller state in quiescence which strongly implies compact\nmagnetosphere and thus rules out magnetar-like fields.", "category": "astro-ph_HE" }, { "text": "QCD, Gravitational Waves, and Pulsars: Investigations of the phase diagram of quantum chromodynamics (QCD) have\nrevealed that exotic new phases, the so called {\\it color superconducting\nphases}, may arise at very high baryon densities. It is speculated that these\nexotic phases may arise in the cores of neutron stars. Focus on neutrons stars\nhas tremendously intensified in recent years with the direct detection of\ngravitational waves (GW) by LIGO/Virgo from BNS merger events which has allowed\nthe possibility of directly probing the properties of the interior of a neutron\nstar. A remarkable phenomenon manifested by rapidly rotating neutron stars is\nin their {\\it avatar} as {\\it Pulsars}. The accuracy of pulsar timing can reach\nthe level of one part in 10$^{15}$, comparable to that of atomic clocks. This\nsuggests that even a tiny deformation of the pulsar can leave its imprints on\nthe pulses by inducing tiny perturbations in the entire moment of inertia (MI)\ntensor affecting the pulse timings, as well as the pulse profile (from wobbling\ninduced by off-diagonal MI components). This may allow a new probe of various\nphase transitions occurring inside a pulsar core through induced density\nfluctuations affecting the MI tensor. Such perturbations also naturally induce\na rapidly changing quadrupole moment of the star, thereby providing a new\nsource of gravitational wave emission. Another remarkable possibility arises\nwhen we consider the effect of an external GW on neutron star. With the\npossibility of detecting any minute changes in its configuration through pulse\nobservations, the neutron star has the potential of performing as a Weber\ndetector of gravitational wave. This brief review will focus on these specific\naspects of a pulsar. Specifically, the focus will be on the type of physics\nwhich can be probed by utilizing the effect of changes in the MI tensor of the\npulsar on pulse properties.", "category": "astro-ph_HE" }, { "text": "Californium-254 and kilonova light curves: Neutron star mergers offer unique conditions for the creation of the heavy\nelements and additionally provide a testbed for our understanding of this\nsynthesis known as the $r$-process. We have performed dynamical nucleosynthesis\ncalculations and identified a single isotope, $^{254}$Cf, which has a\nparticularly high impact on the brightness of electromagnetic transients\nassociated with mergers on the order of 15 to 250 days. This is due to the\nanomalously long half-life of this isotope and the efficiency of fission\nthermalization compared to other nuclear channels. We estimate the fission\nfragment yield of this nucleus and outline the astrophysical conditions under\nwhich $^{254}$Cf has the greatest impact to the light curve. Future\nobservations in the middle-IR which are bright during this regime could\nindicate the production of actinide nucleosynthesis.", "category": "astro-ph_HE" }, { "text": "On the approximation of the black hole shadow with a simple polar curve: A black hole embedded within a bright, optically thin emitting region\nimprints a nearly circular \"shadow\" on its image, corresponding to the\nobserver's line-of-sight into the black hole. The shadow boundary depends on\nthe black hole's mass and spin, providing an observable signature of both\nproperties via high resolution images. However, standard expressions for the\nshadow boundary are most naturally parametrized by Boyer-Lindquist radii rather\nthan by image coordinates. We explore simple, approximate parameterizations for\nthe shadow boundary using ellipses and a family of curves known as limacons. We\ndemonstrate that these curves provide excellent and efficient approximations\nfor all black hole spins and inclinations. In particular, we show that the two\nparameters of the limacon naturally account for the three primary shadow\ndeformations resulting from mass and spin: size, displacement, and asymmetry.\nThese curves are convenient for parametric model fitting directly to\ninterferometric data, they reveal the degeneracies expected when estimating\nblack hole properties from images with practical measurement limitations, and\nthey provide a natural framework for parametric tests of the Kerr metric using\nblack hole images.", "category": "astro-ph_HE" }, { "text": "The Euclidean distribution of Fast Radio Bursts: We investigate whether current data on the distribution of observed flux\ndensities of Fast Radio Bursts (FRBs) are consistent with a constant source\ndensity in Euclidean space. We use the number of FRBs detected in two surveys\nwith different characteristics along with the observed signal-to-noise ratios\nof the detected FRBs in a formalism similar to a V/V_max-test to constrain the\ndistribution of flux densities. We find consistency between the data and a\nEuclidean distribution. Any extension of this model is therefore not\ndata-driven and needs to be motivated separately. As a byproduct we also obtain\nnew improved limits for the FRB rate at 1.4 GHz, which had not been constrained\nin this way before.", "category": "astro-ph_HE" }, { "text": "Suzaku investigation into the nature of the nearest ultraluminous X-ray\n source, M33 X-8: The X-ray spectrum of the nearest ultraluminous X-ray source, M33 X-8,\nobtained by Suzaku during 2010 January 11 -- 13, was closely analyzed to\nexamine its nature. It is, by far, the only data with the highest signal\nstatistic in 0.4 -- 10 keV range. Despite being able to reproduce the X-ray\nspectrum, Comptonization of the disk photons failed to give a physically\nmeaningful solution. A modified version of the multi-color disk model, in which\nthe dependence of the disk temperature on the radius is described as r^(-p)\nwith p being a free parameter, can also approximate the spectrum. From this\nmodel, the innermost disk temperature and bolometric luminosity were obtained\nas T_in = 2.00-0.05+0.06 keV and L_disk = 1.36 x 10^39 (cos i)^(-1) ergs/s,\nrespectively, where i is the disk inclination. A small temperature gradient of\np = 0.535-0.005+0.004, together with the high disk temperature, is regarded as\nthe signatures of the slim accretion disk model, suggesting that M33 X-8 was\naccreting at high mass accretion rate. With a correction factor for the slim\ndisk taken into account, the innermost disk radius, R_in =81.9-6.5+5.9 (cos\ni)^(-0.5) km, corresponds to the black hole mass of M \\sim 10 M_sun (cos\ni)^(-0.5). Accordingly, the bolometric disk luminosity is estimated to be about\n80 (cos i)^(-0.5)% of the Eddington limit. A numerically calculated slim disk\nspectrum was found to reach a similar result. Thus, the extremely\nsuper-Eddington luminosity is not required to explain the nature of M33 X-8.\nThis conclusion is utilized to argue for the existence of intermediate mass\nblack holes with M > 100 M_sun radiating at the sub/trans-Eddington luminosity,\namong ultraluminous X-ray sources with L_disk > 10^(40) ergs/s.", "category": "astro-ph_HE" }, { "text": "Fast TeV variability from misaligned minijets in the jet of M87: The jet of the radio galaxy M87 is misaligned, resulting in a Doppler factor\ndelta~1 for emission of plasma moving parallel to the jet. This makes the\nobserved fast TeV flares on timescales of t_v~5R_g/c harder to understand as\nemission from the jet. In previous work, we have proposed a jets-in-a-jet model\nfor the ultra-fast TeV flares with t_v<60\\%$ of\nthe X-ray opacity of high-$z$ sources (the ionization of He isn't available in\nthe simulation, we used here external knowledge). The high-$z$ asymptotic\noptical depth at 0.5\\,keV in the simulation reaches $0.15\\pm0.07$, while the\nGRB afterglow values tend to $\\approx 0.4$, implying the missing baryons can\naccount for a significant fraction of the observed opacity. The remaining\ndiscrepancy is ascribed mainly to the low average metallicity in the\nsimulation, which drops from 0.06 solar at $z=0$ to 0.01 at $z=3$, and which is\nbelow previously measured values.", "category": "astro-ph_HE" }, { "text": "Instability of twisted magnetar magnetospheres: We present three-dimensional force-free electrodynamics simulations of\nmagnetar magnetospheres that demonstrate the instability of certain degenerate,\nhigh energy equilibrium solutions of the Grad-Shafranov equation. This result\nindicates the existence of an unstable branch of twisted magnetospheric\nsolutions and allows to formulate an instability criterion. The rearrangement\nof magnetic field lines as a consequence of this instability triggers the\ndissipation of up to 30% of the magnetospheric energy on a thin layer above the\nmagnetar surface. During this process, we predict an increase of the mechanical\nstresses onto the stellar crust, which can potentially result in a global\nmechanical failure of a significant fraction of it. We find that the estimated\nenergy release and the emission properties are compatible with the observed\ngiant flare events. The newly identified instability is a candidate for\nrecurrent energy dissipation, which could explain part of the phenomenology\nobserved in magnetars.", "category": "astro-ph_HE" }, { "text": "The link between short Gamma-ray bursts and Gravitational Waves:\n perspectives for the THESEUS mission: The knowledge of the class of short Gamma-Ray Bursts (GRBs), characterised by\na duration of the gamma-ray emission $\\leq 2$ s, experienced an impressive\nboost in the last decade. In particular, the discovery of short GRB afterglows\nin 2005 with Swift and HETE-II provided the first insight into their energy\nscale, environments and host galaxies. The lack of detection of associated\nsupernovae proved that they are not related to the death of massive stars. The\nincreasing evidence for compact object binary progenitors makes short GRBs one\nof the most promising sources of gravitational waves for the forthcoming\nAdvanced LIGO/Virgo science runs. To this end, the spectacular detection of the\nfirst electromagnetic counterpart of the gravitational wave event GW\\,170817\noriginated by the coalescence of a double neutron star (NS) system, represents\na first hystorical milestone. The (weak) short GRB\\,170817A associated to this\nGW event provided the long-sought evidence that at least a fraction of short\nGRBs are originated by NS-NS merging and suggested the intriguing possibility\nthat relativistic jets can be launched in the process of a NS-NS merger. The\nTHESEUS mission, thanks to the diversity of intstrumentation, fast pointing and\nflexible schedule will represent a key facility in the multi-messenger\nastronomy era.", "category": "astro-ph_HE" }, { "text": "Time of Flight and Supernova Progenitor Effects on the Neutrino Halo: We argue that the neutrino halo, a population of neutrinos that have\nundergone direction-changing scattering in the stellar envelope of a\ncore-collapse supernova (CCSNe), is sensitive to neutrino emission history\nthrough time of flight. We show that the constant time approximation, commonly\nused in calculating the neutrino halo, does not capture the spatiotemporal\nevolution of the halo neutrino population and that correcting for time of\nflight can produce conditions which may trigger fast neutrino flavor\nconversion. We also find that there exists a window of time early in all CCSNe\nwhere the neutrino halo population is sufficiently small that it may be\nnegligible. This suggests that collective neutrino oscillation calculations\nwhich neglect the Halo may be well founded at sufficiently early times.", "category": "astro-ph_HE" }, { "text": "X-ray counterpart of gravitational waves due to binary neutron star\n mergers: light curves, luminosity functions, and event rate densities: Zhang (2013) proposed a type of GRB-less X-ray transient associated with\ndouble neutron star (NS-NS) mergers under the conjecture of a rapidly-spinning\nmagnetar merger product with the line of sight off the short GRB jet. We\ninvestigate possible light curves of these transients by considering different\nobserver's viewing angles. We perform Monte Carlo simulations to calculate the\npeak luminosity function (LF) and event rate density of these X-ray transients.\nBy considering that a fraction of massive neutron stars may be supra-massive\nand later collapse into black holes after spinning down, we investigate how the\npredicted LF depends on the equation of state (EoS) of the central object and\nthe geometry of the system. In general, the LF can be fit by two log-normal\ndistributions peaking around $10^{46.4}$ and $10^{49.6}$ $\\rm erg\\,s^{-1}$,\ncorresponding to the trapped and free zones, respectively. For the majority of\nthe EoS models, the current non-detection is consistent with having a free zone\nsolid angle at most a few times of the solid angle of the short GRB jet. The\nevent rate density of these X-ray transients is around a few tens of $\\rm\nGpc^{-3}yr^{-1}$ for luminosity above $10^{45}$ $\\rm erg\\,s^{-1}$. We predict\nthat future X-ray telescopes (such as Einstein Probe) with sensitivity $\\sim\n10^{-11}$ $\\rm erg\\,s^{-1}\\,cm^{-2}$ would detect as many as several tens of\nsuch transients per year per steradian. Within 200 Mpc, the aLIGO average range\nfor NS-NS mergers, the estimated event rate of these transients is about 1\ntransient per year all sky.", "category": "astro-ph_HE" }, { "text": "Long-term monitoring of LS I+61303 with INTEGRAL: LS I+61303 is one of the few high-mass X-ray binaries that have been recently\nobserved at TeV and GeV energies. Here we investigate the hard X-ray spectral\nand timing properties of this source using the IBIS/ISGRI instrument on-board\nthe INTEGRAL satellite. We carry out a systematic analysis based on all\navailable INTEGRAL data since December 28, 2002 up to April 30, 2008. The total\nexposure time analyzed amounts to 2.1 Ms, hence more than doubling the previous\nreported sample. The source is best detected in the 18-60 keV band, with a\nsignificance level of 12.0 sigma. The hard X-ray data are best fit with a\nsimple power law with a photon index of ~ 1.7+-0.2. We detect a periodical\nsignal at 27 +- 4 days, matching the orbital period of 26.496 days previously\nreported at other wavelengths. The hard X-rays orbital lightcurve is obtained\nand compared with those derived at other frequencies.", "category": "astro-ph_HE" }, { "text": "The role of metallicity in high mass X-ray binaries in galaxy formation\n models: Recent theoretical works claim that high-mass X-ray binaries (HMXBs) could\nhave been important sources of energy feedback into the interstellar and\nintergalactic media, playing a major role in the reionization epoch. A\nmetallicity dependence of the production rate or luminosity of the sources is a\nkey ingredient generally assumed but not yet probed. Aims: Our goal is to\nexplore the relation between the X-ray luminosity (Lx) and star formation rate\nof galaxies as a possible tracer of a metallicity dependence of the production\nrates and/or X-ray luminosities of HMXBs. Methods: We developed a model to\nestimate the Lx of star forming galaxies based on stellar evolution models\nwhich include metallicity dependences. We applied our X-ray binary models to\ngalaxies selected from hydrodynamical cosmological simulations which include\nchemical evolution of the stellar populations in a self-consistent way.\nResults: Our models successfully reproduce the dispersion in the observed\nrelations as an outcome of the combined effects of the mixture of stellar\npopulations with heterogeneous chemical abundances and the metallicity\ndependence of the X-ray sources. We find that the evolution of the Lx as a\nfunction of SFR of galaxies could store information on possible metallicity\ndependences of the HMXB sources. A non-metallicity dependent model predicts a\nnon-evolving relation while any metallicity dependence should affect the slope\nand the dispersion as a function of redshift. Our results suggest the\ncharacteristics of the Lx evolution can be linked to the nature of the\nmetallicity dependence of the production rate or the Lx of the stellar sources.\nBy confronting our models with current available observations of strong\nstar-forming galaxies, we find that only chemistry-dependent models reproduce\nthe observed trend for z < 4.", "category": "astro-ph_HE" }, { "text": "Multiwavelength behaviour of the blazar 3C279: decade-long study from\n $\u03b3$-ray to radio: We report the results of decade-long (2008-2018) $\\gamma$-ray to 1 GHz radio\nmonitoring of the blazar 3C 279, including GASP/WEBT, $\\it{Fermi}$ and\n$\\it{Swift}$ data, as well as polarimetric and spectroscopic data. The X-ray\nand $\\gamma$-ray light curves correlate well, with no delay > 3 hours, implying\ngeneral co-spatiality of the emission regions. The $\\gamma$-ray-optical\nflux-flux relation changes with activity state, ranging from a linear to a more\ncomplex dependence. The behaviour of the Stokes parameters at optical and radio\nwavelengths, including 43 GHz VLBA images, supports either a predominantly\nhelical magnetic field or motion of the radiating plasma along a spiral path.\nApparent speeds of emission knots range from 10 to 37c, with the highest values\nrequiring bulk Lorentz factors close to those needed to explain $\\gamma$-ray\nvariability on very short time scales. The Mg II emission line flux in the\n`blue' and `red' wings correlates with the optical synchrotron continuum flux\ndensity, possibly providing a variable source of seed photons for inverse\nCompton scattering. In the radio bands we find progressive delays of the most\nprominent light curve maxima with decreasing frequency, as expected from the\nfrequency dependence of the $\\tau=1$ surface of synchrotron self-absorption.\nThe global maximum in the 86 GHz light curve becomes less prominent at lower\nfrequencies, while a local maximum, appearing in 2014, strengthens toward\ndecreasing frequencies, becoming pronounced at $\\sim5$ GHz. These tendencies\nsuggest different Doppler boosting of stratified radio-emitting zones in the\njet.", "category": "astro-ph_HE" }, { "text": "Type Ic Supernova of a 22 $\\text{M}_{\\odot}$ Progenitor: Type Ic supernovae (SNe Ic) are a sub-class of core-collapse supernovae that\nexhibit no helium or hydrogen lines in their spectra. Their progenitors are\nthought to be bare carbon-oxygen cores formed during the evolution of massive\nstars that are stripped of their hydrogen and helium envelopes sometime before\ncollapse. SNe Ic present a range of luminosities and spectral properties, from\nluminous GRB-SNe with broad-lined spectra to less luminous events with\nnarrow-line spectra. Modelling SNe\\,Ic reveals a wide range of both kinetic\nenergies, ejecta masses, and $^{56}\\text{Ni}$ masses. To explore this diversity\nand how it comes about, light curves and spectra are computed from the ejecta\nfollowing the explosion of an initially 22 $\\text{M}_{\\odot}$ progenitor that\nwas artificially stripped of its hydrogen and helium shells, producing a bare\nCO core of $\\sim$ 5 $\\text{M}_{\\odot}$, resulting in an ejected mass of $\\sim$\n4 $\\text{M}_{\\odot}$, which is an average value for SNe Ic. Four different\nexplosion energies are used that cover a range of observed SNe. Finally,\n$^{56}\\text{Ni}$ and other elements are artificially mixed in the ejecta using\ntwo approximations to determine how element distribution affects light curves\nand spectra. The combination of different explosion energy and degree of mixing\nproduces spectra that roughly replicate the distribution of near-peak\nspectroscopic features of SNe Ic. High explosion energies combined with\nextensive mixing can produce red, broad-lined spectra, while minimal mixing and\na lower explosion energy produce bluer, narrow-lined spectra.", "category": "astro-ph_HE" }, { "text": "ASAS-SN search for optical counterparts of gravitational-wave events\n from the third observing run of Advanced LIGO/Virgo: We report on the search for electromagnetic counterparts to the nine\ngravitational-wave events with a $>$60\\% probability of containing a neutron\nstar during the third (O3) LIGO-Virgo Collaboration (LVC) observing run with\nthe All-Sky Automated Survey for SuperNovae (ASAS-SN). No optical counterparts\nassociated with a gravitational wave event was found. However, thanks to its\nnetwork of telescopes, the average area visible to at least one ASAS-SN site\nduring the first 10 hours after the trigger contained $\\sim$30\\% of the\nintegrated source location probability. Through a combination of normal\noperations and target-of-opportunity observations, ASAS-SN observations of the\nhighest probability fields began within one hour of the trigger for four of the\nevents. After 24 hours, ASAS-SN observed $>$60\\% of total probability for three\nevents and $>$40\\% for all but one of the events. This is the largest area\ncoverage to a depth of $g = 18.5$ mag from any survey with published coverage\nstatistics for seven of the nine events. With its observing strategy, five\nsites around the world, and a large field of view, ASAS-SN will be one of the\nleading surveys to optically search for nearby neutron star mergers during LVC\nO4.", "category": "astro-ph_HE" }, { "text": "Star formation, supernovae, iron, and alpha: consistent cosmic and\n Galactic histories: Recent versions of the observed cosmic star-formation history (SFH) have\nresolved an inconsistency with the stellar mass density history. We show that\nthe revised SFH also scales up the delay-time distribution (DTD) of Type Ia\nsupernovae (SNe Ia), as determined from the observed volumetric SN Ia rate\nhistory, aligning it with other field-galaxy SN Ia DTD measurements. The\nrevised-SFH-based DTD has a $t^{-1.1 \\pm 0.1}$ form and a\nHubble-time-integrated production efficiency of $N/M_\\star=1.3\\pm0.1$ SNe Ia\nper $1000~{\\rm M_\\odot}$ of formed stellar mass. Using these revised histories\nand updated empirical iron yields of the various SN types, we re-derive the\ncosmic iron accumulation history. Core-collapse SNe and SNe Ia have contributed\nabout equally to the total mass of iron in the Universe today. We find the\ntrack of the average cosmic gas element in the [$\\alpha$/Fe] vs. [Fe/H]\nabundance-ratio plane. The track is broadly similar to the observed main locus\nof Galactic stars in this plane, indicating a Milky Way (MW) SFH similar in\nform to the cosmic one. We easily find a simple MW SFH that makes the track\nclosely match this stellar locus. Galaxy clusters appear to have a\nhigher-normalization DTD. This cluster DTD, combined with a short-burst MW SFH\npeaked at $z=3$, produces a track that matches remarkably well the observed\n\"high-$\\alpha$\" locus of MW stars, suggesting the halo/thick-disk population\nhas had a galaxy-cluster-like formation mode. Thus, a simple two-component SFH,\ncombined with empirical DTDs and SN iron yields, suffices to closely reproduce\nthe MW's stellar abundance patterns.", "category": "astro-ph_HE" }, { "text": "Dissipation of the striped pulsar wind: Rapidly rotating neutron stars blow a relativistic, magnetized wind mainly\ncomposed of electron-positron pairs. The free expansion of the wind terminates\nfar from the neutron star where a weakly magnetized pulsar wind nebula forms,\nimplying efficient magnetic dissipation somewhere upstream. The wind current\nsheet that separates the two magnetic polarities is usually considered as the\nmost natural place for magnetic dissipation via relativistic reconnection, but\nits efficiency remains an open question. Here, the goal of this work is to\nrevisit this issue in light of the most recent progress in the understanding of\nreconnection and pulsar electrodynamics. We perform large two-dimensional\nparticle-in-cell simulations of the oblique rotator to capture the multi-scale\nevolution of the wind. We find that the current sheet breaks up into a\ndynamical chain of magnetic islands separated by secondary thin current sheets.\nThe sheet thickness increases linearly with radius while the Poynting flux\ndecreases monotonically as reconnection proceeds. The radius of complete\nannihilation of the stripes is given by the plasma multiplicity parameter at\nthe light cylinder. Current starvation within the sheets does not occur before\ncomplete dissipation as long as there is enough charges where the sheets form.\nParticles are efficiently heated up to a characteristic energy set by the\nmagnetization parameter at the light cylinder. Energetic pulsed synchrotron\nemission peaks close to the light cylinder, and presents sub-pulse variability\nassociated with the formation of plasmoids in the sheet. This study suggests\nthat the striped component of the wind dissipates far before reaching the\ntermination shock in isolated pulsars, even in very-high-multiplicity systems\nsuch as the Crab pulsar. Pulsars in binary systems may provide the best\nenvironments to study magnetic dissipation in the wind.", "category": "astro-ph_HE" }, { "text": "Acceleration and propagation of ultra high energy cosmic rays: We review the physics of the highest energy cosmic rays. The discovery of\ntheir sources, still unknown, will reveal the most energetic astrophysical\nobjects in the universe and could unveil new physics beyond the standard model\nof particle physics. We discuss the details of propagation of these high energy\nparticles, their interaction with astrophysical photon backgrounds and\nintergalactic magnetic fields, and the production of secondary cosmogenic\nparticles associated to their transport. We examine different models of\nacceleration, reviewing the principal astrophysical objects that could energise\ncosmic rays until the highest energies. Given the uncertainties in the observed\nmass composition, we review the two alternative scenarios of a composition made\nonly by protons or by protons and heavier nuclei; discussing the consequences\nof the two scenarios in terms of sources, acceleration mechanisms and\nproduction of secondary cosmogenic gamma rays and neutrinos.", "category": "astro-ph_HE" }, { "text": "CLUES on Fermi-LAT prospects for the extragalactic detection of munuSSM\n gravitino Dark Matter: The munuSSM is a supersymmetric model that has been proposed to solve the\nproblems generated by other supersymmetric extensions of the standard model of\nparticle physics. Given that R-parity is broken in the munuSSM, the gravitino\nis a natural candidate for decaying dark matter since its lifetime becomes much\nlonger than the age of the Universe. In this model, gravitino dark matter could\nbe detectable through the emission of a monochromatic gamma ray in a two-body\ndecay.\n We study the prospects of the Fermi-LAT telescope to detect such\nmonochromatic lines in 5 years of observations of the most massive nearby\nextragalactic objects.\n The dark matter halo around the Virgo galaxy cluster is selected as a\nreference case, since it is associated to a particularly high signal-to-noise\nratio and is located in a region scarcely affected by the astrophysical diffuse\nemission from the galactic plane.\n The simulation of both signal and background gamma-ray events is carried out\nwith the Fermi Science Tools, and the dark matter distribution around Virgo is\ntaken from a N-body simulation of the nearby extragalactic Universe, with\nconstrained initial conditions provided by the CLUES project.\n We find that a gravitino with a mass range of 0.6 to 2 GeV, and with a\nlifetime range of about 3x10^27 to 2x10^28 s would be detectable by the\nFermi-LAT with a signal-to-noise ratio larger than 3. We also obtain that\ngravitino masses larger than about 4 GeV are already excluded in the munuSSM by\nFermi-LAT data of the galactic halo", "category": "astro-ph_HE" }, { "text": "LeHaMoC: a versatile time-dependent lepto-hadronic modeling code for\n high-energy astrophysical sources: Recent associations of high-energy neutrinos with active galactic nuclei\n(AGN) have revived the interest in leptohadronic models of radiation from\nastrophysical sources. The rapid increase in the amount of acquired\nmulti-messenger data will require soon fast numerical models that may be\napplied to large source samples. We develop a time-dependent leptohadronic\ncode, LeHaMoC, that offers several notable benefits compared to other existing\ncodes, such as versatility and speed. LeHaMoC solves the Fokker-Planck\nequations of photons and relativistic particles (i.e. electrons, positrons,\nprotons, and neutrinos) produced in a homogeneous magnetized source that may\nalso be expanding. The code utilizes a fully implicit difference scheme that\nallows fast computation of steady-state and dynamically evolving physical\nproblems. We first present test cases where we compare the numerical results\nobtained with LeHaMoC against exact analytical solutions and numerical results\ncomputed with ATHE$\\nu$A, a well-tested code of similar philosophy but\ndifferent numerical implementation. We find a good agreement (within 10-30%)\nwith the numerical results obtained with ATHE$\\nu$A without evidence of\nsystematic differences. We then demonstrate the capabilities of the code\nthrough illustrative examples. First, we fit the spectral energy distribution\nfrom a jetted AGN in the context of a synchrotron-self Compton model and a\nproton-synchrotron model using Bayesian inference. Second, we compute the\nhigh-energy neutrino signal and the electromagnetic cascade induced by hadronic\ninteractions in the corona of NGC 1068. LeHaMoC is easily customized to model a\nvariety of high-energy astrophysical sources and has the potential to become a\nwidely utilized tool in multi-messenger astrophysics.", "category": "astro-ph_HE" }, { "text": "AT2017gfo: an anisotropic and three-component kilonova counterpart of\n GW170817: The detection of a kilo/macronova electromagnetic counterpart (AT2017gfo) of\nthe first gravitational wave signal compatible with the merger of two neutron\nstars (GW170817) has confirmed the occurrence of r-process nucleosynthesis in\nthis kind of events. The blue and red components of AT2017gfo have been\ninterpreted as the signature of multi-component ejecta in the merger dynamics.\nHowever, the explanation of AT2017gfo in terms of the properties of the ejecta\nand of the ejection mechanisms is still incomplete. In this work, we analyse\nAT2017gfo with a new semi-analytic model of kilo/macronova inferred from\ngeneral relativistic simulations of the merger and long-term numerical models\nof the merger aftermath. The model accounts for the anisotropic emission from\nthe three known mass ejecta components: dynamic, winds and secular outflows\nfrom the disk. The early multi-band light-curves of AT2017gfo can only be\nexplained by the presence of a relatively low opacity component of the ejecta\nat high latitudes. This points to the key role of weak interactions in setting\nthe ejecta properties and determining the nucleosynthetic yields. Our model\nconstrains also the total ejected mass associated to AT2017gfo to be between\n$0.042$ and $0.077 M_{\\odot}$; the observation angle of the source to be\nbetween $\\pi/12$ and $7\\pi/36 $; and the mass of the disk to be $ \\gtrsim 0.08\nM_{\\odot}$.", "category": "astro-ph_HE" }, { "text": "Bondi on spherically symmetric accretion: Hermann Bondi's 1952 paper \"On spherically symmetrical accretion\" is\nrecognized as one of the foundations of accretion theory. Although Bondi later\nremarked that it was \"not much more than an examination exercise\", his\nmathematical analysis of spherical accretion on to a point mass has found broad\nuse across fields of astrophysics that were embryonic or non-existent at the\ntime of the paper's publication. In this non-technical review, I describe the\nmotivations for Bondi's work, and briefly discuss some of the applications of\nBondi accretion in high energy astrophysics, galaxy formation, and star\nformation.", "category": "astro-ph_HE" }, { "text": "Indications of non-conservative mass-transfer in AMXPs: Context. Since the discovery of the first Accreting Millisecond X-ray Pulsar\nSAX J1808.4-3658 in 1998, the family of these sources kept growing on.\nCurrently, it counts 22 members. All AMXPs are transients with usually very\nlong quiescence periods, implying that mass accretion rate in these systems is\nquite low and not constant. Moreover, for at least three sources, a\nnon-conservative evolution was also proposed.\n Aims. Our purpose is to study the long term averaged mass-accretion rates in\nall the Accreting Millisecond X-ray Pulsars discovered so far, to investigate a\nnon-conservative mass-transfer scenario.\n Methods. We calculated the expected mass-transfer rate under the hypothesis\nof a conservative evolution based on their orbital periods and on the (minimum)\nmass of the secondary (as derived from the mass function), driven by\ngravitational radiation and/or magnetic braking. Using this theoretical\nmass-transfer, we determined the expected accretion luminosity of the systems.\nThus, we achieved the lower limit to the distance of the sources by comparing\nthe computed theoretical luminosity and the observed flux averaged over a time\nperiod of 20 years. Then, the lower limit to the distance of the sources has\nbeen compared to the value of the distance reported in literature to evaluate\nhow reasonable is the hypothesis of a conservative mass-transfer.\n Results. Based on a sample of 18 sources, we found strong evidences of a\nnon-conservative mass-transfer for five sources, for which the estimated\ndistance lower limits are higher than their known distances. We also report\nhints for mass outflows in other six sources. The discrepancy can be fixed\nunder the hypothesis of a non-conservative mass-transfer in which a fraction of\nthe mass transferred onto the compact object is swept away from the system,\nlikely due to the (rotating magnetic dipole) radiation pressure of the pulsar.", "category": "astro-ph_HE" }, { "text": "The First Two Years of Electromagnetic Follow-Up with Advanced LIGO and\n Virgo: We anticipate the first direct detections of gravitational waves (GWs) with\nAdvanced LIGO and Virgo later this decade. Though this groundbreaking technical\nachievement will be its own reward, a still greater prize could be observations\nof compact binary mergers in both gravitational and electromagnetic channels\nsimultaneously. During Advanced LIGO and Virgo's first two years of operation,\n2015 through 2016, we expect the global GW detector array to improve in\nsensitivity and livetime and expand from two to three detectors. We model the\ndetection rate and the sky localization accuracy for binary neutron star (BNS)\nmergers across this transition. We have analyzed a large, astrophysically\nmotivated source population using real-time detection and sky localization\ncodes and higher-latency parameter estimation codes that have been expressly\nbuilt for operation in the Advanced LIGO/Virgo era. We show that for most BNS\nevents the rapid sky localization, available about a minute after a detection,\nis as accurate as the full parameter estimation. We demonstrate that Advanced\nVirgo will play an important role in sky localization, even though it is\nanticipated to come online with only one-third as much sensitivity as the\nAdvanced LIGO detectors. We find that the median 90% confidence region shrinks\nfrom ~500 square degrees in 2015 to ~200 square degrees in 2016. A few distinct\nscenarios for the first LIGO/Virgo detections emerge from our simulations.", "category": "astro-ph_HE" }, { "text": "Possible hard X-ray shortages in bursts from KS 1731-260 and 4U 1705-44: Aims: A hard X-ray shortage, implying the cooling of the corona, was observed\nduring bursts of IGR J17473-272, 4U 1636-536, Aql X-1, and GS 1826-238. Apart\nfrom these four sources, we investigate here an atoll sample, in which the\nnumber of bursts for each source is larger than 5, to explore the possible\nadditional hard X-ray shortage during {\\it Rossi X-ray timing explorer (RXTE)}\nera. Methods: According to the source catalog that shows type-I bursts, we\nanalyzed all the available pointing observations of these sources carried out\nby the {\\it RXTE} proportional counter array (PCA). We grouped and combined the\nbursts according to their outburst states and searched for the possible hard\nX-ray shortage while bursting. Results: We found that the island states of KS\n1731-260 and 4U 1705-44 show a hard X-ray shortage at significant levels of 4.5\nand 4.7 $\\sigma$ and a systematic time lag of $0.9 \\pm 2.1$ s and $2.5 \\pm 2.0$\ns with respect to the soft X-rays, respectively. While in their banana branches\nand other sources, we did not find any consistent shortage.", "category": "astro-ph_HE" }, { "text": "The Role of Fission in Neutron Star Mergers and its Impact on the\n r-Process Peaks: Comparing observational abundance features with nucleosynthesis predictions\nof stellar evolution or explosion simulations can scrutinize two aspects: (a)\nthe conditions in the astrophysical production site and (b) the quality of the\nnuclear physics input utilized. We test the abundance features of r-process\nnucleosynthesis calculations for the dynamical ejecta of neutron star merger\nsimulations based on three different nuclear mass models: The Finite Range\nDroplet Model (FRDM), the (quenched version of the) Extended Thomas Fermi Model\nwith Strutinsky Integral (ETFSI-Q), and the Hartree-Fock-Bogoliubov (HFB) mass\nmodel. We make use of corresponding fission barrier heights and compare the\nimpact of four different fission fragment distribution models on the final\nr-process abundance distribution. In particular, we explore the abundance\ndistribution in the second r-process peak and the rare-earth sub-peak as a\nfunction of mass models and fission fragment distributions, as well as the\norigin of a shift in the third r-process peak position. The latter has been\nnoticed in a number of merger nucleosynthesis predictions. We show that the\nshift occurs during the r-process freeze-out when neutron captures and\n{\\beta}-decays compete and an (n,{\\gamma})-({\\gamma},n) equilibrium is not\nmaintained anymore. During this phase neutrons originate mainly from fission of\nmaterial above A = 240. We also investigate the role of {\\beta}-decay\nhalf-lives from recent theoretical advances, which lead either to a smaller\namount of fissioning nuclei during freeze-out or a faster (and thus earlier)\nrelease of fission neutrons, which can (partially) prevent this shift and has\nan impact on the second and rare-earth peak as well.", "category": "astro-ph_HE" }, { "text": "Supergiant, fast, but not so transient 4U 1907+09: We have investigated the dipping activity observed in the high-mass X-ray\nbinary 4U 1907+09 and shown that the source continues to pulsate in the \"off\"\nstate, noting that the transition between the \"on\" and \"off\" states may be\neither dip-like or flare-like. This behavior may be explained in the framework\nof the \"gated accretion\" scenario proposed to explain the flares in supergiant\nfast X-ray transients (SFXTs). We conclude that 4U 1907+09 might prove to be a\nmissing link between the SFXTs and ordinary accreting pulsars.", "category": "astro-ph_HE" }, { "text": "The Fermi Haze: A Gamma-Ray Counterpart to the Microwave Haze: The Fermi Gamma-Ray Space Telescope reveals a diffuse inverse Compton signal\nin the inner Galaxy with a similar spatial morphology to the microwave haze\nobserved by WMAP, supporting the synchrotron interpretation of the microwave\nsignal. Using spatial templates, we regress out pi0 gammas, as well as IC and\nbremsstrahlung components associated with known soft-synchrotron counterparts.\nWe find a significant gamma-ray excess towards the Galactic center with a\nspectrum that is significantly harder than other sky components and is most\nconsistent with IC from a hard population of electrons. The morphology and\nspectrum are consistent with it being the IC counterpart to the electrons which\ngenerate the microwave haze seen at WMAP frequencies. In addition, the implied\nelectron spectrum is hard; electrons accelerated in supernova shocks in the\ndisk which then diffuse a few kpc to the haze region would have a softer\nspectrum. We describe the full sky Fermi maps used in this analysis and make\nthem available for download.", "category": "astro-ph_HE" }, { "text": "A Systematic Search for Periodically Varying Quasars in Pan-STARRS1: An\n Extended Baseline Test in Medium Deep Survey Field MD09: We present a systematic search for periodically varying quasars and\nsupermassive black hole binary (SMBHB) candidates in the Pan-STARRS1 (PS1)\nMedium Deep Survey's MD09 field. From a color-selected sample of 670 quasars\nextracted from a multi-band deep-stack catalog of point sources, we locally\nselect variable quasars and look for coherent periods with the Lomb-Scargle\nperiodogram. Three candidates from our sample demonstrate strong variability\nfor more than ~3 cycles, and their PS1 light curves are well fitted to\nsinusoidal functions. We test the persistence of the candidates' apparent\nperiodic variations detected during the 4.2 years of the PS1 survey with\narchival photometric data from the SDSS Stripe 82 survey or new monitoring with\nthe Large Monolithic Imager at the Discovery Channel Telescope. None of the\nthree periodic candidates (including PSO J334.2028+1.4075) remain persistent\nover the extended baseline of 7 - 14 years, corresponding to a detection rate\nof < 1 in 670 quasars in a search area of 5 deg^2. Even though SMBHBs should be\na common product of the hierarchal growth of galaxies, and periodic variability\nin SMBHBs has been theoretically predicted, a systematic search for such\nsignatures in a large optical survey is strongly limited by its temporal\nbaseline and the \"red noise\" associated with normal quasar variability. We show\nthat follow-up long-term monitoring (>5 cycles) is crucial to our search for\nthese systems.", "category": "astro-ph_HE" }, { "text": "Identification of AGN in the XMM-Newton X-ray survey of the SMC: Context: Finding Active Galactic Nuclei (AGN) behind the Magellanic Clouds\n(MCs) is difficult because of the high stellar density in these fields.\nAlthough the first AGN behind the Small Magellanic Cloud (SMC) were reported in\nthe 1980s, it is only recently that the number of AGN known behind the SMC has\nincreased by several orders of magnitude. Aims: The mid-infrared colour\nselection technique has been proven to be an efficient means of identifying\nAGN, especially obscured sources. The X-ray regime is complementary in this\nregard and we use XMM-Newton observations to support the identification of AGN\nbehind the SMC. Methods: We present a catalogue of AGN behind the SMC by\ncorrelating an updated X-ray point source catalogue from our XMM-Newton survey\nof the SMC with already known AGN from the literature as well as a list of\ncandidates obtained from the ALLWISE mid-infrared colour selection criterion.\nWe studied the properties of the sample with respect to their redshifts,\nluminosities and X-ray spectral characteristics. We also identified the\nnear-infrared counterpart of the sources from the VISTA observations. Results:\nThe redshift and luminosity distributions of the sample (where known) indicate\nthat we detect sources from nearby Seyfert galaxies to distant and obscured\nquasars. The X-ray hardness ratios are compatible with those typically expected\nfor AGN. The VISTA colours and variability are also consistent in this regard.\nA positive correlation was observed between the integrated X-ray flux (0.2--12\nkeV) and the ALLWISE and VISTA magnitudes. We further present a sample of new\ncandidate AGN and candidates for obscured AGN. All of these make an interesting\nsubset for further follow-up studies. An initial spectroscopic follow-up of 6\nout of the 81 new candidates showed all six sources are active galaxies, albeit\ntwo with narrow emission lines.", "category": "astro-ph_HE" }, { "text": "A dispersion excess from pulsar wind nebulae and supernova remnants:\n Implications for pulsars and FRBs: Young pulsars and the pulsar wind nebulae (PWNe) or supernova remnants (SNRs)\nthat surround them are some of the most dynamic and high-powered environments\nin our Universe. With the rise of more sensitive observations, the number of\npulsar-SNR and PWN associations (hereafter, SNR/PWN) has increased, yet we do\nnot understand to which extent this environment influences the pulsars'\nimpulsive radio signals. We studied the dispersive contribution of SNRs and\nPWNe on Galactic pulsars, and considered their relevance to fast radio bursts\n(FRBs) such as FRB 121102. We investigated the dispersion measure (DM)\ncontribution of SNRs and PWNe by comparing the measured DMs of Galactic pulsars\nin a SNR/PWN to the DM expected only from the intervening interstellar\nelectrons, using the NE2001 model. We find that a two-$\\sigma$ DM contribution\nof SNRs and PWNe to the pulsar signal exists, amounting to $21.1 \\pm 10.6$ pc\ncm$^{-3}$. The control sample of pulsars unassociated with a SNR/PWN shows no\nexcess. We model the SNR and PWN electron densities for each young pulsar in\nour sample and show that these indeed predict an excess of this magnitude. By\nextrapolating to the kind of fast-spinning, high magnetic field, young pulsars\nthat may power FRBs, we show their SNR and PWN are capable of significantly\ncontributing to the observed DM.", "category": "astro-ph_HE" }, { "text": "Returning radiation in strong gravity around black holes: Reverberation\n from the accretion disc: We study reflected X-ray emission that returns to the accretion disc in the\nstrong gravitational fields around black holes using General Relativistic ray\ntracing and radiative transfer calculations. Reflected X-rays that are produced\nwhen the inner regions of the disc are illuminated by the corona are subject to\nstrong gravitational light bending, causing up to 47 per cent of the reflected\nemission to be returned to the disc around a rapidly spinning black hole,\ndepending upon the scale height of the corona. The iron K line is enhanced\nrelative to the continuum by 25 per cent, and the Compton hump by up to a\nfactor of three. Additional light travel time between primary and secondary\nreflections increases the reverberation time lag measured in the iron K band by\n49 per cent, while the soft X-ray lag is increased by 25 per cent and the\nCompton hump response time is increased by 60 per cent. Measured samples of\nX-ray reverberation lags are shown to be consistent with X-rays returning to\nthe accretion disc in strong gravity. Understanding the effects of returning\nradiation is important in interpreting reverberation observations to probe\nblack holes. Reflected X-rays returning to the disc can be uniquely identified\nby blueshifted returning iron K line photons that are Compton scattered from\nthe inner disc, producing excess, delayed emission in the 3.5-4.5keV energy\nrange that will be detectable with forthcoming X-ray observatories,\nrepresenting a unique test of General Relativity in the strong field limit.", "category": "astro-ph_HE" }, { "text": "Multiwavelength flaring activity of PKS1510-089: Aims: In this work, we analyse the multiwavelength brightness variations and\nflaring activity of FSRQ PKS1510-089, aiming to constrain the position of the\nemission sources.\n Methods: We report 7 mm (43 GHz) radio and R-band polarimetric observations\nof PKS1510-089. The radio observations were performed at the Itapetinga Radio\nObservatory, while the polarimetric data were obtained at the Pico dos Dias\nObservatory. The 7 mm observations cover the period between 2011 and 2013,\nwhile the optical polarimetric observations were made between 2009 and 2012.\n Results: At 7 mm, we detected a correlation between four radio and\n$\\gamma$-ray flares with a delay of about 54 days between them; the higher\nfrequency counterpart occurred first. Using optical polarimetry, we detected a\nlarge variation in polarization angle (PA) within two days associated with the\nbeginning of a $\\gamma$-ray flare. Complementing our data with other data\nobtained in the literature, we show that PA presented rotations associated with\nthe occurrence of flares.\n Conclusions: Our results can be explained by a shock-in-jet model, in which a\nnew component is formed in the compact core producing an optical and/or\n$\\gamma$-ray flare, propagates along the jet, and after some time becomes\noptically thin and is detected as a flare at radio frequencies. The variability\nin the polarimetric parameters can also be reproduced; we can explain large\nvariation in both PA and polarization degree (PD), in only one of them, or in\nneither, depending on the differences in PA and PD between the jet and the new\ncomponent.", "category": "astro-ph_HE" }, { "text": "Equation-of-State Constraints on the Neutron-Star Binding Energy and\n Tests of Binary Formation Scenarios: The second supernova that forms double-neutron-star systems is expected to\noccur in a progenitor that is ultra-stripped due to binary interactions. Thus,\nthe secondary neutron star's mass as well as the post-supernova binary's\norbital parameters will depend on the nature of the collapsing progenitor core.\nSince neutron stars are in the strong-gravity regime, their binding energy\nmakes up a significant fraction of their total mass-energy budget. The second\nneutron star's binding energy may thus provide a unique insight as to whether\nits progenitor was a low-mass iron core or an oxygen-neon-magnesium core. I\nobtain relations for the baryonic mass and binding energy incorporating both a\nhadronic equation-of-state catalog as well as recent multi-messenger\nneutron-star observations. With these relations, I obtain the first tight\nconstraints on the baryonic mass and binding energy of three neutron stars that\nare thought to have formed from an ultra-stripped progenitor. With these\nconstraints, I test if each neutron star is consistent with forming from an\nONeMg core that undergoes an electron-capture supernova. From these tests, I\nfind that this scenario can be ruled out for one of three neutron stars.\nNeutron-star properties and the dense-matter equation of state can thus help\ndistinguish binary formation scenarios.", "category": "astro-ph_HE" }, { "text": "Evidence of a non-conservative mass transfer in the ultra-compact X-ray\n source XB 1916-053: The dipping source XB 1916-053 is a compact binary system with an orbital\nperiod of 50 min harboring a neutron star. Using ten new {\\it Chandra}\nobservations and one {\\it Swift/XRT} observation, we are able to extend the\nbaseline of the orbital ephemeris; this allows us to exclude some models that\nexplain the dip arrival times. The Chandra observations provide a good plasma\ndiagnostic of the ionized absorber and allow us to determine whether it is\nplaced at the outer rim of the accretion disk or closer to the compact object.\nFrom the available observations we are able to obtain three new dip arrival\ntimes extending the baseline of the orbital ephemeris from 37 to 40 years. From\nthe analysis of the dip arrival times we confirm an orbital period derivative\nof $\\dot{P}=1.46(3) \\times 10^{-11}$ s s$^{-1}$. We show that the $\\dot{P}$\nvalue and the luminosity values are compatible with a mass accretion rate lower\nthan 10\\% of the mass transfer rate. We show that the mass ratio $q=m_2/m_1$ of\n0.048 explains the apsidal precession period and the nodal precession period.\nThe observed absorption lines are associated with the presence of \\ion{Ne}{x},\n\\ion{Mg}{xii}, \\ion{Si}{xiv}, \\ion{S}{xvi,} and \\ion{Fe}{xxvi} ions. We observe\na redshift in the absorption lines between $1.1 \\times 10^{-3}$ and $1.3 \\times\n10^{-3}$. By interpreting it as gravitational redshift, as recently discussed\nin the literature, we find that the ionized absorber is placed at a distance of\n$10^8$ cm from the neutron star with a mass of 1.4 M$_{\\odot}$ and has a\nhydrogen atom density greater than $10^{15}$ cm$^{-3}$. (Abstract abridged)", "category": "astro-ph_HE" }, { "text": "The Variable Pulsar Wind Nebula of PSR J1809-1917: PSR J1809-1917 is a young ($\\tau=51$ kyr) energetic\n($\\dot{E}=1.8\\times10^{36}$ erg s$^{-1}$) radio pulsar powering a pulsar wind\nnebula (PWN). We report on the results of three Chandra X-ray Observatory\nobservations which show that the PWN consists of a small ($\\sim 20''$) bright\ncompact nebula (CN) and faint extended emission seen up to $2'$ from the\npulsar. The CN is elongated in the northeast-southwest direction and exhibits\nmorphological and flux variability on a timescale of a few months. We also find\nevidence of small arcsecond-scale jets extending from the pulsar along the same\ndirection, and exhibiting a hard power-law (PL) spectrum with photon index\n$\\Gamma_{\\rm jet}=1.2\\pm0.1$. The more extended emission and CN share the same\nsymmetry axis, which is also aligned with the direction toward the TeV\n$\\gamma$-ray source HESS J1809--193, supporting their association. The spectrum\nof the extended nebula (EN) fits an absorbed PL with about the same slope as\nthat of the CN, $\\Gamma_{\\rm CN}\\approx\\Gamma_{\\rm EN}=1.55\\pm0.09$; no\nspectral changes across the EN's 2 pc extent are seen. The total PWN 0.5-8 keV\nluminosity is $L_{\\rm PWN}\\approx 9\\times10^{32}$ erg s$^{-1}$, about half of\nwhich is due to the EN.", "category": "astro-ph_HE" }, { "text": "Magnetosphere of an orbiting neutron star: We conduct force-free simulations of a single neutron star undergoing orbital\nmotion in flat spacetime, mimicking the trajectory of the star about the center\nof mass on a compact binary system. Our attention is focused on the kinetic\nenergy being extracted from the orbit by the acceleration of the magnetic\ndipole moment of the neutron star, and particularly, on how this energy gets\ndistributed within its surrounding magnetosphere. A detailed study of the\nresulting magnetospheric configurations in our setting is presented,\nincorporating as well the effects due to neutron star spin and the misalignment\nof the magnetic and orbital axes. We find many features resembling those of\npulsar magnetospheres for the orbiting neutron star --even in the absence of\nspin--, being of particular interest the development of a spiral current sheet\nthat extends beyond the light cylinder. Then, we use recent advances in pulsar\ntheory to estimate electromagnetic emissions produced at the reconnection\nregions of such current sheets.", "category": "astro-ph_HE" }, { "text": "The Redshift Dependence of the Radio Flux of Gamma-Ray Bursts and Their\n Host Galaxies: Using multiwavelength observations of radio afterglows, we confirm the\nhypothesis that the flux density of gamma-ray bursts (GRBs) at a fixed\nobserving frequency is invariable when the distance of the GRBs increases,\nwhich means the detection rate will be approximately independent of redshift.\nWe study this behavior theoretically and find that it can be well explained by\nthe standard forward shock model involving a thin shell expanding in either a\nhomogeneous interstellar medium (ISM) or a wind environment. We also found that\nshort GRBs and supernova-associated GRBs, which are at relatively smaller\ndistances, marginally match the flux-redshift relationship and they could be\noutliers. We rule out the assumption that the medium density evolves with\nredshift as $n\\propto(1+z)^4$ from the current measurements of $n$ and $z$ for\nshort and long GRBs. In addition, the possible dependence of host flux on the\nredshift is also investigated. We find that a similar redshift independence of\nthe flux exists for host galaxies as well, which implies that the detection\nrate of radio hosts might also be independent of the redshift. It is also\nhinted that most radio hosts have the spectral indices ranging from\n$\\beta_h\\simeq-1$ to 2.5 in statistics. Finally, we predict the detection rates\nof radio afterglows by the next-generation radio telescopes such as the\nFive-hundred meter Aperture Spherical Telescope (FAST) and the Square Kilometer\nArray (SKA).", "category": "astro-ph_HE" }, { "text": "Searching for TeV Dark Matter in Irregular dwarf galaxies with HAWC\n Observatory: We present the results of dark matter (DM) searches in a sample of 31 dwarf\nirregular (dIrr) galaxies within the field of view of the HAWC Observatory.\ndIrr galaxies are DM dominated objects, which astrophysical gamma-ray emission\nis estimated to be negligible with respect to the secondary gamma-ray flux\nexpected by annihilation or decay of Weakly Interacting Massive Particles\n(WIMPs). While we do not see any statistically significant DM signal in dIrr\ngalaxies, we present the exclusion limits ($95\\%~\\text{C.L.}$) for annihilation\ncross-section and decay lifetime for WIMP candidates with masses between $1$\nand $100~\\text{TeV}$. Exclusion limits from dIrr galaxies are relevant and\ncomplementary to benchmark dwarf Spheroidal (dSph) galaxies. In fact, dIrr\ngalaxies are targets kinematically different from benchmark dSph, preserving\nthe footprints of different evolution histories. We compare the limits from\ndIrr galaxies to those from ultrafaint and classical dSph galaxies previously\nobserved with HAWC. We find that the contraints are comparable to the limits\nfrom classical dSph galaxies and $\\thicksim2$ orders of magnitude weaker than\nthe ultrafaint dSph limits.", "category": "astro-ph_HE" }, { "text": "From Observations near the Earth to the Local Interstellar Spectra: Propagation of cosmic rays (CRs) from their sources to the observer is\ndescribed mainly as plain diffusion at high energies, while at lower energies\nthere are other physical processes involved, both in the interstellar space and\nin the heliosphere. The latter was a subject of considerable uncertainty until\nrecently. New data obtained by several CR missions can be used to and the local\ninterstellar spectra (LIS) of CR species that would significantly reduce the\nuncertainties associated with the heliospheric propagation. In this paper we\npresent the LIS of CR protons and helium outside the heliospheric boundary. The\nproposed LIS are tuned to accommodate both, the low energy CR spectra measured\nby Voyager 1, and the high energy observations publicly released by BESS,\nPamela, AMS-01 and AMS-02. The proton and helium LIS are derived by combining\nCR propagation in the Galaxy, as described by GALPROP, with the heliospheric\nmodulation computed using the HelMod Monte Carlo Tool. The proposed LIS are\ntuned to reproduce the modulated spectra for both, high and low, levels of\nsolar activity.", "category": "astro-ph_HE" }, { "text": "The potential role of binary neutron star merger afterglows in\n multimessenger cosmology: Binary neutron star mergers offer a new and independent means of measuring\nthe Hubble constant $H_0$ by combining the gravitational-wave inferred source\nluminosity distance with its redshift obtained from electromagnetic follow-up.\nThis method is limited by intrinsic degeneracy between the system distance and\norbital inclination in the gravitational-wave signal. Observing the afterglow\ncounterpart to a merger can further constrain the inclination angle, allowing\nthis degeneracy to be partially lifted and improving the measurement of $H_0$.\nIn the case of the binary neutron star merger GW170817, afterglow light-curve\nand imagery modeling thus allowed to improve the $H_0$ measurement by a factor\nof 3. However, systematic access to afterglow data is far from guaranteed. In\nfact, though each one allows a leap in $H_0$ precision, these afterglow\ncounterparts should prove rare in forthcoming multimessenger campaigns. We\ncombine models for emission and detection of gravitational-wave and\nelectromagnetic radiation from binary neutron star mergers with realistic\npopulation models and estimates for afterglow inclination angle constraints.\nUsing these models, we quantify how fast $H_0$ will be narrowed-down by\nsuccessive multimessenger events with and without the afterglow. We find that,\nbecause of its rareness and though it greatly refines angle estimates, the\nafterglow counterpart should not significantly contribute to the measurement of\n$H_0$ in the long run.", "category": "astro-ph_HE" }, { "text": "A Wolf-Rayet-like progenitor of supernova SN 2013cu from spectral\n observations of a wind: The explosive fate of massive stripped Wolf-Rayet (W-R) stars is a key open\nquestion in stellar physics. An appealing option is that hydrogen-deficient W-R\nstars are the progenitors of some H-poor supernova (SN) explosions of Types\nIIb, Ib, and Ic. A blue object, having luminosity and colors consistent with\nthose of some W-R stars, has been recently identified at the location of a\nSN~Ib in pre-explosion images but has not yet been conclusively determined to\nhave been the progenitor. Similar previous works have so far only resulted in\nnondetections. Comparison of early photometric observations of Type Ic\nsupernovae with theoretical models suggests that the progenitor stars had radii\n<10^12 cm, as expected for some W-R stars. However, the hallmark signature of\nW-R stars, their emission-line spectra, cannot be probed by such studies. Here,\nwe report the detection of strong emission lines in an early-time spectrum of\nSN 2013cu (iPTF13ast; Type IIb) obtained ~15.5 hr after explosion (\"flash\nspectroscopy\"). We identify W-R-like wind signatures suggesting a progenitor of\nthe WN(h) subclass. The extent of this dense wind may indicate increased mass\nloss from the progenitor shortly prior to its explosion, consistent with recent\ntheoretical predictions.", "category": "astro-ph_HE" }, { "text": "Peculiarly Narrow SED of GRB 090926B with MAXI and Fermi/GBM: The monitor of all-sky X-ray image (MAXI) Gas Slit Camera (GSC) on the\nInternational Space Station (ISS) detected a gamma-ray burst (GRB) on 2009,\nSeptember 26, GRB\\,090926B. This GRB had extremely hard spectra in the X-ray\nenergy range. Joint spectral fitting with the Gamma-ray Burst Monitor on the\nFermi Gamma-ray Space Telescope shows that this burst has peculiarly narrow\nspectral energy distribution and is represented by Comptonized blackbody model.\nThis spectrum can be interpreted as photospheric emission from the low\nbaryon-load GRB fireball. Calculating the parameter of fireball, we found the\nsize of the base of the flow $r_0 = (4.3 \\pm 0.9) \\times 10^{9} \\, Y^{\\prime \\,\n-3/2}$ cm and Lorentz factor of the plasma $\\Gamma = (110 \\pm 10) \\, Y^{\\prime\n\\, 1/4}$, where $Y^{\\prime}$ is a ratio between the total fireball energy and\nthe energy in the blackbody component of the gamma-ray emission. This $r_0$ is\nfactor of a few larger, and the Lorentz factor of 110 is smaller by also factor\nof a few than other bursts that have blackbody components in the spectra.", "category": "astro-ph_HE" }, { "text": "Radio Analysis of SN 2004C Reveals an Unusual CSM Density Profile as a\n Harbinger of Core Collapse: We present extensive multi-frequency VLA and VLBA observations of the\nradio-bright supernova (SN) IIb SN 2004C that span $\\sim(40-2793)$ days\npost-explosion. We interpret the temporal evolution of the radio spectral\nenergy distribution (SED) in the context of synchrotron self-absorbed (SSA)\nemission from the explosion's forward shock as it expands in the circumstellar\nmedium (CSM) previously sculpted by the mass-loss history of the stellar\nprogenitor. VLBA observations and modeling of the VLA data point to a blastwave\nwith average velocity $\\sim0.06c$ that carries an energy of $\\sim 10^{49}$ erg.\nOur modeling further reveals a flat CSM density profile $\\rho_{\\rm{CSM}}\n\\propto R^{-0.03 \\pm0.22}$ up to a break radius $R_{br} \\approx (1.96 \\pm 0.10)\n\\times 10^{16}$ cm, with a steep density gradient following $\\rho_{\\rm{CSM}}\n\\propto R^{-2.3 \\pm 0.5}$ at larger radii. We infer that the flat part of the\ndensity profile corresponds to a CSM shell with mass $\\sim0.021 M_{\\odot}$, and\nthat the progenitor's effective mass-loss rate varied with time over the range\n$(50-500) \\times 10^{-5} M_{\\odot} \\rm{yr}^{-1}$ for an adopted wind velocity\n$v_w =1000$ km $s^{-1}$ and shock microphysical parameters ${\\epsilon}_e = 0.1,\n{\\epsilon}_B = 0.01$. These results add to the mounting observational evidence\nfor departures from the traditional single-wind mass-loss scenarios in evolved,\nmassive stars in the centuries leading up to core collapse. Potentially viable\nscenarios include mass loss powered by gravity waves and/or interaction with a\nbinary companion.", "category": "astro-ph_HE" }, { "text": "Consequences of a Dark Disk for the Fermi and PAMELA Signals in Theories\n with a Sommerfeld Enhancement: Much attention has been given to dark matter explanations of the PAMELA\npositron fraction and Fermi electronic excesses. For those theories with a\nTeV-scale WIMP annihilating through a light force-carrier, the associated\nSommerfeld enhancement provides a natural explanation of the large boost factor\nneeded to explain the signals, and the light force-carrier naturally gives rise\nto hard cosmic ray spectra without excess pi0 gamma rays or anti-protons. The\nSommerfeld enhancement of the annihilation rate, which at low relative\nvelocities v scales as 1/v, relies on the comparatively low velocity dispersion\nof the dark matter particles in the smooth halo. Dark matter substructures in\nwhich the velocity dispersion is smaller than in the smooth halo have even\nlarger annihilation rates. N-body simulations containing only dark matter\npredict the existence of such structures, for example subhalos and caustics,\nand the effects of these substructures on dark matter indirect detection\nsignals have been studied extensively. The addition of baryons into\ncosmological simulations of disk-dominated galaxies gives rise to an additional\nsubstructure component, a dark disk. The disk has a lower velocity dispersion\nthan the spherical halo component by a factor ~6, so the contributions to dark\nmatter signals from the disk can be more significant in Sommerfeld models than\nfor WIMPs without such low-velocity enhancements. We consider the consequences\nof a dark disk on the observed signals of cosmic rays as measured by Fermi and\nPAMELA in models where the WIMP annihilations are into a light boson. We find\nthat both the PAMELA and Fermi results are easily accomodated by scenarios in\nwhich a disk signal is included with the standard spherical halo signal. Limits\nfrom extrapolations to the center of the galaxy can also be modified.", "category": "astro-ph_HE" }, { "text": "Chandra Phase-Resolved Spectroscopy of the High Magnetic Field Pulsar\n B1509-58: We report on a timing and spectral analysis of the young, high magnetic field\nrotation-powered pulsar (RPP) B1509-58 using Chandra continuous-clocking mode\nobservation. The pulsar's X-ray light curve can be fit by the two Gaussian\ncomponents and the pulsed fraction shows moderate energy dependence over the\nChandra band. The pulsed X-ray spectrum is well described by a power law with a\nphoton index 1.16(4), which is harder than the values measured with RXTE PCA\nand NuSTAR. This result supports the log-parabolic model for the broadband\nX-ray spectrum. With the unprecedented angular resolution of Chandra, we\nclearly identified off-pulse X-ray emission from the pulsar, and its spectrum\nis best fit by a power law plus blackbody model. The latter component has a\ntemperature of ~0.14 keV with a bolometric luminosity comparable to the\nluminosities of other young and high magnetic field RPPs, and it lies between\nthe temperature of magnetars and typical RPPs. In addition, we found that the\nnonthermal X-ray emission of PSR B1509-58 is significantly softer in the\noff-pulse phase than in the pulsed phase, with the photon index varying between\n1.0 and 1.8 and anticorrelated with the flux. This is similar to the behavior\nof three other young pulsars. We interpreted it as different contributions of\npair-creation processes at different altitudes from the neutron star surface\naccording to the outer-gap model.", "category": "astro-ph_HE" }, { "text": "Evolution of the radio - X-ray coupling throughout an entire outburst of\n Aquila X-1: The 2009 November outburst of the neutron star X-ray binary Aquila X-1 was\nobserved with unprecedented radio coverage and simultaneous pointed X-ray\nobservations, tracing the radio emission around the full X-ray hysteresis loop\nof the outburst for the first time. We use these data to discuss the disc-jet\ncoupling, finding the radio emission to be consistent with being triggered at\nstate transitions, both from the hard to the soft spectral state and vice\nversa. Our data appear to confirm previous suggestions of radio quenching in\nthe soft state above a threshold X-ray luminosity of about 10% of the Eddington\nluminosity. We also present the first detections of Aql X-1 with Very Long\nBaseline Interferometry (VLBI), showing that any extended emission is\nrelatively diffuse, and consistent with steady jets rather than arising from\ndiscrete, compact knots. In all cases where multi-frequency data were\navailable, the source radio spectrum is consistent with being flat or slightly\ninverted, suggesting that the internal shock mechanism that is believed to\nproduce optically thin transient radio ejecta in black hole X-ray binaries is\nnot active in Aql X-1.", "category": "astro-ph_HE" }, { "text": "X-ray Pulsations from the region of the Supergiant Fast X-ray Transient\n IGR J17544-2619: Phase-targeted RXTE observations have allowed us to detect a transient 71.49\n\\pm 0.02 s signal that is most likely to be originating from the supergiant\nfast X-ray transient IGR J17544-2619. The phase-folded light curve shows a\npossible double-peaked structure with a pulsed flux of ~4.8*10^-12 erg cm^-2\ns^-1 (3-10 keV). Assuming the signal to indicate the spin period of the neutron\nstar in the system, the provisional location of IGR J17544-2619 on the Corbet\ndiagram places the system within the classical wind-fed supergiant XRB region.\nSuch a result illustrates the growing trend of supergiant fast X-ray transients\nto span across both of the original classes of HMXB in Porb - Pspin space.", "category": "astro-ph_HE" }, { "text": "StrayCats: A catalog of NuSTAR Stray Light Observations: We present StrayCats: a catalog of NuSTAR stray light observations of X-ray\nsources. Stray light observations arise for sources 1--4$^{\\circ}$ away from\nthe telescope pointing direction. At this off-axis angle, X-rays pass through a\ngap between optics and aperture stop and so do not interact with the X-ray\noptics but, instead, directly illuminate the NuSTAR focal plane. We have\nsystematically identified and examined over 1400 potential observations\nresulting in a catalog of 436 telescope fields and 78 stray light sources that\nhave been identified. The sources identified include historically known\npersistently bright X-ray sources, X-ray binaries in outburst, pulsars, and\nType I X-ray bursters. In this paper we present an overview of the catalog and\nhow we identified the StrayCats sources and the analysis techniques required to\nproduce high level science products. Finally, we present a few brief examples\nof the science quality of these unique data.", "category": "astro-ph_HE" }, { "text": "Spectral and polarization properties of black hole accretion disc\n emission: including absorption effects: The study of radiation emitted from black hole accretion discs represents a\ncrucial way to understand the main physical properties of these sources, and in\nparticular the black hole spin. Beside spectral analysis, polarimetry is\nbecoming more and more important, motivated by the development of new\ntechniques which will soon allow to perform measurements also in the X- and\n{\\gamma}-rays. Photons emitted from black hole accretion discs in the soft\nstate are indeed expected to be polarized, with an energy dependence which can\nprovide an estimate of the black hole spin. Calculations performed so far,\nhowever, considered scattering as the only process to determine the\npolarization state of the emitted radiation, implicitly assuming that the\ntemperatures involved are such that material in the disc is entirely ionized.\nIn this work we generalize the problem by calculating the ionization structure\nof a surface layer of the disc with the public code CLOUDY , and then by\ndetermining the polarization properties of the emerging radiation using the\nMonte Carlo code STOKES . This allows us to account for absorption effects\nalongside scattering ones. We show that including absorption can deeply modify\nthe polarization properties of the emerging radiation with respect to what is\nobtained in the pure-scattering limit. As a general rule, we find that the\npolarization degree is larger when absorption is more important, which occurs\ne.g. for low accretion rates and/or spins when the ionization of the matter in\nthe innermost accretion disc regions is far from complete.", "category": "astro-ph_HE" }, { "text": "Modelling Continuum Reverberation in AGN: A Spectral-Timing Analysis of\n the UV Variability Through X-ray Reverberation in Fairall 9: Continuum reverberation mapping of AGN can provide new insight into the\nnature and geometry of the accretion flow. Some of the X-rays from the central\ncorona irradiating the disc are absorbed, increasing the local disc\ntemperature. This gives an additional re-processed contribution to the spectral\nenergy distribution (SED) which is lagged and smeared relative to the driving\nX-ray light-curve. We directly calculate this reverberation from the accretion\ndisc, creating fully time dependent SEDs for a given X-ray light-curve. We\napply this to recent intensive monitoring data on Fairall 9, and find that it\nis not possible to produce the observed UV variability by X-ray reprocessing of\nthe observed light-curve from the disc. Instead, we find that the majority of\nthe variability must be intrinsic to the UV emission process, adding to\nevidence from changing look AGN that this region has a structure which is quite\nunlike a Shakura-Sunyaev disc. We filter out this long timescale variability\nand find that reprocessing alone is still insufficient to explain even the fast\nvariability in our assumed geometry of a central source illuminating a flat\ndisc. The amplitude of reprocessing can be increased by any vertical structure\nsuch as the BLR and/or an inner disc wind, giving a better match. Fundamentally\nthough the model is missing the major contributor to the variability, intrinsic\nto the UV/EUV emission rather than arising from reprocessing.", "category": "astro-ph_HE" }, { "text": "The chemical enrichment of long-GRB nurseries up to z=2: We investigate the existence of a metallicity threshold for the production of\nlong gamma-ray bursts (LGRBs). We used the host galaxies of the Swift/BAT6\nsample of LGRBs. We considered the stellar mass, star formation rate (SFR), and\nmetallicity determined from the host galaxy photometry and spectroscopy up to z\n= 2 and used them to compare the distribution of host galaxies to that of field\ngalaxies in the mass-metallicity and fundamental metallicity relation plane. We\nfind that although LGRBs also form in galaxies with relatively large stellar\nmasses, the large majority of host galaxies have metallicities below\nlog(O=H)~8.6. The extension to z = 2 results in a good sampling of stellar\nmasses also above Log(Mstar/Msun)~9.5 and provides evidence that LGRB host\ngalaxies do not follow the fundamental metallicity relation. As shown by the\ncomparison with dedicated numerical simulations of LGRB host galaxy population,\nthese results are naturally explained by the existence of a mild (~0.7 Zsun)\nthreshold for the LGRB formation. The present statistics does not allow us to\ndiscriminate between different shapes of the metallicity cutoff, but the\nrelatively high metallicity threshold found in this work is somewhat in\ndisagreement to most of the standard single-star models for LGRB progenitors.", "category": "astro-ph_HE" }, { "text": "Fermi Observation of the transitional pulsar binary XSS J12270-4859: Because of the disappearance of its accretion disk since the time period\naround 2012 November--December, XSS J12270-4859 has recently been identified\nas, in addition to PSR J1023+0038, another transitional millisecond pulsar\nbinary. We have carried out detailed analysis of the Fermi Large Area Telescope\ndata for the binary. While both spectra before and after the disk-disappearance\ntransition are well described by an exponentially cut-off power law, typical\nfor pulsars' emission in the Fermi's 0.2-300 GeV, a factor of 2 flux decrease\nrelated to the transition is detected. A weak orbital modulation is seen, but\nonly detectable in the after-transition data, same to that found at X-rays. In\nthe long-term light curve of the source before the transition, a factor of 3\nflux variations are seen. Comparing to the properties of J1023+0038, we disucss\nthe implications from these results. We suggest that since the modulation is\naligned with that at X-rays in orbital phase, it possibly arises due to the\noccultation of the gamma-ray emitting region by the companion. The origin of\nthe variations in the long-term light curve is not clear, because the source\nfield also contains unidentified radio or X-ray sources and their contamination\ncan not be excluded. Multi-wavelength observations of the source field will\nhelp identify the origin of the variations by detecting any related flux\nchanges from the in-field sources.", "category": "astro-ph_HE" }, { "text": "Cosmic ray Transport in Magnetohydrodynamic turbulence: This paper studies cosmic ray (CR) transport in magneto hydrodynamic (MHD)\nturbulence. CR transport is strongly dependent on the properties of the\nmagnetic turbulence. We perform test particle simulations to study the\ninteractions of CR with both total MHD turbulence and decomposed MHD modes. The\nspatial diffusion coefficients and the pitch angle scattering diffusion\ncoefficients are calculated from the test particle trajectories in turbulence.\nOur results confirm that the fast modes dominate the CR propagation, whereas\nAlfv\\'en and slow modes are much less efficient and have shown similar pitch\nangle scattering rates. We investigate the cross field transport on large and\nsmall scales. On large/global scales, normal diffusion is observed and the\ndiffusion coefficient is suppressed by $M_A^\\zeta$ compared to the parallel\ndiffusion coefficients, with $\\zeta$ closer to 4 in Alfv\\'en modes than that in\ntotal turbulence as theoretically expected. For the CR transport on scales\nsmaller than the turbulence injection scale, both the local and global magnetic\nreference frames are adopted. Super diffusion is observed on such small scales\nin all the cases. Particularly, CR transport in Alfv\\'en modes show clear\nRichardson diffusion in the local reference frame. Our results have broad\napplications to CRs in various astrophysical environments.", "category": "astro-ph_HE" }, { "text": "Radio quasars and the link with GAIA: Modeling VLBI ejections of nuclei of extragalactic radio sources, indicates\nthat their nuclei contain a binary black hole system. One can derive the\ndistance and the positions of the two black holes in the plane of the sky. We\ncan also use the RMS of the time series of the ICRF2 survey to obtain an\nestimate of the structure and the size of the nuclei. We will discuss the\npossible problems to link VLBI observations and GAIA optical observations of\nradio quasars if they contain a binary black hole system.", "category": "astro-ph_HE" }, { "text": "Observations of the Very Young Type Ia Supernova 2019np with\n Early-excess Emission: Early-time radiative signals from type Ia supernovae (SNe Ia) can provide\nimportant constraints on the explosion mechanism and the progenitor system. We\npresent observations and analysis of SN 2019np, a nearby SN Ia discovered\nwithin 1-2 days after the explosion. Follow-up observations were conducted in\noptical, ultraviolet, and near-infrared bands, covering the phases from\n$\\sim-$16.7 days to $\\sim$+367.8 days relative to its $B-$band peak luminosity.\nThe photometric and spectral evolutions of SN 2019np resembles the average\nbehavior of normal SNe Ia. The absolute B-band peak magnitude and the post-peak\ndecline rate are $M_{\\rm max}(B)=-19.52 \\pm 0.47$mag and $\\Delta m_{\\rm15}(B)\n=1.04 \\pm 0.04$mag, respectively. No Hydrogen line has been detected in the\nnear-infrared and nebular-phase spectra of SN 2019np. Assuming that the\n$^{56}$Ni powering the light curve is centrally located, we find that the\nbolometric light curve of SN 2019np shows a flux excess up to 5.0% in the early\nphase compared to the radiative diffusion model. Such an extra radiation\nperhaps suggests the presence of an additional energy source beyond the\nradioactive decay of central nickel. Comparing the observed color evolution\nwith that predicted by different models such as interactions of SN ejecta with\ncircumstellar matter (CSM)/companion star, a double-detonation explosion from a\nsub-Chandrasekhar mass white dwarf (WD), and surface $^{56}$Ni mixing, the\nlatter one is favored.", "category": "astro-ph_HE" }, { "text": "Mass composition of ultra-high-energy cosmic rays with the Telescope\n Array Surface Detector Data: The results on ultra-high-energy cosmic rays (UHECR) mass composition\nobtained with the Telescope Array surface detector are presented. The analysis\nemploys the boosted decision tree (BDT) multivariate analysis built upon 14\nobservables related to both the properties of the shower front and the lateral\ndistribution function. The multivariate classifier is trained with Monte-Carlo\nsets of events induced by the primary protons and iron. An average atomic mass\nof UHECR is presented for energies $10^{18.0}-10^{20.0}\\ \\mbox{eV}$. The\naverage atomic mass of primary particles shows no significant energy dependence\nand corresponds to $\\langle \\ln A \\rangle = 2.0 \\pm 0.1 (stat.) \\pm 0.44\n(syst.)$. The result is compared to the mass composition obtained by the\nTelescope Array with $\\mbox{X}_{\\mbox{max}}$ technique along with the results\nof other experiments. Possible systematic errors of the method are discussed.", "category": "astro-ph_HE" }, { "text": "A Model for Pair Production Limit Cycles in Pulsar Magnetospheres: It was recently proposed that the electric field oscillation as a result of\nself-consistent $e^{\\pm}$ pair production may be the source of coherent radio\nemission from pulsars. Direct Particle-in-Cell (PIC) simulations of this\nprocess have shown that the screening of the parallel electric field by this\npair cascade manifests as a limit cycle, as the parallel electric field is\nrecurrently induced when pairs produced in the cascade escape from the gap\nregion. In this work, we develop a simplified time-dependent kinetic model of\n$e^{\\pm}$ pair cascades in pulsar magnetospheres that can reproduce the\nlimit-cycle behavior of pair production and electric field screening. This\nmodel includes the effects of a magnetospheric current, the escape of\n$e^{\\pm}$, as well as the dynamic dependence of pair production rate on the\nplasma density and energy. Using this simple theoretical model, we show that\nthe power spectrum of electric field oscillations averaged over many limit\ncycles is compatible with the observed pulsar radio spectrum.", "category": "astro-ph_HE" }, { "text": "Magnetar Activity via the Density-Shear Instability in Hall-MHD: We investigate the density-shear instability in Hall-MHD via numerical\nsimulation of the full non-linear problem, in the context of magnetar activity.\nWe confirm the development of the instability of a plane-parallel magnetic\nfield with an appropriate intensity and electron density profile, in accordance\nwith analytic theory. We find that the instability also appears for a\nmonotonically decreasing electron number density and magnetic field, a\nplane-parallel analogue of an azimuthal or meridional magnetic field in the\ncrust of a magnetar. The growth rate of the instability depends on the Hall\nproperties of the field (magnetic field intensity, electron number density and\nthe corresponding scale-heights), while being insensitive to weak resistivity.\nSince the Hall effect is the driving process for the evolution of the crustal\nmagnetic field of magnetars, we argue that this instability is critical for\nsystems containing strong meridional or azimuthal fields. We find that this\nprocess mediates the formation of localised structures with much stronger\nmagnetic field than the average, which can lead to magnetar activity and\naccelerate the dissipation of the field and consequently the production of\nOhmic heating. Assuming a $5\\times10^{14}$G magnetic field at the base of\ncrust, we anticipate that magnetic field as strong as $10^{15}$G will easily\ndevelop in regions of typical size of a few $10^{2}$ meters, containing\nmagnetic energy of $10^{43}$erg, sufficient to power magnetar bursts. These\nactive regions are more likely to appear in the magnetic equator where the\ntangential magnetic field is stronger.", "category": "astro-ph_HE" }, { "text": "An Ultra-Fast X-ray Disk Wind in the Neutron Star Binary GX 340+0: We present a spectral analysis of a brief Chandra/HETG observation of the\nneutron star low-mass X-ray binary GX~340+0. The high-resolution spectrum\nreveals evidence of ionized absorption in the Fe K band. The strongest feature,\nan absorption line at approximately 6.9 keV, is required at the 5 sigma level\nof confidence via an F-test. Photoionization modeling with XSTAR grids suggests\nthat the line is the most prominent part of a disk wind with an apparent\noutflow speed of v = 0.04c. This interpretation is preferred at the 4 sigma\nlevel over a scenario in which the line is H-like Fe XXVI at a modest\nred-shift. The wind may achieve this speed owing to its relatively low\nionization, enabling driving by radiation pressure on lines; in this sense, the\nwind in GX 340+0 may be the stellar-mass equivalent of the flows in broad\nabsorption line quasars (BALQSOs). If the gas has a unity volume filling\nfactor, the mass ouflow rate in the wind is over 10^-5 Msun/year, and the\nkinetic power is nearly 10^39 erg/s (or, 5-6 times the radiative Eddington\nlimit for a neutron star). However, geometrical considerations - including a\nsmall volume filling factor and low covering factor - likely greatly reduce\nthese values.", "category": "astro-ph_HE" }, { "text": "The blazar TXS 0506+056 associated with a high-energy neutrino: insights\n into extragalactic jets and cosmic ray acceleration: A neutrino with energy of $\\sim$290 TeV, IceCube-170922A, was detected in\ncoincidence with the BL Lac object TXS~0506+056 during enhanced gamma-ray\nactivity, with chance coincidence being rejected at $\\sim 3\\sigma$ level. We\nmonitored the object in the very-high-energy (VHE) band with the MAGIC\ntelescopes for $\\sim$41 hours from 1.3 to 40.4 days after the neutrino\ndetection. Day-timescale variability is clearly resolved. We interpret the\nquasi-simultaneous neutrino and broadband electromagnetic observations with a\nnovel one-zone lepto-hadronic model, based on interactions of electrons and\nprotons co-accelerated in the jet with external photons originating from a\nslow-moving plasma sheath surrounding the faster jet spine. We can reproduce\nthe multiwavelength spectra of TXS 0506+056 with neutrino rate and energy\ncompatible with IceCube-170922A, and with plausible values for the jet power of\n$\\sim 10^{45} - 4 \\times 10^{46} {\\rm erg \\ s^{-1}}$. The steep spectrum\nobserved by MAGIC is concordant with internal $\\gamma\\gamma$ absorption above a\nfew tens of GeV entailed by photohadronic production of a $\\sim$290 TeV\nneutrino, corroborating a genuine connection between the multi-messenger\nsignals. In contrast to previous predictions of predominantly hadronic emission\nfrom neutrino sources, the gamma-rays can be mostly ascribed to inverse Compton\nup-scattering of external photons by accelerated electrons. The X-ray and VHE\nbands provide crucial constraints on the emission from both accelerated\nelectrons and protons. We infer that the maximum energy of protons in the jet\nco-moving frame can be in the range $\\sim 10^{14}$ to $10^{18}$ eV.", "category": "astro-ph_HE" }, { "text": "Theoretical Delay Time Distributions: We briefly discuss the method of population synthesis to calculate\ntheoretical delay time distributions of type Ia supernova progenitors. We also\ncompare the results of the different research groups and conclude that although\none of the main differences in the results for single degenerate progenitors is\nthe retention efficiency with which accreted hydrogen is added to the white\ndwarf core, this cannot explain all the differences.", "category": "astro-ph_HE" }, { "text": "Mass Ratios of Merging Double Neutron Stars as Implied by the Milky Way\n Population: Of the seven known double neutron stars (DNS) with precisely measure masses\nin the Milky Way that will merge within a Hubble time, all but one has a mass\nratio, $q$, close to unity. Recently, precise measurements of three\npost-Keplerian parameters in the DNS J1913$+$1102 constrain this system to have\na significantly non-unity mass ratio of 0.78$\\pm$0.03. One may be tempted to\nconclude that approximately one out of seven (14\\%) DNS mergers detected by\ngravitational wave observatories will have mass ratios significantly different\nfrom unity. However J1913$+$1102 has a relatively long merger time of 470 Myr.\nWe show that when merger times and observational biases are taken into account,\nthe population of Galactic DNSs imply that $\\simeq98\\%$ of all merging DNSs\nwill have $q>$0.9. We then apply two separate fitting formulas informed by 3D\nhydrodynamic simulations of DNS mergers to our results on Galactic DNS masses,\nfinding that either $\\simeq$0.004 ${M_{\\odot}}$ or $\\simeq$0.010 ${M_{\\odot}}$\nof material will be ejected at merger, depending on which formula is used.\nThese ejecta masses have implications for both the peak bolometric luminosities\nof electromagnetic counterparts (which we find to be $\\sim$10$^{41}$ erg\ns$^{-1}$) as well as the $r$-process enrichment of the Milky Way.", "category": "astro-ph_HE" }, { "text": "Obscured $pp$-channel neutrino sources: We explore the possibility that the astrophysical neutrinos are produced in\n$pp$-interactions with a gas cloud near the source acting as a beam dump, which\nis sufficiently dense to significantly attenuate the associated gamma-ray flux\nthrough pair-production on this gas. In this way, such sources could\npotentially supply the astrophysical neutrino flux whilst avoiding the existing\nconstraints on the non-blazar contribution to the extragalactic gamma-ray\nbackground. After defining our model, we implement a Monte Carlo simulation and\napply this to different scenarios. First, we investigate a set of active\ngalaxies which exhibit signs of obscuration. We find that, currently, the\nexpected neutrino flux from these objects in our model is below the existing\nexclusion limits, but can already constrain the amount of protons accelerated\nin such sources. Second, we investigate the diffuse neutrino flux generated by\na population of obscured sources. We find that such a population can indeed\nalleviate the tension with the extragalactic background light. We discuss the\npossibility that ultra-luminous infrared galaxies represent such a source\nclass.", "category": "astro-ph_HE" }, { "text": "Wide band observations of the X-ray burster GS 1826-238: GS 1826-238 is a well-studied X-ray bursting neutron star in a low mass\nbinary system. Thermal Comptonisation by a hot electron cloud is a widely\naccepted mechanism accounting for its high energy emission, while the nature of\nmost of its soft X-ray output is not completely understood. A further low\nenergy component is typically needed to model the observed spectra: pure\nblackbody and Comptonisation-modified blackbody radiation by a lower\ntemperature (a few keV) electron plasma were suggested to explain the low\nenergy data. We studied the steady emission of GS 1826-238 by means of broad\nband (X to soft Gamma-rays) measurements obtained by the INTEGRAL observatory\nin 2003 and 2006. The newly developed, up-to-date Comptonisation model CompTB\nis applied for the first time to study effectively the low-hard state\nvariability of a low-luminosity neutron star in a low-mass X-ray binary system.\nWe confirm that the 3-200 keV emission of \\GS is characterised by\nComptonisation of soft seed photons by a hot electron plasma. A single spectral\ncomponent is sufficient to model the observed spectra. At lower energies, no\ndirect blackbody emission is observed and there is no need to postulate a low\ntemperature Compton region. Compared to the 2003 measurements, the plasma\ntemperature decreased from 20 to 14 keV in 2006, together with the seed photons\ntemperature. The source intensity was also found to be 30% lower in 2006,\nwhilst the average recurrence frequency of the X-ray bursts significantly\nincreased. Possible explanations for this apparent deviation from the typical\nlimit-cycle behaviour of this burster are discussed.", "category": "astro-ph_HE" }, { "text": "High Energy Neutrinos from the Gravitational Wave event GW150914\n possibly associated with a short Gamma-Ray Burst: High-energy neutrino (HEN) and gravitational wave (GW) can probe\nastrophysical sources in addition to electromagnetic observations.\nMultimessenger studies can reveal nature of the sources which may not be\ndiscerned from one type of signal alone. We discuss HEN emission in connection\nwith the Advanced Laser Interferometer Gravitational-wave Observatory (ALIGO)\nevent GW150914 which could be associated with a short gamma-ray burst (GRB)\ndetected by the $Fermi$ Gamma-ray Burst Monitor (GBM) 0.4 s after the GW event\nand within localization uncertainty of the GW event. We calculate HEN flux from\nthis short GRB, GW150914-GBM, and show that non-detection of a high-energy\nstarting event (HESE) by the IceCube Neutrino Observatory can constrain the\ntotal isotropic-equivalent jet energy of this short burst to be less than\n$3\\times 10^{52}$ erg.", "category": "astro-ph_HE" }, { "text": "Simulations of Magnetic Fields in Tidally-Disrupted Stars: We perform the first magnetohydrodynamical simulations of tidal disruptions\nof stars by supermassive black holes. We consider stars with both tangled and\nordered magnetic fields, for both grazing and deeply disruptive encounters.\nWhen the star survives disruption, we find its magnetic field amplifies by a\nfactor of up to twenty, but see no evidence for the a self-sustaining dynamo\nthat would yield arbitrary field growth. For stars that do not survive, and\nwithin the tidal debris streams produced in partial disruptions, we find that\nthe component of the magnetic field parallel to the direction of stretching\nalong the debris stream only decreases slightly with time, eventually resulting\nin a stream where the magnetic pressure is in equipartition with the gas. Our\nresults suggest that the returning gas in most (if not all) stellar tidal\ndisruptions is already highly magnetized by the time it returns to the black\nhole.", "category": "astro-ph_HE" }, { "text": "Spectro-Timing Analysis of a highly variable narrow-line Seyfert 1\n galaxy NGC 4748 with AstroSat and XMM-Newton: We present a detailed timing and spectral study of an extremely variable\nnarrow-line Seyfert~1 galaxy NGC 4748 using observations in the year 2017 and\n2014 performed with AstroSat and XMM-Newton, respectively. Both observations\nshow extremely variable soft and hard X-ray emission that are correlated with\neach other. In the 2014 data set, the source retains its general behaviour of\n\"softer when brighter\" while the 2017 observation exhibits a \"harder when\nbrighter\" nature. Such changing behaviour is rare in AGNs and is usually\nobserved in the black hole binary systems. The \"harder when brighter\" is\nconfirmed with the anti-correlation between the photon index and the 0.3-10 keV\npower-law flux. This suggests a possible change in the accretion mode from\nstandard to the advection-dominated flow. Additionally, both the observations\nshow soft X-ray excess below 2 keV over the power-law continuum. This excess\nwas fitted with a single or multiple blackbody component(s). The origin of soft\nexcess during the 2017 observation is likely due to the cool Comptonization as\nthe photon index changes with time. On the other hand, the broad iron line and\ndelayed UV emission during the 2014 observation strongly suggest that X-ray\nillumination onto the accretion disk and reflection and reprocessing play a\nsignificant role in this AGN.", "category": "astro-ph_HE" }, { "text": "The Konus-Wind catalog of gamma-ray bursts with known redshifts. II.\n Waiting mode bursts simultaneously detected by Swift/BAT: In the Second part of The Konus-Wind Catalog of Gamma-Ray Bursts with Known\nRedshifts (first part: Tsvetkova et al. 2017; T17), we present the results of a\nsystematic study of gamma-ray bursts (GRBs) with reliable redshift estimates\ndetected simultaneously by the Konus-Wind (KW) experiment (in the waiting mode)\nand by the Swift/BAT (BAT) telescope during the period from 2005 January to the\nend of 2018. By taking advantage of the high sensitivity of BAT and the wide\nspectral band of KW we were able to constrain the peak spectral energies, the\nbroadband energy fluences, and the peak fluxes for the joint KW-BAT sample of\n167 weak, relatively soft GRBs (including four short bursts). Based on the GRB\nredshifts, which span the range $0.04 \\leq z \\leq 9.4$, we estimate the\nrest-frame, isotropic-equivalent energy, and peak luminosity. For 14 GRBs with\nreasonably constrained jet breaks, we provide the collimation-corrected values\nof the energetics. This work extends the sample of KW GRBs with known redshifts\nto 338 GRBs, the largest set of cosmological GRBs studied to date over a broad\nenergy band. With the full KW sample, accounting for the instrumental bias, we\nexplore GRB rest-frame properties, including hardness-intensity correlations,\nGRB luminosity evolution, luminosity and isotropic-energy functions, and the\nevolution of the GRB formation rate, which we find to be in general agreement\nwith those reported in T17 and other previous studies.", "category": "astro-ph_HE" }, { "text": "Massive non-thermal radio emitters: new data and their modelling: During recent years some non-thermal radio emitting OB stars have been\ndiscovered to be binary, or multiple systems. The non-thermal emission is due\nto synchrotron radiation that is emitted by electrons accelerated up to high\nenergies. The electron acceleration occurs at the strong shocks created by the\ncollision of radiatively-driven winds. Here we summarize the available radio\ndata and more recent observations for the binary Cyg OB2 No. 9. We also show a\nnew emission model which is being developed to compare the theoretical total\nradio flux and the spectral index with the observed radio light curves. This\ncomparison will be useful in order to solve fundamental questions, such as the\ndetermination of the stellar mass loss rates, which are perturbed by clumping.", "category": "astro-ph_HE" }, { "text": "Puffy accretion disks: sub-Eddington, optically thick, and stable: We report on a new class of solutions of black hole accretion disks that we\nhave found through three-dimensional, global, radiative magnetohydrodynamic\nsimulations in general relativity.\n It combines features of the canonical thin, slim and thick disk models but\ndiffers in crucial respects from each of them. We expect these new solutions to\nprovide a more realistic description of black hole disks than the slim disk\nmodel. We are presenting a disk solution for a non-spinning black hole at a\nsub-Eddington mass accretion rate, $\\dot M=0.6\\,\\dot M_{\\rm Edd}$. By the\ndensity scale-height measure the disk appears to be thin, having a high density\ncore near the equatorial plane of height $h_{\\rho} \\sim 0.1 \\,r$, but most of\nthe inflow occurs through a highly advective, turbulent, optically thick,\nKeplerian region that sandwiches the core and has a substantial geometrical\nthickness comparable to the radius, $H \\sim r$. The accreting fluid is\nsupported above the midplane in large part by the magnetic field, with the gas\nand radiation to magnetic pressure ratio $\\beta \\sim 1$, this makes the disk\nthermally stable, even though the radiation pressure strongly dominates over\ngas pressure. A significant part of the radiation emerging from the disk is\ncaptured by the black hole, so the disk is less luminous than a thin disk would\nbe at the same accretion rate.", "category": "astro-ph_HE" }, { "text": "Cosmic ray Transport in Magnetohydrodynamic turbulence: This paper studies cosmic ray (CR) transport in magneto hydrodynamic (MHD)\nturbulence. CR transport is strongly dependent on the properties of the\nmagnetic turbulence. We perform test particle simulations to study the\ninteractions of CR with both total MHD turbulence and decomposed MHD modes. The\nspatial diffusion coefficients and the pitch angle scattering diffusion\ncoefficients are calculated from the test particle trajectories in turbulence.\nOur results confirm that the fast modes dominate the CR propagation, whereas\nAlfv\\'en and slow modes are much less efficient and have shown similar pitch\nangle scattering rates. We investigate the cross field transport on large and\nsmall scales. On large/global scales, normal diffusion is observed and the\ndiffusion coefficient is suppressed by $M_A^\\zeta$ compared to the parallel\ndiffusion coefficients, with $\\zeta$ closer to 4 in Alfv\\'en modes than that in\ntotal turbulence as theoretically expected. For the CR transport on scales\nsmaller than the turbulence injection scale, both the local and global magnetic\nreference frames are adopted. Super diffusion is observed on such small scales\nin all the cases. Particularly, CR transport in Alfv\\'en modes show clear\nRichardson diffusion in the local reference frame. Our results have broad\napplications to CRs in various astrophysical environments.", "category": "astro-ph_HE" }, { "text": "The Spin of New Black Hole Candidate: MAXI J1803-298 Observed by NuSTAR\n and NICER: MAXI J1803-298, a newly-discovered Galactic transient and black hole\ncandidate, was first detected by \\emph{MAXI}/GSC on May 1st, 2021. In this\npaper, we present a detailed spectral analysis of MAXI J1803-298. Utilizing the\nX-ray reflection fitting method, we perform a joint fit to the spectra of MAXI\nJ1803-298, respectively, observed by \\emph{NuSTAR} and \\emph{NICER}/XTI on the\nsame day over the energy range between 0.7-79.0 keV, and found its spin (and\nthe inclination angle i) can be constrained to be close to an extreme value,\n0.991 ($i\\sim$ $70 ^{\\circ}$), at 68\\% confidence interval. The results suggest\nthat MAXI J1803-298 may be a fast-rotating black hole with a large inclination\nangle.", "category": "astro-ph_HE" }, { "text": "Discovery of a recurrent spectral evolutionary cycle in the\n ultra-luminous X-ray sources Holmberg II X-1 and NGC 5204 X-1: Most ultra-luminous X-ray sources (ULXs) are now thought to be powered by\nstellar-mass compact objects accreting at super-Eddington rates. While the\ndiscovery of evolutionary cycles have marked a breakthrough in our\nunderstanding of the accretion flow changes in the sub-Eddington regime in\nGalactic Black Hole Binaries, their evidence in the super-Eddington regime\nremained elusive. However, recent circumstantial evidence had hinted the\npresence of a recurrent evolutionary cycle in two archetypal ULXs: Holmberg II\nX-1 and NGC 5204 X-1. Here we build on our previous work and exploit the\nlong-term high-cadence monitoring of Swift-XRT in order to provide evidence of\nthe evolutionary cycle in these two sources and investigate the main physical\nparameters inducing their spectral transitions. We study the long-term\nevolution of both sources using hardness-intensity diagrams (HID) and by means\nof Lomb-Scargle periodograms and Gaussian processes modelling to look for\nperiodic variability. We show that both sources follow a recurrent evolutionary\npattern in the HID that can be characterized by the hard ultraluminous (HUL)\nand soft ultraluminous (SUL) spectral regimes, and a third state with\ncharacteristics similar to the supersoft ultraluminous (SSUL) state. The\ntransitions between the soft states seem aperiodic, as revealed by timing\nanalysis of the light curve of Holmberg II X-1, albeit further investigation is\nwarranted. The light curve of NGC 5204 X-1 shows a periodicity of $\\sim$ 200\ndays, possibly associated with the duration of the evolutionary cycle. We\nsupport a scenario in which the spectral changes from HUL to SUL are due to a\nperiodic increase of the mass-transfer rate and subsequent narrowing of the\nopening angle of the supercritical funnel. The narrower funnel, combined with\nstochastic variability imprinted by the wind, might explain the SUL--SSUL\nspectral changes.", "category": "astro-ph_HE" }, { "text": "Accretion Properties and Estimation of Spin of Galactic Black Hole\n Candidate Swift J1728.9-3613 with NuSTAR during its 2019 outburst: Black hole X-ray binaries (BHXRBs) play a crucial role in understanding the\naccretion of matter onto a black hole. Here, we focus on exploring the\ntransient BHXRB \\source~discovered by Swift/BAT and MAXI/GSC during its January\n2019 outburst. We present measurements on its accretion properties, long\ntime-scale variability, and spin. To probe these properties we make use of\nseveral NICER observations and an unexplored data set from NuSTAR, as well as\nlong term light curves from MAXI/GSC. In our timing analysis we provide\nestimates of the cross-correlation functions between light curves in various\nenergy bands. In our spectral analysis we employ numerous phenomenological\nmodels to constrain the parameters of the system, including flavours of the\nrelativistic reflection model Relxill to model the Fe K$\\alpha$ line and the\n$>15$ keV reflection hump. Our analysis reveals that: (i) Over the course of\nthe outburst the total energy released was $\\sim 5.2 \\times 10^{44}$~ergs,\ncorresponding to roughly 90\\% the mass of Mars being devoured. (ii) We find a\ncontinuum lag of $8.4 \\pm 1.9$ days between light curves in the $2-4$ keV and\n$10-20$ keV bands which could be related to the viscous inflow time-scale of\nmatter in the standard disc. (iii) Spectral analysis reveals a spin parameter\nof $\\sim 0.6 - 0.7$ with an inclination angle of $\\sim 45^{\\circ}-70^{\\circ}$,\nand an accretion rate during the NuSTAR observation of $\\sim 17\\% ~L_{\\rm\nEdd}$.", "category": "astro-ph_HE" }, { "text": "Dark Matter Density Spikes around Primordial Black Holes: We show that density spikes begin to form from dark matter particles around\nprimordial black holes immediately after their formation at the\nradiation-dominated cosmological stage. This follows from the fact that in the\nthermal velocity distribution of particles there are particles with low\nvelocities that remain in finite orbits around black holes and are not involved\nin the cosmological expansion. The accumulation of such particles near black\nholes gives rise to density spikes. These spikes are considerably denser than\nthose that are formed later by the mechanism of secondary accretion. The\ndensity spikes must be bright gamma-ray sources. Comparison of the calculated\nsignal from particle annihilation with the Fermi-LAT data constrains the\npresent-day cosmological density parameter for primordial black holes with\nmasses $M_{\\rm BH}\\geq10^{-8}M_\\odot$ from above by values from $\\Omega_{\\rm\nBH}\\leq1$ to $\\Omega_{\\rm BH}\\leq10^{-8}$, depending on $M_{\\rm BH}$. These\nconstraints are several orders of magnitude more stringent than other known\nconstraints.", "category": "astro-ph_HE" }, { "text": "Early optical emission in support of synchrotron radiation in\n $\u03b3$-ray bursts: The origin of prompt emission in $\\gamma$-ray bursts (GRBs) is highly debated\ntopic. The observed spectra are supposed to play a crucial role in constraining\nthe location of the emitting region, the strength of the magnetic field and the\ndistribution of the accelerated particles. The apparent inconsistency of the\nprompt emission spectra with the synchrotron radiation scenario has resulted in\nconsidering more complex models. The inclusion of the soft X-ray data (down to\n0.5 keV) in GRB spectra have led to the discovery of low-energy breaks in their\nspectra. More importantly, the distribution of spectral slopes has been shifted\ntowards the prediction of the synchrotron radiation scenario if the break is\nassociated with the synchrotron cooling frequency. We discuss the recent study\nthat systematically extend the range of investigation down to the optical\ndomain. It was shown that the optical-to-gamma-rays spectra are consistent with\nthe synchrotron model. In addition, widely used empirical model made of thermal\nand non-thermal components has been tested. We conclude that most of the\nspectra are consistent with the synchrotron scenario while the two-component\nmodel faces difficulties to account for the optical radiation in\npresence/absence of the contaminating afterglow emission. We comment on the\nparameter space of GRB emitting region derived from the best fit parameters of\nthe synchrotron model. In a basic one-shot particle acceleration model it\ncorresponds to the quite contrived solutions for the magnetic field strength\n($\\sim$ 10 G) and for the radius of the emitting region ($R_\\gamma \\ge 10^{16}$\ncm). Possible modifications of the basic model would be necessary to have a\nfully consistent picture.", "category": "astro-ph_HE" }, { "text": "A NICER Discovery of a Low-Frequency Quasi-Periodic Oscillation in the\n Soft-Intermediate State of MAXI J1535-571: We present the discovery of a low-frequency $\\approx 5.7$ Hz quasi-periodic\noscillation (QPO) feature in observations of the black hole X-ray binary MAXI\nJ1535-571 in its soft-intermediate state, obtained in September-October 2017 by\nthe Neutron Star Interior Composition Explorer (NICER). The feature is\nrelatively broad (compared to other low-frequency QPOs; quality factor\n$Q\\approx 2$) and weak (1.9% rms in 3-10 keV), and is accompanied by a weak\nharmonic and low-amplitude broadband noise. These characteristics identify it\nas a weak Type A/B QPO, similar to ones previously identified in the\nsoft-intermediate state of the transient black hole X-ray binary XTE J1550-564.\nThe lag-energy spectrum of the QPO shows increasing soft lags towards lower\nenergies, approaching 50 ms at 1 keV (with respect to a 3-10 keV continuum).\nThis large phase shift has similar amplitude but opposite sign to that seen in\nRossi X-ray Timing Explorer data for a Type B QPO from the transient black hole\nX-ray binary GX 339-4. Previous phase-resolved spectroscopy analysis of the\nType B QPO in GX 339-4 pointed towards a precessing jet-like corona\nilluminating the accretion disk as the origin of the QPO signal. We suggest\nthat this QPO in MAXI J1535-571 may have the same origin, with the different\nlag sign depending on the scale height of the emitting region and the observer\ninclination angle.", "category": "astro-ph_HE" }, { "text": "Spectral evolution responsible for the transition from positive lags to\n negative lags in Gamma-ray Bursts: It was well known that most of gamma-ray bursts (GRBs) are dominated by\npositive spectral lags, while a small fraction of GRBs show negative lags.\nHowever, Wei et al. firstly identified a well-defined transition from positive\nlags to negative lags in GRB 160625B, and then got robust limits on possible\nviolation of Lorentz Invariance (LIV) based on the observation. Recently, such\na transition has been found in three different emission episodes in \\thisgrb by\nGunapati et al., which provides us a great opportunity to investigate whether\nthe transition results from LIV-induced observed spectral lags. Our analysis\nshows that the LIV model can not be compatible with the current observations,\nwhereas, only the spectral evolution induced spectral lags could responsible\nfor the transition. So, spectral evolution can also explain the positive to\nnegative lag in GRB 190530A.", "category": "astro-ph_HE" }, { "text": "Gravitational-wave Signals From Three-dimensional Supernova Simulations\n With Different Neutrino-Transport Methods: We compare gravitational-wave (GW) signals from eight three-dimensional\nsimulations of core-collapse supernovae, using two different progenitors with\nzero-age main sequence masses of 9 and 20 solar masses. The collapse of each\nprogenitor was simulated four times, at two different grid resolutions and with\ntwo different neutrino transport methods, using the Aenus-Alcar code. The main\ngoal of this study is to assess the validity of recent concerns that the\nso-called \"Ray-by-Ray+\" (RbR+) approximation is problematic in core-collapse\nsimulations and can adversely affect theoretical GW predictions. Therefore,\nsignals from simulations using RbR+ are compared to signals from corresponding\nsimulations using a fully multidimensional (FMD) transport scheme. The 9\nsolar-mass progenitor successfully explodes, whereas the 20 solar-mass model\ndoes not. Both the standing accretion shock instability and hot-bubble\nconvection develop in the postshock layer of the non-exploding models. In the\nexploding models, neutrino-driven convection in the postshock flow is\nestablished around 100 ms after core bounce and lasts until the onset of shock\nrevival. We can, therefore, judge the impact of the numerical resolution and\nneutrino transport under all conditions typically seen in non-rotating\ncore-collapse simulations. We find excellent qualitative agreement in all GW\nfeatures. We find minor quantitative differences between simulations, but find\nno systematic differences between simulations using different transport\nschemes. Resolution-dependent differences in the hydrodynamic behaviour of\nlow-resolution and high-resolution models have a greater impact on the GW\nsignals than consequences of the different transport methods. Furthermore,\nincreasing the resolution decreases the discrepancies between models with\ndifferent neutrino transport.", "category": "astro-ph_HE" }, { "text": "An imperfect double: probing the physical origin of the low-frequency\n QPO and its harmonic in black hole binaries: We extract the spectra of the strong low-frequency quasi-periodic oscillation\n(QPO) and its harmonic during the rising phase of an outburst in the black-hole\nbinary XTE J1550-564. We compare these frequency resolved spectra to the\ntime-averaged spectrum and the spectrum of the rapid (<0.1s) variability. The\nspectrum of the time averaged emission can be described by a disc, a Compton\nupscattered tail, and its reflection. The QPO spectrum contains no detectable\ndisc, and the Compton spectrum is generally harder than in the time averaged\nemission, and shows less reflection, making it very similar to the spectrum of\nthe rapid variability. The harmonic likewise contains no detectable disc\ncomponent, but has a Compton spectrum which is systematically softer than the\nQPO, softer even than the Compton tail in the time averaged emission. We\ninterpret these results in the context of the Lense-Thirring model for the QPO,\nwhere a precessing hot flow replaces the inner disc, and the harmonic is\nproduced by the angular dependence of Compton scattering within the hot flow.\nWe extend these models to include stratification of the hot flow, so that it is\nsofter (lower optical depth) at larger radii closer to the truncated disc, and\nharder (higher optical depth) in the innermost parts of the flow where the\nrapid variability is produced. The different optical depth with radius gives\nrise to different angular dependence of the Comptonised emission, weighting the\nfundamental to the inner parts of the hot flow, and the harmonic to the outer.\nThis is the first model which can explain both the spectrum of the QPO and its\nharmonic in a self consistent geometry.", "category": "astro-ph_HE" }, { "text": "Multi-messenger Observations of Tidal Disruption Events: Using the Zwicky Transient Facility (ZTF) and other observatories, we have\nidentified three candidate Tidal Disruption Events (TDEs) in spatial and\ntemporal coincidence with high-energy neutrinos detected by IceCube: AT2019dsg,\nAT2019fdr and AT2019aalc. All three of these events have been shown to be able\nto produce high-energy neutrinos. In these proceedings, I will give an overview\nof Tidal Disruption Events, outline our follow-up program with ZTF, describe\nthe observations carried out for each of those coincident events and highlight\ntheir similarities and differences.", "category": "astro-ph_HE" }, { "text": "Binary Neutron Star Mergers and Short Gamma-Ray Bursts: Effects of\n Magnetic Field Orientation, Equation of State, and Mass Ratio: We present fully GRMHD simulations of the merger of binary neutron star (BNS)\nsystems. We consider BNSs producing a hypermassive neutron star (HMNS) that\ncollapses to a spinning black hole (BH) surrounded by a magnetized accretion\ndisk in a few tens of ms. We investigate whether such systems may launch\nrelativistic jets and power short gamma-ray bursts. We study the effects of\ndifferent equations of state (EOSs), different mass ratios, and different\nmagnetic field orientations. For all cases, we present a detailed investigation\nof the matter dynamics and of the magnetic field evolution, with particular\nattention to its global structure and possible emission of relativistic jets.\nThe main result of this work is that we found the formation of an organized\nmagnetic field structure. This happens independently of EOS, mass ratio, and\ninitial magnetic field orientation. We also show that those models that produce\na longer-lived HMNS lead to a stronger magnetic field before collapse to BH.\nSuch larger fields make it possible, for at least one of our models, to resolve\nthe MRI and hence further amplify the magnetic field. However, by the end of\nour simulations, we do not observe a magnetically dominated funnel and hence\nneither a relativistic outflow. With respect to the recent simulations of Ruiz\net al 2016, we evolve models with lower and more realistic initial magnetic\nfield strengths and, because of computational reasons, we do not evolve the\naccretion disk for the long timescales that seem to be required in order to see\na relativistic outflow. Since all our models produce a similar ordered magnetic\nfield structure, we expect that the results found in Ruiz et al 2016, where\nthey only considered an equal-mass system with an ideal fluid EOS, should be\ngeneral and, at least from a qualitative point of view, independent from\nmass-ratio, magnetic field orientation, and EOS.", "category": "astro-ph_HE" }, { "text": "An independent determination of the distance to supernova SN 1987A by\n means of the light echo AT 2019xis: Accurate distance determination to astrophysical objects is essential for the\nunderstanding of their intrinsic brightness and size. The distance to SN 1987A\nhas been previously measured by the expanding photosphere method, and by using\nthe angular size of the circumstellar rings with absolute sizes derived from\nlight curves of narrow UV emission lines, with reported distances ranging from\n46.77 kpc to 55 kpc. In this study, we independently determined the distance to\nSN 1987A using photometry and imaging polarimetry observations of AT 2019xis, a\nlight echo of SN 1987A, by adopting a radiative transfer model of the light\necho developed in Ding et al. (2021). We obtained distances to SN 1987A in the\nrange from 49.09 $\\pm$ 2.16 kpc to 59.39 $\\pm$ 3.27 kpc, depending on the\ninterstellar polarization and extinction corrections, which are consistent with\nthe literature values. This study demonstrates the potential of using light\nechoes as a tool for distance determination to astrophysical objects in the\nMilky Way, up to kiloparsec level scales.", "category": "astro-ph_HE" }, { "text": "Anomalous Anisotropies of Cosmic Rays from Turbulent Magnetic Fields: The propagation of cosmic rays (CRs) in turbulent interstellar magnetic\nfields is typically described as a spatial diffusion process. This formalism\npredicts only a small deviation from an isotropic CR distribution in the form\nof a dipole in the direction of the CR density gradient or relative background\nflow. We show that the existence of a global CR dipole moment necessarily\ngenerates a spectrum of higher multipole moments in the local CR distribution.\nThese \"anomalous\" anisotropies are a direct consequence of Liouville's theorem\nin the presence of a local turbulent magnetic field. We show that the\npredictions of this model are in excellent agreement with the observed power\nspectrum of multi-TeV CRs.", "category": "astro-ph_HE" }, { "text": "Formation scenarios and mass-radius relation for neutron stars: Neutron star crust, formed via accretion of matter from a companion in a\nlow-mass X-ray binary (LMXB), has an equation of state (EOS) stiffer than that\nof catalyzed matter. At a given neutron star mass, M, the radius of a star with\nan accreted crust is therefore larger, by DR(M), than for usually considered\nstar built of catalyzed matter. Using a compressible liquid drop model of\nnuclei, we calculate, within the one-component plasma approximation, the EOSs\ncorresponding to different nuclear compositions of ashes of X-ray bursts in\nLMXB. These EOSs are then applied for studying the effect of different\nformation scenarios on the neutron-star mass-radius relation. Assuming the SLy\nEOS for neutron star's liquid core, derived by Douchin & Haensel (2001), we\nfind that at M=1.4 M_sun the star with accreted crust has a radius more than\n100 m larger that for the crust of catalyzed matter. Using smallness of the\ncrust mass compared to M, we derive a formula that relates DR(M) to the\ndifference in the crust EOS. This very precise formula gives also analytic\ndependence of DR on M and R of the reference star built of catalyzed matter.\nThe formula is valid for any EOS of the liquid core. Rotation of neutron star\nmakes DR(M) larger. We derive an approximate but very precise formula that\ngives difference in equatorial radii, DR_eq(M), as a function of stellar\nrotation frequency.", "category": "astro-ph_HE" }, { "text": "Telescope Array 10 Year Composition: Estimates of the composition of ultra high energy cosmic rays (UHECRs) can be\ninferred by recording the depth of air shower maximum, $X_{\\mathrm{max}}$, for\nmany showers and comparing the distributions to those predicted by Monte Carlo\nsimulations. Traditionally, UHECR composition has relied upon comparison of the\nfirst and second moments of the $X_{\\mathrm{max}}$ distributions to estimate\nthe compatibility between data and simulations, but with the large UHECR\ndatasets being built the current generation experiments better tests which\ncompare full distributions can be employed. Such tests can be used to\nunderstand the accuracy with which UHECR composition can actually be understood\nat the current level of statistics and quantitatively measure the significance\nof agreement or disagreement with models in order to reject them. In this paper\nwe present the most recent results of 10 years of Telescope Array hybrid\n$X_{\\mathrm{max}}$ measurements which is found to agree with a predominantly\nlight composition. In previously published results we have demonstrated the\nagreement of Telescope Array hybrid $X_{\\mathrm{max}}$ data with single element\nmodels using systematic shifting of the data in order to ensure the shapes of\nthe distributions are being compared. Here we present multi-component source\nmodels fit to hybrid $X_{\\mathrm{max}}$ data and report on the relative\nfractions of those sources that best fit the data. Below $10^{19.1}$ eV TA\nhybrid data is found to be compatible with mixtures composed of predominantly\nlight elements such as protons and helium.", "category": "astro-ph_HE" }, { "text": "Search for Coincident Gravitational Wave and Long Gamma-Ray Bursts from\n 4-OGC and the Fermi-GBM/Swift-BAT Catalog: The recent discovery of a kilonova associated with an apparent long-duration\ngamma-ray burst has challenged the typical classification that long gamma-ray\nbursts originate from the core collapse of massive stars and short gamma-ray\nbursts are from compact binary coalescence. The kilonova indicates a neutron\nstar merger origin and suggests the viability of gravitational-wave and long\ngamma-ray burst multimessenger astronomy. Gravitational waves play a crucial\nrole by providing independent information for the source properties. This work\nrevisits the archival 2015-2020 LIGO/Virgo gravitational-wave candidates from\nthe 4-OGC catalog which are consistent with a binary neutron star or neutron\nstar-black hole merger and the long-duration gamma-ray bursts from the\nFermi-GBM and Swift-BAT catalogs. We search for spatial and temporal\ncoincidence with up to 10 s time lag between gravitational-wave candidates and\nthe onset of long-duration gamma-ray bursts. The most significant candidate\nassociation has only a false alarm rate of once every two years; given the\nLIGO/Virgo observational period, this is consistent with a null result. We\nreport an exclusion distance for each search candidate for a fiducial\ngravitational-wave signal and conservative viewing angle assumptions.", "category": "astro-ph_HE" }, { "text": "Resolving the X-ray obscuration in a low flux observation of the quasar\n PDS 456: Simultaneous XMM-Newton, NuSTAR and HST observations, performed in March\n2017, of the nearby ($z=0.184$) luminous quasar PDS 456 are presented. PDS 456\nhad a low X-ray flux compared to past observations, where the first of the two\nnew XMM-Newton observations occurred during a pronounced dip in the X-ray\nlightcurve. The broad-band X-ray spectrum is highly absorbed, attenuated by a\nsoft X-ray absorber of column density $N_{\\rm H}=6\\times10^{22}$ cm$^{-2}$. An\nincrease in obscuration occurs during the dip, which may be due to an X-ray\neclipse. In addition, the persistent, fast Fe K outflow is present, with\nvelocity components of $-0.25c$ and $-0.4c$. The soft absorber is less ionized\n($\\log\\xi=3$) compared to the iron K outflow ($\\log\\xi=5$) and is outflowing\nwith a velocity of approximately $-0.2c$. A soft X-ray excess is present below\n1 keV against the highly absorbed continuum and can be attributed to the\nre-emission from a wide angle wind. The complex X-ray absorption present in PDS\n456 suggests that the wind is inhomogeneous, whereby the soft X-ray absorber\noriginates from denser clumps or filaments which may form further out along the\noutflow. In contrast to the X-ray observations, the simultaneous UV spectrum of\nPDS 456 is largely unabsorbed, where only a very weak broad absorption trough\nis present bluewards of Ly$\\alpha$, compared to a past observation in 2000 when\nthe trough was significantly stronger. The relative weakness of the UV\nabsorption may be due to the soft X-ray absorber being too highly ionized and\nalmost transparent in the UV band.", "category": "astro-ph_HE" }, { "text": "Proton synchrotron, plausible explanation for delayed VHE activity of 3C\n 279 in 2018: A nearly 11-day delayed very-high-energy(VHE) activity compared to the\nFermi-LAT flare from quasar 3C 279 was reported by H.E.S.S. on 28 January 2018.\n3C 279 has long been considered a candidate site for particle acceleration;\nhence such events may embed information about the high-energy phenomena. We\npropose the production channel being leptonic for the multi-wavelength flare,\nUV-Optical-Xrays-$\\gamma$-rays, whereas the delayed VHE activity originated\nfrom the proton synchrotron. Our model requires the magnetic field to be 2.3 G\nand the proton luminosity (L$_{p}$) $1.56\n \\times 10^{46}$ erg/sec, whereas the lepton luminosity (L$_e$) $3.9\n \\times 10^{43}$ erg/sec.", "category": "astro-ph_HE" }, { "text": "Properties of unique hard X-ray dips observed from GRS 1915+105 and IGR\n J17091-3624 and their implications: We report a comprehensive study on spectral and timing properties of hard\nX-ray dips uniquely observed in some so-called variability classes of the\nmicro-quasars GRS 1915+105 and IGR J17091-3624. These dips are characterized by\na sudden decline in the 2.0-60.0 keV X-ray intensity by a factor of 4-12\nsimultaneous with the increase in hardness ratio by a factor of 2-4. Using 31\nobservations of GRS 1915+105 with RXTE/PCA, we show that different behavior are\nobserved in different types of variability classes, and we find that a\ndichotomy is observed between classes with abrupt transitions vs those with\nsmoother evolution. For example, both energy-lag spectra and frequency-lag\nspectra of hard X-ray dips in classes with abrupt transitions and shorter dip\nintervals show hard-lag (hard photons lag soft photons), while both lag spectra\nduring hard dips in classes with smoother evolution and longer dip intervals\nshow soft-lag. Both lag time-scales are of the order of 100-600 msec. We also\nshow that timing and spectral properties of hard X-ray dips observed in light\ncurves of IGR J17091-3624 during its 2011 outburst are consistent with the\nproperties of the abrupt transitions in GRS 1915+105 rather than smooth\nevolution. A global correlation between the X-ray intensity cycle time and hard\ndip time is observed for both abrupt and smooth transition which may be due to\ntwo distinct physical processes whose time-scales are eventually correlated. We\ndiscuss implications of our results in the light of some generic models.", "category": "astro-ph_HE" }, { "text": "Gamma-Ray Burst observations with Fermi: After seven years of science operation, the Fermi mission has brought great\nadvances in the study of Gamma-ray Bursts (GRBs). Over 1600 GRBs have been\ndetected by the Gamma-ray Burst Monitor, and more than 100 of these are also\ndetected by the Large Area Telescope above 30 MeV. We will give an overview of\nthese observations, presenting the common properties in the GRB temporal and\nspectral behavior at high energies. We will also highlight the unique\ncharacteristics of some individual bursts. The main physical implications of\nthese results will be discussed, along with open questions regarding GRB\nmodeling in their prompt and temporally-extended emission phases.", "category": "astro-ph_HE" }, { "text": "Strongly Lensed Supermassive Black Hole Binaries as Nanohertz\n Gravitational-Wave Sources: Supermassive black hole binary systems (SMBHBs) should be the most powerful\nsources of gravitational waves (GWs) in the Universe. Once Pulsar Timing Arrays\n(PTAs) detect the stochastic GW background from their cosmic merger history,\nsearching for individually resolvable binaries will take on new importance.\nSince these individual SMBHBs are expected to be rare, here we explore how\nstrong gravitational lensing can act as a tool for increasing their detection\nprospects by magnifying fainter sources and bringing them into view. Unlike for\nelectromagnetic waves, when the geometric optics limit is nearly always valid,\nfor GWs the wave-diffraction-interference effects can become important when the\nwavelength of the GWs is larger than the Schwarzchild radius of the lens, i.e.\n$M_{\\rm lens} \\sim 10^8\\,(\\frac{f}{mHz})^{-1}\\,M_\\odot$. For the GW frequency\nrange explored in this work, the geometric optics limit holds. We investigate\nGW signals from SMBHBs that might be detectable with current and future PTAs\nunder the assumption that quasars serve as bright beacons that signal a recent\nmerger. Using the black hole mass function derived from quasars and a\nphysically motivated magnification distribution, we expect to detect a few\nstrongly lensed binary systems out to $z \\approx 2$. Additionally, for a range\nof fixed magnifications $2 \\leq \\mu \\leq 100$, strong lensing adds up to\n$\\sim$30 more detectable binaries for PTAs. Finally, we investigate the\npossibility of observing both time-delayed electromagnetic signals and GW\nsignals from these strongly lensed binary systems -- that will provide us with\nunprecedented multimessenger insights into their orbital evolution.", "category": "astro-ph_HE" }, { "text": "Properties of Low Luminosity Afterglow Gamma-ray Bursts: Aims: We characterize a sample of Gamma-Ray Bursts with low luminosity X-ray\nafterglows (LLA GRBs), and study their properties. Method: We select a sample\nconsisting of the 12\\% faintest X-ray afterglows from the total population of\nlong GRBs (lGRBs) with known redshift. We study their intrinsic properties\n(spectral index, decay index, distance, luminosity, isotropic radiated energy\nand peak energy) to assess whether they belong to the same population than the\nbrighter afterglow events. Results: We present strong evidences that these\nevents belong to a population of nearby events, different from that of the\ngeneral population of lGRBs. These events are faint during their prompt phase,\nand include the few possible outliers of the Amati relation. Out of 14 GRB-SN\nassociations, 9 are in LLA GRB sample, prompting for caution when using SN\ntemplates in observational and theoretical models for the general lGRBs\npopulation.", "category": "astro-ph_HE" }, { "text": "Neutrino analysis of the September 2010 Crab Nebula flare and\n time-integrated constraints on neutrino emission from the Crab using IceCube: We present the results for a search of high-energy muon neutrinos with the\nIceCube detector in coincidence with the Crab nebula flare reported on\nSeptember 2010 by various experiments. Due to the unusual flaring state of the\notherwise steady source we performed a prompt analysis of the 79-string\nconfiguration data to search for neutrinos that might be emitted along with the\nobserved gamma-rays. We performed two different and complementary data\nselections of neutrino events in the time window of 10 days around the flare.\nOne event selection is optimized for discovery of E^-2 neutrino spectrum\ntypical of 1st order Fermi acceleration. A similar event selection has also\nbeen applied to the 40-string data to derive the time-integrated limits to the\nneutrino emission from the Crab. The other event selection was optimized for\ndiscovery of neutrino spectra with softer spectral index and TeV energy\ncut-offs as observed for various galactic sources in gamma-rays. The 90% CL\nbest upper limits on the Crab flux during the 10 day flare are 4.73 x 10^-11\ncm-2 s-1 TeV-1 for an E^-2 neutrino spectrum and 2.50 x 10^-10 cm-2 s-1 TeV-1\nfor a softer neutrino spectra of E-2.7, as indicated by Fermi measurements\nduring the flare. IceCube has also set a time-integrated limit on the neutrino\nemission of the Crab using 375.5 days of livetime of the 40-string\nconfiguration data. This limit is compared to existing models of neutrino\nproduction from the Crab and its impact on astrophysical parameters is\ndiscussed. The most optimistic predictions of some models are already rejected\nby the IceCube neutrino telescope with more than 90% CL.", "category": "astro-ph_HE" }, { "text": "Properties of intra-cluster low-mass X-ray binaries in Fornax globular\n clusters: We present a study of the intra-cluster population of low-mass X-ray binaries\n(LMXB) residing in globular clusters (GC) in the central 1 $deg^2$ of the\nFornax galaxy cluster. Differently from previous studies, which were restricted\nto the innermost regions of individual galaxies, this work is aimed at\ncomparing the properties of the intra-cluster population of GC-LMXBs with those\nof the host galaxy. We use VLT Survey Telescope (VST) and Chandra observations.\nWe identify 168 LMXBs residing in GCs and divide it into host-galaxy and\nintra-cluster objects based on their distance from the nearest galaxy in terms\nof effective radius. We found 82 intra-cluster GC-LMXBs and 86 objects that are\nhosted in galaxies. We perform a Gaussian mixture model to divide the\npopulation into red and blue GCs. As has been found for the innermost regions\nof galaxies, LMXBs tend to form in red and bright GCs in intra-cluster space as\nwell. We find, however, that the likelihood of a red GC to host an LMXB\ndecreases with galactocentric distance, but it remains approximately constant\nfor the blue GC population. Investigating the X-ray properties of the LMXBs\nresiding in GCs, we find a difference in the X-ray luminosity function between\nthe intra-cluster and host-galaxy sample: both follow a power-law down to $\\sim\n8.5\\times 10^{37}$ erg s$^{-1}$, which is consistent with field LMXBs for the\nintra-cluster sample, while the latter agree with previous estimates for LMXBs\nin GCs. We detect a tentative difference in the hardness ratio of two\npopulations, where the intra-cluster GC-LMXBs appear to have harder spectra\nthan the host-galaxy objects. We find the same trend when we compare red and\nblue GC-LMXBs: the spectra of the blue sample are harder spectra than those of\nthe red sample. This result could suggest a relation between the spectral\nproperties of LMXBs and the host GC colour and therefore its metallicity.", "category": "astro-ph_HE" }, { "text": "Rapid Infrared Variability of Three Radio-loud Narrow-line Seyfert 1\n Galaxies: A View from the Wide-field Infrared Survey Explorer: Using newly released data from the Wide-field Infrared Survey Explorer, we\nreport the discovery of rapid infrared variability in three radio-loud\nnarrow-line Seyfert 1 galaxies (NLS1s) selected from the 23 sources in the\nsample of Yuan et al. (2008). J0849+5108 and J0948+0022 clearly show intraday\nvariability, while J1505+0326 has a longer measurable time scale within 180\ndays. Their variability amplitudes, corrected for measurement errors, are $\\sim\n0.1-0.2$ mag. The detection of intraday variability restricts the size of the\ninfrared-emitting region to $\\sim 10^{-3}$ pc, significantly smaller than the\nscale of the torus but consistent with the base of a jet. The three variable\nsources are exceptionally radio-loud, have the highest radio brightness\ntemperature among the whole sample, and all show detected $\\gamma$-ray emission\nin Fermi/LAT observations. Their spectral energy distributions resemble those\nof low-energy-peaked blazars, with a synchrotron peak around infrared\nwavelengths. This result strongly confirms the view that at least some\nradio-loud NLS1s are blazars with a relativistic jet close to our line of\nsight. The beamed synchrotron emission from the jet contributes significantly\nto and probably dominates the spectra in the infrared and even optical bands.", "category": "astro-ph_HE" }, { "text": "Assessing equation of state-independent relations for neutron stars with\n nonparametric models: Relations between neutron star properties that do not depend on the nuclear\nequation of state offer insights on neutron star physics and have practical\napplications in data analysis. Such relations are obtained by fitting to a\nrange of phenomenological or nuclear physics equation of state models, each of\nwhich may have varying degrees of accuracy. In this study we revisit\ncommonly-used relations and re-assess them with a very flexible set of\nphenomenological nonparametric equation of state models that are based on\nGaussian Processes. Our models correspond to two sets: equations of state which\nmimic hadronic models, and equations of state with rapidly changing behavior\nthat resemble phase transitions. We quantify the accuracy of relations under\nboth sets and discuss their applicability with respect to expected upcoming\nstatistical uncertainties of astrophysical observations. We further propose a\ngoodness-of-fit metric which provides an estimate for the systematic error\nintroduced by using the relation to model a certain equation-of-state set.\nOverall, the nonparametric distribution is more poorly fit with existing\nrelations, with the I-Love-Q relations retaining the highest degree of\nuniversality. Fits degrade for relations involving the tidal deformability,\nsuch as the Binary-Love and compactness-Love relations, and when introducing\nphase transition phenomenology. For most relations, systematic errors are\ncomparable to current statistical uncertainties under the nonparametric\nequation of state distributions.", "category": "astro-ph_HE" }, { "text": "Very High-Energy ($>$50 GeV) Gamma-ray Flux Variability of Bright Fermi\n Blazars: Understanding the high-energy emission processes and variability patterns are\ntwo of the most challenging research problems associated with relativistic\njets. In particular, the long-term (months-to-years) flux variability at very\nhigh energies (VHE, $>$50 GeV) has remained an unexplored domain so far. This\nis possibly due to the decreased sensitivity of the Fermi Large Area Telescope\n(LAT) above a few GeV, hence low photon statistics, and observing constraints\nassociated with the ground-based Cherenkov telescopes. This paper reports the\nresults obtained from the 0.05$-$2 TeV Fermi-LAT data analysis of a sample of\n29 blazars with the primary objective to explore their months-to-year long VHE\nflux variability behavior. This systematic search has led to, for the first\ntime, the detection of significant flux variations in 5 blazars at $>$99\\%\nconfidence level, whereas, 8 of them exhibit variability albeit at a lower\nconfidence level ($\\sim$95\\%-99\\%). A comparison of the 0.05$-$2 TeV flux\nvariations with that observed at 0.1$-$50 GeV band has revealed similar\nvariability behavior for most of the sources. However, complex variability\npatterns that are not reflected contemporaneously in both energy bands were\nalso detected, thereby providing tantalizing clues about the underlying\nradiative mechanisms. These results open up a new dimension to unravel the VHE\nemission processes operating in relativistic jets, hence sowing the seeds for\ntheir future observations with the upcoming Cherenkov Telescope Array.", "category": "astro-ph_HE" }, { "text": "Very Rapid High-Amplitude Gamma-ray Variability in Luminous Blazar PKS\n 1510-089 Studied with Fermi-LAT: Here we report on the detailed analysis of the gamma-ray light curve of a\nluminous blazar PKS1510-089 observed in the GeV range with the Large Area\nTelescope (LAT) onboard the Fermi satellite during the period 2011 September --\nDecember. By investigating the properties of the detected three major flares\nwith the shortest possible time binning allowed by the photon statistics, we\nfind a variety of temporal characteristics and variability patterns. This\nincludes a clearly asymmetric profile (with a faster flux rise and a slower\ndecay) of the flare resolved on sub-daily timescales, a superposition of many\nshort uncorrelated flaring events forming the apparently coherent\nlonger-duration outburst, and a huge single isolated outburst unresolved down\nto the timescale of three-hours. In the latter case we estimate the\ncorresponding gamma-ray flux doubling timescale to be below one hour, which is\nextreme and never previously reported for any active galaxy in the GeV range.\nThe other unique finding is that the total power released during the studied\nrapid and high-amplitude flares constitute the bulk of the power radiatively\ndissipated in the source, and a significant fraction of the total kinetic\nluminosity of the underlying relativistic outflow. Our analysis allows us to\naccess directly the characteristic timescales involved in shaping the energy\ndissipation processes in the source, and to provide constraints on the location\nand the structure of the blazar emission zone in PKS1510-089.", "category": "astro-ph_HE" }, { "text": "Application of the space-based optical interferometer towards measuring\n cosmological distances of quasars: Measuring the quasar distance through joint analysis of spectroastrometry\n(SA) and reverberation mapping (RM) observations is a new method for driving\nthe development of cosmology. In this paper, we carry out detailed simulation\nand analysis to study the effect of four basic observational parameters\n(baseline length, exposure time, equivalent diameter and spectral resolution)\non the data quality of differential phase curves (DPCs), furthermore on the\naccuracy of distance measurement. In our simulation, we adopt an axis\nsymmetrical disc model of broad line region (BLR) to generate differential\nphase signals. We find that the differential phases and their Poisson errors\ncould be amplified by extending the baseline, while the influence of OPD\n(optical path difference) errors can be reduced during fitting the BLR model.\nLonger exposure time or larger equivalent diameter helps reduce the absolute\nPoisson error. Therefore, the relative error of DPCs could be reduce by\nincreasing any of the above three parameters, then the accuracy of distance\nmeasurement could be improved. In contrast, the uncertainty of $D_{\\rm{A}}$ (\nabsolute angular distances) could be improved with higher spectral resolution,\nalthough the relative error of DPCs would be amplified. We show how the\nuncertainty of distance measurement varies with the relative error of DPCs. For\nour specific set of model parameters, without considering more complicated\nstructures and kinematics of BLRs in our simulation, it is found that the\nrelative error of DPCs $<$ 20$\\%$ is a limit for accurate distance measurement.\nThe relative error of DPCs have a lower limit (roughly 5$\\%$) and the\nuncertainty of distance measurement can be better than 2$\\%$.", "category": "astro-ph_HE" }, { "text": "Unexpected late-time temperature increase observed in two neutron star\n crust cooling sources -- XTE~J1701-462 and EXO~0748-676: Transient LMXBs that host neutron stars (NSs) provide excellent laboratories\nfor probing the dense matter physics present in NS crusts. During accretion\noutbursts in LMXBs, exothermic reactions may heat the NS crust, disrupting the\ncrust-core equilibrium. When the outburst ceases, the crust cools to restore\nthermal equilibrium with the core. Monitoring this evolution allows us to probe\nthe dense matter physics in the crust. Properties of the deeper crustal layers\ncan be probed at later times after the end of the outburst. We report on the\nunexpected late-time temperature evolution (>2000 days after the end of their\noutbursts) of two NSs in LMXBs, XTE J1701-462 and EXO 0748-676. Although both\nthese sources exhibited very different outbursts (in terms of duration and the\naverage accretion rate), they exhibit an unusually steep decay of ~7 eV in the\nobserved effective temperature (occurring in a time span of ~700 days) around\n~2000 days after the end of their outbursts. Furthermore, they both showed an\neven more unexpected rise of ~3 eV in temperature (over a time period of\n~500-2000 days) after this steep decay. This rise was significant at the\n2.4{\\sigma} and 8.5{\\sigma} level for XTE J1701-462 and EXO 0748-676,\nrespectively. The physical explanation for such behaviour is unknown and cannot\nbe straightforwardly be explained within the cooling hypothesis. In addition,\nthis observed evolution cannot be well explained by low-level accretion either\nwithout invoking many assumptions. We investigate the potential pathways in the\ntheoretical heating and cooling models that could reproduce this unusual\nbehaviour, which so far has been observed in two crust-cooling sources. Such a\ntemperature increase has not been observed in the other NS crust-cooling\nsources at similarly late times, although it cannot be excluded that this might\nbe a result of the inadequate sampling obtained at such late times.", "category": "astro-ph_HE" }, { "text": "Towards a dynamical mass of the ultraluminous X-ray source NGC 5408 X-1: We obtained multi-epoch Very Large Telescope (VLT) optical spectroscopic data\nin 2011 and 2012 on the ultraluminous X-ray source (ULX) NGC 5408 X-1. We\nconfirm that the HeII\\lambda4686 line has a broad component with an average\nFWHM of v=780\\pm64 km/s with a variation of ~13% during observations spanning\nover 4 years, and is consistent with the origin in the accretion disc. The\ndeepest optical spectrum does not reveal any absorption line from a donor star.\nOur aim was to measure the radial velocity curve and estimate the parameters of\nthe binary system. We find an upper limit on the semi-amplitude of the radial\nvelocity of K=132\\pm42 km/s. A search for a periodic signal in the data\nresulted in no statistically significant period. The mass function and\nconstraints on the binary system imply a black hole mass of less than ~510\nM_sun. Whilst, a disc irradiation model may imply a black hole mass smaller\nthan ~431-1985 M_sun, depending on inclination. Our data can also be consistent\nwith an unexplored orbital period range from a couple of hours to a few days,\nthus with a stellar-mass black hole and a subgiant companion.", "category": "astro-ph_HE" }, { "text": "On the Diversity of Fallback Rates from Tidal Disruption Events with\n Accurate Stellar Structure: The tidal disruption of stars by supermassive black holes (SMBHs) can be used\nto probe the SMBH mass function, the properties of individual stars, and\nstellar dynamics in galactic nuclei. Upcoming missions will detect thousands of\nTDEs, and accurate theoretical modeling is required to interpret the data with\nprecision. Here we analyze the influence of more realistic stellar structure on\nthe outcome of TDEs; in particular, we compare the fallback rates -- being the\nrate at which tidally-disrupted debris returns to the black hole -- from\nprogenitors generated with the stellar evolution code {\\sc mesa} to $\\gamma =\n4/3$ and $\\gamma = 5/3$ polytropes. We find that {\\sc mesa}-generated density\nprofiles yield qualitatively-different fallback rates as compared to polytropic\napproximations, and that only the fallback curves from low-mass ($1M_{\\odot}$\nor less), zero-age main-sequence stars are well fit by either a $\\gamma = 4/3$\nor $5/3$ polytrope. Stellar age has a strong affect on the shape of the\nfallback curve, and can produce characteristic timescales (e.g., the time to\nthe peak of the fallback rate) that greatly differ from the polytropic values.\nWe use these differences to assess the degree to which the inferred black hole\nmass from the observed lightcurve can deviate from the true value, and find\nthat the discrepancy can be at the order of magnitude level. Accurate stellar\nstructure also leads to a substantial variation in the critical impact\nparameter at which the star is fully disrupted, and can increase the\nsusceptibility of the debris stream to fragmentation under its own\nself-gravity. These results suggest that detailed modeling is required to\naccurately interpret observed lightcurves of TDEs.", "category": "astro-ph_HE" }, { "text": "Spherical configuration of a super-dense hot compact object with\n particular EoS: The equation of state (EoS) $P = P (\\rho, ...)$ -- pressure as a function of\ndensity and other thermodynamical quantities -- is what generates\nparticularities of mass--radius distribution $M (R)$ for super--dense compact\nstellar bodies, the remnants of cosmic cataclysms. In view of recent nuclear\nexperiments, we propose one particular EoS, which admits the critical state\ncharacterized by density $\\rho_c$ and temperature $T_c$, and which under\ncertain conditions permits a radial distribution of the super--dense matter in\n\"liquid\" phase. We establish such conditions and demonstrate that a stable\nconfiguration is indeed possible (only) for temperatures smaller than the\ncritical one. Using Tolman--Oppenheimer--Volkoff equations for hydrostatic\nequilibrium, we derive the mass--radius relation for the super--dense compact\nobjects with masses smaller than the Sun, $M \\ll M_{\\odot}$. The obtained\nresults are within the constraints established by both heavy--ion collision\nexperiments and theoretical studies of neutron--rich matter.", "category": "astro-ph_HE" }, { "text": "X-ray/UV/optical variability of NGC 4593 with Swift: Reprocessing of\n X-rays by an extended reprocessor: We report the results of intensive X-ray, UV and optical monitoring of the\nSeyfert 1 galaxy NGC 4593 with Swift. There is no intrinsic flux-related\nspectral change in the the variable components in any band with small apparent\nvariations due only to contamination by a second constant component, possibly a\n(hard) reflection component in the X-rays and the (red) host galaxy in the\nUV/optical bands. Relative to the shortest wavelength band, UVW2, the lags of\nthe other UV and optical bands are mostly in agreement with the predictions of\nreprocessing of high energy emission from an accretion disc. The U-band lag is,\nhowever, far larger than expected, almost certainly because of reprocessed\nBalmer continuum emission from the more distant broad line region gas. The UVW2\nband is well correlated with the X-rays but lags by ~6x more than expected if\nthe UVW2 results from reprocessing of X-rays on the accretion disc. However, if\nthe lightcurves are filtered to remove variations on timescales >5d, the lag\napproaches the expectation from disc reprocessing. MEMEcho analysis shows that\ndirect X-rays can be the driver of most of the variations in the UV/optical\nbands as long as the response functions for those bands all have long tails (up\nto 10d) in addition to a strong peak (from disc reprocessing) at short lag\n(<1d). We interpret the tails as due to reprocessing from the surrounding gas.\nComparison of X-ray to UVW2 and UVW2 to V-band lags for 4 AGN, including NGC\n4593, shows that all have UVW2 to V-band lags which exceed the expectations\nfrom disc resprocessing by factor < 2. However the X-ray to UVW2 lags are,\nmostly, in greater excess from the expectations from disc reprocessing and\ndiffer between AGN. The largest excess is in NGC 4151. Absorption and\nscattering may be affecting X-ray to UV lags.", "category": "astro-ph_HE" }, { "text": "Evidence for powerful winds and the associated reverse shock as the\n origin of the Fermi bubbles: The Fermi bubbles are large gamma-ray-emitting structures. They are symmetric\nabout the Galactic Centre (GC), and their creation is therefore attributed to\nintensive energy injection at the GC. In this study, we focus on the\nnon-equilibrium X-ray gas structures associated with the bubbles. We show that\na combination of the density, temperature, and shock age profiles of the X-ray\ngas can be used to distinguish the energy injection mechanisms. By comparing\nthe results of numerical simulations with observations, we indicate that the\nbubbles were created by a fast wind from the GC because it generates a strong\nreverse shock and reproduces the observed temperature peak there. On the other\nhand, instantaneous energy injection at the GC cannot reproduce the temperature\nprofile. The wind had a speed of ~1000 km/s, and blew for ~10^7 yr. Because the\nmass flux of the wind is large, the entrainment of interstellar gas by\nwide-angle outflows from the black hole is required. Thus, the wind may be the\nsame as active galactic nuclei outflows often observed in other galaxies and\nthought to regulate the growth of galaxies and their central black holes.", "category": "astro-ph_HE" }, { "text": "Black hole lightning due to particle acceleration at subhorizon scales: Supermassive black holes with masses of millions to billions of solar masses\nare commonly found in the centers of galaxies. Astronomers seek to image jet\nformation using radio interferometry, but still suffer from insufficient\nangular resolution. An alternative method to resolve small structures is to\nmeasure the time variability of their emission. Here, we report on gamma-ray\nobservations of the radio galaxy IC 310 obtained with the MAGIC telescopes\nrevealing variability with doubling time scales faster than 4.8 min. Causality\nconstrains the size of the emission region to be smaller than 20\\% of the\ngravitational radius of its central black hole. We suggest that the emission is\nassociated with pulsar-like particle acceleration by the electric field across\na magnetospheric gap at the base of the radio jet.", "category": "astro-ph_HE" }, { "text": "Optical and Infrared Photometry of the Nearby SN 2017cbv: On 2017 March 11, the DLT40 Transient Discovery Survey discovered SN 2017cbv\nin NGC5643, a Type 2 Seyfert Galaxy in the Lupus Constellation. SN 2017cbv went\non to become a bright Type Ia supernova, with a $V_{max}$ of 11.51 $\\pm$ 0.05\nmag. We present early time optical and infrared photometry of SN 2017cbv\ncovering the rise and fall of over 68 days. We find that SN 2017cbv has a broad\nlight curve $\\Delta m_{15}(B)$ = 0.88 $\\pm$ 0.07, a $B$-band maximum at\n2457840.97 $\\pm$ 0.43, a negligible host galaxy reddening where $E(B-V)_{host}$\n$\\approx$ 0, and a distance modulus of 30.49 $\\pm$ 0.32 to the SN,\ncorresponding to a distance of $12.58_{-1.71}^{+1.98}$ Mpc. We also present the\nresults of two different numerical models we used for analysis in this paper:\nSALT2, an empirical model for Type Ia supernova optical light curves that\naccounts for variability components; and SNooPy, the CSP-II light-curve model\nthat covers both optical and near-infrared wavelengths and is used for distance\nestimates.", "category": "astro-ph_HE" }, { "text": "Rapid spectral transition of the black hole binary V404 Cyg: During the June 2015 outburst of the black hole binary V404 Cyg, rapid\nchanges in the X-ray brightness and spectra were common. The INTEGRAL\nmonitoring campaign detected spectacular Eddington-limited X-ray flares, but\nalso rapid variations at much lower flux levels. On 2015 June 21 at 20 h 50\nmin, the 3-10 keV JEM-X data as well as simultaneous optical data started to\ndisplay a gradual brightening from one of these low-flux states. This was\nfollowed 15 min later by an order-of-magnitude increase of flux in the 20-40\nkeV IBIS/ISGRI light curve in just 15 s. The best-fitting model for both the\npre- and post-transition spectra required a Compton-thick partially covering\nabsorber. The absorber parameters remained constant, but the spectral slope\nvaried significantly during the event, with the photon index decreasing from\n$\\Gamma \\approx 3.7$ to $\\Gamma \\approx 2.3$. We propose that the rapid 20-40\nkeV flux increase was either caused by a spectral state transition that was\nhidden from our direct view, or that there was a sudden reduction in the amount\nof Compton down-scattering of the primary X-ray emission in the disk outflow.", "category": "astro-ph_HE" }, { "text": "Astro 2020 Science White Paper: Cosmic-ray Antinuclei as Messengers for\n Dark Matter: The origin of dark matter is a driving question of modern physics. Low-energy\nantideuterons provide a \"smoking gun\" signature of dark matter annihilation or\ndecay, essentially free of astrophysical background. Low-energy antiprotons are\na vital partner for this analysis, and low-energy antihelium could provide\nfurther discovery space for new physics. In the coming decade, AMS-02 will\ncontinue accumulating the large statistics and systematic understanding\nnecessary for it to probe rare antinuclei signatures, and GAPS, which is the\nfirst experiment optimized specifically for low-energy cosmic antinuclei, will\nbegin several Antarctic balloon campaigns. The connection of cosmic-ray\nantinuclei and dark matter is reviewed and the outlook in light of experimental\nprogress is presented.", "category": "astro-ph_HE" }, { "text": "Binary evolution with LOFT: This is a White Paper in support of the mission concept of the Large\nObservatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We\ndiscuss the potential of LOFT for the study of very faint X-ray binaries,\norbital period distribution of black hole X-ray binaries and neutron star spin\nup. For a summary, we refer to the paper.", "category": "astro-ph_HE" }, { "text": "Old Data, New Forensics: The First Second of SN 1987A Neutrino Emission: The next Milky Way supernova will be an epochal event in multi-messenger\nastronomy, critical to tests of supernovae, neutrinos, and new physics.\nRealizing this potential depends on having realistic simulations of core\ncollapse. We investigate the neutrino predictions of nearly all modern models\n(1-, 2-, and 3-d) over the first $\\simeq$1 s, making the first detailed\ncomparisons of these models to each other and to the SN 1987A neutrino data.\nEven with different methods and inputs, the models generally agree with each\nother. However, even considering the low neutrino counts, the models generally\ndisagree with data. What can cause this? We show that neither neutrino\noscillations nor different progenitor masses appear to be a sufficient\nsolution. We outline urgently needed work.", "category": "astro-ph_HE" }, { "text": "Strong Calcium Emission Indicates that the Ultraviolet-Flashing Type Ia\n SN 2019yvq was the Result of a Sub-Chandrasekhar Mass Double-Detonation\n Explosion: We present nebular spectra of the Type Ia supernova (SN Ia) SN 2019yvq, which\nhad a bright flash of blue and ultraviolet light after exploding, followed by a\nrise similar to other SNe Ia. Although SN 2019yvq displayed several other rare\ncharacteristics such as persistent high ejecta velocity near peak brightness,\nit was not especially peculiar and if the early \"excess\" emission were not\nobserved, it would likely be included in cosmological samples. The excess flux\ncan be explained by several different physical models linked to the details of\nthe progenitor system and explosion mechanism. Each has unique predictions for\nthe optically thin emission at late times. In our nebular spectra, we detect\nstrong [Ca II] $\\lambda\\lambda$7291, 7324 and Ca NIR triplet emission,\nconsistent with a double-detonation explosion. We do not detect H, He, or [O I]\nemission, predictions for some single-degenerate progenitor systems and violent\nwhite dwarf mergers. The amount of swept-up H or He is <2.8 x 10^-4 and 2.4 x\n10^-4 Msun, respectively. Aside from strong Ca emission, the SN 2019yvq nebular\nspectrum is similar to those of typical SNe Ia with the same light-curve shape.\nComparing to double-detonation models, we find that the Ca emission is\nconsistent with a model with a total progenitor mass of 1.15 Msun. However, we\nnote that a lower progenitor mass better explains the early light-curve and\npeak luminosity. The unique properties of SN 2019yvq suggest that thick\nHe-shell double-detonations only account for $1.1^{+2.1}_{-1.1}\\%$ of the total\n\"normal\" SN Ia rate. SN 2019yvq is one of the best examples yet that multiple\nprogenitor channels appear necessary to reproduce the full diversity of\n\"normal\" SNe Ia.", "category": "astro-ph_HE" }, { "text": "Extreme Gravity and Fundamental Physics: Future gravitational-wave observations will enable unprecedented and unique\nscience in extreme gravity and fundamental physics answering questions about\nthe nature of dynamical spacetimes, the nature of dark matter and the nature of\ncompact objects.", "category": "astro-ph_HE" }, { "text": "X-ray Polarization of Gamma-Ray Bursts: The degree and the temporal evolution of linear polarization in the prompt\nand afterglow emission of gamma-ray bursts is a very robust diagnostic of some\nkey features of gamma-ray bursts jets and their micro and macro physics. In\nthis contribution, I review the current status of the theory of polarized\nemission from GRB jets during the prompt, optical flash, and afterglow\nemission. I compare the theoretical predictions to the available observations\nand discuss the future prospect from both the theoretical and observational\nstandpoints.", "category": "astro-ph_HE" }, { "text": "Impact of the Cherenkov Telescope Array (CTA) altitude on Dark Matter\n searches in the Milky Way Halo: Observations of dwarf galaxies and of the Milky Way halo with current\nground-based Cherenkov telescopes have resulted in interesting limits on the\ncross-section for dark matter (DM) self- annihilation for WIMP masses above\nsome 100 GeV. The future Cherenkov Telescope Array (CTA) is expected to further\nexplore the parameter space of dark matter candidates that are predicted in\nextensions of the standard model of particle physics. Due to its low energy\nthreshold (of order of few tens of GeV) and high sensitivity, CTA will also\nprobe lower WIMP masses than current experiments, but the actual performance in\nthis regime will be influenced by the altitude of the observatory above sea\nlevel. Using the response of possible CTA candidate arrays to simulated photons\nand hadrons, we estimate how searches for a WIMP annihilation signal from the\nMilky Way halo will be influenced by altitude of different possible CTA sites.", "category": "astro-ph_HE" }, { "text": "Spectroscopic r-Process Abundance Retrieval for Kilonovae II:\n Lanthanides in the Inferred Abundance Patterns of Multi-Component Ejecta from\n the GW170817 Kilonova: In kilonovae, freshly-synthesized $r$-process elements imprint features on\noptical spectra, as observed in AT2017gfo, the counterpart to the GW170817\nbinary neutron star merger. However, measuring the $r$-process compositions of\nthe merger ejecta is computationally challenging. Vieira et al. (2023)\nintroduced Spectroscopic $r$-Process Abundance Retrieval for Kilonovae (SPARK),\na software tool to infer elemental abundance patterns of the ejecta, and\nassociate spectral features with particular species. Previously, we applied\nSPARK to the 1.4 day spectrum of AT2017gfo and inferred its abundance pattern\nfor the first time, characterized by electron fraction $Y_e=0.31$, a\nsubstantial abundance of strontium, and a dearth of lanthanides and heavier\nelements. This ejecta is consistent with wind from a remnant hypermassive\nneutron star and/or accretion disk. We now extend our inference to spectra at\n2.4 and 3.4 days, and test the need for multicomponent ejecta, where we\nstratify the ejecta in composition. The ejecta at 1.4 and 2.4 days is described\nby the same single blue component. At 3.4 days, a new redder component with\nlower $Y_e=0.16$ and a significant abundance of lanthanides emerges. This new\nredder component is consistent with dynamical ejecta and/or neutron-rich ejecta\nfrom a magnetized accretion disk. As expected from photometric modelling, this\ncomponent emerges as the ejecta expands, the photosphere recedes, and the\nearlier bluer component dims. At 3.4 days, we find an ensemble of lanthanides,\nwith the presence of cerium most concrete. This presence of lanthanides has\nimportant implications for the contribution of kilonovae to the $r$-process\nabundances observed in the Universe.", "category": "astro-ph_HE" }, { "text": "Multimessenger signal from phase transition of neutron star to quark\n star: Aim: To study the multimessenger nature of the signal that can result from\nthe phase transition of a neutron star to a quark star and their corresponding\nastrophysical observations. Methods: The phase transition process is initiated\nby the abrupt pressure and density changes at the star center, giving rise to a\nshock which deconfines matter followed by a weak front converting excess down\nto strange quarks to attain absolute stability. This process's effects are\ninvestigated by understanding how the energy escapes from the star in the form\nof neutrino-antineutrino annihilation. For such annihilation process, the\ncorresponding energy deposition rate is calculated. Structural changes due to\nthe energy loss have been investigated in the likes of misalignment angle\nevolution of the star and its astrophysical observation through gravitational\nwaves. Results: The energy and time signature for the neutrino-antineutrino\nannihilation is compared with the observed isotropic energy for a short\ngamma-ray burst. The misalignment angle evolves to align the star's tilt axis,\nwhich can lead to the sudden increase or decrease of radio intensity from the\npulsar. The corresponding gravitational wave emission, both continuous and\nburst, all lead towards multimessenger signals coming from the phase\ntransition.", "category": "astro-ph_HE" }, { "text": "Constraining Progenitors of Observed LMXBs Using CARB Magnetic Braking: We present a new method for constraining the mass transfer evolution of low\nmass X-ray binaries (LMXBs) - a reverse population synthesis technique. This is\ndone using the detailed 1D stellar evolution code MESA (Modules for Experiments\nin Stellar Astrophysics) to evolve a high-resolution grid of binary systems\nspanning a comprehensive range of initial donor masses and orbital periods. We\nuse the recently developed Convection And Rotation Boosted (CARB) magnetic\nbraking scheme. The CARB magnetic braking scheme is the only magnetic braking\nprescription capable of reproducing an entire sample of well studied persistent\nLMXBs -- those with their mass ratios, periods and mass transfer rates that\nhave been observationally determined. Using the reverse population synthesis\ntechnique, where we follow any simulated system that successfully reproduces an\nobserved LMXB backwards, we have constrained possible progenitors for each\nobserved well-studied persistent LMXB. We also determined that the minimum\nnumber of LMXB formations in the Milky Way is 1500 per Gyr if we exclude Cyg\nX-2. For Cyg X-2, the most likely formation rate is 9000 LMXB per Gyr. The\ntechnique we describe can be applied to any observed LMXB with well-constrained\nmass ratios, period and mass transfer rate. With the upcoming GAIA DR3\ncontaining information on binary systems, this technique can be applied to the\ndata release to search for progenitors of observed persistent LMXBs.", "category": "astro-ph_HE" }, { "text": "Novel dark matter constraints from antiprotons in the light of AMS-02: We evaluate dark matter (DM) limits from cosmic-ray antiproton observations\nusing the recent precise AMS-02 measurements. We properly take into account\ncosmic-ray propagation uncertainties fitting at the same time DM and\npropagation parameters, and marginalizing over the latter. We find a\nsignificant (~4.5 sigma) indication of a DM signal for DM masses near 80 GeV,\nwith a hadronic annihilation cross-section close to the thermal value, sigma v\n~3e-26 cm3s-1. Intriguingly, this signal is compatible with the DM\ninterpretation of the Galactic center gamma-ray excess. Confirmation of the\nsignal will require a more accurate study of the systematic uncertainties,\ni.e., the antiproton production cross-section, and modelling of the solar\nmodulation effect. Interpreting the AMS-02 data in terms of upper limits on\nhadronic DM annihilation, we obtain strong constraints excluding a thermal\nannihilation cross-section for DM masses below about 50 GeV and in the range\nbetween approximately 150 and 500 GeV, even for conservative propagation\nscenarios. Except for the range around 80 GeV, our limits are a factor 4\nstronger than the limits from gamma-ray observations of dwarf galaxies.", "category": "astro-ph_HE" }, { "text": "Cosmic-ray Antinuclei as Messengers of New Physics: Status and Outlook\n for the New Decade: The precise measurement of cosmic-ray antinuclei serves as an important means\nfor identifying the nature of dark matter and other new astrophysical\nphenomena, and could be used with other cosmic-ray species to understand\ncosmic-ray production and propagation in the Galaxy. For instance, low-energy\nantideuterons would provide a \"smoking gun\" signature of dark matter\nannihilation or decay, essentially free of astrophysical background. Studies in\nrecent years have emphasized that models for cosmic-ray antideuterons must be\nconsidered together with the abundant cosmic antiprotons and any potential\nobservation of antihelium. Therefore, a second dedicated Antideuteron Workshop\nwas organized at UCLA in March 2019, bringing together a community of theorists\nand experimentalists to review the status of current observations of cosmic-ray\nantinuclei, the theoretical work towards understanding these signatures, and\nthe potential of upcoming measurements to illuminate ongoing controversies.\nThis review aims to synthesize this recent work and present implications for\nthe upcoming decade of antinuclei observations and searches. This includes\ndiscussion of a possible dark matter signature in the AMS-02 antiproton\nspectrum, the most recent limits from BESS Polar-II on the cosmic antideuteron\nflux, and reports of candidate antihelium events by AMS-02; recent collider and\ncosmic-ray measurements relevant for antinuclei production models; the state of\ncosmic-ray transport models in light of AMS-02 and Voyager data; and the\nprospects for upcoming experiments, such as GAPS. This provides a roadmap for\nprogress on cosmic antinuclei signatures of dark matter in the coming years.", "category": "astro-ph_HE" }, { "text": "A Numerical Model for the Multi-wavelength Lightcurves of PSR J0030+0451: Recent modeling of Neutron Star Interior Composition Explorer(NICER)\nobservations of the millisecond pulsar PSR J0030+0451 suggests that the\nmagnetic field of the pulsar is non-dipolar. We construct a magnetic field\nconfiguration where foot points of the open field lines closely resemble the\nhotspot configuration from NICER observations. Using this magnetic field as\ninput, we perform force-free simulations of the magnetosphere of PSR\nJ0030+0451, showing the three-dimensional structure of its plasma-filled\nmagnetosphere. Making simple and physically motivated assumptions about the\nemitting regions, we are able to construct the multi-wavelength lightcurves\nthat qualitatively agree with the corresponding observations. The agreement\nsuggests that multipole magnetic structures are the key to modeling this type\nof pulsars, and can be used to constrain the magnetic inclination angle and the\nlocation of radio emission.", "category": "astro-ph_HE" }, { "text": "Spheroidal force-free neutron star magnetospheres: Abridged. Fast rotating and self-gravitating astrophysical objects suffer\nstrong deformations from centrifugal forces. If moreover they are magnetized,\nthey generate an electromagnetic wave that is perturbed accordingly. When\nstellar objects are also surrounded by an ideal plasma, a magnetosphere is\nformed. We study the electromagnetic configuration of a force-free\nmagnetosphere encompassing an ideal spheroidal rotating conductor as an inner\nboundary. We put special emphasize to millisecond period neutron star\nmagnetospheres, those showing a significant oblate shape. Force-free solutions\nare computed by numerical integration of the time-dependent Maxwell equations\nin spheroidal coordinates. Relevant quantities such as the magnetic field\nstructure, the spin down luminosity, the polar cap rims and the current density\nare shown. We find that the force-free magnetic field produced by spheroidal\nstars remains very similar to their spherical counterpart. However, the spin\ndown luminosity slightly decreases with increasing oblateness or prolateness.\nMoreover the polar cap area increases and always mostly encompasses the\nequivalent spherical star polar cap rims. The polar cap current density is also\ndrastically affected.", "category": "astro-ph_HE" }, { "text": "A study of the accretion mechanisms of the High Mass X-ray Binary IGR\n J00370+6122: IGR J00370+6122 is a high-mass X-ray binary, of which the primary is a B1 Ib\nstar, whereas the companion is suggested to be a neutron star by the detection\nof 346-s pulsation in one-off 4-ks observation. To better understand the nature\nof the compact companion, the present work performs timing and spectral studies\nof the X-ray data of this object, taken with XMM-Newton, Swift, Suzaku, RXTE,\nand INTEGRAL. In the XMM-Newton data, a sign of coherent 674 s pulsation was\ndetected, for which the previous 346-s period may be the 2nd harmonic. The\nspectra exhibited the \"harder when brighter\" trend in the 1$-$10 keV range, and\na flat continuum without clear cutoff in the 10$-$80 keV range. These\nproperties are both similar to those observed from several low-luminosity\naccreting pulsars, including X Persei in particular. Thus, the compact object\nin IGR J00370+6122 is considered to be a magnetized neutron star with a rather\nlow luminosity. The orbital period was refined to $15.6649 \\pm 0.0014$ d. Along\nthe orbit, the luminosity changes by 3 orders of magnitude, involving a sudden\ndrop from $\\sim 4 \\times 10^{33}$ to $\\sim 1\\times10^{32}$ erg s$^{-1}$ at an\norbital phase of 0.3 (and probably vice verse at 0.95). Although these\nphenomena cannot be explained by a simple Hoyle-Lyttleton accretion from the\nprimary's stellar winds, they can be explained when incorporating the propeller\neffect with a strong dipole magnetic field of $\\sim 5 \\times10^{13}$ G.\nTherefore, the neutron star in IGR J00370+6122 may have a stronger magnetic\nfield compared to ordinary X-ray pulsars.", "category": "astro-ph_HE" }, { "text": "A Ray-Tracing Algorithm for Spinning Compact Object Spacetimes with\n Arbitrary Quadrupole Moments. II. Neutron Stars: A moderately spinning neutron star acquires an oblate shape and a spacetime\nwith a significant quadrupole moment. These two properties affect its apparent\nsurface area for an observer at infinity, as well as the lightcurve arising\nfrom a hot spot on its surface. In this paper, we develop a ray-tracing\nalgorithm to calculate the apparent surface areas of moderately spinning\nneutron stars making use of the Hartle-Thorne metric. This analytic metric\nallows us to calculate various observables of the neutron star in a way that\ndepends only on its macroscopic properties and not on the details of its\nequation of state. We use this algorithm to calculate the changes in the\napparent surface area, which could play a role in measurements of neutron star\nradii and, therefore, in constraining their equation of state. We show that\nwhether the spinning neutron star appears larger or smaller than its\nnon-rotating counterpart depends primarily on its equatorial radius. For\nneutron stars with radii ~10 km, the corrections to the Schwarzschild spacetime\ncause the apparent surface area to increase with spin frequency. In contrast,\nfor neutron stars with radii ~15 km, the oblateness of the star dominates the\nspacetime corrections and causes the apparent surface area to decrease with\nincreasing spin frequency. In all cases, the change in the apparent geometric\nsurface area for the range of observed spin frequencies is < 5% and hence only\na small source of error in the measurement of neutron star radii.", "category": "astro-ph_HE" }, { "text": "Possible changes of state and relevant timescales for a neutron star in\n LS I +61\u00b0303: The properties of the short, energetic bursts recently observed from the\ngamma-ray binary LS I +61{\\deg}303, are typical of those showed by high\nmagnetic field neutron stars, and thus provide a strong indication in favor of\na neutron star being the compact object in the system. Here, we discuss the\ntransitions among the states accessible to a neutron star in a system like LS I\n+61{\\deg}303, such as the ejector, propeller and accretor phases, depending on\nthe NS spin period, magnetic field and rate of mass captured. We show how the\nobserved bolometric luminosity (>= few x 1E35 erg/s), and its broad-band\nspectral distribution, indicate that the compact object is most probably close\nto the transition between working as an ejector all along its orbit, and being\npowered by the propeller effect when it is close to the orbit periastron, in a\nso-called flip-flop state. By assessing the torques acting onto the compact\nobject in the various states, we follow the spin evolution of the system,\nevaluating the time spent by the system in each of them. Even taking into\naccount the constraint set by the observed gamma-ray luminosity, we found that\nthe total age of the system is compatible with being ~5-10 kyr, comparable to\nthe typical spin-down ages of high-field neutron stars. The results obtained\nare discussed in the context of the various evolutionary stages expected for a\nneutron star with a high mass companion.", "category": "astro-ph_HE" }, { "text": "Measurement of Anisotropy and Search for UHECR Sources: Ultra-high energy cosmic rays (UHECRs) are particles, likely protons and/or\nnuclei, with energies up to $10^{20}$ eV that are observed through the giant\nair showers they produce in the atmosphere. These particles carry the\ninformation on the most extreme phenomena in the Universe. At these energies,\neven charged particles could be magnetically rigid enough to keep track of, or\neven point directly to, the original positions of their sources on the sky. The\ndiscovery of anisotropy of UHECRs would thus signify opening of an entirely new\nwindow onto the Universe. With the construction and operation of the new\ngeneration of cosmic ray experiments -- the Pierre Auger Observatory in the\nSouthern hemisphere and the Telescope Array in the Northern one -- the study of\nthese particles, the most energetic ever detected, has experienced a jump in\nstatistics as well as in the data quality, allowing for a much better\nsensitivity in searching for their sources. In this review, we summarize the\nsearches for anisotropies and the efforts to identify the sources of UHECRs\nwhich have been carried out using these new data.", "category": "astro-ph_HE" }, { "text": "Antiproton Flux in Cosmic Ray Propagation Models with Anisotropic\n Diffusion: Recently a cosmic ray propagation model has been introduced, where\nanisotropic diffusion is used as a mechanism to allow for $\\mathcal{O}(100)$\nkm/s galactic winds. This model predicts a reduced antiproton background flux,\nsuggesting an excess is being observed. We implement this model in GALPROP\nv50.1 and perform a $\\chi^2$ analysis for B/C, $^{10}$Be/$^{9}$Be, and the\nrecent PAMELA $\\bar{p}/p$ datasets. By introducing a power-index parameter\n$\\alpha$ that dictates the dependence of the diffusion coefficient $D_{xx}$ on\nheight $|z|$ away from the galactic plane, we confirm that isotropic diffusion\nmodels with $\\alpha=0$ cannot accommodate high velocity convective winds\nsuggested by ROSAT, while models with $\\alpha=1$ ($D_{xx}\\propto |z|$) can give\na very good fit. A fit to B/C and $^{10}$Be/$^{9}$Be data predicts a lower\n$\\bar{p}/p$ flux ratio than the PAMELA measurement at energies between\napproximately 2 GeV to 20 GeV. A combined fit including in addition the\n$\\bar{p}/p$ data is marginal, suggesting only a partial contribution to the\nmeasured antiproton flux.", "category": "astro-ph_HE" }, { "text": "On the GeV excess in the diffuse \u03b3-ray emission towards the\n Galactic Center: The Fermi-LAT \\gamma-ray data have been used to study the morphological and\nspectral features of the so-called GeV excess - a diffuse radiation component\nrecently discovered towards the Galactic centre. We used the likelihood method\nto analyze Fermi-LAT data. Our study does confirm the existence of such an\nextra component in the diffuse \\gamma-ray emission at GeV energies. Based on a\ndetailed morphological analysis, a spatial template that fits the data best was\ngenerated and adopted. Using this template, the energy distribution of\n\\gamma-rays was derived in the 0.3-30 GeV energy interval. The spectrum\nappeared to have less distinct ('bump'-like) structure than previous reported.\nWe argue that the morphology of this radiation component has a bipolar rather\nthan a spherically symmetric structure as has been assumed a priori in previous\nstudies. This finding excludes the associations of the GeV excess with Dark\nMatter. We briefly discuss the radiation mechanisms and possible source\npopulations that could be responsible for this new component of diffuse gamma\nradiation.", "category": "astro-ph_HE" }, { "text": "Discovery of a jet-like structure with overionized plasma in the SNR\n IC443: IC443 is a supernova remnant located in a quite complex environment since it\ninteracts with nearby clouds. Indications for the presence of overionized\nplasma have been found though the possible physical causes of overionization\nare still debated. Moreover, because of its peculiar position and proper\nmotion, it is not clear if the pulsar wind nebula (PWN) within the remnant is\nthe relic of the IC443 progenitor star or just a rambling one seen in\nprojection on the remnant. Here we address the study of IC443 plasma in order\nto clarify the relationship PWN-remnant, the presence of overionization and the\norigin of the latter. We analyzed two \\emph{XMM-Newton} observations producing\nbackground-subtracted, vignetting-corrected and mosaicked images in two\ndifferent energy bands and we performed a spatially resolved spectral analysis\nof the X-ray emission. We identified an elongated (jet-like) structure with\nMg-rich plasma in overionization. The head of the jet is interacting with a\nmolecular cloud and the jet is aligned with the position of the PWN at the\ninstant of the supernova explosion. Interestingly, the direction of the jet of\nejecta is somehow consistent with the direction of the PWN jet. Our discovery\nof a jet of ejecta in IC443 enlarge the sample of core-collapse SNRs with\ncollimated ejecta structures. IC443's jet is the first one which shows\noverionized plasma, possibly associated with the adiabatic expansion of ejecta.\nThe match between the jet's direction and the original position of the PWN\nstrongly supports the association between the neutron star and IC443.", "category": "astro-ph_HE" }, { "text": "Growth of magnetic fields in accreting millisecond pulsars: R-modes can generate strong magnetic fields in the core of accreting\nmillisecond neutron stars (NSs). The diffusion of these fields outside the core\ncauses the growth of the external magnetic field and thus it affects the\nevolution of the spin down rates $\\dot{P}$ of the millisecond pulsars (MSPs).\nThe diffusion of the internal magnetic field provides a new evolutionary path\nfor the MSPs. This scenario could explain the large $\\dot{P}$ of the pulsars\nJ1823-3021A and J1824-2452A.", "category": "astro-ph_HE" }, { "text": "Fermi LAT Measurements of the Diffuse Gamma-Ray Emission at Intermediate\n Galactic Latitudes: The diffuse Galactic gamma-ray emission is produced by cosmic rays (CRs)\ninteracting with the interstellar gas and radiation field. Measurements by the\nEnergetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton\nGamma-Ray Observatory indicated excess gamma-ray emission > 1 GeV relative to\ndiffuse Galactic gamma-ray emission models consistent with directly measured CR\nspectra (the so-called ``EGRET GeV excess''). The excess emission was observed\nin all directions on the sky, and a variety of explanations have been proposed,\nincluding beyond-the-Standard-Model scenarios like annihilating or decaying\ndark matter. The Large Area Telescope (LAT) instrument on the Fermi Gamma-ray\nSpace Telescope has measured the diffuse gamma-ray emission with unprecedented\nsensitivity and resolution. We report on LAT measurements of the diffuse\ngamma-ray emission for energies 100 MeV to 10 GeV and Galactic latitudes 10\ndeg. <= |b| <= 20 deg. The LAT spectrum for this region of the sky is well\nreproduced by the diffuse Galactic gamma-ray emission models mentioned above\nand inconsistent with the EGRET GeV excess.", "category": "astro-ph_HE" }, { "text": "Joint constraints on the field-cluster mixing fraction, common envelope\n efficiency, and globular cluster radii from a population of binary hole\n mergers via deep learning: The recent release of the second Gravitational-Wave Transient Catalog\n(GWTC-2) has increased significantly the number of known GW events, enabling\nunprecedented constraints on formation models of compact binaries. One pressing\nquestion is to understand the fraction of binaries originating from different\nformation channels, such as isolated field formation versus dynamical formation\nin dense stellar clusters. In this paper, we combine the $\\texttt{COSMIC}$\nbinary population synthesis suite and the $\\texttt{CMC}$ code for globular\ncluster evolution to create a mixture model for black hole binary formation\nunder both formation scenarios. For the first time, these code bodies are\ncombined self-consistently, with $\\texttt{CMC}$ itself employing\n$\\texttt{COSMIC}$ to track stellar evolution. We then use a deep-learning\nenhanced hierarchical Bayesian analysis to constrain the mixture fraction $f$\nbetween formation models, while simultaneously constraining the common envelope\nefficiency $\\alpha$ assumed in $\\texttt{COSMIC}$ and the initial cluster virial\nradius $r_v$ assumed in $\\texttt{CMC}$. Under specific assumptions about other\nuncertain aspects of isolated binary and globular cluster evolution, we report\nthe median and $90\\%$ confidence interval of three physical parameters\n$(f,\\alpha,r_v)=(0.20^{+0.32}_{-0.18},2.26^{+2.65}_{-1.84},2.71^{+0.83}_{-1.17})$.\nThis simultaneous constraint agrees with observed properties of globular\nclusters in the Milky Way and is an important first step in the pathway toward\nlearning astrophysics of compact binary formation from GW observations.", "category": "astro-ph_HE" }, { "text": "Multi-wavelength temporal and spectral study of PKS 0402-362: We study the long-term behavior of the bright gamma-ray blazar PKS 0402-362.\nOver a span of approximately 12.5 years, from August 2008 to January 2021, we\ngathered Fermi-LAT temporal data and identified three distinct periods of\nintense $\\gamma$-ray activity. Notably, the second period exhibited the highest\nbrightness ever observed in this particular source. We observed most of the\n$\\gamma$-ray flare peaks to be asymmetric in profile suggesting a slow cooling\ntime of particles or the varying Doppler factor as the main cause of these\nflares. The $\\gamma$-ray spectrum is fitted with power-law and log-parabola\nmodels, and in both cases, the spectral index is very steep. The lack of time\nlags between optical-IR and $\\gamma$-ray emissions indicates the presence of a\nsingle-zone emission model. Using this information, we modeled the broadband\nSEDs with a simple one-zone leptonic model using the publicly available code\n`GAMERA'. The particle distribution index is found to be the same as expected\nin diffusive shock acceleration suggesting it as the main mechanism of particle\nacceleration to very high energy up to 4 - 6 GeV. During the different flux\nphases, we observed that the thermal disk dominates the optical emission,\nindicating that this source presents a valuable opportunity to investigate the\nconnection between the disk and the jet.", "category": "astro-ph_HE" }, { "text": "New limits on the local Lorentz invariance violation of gravity in the\n Standard-Model Extension with pulsars: Lorentz Violation (LV) is posited as a possible relic effect of quantum\ngravity at low energy scales. The Standard-Model Extension provides an\neffective field-theoretic framework for examining possible deviations\nattributed to LV. With their high observational accuracy, pulsars serve as\nideal laboratories for probing LV. In the presence of LV, both the spin\nprecession of solitary pulsars and orbital dynamics of binary pulsars would\nundergo modifications. Observations of pulse profiles and times of arrival\n(TOAs) of pulses allow for an in-depth investigation of these effects, leading\nto the establishment of strict limits on LV coefficients. We revisit the\nproject of limiting local LV with updated pulsar observations. We employ a new\nparameter estimation method and utilize state-of-the-art pulsar timing\nobservation data and get new limits on 8 linear combinations of LV coefficients\nbased on 25 tests from 12 different systems. Compared to previous limits from\npulsars, precision has improved by a factor of two to three. Additionally, we\nexplore prospects for further improvements from pulsars. Simulation results\nindicate that more observations of spin precession in solitary millisecond\npulsars could significantly enhance the accuracy of spatial LV coefficients,\npotentially by three to four orders of magnitude. As observational data\naccumulate, pulsars are anticipated to increasingly contribute to the tests of\nLV.", "category": "astro-ph_HE" }, { "text": "First 100 ms of a long-lived magnetized neutron star formed in a binary\n neutron star merger: The recent multimessenger observation of the short gamma-ray burst (SGRB) GRB\n170817A together with the gravitational wave (GW) event GW170817 provides\nevidence for the long-standing hypothesis associating SGRBs with binary neutron\nstar (BNS) mergers. The nature of the remnant object powering the SGRB, which\ncould have been either an accreting black hole (BH) or a long-lived magnetized\nneutron star (NS), is, however, still uncertain. General relativistic\nmagnetohydrodynamic (GRMHD) simulations of the merger process represent a\npowerful tool to unravel the jet launching mechanism, but so far most\nsimulations focused the attention on a BH as the central engine, while the\nlong-lived NS scenario remains poorly investigated. Here, we explore the latter\nby performing a GRMHD BNS merger simulation extending up to ~100 ms after\nmerger, much longer than any previous simulation of this kind. This allows us\nto (i) study the emerging structure and amplification of the magnetic field and\nobserve a clear saturation at magnetic energy $E_\\mathrm{mag} \\sim 10^{51}$\nerg, (ii) follow the magnetically supported expansion of the outer layers of\nthe remnant NS and its evolution into an ellipsoidal shape without any\nsurrounding torus, and (iii) monitor density, magnetization, and velocity along\nthe axis, observing no signs of jet formation. We also argue that the\nconditions at the end of the simulation disfavor later jet formation on\nsubsecond timescales if no BH is formed. Furthermore, we examine the rotation\nprofile of the remnant, the conversion of rotational energy associated with\ndifferential rotation, the overall energy budget of the system, and the\nevolution of the GW frequency spectrum. Finally, we perform an additional\nsimulation where we induce the collapse to a BH ~70 ms after merger, in order\nto gain insights on the prospects for massive accretion tori in case of a late\ncollapse. We find that...", "category": "astro-ph_HE" }, { "text": "Electron capture of strongly screening nuclides $^{56}$Fe, $^{56}$Co,\n $^{56}$Ni , $^{56}$Mn ,$^{56}$Cr and $^{56}$V in presupernova: According to the Shell-Model Monte Carlo method, basing on the Random Phase\nApproximation and the linear response theory, we carried out an estimation on\nelectron capture of strongly screening nuclides $^{56}$Fe, $^{56}$Co, $^{56}$Ni\n, $^{56}$Mn ,$^{56}$Cr and $^{56}$V in strong electron screening (SES)in\npresupernova. The\n EC rates are decreased greatly and even exceed $21.5\\%$ in SES. We also\ncompare our results with those of Aufderheide (AFUD), which calculated by the\nmethod of Aufderheide in SES. Our results are agreed reasonably well with AUFD\nat higher density-temperature surroundings (e.g. $\\rho_7>60, T_9=15.40$) and\nthe maximum error is $\\sim $0.5$\\%$. However, the maximum error is $\\sim\n$13.0$\\%$ at lower density surroundings (e.g. $^{56}$Cr at $\\rho_7=10,\nT_9=15.40, Y_e=0.41$ ). On the other hand, we also compared our results in SES\nwith those of FFN's and Nabi's, which is in the case without SES. The\ncomparisons show that our results are lower more than one order magnitude than\nFFN's, but about $7.23\\%$ than Nabi's.", "category": "astro-ph_HE" }, { "text": "Disentangling the Air Shower Components Using Scintillation and Water\n Cherenkov Detectors: We consider a ground array of scintillation and water Cherenkov detectors\nwith the purpose of determining the muon content of air showers. The different\nresponse characteristics of these two types of detectors to the components of\nthe air shower provide a way to infer their relative contributions. We use a\ndetailed simulation to estimate the impact of parameters, such as scintillation\ndetector size, in the determination of the size of the muon component.", "category": "astro-ph_HE" }, { "text": "Mass Distribution and Maximum Mass of Neutron Stars: Effects of Orbital\n Inclination Angle: Matter at ultra-high densities finds a physical realization inside neutron\nstars. One key property is their maximum mass, which has far-reaching\nimplications for astrophysics and the equation of state of ultra dense matter.\nIn this work, we employ Bayesian analysis to scrutinize the mass distribution\nand maximum mass threshold of galactic neutron stars. We compare two distinct\nmodels to assess the impact of assuming a uniform distribution for the most\nimportant quantity, the cosine of orbital inclination angles ($i$), which has\nbeen a common practice in previous analyses. This prevailing assumption yields\na maximum mass of $2.25$~$M_\\odot$ (2.15--3.32~$M_\\odot$ within $90\\%$\nconfidence), with a strong peak around the maximum value. However, in the\nsecond model, which indirectly includes observational constraints of $i$, the\nanalysis supports a mass limit of $2.56^{+0.87}_{-0.58}~M_\\odot$ ($2\\sigma$\nuncertainty), a result that points in the same direction as some recent results\ngathered from gravitational wave observations, although their statistics are\nstill limited. This work stresses the importance of an accurate treatment of\norbital inclination angles, and contributes to the ongoing debate about the\nmaximum neutron star mass, further emphasizing the critical role of\nuncertainties in the individual neutron star mass determinations.", "category": "astro-ph_HE" }, { "text": "Interacting Kilonovae: Long-lasting Electromagnetic Counterparts to\n Binary Mergers in the Accretion Disks of Active Galactic Nuclei: We investigate the dynamics and electromagnetic (EM) signatures of neutron\nstar-neutron star (NS-NS) or neutron star-black hole (NS-BH) merger ejecta that\noccurs in the accretion disk of an active galactic nucleus (AGN). We find that\nthe interaction between ejecta and disk gas leads to important effects on the\ndynamics and radiation. We show five stages of the ejecta dynamics:\ngravitational slowing down, coasting, Sedov-Taylor deceleration in the disk,\nre-acceleration after the breakout from the disk surface, and\nmomentum-conserved snowplow phase. Meanwhile, the radiation from the ejecta is\nso bright that its typical peak luminosity reaches a few times\n$10^{43}-10^{44}~\\rm erg~s^{-1}$. Since most of the radiation energy has\nconverted from the kinetic energy of merger ejecta, we call such an explosive\nphenomenon an interacting kilonova (IKN). It should be emphasized that IKNe are\nvery promising, bright EM counterparts to NS-NS/BH-NS merger events in AGN\ndisks. The bright peak luminosity and long rising time (i.e., ten to twenty\ndays in UV bands, thirty to fifty days in optical bands, and one hundred days\nto hundreds of days in IR bands) allow most survey telescopes to have ample\ntime to detect an IKN. However, the peak brightness, peak time, and evolution\npattern of the light curve of an IKN are similar to a superluminous supernova\nin a galactic nucleus and a tidal disruption event making it difficult to\ndistinguish between them. But it also suggests that IKNe might have been\npresent in recorded AGN transients.", "category": "astro-ph_HE" }, { "text": "The temperature of hot gas halos of early-type galaxies: Recently, the temperature T and luminosity L_X of the hot gas halos of early\ntype galaxies have been derived with unprecedented accuracy from Chandra data,\nfor 30 galaxies covering a wider range of galactic luminosity (and central\nvelocity dispersion sigma_c) than before. This work investigates the origin of\nthe observed temperatures, by examining the relationship between them and the\ngalaxy structure, the gas heating due to Type Ia supernovae (SNIa's) and the\ngravitational potential, and the dynamical status of the gas flow. In galaxies\nwith sigma_c<200 km/s, the T's are close to a fiducial average temperature for\nthe gas when in outflow; at 200100 GeV) gamma rays coming from AGN can pair-produce\non the intergalactic background light generating an electromagnetic cascade. If\nthe Intergalactic Magnetic Field (IGMF) is sufficiently strong, this cascade\nmay result in an extended isotropic emission of photons around the source, or\nhalo. Using VERITAS observations of the blazar Markarian 421, we search for\nextended emission by comparing the source angular distribution (${\\theta}^2$)\nfrom a quiescent period with one coming from a flare period, which can be\nconsidered as halo-free. ${\\chi}^2$ test showed no significant statistical\ndifferences between the samples, suggesting that the effect is either\nnon-existent or too weak to be detected. We calculated upper limits for the\nextended flux considering different angle ranges, the most stringent being <8%\nof the Crab Nebulae flux (C.U), in the range $0\\deg \\leq {\\theta} \\leq 0.1\\deg$ .", "category": "astro-ph_HE" }, { "text": "Enforcing causality in nonrelativistic equations of state at finite\n temperature: We present a thermodynamically consistent method by which equations of state\nbased on nonrelativistic potential models can be modified so that they respect\ncausality at high densities, both at zero and finite temperature (entropy). We\nillustrate the application of the method using the high density phase\nparametrization of the well known APR model in its pure neutron matter\nconfiguration as an example. We also show that, for models with only contact\ninteractions, the adiabatic speed of sound is independent of the temperature in\nthe limit of very large temperature. This feature is approximately valid for\nmodels with finite-range interactions as well, insofar as the temperature\ndependence they introduce to the Landau effective mass is weak. In addition,\nour study reveals that in first principle nonrelativistic models of hot and\ndense matter, contributions from higher than two-body interactions must be\nscreened at high density to preserve causality.", "category": "astro-ph_HE" }, { "text": "Low-Mass X-ray Binaries in Globular Clusters: Puzzles and Solutions: In dense stellar systems, dynamical interactions between objects lead to\nfrequent formation of exotic stellar objects, unusual binaries, and systems of\nhigher multiplicity. They are especially important for the formation of low\nmass X-ray binaries (LMXBs), which are not only formed 100 times more\nefficiently than in the field, but also have a puzzling dependence on\nmetallicity. In this contribution we review how compact objects are formed and\nretained, the mechanisms of dynamical formation and the specifics of the\nevolution of mass-transferring binaries with neutron stars and black holes in\nglobular clusters - those two kinds of compact objects have different favored\npaths to become luminous in X-ray. We describe how stellar evolution affects\nostensibly purely dynamical formation, producing the observed metallicity\ndependence for LMXBs. We also discuss the next puzzle to be solved on our\njourney to understand the link between LMXBs and millisecond pulsars formation.", "category": "astro-ph_HE" }, { "text": "Mode switching and oscillations in PSR B1828-11: The young pulsar PSR B1828-11 has long been known to show correlated shape\nand spin-down changes with timescales of roughly 500 and 250 days, perhaps\nassociated with large-scale magnetospheric switching. Here we present\nmulti-hour observations with the Parkes and Green Bank Telescopes at multiple\nphases across the roughly 500-day cycle and show that the pulsar undergoes\nmode-changing between two stable, extreme profile states. The fraction of time\nspent in each profile state naturally accounts for the observed overall \"shape\nparameter\" (defined to be 0 for wide profiles and 1 for narrow ones); this and\nthe variable rate of the mode transitions are directly related to the spin-down\nchanges. We observe that the mode transition rate could plausibly function as\nan additional parameter governing the chaotic behaviour in this object which\nwas proposed earlier by Seymour and Lorimer. Free precession is not needed to\naccount for the variations.", "category": "astro-ph_HE" }, { "text": "Multi-frequency VLBI Observations of the M 84 Inner Jet/Counterjet: Observational studies of inner-most regions of the edge-on jets in nearby\nactive galactic nuclei (AGN) are crucial to understand their kinematics and\nmorphology. For the inner jet of the nearby low luminosity AGN in M 84, we\npresent new high-sensitivity observations with very long baseline\ninterferometry since 2019, as well as archival Very Long Baseline Array\nobservations in 2014. We find that the compact core in M 84 has an\ninverted-to-flat spectrum from 1.5 to 88 GHz. Based on the turnover frequency\nof $4.2\\pm 0.2$ GHz in the spectrum, we estimated a magnetic field strength of\n1-10mG and an electron number density of $\\sim 10^5 cm^{-3}$ in the core\nregion. Three inner jet components within $\\sim 3$ mas from the core are\nidentified and traced in the images at 22 GHz, whose apparent speeds are 0.11\nc, 0.27 c, and 0.32 c, respectively. We calculate the viewing angle of $\\sim58$\ndegree for the inner jet based on the proper motion and the flux ratio of\njet-to-counterjet. A propagating sinusoidal model with a wavelength of $\\sim\n3.4$ mas is used to fit the helical morphology of the jet extended to 20 mas\n($\\sim 2.2\\times 10^4$ Schwarzschild Radii).", "category": "astro-ph_HE" }, { "text": "Fermi-LAT detection of G118.4+37.0: a supernova remnant in the Galactic\n halo seen around the Calvera pulsar: The discovery of a non-thermal radio ring of low surface brightness about one\ndegree in diameter has been recently reported around the location in the sky of\nthe Calvera pulsar, at a high Galactic latitude. The radio properties point to\nit likely being a new supernova remnant (SNR), G118.4+37.0. We report an\nanalysis of almost 14 years of observations of this region by the gamma-ray\nLarge Area Telescope onboard the Fermi satellite. We detect extended GeV\nemission consistent with the size and location of the radio source, which\nconfirms the presence of relativistic particles. The spectrum of the\nhigh-energy emission is fully compatible with an origin in the same\nrelativistic particles producing the radio emission. These features and its\nsimilarities to other isolated SNRs establish this source as the remnant of a\nsupernova. A simple model of the non-thermal emission from radio to GeV\nenergies resulting from leptonic emission from electrons produced by the SNR is\npresented. G118.4+37.0 and other similar isolated remnants could be part of a\nradio-dim SNR population evolving in low density environments showing hard GeV\nemission of leptonic origin. Future deeper surveys in radio and gamma-rays\ncould discover new members of the group.", "category": "astro-ph_HE" }, { "text": "Hydrodynamics of Core-Collapse Supernovae at the Transition to\n Explosion. I. Spherical Symmetry: We study the transition to runaway expansion of an initially stalled\ncore-collapse supernova shock. The neutrino luminosity, mass accretion rate,\nand neutrinospheric radius are all treated as free parameters. In spherical\nsymmetry, this transition is mediated by a global non-adiabatic instability\nthat develops on the advection time and reaches non-linear amplitude. Here we\nperform high-resolution, time-dependent hydrodynamic simulations of stalled\nsupernova shocks with realistic microphysics to analyze this transition. We\nfind that radial instability is a sufficient condition for runaway expansion if\nthe neutrinospheric parameters do not vary with time and if heating by the\naccretion luminosity is neglected. For a given unstable mode, transition to\nrunaway occurs when fluid in the gain region reaches positive specific energy.\nWe find approximate instability criteria that accurately describe the behavior\nof the system over a wide region of parameter space. The threshold neutrino\nluminosities are in general different than the limiting value for a\nsteady-state solution. We hypothesize that multidimensional explosions arise\nfrom the excitation of unstable large-scale modes of the turbulent background\nflow, at threshold luminosities that are lower than in the laminar case.", "category": "astro-ph_HE" }, { "text": "A NICER look at the state transitions of the black hole candidate MAXI\n J1535-571 during its reflares: The black hole candidate and X-ray binary MAXI J1535-571 was discovered in\nSeptember 2017. During the decay of its discovery outburst, and before\nreturning to quiescence, the source underwent at least four reflaring events,\nwith peak luminosities of $\\sim$10$^{35-36}$ erg s$^{-1}$ (d/4.1 kpc)$^2$. To\ninvestigate the nature of these flares, we analysed a sample of NICER\nobservations taken with almost daily cadence. In this work we present the\ndetailed spectral and timing analysis of the evolution of the four reflares.\nThe higher sensitivity of NICER at lower energies, in comparison with other\nX-ray detectors, allowed us to constrain the disc component of the spectrum at\n$\\sim$0.5 keV. We found that during each reflare the source appears to trace\nout a q-shaped track in the hardness-intensity diagram similar to those\nobserved in black hole binaries during full outbursts. MAXI J1535-571 transits\nbetween the hard state (valleys) and softer states (peaks) during these flares.\nMoreover, the Comptonised component is undetected at the peak of the first\nreflare, while the disc component is undetected during the valleys. Assuming\nthe most likely distance of 4.1 kpc, we find that the hard-to-soft transitions\ntake place at the lowest luminosities ever observed in a black hole transient,\nwhile the soft-to-hard transitions occur at some of the lowest luminosities\never reported for such systems.", "category": "astro-ph_HE" }, { "text": "Inclination Dependence of The Time-Lag -- Photon-Index Correlation in\n BHXRBs and its Explanation with a Simple Jet Model: Recently, we reported an observational correlation between a) the time-lag of\nthe hard (9 - 15 keV) with respect to the soft (2 - 5 keV) X-ray photons in\nblack-hole X-ray binaries (BHXRBs) and b) the power-law photon index $\\Gamma$\nof the X-ray spectrum. This was physically explained with a simple jet model,\ni.e., a model where the Comptonization (the Compton upscattering of soft\nphotons) happens in the jet. Here, we report the inclination dependence of this\ncorrelation, which we also explain with our jet model. Photons that emerge at\ndifferent polar angles from the jet axis have different spectra and different\ntime-lags. Because of this, we can explain quantitatively the type-B QPOs of GX\n339-4 as resulting from a precessing jet.", "category": "astro-ph_HE" }, { "text": "Superluminal Waves and the Structure of Pulsar Wind Termination Shocks: The termination shock of a pulsar wind is located roughly where the ram\npressure matches that of the surrounding medium. Downstream of the shock, MHD\nmodels of the diffuse nebular emission suggest the plasma is weakly magnetized.\nHowever, the transition from a Poynting-dominated MHD wind to a\nparticle-dominated flow is not well understood. We discuss a solution of this\n\"sigma problem\" in which a striped wind converts into a strong, superluminal\nelectromagnetic wave. This mode slows down as it propagates radially, and its\nram pressure tends to a constant value at large radius, a property we use to\nmatch the solution to the surrounding nebula. The wave thus forms a pre-cursor\nto the termination shock, which occurs at the point where the wave dissipates.\nPossible damping and dissipation mechanisms are discussed qualitatively.", "category": "astro-ph_HE" }, { "text": "Impact of Rastall gravity on mass, radius and sound speed of the pulsar\n PSR J0740+6620: Millisecond pulsars are perfect laboratories to test possible matter-geometry\ncoupling and its physical implications in light of recent Neutron Star Interior\nComposition Explorer (NICER) observations. We apply Rastall field equations of\ngravity, where matter and geometry are nonminimally coupled, to Krori-Barua\ninterior spacetime whereas the matter source is assumed to be anisotropic\nfluid. We show that all physical quantities inside the star can be expressed in\nterms of Rastall, $\\epsilon$, and compactness, $C=2GM/Rc^2$, parameters. Using\nNICER and X-ray Multi-Mirror X-ray observational constraints on the mass and\nradius of the pulsar PSR J0740+6620 we determine Rastall parameter to be at\nmost $\\epsilon=0.041$ in the positive range. The obtained solution provides a\nstable compact object; in addition the squared sound speed does not violate the\nconjectured sound speed $c_s^2\\leq c^2/3$ unlike the general relativistic\ntreatment. We note that no equations of state are assumed; the model however\nfits well with linear patterns with bag constants. In general, for\n$\\epsilon>0$, the theory predicts a slightly larger size star in comparison to\ngeneral relativity for the same mass. This has been explained as an additional\nforce, due to matter-geometry coupling, in the hydrodynamic equilibrium\nequation, which contributes to partially diminish the gravitational force\neffect. Consequently, we calculate the maximal compactness as allowed by the\nstrong energy condition to be $C = 0.735$ which is $\\sim 2\\%$ higher than\ngeneral relativity prediction. Moreover, for the surface density at saturation\nnuclear density $\\rho_{\\text{nuc}} = 2.7\\times 10^{14}$ g/cm$^3$ we estimate\nthe maximum mass $M=4 M_\\odot$ at radius $R=16$ km.", "category": "astro-ph_HE" }, { "text": "Identification of an X-ray Pulsar in the BeXRB system IGR J18219$-$1347: We report on observations of the candidate Be/X-ray binary IGR J18219$-$1347\nwith \\textit{Swift}/XRT, \\textit{NuSTAR}, and \\textit{NICER} during Type-I\noutbursts in March and June 2020. Our timing analysis revealed the spin period\nof a neutron star with $P_\\textrm{spin}=52.46$ s. This periodicity, combined\nwith the known orbital period of $72.4$ d, indicates that the system is a\nBeXRB. Furthermore, by comparing the infrared counterpart's spectral energy\ndistribution to known BeXRBs, we confirm this classification and set a distance\nof approximately $10-15$ kpc for the source. The source's broadband X-ray\nspectrum ($1.5-50$ keV) is described by an absorbed power-law with photon index\n$\\Gamma$\\,$\\sim$\\,$0.5$ and cutoff energy at $\\sim$\\,$13$ keV.", "category": "astro-ph_HE" }, { "text": "A real-time transient detector and the Living Swift-XRT Point Source\n catalogue: We present the Living Swift-XRT Point Source catalogue (LSXPS) and real-time\ntransient detector. This system allows us for the first time to carry out\nlow-latency searches for new transient X-ray events fainter than those\navailable to the current generation of wide-field imagers, and report their\ndetection in near real-time. Previously, such events could only be found in\ndelayed searches, e.g. of archival data; our low-latency analysis now enables\nrapid and ongoing follow up of these events, enabling the probing of timescales\npreviously inaccessible. The LSXPS is, uniquely among X-ray catalogues, updated\nin near real-time, making this the first up-to-date record of the point sources\ndetected by a sensitive X-ray telescope: the Swift-X-ray Telescope (XRT). The\nassociated upper limit calculator likewise makes use of all available data\nallowing contemporary upper limits to be rapidly produced on-demand. These\nfacilities, which enable the low-latency transient system are also fully\navailable to the community, providing a powerful resource for time-domain and\nmulti-messenger astrophysics.", "category": "astro-ph_HE" }, { "text": "SN 2020udy: a SN Iax with strict limits on interaction consistent with a\n helium-star companion: Early observations of transient explosions can provide vital clues to their\nprogenitor origins. In this paper we present the nearby Type Iax (02cx-like)\nsupernova (SN), SN 2020udy that was discovered within hours ($\\sim$7 hr) of\nestimated first light. An extensive dataset of ultra-violet, optical, and\nnear-infrared observations was obtained, covering out to $\\sim$150 d after\nexplosion. SN 2020udy peaked at -17.86$\\pm$0.43 mag in the r band and evolved\nsimilarly to other 'luminous' SNe Iax, such as SNe 2005hk and 2012Z. Its\nwell-sampled early light curve allows strict limits on companion interaction to\nbe placed. Main-sequence companion stars with masses of 2 and 6 M$_\\odot$ are\nruled out at all viewing angles, while a helium-star companion is allowed from\na narrow range of angles (140-180$^\\circ$ away from the companion). The spectra\nand light curves of SN2020udy are in good agreement with those of the 'N5def'\ndeflagration model of a near Chandrasekhar-mass carbon-oxygen white dwarf.\nHowever, as has been seen in previous studies of similar luminosity events, SN\n2020udy evolves slower than the model. Broad-band linear polarisation\nmeasurements taken at and after peak are consistent with no polarisation, in\nagreement with the predictions of the companion-star configuration from the\nearly light curve measurements. The host galaxy environment is low metallicity\nand is consistent with a young stellar population. Overall, we find the most\nplausible explosion scenario to be the incomplete disruption of a CO white\ndwarf near the Chandrasekhar-mass limit, with a helium-star companion.", "category": "astro-ph_HE" }, { "text": "On the morphology of outbursts of accreting millisecond X-ray pulsar\n Aquila X-1: We present the X-ray light curves of the last two outbursts --2014 & 2016--\nof the well known accreting millisecond X-ray pulsar (AMXP) Aquila X-1 using\nthe monitor of all sky X-ray image (MAXI) observations in the $2-20$ keV band.\nAfter calibrating the (MAXI) count rates to the all-sky monitor (ASM) level, we\nreport that the 2016 outburst is the most energetic event of Aql X-1, ever\nobserved from this source. We show that 2016 outburst is a member of the\nlong-high class according to the classification presented by G\\\"ung\\\"or et al.\nwith $\\sim68$ cnt/s maximum flux and $\\sim60$ days duration time and the\nprevious outburst, 2014, belongs to the short-low class with $\\sim 25$ cnt/s\nmaximum flux and $\\sim 30$ days duration time. In order to understand\ndifferences between outbursts, we investigate the possible dependence of the\npeak intensity to the quiescent duration leading to the outburst and find that\nthe outbursts following longer quiescent episodes tend to reach higher peak\nenergetic.", "category": "astro-ph_HE" }, { "text": "The 450 days X-ray monitoring of the changing-look AGN 1ES 1927+654: 1ES 1927+654 is a nearby active galactic nucleus (AGN) which underwent a\nchanging-look event in early 2018, developing prominent broad Balmer lines\nwhich were absent in previous observations. We have followed up this object in\nthe X-rays with an ongoing campaign that started in May 2018, and that includes\n265 NICER (for a total of 678ks) and 14 Swift/XRT (26ks) observations, as well\nas three simultaneous XMM-Newton/NuSTAR (158/169 ks) exposures. In the X-rays,\n1ES 1927+654 shows a behaviour unlike any previously known AGN. The source is\nextremely variable both in spectral shape and flux, and does not show any\ncorrelation between X-ray and UV flux on timescales of hours or weeks/months.\nAfter the outburst the power-law component almost completely disappeared, and\nthe source showed an extremely soft continuum dominated by a blackbody\ncomponent. The temperature of the blackbody increases with the luminosity,\ngoing from $kT\\sim 80$eV (for a 0.3--2keV luminosity of $L_{0.3-2}\\sim\n10^{41.5}\\rm\\,erg\\,s^{-1}$) to $\\sim 200$eV (for $L_{0.3-2}\\sim\n10^{44}\\rm\\,erg\\,s^{-1}$). The spectra show evidence of ionized outflows, and\nof a prominent feature at $\\sim 1$keV, which can be reproduced by a broad\nemission line. The unique characteristics of 1ES 1927+654 in the X-ray band\nsuggest that it belongs to a new type of changing-look AGN. Future X-ray\nsurveys might detect several more objects with similar properties.", "category": "astro-ph_HE" }, { "text": "Modelling the Type Ic SN 2004aw: a Moderately Energetic Explosion of a\n Massive C+O Star without a GRB: An analysis of the Type Ic supernova (SN) 2004aw is performed by means of\nmodels of the photospheric and nebular spectra and of the bolometric light\ncurve. SN2004aw is shown not to be ``broad-lined'', contrary to previous\nclaims, but rather a ``fast-lined'' SN Ic. The spectral resemblance to the\nnarrow-lined Type Ic SN1994I, combined with the strong nebular [O I] emission\nand the broad light curve, point to a moderately energetic explosion of a\nmassive C+O star. The ejected 56Ni mass is ~0.2 Msun. The ejecta mass as\nconstrained by the models is ~3-5 Msun, while the kinetic energy is estimated\nas KE ~3-6 e51 ergs. The ratio KE/Mej, the specific energy which influences the\nshape of the spectrum, is therefore ~1. The corresponding zero-age\nmain-sequence mass of the progenitor star may have been ~23-28 Msun. Tests show\nthat a flatter outer density structure may have caused a broad-lined spectrum\nat epochs before those observed without affecting the later epochs when data\nare available, implying that our estimate of KE is a lower limit. SN2004aw may\nhave been powered by either a collapsar or a magnetar, both of which have been\nproposed for gamma-ray burst-supernovae. Evidence for this is seen in the\ninnermost layers, which appear to be highly aspherical as suggested by the\nnebular line profiles. However, any engine was not extremely powerful, as the\nouter ejecta are more consistent with a spherical explosion and no gamma-ray\nburst was detected in coincidence with SN2004aw.", "category": "astro-ph_HE" }, { "text": "A ring-like accretion structure in M87 connecting its black hole and jet: The nearby radio galaxy M87 is a prime target for studying black hole\naccretion and jet formation^{1,2}. Event Horizon Telescope observations of M87\nin 2017, at a wavelength of 1.3 mm, revealed a ring-like structure, which was\ninterpreted as gravitationally lensed emission around a central black hole^3.\nHere we report images of M87 obtained in 2018, at a wavelength of 3.5 mm,\nshowing that the compact radio core is spatially resolved. High-resolution\nimaging shows a ring-like structure of 8.4_{-1.1}^{+0.5} Schwarzschild radii in\ndiameter, approximately 50% larger than that seen at 1.3 mm. The outer edge at\n3.5 mm is also larger than that at 1.3 mm. This larger and thicker ring\nindicates a substantial contribution from the accretion flow with absorption\neffects in addition to the gravitationally lensed ring-like emission. The\nimages show that the edge-brightened jet connects to the accretion flow of the\nblack hole. Close to the black hole, the emission profile of the jet-launching\nregion is wider than the expected profile of a black-hole-driven jet,\nsuggesting the possible presence of a wind associated with the accretion flow.", "category": "astro-ph_HE" }, { "text": "Predicting the X-ray polarization of type-2 Seyfert galaxies: Infrared, optical and ultraviolet spectropolarimetric observations have\nproven to be ideal tools for the study of the hidden nuclei of type-2 active\ngalactic nuclei (AGN) and for constraining the composition and morphology of\nthe sub-parsec scale emission components. In this paper, we extend the analysis\nto the polarization of the X-rays from type-2 AGN. Combining two radiative\ntransfer codes, we performed the first simulations of photons originating in\nthe gravity dominated vicinity of the black hole and scattering in structures\nall the way out to the parsec-scale torus and polar winds. We demonstrate that,\nwhen strong gravity effects are accounted for, the X-ray polarimetric signal of\nSeyfert-2s carries as much information about the central AGN components as\nspectropolarimetric observations of Seyfert-1s. The spectropolarimetric\nmeasurements can constrain the spin of the central supermassive black hole even\nin edge-on AGN, the hydrogen column density along the observer's line-of-sight,\nand the composition of the polar outflows. However, the polarization state of\nthe continuum source is washed out by multiple scattering, and should not be\nmeasurable unless the initial polarization is exceptionally strong. Finally, we\nestimate that modern X-ray polarimeters, either based on the photo-electric\neffect or on Compton scattering, will require long observational times on the\norder of a couple of mega-seconds to be able to properly measure the\npolarization of type-2 AGN.", "category": "astro-ph_HE" }, { "text": "Oscillation of high-energy neutrinos from choked jets in stellar and\n merger ejecta: We present a comprehensive study on oscillation of high-energy neutrinos from\ntwo different environments: blue supergiant progenitors that may harbor\nlow-power gamma-ray burst (GRB) jets and neutron star merger ejecta that would\nbe associated with short gamma-ray bursts. We incorporate the radiation\nconstraint that gives a necessary condition for nonthermal neutrino production,\nand account for the time evolution of the jet, which allows us to treat\nneutrino oscillation in matter more accurately. For massive star progenitors,\nneutrino injection inside the star can lead to nonadiabatic oscillation\npatterns in the 1 TeV - 10 TeV and is also visible in the flavor ratio. For\nneutron star merger ejecta, we find a similar behavior in the 100 GeV - 10 TeV\nregion and the oscillation may result in a $\\nu_e$ excess around 1 TeV. These\nfeatures, which enable us to probe the progenitors of long and short GRBs,\ncould be seen by future neutrino detectors with precise flavor ratio\nmeasurements. We also discuss potential contributions to the diffuse neutrino\nflux measured by IceCube, and find parameter sets allowing choked low-power GRB\njets to account for the neutrino flux in the 10 TeV - 100 TeV range without\nviolating the existing constraints.", "category": "astro-ph_HE" }, { "text": "X-ray line formation in the spectrum of SS 433: The mechanisms for the formation of X-ray lines in the spectrum of SS 433 are\ninvestigated by taking into account the radiative transfer inside the jets. The\nresults of Monte Carlo numerical simulations are presented. The effect of a\ndecrease in line intensity due to scattering inside the jet turns out to be\npronounced, but it does not exceed 60% in magnitude on the entire grid of\nparameters. The line broadening due to scattering, nutational motion, and the\ncontribution of satellites can lead to overestimates of the jet opening angle\n$\\Theta$ from the line widths in Chandra X-ray observations. The fine structure\nof the lines turns out to be very sensitive to the scattering effects. This\nmakes its investigation by planned X-ray observatories equipped with\nhigh-resolution spectrometers (primarily Astro-H) a powerful tool for\ndiagnosing the parameters of the jets in SS 433.", "category": "astro-ph_HE" }, { "text": "A radio structure resolved at the deca-parsec scale in radio-quiet\n quasar PDS 456 with an extremely powerful X-ray outflow: Active galactic nuclei (AGN) accreting at rates close to the Eddington limit\ncan host radiatively driven mildly relativistic outflows. Some of these X-ray\nabsorbing but powerful outflows may produce strong shocks resulting in a\nsignificant non-thermal emission. This outflow-driven radio emission may be\ndetectable in the radio-quiet quasar PDS 456 since it has a bolometric\nluminosity reaching the Eddington limit and a relativistic wide-aperture X-ray\noutflow with a kinetic power high enough to quench the star formation in its\nhost galaxy. To investigate this possibility, we performed very-long-baseline\ninterferometric (VLBI) observations of the quasar with the European VLBI\nNetwork (EVN) at 5 GHz. The EVN image with the full resolution reveals two\nfaint and diffuse radio components with a projected separation of about 20 pc\nand an average brightness temperature of around two million Kelvin. In relation\nto the optical sub-mas-accuracy position measured by the Gaia mission, the two\ncomponents are very likely on opposite sides of an undetected radio core. The\nVLBI structure at the deca-pc scale can thus be either a young jet or a\nbidirectional radio-emitting outflow, launched in the vicinity of a strongly\naccreting central engine. Two diffuse components at the hecto-pc scale, likely\nthe relic radio emission from the past AGN activity, are tentatively detected\non each side in the low-resolution EVN image.", "category": "astro-ph_HE" }, { "text": "Observations of diffuse fluxes of cosmic neutrinos: In this contribution the current observational results for the diffuse flux\nof high-energy astrophysical neutrinos are reviewed. In order to understand the\nscience implications, the measurements in different detection channels are\ndiscussed and results are compared. The discussion focuses is the energy\nspectrum, the flavor ratio and large scale anisotropy.", "category": "astro-ph_HE" }, { "text": "The Compton Spectrometer and Imager: The Compton Spectrometer and Imager (COSI) is a NASA Small Explorer (SMEX)\nsatellite mission in development with a planned launch in 2027. COSI is a\nwide-field gamma-ray telescope designed to survey the entire sky at 0.2-5 MeV.\nIt provides imaging, spectroscopy, and polarimetry of astrophysical sources,\nand its germanium detectors provide excellent energy resolution for emission\nline measurements. Science goals for COSI include studies of 0.511 MeV emission\nfrom antimatter annihilation in the Galaxy, mapping radioactive elements from\nnucleosynthesis, determining emission mechanisms and source geometries with\npolarization measurements, and detecting and localizing multimessenger sources.\nThe instantaneous field of view for the germanium detectors is >25% of the sky,\nand they are surrounded on the sides and bottom by active shields, providing\nbackground rejection as well as allowing for detection of gamma-ray bursts and\nother gamma-ray flares over most of the sky. In the following, we provide an\noverview of the COSI mission, including the science, the technical design, and\nthe project status.", "category": "astro-ph_HE" }, { "text": "Comments on \"Imaging Reanalyses of EHT Data\": On June 14, 2022, the EHT collaboration (hereafter EHTC) made the web page\n(https://eventhorizontelescope.org/blog/imaging-reanalyses-eht-data) with the\ntitle \"Imaging Reanalyses of EHT Data,\" in which they made comments on our\nrecent Miyoshi et al .2022 published in the Astrophysical Journal.\n We investigated the EHTC comments and found that all of the five points\nraised by the EHTC are subjective and unsubstantiated claims. Thus they do not\nprove the correctness of the result of EHTC. Sincerely we hope that the EHTC\nwill publish, not a collection of unsubstantiated claims, but a discussion\nbased on scientific arguments.", "category": "astro-ph_HE" }, { "text": "Gamma Ray Lines from a Universal Extra Dimension: Indirect Dark Matter searches are based on the observation of secondary\nparticles produced by the annihilation or decay of Dark Matter. Among them,\ngamma-rays are perhaps the most promising messengers, as they do not suffer\ndeflection or absorption on Galactic scales, so their observation would\ndirectly reveal the position and the energy spectrum of the emitting source.\nHere, we study the detailed gamma-ray energy spectrum of Kaluza--Klein Dark\nMatter in a theory with 5 Universal Extra Dimensions. We focus in particular on\nthe two body annihilation of Dark Matter particles into a photon and another\nparticle, which produces monochromatic photons, resulting in a line in the\nenergy spectrum of gamma rays. Previous calculations in the context of the five\ndimensional UED model have computed the line signal from annihilations into\n\\gamma \\gamma, but we extend these results to include \\gamma Z and \\gamma H\nfinal states. We find that these spectral lines are subdominant compared to the\npredicted \\gamma \\gamma signal, but they would be important as follow-up\nsignals in the event of the observation of the \\gamma \\gamma line, in order to\ndistinguish the 5d UED model from other theoretical scenarios.", "category": "astro-ph_HE" }, { "text": "Estimation of Plasma Parameters in an Accretion Column near the Surface\n of Accreting White Dwarfs from Their Flux Variability: We consider the behavior of matter in the accretion column that emerges under\naccretion in binary systems near the surface of a white dwarf. The plasma\nheated in a standing shock wave near the white dwarf surface efficiently\nradiates in the X-ray energy band. We suggest a method for estimating\npost-shock plasma parameters, such as the density, temperature, and height of\nthe hot zone, from the power spectrum of its X-ray luminosity variability. The\nmethod is based on the fact that the flux variability amplitude for the hot\nregion at various Fourier frequencies depends significantly on its cooling\ntime, which is determined by the parameters of the hot zone in the accretion\ncolumn. This allows the density and temperature of the hot matter to be\nestimated. We show that the characteristic cooling time can be efficiently\ndetermined from the break frequency in the power spectrum of the X-ray flux\nvariability for accreting white dwarfs. The currently available X-ray\ninstruments do not allow such measurements to be made because of an\ninsufficient collecting area, but this will most likely become possible with\nnew-generation large-area X-ray spectrometers.", "category": "astro-ph_HE" }, { "text": "Application of the MST clustering to the high energy gamma-ray sky. IV -\n Blazar candidates found as possible counterparts of photon clusters: We present the results of a cluster search in the Fermi-LAT Pass 8 gamma-ray\nsky by means of the Minimum Spanning Tree algorithm, at energies higher than 10\nGeV and at Galactic latitudes higher than 25 degrees. The selected clusters\nhave a minimum number of photons higher than or equal to 5, a high degree of\nconcentration, and are without a clear corresponding counterpart in blazar\ncatalogues. A sample of 30 possible gamma-ray sources was obtained. These\nobjects were verified by applying the standard Maximum Likelihood analysis on\nthe Fermi-LAT data. A search for possible radio counterparts in a circle having\na radius of 6 arcmin was performed, finding several interesting objects, the\nmajority of them without optical spectroscopical data. These can be considered\nas new blazar candidates. Some of them were already noticed as possible blazars\nor Active Galactic Nuclei in previous surveys, but never associated with high\nenergy emission. These possible counterparts are reported and their properties\nare discussed.", "category": "astro-ph_HE" }, { "text": "IceCube as a Multi-messenger Follow-up Observatory for Astrophysical\n Transients: The recent association between IC-170922A and the blazar TXS0506+056\nhighlights the importance of real-time observations for identifying possible\nastrophysical neutrino sources. Thanks to its near-100\\% duty cycle, 4$\\pi$\nsteradian field of view, and excellent sensitivity over many decades of energy,\nIceCube is well suited both to generate alerts for follow-up by other\ninstruments and to rapidly follow up alerts generated by other instruments.\nDetection of neutrinos in coincidence with transient astrophysical phenomena\nserves as a smoking gun for hadronic processes and supplies essential\ninformation about the identities and mechanisms of cosmic-ray accelerators. In\n2016, the IceCube Neutrino Observatory established a pipeline to rapidly search\nfor neutrinos from astrophysical transients on timescales ranging from a\nfraction of a second to multiple weeks. Since then, 67 dedicated analyses have\nbeen performed searching for associations between IceCube neutrinos and\nastrophysical transients reported by radio, optical, X-ray, and gamma-ray\ninstruments in addition to searching for lower energy neutrino signals in\nassociation with IceCube's own high-energy alerts. We present the event\nselection, maximum likelihood analysis method, and sensitivity of the IceCube\nreal-time pipeline. We also summarize the results of all follow-up analyses to\ndate.", "category": "astro-ph_HE" }, { "text": "Cosmic Gamma Ray Bursts: Gamma ray bursts (GRBs) are astronomical phenomena detected at highest\nenergies. The gamma ray photons carry energies on the order of\nmega-electronovolts and arrive to us from the point-like sources that are\nuniformly distributed on the sky. A typical burst has a form of a pulse that\nlasts for about a minute. As the Earth atmosphere is not transparent to the\nvery high energy radiation, the bursts are detected by means of the telescopes\nonboard satellites that are placed on the orbit. The total energetics of GRB\nevents, which is given by the integrated energy flux by the detector unit area,\nimplies that we are witnessing very powerful explosions, where an enormously\ngreat power is released within a short time. There is only one way to obtain\nsuch huge energies in cosmos: the disruption of a star.", "category": "astro-ph_HE" }, { "text": "Multimessenger observations of counterparts to IceCube-190331A: High-energy neutrinos are a promising tool for identifying astrophysical\nsources of high and ultra-high energy cosmic rays (UHECR). Prospects of\ndetecting neutrinos at high energies ($\\gtrsim$TeV) from blazars have been\nboosted after the recent association of IceCube-170922A and TXS 0506+056. We\ninvestigate the high-energy neutrino, IceCube-190331A, a high-energy starting\nevent (HESE) with a high likelihood of being astrophysical in origin. We\ninitiated a Swift/XRT and UVOT tiling mosaic of the neutrino localisation, and\nfollowed up with ATCA radio observations, compiling a multiwavelength SED for\nthe most likely source of origin. NuSTAR observations of the neutrino location\nand a nearby X-ray source were also performed. We find two promising\ncounterpart in the 90% confidence localisation region and identify the\nbrightest as the most likely counterpart. However, no Fermi/LAT $\\gamma$-ray\nsource and no prompt Swift/BAT source is consistent with the neutrino event. At\nthis point it is unclear whether any of the counterparts produced\nIceCube-190331A. We note that the Helix Nebula is also consistent with the\nposition of the neutrino event, and we calculate that associated particle\nacceleration processes cannot produce the required energies to generate a\nhigh-energy HESE neutrino.", "category": "astro-ph_HE" }, { "text": "On discrepancy between ATIC and Fermi data: Either ATIC or Fermi-LAT data can be fitted together with the PAMELA data by\nthree components: primary background ~ E^{-3.3}, secondary background ~\nE^{-3.6}, and an additional source of electrons ~ E^{-g_a} Exp(-E/E_{cut}). We\nfind that the best fits for ATIC + PAMELA and for Fermi + PAMELA are\napproximately the same, g_a ~ 2 and E_{cut} ~ 500 GeV. However, the ATIC data\nhave a narrow bump between 300 GeV and 600 GeV which contradicts the smooth\nFermi spectrum. An interpretation of the ATIC bump as well as the featureless\nFermi spectrum in terms of dark matter models and pulsars is discussed.", "category": "astro-ph_HE" }, { "text": "The Morphologies and Kinematics of Supernova Remnants: We review the major advances in understanding the morphologies and kinematics\nof supernova remnants (SNRs). Simulations of SN explosions have improved\ndramatically over the last few years, and SNRs can be used to test models\nthrough comparison of predictions with SNRs' observed large-scale compositional\nand morphological properties as well as the three-dimensional kinematics of\nejecta material. In particular, Cassiopeia A -- the youngest known\ncore-collapse SNR in the Milky Way -- offers an up-close view of the complexity\nof these explosive events that cannot be resolved in distant, extragalactic\nsources. We summarize the progress in tying SNRs to their progenitors'\nexplosions through imaging and spectroscopic observations, and we discuss\nexciting future prospects for SNR studies, such as X-ray microcalorimeters", "category": "astro-ph_HE" }, { "text": "Spectral and Timing Properties of IGR J17091-3624 in the Rising Hard\n State During its 2016 Outburst: We present a spectral and timing study of the NuSTAR and Swift observations\nof the black hole candidate IGR J17091-3624 in the hard state during its\noutburst in 2016. Disk reflection is detected in each of the NuSTAR spectra\ntaken in three epochs. Fitting with relativistic reflection models reveals that\nthe accretion disk is truncated during all epochs with $R_{\\rm in}>10~r_{\\rm\ng}$, with the data favoring a low disk inclination of $\\sim\n30^{\\circ}-40^{\\circ}$. The steepening of the continuum spectra between epochs\nis accompanied by a decrease in the high energy cut-off: the electron\ntemperature $kT_{\\rm e}$ drops from $\\sim 64$ keV to $\\sim 26$ keV, changing\nsystematically with the source flux. We detect type-C QPOs in the power spectra\nwith frequency varying between 0.131 Hz and 0.327 Hz. In addition, a secondary\npeak is found in the power spectra centered at about 2.3 times the QPO\nfrequency during all three epochs. The nature of this secondary frequency is\nuncertain, however a non-harmonic origin is favored. We investigate the\nevolution of the timing and spectral properties during the rising phase of the\noutburst and discuss their physical implications.", "category": "astro-ph_HE" }, { "text": "An unexpected drop in the magnetic field of the X-ray pulsar V0332+53\n after the bright outburst occurred in 2015: How the accreted mass settling on the surface of a neutron star affects the\ntopology of the magnetic field and how the secular evolution of the binary\nsystem depends on the magnetic field change is still an open issue. We report\nevidence for a clear drop in the observed magnetic field in the accreting\npulsar V0332+53 after undergoing a bright 3-month long X-ray outburst. We\ndetermine the field from the position of the fundamental cyclotron line in its\nX-ray spectrum and relate it to the luminosity. For equal levels of luminosity,\nin the declining phase we measure a systematically lower value of the cyclotron\nline energy with respect to the rising phase. This results in a drop of ~1.7 x\n10^11 G of the observed field between the onset and the end of the outburst.\nThe settling of the accreted plasma onto the polar cap seems to induce a\ndistortion of the magnetic field lines weakening their intensity along the\naccretion columns. Therefore the dissipation rate of the magnetic field could\nbe much faster than previously estimated, unless the field is able to restore\nits original configuration on a time-scale comparable with the outbursts\nrecurrence time.", "category": "astro-ph_HE" }, { "text": "ASASSN-18ey: The Rise of a New Black-Hole X-ray Binary: We present the discovery of ASASSN-18ey (MAXI J1820+070), a new black hole\nlow-mass X-ray binary discovered by the All-Sky Automated Survey for SuperNovae\n(ASAS-SN). A week after ASAS-SN discovered ASASSN-18ey as an optical transient,\nit was detected as an X-ray transient by MAXI/GCS. Here, we analyze ASAS-SN and\nAsteroid Terrestrial-impact Last Alert System (ATLAS) pre-outburst optical\nlight curves, finding evidence of intrinsic variability for several years prior\nto the outburst. While there was no long-term rise leading to outburst, as has\nbeen seen in several other systems, the start of the outburst in the optical\npreceded that in the X-rays by $7.20\\pm0.97~\\rm days$. We analyze the\nspectroscopic evolution of ASASSN-18ey from pre-maximum to $> 100~\\rm days$\npost-maximum. The spectra of ASASSN-18ey exhibit broad, asymmetric,\ndouble-peaked H$\\alpha$ emission. The Bowen blend ($\\lambda \\approx 4650$\\AA)\nin the post-maximum spectra shows highly variable double-peaked profiles,\nlikely arising from irradiation of the companion by the accretion disk, typical\nof low-mass X-ray binaries. The optical and X-ray luminosities of ASASSN-18ey\nare consistent with black hole low-mass X-ray binaries, both in outburst and\nquiescence.", "category": "astro-ph_HE" }, { "text": "What are Published X-ray lightcurves telling us about Young Supernova\n Expansion?: Massive stars lose mass in the form of stellar winds and outbursts. This\nmaterial accumulates around the star. When the star explodes as a supernova\n(SN) the resulting shock wave expands within this circumstellar medium. The\nX-ray emission resulting from the interaction depends, among other parameters,\non the density of this medium, and therefore the variation in the X-ray\nluminosity can be used to study the variation in the density structure of the\nmedium. In this paper we explore the X-ray emission and lightcurves of all\nknown SNe, in order to study the nature of the medium into which they are\nexpanding. In particular we wish to investigate whether young SNe are expanding\ninto a steady wind medium, as is most often assumed in the literature. We find\nthat in the context of the theoretical arguments that have been generally used\nin the literature, many young SNe, and especially those of Type IIn, which are\nthe brightest X-ray luminosity class, do not appear to be expanding into steady\nwinds. Some IIns appear to have very steep X-ray luminosity declines,\nindicating density declines much steeper than r$^{-2}$. However, other IIns\nshow a constant or even increasing X-ray luminosity over periods of months to\nyears. Many other SNe do not appear to have declines consistent with expansion\nin a steady wind. SNe with lower X-ray luminosities appear to be more\nconsistent with steady wind expansion, although the numbers are not large\nenough to make firm statistical comments. The numbers do indicate that the\nexpansion and density structure of the circumstellar medium must be\ninvestigated before assumptions can be made of steady wind expansion. Unless a\nsteady wind can be shown, mass-loss rates deduced using this assumption may\nneed to be revised.", "category": "astro-ph_HE" }, { "text": "Thick accretion disk and Its super Eddington luminosity around spinning\n blackholes: In the general accretion disk model theory, the accretion disk surrounding an\nastronomical object comprises fluid rings obeying Keplerian motion. However, we\nshould consider relativistic and rotational effects as we close in toward the\ncenter of accretion disk surrounding spinning compact massive objects such as a\nblack hole or a neutron star. In this study, we explore the geometry of the\ninner portion of the accretion disk in the context of Mukhopadhyay's pseudo\nNewtonian potential approximation for the full general relativity theory. We\nfound that the shape of the accretion disk \"puffs up\" or becomes thicker and\nthe luminosity of the disk could exceed the Eddington luminosity near the\nsurface of the compact spinning black hole.", "category": "astro-ph_HE" }, { "text": "Rapid Generation of Kilonova Light Curves Using Conditional Variational\n Autoencoder: The discovery of the optical counterpart, along with the gravitational waves\nfrom GW170817, of the first binary neutron star merger, opened up a new era for\nmulti-messenger astrophysics. Combining the GW data with the optical\ncounterpart, also known as AT2017gfo, classified as a kilonova, has revealed\nthe nature of compact binary merging systems by extracting enriched information\nabout the total binary mass, the mass ratio, the system geometry, and the\nequation of state. Even though the detection of kilonova brought about a\nrevolution in the domain of multi-messenger astronomy, since there has been\nonly one kilonova from a gravitational wave detected binary neutron star merger\nevent so far, this limits the exact understanding of the origin and propagation\nof the kilonova. Here, we use a conditional variational autoencoder trained on\nlight curve data from two kilonova models having different temporal lengths,\nand consequently, generate kilonova light curves rapidly based on physical\nparameters of our choice with good accuracy. Once trained, the time scale for\nlight curve generation is of the order of a few milliseconds, thus speeding up\ngenerating light curves by $1000$ times compared to the simulation. The mean\nsquared error between the generated and original light curves is typically\n$0.015$ with a maximum of $0.08$ for each set of considered physical parameter;\nwhile having a maximum of $\\approx0.6$ error across the whole parameter space.\nHence, implementing this technique provides fast and reliably accurate results.", "category": "astro-ph_HE" }, { "text": "The implications of large binding energies of massive stripped core\n collapse supernova progenitors on the explosion mechanism: We examine the binding energies of massive stripped-envelope core collapse\nsupernova (SECCSN) progenitors with the stellar evolution code MESA, and find\nthat the jittering jets explosion mechanism is preferred for explosions where\ncarbon-oxygen cores with masses of $>20 M_\\odot$ collapse to leave a neutron\nstar (NS) remnant. We calculate the binding energy at core collapse under the\nassumption that the remnant is a NS. Namely, stellar gas above mass coordinate\nof $~1.5-2.5 M_\\odot$ is ejected in the explosion. We find that the typical\nbinding energy of the ejecta of stripped-envelope progenitors with\ncarbon-oxygen core masses of $M_{CO} > 20 M_\\odot$ is $E_{bind}>2 \\times\n10^{51} erg$. We claim that jets are most likely to explode such cores as\njet-driven explosion mechanisms can supply high energies to the explosion. We\napply our results to SN 2020qlb, which is a SECCSN with a claimed core mass of\n$~30-50 M_\\odot$, and conclude that the jittering jets explosion mechanism best\naccounts for such an explosion that leaves a NS.", "category": "astro-ph_HE" }, { "text": "Energy Dependent Time Delays of kHz Oscillations due to Thermal\n Comptonization: We study the energy dependent photon variability from a thermal Comptonizing\nplasma that is oscillating at kHz frequencies. In particular, we solve the\nlinearised time dependent Kompaneets equation and consider the oscillatory\nperturbation to be either in the soft photon source or in the heating rate of\nthe plasma. For each case, we self consistently consider the energy balance of\nthe plasma and the soft photon source. The model incorporates the possibility\nof a fraction of the Comptonized photons impinging back into the soft photon\nsource. We find that when the oscillation is due to the soft photon source, the\nvariation of the fractional root mean sqaure (r.m.s) is nearly constant with\nenergy and the time-lags are hard. However, for the case when the oscillation\nis due to variation in the heating rate of the corona, and when a significant\nfraction of the photons impinge back into the soft photon source, the r.m.s\nincreases with energy and the time lags are soft. As an example, we compare the\nresults with the $\\sim 850$ Hz oscillation observed on March 3, 1996 for 4U\n1608-52 and show that both the observed soft time-lags as well as the r.m.s\nversus energy can be well described by such a model where the size of the\nComptonizing plasma is $\\sim 1$ km. Thus, modelling of the time lags as due to\nComptonization delays, can provide tight constraints on the size and geometry\nof the system. Detailed analysis would require well constrained spectral\nparameters.", "category": "astro-ph_HE" }, { "text": "How much H and He is \"hidden\" in SNe Ib/c? -- II. Intermediate-mass\n objects: a 22 M$_{\\odot}$ progenitor case study: Stripped envelope supernovae are a sub-class of core collapse supernovae\nshowing several stages of H/He shell stripping that determines the class:\nH-free/He-poor SNe are classified as Type Ic, H-poor/He-rich are Type Ib, and\nH/He-rich are Type IIb. Stripping H/He with only stellar wind requires\nsignificantly higher mass loss rates than observed while binary-involved mass\ntransfer may usually not strip enough to produce H/He free SNe. Type Ib/c SNe\nare sometimes found to include weak H/He transient lines as a product of a\ntrace amount of H/He left over from stripping processes. The extent and mass of\nthe H/He required to produce these lines is not well known. In this work, a 22\nM$_{\\odot}$ progenitor model is stripped of the H/He shells in five steps prior\nto collapse and then exploded at four explosion energies. Requiring both\noptical and NIR He I lines for helium identification does not allow much He\nmass to be hidden in SE--SNE. Increasing the mass of He above the CO core\ndelays the visibility of O I 7774 in early spectra. Our SN Ib-like models are\ncapable of reproducing the spectral evolution of a set of observed SNe with\nreasonable estimated $E_\\mathrm{k}$ accuracy. Our SN\\,IIb-like models can\npartially reproduce low energy observed SN IIb, but we find no observed\ncomparison for the SN IIb-like models with high $E_\\mathrm{k}$.", "category": "astro-ph_HE" }, { "text": "Magnetic Reconnection in Black-Hole Magnetospheres: Lepton Loading into\n Jets, Superluminal Radio Blobs, and Multi-wavelength Flares: Supermassive black holes in active galactic nuclei launch relativistic jets,\nas indicated by observed superluminal radio blobs. The energy source of these\njets is widely discussed in the theoretical framework of Blandford-Znajek\nprocess, the electromagnetic energy extraction from rotating black holes (BHs),\nwhile formation mechanism of the radio blobs in the\nelectromagnetically-dominated jets has been a long-standing problem. Recent\nhigh-resolution magnetohydrodynamic simulations of magnetically arrested disks\nexhibited magnetic reconnection in a transient magnetically-dominated part of\nthe equatorial disk near the BH horizon, which led to a promising scenario of\nefficient MeV gamma-ray production and subsequent electron-positron pair\nloading into BH magnetosphere. We develop this scenario to build a theoretical\nframework on energetics, timescales and particle number density of the\nsuperluminal radio blobs and discuss observable signatures in other wavebands.\nWe analytically show that the non-thermal electrons emit broadband photons from\noptical to multi-MeV bands. The electron-positron pairs produced in the\nmagnetosphere are optically thick for synchrotron-self absorption, so that the\ninjected energy is stored in the plasma. The stored energy is enough to power\nthe superluminal radio blobs observed in M87. This scenario predicts rather dim\nradio blobs around Sgr A*, which are consistent with no clear detection by\ncurrent facilities. In addition, this scenario inevitably produces strong X-ray\nflares in a short timescale, which will be detectable by future X-ray\nsatellites.", "category": "astro-ph_HE" }, { "text": "Mixing of charged and neutral Bose condensates at nonzero temperature\n and magnetic field: It is expected that in the interior of compact stars a proton superconductor\ncoexists with and couples to a neutron superfluid. Starting from a\nfield-theoretical model for two complex scalar fields - one of which is\nelectrically charged - we derive a Ginzburg-Landau potential which includes\nentrainment between the two fluids and temperature effects from thermal\nexcitations of the two scalar fields and the gauge field. The Ginzburg-Landau\ndescription is then used for an analysis of the phase structure in the presence\nof an external magnetic field. In particular, we study the effect of the\nsuperfluid on the flux tube phase by computing the various critical magnetic\nfields and deriving an approximation for the flux tube interaction. As a\nresult, we point out differences to the naive expectations from an isolated\nsuperconductor, for instance the existence of a first-order flux tube onset,\nresulting in a more complicated phase structure in the region between type-I\nand type-II superconductivity.", "category": "astro-ph_HE" }, { "text": "Comptonization in 1E 1740.7-2942 spectra from 2 to 200 keV: Studies of the long-term spectral variations have been used to constrain the\nemission processes of black hole candidates. However, a common scenario which\nis able to explain the emission from soft to hard X-rays has been proposed only\nrecently. Here, we use XMM and INTEGRAL data on 1E 1740.7-2942 in order to\ndemonstrate that Comptonization plays an important role in producing high\nenergy photons, as predicted by the current modeling scenario.", "category": "astro-ph_HE" }, { "text": "Forbush Decreases during the DeepMin and MiniMax of Solar Cycle 24: After a prolong and deep solar minimum at the end of solar cycle 23, the\ncurrent cycle 24 is one of the lowest cycles. The two periods of deep minimum\nand mini-maximum of the cycle 24 are connected by a period of increasing solar\nactivity. In this work, the Forbush decreases of cosmic ray intensity during\nthe period from January 2008 to December 2014 are studied. A statistical\nanalysis of 749 events using the IZMIRAN database of Forbush effects obtained\nby processing the data of the worldwide neutron monitor network using the\nglobal survey method is performed. A further study of the events that happened\non the Sun and affected the interplanetary space, and finally provoked the\ndecreases of the galactic cosmic rays near Earth is performed. A statistical\nanalysis of the amplitude of the cosmic ray decreases with solar and\ngeomagnetic parameters is carried out. The results will be useful for space\nweather studies and especially for Forbush decreases forecasting.", "category": "astro-ph_HE" }, { "text": "Astrophysical black holes: In this chapter, we introduce the concept of a black hole (BH) and recount\nthe initial theoretical predictions. We then review the possible types of BHs\nin nature, from primordial, to stellar-mass, to supermassive BHs. Finally, we\nfocus on the latter category and on their intricate relation with their host\ngalaxies.", "category": "astro-ph_HE" }, { "text": "Evidence for changes in the radiative efficiency of transient black hole\n X-ray binaries: We have used pointed RXTE data to examine the long-term X-ray light curves of\nsix transient black hole X-ray binaries during their decay from outburst to\nquiescence. In most cases there is a period of exponential decay as the source\napproaches the soft-to-hard state transition, and another period of exponential\ndecay following this transition as the source decays in the hard state. The\ne-folding times change around the time of the state transition, from typically\napprox 12 days at the end of the soft state to approx 7 days at the beginning\nof the hard state. This factor ~2 change in the decay timescale is expected if\nthere is a change from radiatively efficient emission in the soft state to\nradiatively inefficient emission in the hard state, overlying an exponential\ndecay in the mass accretion rate. This adds support to the idea that the X-ray\nemitting region is governed by radiatively inefficient accretion (such as an\nadvection-dominated or jet-dominated accretion flow) during the fading hard\nstate.", "category": "astro-ph_HE" }, { "text": "Search for relativistic fractionally charged particles in space: More than a century after the performance of the oil drop experiment, the\npossible existence of fractionally charged particles FCP still remains\nunsettled. The search for FCPs is crucial for some extensions of the Standard\nModel in particle physics. Most of the previously conducted searches for FCPs\nin cosmic rays were based on experiments underground or at high altitudes.\nHowever, there have been few searches for FCPs in cosmic rays carried out in\norbit other than AMS-01 flown by a space shuttle and BESS by a balloon at the\ntop of the atmosphere. In this study, we conduct an FCP search in space based\non on-orbit data obtained using the DArk Matter Particle Explorer (DAMPE)\nsatellite over a period of five years. Unlike underground experiments, which\nrequire an FCP energy of the order of hundreds of GeV, our FCP search starts at\nonly a few GeV. An upper limit of $6.2\\times 10^{-10}~~\\mathrm{cm^{-2}sr^{-1}\ns^{-1}}$ is obtained for the flux. Our results demonstrate that DAMPE exhibits\nhigher sensitivity than experiments of similar types by three orders of\nmagnitude that more stringently restricts the conditions for the existence of\nFCP in primary cosmic rays.", "category": "astro-ph_HE" }, { "text": "Gamma-Ray Observations of the Supernova Remnant RX J0852.0-4622 with the\n Fermi LAT: We report on gamma-ray observations of the supernova remnant (SNR) RX\nJ0852.0-4622 with the Large Area Telescope (LAT) aboard the Fermi Gamma-ray\nSpace Telescope. In the Fermi LAT data, we find a spatially extended source at\nthe location of the SNR. The extension is consistent with the SNR size seen in\nother wavelengths such as X-rays and TeV gamma rays, leading to the\nidentification of the gamma-ray source with the SNR. The spectrum is well\ndescribed as a power law with a photon index of Gamma = 1.85 +/- 0.06 (stat)\n(+0.18,-0.19) (sys), which smoothly connects to the H.E.S.S. spectrum in the\nTeV energy band. We discuss the gamma-ray emission mechanism based on\nmultiwavelength data. The broadband data can be fit well by a model in which\nthe gamma rays are of hadronic origin. We also consider a scenario with inverse\nCompton scattering of electrons as the emission mechanism of the gamma rays.\nAlthough the leptonic model predicts a harder spectrum in the Fermi LAT energy\nrange, the model can fit the data considering the statistical and systematic\nerrors.", "category": "astro-ph_HE" }, { "text": "The Gamma-Ray Burst - Supernova Connection: A preponderance of evidence links long-duration, soft-spectrum gamma-ray\nbursts (GRBs) with the death of massive stars. The observations of the\nGRB-supernova (SN) connection present the most direct evidence of this physical\nlink. We summarize 30 GRB-SN associations and focus on five ironclad cases,\nhighlighting the subsequent insight into the progenitors enabled by detailed\nobservations. We also address the SN association (or lack thereof) with several\nsub-classes of GRBs, finding that the X-ray Flash (XRF) population is likely\nassociated with massive stellar death whereas short-duration events likely\narise from an older population not readily capable of producing a SN concurrent\nwith a GRB. Interestingly, a minority population of seemingly long-duration,\nsoft-spectrum GRBs show no evidence for SN-like activity; this may be a natural\nconsequence of the range of Ni-56 production expected in stellar deaths.", "category": "astro-ph_HE" }, { "text": "The Guided Moments formalism: a new efficient full-neutrino treatment\n for astrophysical simulations: We present the new Guided Moments ($\\texttt{GM}$) formalism for neutrino\nmodeling in astrophysical scenarios like core-collapse supernovae and neutron\nstar mergers. The truncated moments approximation ($\\texttt{M1}$) and\nMonte-Carlo ($\\texttt{MC}$) schemes have been proven to be robust and accurate\nin solving the Boltzmann's equation for neutrino transport. However, it is\nwell-known that each method exhibits specific strengths and weaknesses in\nvarious physical scenarios. The $\\texttt{GM}$ formalism effectively solves\nthese problems, providing a comprehensive scheme capable of accurately\ncapturing the optically thick limit through the exact $\\texttt{M1}$ closure and\nthe optically thin limit through a $\\texttt{MC}$ based approach. In addition,\nthe $\\texttt{GM}$ method also approximates the neutrino distribution function\nwith a reasonable computational cost, which is crucial for the correct\nestimation of the different neutrino-fluid interactions. Our work provides a\ncomprehensive discussion of the formulation and application of the\n$\\texttt{GM}$ method, concluding with a thorough comparison across several test\nproblems involving the three schemes ($\\texttt{M1}$, $\\texttt{MC}$,\n$\\texttt{GM}$) under consideration.", "category": "astro-ph_HE" }, { "text": "X-ray Measurement of the Spin-Down of Calvera: a Radio- and\n Gamma-ray-Quiet Pulsar: We measure spin-down of the 59 ms X-ray pulsar Calvera by comparing the\nXMM-Newton discovery data from 2009 with new Chandra timing observations taken\nin 2013. Its period derivative is P-dot = (3.19+/-0.08)e-15, which corresponds\nto spin-down luminosity E-dot = 6.1e35 erg/s, characteristic age tau_c =\nP/2P-dot = 2.9e5 yr, and surface dipole magnetic field strength B_s = 4.4e11 G.\nThese values rule out a mildly recycled pulsar, but Calvera could be an\norphaned central compact object (anti-magnetar), with a magnetic field that was\ninitially buried by supernova debris and is now reemerging and approaching\nnormal strength. We also performed unsuccessful searches for high-energy\ngamma-rays from Calvera in both imaging and timing of >100 MeV Fermi photons.\nEven though the distance to Calvera is uncertain by an order of magnitude, an\nupper limit of d < 2 kpc inferred from X-ray spectra implies a gamma-ray\nluminosity limit of < 3.3e32 erg/s, which is less than that of any pulsar of\ncomparable E-dot. Calvera shares some properties with PSR J1740+1000, a young\nradio pulsar that we show by virtue of its lack of proper motion was born\noutside of the Galactic disk. As an energetic, high-Galactic-latitude pulsar,\nCalvera is unique in being undetected in both radio and gamma-rays to faint\nlimits, which should place interesting constraints on models for particle\nacceleration and beam patterns in pulsar magnetospheres.", "category": "astro-ph_HE" }, { "text": "On the Rate of Neutron Star Binary Mergers from Globular Clusters: The first detection of gravitational waves from a neutron star - neutron star\n(NS-NS) merger, GW170817, and the increasing number of observations of short\ngamma-ray bursts (SGRBs) have greatly motivated studies of the origins of NS-NS\nand neutron star - black hole (NS-BH) binaries. We calculate the merger rates\nof NS-NS and NS-BH binaries from globular clusters (GCs) using realistic GC\nsimulations with the \\texttt{CMC} cluster catalog. We use a large sample of\nmodels with a range of initial numbers of stars, metallicities, virial radii\nand galactocentric distances, representative of the present-day Milky Way GCs,\nto quantify the inspiral times and volumetric merger rates as a function of\nredshift, both inside and ejected from clusters. We find that over the complete\nlifetime of most GCs, stellar BHs dominate the cluster cores and prevent the\nmass segregation of NSs, thereby reducing the dynamical interaction rates of\nNSs so that at most a few NS binary mergers are ever produced. We estimate the\nmerger rate in the local universe to be $\\sim\\rm{0.02\\,Gpc^{-3}\\,yr^{-1}}$ for\nboth NS-NS and NS-BH binaries, or a total of $\\sim 0.04$~Gpc$^{-3}$~yr$^{-1}$\nfor both populations. These rates are about 5 orders of magnitude below the\ncurrent empirical merger rate from LIGO/Virgo. We conclude that dynamical\ninteractions in GCs do not play a significant role in enhancing the NS-NS and\nNS-BH merger rates.", "category": "astro-ph_HE" }, { "text": "Precursors of Short Gamma-Ray Bursts in the SPI-ACS/INTEGRAL Experiment: We have analyzed the light curves of 519 short gamma-ray bursts (GRBs)\ndetected in the SPI-ACS/INTEGRAL experiment from December 2002 to May 2014 to\nsearch for precursors. Both the light curves of 519 individual events and the\naveraged light curve of 372 brightest bursts have been analyzed. In a few\ncases, we have found and thoroughly studied precursor candidates based on\nSPI-ACS/INTEGRAL, GBM/Fermi, and LAT/Fermi data. A statistical analysis of the\naveraged light curve for the entire sample of short bursts has revealed no\nregular precursor. Upper limits for the relative intensity of precursors have\nbeen estimated. No convincing evidence for the existence of precursors of short\nGRBs has been found. We show that the fraction of short GRBs with precursors is\nless than 0.4% of all short bursts.", "category": "astro-ph_HE" }, { "text": "High-energy gamma-ray observations of Geminga with the Fermi Large Area\n Telescope: Geminga is the second brightest persistent source in the GeV gamma-ray sky.\nDiscovered in 1975 by SAS-2 mission, it was identified as a pulsar only in the\n90s, when ROSAT detected the 237 ms X-ray periodicity, that was later also\nfound by EGRET in gamma rays. Even though Geminga has been one of the most\nintensively studied isolated neutron star during the last 30 years, its\ninterest remains intact especially at gamma-ray energies, where instruments\nlike the Large Area Telescope (LAT) aboard the Fermi mission will provide an\nunprecedented view of this pulsars. We will report on the preliminary results\nobtained on the analysis of the first year of observations. We have been able\nto do precise timing of Geminga using solely gamma rays, producing a timing\nsolution and allowing a deep study of the evolution of the light curve with\nenergy. We have also measured and studied the high-energy cutoff in the\nphase-averaged spectrum and produced a detailed study of the spectral evolution\nwith phase.", "category": "astro-ph_HE" }, { "text": "Nonthermal X-ray Emission from the N11 Superbubble in the Large\n Magellanic Cloud: We present the results of X-ray spectral analysis of the superbubble around\nthe OB association LH9 in the H II complex N11 in the Large Magellanic Cloud.\nUsing observations from Suzaku, we have modeled the X-ray emission over the\nenergy range 0.2-7.5 keV. We constrained the thermal spectrum below 2 keV using\na thermal plasma model found in a previous XMM-Newton study of the N11 region.\nWe find that the hard X-ray emission (> 2 keV) requires a nonthermal power-law\ncomponent. The photon index of this component was found to be $\\Gamma=1.72\\pm\n0.15$. We performed an energy budget analysis for N11 using the known stellar\ncontent of LH9. We found that the observed thermal and kinetic energy in the\nsuperbubble is only half of the expected mechanical energy injected by stars.", "category": "astro-ph_HE" }, { "text": "On the physical meaning of the 2.1 keV absorption feature in 4U 1538-52: The improvement of the capabilities of nowadays X-ray observatories, like\nChandra or XMM-Newton, offers the possibility to detect both absorption and\nemission lines and to study the nature of the matter surrounding the neutron\nstar in X-ray binaries and the phenomena that produce these lines. The aim of\nthis work is to discuss the different physical scenarios in order to explain\nthe meaning of the significant absorption feature present in the X-ray spectrum\nof 4U 1538-52. Using the last available calibrations, we discard the\npossibility that this feature is due to calibration, gain effects or be\nproduced by the X-ray background or a dust region. Giving the energy resolution\nof the XMM-Newton telescope we could not establish if the line is formed in the\natmosphere of the neutron star or by the dispersion of the stellar wind of the\noptical counterpart.", "category": "astro-ph_HE" }, { "text": "Simple waves in relativistic fluids: We consider the Riemann problem for relativistic flows of polytropic fluids\nand find relations for the flow characteristics. Evolution of physical\nquantities take especially simple form for the case of cold magnetized plasmas.\nWe find exact, explicit analytical solutions for one dimensional expansion of\nmagnetized plasma into vacuum, valid for arbitrary magnetization. We also\nconsider expansion into cold unmagnetized external medium both for stationary\ninitial conditions and for initially moving plasma, as well as reflection of\nrarefaction wave from a wall. We also find self-similar structure of\nthree-dimensional magnetized outflows into vacuum, valid close to the\nplasma-vacuum interface.\n The key results of this work, the self-similar solutions, were incorporated\npost-initial submission into appendices of the published version of Granot et\nal. (2010).", "category": "astro-ph_HE" }, { "text": "Expansion and Age of the Supernova Remnant G350.1-0.3: High-Velocity\n Iron Ejecta from a Core-Collapse Event: We report Chandra observations of the highly asymmetric core-collapse\nsupernova remnant G350.1-0.3. We document expansion over 9 years away from the\nroughly stationary central compact object, with sky-plane velocities up to\n$5000 d_{4.5}$ km s$^{-1}$ ($d_{4.5}$ is the distance in units of 4.5 kpc),\nredshifts ranging from 900 km s$^{-1}$ to 2600 km s$^{-1}$, and\nthree-dimensional space velocities approaching 6000 km s$^{-1}$. Most of the\nbright emission comes from heavy-element ejecta particularly strong in iron.\nIron-enhanced ejecta are seen at 4000 - 6000 km s$^{-1}$, strongly suggesting\nthat the supernova was not a common Type IIP event. While some fainter regions\nhave roughly solar abundances, we cannot identify clear blast-wave features.\nOur expansion proper motions indicate that G350.1-0.3 is no more than about 600\nyears old, independent of distance: the third youngest known core-collapse\nsupernova in the Galaxy, and one of the most asymmetric.", "category": "astro-ph_HE" }, { "text": "Multiple Peaks and a Long Precursor in the Type IIn Supernova 2021qqp:\n An Energetic Explosion in a Complex Circumsteller Environment: We present detailed optical photometry and spectroscopy of the Type IIn\nsupernova (SN) 2021qqp. Its unusual light curve is marked by a long gradual\nbrightening (i.e., precursor) for about 300 days, a rapid increase in\nbrightness for about 60 days, and then a sharp increase of about 1.6 mag in\nonly a few days to a first peak of $M_r\\approx -19.5$ mag. The light curve then\nturns over and declines rapidly, until it re-brightens to a second distinct and\nsharp peak with $M_r\\approx -17.3$ mag centered at about 335 days after the\nfirst peak. The spectra are dominated by Balmer-series lines with a complex\nmorphology that includes a narrow component with a width of $\\approx 1300$ km\ns$^{-1}$ (first peak) and $\\approx 2500$ km s$^{-1}$ (second peak) that we\nassociate with the circumstellar medium (CSM), and a P Cygni component with an\nabsorption velocity of $\\approx 8500$ km s$^{-1}$ (first peak) and $\\approx\n5600$ km s$^{-1}$ (second peak) that we associate with the SN-CSM interaction\nshell. Using the bolometric light curve and velocity evolution, we construct an\nanalytical model to extract the CSM profile and SN properties. We find two\nsignificant mass-loss episodes with peak mass loss rates of $\\approx 10$\nM$_\\odot$ yr$^{-1}$ and $\\approx 5$ M$_\\odot$ yr$^{-1}$ about 0.8 and 2 years\nbefore explosion, and a total CSM mass of $\\approx 2-4\\,M_\\odot$. We show that\nthe most recent mass-loss episode can explain the precursor for the year\npreceding the explosion. The SN ejecta mass is constrained to be $M_{\\rm\nSN}\\approx 5-30\\,M_\\odot$ for an explosion energy of $E_{\\rm SN}\\approx\n(3-10)\\times10^{51}\\,{\\rm erg}$. We discuss eruptive massive stars (luminous\nblue variable, pulsational pair instability) and an extreme stellar merger with\na compact object as possible progenitor channels for generating the energetic\nexplosion in the complex CSM environment.", "category": "astro-ph_HE" }, { "text": "Disentangling multiple high-energy emission components in the Vela X\n pulsar wind nebula with the Fermi Large Area Telescope: Vela X is a pulsar wind nebula in which two relativistic particle populations\nwith distinct spatial and spectral distributions dominate the emission at\ndifferent wavelengths. An extended $2^\\circ \\times 3^\\circ$ nebula is seen in\nradio and GeV gamma rays. An elongated cocoon prevails in X-rays and TeV gamma\nrays. We use 9.5 years of data from the Fermi Large Area Telescope (LAT) to\ndisentangle gamma-ray emission from the two components in the energy range from\n10 GeV to 2 TeV, bridging the gap between previous measurements at GeV and TeV\nenergies. We determine the morphology of emission associated to Vela X\nseparately at energies < 100 GeV and > 100 GeV, and compare it to the\nmorphology seen at other wavelengths. Then, we derive the spectral energy\ndistribution of the two gamma-ray components over the full energy range. The\nbest fit to the LAT data is provided by the combination of the two components\nderived at energies < 100 GeV and > 100 GeV. The first component has a soft\nspectrum, spectral index $2.19\\pm0.16^{+0.05}_{-0.22}$, and extends over a\nregion of radius $1.36^\\circ\\pm0.04^\\circ$, consistent with the radio nebula.\nThe second component has a harder spectrum, spectral index\n$0.9\\pm0.3^{+0.3}_{-0.1}$, and is concentrated over an area of radius\n$0.63^\\circ\\pm0.03^\\circ$, coincident with the X-ray cocoon that had already\nbeen established to account for the bulk of the emission at TeV energies. The\nspectrum measured for the low-energy component corroborates previous evidence\nfor a roll-over of the electron spectrum at energies of a few tens of GeV\npossibly due to diffusive escape. The high-energy component has a very hard\nspectrum: if the emission is produced by electrons with a power-law spectrum\nthe electrons must be uncooled, and there is a hint that their spectrum may be\nharder than predictions by standard models of Fermi acceleration at\nrelativistic shocks. (Abridged)", "category": "astro-ph_HE" }, { "text": "Radio-Frequency Searches for Dark Matter in Dwarf Galaxies: Dwarf spheroidal galaxies have long been discussed as optimal targets for\nindirect dark matter searches. However, the majority of such studies have been\nconducted with gamma-ray instruments. In this review, we discuss the very\nrecent progress that has been made in radio-based indirect dark matter\nsearches. We look at existing work on this topic and discuss the future\nprospects that motivate continued work in this newly developing field that\npromises to become, in the light of the up-coming Square Kilometre Array, a\nprominent component of the hunt for dark matter.", "category": "astro-ph_HE" }, { "text": "Ultra-high energy cosmic ray correlations with Active Galactic Nuclei in\n the world dataset: Pierre Auger collaboration have recently put forward the hypothesis that the\narrival directions of the highest energy cosmic rays correlate with the subset\nof local active galactic nuclei (AGN). We perform a blind test of AGN\nhypothesis using publicly available event sets collected by Yakutsk, AGASA and\nHiRes experiments. The consistency of the procedure requires the event energies\nto be normalized towards the common energy scale. The number of correlating\nevents in resulting data-set is 3 of 21 which is consistent with expected\nrandom background.", "category": "astro-ph_HE" }, { "text": "Cosmic ray streaming in the turbulent interstellar medium: We study the streaming instability of GeV$-100~$GeV cosmic rays (CRs) and its\ndamping in the turbulent interstellar medium (ISM). We find that the damping of\nstreaming instability is dominated by ion-neutral collisional damping in weakly\nionized molecular clouds, turbulent damping in the highly ionized warm medium,\nand nonlinear Landau damping in the Galactic halo. Only in the Galactic halo,\nis the streaming speed of CRs close to the Alfv\\'{e}n speed. Alfv\\'{e}nic\nturbulence plays an important role in both suppressing the streaming\ninstability and regulating the diffusion of streaming CRs via magnetic field\nline tangling, with the effective mean free path of streaming CRs in the\nobserver frame determined by the Alfv\\'{e}nic scale in super-Alfv\\'{e}nic\nturbulence. The resulting diffusion coefficient is sensitive to Alfv\\'{e}n Mach\nnumber, which has a large range of values in the multi-phase ISM.\nSuper-Alfv\\'{e}nic turbulence contributes to additional confinement of\nstreaming CRs, irrespective of the dominant damping mechanism.", "category": "astro-ph_HE" }, { "text": "Modeling High-energy and Very-high-energy gamma-rays from the Terzan 5\n Cluster: The Fermi Large Area Telescope (LAT) has recently detected a population of\nglobular clusters (GCs) in high-energy (HE) gamma-rays. Their spectral\nproperties and energetics are consistent with cumulative emission from a\npopulation of millisecond pulsars (MSPs) hosted by these clusters. For example,\nthe HE spectra exhibit fairly hard power-law indices and cutoffs around a few\nGeV, typical of pulsed spectra measured for the gamma-ray pulsar population.\nThe energetics may be used to constrain the number of visible MSPs in the\ncluster (N_vis), assuming canonical values for the average gamma-ray efficiency\nand spin-down power. This interpretation is indeed strengthened by the fact\nthat the first gamma-ray MSP has now been identified in the GC NGC 6624, and\nthis MSP is responsible for almost all of the HE emission from this cluster. On\nthe other hand, it has been argued that the MSPs are also sources of\nrelativistic leptons which may be reaccelerated in shocks originating in\ncollisions of stellar winds in the cluster core, and may upscatter bright\nstarlight and cosmic microwave background photons to very high energies.\nTherefore, this unpulsed component may give an independent constraint on the\ntotal number of MSPs (N_tot) hosted in the GC, for a given cluster magnetic\nfield B and diffusion coefficient k_0. Lastly, the transport properties of the\nenergetic leptons may be further constrained using multiwavelength data, e.g.,\nto infer the radial dependence of k_0 and B. We present results on our modeling\nof the pulsed and unpulsed gamma-ray fluxes from the GC Terzan 5.", "category": "astro-ph_HE" }, { "text": "MeV Pulsars: Modeling Spectra and Polarization: A sub-population of energetic rotation-powered pulsars show high fluxes of\npulsed non-thermal hard X-ray emission. While this MeV pulsar population\nincludes some radio-loud pulsars like the Crab, a significant number have no\ndetected radio or GeV emission, a mystery since gamma- ray emission is a common\ncharacteristic of pulsars with high spin-down power. Their steeply rising hard\nX-ray spectral energy distributions (SEDs) suggest peaks at 0.1 - 1 MeV but\nthey have not been detected above 200 keV. Several upcoming and planned\ntelescopes may shed light on the MeV pulsars. The Neutron star Interior\nComposition ExploreR (NICER) will observe pulsars in the 0.2 - 12 keV band and\nmay discover additional MeV pulsars. Planned telescopes, such as All-Sky\nMedium-Energy Gamma-Ray Observatory (AMEGO) and e-ASTROGAM, will detect\nemission above 0.2 MeV and polarization in the 0.2 - 10 MeV band. We present a\nmodel for the spectrum and polarization of MeV pulsars where the X-ray emission\ncomes from electron- positron pairs radiating in the outer magnetosphere and\ncurrent sheet. This model predicts that the peak of the SED increases with\nsurface magnetic field strength if the pairs are produced in polar cap\ncascades. For small inclination angles, a range of viewing angles can miss both\nthe radio pulse and the GeV pulse from particles accelerating near the current\nsheet. Characterizing the emission and geometry of MeV pulsars can thus provide\nclues to the source of pairs and acceleration in the magnetosphere.", "category": "astro-ph_HE" }, { "text": "A re-analysis of the NuSTAR and XMM-Newton broad-band spectrum of\n Ser~X-1: Context: Ser X-1 is a well studied LMXB which clearly shows a broad iron\nline. Recently, Miller et al. (2103) have presented broad-band, high quality\nNuSTAR data of SerX-1.Using relativistically smeared self-consistent reflection\nmodels, they find a value of R_in close to 1.0 R_ISCO (corresponding to 6 R_g),\nand a low inclination angle, less than 10 deg. Aims: The aim of this paper is\nto probe to what extent the choice of reflection and continuum models (and\nuncertainties therein) can affect the conclusions about the disk parameters\ninferred from the reflection component. To this aim we re-analyze all the\navailable public NuSTAR and XMM-Newton. Ser X-1 is a well studied source, its\nspectrum has been observed by several instruments, and is therefore one of the\nbest sources for this study. Methods: We use slightly different continuum and\nreflection models with respect to those adopted in literature for this source.\nIn particular we fit the iron line and other reflection features with\nself-consistent reflection models as reflionx (with a power-law illuminating\ncontinuum modified with a high energy cutoff to mimic the shape of the incident\nComptonization spectrum) and rfxconv. With these models we fit NuSTAR and\nXMM-Newton spectra yielding consistent spectral results. Results: Our results\nare in line with those already found by Miller et al. (2013) but less extreme.\nIn particular, we find the inner disk radius at about 13 R_g and an inclination\nangle with respect to the line of sight of about 27 deg. We conclude that,\nwhile the choice of the reflection model has little impact on the disk\nparameters, as soon as a self-consistent model is used, the choice of the\ncontinuum model can be important in the precise determination of the disk\nparameters from the reflection component. Hence broad-band X-ray spectra are\nhighly preferable to constrain the continuum and disk parameters.", "category": "astro-ph_HE" }, { "text": "Embedded Spiral Patterns in the Cool Core of the Massive Cluster of\n Galaxies Abell 1835: We present the properties of intracluster medium (ICM) in the cool core of\nthe massive cluster of galaxies Abell 1835 obtained with the data by $ Chandra$\n$X$-$ray$ $Observatory$. We find distinctive spiral patterns with the radius of\n70 kpc (or 18 arcsec) as a whole in the residual image of X-ray surface\nbrightness after the 2-dimensional ellipse model of surface brightness is\nsubtracted. The size is smaller by a factor of 2 -- 4 than that of other\nclusters known to have a similar pattern. The spiral patterns consist of two\narms. One of them appears as positive, and the other does as negative excesses\nin the residual image. Their X-ray spectra show that the ICM temperatures in\nthe positive- and negative-excess regions are $5.09^{+0.12}_{-0.13}$ keV and\n$6.52^{+0.18}_{-0.15}$ keV, respectively. In contrast, no significant\ndifference is found in the abundance or pressure, the latter of which suggests\nthat the ICM in the two regions of the spiral patterns is in pressure\nequilibrium or close. The spatially-resolved X-ray spectroscopy of the central\nregion ($r<40$ arcsec) divided into 92 sub-regions indicates that Abell 1835 is\na typical cool core cluster. We also find that the spiral patterns extend from\nthe cool core out to the hotter surrounding ICM. The residual image reveals\nsome lumpy sub-structure in the cool core. The line-of-sight component of the\ndisturbance velocity responsible for the sub-structures is estimated to be\nlower than 600 km/s. Abell 1835 may be now experiencing an off-axis minor\nmerger.", "category": "astro-ph_HE" }, { "text": "Resolving the puzzle of type IIP SN 2016X: The enigmatic type IIP SN 2016X demonstrates the unprecedented asphericity in\nthe nebular H-alpha line profile, the absence of nebular [O I] emission, and\nthe unusual occultation effect due to the internal dust. The hydrodynamic\nmodelling of the bolometric light curve and expansion velocities suggests that\nthe event is an outcome of the massive star explosion that ejected 28 Msun with\nthe kinetic energy of 1.7x10^51 erg and 0.03 Msun of radioactive Ni-56. We\nrecover the bipolar distribution of Ni-56 from the H-alpha profile via the\nsimulation of the emissivity produced by non-spherical Ni-56 ejecta. The\nconspicuous effect of the dust absorption in the H-alpha profile rules out the\noccultation by the dusty sphere or dusty thick disk but turns out consistent\nwith the thin dusty disk-like structure in the plane perpendicular to the\nbipolar axis. We speculate that the absence of the nebular [O I] emission might\noriginate from the significant cooling of the oxygen-rich matter mediated by CO\nand SiO molecules.", "category": "astro-ph_HE" }, { "text": "Simulation study on origin of multi-core events in cosmic rays extensive\n air showers: Some experiments have found multi-core events in cosmic rays extensive air\nshowers which should be interpreted by hadronic interaction theory. In this\npaper, the multi-core events are reproduced by Monte Carlo simulation with\nCORSIKA. The origin of each sub-cores is tracked back from the observation\nlevel. The interaction mechanism and original particles of sub-core are studied\nin this paper.", "category": "astro-ph_HE" }, { "text": "A Proper Motion for the Pulsar Wind Nebula G359.23-0.82, \"the Mouse,\"\n Associated with the Energetic Radio Pulsar J1747-2958: The \"Mouse\" (PWN G359.23-0.82) is a spectacular bow shock pulsar wind nebula,\npowered by the radio pulsar J1747-2958. The pulsar and its nebula are presumed\nto have a high space velocity, but their proper motions have not been directly\nmeasured. Here we present 8.5 GHz interferometric observations of the Mouse\nnebula with the Very Large Array, spanning a time baseline of 12 yr. We measure\neastward proper motion for PWN G359.23-0.82 (and hence indirectly for PSR\nJ1747-2958) of 12.9+/-1.8 mas/yr, which at an assumed distance of 5 kpc\ncorresponds to a transverse space velocity of 306+/-43 km/s. Considering\npressure balance at the apex of the bow shock, we calculate an in situ hydrogen\nnumber density of approximately 1.0(-0.2)(+0.4) cm^(-3) for the interstellar\nmedium through which the system is traveling. A lower age limit for PSR\nJ1747-2958 of 163(-20)(+28) kyr is calculated by considering its potential\nbirth site. The large discrepancy with the pulsar's spin-down age of 25 kyr is\npossibly explained by surface dipole magnetic field growth on a timescale ~15\nkyr, suggesting possible future evolution of PSR J1747-2958 to a different\nclass of neutron star. We also argue that the adjacent supernova remnant\nG359.1-0.5 is not physically associated with the Mouse system but is rather an\nunrelated object along the line of sight.", "category": "astro-ph_HE" }, { "text": "Time Evolution of Pulsar Magnetosphere I - An Implicit Approach: We apply a computationally efficient technique to validate the global\nstructure of the pulsar magnetosphere. In this first of a series of studies, a\n3D, computationally intensive, implicit Crank-Nicolson finite-difference scheme\nis developed. The region of magnetic influence is evolved under the\napproximation of force-free electrodynamics. The main objective of this paper\nis to present our code and use it to demonstrate and verify the now widely -\naccepted global features of a pulsar magnetosphere. Our results qualitatively\nagree with previously developed time-dependent models for an oblique rotator.\nIn line with earlier studies, we also demonstrate that our simulations can run\nfor many stellar rotations. Once we extend our code, we believe that our\nimplicit approach can be extremely useful to investigate magnetospheres filled\nwith resistive plasma, develop better resolution current sheets and investigate\nsmall scale microphysics of pair creation using particle-in-cell techniques.", "category": "astro-ph_HE" }, { "text": "Assessing luminosity correlations via cluster analysis: Evidence for\n dual tracks in the radio/X-ray domain of black hole X-ray binaries: [abridged] The radio:X-ray correlation for hard and quiescent state black\nhole X-ray binaries is critically investigated in this paper. New observations\nof known sources, along with newly discovered ones, have resulted in an\nincreasingly large number of outliers lying well outside the scatter about the\nquoted best-fit relation. Here, we employ and compare state of the art data\nclustering techniques in order to identify and characterize different data\ngroupings within the radio:X-ray luminosity plane for 18 hard and quiescent\nstate black hole X-ray binaries with nearly simultaneous multi-wavelength\ncoverage. Linear regression is then carried out on the clustered data to infer\nthe parameters of a relationship of the form {ell}_{r}=alpha+beta {ell}_x\nthrough a Bayesian approach (where {ell} denotes log lum). We conclude that the\ntwo cluster model, with independent linear fits, is a significant improvement\nover fitting all points as a single cluster. While the upper track slope\n(0.63\\pm0.03) is consistent, within the errors, with the fitted slope for the\n2003 relation (0.7\\pm0.1), the lower track slope (0.98\\pm0.08) is not\nconsistent with the upper track, nor it is with the widely adopted value of\n~1.4 for the neutron stars. The two luminosity tracks do not reflect systematic\ndifferences in black hole spins as estimated either from reflection, or\ncontinuum fitting method. These results are insensitive to the selection of\nsub-samples, accuracy in the distances, and to the treatment of upper limits.\nBesides introducing a further level of complexity in understanding the\ninterplay between synchrotron and Comptonised emission from black hole X-ray\nbinaries, the existence of two tracks in the radio:X-ray domain underscores\nthat a high level of caution must be exercised when employing black hole\nluminosity relations for the purpose of estimating a third parameter, such as\ndistance or mass.", "category": "astro-ph_HE" }, { "text": "Soft gamma-ray constraints on a bright flare from the Galactic Center\n supermassive black hole: Sagittarius A* (Sgr A*) is the supermassive black hole residing at the center\nof the Milky Way. It has been the main target of an extensive multiwavelength\ncampaign we carried out in April 2007. Herein, we report the detection of a\nbright flare from the vicinity of the horizon, observed simultaneously in\nX-rays (XMM/EPIC) and near infrared (VLT/NACO) on April 4th for 1-2 h. For the\nfirst time, such an event also benefitted from a soft gamma-rays\n(INTEGRAL/ISGRI) and mid infrared (VLT/VISIR) coverage, which enabled us to\nderive upper limits at both ends of the flare spectral energy distribution\n(SED). We discuss the physical implications of the contemporaneous light curves\nas well as the SED, in terms of synchrotron, synchrotron self-Compton and\nexternal Compton emission processes.", "category": "astro-ph_HE" }, { "text": "Tidal Instability and Superhump by a Wave-Wave Resonant Model: On a disk deformed to a non-axisymmetric form, a set of oscillations can be\nexcited by their resonant interaction through the disk deformation (Kato et al.\n2011). This resonant instability process has been proposed to suggest a\npossible cause of the high-frequency quasi-periodic oscillations (HF QPOs)\nobserved in black-hole low-mass X-ray binaries. In the present paper, we\nexamine whether the above-mentioned wave-wave resonant process can describe the\ntidal instability and superhump in dwarf novae. The results show that the\nprocess seems to well describe the observations. If this process is really the\ncause of the tidal instability and superhump, a two-armed oscillation with high\nfrequency roughly on the magnitude of three times the orbital frequency is\npresent on disks, although its expected amplitude may be small.", "category": "astro-ph_HE" }, { "text": "Constraints on the source of ultra-high energy cosmic rays using\n anisotropy vs chemical composition: The joint analysis of anisotropy signals and chemical composition of\nultra-high energy cosmic rays offers strong potential for shedding light on the\nsources of these particles. Following up on an earlier idea, this paper studies\nthe anisotropies produced by protons of energy >E/Z, assuming that anisotropies\nat energy >E have been produced by nuclei of charge Z, which share the same\nmagnetic rigidity. We calculate the number of secondary protons produced\nthrough photodisintegration of the primary heavy nuclei. Making the extreme\nassumption that the source does not inject any proton, we find that the\nsource(s) responsible for anisotropies such as reported by the Pierre Auger\nObservatory should lie closer than ~20-30, 80-100 and 180-200 Mpc if the\nanisotropy signal is mainly composed of oxygen, silicon and iron nuclei\nrespectively. A violation of this constraint would otherwise result in the\nsecondary protons forming a more significant anisotropy signal at lower\nenergies. Even if the source were located closer than this distance, it would\nrequire an extraordinary metallicity >120, 1600, 1100 times solar metallicity\nin the acceleration zone of the source, for oxygen, silicon and iron\nrespectively, to ensure that the concomitantly injected protons not to produce\na more significant low energy anisotropy. This offers interesting prospects for\nconstraining the nature and the source of ultra-high energy cosmic rays with\nthe increase in statistics expected from next generation detectors.", "category": "astro-ph_HE" }, { "text": "Fragmentation cross-sections and model uncertainties in Cosmic Ray\n propagation physics: Abundances and energy spectra of cosmic ray nuclei are being measured with\nhigh accuracy by the AMS experiment. These observations can provide tight\nconstraints to the propagation models of galactic cosmic rays. In the view of\nthe release of these data, I present an evaluation of the model uncertainties\nassociated to the cross-sections for secondary production of Li-Be-B nuclei in\ncosmic rays. I discuss the role of cross section uncertainties in the\ncalculation of the boron-to-carbon and beryllium-to-boron ratios, as well as\ntheir impact in the determination of the cosmic-ray transport parameters.", "category": "astro-ph_HE" }, { "text": "Diffuse flux of PeV neutrinos from centrifugally accelerated protons in\n active galactic nuclei: Evidence for high-energy astrophysical PeV neutrinos has been found in the\nIceCube experiment from an analysis with a 7.5 year (2010 - 2017) data. Active\ngalactic nuclei (AGN) are among the most prominent objects in the universe, and\nare widely speculated to be emitters of ultra-high-energy (UHE) cosmic rays\nwith proton domination. Based on the standard two-step LLCD mechanism of\nparticle acceleration, a transformation of energy occurs from AGN's central\nsuper-massive black hole (SMBH) rotation to high-energy protons. Protons can be\naccelerated up to $\\sim 0.1$ EeV energies and above, and might generate PeV\nneutrinos in the energy range $1$--$10$~ PeV through plausible hadronic\ninteractions. The theoretically estimated revised extragalactic diffuse muon\nneutrino flux employing the \"luminosity-dependent density evolution (LDDE)\"\nmodel for the AGN luminosity function (LF) is found consistent with the IceCube\nlevel if only a fraction, $6.56\\%$ of the total bolometric luminosity (BL) of\nAGN is being realizable to power the PeV neutrinos. In the $\\Lambda$~CDM\ncosmological framework with the LDDE modeled LF and photon index distribution,\nabout $5.18\\%$ of the total BL is enough to power the IceCube neutrinos.", "category": "astro-ph_HE" }, { "text": "Newtonian analogue of corresponding spacetime dynamics of rotating black\n holes: Implication on black hole accretion: Based on the conserved Hamiltonian for a test particle, we have formulated a\nNewtonian analogue of Kerr spacetime in the `low energy limit of the test\nparticle motion' that, in principle, can be comprehensively used to describe\ngeneral relativistic (GR) features of Kerr spacetime, however, with less\naccuracy for high spin. The derived potential, which has an explicit velocity\ndependence, contains the entire relativistic features of corresponding\nspacetime including the frame dragging effect, unlike other prevailing\npseudo-Newtonian potentials (PNPs) for the Kerr metric where such an effect is\neither totally missing or introduced in a ad hoc manner. The particle dynamics\nwith this potential precisely reproduce the GR results within a maximum ~ 10 %\ndeviation in energy for a particle orbiting circularly in the vicinity of a\nrapidly corotating black hole. GR epicyclic frequencies are also well\nreproduced with the potential, though with a relatively higher percentage of\ndeviation. For counterrotating cases, the obtained potential replicate the GR\nresults with precise accuracy. The Kerr-Newtonian potential also approximates\nthe radius of marginally stable and marginally bound circular orbits with\nreasonable accuracy for a < 0.7. Importantly, the derived potential can imitate\nthe experimentally tested GR effects like perihelion advancement and bending of\nlight with reasonable accuracy. The formulated Kerr-Newtonian potential thus\ncan be useful to study complex accreting plasma dynamics and its implications\naround rotating BHs in the Newtonian framework, avoiding GR gas dynamical\nequations.", "category": "astro-ph_HE" }, { "text": "Potential Gamma-ray Emissions from Low-Mass X-ray Binary Jets: By proposing a pure leptonic radiation model, we study the potential\ngamma-ray emissions from jets of the low-mass X-ray binaries. In this model,\nthe relativistic electrons that are accelerated in the jets are responsible for\nradiative outputs. Nevertheless, dynamics of jets are dominated by the magnetic\nand proton-matter kinetic energies. The model involves all kinds of related\nradiative processes and considers the evolution of relativistic electrons along\nthe jet by numerically solving the kinetic equation. Numerical results show\nthat the spectral energy distributions can extend up to TeV bands, in which\nsynchrotron radiation and synchrotron self-Compton scattering are dominant\ncomponents. As an example, we apply the model to the low-mass X-ray binary GX\n339-4. The results can not only reproduce the currently available observations\nfrom GX 339-4, but also predict detectable radiation at GeV and TeV bands by\nFermi and CTA telescopes. The future observations with Fermi and CTA can be\nused to test our model, which could be employed to distinguish the origin of\nX-ray emissions.", "category": "astro-ph_HE" }, { "text": "Kinetic modeling of the electromagnetic precursor from an axisymmetric\n binary pulsar coalescence: The recent detection of gravitational waves associated with a binary neutron\nstar merger revives interest in interacting pulsar magnetospheres. Current\nmodels predict that a significant amount of magnetic energy should be released\nprior to the merger, leading to electromagnetic precursor emission. In this\npaper, we revisit this problem in the light of the recent progress in kinetic\nmodeling of pulsar magnetospheres. We limit our work to the case of aligned\nmagnetic moments and rotation axes, and thus neglect the orbital motion. We\nperform global two-dimensional axisymmetric particle-in-cell simulations of two\npulsar magnetospheres merging at a rate consistent with the emission of\ngravitational waves. Both symmetric and asymmetric systems are investigated.\nSimulations show a significant enhancement of magnetic dissipation within the\nmagnetospheres as both stars get closer. Even though the magnetospheric\nconfiguration depends on the relative orientations of the pulsar spins and\nmagnetic axes, all configurations present nearly the same radiative signature,\nindicating that a common dissipation mechanism is at work. The relative motion\nof both pulsars drives magnetic reconnection at the boundary between the two\nmagnetospheres, leading to efficient particle acceleration and high-energy\nsynchrotron emission. Polar-cap discharge is also strongly enhanced in\nasymmetric configurations, resulting in vigorous pair production and\npotentially additional high-energy radiation. We observe an increase in the\npulsar radiative efficiency by two orders of magnitude over the last orbit\nbefore the merger exceeding the spindown power of an isolated pulsar. The\nexpected signal is too weak to be detected at high energies even in the nearby\nuniverse. However, if a small fraction of this energy is channeled into radio\nwaves, it could be observed as a non-repeating fast radio burst.", "category": "astro-ph_HE" }, { "text": "The proton-air inelastic cross-section measurement at $\\sqrt{s} \\approx\n $ 2 TeV from EAS-TOP experiment: The proton-air inelastic cross section value \\sigmapairin=338$\\pm$21({\\it\nstat})$\\pm$19({\\it syst})-28({\\it syst}) mb at $\\sqrt{s} \\approx $ 2 TeV has\nbeen measured by the EAS-TOP Extensive Air Shower experiment. The absorption\nlength of cosmic ray proton primaries cascades reaching the maximum development\nat the observation level is obtained from the flux attenuation for different\nzenith angles (i.e. atmospheric depths). The analysis, including the effects of\nthe heavier primaries contribution and systematic uncertainties, is described.\nThe experimental result is compared with different high energy interaction\nmodels and the relationships with the {\\it pp} ($\\bar pp$) total cross section\nmeasurements are discussed.", "category": "astro-ph_HE" }, { "text": "Low X-ray emission challenges supernovae remnants as the source of\n cosmic-ray electrons: The X-ray synchrotron emission of each of the young supernova-remnants (SNRs)\nSN1006, Kepler, Tycho, RCW86 and Cas A, is roughly given by $\\nu L_{\\nu}\\sim\n10^{45}\\rm{erg}/t$, where $t$ is the remnant's age. The electrons emitting the\nX-ray emission cool fast, implying that the X-ray emission is calorimetric and\nequal to half of the cosmic ray (CR) electron acceleration efficiency (per\nlogarithmic interval of particle energies, at multi TeV energies). Assuming\nSedov-Taylor expansion, the resulting CR electron yield per SNR is estimated to\nbe $E^2dN_e/dE\\approx 6\\nu L_{\\nu}t \\sim 10^{46}\\rm erg$. This is about two\norders of magnitudes below the required amount for explaining the observed\nelectron CRs at $E\\sim 10\\rm GeV$. Possible resolutions are 1. a soft\nacceleration spectrum allowing much more energy at $E\\sim 10\\rm GeV$ compared\nto $E\\sim 10\\rm TeV$, 2. an increased acceleration efficiency at later phases\nof the SNR evolution (unlikely), or 3. SNRs are not the source of CR electrons.", "category": "astro-ph_HE" }, { "text": "Coherent Network Analysis of Gravitational Waves from Three-Dimensional\n Core-Collapse Supernova Models: Using predictions from three-dimensional (3D) hydrodynamics simulations of\ncore-collapse supernovae (CCSNe), we present a coherent network analysis to\ndetection, reconstruction, and the source localization of the\ngravitational-wave (GW) signals. We use the {\\tt RIDGE} pipeline for the\nanalysis, in which the network of LIGO Hanford, LIGO Livingston, VIRGO, and\nKAGRA is considered. By combining with a GW spectrogram analysis, we show that\nseveral important hydrodynamics features in the original waveforms persist in\nthe waveforms of the reconstructed signals. The characteristic excess in the\nspectrograms originates not only from rotating core-collapse, bounce and the\nsubsequent ring down of the proto-neutron star (PNS) as previously identified,\nbut also from the formation of magnetohydrodynamics jets and non-axisymmetric\ninstabilities in the vicinity of the PNS. Regarding the GW signals emitted near\nat the rotating core bounce, the horizon distance extends up to $\\sim$ 18 kpc\nfor the most rapidly rotating 3D model in this work. Following the rotating\ncore bounce, the dominant source of the GW emission shifts to the\nnon-axisymmetric instabilities. The horizon distances extend maximally up to\n$\\sim$ 40 kpc seen from the spin axis. With an increasing number of 3D models\ntrending towards explosion recently, our results suggest that in addition to\nthe best studied GW signals due to rotating core-collapse and bounce, the time\nis ripe to consider how we can do science from GWs of CCSNe much more seriously\nthan before. Particularly the quasi-periodic signals due to the\nnon-axisymmetric instabilities and the detectability should deserve further\ninvestigation to elucidate the inner-working of the rapidly rotating CCSNe.", "category": "astro-ph_HE" }, { "text": "Why Haven't Many of the Brightest Radio Loud Blazars Been Detected by\n Fermi ?: We use the complete MOJAVE 1.5 Jy sample of active galactic nuclei (AGN) to\nexamine the gamma-ray detection statistics of the brightest radio-loud blazars\nin the northern sky. We find that 23% of these AGN were not detected above 0.1\nGeV by the Fermi LAT during the 4-year 3FGL catalog period partly because of an\ninstrumental selection effect, and partly due to their lower Doppler boosting\nfactors. Blazars with synchrotron peaks in their spectral energy distributions\nlocated below $10^{13.4}$ Hz also tend to have high-energy peaks that lie below\nthe 0.1 GeV threshold of the LAT, and are thus less likely to be detected by\nFermi. The non-detected AGN in the 1.5 Jy sample also have significantly lower\n15 GHz radio modulation indices and apparent jet speeds, indicating that they\nhave lower than average Doppler factors. Since the effective amount of\nrelativistic Doppler boosting is enhanced in gamma-rays (particularly in the\ncase of external inverse-Compton scattering), this makes them less likely to\nappear in the 3FGL catalog. Based on their observed properties, we have\nidentified several bright radio-selected blazars that are strong candidates for\nfuture detection by Fermi.", "category": "astro-ph_HE" }, { "text": "Optical Observations of the Type Ia Supernova 2011fe in M101 for Nearly\n 500 Days: We present well-sampled optical observations of the bright Type Ia supernova\n(SN~Ia) SN 2011fe in M101. Our data, starting from $\\sim16$ days before maximum\nlight and extending to $\\sim463$ days after maximum, provide an unprecedented\ntime series of spectra and photometry for a normal SN~Ia. Fitting the\nearly-time rising light curve, we find that the luminosity evolution of SN\n2011fe follows a $t^n$ law, with the index $n$ being close to 2.0 in the $VRI$\nbands but slightly larger in the $U$ and $B$ bands. Combining the published\nultraviolet (UV) and near-infrared (NIR) photometry, we derive the contribution\nof UV/NIR emission relative to the optical. SN 2011fe is found to have stronger\nUV emission and reaches its UV peak a few days earlier than other SNe~Ia with\nsimilar $\\Delta m_{15}(B)$, suggestive of less trapping of high-energy photons\nin the ejecta. Moreover, the $U$-band light curve shows a notably faster\ndecline at late phases ($t\\approx 100$--300 days), which also suggests that the\nejecta may be relatively transparent to UV photons. These results favor the\nnotion that SN 2011fe might have a progenitor system with relatively lower\nmetallicity. On the other hand, the early-phase spectra exhibit prominent\nhigh-velocity features (HVFs) of O~I $\\lambda$7773 and the Ca~II~NIR triplet,\nbut only barely detectable in Si~II~6355. This difference can be caused either\nby an ionization/temperature effect or an abundance enhancement scenario for\nthe formation of HVFs; it suggests that the photospheric temperature of SN\n2011fe is intrinsically low, perhaps owing to incomplete burning during the\nexplosion of the white dwarf.", "category": "astro-ph_HE" }, { "text": "Metallicity Effect on LMXB Formation in Globular Clusters: We present comprehensive observational results of the metallicity effect on\nthe fraction of globular clusters (GC) that contain low-mass X-ray binaries\n(LMXB), by utilizing all available data obtained with Chandra for LMXBs and HST\nACS for GCs. Our primary sample consists of old elliptical galaxies selected\nfrom the ACS Virgo and Fornax surveys. To improve statistics at both the lowest\nand highest X-ray luminosity, we also use previously reported results from\nother galaxies. It is well known that the LMXB fraction is considerably higher\nin red, metal-rich, than in blue, metal-poor GCs. In this paper, we test\nwhether this metallicity effect is X-ray luminosity-dependent, and find that\nthe effect holds uniformly in a wide luminosity range. This result is\nstatistically significant (at >= 3 sigma) in LMXBs with luminosities in the\nrange LX = 2 x 10^37 - 5 x 10^38 erg s-1, where the ratio of LMXB fractions in\nmetal-rich to metal-poor GCs is R = 3.4 +- 0.5. A similar ratio is also found\nat lower (down to 10^36 erg s-1) and higher luminosities (up to the ULX\nregime), but with less significance (~2 sigma confidence). Because different\ntypes of LMXBs dominate in different luminosities, our finding requires a new\nexplanation for the metallicity effect in dynamically formed LMXBs. We confirm\nthat the metallicity effect is not affected by other factors such as stellar\nage, GC mass, stellar encounter rate, and galacto-centric distance.", "category": "astro-ph_HE" }, { "text": "Precision ephemerides for gravitational-wave searches -- III. Revised\n system parameters of Sco X-1: Neutron stars in low-mass X-ray binaries are considered promising candidate\nsources of continuous gravitational-waves. These neutron stars are typically\nrotating many hundreds of times a second. The process of accretion can\npotentially generate and support non-axisymmetric distortions to the compact\nobject, resulting in persistent emission of gravitational-waves. We present a\nstudy of existing optical spectroscopic data for Sco X-1, a prime target for\ncontinuous gravitational-wave searches, with the aim of providing revised\nconstraints on key orbital parameters required for a directed search with\nadvanced-LIGO data. From a circular orbit fit to an improved radial velocity\ncurve of the Bowen emission components, we derived an updated orbital period\nand ephemeris. Centre of symmetry measurements from the Bowen Doppler tomogram\nyield a centre of the disc component of 90 km $\\mathrm{s^{-1}}$, which we\ninterpret as a revised upper limit to the projected orbital velocity of the NS\n$K_1$. By implementing Monte Carlo binary parameter calculations, and imposing\nnew limits on $K_1$ and the rotational broadening, we obtained a complete set\nof dynamical system parameter constraints including a new range for $K_1$ of\n40--90 km $\\mathrm{s^{-1}}$. Finally, we discussed the implications of the\nupdated orbital parameters for future continuous-waves searches.", "category": "astro-ph_HE" }, { "text": "Charged current neutrino interactions in core-collapse supernovae in a\n virial expansion: Core-collapse supernovae may depend sensitively on charged current neutrino\ninteractions in warm, low density neutron rich matter. A proton in neutron rich\nmatter is more tightly bound than is a neutron. This energy shift \\Delta U\nincreases the electron energy in \\nu_e + n --> p + e, increasing the available\nphase space and absorption cross section. Likewise \\Delta U decreases the\npositron energy in \\bar \\nu_e + p --> n + e^+, decreasing the phase space and\ncross section. We have calculated \\Delta U using a model independent virial\nexpansion and we find \\Delta U is much larger, at low densities, than the\npredictions of many mean field models. Therefore \\Delta U could have a\nsignificant impact on charged current neutrino interactions in supernovae.\nPreliminary simulations of the accretion phase of core-collapse supernovae find\nthat \\Delta U increases \\bar \\nu_e energies and decreases the \\nu_e luminosity.", "category": "astro-ph_HE" }, { "text": "Search for anisotropic Lorentz invariance violation with \u03b3-rays: While Lorentz invariance, the fundamental symmetry of Einstein's theory of\nGeneral Relativity, has been tested to a great level of detail, Grand Unified\nTheories that combine gravity with the other three fundamental forces may\nresult in a violation of Lorentz symmetry at the Planck scale. These energies\nare unattainable experimentally. However, minute deviations from Lorentz\ninvariance may still be present at much lower energies. These deviations can\naccumulate over large distances, making astrophysical measurements the most\nsensitive tests of Lorentz symmetry. One effect of Lorentz invariance violation\nis an energy dependent photon dispersion of the vacuum resulting in differences\nof the light travel time from distant objects. The Standard-Model Extension\n(SME) is an effective theory to describe the low-energy behavior of a more\nfundamental Grand Unified Theory, including Lorentz and CPT violating terms. In\nthe SME the Lorentz violating operators can in part be classified by their\nmass-dimension d, with the lowest order being d=5. However, measurements of\nphoton polarization have constrained operators with d=5 setting lower limits on\nthe energy at which they become dominant well beyond the Planck scale. On the\nother hand, these operators also violate CPT, and thus d=6 could be the leading\norder. In this paper we present constraints on all 25 real coefficients\ndescribing anisotropic non-birefringent Lorentz invariance violation at mass\ndimension d=6 in the SME. We used Fermi-LAT observations of 25 active galactic\nnuclei to constrain photon dispersion and combined our results with previously\npublished limits in order to simultaneously constrain all 25 coefficients. This\nrepresents the first set of constraints on these coefficients of a\nmass-dimension d=6, whereas previous measurements were only able to constrain\nlinear combinations of all 25 coefficients.", "category": "astro-ph_HE" }, { "text": "Detecting Stellar Lensing of Gravitational Waves with Ground-Based\n Observatories: We investigate the ability of ground based gravitational wave observatories\nto detect gravitational wave lensing events caused by stellar mass lenses. We\nshow that LIGO and Virgo possess the sensitivities required to detect lenses\nwith masses as small as $\\sim 30 M_\\odot$ provided that the gravitational wave\nis observed with a signal-to-noise ratio of $\\sim30$. Third generation\nobservatories will allow detection of gravitational wave lenses with masses of\n$\\sim 1 M_\\odot$. Finally, we discuss the possibility of lensing by multiple\nstars, as is the case if the gravitational radiation is passing through\ngalactic nucleus or a dense star cluster.", "category": "astro-ph_HE" }, { "text": "Effects of Compton Cooling on outflows in a Two Component Accretion Flow\n around a Black Hole: Results of a Coupled Monte Carlo-TVD Simulation: The effect of cooling on the outflow rate from an accretion disk around a\nblack hole is investigated using a coupled Monte Carlo Total Variation\nDiminishing code. A correlation between the spectral states and the outflow\nrates is found as a consequence.", "category": "astro-ph_HE" }, { "text": "The Definitive X-ray Light Curve of Swift J164449.3+573451: On March 28, 2011, the Swift Burst Alert Telescope triggered on an object\nthat had no analog in over six years of Swift operations. Follow-up\nobservations by the Swift X-ray Telescope (XRT) found a new, bright X-ray\nsource covering 3 orders of magnitude in flux over the first few days, that was\nmuch more persistent (and variable) than gamma-ray burst afterglows.\nGround-based spectroscopy found a redshift of 0.35, implying extremely high\nluminosity, with integrated isotropic-equivalent energy output in the X-ray\nband alone exceeding $10^{53}$ ergs in the first two weeks after discovery.\nStrong evidence for a collimated outflow or beamed emission was found. The\nobservational properties of this object are unlike anything ever before\nobserved. We interpret these unique properties as the result of emission from a\nrelativistic jet produced in the aftermath of the tidal disruption of a main\nsequence star by a massive black hole (BH) in the center of the host galaxy.\nThe source decayed slowly as the stellar remnants were accreted onto the BH,\nbefore abruptly shutting off. Here we present the definitive XRT team light\ncurve for Swift J164449.3+573451 and discuss its implications. We show that the\nunabsorbed flux decayed roughly as a $t^{-1.5}$ power law up to August 17,\n2012. The steep turnoff of an order of magnitude in 24 hours seems to be\nconsistent with the shutdown of the jet as the accretion disk transitioned from\na thick disk to a thin disk. Swift continues to monitor this source in case the\njet reactivates.", "category": "astro-ph_HE" }, { "text": "An extended Heitler-Matthews model for the full hadronic cascade in\n cosmic air showers: The Heitler-Matthews model for hadronic air showers will be extended to all\nthe generations of electromagnetic subshowers in the hadronic cascade. The\nanalysis is outlined in detail for showers initiated by primary protons. For\nshowers initiated by iron primaries the part of the analysis is given for as\nfar as it differs from the analysis for a primary proton. Predictions for\nshower sizes and the depth of maximum shower size are compared with results of\nMonte Carlo simulations. The depth of maximum as it follows from the\nextrapolation of the Heitler-Matthews model restricted to the first generation\nof electromagnetic subshowers is too small with respect to Monte Carlo\npredictions. It is shown that the inclusion of all the generations of\nelectromagnetic subshowers leads to smaller predictions for the depth of\nmaximum and to smaller predictions for the elongation rate. The discrepancy\nbetween discrete model predictions and Monte Carlo predictions for the depth of\nmaximum can therefore not be explained from the number of generations that is\ntaken into consideration. An alternative explanation will be proposed.", "category": "astro-ph_HE" }, { "text": "Search for gamma-ray emission from four accreting millisecond pulsars\n with Fermi/LAT: We report our search for \\gamma-ray emission in the energy range from 100 MeV\nto 300 GeV from four Accreting Millisecond Pulsars (AMPs), SAX J1808.4-3658,\nIGR J00291+5934, XTE J1814-338, and XTE J0929-314. The data are from four-year\nobservations carried out by Large Area Telescope (LAT) onboard the Fermi\n\\gamma-ray Space Telescope. The AMPs were not detected, and their \\gamma-ray\nluminosity upper limits we obtain are 5.1*10^33 ergs/s for SAX J1808.4-3658,\n2.1*10^33 ergs/s for IGR J00291+5934, 1.2*10^34 ergs/s for XTE J1814-338, and\n2.2*10^33 ergs/s for XTE J0929-314. We compare our results with \\gamma-ray\nirradiation luminosities required for producing optical modulations seen from\nthe companions in the AMPs, which has been suggested by Takata et al. (2012),\nand our upper limits have excluded \\gamma-ray emission as the heating source in\nthese systems except XTE J0929-314, the upper limit of which is not deep\nenough. Our results also do not support the model proposed by Takata et al.\n(2012) that relatively strong \\gamma-ray emission could arise from the outer\ngap of a high-mass neutron star controlled by the photon-photon pair-creation\nfor the AMPs. Two AMPs, SAX J1808.4-3658 and IGR J00291+5934, have the\nmeasurements of their spin-down rates, and we derive the upper limits of their\n\\gamma-ray conversion efficiencies, which are 57% and 3%, respectively. We\ndiscuss the implications to the AMP systems by comparing the efficiency upper\nlimit values with that of 20 \\gamma-ray millisecond pulsars (MSP) detected by\nFermi and the newly discovered transitional MSP binary J1023+0038.", "category": "astro-ph_HE" }, { "text": "Gamma-Ray Observational Properties of TeV-Detected Blazars: The synergy between the Fermi-LAT and ground-based Cherenkov telescope arrays\ngives us the opportunity for the first time to characterize the high-energy\nemission from blazars over 5 decades in energy, from 100 MeV to 10 TeV. In this\nstudy, we perform a Fermi-LAT spectral analysis for TeV-detected blazars and\ncombine it with archival TeV data. We examine the observational properties in\nthe gamma-ray band of our sample of TeV-detected blazars and compare the\nresults with X-ray and GeV-selected populations. The spectral energy\ndistributions (SEDs) that result from combining Fermi-LAT and ground-based\nspectra are studied in detail. Simple parameterizations such as a power-law\nfunction do not always reproduce the high-energy SEDs, where spectral features\nthat could indicate intrinsic absorption are observed.", "category": "astro-ph_HE" }, { "text": "GRB 091208B: First Detection of the Optical Polarization in Early\n Forward Shock Emission of a Gamma-Ray Burst Afterglow: We report that the optical polarization in the afterglow of GRB 091208B is\nmeasured at t = 149 - 706 s after the burst trigger, and the polarization\ndegree is P = 10.4% +/- 2.5%. The optical light curve at this time shows a\npower-law decay with index -0.75 +/- 0.02, which is interpreted as the forward\nshock synchrotron emission, and thus this is the first detection of the\nearly-time optical polarization in the forward shock (rather than that in the\nreverse shock reported by Steele et al. (2009). This detection disfavors the\nafterglow model in which the magnetic fields in the emission region are random\non the plasma skin depth scales, such as amplified by the plasma instabilities,\ne.g., Weibel instability. We suggest that the fields are amplified by the\nmagnetohydrodynamic instabilities, which would be tested by future observations\nof the temporal changes of the polarization degrees and angles for other\nbursts.", "category": "astro-ph_HE" }, { "text": "The high-energy emission of millisecond pulsars: This chapter provides a phenomenological appraisal of the high-energy\nemission of millisecond pulsars. We comment on some of their properties as a\npopulation, as well as consider the especial cases of transitional pulsars,\nother redbacks, and black widow systems.", "category": "astro-ph_HE" }, { "text": "Constraints on the optical precursor to the naked-eye burst GRB080319B\n from Pi of the Sky observations: I present the results of the search for an optical precursor to the naked-eye\nburst - GRB080319B, which reached 5.87m optical peak luminosity in the \"Pi of\nthe Sky\" data. A burst of such a high brightness could have been preceded by an\noptical precursor luminous enough to be in detection range of our experiment.\nThe \"Pi of the Sky\" cameras observed the coordinates of the GRB for about 20\nminutes prior to the explosion, thus provided crucial data for the precursor\nsearch. No signal within 3 sigma limit was found. A limit of 12m (V-band\nequivalent) was set based on the data combined from two cameras, the most\nrobust limit to my knowledge for this precursor.", "category": "astro-ph_HE" }, { "text": "X-ray Polarimetry of the accreting pulsar 1A~0535+262 in the\n supercritical state with PolarLight: The X-ray pulsar 1A 0535+262 exhibited a giant outburst in 2020, offering us\na unique opportunity for X-ray polarimetry of an accreting pulsar in the\nsupercritical state. Measurement with PolarLight yielded a non-detection in 3-8\nkeV; the 99% upper limit of the polarization fraction (PF) is found to be 0.34\naveraged over spin phases, or 0.51 based on the rotating vector model. No\nuseful constraint can be placed with phase resolved polarimetry. These upper\nlimits are lower than a previous theoretical prediction of 0.6-0.8, but\nconsistent with those found in other accreting pulsars, like Her X-1, Cen X-3,\n4U 1626-67, and GRO J1008-57, which were in the subcritical state, or at least\nnot confidently in the supercritical state, during the polarization\nmeasurements. Our results suggest that the relatively low PF seen in accreting\npulsars cannot be attributed to the source not being in the supercritical\nstate, but could be a general feature.", "category": "astro-ph_HE" }, { "text": "{\\it Suzaku} observation of Galactic supernova remnant CTB 37A\n (G348.5+0.1): We present here the results of the observation of CTB 37A obtained with the\nX-ray Imaging Spectrometer onboard the {\\it Suzaku} satellite. The X-ray\nspectrum of CTB 37A is well fitted by two components, a single-temperature\nionization equilibrium component (VMEKAL) with solar abundances, an electron\ntemperature of $kT_{\\rm e}\\sim0.6$ keV, absorbing column density of $N_{\\rm\nH}\\sim3\\times10^{22}$ ${\\rm cm^{-2}}$ and a power-law component with photon\nindex of $\\Gamma$ $\\sim 1.6$. The X-ray spectrum of CTB 37A is characterized by\nclearly detected K-shell emission lines of Mg, Si, S, and Ar. The plasma with\nsolar abundances supports the idea that the X-ray emission originates from the\nshocked interstellar material. The ambient gas density, and age of the remnant\nare estimated to be $\\sim1f^{-1/2}$${\\rm cm^{-3}}$ and\n$\\sim3\\times10^{4}f^{1/2}$ yr, respectively. The center-filling X-ray emission\nsurrounded by a shell-like radio structure and other X-ray properties indicate\nthat this remnant would be a new member of mixed-morphology supernova remnant\nclass.", "category": "astro-ph_HE" }, { "text": "A flaring magnetar in FRB 121102?: The persistent radio counterpart of FRB 121102 is estimated to have $N\\sim\n10^{52}$ particles, energy $E_N\\sim 10^{48}$ erg, and size $R\\sim 10^{17}$ cm.\nThe source can be a nebula inflated and heated by an intermittent outflow from\na magnetar --- a neutron star powered by its magnetic (rather than rotational)\nenergy. The object is young and frequently liberating energy in magnetic flares\ndriven by accelerated ambipolar diffusion in the neutron star core, feeding the\nnebula and producing bright millisecond bursts. The particle number in the\nnebula is consistent with ion ejecta from giant flares. The nebula may also\ncontain the freeze-out of electron-positron pairs $N_\\pm\\sim 10^{51}$ created\nmonths after the neutron star birth; the same mechanism offers an explanation\nfor $N_\\pm$ in the Crab nebula. The persistent source around FRB 121102 is\nlikely heated by magnetic dissipation and internal waves excited by the\nmagnetar ejecta. The volumetric heating by waves explains the nebula's enormous\nefficiency in producing radio emission. The repeating radio bursts are\nsuggested to occur much closer to the magnetar, whose flaring magnetosphere\ndrives ultrarelativistic internal shocks into the magnetar wind. The shocks are\nmediated by Larmor rotation that forms a GHz maser with the observed ms\nduration. Furthermore, the flare ejecta can become charge-starved and then\nconvert to electromagnetic waves.", "category": "astro-ph_HE" }, { "text": "$JetCurry$ I. Reconstructing Three-Dimensional Jet Geometry from\n Two-Dimensional Images: We present a three-dimensional (3-D) visualization of jet geometry using\nnumerical methods based on a Markov Chain Monte Carlo (MCMC) and limited memory\nBroyden-Fletcher-Goldfarb-Shanno (BFGS) optimized algorithm. Our aim is to\nvisualize the 3-D geometry of an active galactic nucleus (AGN) jet using\nobservations, which are inherently two-dimensional (2-D) images. Many AGN jets\ndisplay complex structures that include hotspots and bends. The structure of\nthese bends in the jet's frame may appear quite different than what we see in\nthe sky frame, where it is transformed by our particular viewing geometry. The\nknowledge of the intrinsic structure will be helpful in understanding the\nappearance of the magnetic field and hence emission and particle acceleration\nprocesses over the length of the jet. We present the $JetCurry$ algorithm to\nvisualize the jet's 3-D geometry from its 2-D image. We discuss the underlying\ngeometrical framework and outline the method used to decompose the 2-D image.\nWe report the results of our 3-D visualization of the jet of M87, using the\ntest case of the knot D region. Our 3-D visualization is broadly consistent\nwith the expected double helical magnetic field structure of the knot D region\nof the jet. We also discuss the next steps in the development of the $JetCurry$\nalgorithm.", "category": "astro-ph_HE" }, { "text": "Quark matter with strong magnetic field and possibility of the third\n family of compact stars: We consider the possibility for the existence of the third family of compact\nobjects, considering the effect of strong magnetic fields inside the hybrid\nstars. As a result, we demonstrate such new sequences of stable equilibrium\nconfigurations for some hadronic equations of state. Through the analysis of\nthe adiabatic index inside stars, we find the conditions for appearing the\nthird family of compact objects, i.e., for hadronic stars without quarks, that\nthe maximum mass should be small, the central density for the maximum mass\nshould be also small, and the radius for the the maximum mass should be large.\nEven for soft hadronic equations of state, the two solar-mass stars might\nsurvive as the third family of compact objects, once quark matter with strong\nmagnetic field, such as $\\sim {\\cal O}(10^{19} {\\rm G})$, is taken into\naccount. It might give a hint to solve the so-called hyperon puzzle in nuclear\nphysics.", "category": "astro-ph_HE" }, { "text": "Low-frequency gravitational wave memory from gamma-ray burst afterglows\n with energy injection: Ultrarelativistic gamma-ray burst (GRB) jets are strong gravitational wave\n(GW) sources with memory-type signals. The plateau (or shallow decay) phases\ndriven by the energy injection might appear in the early X-ray afterglows of\nGRBs. In this paper, we investigate the GW signal as well as X-ray afterglow\nemission in the framework of GRB jets with energy injection, and both short-\nand long-duration GRBs are considered. We find that, regardless of the case,\nbecause of the antibeaming and time delay effects, a rising slope emerging in\nthe waveform of GW signal due to the energy injection lags far behind the\nenergy ejection, and the typical frequency of the characteristic amplitudes\nfalls within a low-frequency region of $\\sim10^{-4}-10^{-6} \\,{\\rm Hz}$; and we\nconsider that the GW memory triggered by GRB jets with energy injection are\npreviously unaware and the nearby GRBs with strong energy injection might\ndisturb the measurement of the stochastic GW background. Such GW memory\ndetection would provide a direct test for models of energy injection in the\nscenario of GRB jets.", "category": "astro-ph_HE" }, { "text": "Multifrequency studies of the narrow-line Seyfert 1 galaxy SBS 0846+513: The narrow-line Seyfert 1 galaxy SBS 0846+513 was first detected by the Large\nArea Telescope (LAT) on-board Fermi in 2011 June-July when it underwent a\nperiod of flaring activity. Since then, as Fermi continues to accumulate data\non this source, its flux has been monitored on a daily basis. Two further\ngamma-ray flaring episodes from SBS 0846+513 were observed in 2012 May and\nAugust, reaching a daily peak flux integrated above 100 MeV of (50+/-12)x10^-8\nph/cm^2/s, and (73+/-14)x10^-8 ph/cm^2/s on May 24 and August 7, respectively.\nThree outbursts were detected at 15 GHz by the Owens Valley Radio Observatory\n40-m telescope in 2012 May, 2012 October, and 2013 January, suggesting a\ncomplex connection with the gamma-ray activity. The most likely scenario\nsuggests that the 2012 May gamma-ray flare may not be directly related to the\nradio activity observed over the same period, while the two gamma-ray flaring\nepisodes may be related to the radio activity observed at 15 GHz in 2012\nOctober and 2013 January. The gamma-ray flare in 2012 May triggered Swift\nobservations that confirmed that SBS 0846+513 was also exhibiting high activity\nin the optical, UV and X-ray bands, thus providing a firm identification\nbetween the gamma-ray source and the lower-energy counterpart. We compared the\nspectral energy distribution (SED) of the flaring state in 2012 May with that\nof a quiescent state. The two SEDs, modelled as an external Compton component\nof seed photons from a dust torus, could be fitted by changing the electron\ndistribution parameters as well as the magnetic field. No significant evidence\nof thermal emission from the accretion disc has been observed. Interestingly,\nin the 5 GHz radio luminosity vs. synchrotron peak frequency plot SBS 0846+513\nseems to lie in the flat spectrum radio quasar part of the so-called `blazar\nsequence'.", "category": "astro-ph_HE" }, { "text": "Modeling the Spectrum of IGR J17177-3656: The correlation between radio and X-ray luminosity in the hard state of black\nhole X-ray binaries is important for unveiling the relation between the\naccretion flow and the jets. In this paper, we have modeled the\nquasi-simultaneous multi-band observations of a recently discovered transient\nX-ray source, IGR J17177-3656. It is found that the source is probably an\noutlier following the steep radio/X-ray correlation rather than an outlier in\nthe transition region as suggested by Paizis et al. (2011). It is also found\nthat the multi-band spectrum can be successfully modeled by the luminous hot\naccretion flow (LHAF) but less likely by the advection dominated accretion flow\n(ADAF). Our results support the point that LHAF can explain the steep\nradio/X-ray correlation.", "category": "astro-ph_HE" }, { "text": "Observational Signatures of Black Hole Accretion: Rotating vs. Spherical\n Flows with Tilted Magnetic Fields: We study the observational signatures of magnetically arrested black hole\naccretion with non-rotating inflow onto a rotating black hole; we consider a\nrange of angles between the black hole spin and the initial magnetic field\norientation. We compare the results of our General Relativistic\nMagneto-Hydrodynamic simulations to more commonly used rotating initial\nconditions and to the Event Horizon Telescope (EHT) observations of M87. We\nfind that the mm intensity images, polarization images, and synchrotron\nemission spectra are very similar among the different simulations when\npost-processed with the same electron temperature model; observational\ndifferences due to different electron temperature models are significantly\nlarger than those due to the different realizations of magnetically arrested\naccretion. The orientation of the mm synchrotron polarization is particularly\ninsensitive to the initial magnetic field orientation, the electron temperature\nmodel, and the rotation of the inflowing plasma. The largest difference among\nthe simulations with different initial rotation and magnetic tilt is in the\nstrength and stability of the jet; spherical inflow leads to kink-unstable\njets. We discuss the implications of our results for current and future EHT\nobservations and for theoretical models of event-horizon-scale black hole\naccretion.", "category": "astro-ph_HE" }, { "text": "The short bursts in SGR 1806-20, 1E 1048-5937 and SGR 0501+4516: We analyzed temporal and spectral properties, focusing on the short bursts,\nfor three anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs),\nincluding SGR 1806-20, 1E 1048-5937 and SGR 0501+4516. Using the data from\nXMM-Newton, we located the short bursts by Bayesian blocks algorithm. The short\nbursts' duration distributions for three sources were fitted by two lognormal\nfunctions. The spectra of shorter bursts ($< 0.2~\\rm s$) and longer bursts\n($\\geq 0.2~\\rm s$) can be well fitted in two blackbody components model or\noptically thin thermal bremsstrahlung model for SGR 0501+4516. We also found\nthat there is a positive correlation between the burst luminosity and the\npersistent luminosity with a power law index $\\gamma = 1.23 \\pm 0.18 $. The\nenergy ratio of this persistent emission to the time averaged short bursts is\nin the range of $10 - 10^3$, being comparable to the case in Type I X-ray\nburst.", "category": "astro-ph_HE" }, { "text": "Disentangling the Origin and Heating Mechanism of Supernova Dust:\n Late-Time Spitzer Spectroscopy of the Type IIn SN 2005ip: This paper presents late-time near-infrared and {\\it Spitzer} mid-infrared\nphotometric and spectroscopic observations of warm dust in the Type IIn SN\n2005ip in NGC 2906. The spectra show evidence for two dust components with\ndifferent temperatures. Spanning the peak of the thermal emission, these\nobservations provide strong constraints on the dust mass, temperature, and\nluminosity, which serve as critical diagnostics for disentangling the origin\nand heating mechanism of each component. The results suggest the warmer dust\nhas a mass of $\\sim 5 \\times 10^{-4}~$\\msolar, originates from newly formed\ndust in the ejecta, or possibly the cool, dense shell, and is continuously\nheated by the circumstellar interaction. By contrast, the cooler component\nlikely originates from a circumstellar shock echo that forms from the heating\nof a large, pre-existing dust shell $\\sim 0.01 - 0.05$~\\msolar~by the late-time\ncircumstellar interaction. The progenitor wind velocity derived from the blue\nedge of the He 1 1.083 \\micron~P Cygni profile indicates a progenitor eruption\nlikely formed this dust shell $\\sim$100 years prior to the supernova explosion,\nwhich is consistent with a Luminous Blue Variable (LBV) progenitor star.", "category": "astro-ph_HE" }, { "text": "Multi-messenger and real-time astrophysics with the Baikal-GVD telescope: The Baikal-GVD deep underwater neutrino experiment participates in the\ninternational multi-messenger program on discovering the astrophysical sources\nof high energy fluxes of cosmic particles, while being at the stage of\ndeployment with a gradual increase of its effective volume to the scale of a\ncubic kilometer. In April 2021 the effective volume of the detector has been\nreached 0.4 km3 for cascade events with energy above 100 TeV generated by\nneutrino interactions in Lake Baikal. The alarm system in real-time monitoring\nof the celestial sphere was launched at the beginning of 2021, that allows to\nform the alerts of two ranks like \"muon neutrino\" and \"VHE cascade\". Recent\nresults of fast follow-up searches for coincidences of Baikal-GVD high energy\ncascades with ANTARES/TAToO high energy neutrino alerts and IceCube GCN\nmessages will be presented, as well as preliminary results of searches for high\nenergy neutrinos in coincidence with the magnetar SGR 1935+2154 activity in\nperiod of radio and gamma burst in 2020.", "category": "astro-ph_HE" }, { "text": "Geodesic models of quasi-periodic-oscillations as probes of quadratic\n gravity: Future very-large-area X-ray instruments (for which the effective area is\nlarger than $>3$m$^2$) will be able to measure the frequencies of\nquasi-periodic oscillations~(QPOs) observed in the X-ray flux from accreting\ncompact objects with sub-percent precision. If correctly modeled, QPOs can\nprovide a novel way to test the strong-field regime of gravity. By using the\nrelativistic precession model and a modified version of the epicyclic resonance\nmodel, we develop a method to test general relativity against a generic class\nof theories with quadratic curvature corrections. With the instrumentation\nbeing studied for future missions such as eXTP, LOFT, or STROBE-X, a\nmeasurement of at least two QPO triplets from a stellar mass black hole can set\nstringent constraints on the coupling parameters of quadratic gravity.", "category": "astro-ph_HE" }, { "text": "The GALEX Time Domain Survey. II. Wavelength-Dependent Variability of\n Active Galactic Nuclei in the Pan-STARRS1 Medium Deep Survey: We analyze the wavelength-dependent variability of a sample of\nspectroscopically confirmed active galactic nuclei (AGN) selected from near-UV\n($NUV$) variable sources in the GALEX Time Domain Survey that have a large\namplitude of optical variability (difference-flux S/N $>$ 3) in the Pan-STARRS1\nMedium Deep Survey (PS1 MDS). By matching GALEX and PS1 epochs in 5 bands\n($NUV$, $g_{P1}$, $r_{P1}$, $i_{P1}$, $z_{P1}$) in time, and taking their flux\ndifference, we create co-temporal difference-flux spectral energy distributions\n($\\Delta f$SEDs) using two chosen epochs for each of the 23 objects in our\nsample on timescales of about a year.\n We confirm the \"bluer-when-brighter\" trend reported in previous studies, and\nmeasure a median spectral index of the $\\Delta f$SEDs of $\\alpha_{\\lambda}$ =\n2.1 that is consistent with an accretion disk spectrum. We further fit the\n$\\Delta f$SEDs of each source with a standard accretion disk model in which the\naccretion rate changes from one epoch to the other. In our sample, 17 out of 23\n($\\sim$74 %) sources are well described by this variable accretion-rate disk\nmodel, with a median average characteristic disk temperature $\\bar{T}^*$ of\n$1.2\\times 10^5$~K that is consistent with the temperatures expected given the\ndistribution of accretion rates and black hole masses inferred for the sample.\nOur analysis also shows that the variable accretion rate model is a better fit\nto the $\\Delta f$SEDs than a simple power law.", "category": "astro-ph_HE" }, { "text": "Probing AGN variability with the Cherenkov Telescope Array: Relativistic jets launched by Active Galactic Nuclei are among the most\npowerful particle accelerators in the Universe. The emission over the entire\nelectromagnetic spectrum of these relativistic jets can be extremely variable\nwith scales of variability from less than few minutes up to several years.\nThese variability patterns, which can be very complex, contain information\nabout the acceleration processes of the particles and the area(s) of emission.\nThanks to its sensitivity, five-to twenty-times better than the current\ngeneration of Imaging Atmospheric Cherenkov Telescopes depending on energy, the\nCherenkov Telescope Array will be able to follow the emission from these\nobjects with a very accurate time sampling and over a wide spectral coverage\nfrom 20 GeV to > 20 TeV and thus reveal the nature of the acceleration\nprocesses at work in these objects. We will show the first results of our\nlightcurve simulations and long-term behavior of AGN as will be observed by\nCTA, based on state-of-art particle acceleration models.", "category": "astro-ph_HE" }, { "text": "Influence of a Galactic Gamma-Ray Burst on Ocean Plankton: The hypothesis that one or more biodiversity drops in the Phanerozoic eon,\nevident in the geological record, might have been caused by the most powerful\nkind of stellar explosion so far known Gamma Ray Bursts (GRB) has been\ndiscussed in several works. These stellar explosions could have left an imprint\nin the biological evolution on Earth and in other habitable planets. In this\nwork we calculate the short-term lethality that a GRB would produce in the\naquatic primary producers on Earth. This effect on life appears because of\nultraviolet (UV) retransmission in the atmosphere of a fraction of the gamma\nenergy, resulting in an intense UV flash capable of penetrating tens of meters\nin the water column in the ocean. We focus on the action of the UV flash on\nphytoplankton, as they are the main contributors to global aquatic primary\nproductivity. Our results suggest that the UV flash could cause a significant\nreduction of phytoplankton biomass in the upper mixed layer of the World Ocean.", "category": "astro-ph_HE" }, { "text": "First search for atmospheric and extraterrestrial neutrino-induced\n cascades with the IceCube detector: We report on the first search for atmospheric and for diffuse astrophysical\nneutrino-induced showers (cascades) in the IceCube detector using 257 days of\ndata collected in the year 2007-2008 with 22 strings active. A total of 14\nevents with energies above 16 TeV remained after event selections in the\ndiffuse analysis, with an expected total background contribution of $8.3\\pm\n3.6$. At 90% confidence we set an upper limit of\n$E^2\\Phi_{90%CL}<3.6\\times10^{-7} GeV \\cdot cm^{-2} \\cdot s^{-1}\\cdot sr^{-1} $\non the diffuse flux of neutrinos of all flavors in the energy range between 24\nTeV and 6.6 PeV assuming that $\\Phi \\propto E^{-2}$ and that the flavor\ncomposition of the $\\nu_e : \\nu_\\mu : \\nu_\\tau$ flux is $1 : 1 : 1$ at the\nEarth. The atmospheric neutrino analysis was optimized for lower energies. A\ntotal of 12 events were observed with energies above 5 TeV. The observed number\nof events is consistent with the expected background, within the uncertainties.", "category": "astro-ph_HE" }, { "text": "Magnetically-driven crustquakes in neutron stars: Crustquake events may be connected with both rapid spin-up `glitches' within\nthe regular slowdown of neutron stars, and high-energy magnetar flares. We\nargue that magnetic field decay builds up stresses in a neutron star's crust,\nas the elastic shear force resists the Lorentz force's desire to rearrange the\nglobal magnetic-field equilibrium. We derive a criterion for crust-breaking\ninduced by a changing magnetic-field configuration, and use this to investigate\nstrain patterns in a neutron star's crust for a variety of different\nmagnetic-field models. Universally, we find that the crust is most liable to\nbreak if the magnetic field has a strong toroidal component, in which case the\nepicentre of the crustquake is around the equator. We calculate the energy\nreleased in a crustquake as a function of the fracture depth, finding that it\nis independent of field strength. Crust-breaking is, however, associated with a\ncharacteristic local field strength of $2.4\\times 10^{14}$ G for a breaking\nstrain of $0.001$, or $2.4\\times 10^{15}$ G at a breaking strain of $0.1$. We\nfind that even the most luminous magnetar giant flare could have been powered\nby crustal energy release alone.", "category": "astro-ph_HE" }, { "text": "Understanding the Unusual X-Ray Emission Properties of the Massive,\n Close Binary WR 20a: A High Energy Window into the Stellar Wind Initiation\n Region: The problem of explaining the X-ray emission properties of the massive, close\nbinary WR 20a is discussed. Located near the cluster core of Westerlund 2, WR\n20a is composed of two nearly identical Wolf- Rayet stars of 82 and 83 solar\nmasses orbiting with a period of only 3.7 days. Although Chandra observations\nwere taken during the secondary optical eclipse, the X-ray light curve shows no\nsigns of a flux decrement. In fact, WR 20a appears slightly more X-ray luminous\nand softer during the optical eclipse, opposite to what has been observed in\nother binary systems. To aid in our interpretation of the data, we compare with\nthe results of hydrodynamical simulations using the adaptive mesh refinement\ncode Mezcal that includes radiative cooling and a radiative acceleration force\nterm. It is shown that the X-ray emission can be successfully explained in\nmodels where the wind-wind collision interface in this system occurs while the\noutflowing material is still being accelerated. Consequently, WR 20a serves as\na critical test-case for how radiatively-driven stellar winds initiate and\ninteract. Our models not only procure a robust description of current Chandra\ndata, which cover the orbital phases between 0.3 to 0.6, but provide detailed\npredictions over the entire orbit.", "category": "astro-ph_HE" }, { "text": "GeV observations of the extended pulsar wind nebulae constrain the\n pulsar interpretations of the cosmic-ray positron excess: It has long been suggested that nearby pulsars within $\\sim 1 \\,{\\rm kpc}$\nare the leading candidate of the 10-500 GeV cosmic-ray positron excess measured\nby PAMELA and other experiments. The recent measurement of surface brightness\nprofile of TeV nebulae surrounding Geminga and PSR~B0656+14 by the\nHigh-Altitude Water Cherenkov Observatory (HAWC) suggests inefficient diffusion\nof particles from the sources, giving rise to a debate on the pulsar\ninterpretation of the cosmic-ray positron excess. Here we argue that GeV\nobservations provide more direct constraints on the positron density in the TeV\nnebulae in the energy range of 10-500 GeV and hence on the origin of the\nobserved positron excess. Motivated by this, we search for GeV emission from\nthe TeV nebulae with the \\textsl{Fermi} Large Area Telescope (LAT). No\nspatially-extended GeV emission is detected from these two TeV nebulae in the\nframework of two-zone diffusion spatial templates, suggesting a relatively low\ndensity of GeV electrons/positrons in the TeV nebulae. A joint modelling of the\ndata from HAWC and \\textsl{Fermi}-LAT disfavors Geminga and PSR~B0656+14 as the\ndominant source of the positron excess at $\\sim 50-500$ GeV for the usual\nKolmogorov-type diffusion, while for an energy-independent diffusion, a\ndominant part of the positron excess contributed by them cannot be ruled out by\nthe current data.", "category": "astro-ph_HE" }, { "text": "Electron-positron flows around magnetars: The twisted magnetospheres of magnetars must sustain a persistent flow of\nelectron-positron plasma. The flow dynamics is controlled by the radiation\nfield around the hot neutron star. The problem of plasma motion in the\nself-consistent radiation field is solved using the method of virtual beams.\nThe plasma and radiation exchange momentum via resonant scattering and\nself-organize into the \"radiatively locked\" outflow with a well-defined,\ndecreasing Lorentz factor. There is an extended zone around the magnetar where\nthe plasma flow is ultra-relativistic; its Lorentz factor is self-regulated so\nthat it can marginally scatter thermal photons. The flow becomes slow and\nopaque in an outer equatorial zone, where the decelerated plasma accumulates\nand annihilates; this region serves as a reflector for the thermal photons\nemitted by the neutron star. The e+- flow carries electric current, which is\nsustained by a moderate induced electric field. The electric field maintains a\nseparation between the electron and positron velocities, against the will of\nthe radiation field. The two-stream instability is then inevitable, and the\ninduced turbulence can generate low-frequency emission. In particular, radio\nemission may escape around the magnetic dipole axis of the star. Most of the\nflow energy is converted to hard X-ray emission, which is examined in the\naccompanying paper.", "category": "astro-ph_HE" }, { "text": "A Consistent Modeling of Neutrino-driven Wind with Accretion Flow onto a\n Protoneutron Star and its Implications for $^{56}$Ni Production: Details of the explosion mechanism of core-collapse supernovae (CCSNe) are\nnot yet fully understood. There is now an increasing number of successful\nexamples of reproducing explosions in the first-principles simulations, which\nhave shown a slow increase of explosion energy. However, it was recently\npointed out that the growth rates of the explosion energy of these simulations\nare insufficient to produce enough $^{56}$Ni mass to account for observations.\nWe refer to this issue as the `nickel mass problem' (Ni problem, hereafter) in\nthis paper. The neutrino-driven wind is suggested as one of the most promising\ncandidates for the solution to the Ni problem in previous literature, but a\nmulti-dimensional simulation for this is computationally too expensive to allow\nlong-term investigations. In this paper, we first built a consistent model of\nthe neutrino-driven wind with an accretion flow onto a protoneutron star (PNS),\nby connecting a steady-state solution of the neutrino-driven wind and a\nphenomenological mass accretion model. Comparing the results of our model with\nthe results of first-principles simulations, we find that the total ejectable\namount of the neutrino-driven wind is roughly determined within $\\sim$ 1 sec\nfrom the onset of the explosion and the supplementable amount at a late phase\n($t_e \\gtrsim 1$ sec) remains $M_\\mathrm{ej} \\lesssim 0.01M_\\odot$ at most. Our\nconclusion is that it is difficult to solve the Ni problem, by continuous\ninjection of $^{56}$Ni by the neutrino-driven wind. We suggest that the total\namount of synthesized $^{56}$Ni can be estimated robustly if simulations are\nfollowed up to $\\sim 2$ seconds.", "category": "astro-ph_HE" }, { "text": "Ejection of Double knots from the radio core of PKS 1510--089 during the\n strong gamma-ray flares in 2015: PKS 1510--089 is a bright and active $\\gamma$-ray source that showed strong\nand complex $\\gamma$-ray flares in mid-2015 during which the Major Atmospheric\nGamma Imaging Cherenkov telescopes detected variable very high energy (VHE;\nphoton energies $>$100 GeV) emission. We present long-term multi-frequency\nradio, optical, and $\\gamma$-ray light curves of PKS 1510--089 from 2013 to\n2018, and results of an analysis of the jet kinematics and linear polarization\nusing 43 GHz Very Long Baseline Array data observed between late 2015 and\nmid-2017. We find that a strong radio flare trails the $\\gamma$-ray flares in\n2015, showing an optically thick spectrum at the beginning and becoming\noptically thin over time. Two laterally separated knots of emission are\nobserved to emerge from the radio core nearly simultaneously during the\n$\\gamma$-ray flares. We detect an edge-brightened linear polarization near the\ncore in the active jet state in 2016, similar to the quiescent jet state in\n2008--2013. These observations indicate that the $\\gamma$-ray flares may\noriginate from compression of the knots by a standing shock in the core and the\njet might consist of multiple complex layers showing time-dependent behavior,\nrather than of a simple structure of a fast jet spine and a slow jet sheath.", "category": "astro-ph_HE" }, { "text": "Multi-messenger searches via IceCube's high-energy neutrinos and\n gravitational-wave detections of LIGO/Virgo: We summarize initial results for high-energy neutrino counterpart searches\ncoinciding with gravitational-wave events in LIGO/Virgo's GWTC-2 catalog using\nIceCube's neutrino triggers. We did not find any statistically significant\nhigh-energy neutrino counterpart and derived upper limits on the\ntime-integrated neutrino emission on Earth as well as the isotropic equivalent\nenergy emitted in high-energy neutrinos for each event.", "category": "astro-ph_HE" }, { "text": "Analysis of a State Changing Supersoft X-ray Source in M31: We report on observations of a luminous supersoft X-ray source (SSS) in M31,\nr1-25, that has exhibited spectral changes to harder X-ray states. We document\nthese spectral changes. In addition, we show that they have important\nimplications for modeling the source. Quasisoft states in a source that has\nbeen observed as an SSS represent a newly- discovered phenomenon. We show how\nsuch state changers could prove to be examples of unusual black hole or neutron\nstar accretors. Future observations of this and other state changers can\nprovide the information needed to determine the nature(s) of these intriguing\nnew sources.", "category": "astro-ph_HE" }, { "text": "Primordial Black Holes having Gravitomagnetic Monopole: A primordial black hole (PBH) is thought to be made of the regular matter or\nordinary mass ($M$) only, and hence could have already been decayed due to the\nHawking radiation if its initial ordinary mass were $\\lesssim 5 \\times 10^{11}$\nkg. Here, we study the role of gravitomagnetic monopole for the evaporation of\nPBHs, and propose that the lower energy PBHs (equivalent to ordinary mass $M <<\n5\\times 10^{11}$ kg) could still exist in our present Universe, if it has\ngravitomagnetic monopole. If a PBH was initially made of both regular matter\nand gravitomagnetic monopole, the regular matter could decay away due to the\nHawking radiation. The remnant gravitomagnetic monopole might not entirely\ndecay, which could still be found as a PBH in the form of the pseudo\n`mass-energy'. If a PBH with $M \\gtrsim 5 \\times 10^{11}$ kg is detected, one\nmay not be able to conclude if it has gravitomagnetic monopole. But, a\nplausible detection of a relatively low energy (equivalent to $2.176 \\times\n10^{-8}$ kg $< M \\lesssim 5\\times10^{11}$ kg) PBH in future may imply the\nexistence of a gravitomagnetic monopole PBH, which may or may not contain the\nordinary mass.", "category": "astro-ph_HE" }, { "text": "Discovery and Follow-up of ASASSN-23bd (AT 2023clx): The Lowest Redshift\n and Least Luminous Tidal Disruption Event To Date: We report the All-Sky Automated Survey for SuperNovae discovery of the tidal\ndisruption event (TDE) ASASSN-23bd (AT 2023clx) in NGC 3799, a LINER galaxy\nwith no evidence of strong AGN activity over the past decade. With a redshift\nof $z = 0.01107$ and a peak UV/optical luminosity of $(5.4\\pm0.4)\\times10^{42}$\nerg s$^{-1}$, ASASSN-23bd is the lowest-redshift and least-luminous TDE\ndiscovered to date. Spectroscopically, ASASSN-23bd shows H$\\alpha$ and He I\nemission throughout its spectral time series, and the UV spectrum shows\nnitrogen lines without the strong carbon and magnesium lines typically seen for\nAGN. Fits to the rising ASAS-SN light curve show that ASASSN-23bd started to\nbrighten on MJD 59988$^{+1}_{-1}$, $\\sim$9 days before discovery, with a nearly\nlinear rise in flux, peaking in the $g$ band on MJD $60000^{+3}_{-3}$. Scaling\nrelations and TDE light curve modelling find a black hole mass of $\\sim$10$^6$\n$M_\\odot$, which is on the lower end of supermassive black hole masses.\nASASSN-23bd is a dim X-ray source, with an upper limit of\n$L_{0.3-10\\,\\mathrm{keV}} < 1.0\\times10^{40}$ erg s$^{-1}$ from stacking all\n\\emph{Swift} observations prior to MJD 60061, but with soft ($\\sim 0.1$ keV)\nthermal emission with a luminosity of $L_{0.3-2\n\\,\\mathrm{keV}}\\sim4\\times10^{39}$ erg s$^{-1}$ in \\emph{XMM-Newton}\nobservations on MJD 60095. The rapid $(t < 15$ days) light curve rise, low\nUV/optical luminosity, and a luminosity decline over 40 days of $\\Delta\nL_{40}\\approx-0.7$ make ASASSN-23bd one of the dimmest TDEs to date and a\nmember of the growing ``Low Luminosity and Fast'' class of TDEs.", "category": "astro-ph_HE" }, { "text": "Estimating the maximum gravitational mass of nonrotating neutron stars\n from the GW170817/GRB 170817A/AT2017gfo observation: Assuming that the differential rotation of the massive neutron star (NS)\nformed in the double NS (DNS) mergers has been effectively terminated by the\nmagnetic braking and a uniform rotation has been subsequently established\n(i.e., a supramassive NS is formed), we analytically derive in this work an\napproximated expression for the critical total gravitational mass ($M_{\\rm\ntot,c}$) to form supramassive NS (SMNS) in the DNS mergers, benefited from some\nequation of state (EoS) insensitive relationships. The maximum gravitational\nmass of the nonrotating NSs ($M_{\\rm TOV}$) as well as the dimensionless\nangular momentum of the remnant ($j$) play the dominant roles in modifying\n$M_{\\rm tot,c}$, while the radius and mass differences of the premerger NSs do\nnot. The GW170817/GRB 170817A/AT2017gfo observations have provided so far the\nbest opportunity to quantitatively evaluate $M_{\\rm TOV}$. Supposing the\ncentral engine for GRB 170817A is a black hole quickly formed in the collapse\nof an SMNS, we find $M_{\\rm TOV}=2.13^{+0.09}_{-0.08}M_\\odot$ (68.3%\ncredibility interval, including also the uncertainties of the EoS insensitive\nrelationships), which is consistent with the constraints set by current NS mass\nmeasurements.", "category": "astro-ph_HE" }, { "text": "The coalescence rates of double black holes: We present the summary of the recent investigations of double black hole\nbinaries in context of their formation and merger rates. In particular we\ndiscuss the spectrum of black hole masses, the formation scenarios in the local\nUniverse and the estimates of detection rates for gravitational radiation\ndetectors like LIGO and VIRGO. Our study is based on observed properties of\nknown Galactic and extra-galactic stellar mass black holes and evolutionary\npredictions. We argue that the binary black holes are the most promising source\nof gravitational radiation.", "category": "astro-ph_HE" }, { "text": "Patterns in the multi-wavelength behavior of candidate neutrino blazars: Motivated by the identification of the blazar TXS 0506+056 as the first\npromising high-energy neutrino counterpart candidate, we search for additional\nneutrino blazars candidates among the Fermi-LAT detected blazars.\n We investigate the multi-wavelength behavior from radio to GeV gamma rays of\nblazars found to be in spatial coincidence with single high-energy neutrinos\nand lower-energy neutrino flare candidates. In addition, we compare the average\ngamma-ray emission of the potential neutrino-emitting sources to the entire\nsample of gamma-ray blazars. We find that neutrino-emitting blazar candidates\nare statistically compatible with both hypothesis of a linear correlation and\nof no correlation between neutrino and gamma-ray energy flux.", "category": "astro-ph_HE" }, { "text": "Analysis of the duration$-$hardness ratio plane of gamma-ray bursts with\n skewed distributions: It was recently shown that the $T_{90}-H_{32}$ distributions of gamma-ray\nbursts from CGRO/BATSE and Fermi/GBM are well described by a mixture of only\ntwo skewed components, making the presumed third, intermediate class\nunnecesary. The Swift/BAT, Konus-Wind, RHESSI and Suzaku/WAM data sets are\nfound to be consistent with a two-class description as well.", "category": "astro-ph_HE" }, { "text": "Deep Synoptic Array science I: discovery of the host galaxy of FRB\n 20220912A: We report the detection and interferometric localization of the repeating\nfast radio burst (FRB) source FRB 20220912A during commissioning observations\nwith the Deep Synoptic Array (DSA-110). Two bursts were detected from FRB\n20220912A, one each on 2022 October 18 and 2022 October 25. The best-fit\nposition is (R.A. J2000, decl. J2000) = (23:09:04.9, +48:42:25.4), with a 90%\nconfidence error ellipse of $\\pm2$ arcsec and $\\pm1$ arcsec in right ascension\nand declination respectively. The two bursts have disparate polarization\nproperties and temporal profiles. We find a Faraday rotation measure that is\nconsistent with the low value of $+0.6$ rad m$^{-2}$ reported by CHIME/FRB. The\nDSA-110 localization overlaps with the galaxy PSO J347.2702+48.7066 at a\nredshift $z=0.0771$, which we identify as the likely host. PSO\nJ347.2702$+$48.7066 has a stellar mass of approximately $10^{10}M_{\\odot}$,\nmodest internal dust extinction, and a star-formation rate likely in excess of\n$0.1\\,M_{\\odot}$ yr$^{-1}$. The host-galaxy contribution to the dispersion\nmeasure is likely $\\lesssim50$ pc cm$^{-3}$. The FRB 20220912A source is\ntherefore likely viewed along a tenuous plasma column through the host galaxy.", "category": "astro-ph_HE" }, { "text": "The XMM-Newton Slew view of IGRJ17361-4441: a transient in the globular\n cluster NGC 6388: IGRJ17361-4441 is a hard transient recently observed by the INTEGRAL\nsatellite. The source, close to the center of gravity of the globular cluster\nNGC 6388, quickly became the target of follow-up observations conducted by the\nChandra, Swift/XRT and RXTE observatories. Here, we concentrate in particular\non a set of observations conducted by the XMM-Newton satellite during two\nslews, in order to get the spectral information of the source and search for\nspectral variations. The spectral parameters determined by the recent\nXMM-Newton slew observations were compared to the previously known results. The\nmaximum unabsorbed $X$-ray flux in the 0.5-10 keV band as detected by the\nXMM-Newton slew observations is $\\simeq 4.5\\times 10^{-11}$ erg cm$^{-2}$\ns$^{-1}$, i.e. consistent with that observed by the Swift/XRT satellite 15 days\nearlier. The spectrum seems to be marginally consistent ($\\Gamma\\simeq\n0.93-1.63$) with that derived from the previous high energy observation.", "category": "astro-ph_HE" }, { "text": "Radio Monitoring of the Tidal Disruption Event Swift J164449.3+573451.\n IV. Continued Fading and Non-Relativistic Expansion: We present continued radio and X-ray observations of the previously\nrelativistic tidal disruption event (TDE) Swift J164449.3+573451 (\\sw)\nextending to about 9.4 years post disruption, as part of ongoing campaigns with\nthe Jansky Very Large Array (VLA) and the \\textit{Chandra} X-ray observatory.\nWe find that the X-ray emission has faded below detectable levels, with an\nupper limit of $\\lesssim 3.5\\times 10^{-15}$ erg cm$^{-2}$ s$^{-1}$ in a 100 ks\nobservation, while the radio emission continues to be detected and steadily\nfade. Both are consistent with forward shock emission from a non-relativistic\noutflow, although we find that the radio spectral energy distribution is better\nfit at these late times with an electron power law index of $p\\approx 3$ (as\nopposed to $p\\approx 2.5$ at earlier times). With the revised spectral index we\nfind $\\epsilon_B\\approx 0.01$ using the radio and X-ray data, and a density of\n$\\approx 0.04$ cm$^{3}$ at a radius of $R\\approx 0.65$ pc ($R_{\\rm sch}\\approx\n2\\times 10^6$ R$_\\odot$) from the black hole. The energy scale of the blastwave\nis $\\approx 10^{52}$ erg. We also report detections of \\sw\\ at 3 GHz from the\nfirst two epochs of the VLA Sky Survey (VLASS), and find that $\\sim 10^2$\noff-axis \\sw-like events to $z\\sim 0.5$ may be present in the VLASS data.\nFinally, we find that \\sw\\ itself will remain detectable for decades at radio\nfrequencies, although observations at sub-GHz frequencies will become\nincreasingly important to characterize its dynamical evolution.", "category": "astro-ph_HE" }, { "text": "How unique are pulsar wind nebulae models? Implementation of a\n multi-parameter, automatic fitting for time-dependent spectra: Due to the computational cost of calculating a great number of variations of\nthe parameters, detailed radiative models of pulsar wind nebulae (PWNe) do not\nusually contain fitting algorithms. As a consequence, most of the models in the\nliterature are, in fact, qualitative fits based on visual inspection. This is\nparticularly true when complex, time-dependent models are considered. Motivated\nby improvements in the computational efficiency of the current PWN models that\nwere obtained in the last years, we here explore the inclusion of automatic\nfitting algorithms into a fully time-dependent model. Incorporating an\nefficient fitting tool based on the Nelder-Mead algorithm, we blindly find\nfitting solutions for the Crab nebula and 3C 58 with a time-dependent radiation\nmodel to compute the spectral and dynamical evolution of young and middle-aged\nPWNe. This inclusion allows us, in addition of more faithfully determining the\nquality of the fit, to tackle whether there exist degeneracy in the selected\nPWNe models. We find both for Crab and 3C58, that the fits are well determined,\nand that no other significantly different set of model parameters is able to\ncope with experimental data equally well. The code is also able to consider the\nsystem's age as a free parameter, recursively determining all other needed\nmagnitudes depending on age accordingly. We use this feature to consider\nwhether a detailed multi-frequency spectra can constrain the nebula age,\nfinding that in fact this is the case for the two PWNe studied.", "category": "astro-ph_HE" }, { "text": "Cosmic Neutron Star Merger Rate and Gravitational Waves constrained by\n the R Process Nucleosynthesis: The cosmic evolution of the neutron star merger (NSM) rate can be deduced\nfrom the observed cosmic star formation rate. This allows to estimate the rate\nexpected in the horizon of the gravitational wave detectors advanced Virgo and\nad LIGO and to compare those rates with independent predictions. In this\ncontext, the rapid neutron capture process, or r process, can be used as a\nconstraint assuming NSM is the main astrophysical site for this nucleosynthetic\nprocess. We compute the early cosmic evolution of a typical r process element,\nEuropium. Eu yields from NSM are taken from recent nucleosynthesis\ncalculations. The same approach allows to compute the cosmic rate of Core\nCollapse SuperNovae (CCSN) and the associated evolution of Eu. We find that the\nbulk of Eu observations at high iron abundance can be rather well fitted by\neither CCSN or NSM scenarios. However, at lower metallicity, the early Eu\ncosmic evolution favors NSM as the main astrophysical site for the r process. A\ncomparison between our calculations and spectroscopic observations at very low\nmetallicities allows to constrain the coalescence timescale in the NSM scenario\nto about 0.1 to 0.2 Gyr. These values are in agreement with the coalescence\ntimescales of some observed binary pulsars. Finally, the cosmic evolution of Eu\nis used to put constraints on the NSM rate, the merger rate in the horizon of\nthe gravitational wave detectors advanced Virgo/ad LIGO, as well as the\nexpected rate of electromagnetic counterparts to mergers (kilonovae) in large\nnear-infrared surveys.", "category": "astro-ph_HE" }, { "text": "NICER observations of the Crab pulsar glitch of 2017 November: Context: The Crab pulsar underwent its largest timing glitch on 2017 Nov 8.\nThe event was discovered at radio wavelengths, and was followed at soft X-ray\nenergies by observatories, such as XPNAV and NICER. aims: This work aims to\ncompare the glitch behavior at the two wavelengths mentioned above. Preliminary\nwork in this regard has been done by the X-ray satellite XPNAV. NICER with its\nfar superior sensitivity is expected to reveal much more detailed behavior.\nmethods: NICER has accumulated more than $301$ kilo seconds of data on the Crab\npulsar, equivalent to more than $3.3$ billion soft X-ray photons. These data\nwere first processed using the standard NICER analysis pipeline. Then the\narrival times of the X-ray photons were referred to the solar system's\nbarycenter. Then specific analysis was done to study the specific behavior\noutlined in the following sections, while taking dead time into account.\nresults: The variation of the rotation frequency of the Crab pulsar and its\ntime derivative during the glitch is almost exactly similar at the radio and\nX-ray energies. The following properties of the Crab pulsar remain essentially\nconstant before and after the glitch: the total X-ray flux; the flux, widths,\nand peaks of the two components of its integrated profile; and the soft X-ray\nspectrum. There is no evidence for giant pulses at X-ray energies. However, the\ntiming noise of the Crab pulsar shows quasi sinusoidal variation before the\nglitch, with increasing amplitude, which is absent after the glitch.\nconclusions: Even the strongest glitch in the Crab pulsar appears not to affect\nall but one of the properties mentioned above, at either frequency. The fact\nthat the timing noise appears to change due to the glitch is an important clue\nto unravel as this is still an unexplained phenomenon.", "category": "astro-ph_HE" }, { "text": "Study of measured pulsar masses and their possible conclusions: We study the statistics of 61 measured masses of neutron stars (NSs) in\nbinary pulsar systems, including 18 double NS (DNS) systems, 26 radio pulsars\n(10 in our Galaxy) with white dwarf (WD) companions, 3 NSs with main-sequence\ncompanions, 13 NSs in X-ray binaries, and one undetermined system. We derive a\nmean value of M = 1.46 +/- 0.30 solar masses. When the 46 NSs with measured\nspin periods are divided into two groups at 20 milliseconds, i.e., the\nmillisecond pulsar (MSP) group and others, we find that their mass averages\nare, respectively, M=1.57 +/- 0.35 solar masses and M=1.37+/- 0.23 solar\nmasses. In the framework of the pulsar recycling hypothesis, this suggests that\nan accretion of approximately 0.2 solar mass is sufficient to spin up a neutron\nstar and place it in the millisecond pulsar group. An empirical relation\nbetween the accreting mass and MSP spin period is \\Delta M=0.43 (solar\nmass)(P/1 ms)^{-2/3}. UNlike the standard recycling process, if a MSP is formed\nby the accretion induced collapse (AIC) of a white dwarf with a mass less than\nChandrasekha limit, e.g. 1.35 solar mass, then the binary MSPs involved in AICs\nis not be higher than 20%, which imposes a constraint on the AIC origin of\nMSPs.", "category": "astro-ph_HE" }, { "text": "Type IIP supernova SN 2016X in radio frequencies: Context: The study of radio emission from core-collapse supernovae (SNe)\nprobes the interaction of the ejecta with the circumstellar medium (CSM) and\nreveals details of the mass-loss history of the progenitor. Aims: We report\nobservations of the type IIP supernova SN\\,2016X during the plateau phase, at\nages between 21 and 75 days, obtained with the Karl G. Jansky Very Large Array\n(VLA) radio observatory. Methods: We modelled the radio spectra as\nself-absorbed synchrotron emission, and we characterised the shockwave and the\nmass-loss rate of the progenitor. We also combined our results with previously\nreported X-ray observations to verify the energy equipartition assumption.\nResults: The properties of the shockwave are comparable to other type IIP\nsupernovae. The shockwave expands according to a self-similar law $R \\propto\nt^m$ with $m=0.76 \\pm 0.08$, which is notably different from a constant\nexpansion. The corresponding shock velocities are approximately 10700 - 8000 km\ns$^{-1}$ during the time of our observations. The constant mass-loss rate of\nthe progenitor is $\\dot{M}=$ (7.8 $\\pm$ 0.9)\\,$\\times 10^{-7} \\alpha^{-8/19}\n(\\epsilon_B/0.1)^{-1} M_{\\odot}$ yr$^{-1}$, for an assumed wind velocity of 10\nkm s$^{-1}$. We observe spectral steepening in the optically thin regime at the\nearlier epochs, and we demonstrate that it is caused by electron cooling via\nthe inverse Compton effect. We show that the shockwave is characterised by a\nmoderate deviation from energy equipartition by a factor of $\\epsilon_e /\n\\epsilon_B \\approx 28$, being the second type IIP supernova to show such a\nfeature.", "category": "astro-ph_HE" }, { "text": "Nuclei in Strongly Magnetised Neutron Star Crusts: We discuss the ground state properties of matter in outer and inner crusts of\nneutron stars under the influence of strong magnetic fields. In particular, we\ndemonstrate the effects of Landau quantization of electrons on compositions of\nneutron star crusts. First we revisit the sequence of nuclei and the equation\nof state of the outer crust adopting the Baym, Pethick and Sutherland (BPS)\nmodel in the presence of strong magnetic fields and most recent versions of the\ntheoretical and experimental nuclear mass tables. Next we deal with nuclei in\nthe inner crust. Nuclei which are arranged in a lattice, are immersed in a\nnucleonic gas as well as a uniform background of electrons in the inner crust.\nThe Wigner-Seitz approximation is adopted in this calculation and each lattice\nvolume is replaced by a spherical cell. The coexistence of two phases of\nnuclear matter - liquid and gas, is considered in this case. We obtain the\nequilibrium nucleus corresponding to each baryon density by minimizing the free\nenergy of the cell. We perform this calculation using Skyrme nucleon-nucleon\ninteraction with different parameter sets. We find nuclei with larger mass and\ncharge numbers in the inner crust in the presence of strong magnetic fields\nthan those of the zero field case for all nucleon-nucleon interactions\nconsidered here. However, SLy4 interaction has dramatic effects on the proton\nfraction as well as masses and charges of nuclei. This may be attributed to the\nbehaviour of symmetry energy with density in the sub-saturation density regime.\nFurther we discuss the implications of our results to shear mode oscillations\nof magnetars.", "category": "astro-ph_HE" }, { "text": "New complex EAS installation of the Tien Shan Mountain Cosmic Ray\n Station: We present a description of the new complex installation for the study of\nextensive air showers which was created at the Tien Shan mountain cosmic ray\nstation, as well as the results of the test measurements made there in\n2014-2016. At present, the system for registration of electromagnetic shower\ncomponent consists of $\\sim$100 detector points built on the basis of plastic\nscintillator plates with the sensitive area of 0.25m$^2$ and 1m$^2$, spread\nequidistantly over $\\sim$10$^4$m$^2$ space. The dynamic range of scintillation\namplitude measurements is currently about $(3-7)\\cdot 10^4$, and there is a\nprospect of it being extended up to $\\sim$10$^6$. The direction of shower\narrival is defined by signal delays from a number of the scintillators placed\ncross-wise at the periphery of the detector system. For the investigation of\nnuclear active shower components there was created a multi-tier 55m$^2$\nionization-neutron calorimeter with a sum absorber thickness of\n$\\sim$1000g/cm$^2$, typical spatial resolution of the order of 10cm, and\ndynamic range of ionization measurement channel about $\\sim$10$^5$. Also, the\nuse of saturation-free neutron detectors is anticipated for registration of the\nhigh- and low-energy hadron components in the region of shower core. A complex\nof underground detectors is designed for the study of muonic and penetrative\nnuclear-active components of the shower.\n The full stack of data acquisition, detector calibration, and shower\nparameters restoration procedures are now completed, and the newly obtained\nshower size spectrum and lateral distribution of shower particles occur in\nagreement with conventional data. Future studies in the field of\n$10^{14}-10^{17}$eV cosmic ray physics to be held at the new shower\ninstallation are discussed.", "category": "astro-ph_HE" }, { "text": "Observational characteristics of accretion onto black holes: These notes resulted from a series of lectures at the IAC winter school. They\nare designed to help students, especially those just starting in subject, to\nget hold of the fundamental tools used to study accretion powered sources. As\nsuch, the references give a place to start reading, rather than representing a\ncomplete survey of work done in the field. I outline Compton scattering and\nblackbody radiation as the two predominant radiation mechanisms for accreting\nblack holes, producing the hard X-ray tail and disc spectral components,\nrespectively. The interaction of this radiation with matter can result in\nphoto-electric absorption and/or reflection. While the basic processes can be\nfound in any textbook, here I focus on how these can be used as a toolkit to\ninterpret the spectra and variability of black hole binaries (hereafter BHB)\nand Active Galactic Nuclei (AGN). I also discuss how to use these to physically\ninterpret real data using the publicly available XSPEC spectral fitting package\n(Arnaud et al 1996), and how this has led to current models (and controversies)\nof the accretion flow in both BHB and AGN.", "category": "astro-ph_HE" }, { "text": "Fermi LAT Observations of LS 5039: The first results from observations of the high mass X-ray binary LS 5039\nusing the Fermi Gamma-ray Space Telescope data between 2008 August and 2009\nJune are presented. Our results indicate variability that is consistent with\nthe binary period, with the emission being modulated with a period of 3.903 +/-\n0.005 days; the first detection of this modulation at GeV energies. The light\ncurve is characterized by a broad peak around superior conjunction in agreement\nwith inverse Compton scattering models. The spectrum is represented by a power\nlaw with an exponential cutoff, yielding an overall flux (100 MeV - 300 GeV) of\n4.9 +/- 0.5(stat) +/- 1.8(syst) x 10^-7 photon cm^-2 s^-1, with a cutoff at 2.1\n+/- 0.3(stat) +/- 1.1(syst) GeV and photon index Gamma = 1.9 +/- 0.1(stat) +/-\n0.3(syst). The spectrum is observed to vary with orbital phase, specifically\nbetween inferior and superior conjunction. We suggest that the presence of a\ncutoff in the spectrum may be indicative of magnetospheric emission similar to\nthe emission seen in many pulsars by Fermi.", "category": "astro-ph_HE" }, { "text": "Minutes-delayed jets from a neutron star companion in core collapse\n supernovae: I study cases where a neutron star (NS; or a black hole) companion to a type\nIb or type Ic (stripped-envelope) core collapse supernova (CCSN) accretes mass\nfrom the explosion ejecta and launches jets minutes to hours after explosion.\nThe NS orbits at a pre-explosion radius of a=1-5Ro. I find that when the ejecta\nvelocity drops to be <1000-1500km/s the ejecta gas that the NS accretes\npossesses sufficient specific angular momentum to form an accretion disk around\nthe NS. The NS accretes a fraction of 3e-5 to 3e-4 of the ejecta mass through\nan accretion disk over a time period of 10min to few hours. If the jets carry\nabout ten per cent of the accretion energy, then their total energy is a\nfraction of about 0.003-0.03 of the kinetic energy of the ejecta. The\nimplications of these jets from a NS (or a black hole) companion to a CCSN are\nthe shaping the inner ejecta to have a bipolar morphology, energising the light\ncurve of the CCSN, and in some cases the possible enrichment of the inner\nejecta with r-process elements.", "category": "astro-ph_HE" }, { "text": "First discovery of new pulsars and RRATs with CHIME/FRB: We report the discovery of seven new Galactic pulsars with the Canadian\nHydrogen Intensity Mapping Experiment's Fast Radio Burst backend (CHIME/FRB).\nThese sources were first identified via single pulses in CHIME/FRB, then\nfollowed up with CHIME/Pulsar. Four sources appear to be rotating radio\ntransients (RRATs), pulsar-like sources with occasional single pulse emission\nwith an underlying periodicity. Of those four sources, three have detected\nperiods ranging from 220 ms to 2.726 s. Three sources have more persistent but\nstill intermittent emission and are likely intermittent or nulling pulsars. We\nhave determined phase-coherent timing solutions for the latter three. These\nseven sources are the first discovery of previously unknown Galactic sources\nwith CHIME/FRB and highlight the potential of fast radio burst detection\ninstruments to search for intermittent Galactic radio sources.", "category": "astro-ph_HE" }, { "text": "An improved infrastructure for the IceCube realtime system: The IceCube realtime alert system has been operating since 2016. It provides\nprompt alerts on high-energy neutrino events to the astroparticle physics\ncommunity. The localization regions for the incoming direction of neutrinos are\npublished through NASA's Gamma-ray Coordinate Network (GCN). The IceCube\nrealtime system consists of infrastructure dedicated to the selection of alert\nevents, the reconstruction of their topology and arrival direction, the\ncalculation of directional uncertainty contours and the distribution of the\nevent information through public alert networks. Using a message-based workflow\nmanagement system, a dedicated software (SkyDriver) provides a representational\nstate transfer (REST) interface to parallelized reconstruction algorithms. In\nthis contribution, we outline the improvements of the internal infrastructure\nof the IceCube realtime system that aims to streamline the internal handling of\nneutrino events, their distribution to the SkyDriver interface, the collection\nof the reconstruction results as well as their conversion into human- and\nmachine-readable alerts to be publicly distributed through different alert\nnetworks. An approach for the long-term storage and cataloging of alert events\naccording to findability, accessibility, interoperability and reusability\n(FAIR) principles is outlined.", "category": "astro-ph_HE" }, { "text": "Prospects for ultra-high-energy particle acceleration at relativistic\n shocks: We study the acceleration of charged particles by ultra-relativistic shocks\nusing test-particle Monte-Carlo simulations. Two field configurations are\nconsidered: (i) shocks with uniform upstream magnetic field in the plane of the\nshock, and (ii) shocks in which the upstream magnetic field has a cylindrical\ngeometry. Particles are assumed to diffuse in angle due to frequent\nnon-resonant scattering on small-scale fields. The steady-state distribution of\nparticles' Lorentz factors is shown to approximately satisfy $dN/d\\gamma\n\\propto \\gamma^{-2.2}$ provided the particle motion is scattering dominated on\nat least one side of the shock. For scattering dominated transport, the\nacceleration rate scales as $t_{\\rm acc}\\propto t^{1/2}$, though recovers Bohm\nscaling $t_{\\rm acc}\\propto t$ if particles become magnetised on one side of\nthe shock. For uniform field configurations, a limiting energy is reached when\nparticles are magnetised on both sides of the shock. For the cylindrical field\nconfiguration, this limit does not apply, and particles of one sign of charge\nwill experience a curvature drift that redirects particles upstream. For the\nnon-resonant scattering model considered, these particles preferentially escape\nonly when they reach the confinement limit determined by the finite system\nsize, and the distribution approaches the escapeless limit $dN/d\\gamma \\propto\n\\gamma^{-1}$. The cylindrical field configuration resembles that expected for\njets launched by the Blandford $\\&$ Znajek mechanism, the luminous jets of AGN\nand GRBs thus provide favourable sites for the production of ultra-high energy\ncosmic rays.", "category": "astro-ph_HE" }, { "text": "A search for gamma-ray prompt emission associated with the Lorimer Burst\n FRB010724: No transient electromagnetic emission has yet been found in association to\nfast radio bursts (FRBs), the only possible exception (3sigma confidence) being\nthe putative gamma-ray signal detected in Swift/BAT data in the energy band\n15-150 keV at the time and position of FRB131104. Systematic searches for hard\nX/gamma-ray counterparts to other FRBs ended up with just lower limits on the\nradio/gamma-ray fluence ratios. In 2001, at the time of the earliest discovered\nFRBs, the BeppoSAX Gamma-Ray Burst Monitor (GRBM) was one of the most sensitive\nopen sky gamma-ray monitors in the 40-700~keV energy band. During its lifetime,\none of the FRBs with the highest radio fluence ever recorded, FRB010724 (800 +-\n400 Jy ms), also known as the Lorimer burst, was promptly visible to the GRBM.\nUpon an accurate modeling of the GRBM background, eased by its equatorial\norbit, we searched for a possible gamma-ray signal in the first 400 s following\nthe FRB, similar to that claimed for FRB131104 and found no significant\nemission down to a 5-sigma limit in the range (0.24-4.7)x10^-6 erg cm^-2\n(corresponding to 1 and 400 s integration time, respectively), in the energy\nband 40-700 keV. This corresponds to eta = F_radio/F_gamma>10^{8-9} Jy ms\nerg^-1 cm^2, i.e. the deepest limit on the ratio between radio and gamma-ray\nfluence, which rules out a gamma-ray counterpart similar to that of FRB131104.\nWe discuss the implications on the possible mechanisms and progenitors that\nhave been proposed in the literature, also taking into account its relatively\nlow dispersion measure (375 +- 3 pc cm^-3) and an inferred redshift limit of\nz<0.4.", "category": "astro-ph_HE" }, { "text": "Type IIn supernovae as sources of high energy neutrinos: It is shown that high-energy astrophysical neutrinos observed in the IceCube\nexperiment can be produced by protons accelerated in extragalactic Type IIn\nsupernova remnants by shocks propagating in the dense circumstellar medium. The\nnonlinear diffusive shock acceleration model is used for description of\nparticle acceleration.", "category": "astro-ph_HE" }, { "text": "Evidence for Unresolved Gamma-Ray Point Sources in the Inner Galaxy: We present a new method to characterize unresolved point sources (PSs),\ngeneralizing traditional template fits to account for non-Poissonian photon\nstatistics. We apply this method to Fermi Large Area Telescope gamma-ray data\nto characterize PS populations at high latitudes and in the Inner Galaxy. We\nfind that PSs (resolved and unresolved) account for ~50% of the total\nextragalactic gamma-ray background in the energy range ~1.9 to 11.9 GeV. Within\n10$^\\circ$ of the Galactic Center with $|b| \\geq 2^\\circ$, we find that ~5-10%\nof the flux can be accounted for by a population of unresolved PSs, distributed\nconsistently with the observed ~GeV gamma-ray excess in this region. The excess\nis fully absorbed by such a population, in preference to dark-matter\nannihilation. The inferred source population is dominated by near-threshold\nsources, which may be detectable in future searches.", "category": "astro-ph_HE" }, { "text": "Can FSRQ 3C 345 be a Very High Energy Blazar Candidate?: The recent detection of very high energy (VHE) emissions from flat spectrum\nradio quasars (FSRQs) at high redshifts has revealed that the universe is more\ntransparent to VHE $\\gamma$-rays than it was expected. It has also questioned\nthe plausible VHE emission mechanism responsible for these objects.\nParticularly for FSRQs, the $\\gamma$-ray emission is attributed to the external\nCompton process (EC). We perform a detailed spectral study of\n\\emph{Fermi}-detected FSRQ 3C 345 using synchrotron, synchrotron self Compton\n(SSC) and EC emission mechanisms. The simultaneous data available in optical,\nUV, X-ray, and $\\gamma$-ray energy bands is statistically fitted under these\nemission mechanisms using the $\\chi^2$-minimization technique. Three high flux\nstates and one low flux state are chosen for spectral fitting. The broadband\nspectral energy distribution (SED) during these flux states is fitted under\ndifferent target photon temperatures, and the model VHE flux is compared with\nthe 50\\hspace{0.05cm}hr CTA sensitivity. Our results indicate a significant VHE\nemission could be attained during the high flux state from MJD 59635-59715 when\nthe target photon temperature is within 900K to 1200K. Furthermore, our study\nshows a clear trend of variation in the bulk Lorentz factor of the emission\nregion as the source transits through different flux states. We also note that\nduring high $\\gamma$-ray flux states, an increase in external photon\ntemperature demands high bulk Lorentz factors, while this behaviour reverses in\ncase of low $\\gamma$-ray flux state.", "category": "astro-ph_HE" }, { "text": "Jets at lowest mass accretion rates: We present results of recent observations and theoretical modeling of data\nfrom black holes accreting at very low luminosities (L/L_Edd ~ 10^{-8}). We\ndiscuss our newly developed time-dependent model for episodic ejection of\nrelativistic plasma within a jet framework, and a successful application of\nthis model to describe the origin of radio flares seen in Sgr A*, the Galactic\ncenter black hole. Both the observed time lags and size-frequency relationships\nare reproduced well by the model. We also discuss results from new Spitzer data\nof the stellar black hole X-ray binary system A0620-00. Complemented by long\nterm SMARTS monitoring, these observations indicate that once the contribution\nfrom the accretion disk and the donor star are properly included, the residual\nmid-IR spectral energy distribution of A0620-00 is quite flat and consistent\nwith a non-thermal origin. The results above suggest that a significant\nfraction of the observed spectral energy distribution originating near black\nholes accreting at low luminosities could result from a mildly relativistic\noutflow. The fact that these outflows are seen in both stellar-mass black holes\nas well as in supermassive black holes at the heart of AGNs strengthens our\nexpectation that accretion and jet physics scales with mass.", "category": "astro-ph_HE" }, { "text": "The Long-term Variability of the X-ray Sources in M82: We investigate the long-term variability exhibited by the X-ray point sources\nin the starburst galaxy M82. By combining 9 Chandra observations taken between\n1999 and 2007, we detect 58 X-ray point sources within the D25 isophote of M82\ndown to a luminosity of ~ 10^37 ergs/s. Of these 58 sources, we identify 3\nsupernova remnant candidates and one supersoft source. Twenty-six sources in\nM82 exhibit long-term (i.e., days to years) flux variability and 3 show\nlong-term spectral variability. Furthermore, we classify 26 sources as\nvariables and 10 as persistent sources. Among the total 26 variables, 17 varied\nby a flux ratio of > 3 and 6 are transient candidates. By comparing with other\nnearby galaxies, M82 shows extremely strong long-term X-ray variability that\n47% of the X-ray sources are variables with a flux ratio of > 3. The strong\nX-ray variability of M82 suggests that the population is dominated by X-ray\nbinaries.", "category": "astro-ph_HE" }, { "text": "SMBH Seeds: Model Discrimination with High Energy Emission Based on\n Scaling Relation Evolution: We explore the expected X-ray (0.5-2keV) signatures from super massive black\nholes (SMBHs) at high redshifts ($z\\sim5-12$) assuming various models for their\nseeding mechanism and evolution. The seeding models are approximated through\ndeviations from the M$_{BH}-\\sigma$ relation observed in the local universe. We\nuse results from N-body simulations of the large-scale structure to estimate\nthe density of observable SMBHs. We focus on two families of seeding models:\n(\\textit{i}) light seed BHs from remnants of Pop-III stars; and (\\textit{ii})\nheavy seeds from the direct collapse of gas clouds. We investigate several\nmodels for the accretion history, such as sub-Eddington accretion, slim disk\nmodels allowing mild super-Eddington accretion and torque-limited growth\nmodels. We consider observations with two instruments: (\\textit{i}) the Chandra\nX-ray observatory, and (\\textit{ii}) the proposed Lynx. We find that all the\nsimulated models are in agreement with the current results from Chandra Deep\nField South (CDFS) - \\textit{i.e.,} consistent with zero to a few observed\nSMBHs in the field of view. In deep Lynx exposures, the number of observed\nobjects is expected to become statistically significant. We demonstrate the\ncapability to limit the phase space of plausible scenarios of the birth and\nevolution of SMBHs by performing deep observations at a flux limit of\n$1\\times10^{-19}\\mathrm{erg\\,cm^{-2}\\,s^{-1}}$. Finally, we estimate the\nexpected contribution from each model to the unresolved cosmic X-ray background\n(CXRB), and show that our models are in agreement with current limits on the\nCXRB and the expected contribution from unresolved quasars. We find that an\nanalysis of CXRB contributions down to the Lynx confusion limit yields valuable\ninformation that can help identify the correct scenario for the birth and\nevolution of SMBHs.", "category": "astro-ph_HE" }, { "text": "The Role of Nucleon Strangeness in Supernova Explosions: Recent hydrodynamical simulations of supernova (SN) evolution have\nhighlighted the importance of a thorough control over microscopic physics\nresponsible for such internal processes as neutrino heating. In particular, it\nhas been suggested that modifications to the neutrino-nucleon elastic cross\nsection can potentially play a crucial role in producing successful supernova\nexplosions. One possible source of such corrections can be found in a nonzero\nvalue for the nucleon's strange helicity content $\\Delta s$. In the present\nanalysis, however, we show that theoretical and experimental progress over the\npast decade has suggested a comparatively small magnitude for $\\Delta s$, such\nthat its sole effect is not sufficient to provide the physics leading to\nsupernova explosions.", "category": "astro-ph_HE" }, { "text": "Orbital Modulation of Gamma Rays from PSR~J2339$-$0533: We report on orbital modulation of the 100-600 MeV gamma-ray emission of the\n$P_{\\rm B}=4.6$ hr millisecond pulsar binary PSR J2339$-$0533 using 11 yr of\nFermi Large Area Telescope data. The modulation has high significance (chance\nprobability $p\\approx 10^{-7}$), is approximately sinusoidal, peaks near pulsar\nsuperior conjunction, and is detected only in the low-energy 100-600 MeV band.\nThe modulation is confined to the on-pulse interval, suggesting that the\nvariation is in the 2.9-ms pulsed signal itself. This contrasts with the few\nother known systems exhibiting GeV orbital modulations, as these are unpulsed\nand generally associated with beamed emission from an intrabinary shock. The\norigin of the modulated pulsed signal is not yet clear, although we describe\nseveral scenarios, including Compton upscattering of photons from the heated\ncompanion. This would require high coherence in the striped pulsar wind.", "category": "astro-ph_HE" }, { "text": "Multi-Messenger Searches in Astrophysics: Multi-messenger astronomy has experienced an explosive development in the\npast few years. While not being a particularly young field, it has recently\nattracted a lot of attention by several major discoveries and unprecedented\nobservation campaigns covering the entity of the electromagnetic spectrum as\nwell as observations of cosmic rays, neutrinos, and gravitational waves. The\nexploration of synergies is in full steam and requires close cooperation\nbetween different instruments. Here I give an overview over the subject of\nmulti-messenger astronomy and its virtues compared to classical \"single\nmessenger\" observations, present the recent break throughs of the field, and\ndiscuss some of its organisational and technical challenges.", "category": "astro-ph_HE" }, { "text": "A Comparison of Gamma-ray Burst Subgroups Measured by RHESSI and BATSE: A sample of almost 400 Gamma-ray bursts (GRBs) detected by the RHESSI\nsatellite is studied statistically. We focus on GRB duration and hardness ratio\nand use the statistical chi^2 test and the F-test to compare the number of GRB\nsubgroups in the RHESSI database with that of the BATSE database. Although some\nprevious articles based on the BATSE catalog claim the existence of an\nintermediate GRB subgroup, besides long and short, we have not found a\nstatistically significant intermediate subgroup in the RHESSI data.", "category": "astro-ph_HE" }, { "text": "Constraints from LIGO O3 data on gravitational-wave emission due to\n r-modes in the glitching pulsar PSR J0537-6910: We present a search for continuous gravitational-wave emission due to r-modes\nin the pulsar PSR J0537-6910 using data from the LIGO-Virgo Collaboration\nobserving run O3. PSR J0537-6910 is a young energetic X-ray pulsar and is the\nmost frequent glitcher known. The inter-glitch braking index of the pulsar\nsuggests that gravitational-wave emission due to r-mode oscillations may play\nan important role in the spin evolution of this pulsar. Theoretical models\nconfirm this possibility and predict emission at a level that can be probed by\nground-based detectors. In order to explore this scenario, we search for r-mode\nemission in the epochs between glitches by using a contemporaneous timing\nephemeris obtained from NICER data. We do not detect any signals in the\ntheoretically expected band of 86-97 Hz, and report upper limits on the\namplitude of the gravitational waves. Our results improve on previous amplitude\nupper limits from r-modes in J0537-6910 by a factor of up to 3 and place\nstringent constraints on theoretical models for r-mode driven spin-down in PSR\nJ0537-6910, especially for higher frequencies at which our results reach below\nthe spin-down limit defined by energy conservation.", "category": "astro-ph_HE" }, { "text": "Radius-expansion burst spectra from 4U 1728-34: an ultracompact binary?: Recent theoretical and observational studies have shown that ashes from\nthermonuclear burning may be ejected during radius-expansion bursts, giving\nrise to photoionisation edges in the X-ray spectra. We report a search for such\nfeatures in Chandra spectra observed from the low-mass X-ray binary 4U 1728-34.\nWe analysed the spectra from four radius-expansion bursts detected in 2006\nJuly, and two in 2002 March, but found no evidence for discrete features. We\nestimate upper limits for the equivalent widths of edges of a few hundred eV,\nwhich for the moderate temperatures observed during the bursts, are comparable\nwith the predictions. During the 2006 July observation 4U 1728-34 exhibited\nweak, unusually frequent bursts (separated by <2 hr in some cases), with\nprofiles and alpha-values characteristic of hydrogen-poor fuel. Recurrence\ntimes as short as those measured are insufficient to exhaust the accreted\nhydrogen at solar composition, suggesting that the source accretes hydrogen\ndeficient fuel, for example from an evolved donor. The detection for the first\ntime of a 10.77 min periodic signal in the persistent intensity, perhaps\narising from orbital modulation, supports this explanation, and suggests that\nthis system is an ultracompact binary similar to 4U 1820-30.", "category": "astro-ph_HE" }, { "text": "X-ray bolometric corrections for Compton-thick active galactic nuclei: We present X-ray bolometric correction factors, $\\kappa_{Bol}$ ($\\equiv\nL_{Bol}/L_X$), for Compton-thick (CT) active galactic nuclei (AGN) with the aim\nof testing AGN torus models, probing orientation effects, and estimating the\nbolometric output of the most obscured AGN. We adopt bolometric luminosities,\n$L_{Bol}$, from literature infrared (IR) torus modeling and compile published\nintrinsic 2--10 keV X-ray luminosities, $L_{X}$, from X-ray torus modeling of\nNuSTAR data. Our sample consists of 10 local CT AGN where both of these\nestimates are available. We test for systematic differences in $\\kappa_{Bol}$\nvalues produced when using two widely used IR torus models and two widely used\nX-ray torus models, finding consistency within the uncertainties. We find that\nthe mean $\\kappa_{Bol}$ of our sample in the range\n$L_{Bol}\\approx10^{42}-10^{45}$ erg/s is log$_{10}\\kappa_{Bol}=1.44\\pm0.12$\nwith an intrinsic scatter of $\\sim0.2$ dex, and that our derived $\\kappa_{Bol}$\nvalues are consistent with previously established relationships between\n$\\kappa_{Bol}$ and $L_{Bol}$ and $\\kappa_{Bol}$ and Eddington ratio. We\ninvestigate if $\\kappa_{Bol}$ is dependent on $N_H$ by comparing our results on\nCT AGN to published results on less-obscured AGN, finding no significant\ndependence. Since many of our sample are megamaser AGN, known to be viewed\nedge-on, and furthermore under the assumptions of AGN unification whereby\nunobscured AGN are viewed face-on, our result implies that the X-ray emitting\ncorona is not strongly anisotropic. Finally, we present $\\kappa_{Bol}$ values\nfor CT AGN identified in X-ray surveys as a function of their observed $L_X$,\nwhere an estimate of their intrinsic $L_{X}$ is not available, and redshift,\nuseful for estimating the bolometric output of the most obscured AGN across\ncosmic time.", "category": "astro-ph_HE" }, { "text": "A Fast-Evolving, Luminous Transient Discovered by K2/Kepler: For decades optical time-domain searches have been tuned to find ordinary\nsupernovae, which rise and fall in brightness over a period of weeks. Recently,\nsupernova searches have improved their cadences and a handful of fast-evolving\nluminous transients (FELTs) have been identified. FELTs have peak luminosities\ncomparable to Type Ia supernovae, but rise to maximum in $<10$ days and fade\nfrom view in $<$month. Here we present the most extreme example of this class\nthus far, KSN2015K, with a rise time of only 2.2 days and a time above\nhalf-maximum ($t_{1/2}$) of only 6.8 days. Here we show that, unlike Type Ia\nsupernovae, the light curve of KSN2015K was not powered by the decay of\nradioactive elements. We further argue that it is unlikely that it was powered\nby continuing energy deposition from a central remnant (a magnetar or black\nhole). Using numerical radiation hydrodynamical models, we show that the light\ncurve of KSN2015K is well fit by a model where the supernova runs into external\nmaterial presumably expelled in a pre-supernova mass loss episode. The rapid\nrise of KSN2015K therefore probes the venting of photons when a hypersonic\nshock wave breaks out of a dense extended medium.", "category": "astro-ph_HE" }, { "text": "Pulse energy distribution for RRAT J0139+33 according to observations at\n the frequency 111 MHz: Using five year monitoring observations, we did a blind search for pulses for\nrotating radio transient (RRAT) J0139+33 and PSR B0320+39. At the interval \\pm\n1.5m of the time corresponding to the source passing through the meridian, we\ndetected 39377 individual pulses for the pulsar B0320+39 and 1013 pulses for\nRRAT J0139+33. The share of registered pulses from the total number of observed\nperiods for the pulsar B0320+39 is 74%, and for the transient J0139+33 it is\n0.42%. Signal-to-noise ratio (S/N) for the strongest registered pulses is\napproximately equal to: S/N = 262 (for B0320+39) and S/N = 154 (for J0139+33).\n Distributions of the number of detected pulses in S/N units for the pulsar\nand for the rotating transient are obtained. The distributions could be\napproximated with a lognormal and power dependencies. For B0320+39 pulsar, the\ndependence is lognormal, it turns into a power dependence at high values of\nS/N, and for RRAT J0139+33, the distribution of pulses by energy is described\nby a broken (bimodal) power dependence with an exponent of about 0.4 and 1.8\n(S/N < 19 and S/N > 19).\n We have not detected regular (pulsar) emission of J0139+33. Analysis of the\nobtained data suggests that RRAT J0139+33 is a pulsar with giant pulses.", "category": "astro-ph_HE" }, { "text": "Forbidden Line Emission from Type Ia Supernova Remnants Containing\n Balmer-Dominated Shells: Balmer-dominated shells in supernova remnants (SNRs) are produced by\ncollisionless shocks advancing into a partially neutral medium, and are most\nfrequently associated with Type Ia supernovae. We have analyzed Hubble Space\nTelescope (HST) images and VLT/MUSE or AAT/WiFeS observations of five Type Ia\nSNRs containing Balmer-dominated shells in the LMC: 0509-67.5, 0519-69.0,\nN103B, DEM L71, and 0548-70.4. Contrary to expectations, we find bright\nforbidden line emission from small dense knots embedded in four of these SNRs.\nThe electron densities in some knots are higher than 10$^4$ cm$^{-3}$. The size\nand density of these knots are not characteristic for interstellar medium (ISM)\n-- they most likely originate from a circumstellar medium (CSM) ejected by the\nSN progenitor. Physical property variations of dense knots in the SNRs appear\nto reflect an evolutionary effect. The recombination timescales for high\ndensities are short, and HST images of N103B taken 3.5 yr apart already show\nbrightness changes in some knots. VLT/MUSE observations detect [Fe XIV] line\nemission from reverse shocks into SN ejecta as well as forward shocks into the\ndense knots. Faint [O III] line emission is also detected from the Balmer shell\nin 0519-69.0, N103B, and DEM L71. We exclude the postshock origin because the\n[O III] line is narrow. For the preshock origin, we considered three\npossibilities: photoionization precursor, cosmic ray precursor, and neutral\nprecursor. We conclude that the [O III] emission arises from oxygen that has\nbeen photoionized by [He II] $\\lambda$304 photons and is then collisionally\nexcited in a shock precursor heated mainly by cosmic rays.", "category": "astro-ph_HE" }, { "text": "Ultra-wide Bandwidth Observations of 19 pulsars with Parkes telescope: Flux densities are basic observation parameters to describe pulsars. In the\nmost updated pulsar catalog, 24% of the listed radio pulsars have no flux\ndensity measurement at any frequency. Here, we report the first flux density\nmeasurements, spectral indices, pulse profiles, and correlations of the\nspectral index with pulsar parameters for 19 pulsars employing the\nUltra-Wideband Low (UWL) receiver system installed on the Parkes radio\ntelescope. The results for spectral indices of 17 pulsars are in the range\nbetween -0.6 and -3.10. The polarization profiles of thirteen pulsars are\nshown. There is a moderate correlation between the spectral index and spin\nfrequency. For most pulsars detected, the S/N ratio of pulse profile is not\nhigh, so DM, Faraday rotation measure (RM), and polarization can not be\ndetermined precisely. Twenty-nine pulsars were not detected in our\nobservations. We discuss the possible explanations for why these pulsars were\nnot detected.", "category": "astro-ph_HE" }, { "text": "Absorption effects in the blazar's gamma-ray spectra due to luminous\n stars crossing the jet: Gamma-ray emission in active galaxies is likely produced within the inner\njet, or in the close vicinity of the supermassive black hole (SMBH) at\nsub-parsec distances. Gamma rays have to pass through the surrounding massive\nstellar cluster which luminous stars can accidentally appear close to the\nobserver's line of sight. In such a case, soft radiation of massive stars can\ncreate enough target for transient absorption of the gamma rays in multi-GeV to\nTeV energy range. We consider the effect of such stellar encounters on the\ngamma-ray spectrum produced within the massive stellar cluster surrounding a\ncentral SMBH. We predict characteristic, time-dependent effects on the\ngamma-ray spectra due to the encounter with the single luminous star and also\nstellar binary system. We conclude that during the encounter, the gamma-ray\nspectrum of an active galaxy should steepen at tens of GeV and harden in the\nrange of hundreds of GeV. As an example, we consider such effects on the\nspectra observed from a typical blazar, 1ES\\ 1959+650 (in an active state) and\nalso in the case of a radio galaxy M87 (in a low state). It is shown that\nobservation of such transient characteristic features in the gamma-ray spectra,\nobserved from blazars and radio galaxies, lays within the sensitivity of the\nfuture Cherenkov Telescope Array.", "category": "astro-ph_HE" }, { "text": "Opacities of Singly and Doubly Ionised Neodymium and Uranium for\n Kilonova Emission Modeling: Even though the electromagnetic counterpart AT2017gfo to the binary neutron\nstar merger GW170817 is powered by the radioactive decay of r-process nuclei,\nonly few tentative identifications of light r-process elements have been made\nso far. One of the major limitations for the identification of heavy nuclei is\nincomplete or missing atomic data. While substantial progress has been made on\nlanthanide atomic data over the last few years, for actinides there has been\nless emphasis, with the first complete set of opacity data only recently\npublished. We perform atomic structure calculations of neodymium $(Z=60)$ as\nwell as the corresponding actinide uranium $(Z=92)$. Using two different codes\n(FAC and HFR) for the calculation of the atomic data, we investigate the\naccuracy of the calculated data (energy levels and electric dipole transitions)\nand their effect on kilonova opacities. For the FAC calculations, we optimise\nthe local central potential and the number of included configurations and use a\ndedicated calibration technique to improve the agreement between theoretical\nand available experimental atomic energy levels (AELs). For ions with vast\namounts of experimental data available, the presented opacities agree quite\nwell with previous estimations. On the other hand, the optimisation and\ncalibration method cannot be used for ions with only few available AELs. For\nthese cases, where no experimental nor benchmarked calculations are available,\na large spread in the opacities estimated from the atomic data obtained with\nthe various atomic structure codes is observed.We find that the opacity of\nuranium is almost double the neodymium opacity.", "category": "astro-ph_HE" }, { "text": "The First Fermi LAT Gamma-Ray Burst Catalog: In three years of observations since the beginning of nominal science\noperations in August 2008, the Large Area Telescope (LAT) on board the Fermi\nGamma Ray Space Telescope has observed high-energy (>20 MeV) \\gamma-ray\nemission from 35 gamma-ray bursts (GRBs). Among these, 28 GRBs have been\ndetected above 100 MeV and 7 GRBs above ~ 20 MeV. The first Fermi-LAT catalog\nof GRBs is a compilation of these detections and provides a systematic study of\nhigh-energy emission from GRBs for the first time. To generate the catalog, we\nexamined 733 GRBs detected by the Gamma-Ray Burst Monitor (GBM) on Fermi and\nprocessed each of them using the same analysis sequence. Details of the\nmethodology followed by the LAT collaboration for GRB analysis are provided. We\nsummarize the temporal and spectral properties of the LAT-detected GRBs. We\nalso discuss characteristics of LAT-detected emission such as its delayed onset\nand longer duration compared to emission detected by the GBM, its power-law\ntemporal decay at late times, and the fact that it is dominated by a power-law\nspectral component that appears in addition to the usual Band model.", "category": "astro-ph_HE" }, { "text": "The variability timescales and brightness temperatures of radio flares\n from stars to supermassive black holes: In this paper we compile the analysis of ~ 200 synchrotron flare events from\n~ 90 distinct objects/events for which the distance is well established, and\nhence the peak luminosity can be accurately estimated. For each event we\nmeasure this peak and compare it to the rise and decay timescales, as fit by\nexponential functions, which allows us in turn to estimate a minimum brightness\ntemperature for all the events. The astrophysical objects from which the flares\noriginate vary from flare stars to supermassive black holes in active galactic\nnuclei, and include both repeating phenomena and single cataclysmic events\n(such as supernovae and gamma ray burst afterglows). The measured timescales\nvary from minutes to longer than years, and the peak radio luminosities range\nover 22 orders of magnitude. Despite very different underlying phenomena,\nincluding relativistic and non-relativistic regimes, and highly collimated\nversus isotropic phenomena, we find a broad correlation between peak radio\nluminosity and rise/decay timescales, approximately of the form L ~ t^5. This\nrather unexpectedly demonstrates that the estimated minimum brightness\ntemperature, when based upon variability timescales, and with no attempt to\ncorrect for relativistic boosting, is a strongly rising function of source\nluminosity. It furthermore demonstrates that variability timescales could be\nused as an early diagnostic of source class in future radio transient surveys.\nAs an illustration of radio transients parameter space, we compare the\nsynchrotron events with coherent bursts at higher brightness temperatures to\nillustrate which regions of radio transient parameter space have been explored.", "category": "astro-ph_HE" }, { "text": "Suzaku and BeppoSAX X-ray Spectra of the Persistently Accreting\n Neutron-Star Binary 4U 1705-44: We present an analysis of the broad-band spectra of 4U~1705--44 obtained with\n{\\it Suzaku} in 2006--2008 and by {\\it BeppoSAX} in 2000. The source exhibits\ntwo distinct states: the hard state shows emission from 1 to 150 keV, while the\nsoft state is mostly confined to be $<40$ keV. We model soft-state continuum\nspectra with two thermal components, one of which is a multicolor accretion\ndisk and the other is a single-temperature blackbody to describe the boundary\nlayer, with additional weak Comptonization represented by either a simple power\nlaw or the SIMPL model by Steiner et al. The hard-state continuum spectra are\nmodeled by a single-temperature blackbody for the boundary layer plus strong\nComptonization, modeled by a cutoff power law. While we are unable to draw firm\nconclusions about the physical properties of the disk in the hard state, the\naccretion disk in the soft state appears to approximately follow $L\\propto\nT^{3.2}$. The deviation from $L\\propto T^4$, as expected from a constant inner\ndisk radius, might be caused by a luminosity-dependent spectral hardening\nfactor and/or real changes of the inner disk radius in some part of the soft\nstate. The boundary layer apparent emission area is roughly constant from the\nhard to the soft states, with a value of about 1/11 of the neutron star\nsurface. The magnetic field on the surface of the NS in 4U~1705--44 is\nestimated to be less than about $1.9\\times 10^8$ G, assuming that the disk is\ntruncated by the ISCO or by the neutron star surface. Broad relativistic Fe\nlines are detected in most spectra and are modeled with the diskline model. The\nstrength of the Fe lines is found to correlate well with the boundary layer\nemission in the soft state. In the hard state, the Fe lines are probably due to\nillumination of the accretion disk by the strong Comptonization emission.", "category": "astro-ph_HE" }, { "text": "Bumpy Declining Light Curves Are Common in Hydrogen-poor Superluminous\n Supernovae: Recent work has revealed that the light curves of hydrogen-poor (Type I)\nsuperluminous supernovae (SLSNe), thought to be powered by magnetar central\nengines, do not always follow the smooth decline predicted by a simple magnetar\nspin-down model. Here we present the first systematic study of the prevalence\nand properties of \"bumps\" in the post-peak light curves of 34 SLSNe. We find\nthat the majority (44-76%) of events cannot be explained by a smooth magnetar\nmodel alone. We do not find any difference in supernova properties between\nevents with and without bumps. By fitting a simple Gaussian model to the\nlight-curve residuals, we characterize each bump with an amplitude,\ntemperature, phase, and duration. We find that most bumps correspond with an\nincrease in the photospheric temperature of the ejecta, although we do not see\ndrastic changes in spectroscopic features during the bump. We also find a\nmoderate correlation ($\\rho\\approx0.5$; $p\\approx0.01$) between the phase of\nthe bumps and the rise time, implying that such bumps tend to happen at a\ncertain \"evolutionary phase,\" $(3.7\\pm1.4)t_\\mathrm{rise}$. Most bumps are\nconsistent with having diffused from a central source of variable luminosity,\nalthough sources further out in the ejecta are not excluded. With this\nevidence, we explore whether the cause of these bumps is intrinsic to the\nsupernova (e.g., a variable central engine) or extrinsic (e.g., circumstellar\ninteraction). Both cases are plausible, requiring low-level variability in the\nmagnetar input luminosity, small decreases in the ejecta opacity, or a thin\ncircumstellar shell or disk.", "category": "astro-ph_HE" }, { "text": "Nonlinear variations in axisymmetric accretion: We subject the stationary solutions of inviscid and axially symmetric\nrotational accretion to a time-dependent radial perturbation, which includes\nnonlinearity to any arbitrary order. Regardless of the order of nonlinearity,\nthe equation of the perturbation bears a form that is similar to the metric\nequation of an analogue acoustic black hole. We bring out the time dependence\nof the perturbation in the form of a Li\\'enard system, by requiring the\nperturbation to be a standing wave under the second order of nonlinearity. We\nperform a dynamical systems analysis of the Li\\'enard system to reveal a saddle\npoint in real time, whose implication is that instabilities will develop in the\naccreting system when the perturbation is extended into the nonlinear regime.\nWe also model the perturbation as a high-frequency travelling wave, and carry\nout a Wentzel-Kramers-Brillouin analysis, treating nonlinearity iteratively as\na very feeble effect. Under this approach both the amplitude and the energy\nflux of the perturbation exhibit growth, with the acoustic horizon segregating\nthe regions of stability and instability.", "category": "astro-ph_HE" }, { "text": "Classification of pulsars with Dirichlet process Gaussian mixture model: Young isolated neutron stars (INS) most commonly manifest themselves as\nrotationally powered pulsars (RPPs) which involve conventional radio pulsars as\nwell as gamma-ray pulsars (GRPs) and rotating radio transients (RRATs). Some\nother young INS families manifest themselves as anomalous X-ray pulsars (AXPs)\nand soft gamma-ray repeaters (SGRs) which are commonly accepted as magnetars,\ni.e. magnetically powered neutron stars with decaying superstrong fields. Yet\nsome other young INS are identified as central compact objects (CCOs) and X-ray\ndim isolated neutron stars (XDINSs) which are cooling objects powered by their\nthermal energy. Older pulsars, as a result of a previous long episode of\naccretion from a companion, manifest themselves as millisecond pulsars and more\ncommonly appear in binary systems. We use Dirichlet process Gaussian mixture\nmodel (DPGMM), an unsupervised machine learning algorithm, for analyzing the\ndistribution of these pulsar families in the parameter space of period and\nperiod derivative. We compare the average values of the characteristic age,\nmagnetic dipole field strength, surface temperature and transverse velocity of\nall discovered clusters. We verify that DPGMM is robust and provides hints for\ninferring relations between different classes of pulsars. We discuss the\nimplications of our findings for the magneto-thermal spin evolution models and\nfallback discs.", "category": "astro-ph_HE" }, { "text": "Annihilation of positrons from AGN jets as a possible source of cosmic\n gamma-ray background at energies below 511 keV: The origin of the diffuse gamma-ray background in the range from hundreds keV\nto several MeV is not known conclusively. From current models and observations\nit is believed that, at least partially, this background is formed by blazars\nand remnants of supernovae (SN) of type Ia in distant galaxies. However, these\ncontributions are not sufficient to reproduce the observed level of the signal.\nIn this work we propose another source which could contribute to this\nbackground, namely the jets of active galactic nuclei (AGN). The composition of\njets is not known, but there are observational hints that the fraction of\npositrons there is substantial. Positrons are partially evacuated to the\nintergalactic medium and partially mix with the circumgalactic medium and\nannihilate there comparatively quickly. Using the AGN luminosity function, we\nestimated the positron production rate and the contribution of the positron\nannihilation to the cosmic background below 511 keV. We also estimated the\nanalogous contribution from positron annihilation within SN Ia remnants in\ndistant galaxies. The contribution of AGNs is estimated to be a factor of 5 -\n10 smaller than the observed background intensity, and the contribution from\nSNe is yet smaller by one order of magnitude. Nevertheless, the contribution of\nAGNs appeared to be larger than the contribution of blazars estimated from\nSwift-BAT and Fermi-LAT observations. The main uncertainty in our model is the\nfraction of positrons remaining in the circumgalactic medium which makes our\nestimation an upper limit.", "category": "astro-ph_HE" }, { "text": "Jet production in black-hole X-ray binaries and active galactic nuclei:\n mass feeding and advection of magnetic fields: Relativistic jets are observed only in the low/hard and intermediate states\nof X-ray binaries (XRBs), and are switched off in the thermal state, but they\nappear to be present in both low-luminosity and luminous active galactic nuclei\n(AGNs). It is widely believed that strong large-scale magnetic fields is a\ncrucial ingredient in jet production; such fields can be attained only through\nefficient advection from the outer disc. We suggest that geometrically thin\naccretion discs with magnetic outflows are present in luminous radio-loud AGNs;\nthis is likely because the interstellar medium provides both mass and\nsufficient magnetic flux to the outer disc. Most angular momentum of such disc\nis removed by the outflows, and the radial velocity of the disc is\nsignificantly increased compared to viscous drift velocity. This facilitates\nefficient magnetic field advection through the disc to produce a strong field\nnear the black hole in luminous AGNs, which helps launch relativistic jets. In\nXRBs, the magnetic fields of the gas from companion stars are too weak to drive\noutflows from outer discs. Jets are therefore switched off in the thermal state\ndue to inefficient magnetic field advection in the disc.", "category": "astro-ph_HE" }, { "text": "Interaction between Molecular Clouds and MeV-TeV Cosmic-ray Protons\n Escaped from Supernova Remnants: Recent discovery of the X-ray neutral iron line (Fe I K\\alpha at 6.40 keV)\naround several supernova remnants (SNRs) show that MeV cosmic-ray (CR) protons\nare distributed around the SNRs and are interacting with neutral gas there. We\npropose that these MeV CRs are the ones that have been accelerated at the SNRs\ntogether with GeV-TeV CRs. In our analytical model, the MeV CRs are still\nconfined in the SNR when the SNR collides with molecular clouds. After the\ncollision, the MeV CRs leak into the clouds and produce the neutral iron line\nemissions. On the other hand, GeV-TeV CRs had already escaped from the SNRs and\nemit gamma-rays through interaction with molecular clouds surrounding the SNRs.\nWe apply this model to the SNRs W28 and W44 and show that it can reproduce the\nobservations of the iron line intensities and the gamma-ray spectra. This can\nbe another support of a hadronic scenario for the gamma-ray emissions from\nthese SNRs.", "category": "astro-ph_HE" }, { "text": "Spin-Down of the Long-Period Accreting Pulsar 4U 2206+54: 4U 2206+54 is a high mass X-ray binary which has been suspected to contain a\nneutron star accreting from the wind of its companion BD +53 2790. Reig et al.\nhave recently detected 5560 s period pulsations in both RXTE and INTEGRAL\nobservations which they conclude are due to the spin of the neutron star. We\npresent observations made with Suzaku which are contemporaneous with their RXTE\nobservation of this source. We find strong pulsations at a period of 5554 +/- 9\ns in agreement with their results. We also present a reanalysis of BeppoSAX\nobservations of 4U 2206+54 made in 1998, in which we find strong pulsations at\na period of 5420 +/- 28 seconds, revealing a spin-down trend in this\nlong-period accreting pulsar. Analysis of these data suggests that the neutron\nstar in this system is an accretion-powered magnetar.", "category": "astro-ph_HE" }, { "text": "Low-Latency Algorithm for Multi-messenger Astrophysics (LLAMA) with\n Gravitational-Wave and High-Energy Neutrino Candidates: We describe in detail the online data analysis pipeline that was used in the\nmulti-messenger search for common sources of gravitational waves (GWs) and\nhigh-energy neutrinos (HENs) during the second observing period (O2) of\nAdvanced LIGO and Advanced Virgo. Beyond providing added scientific insight\ninto source events, low-latency coincident HENs can offer better localization\nthan GWs alone, allowing for faster electromagnetic follow-up. Transitioning\nGW+HEN analyses to low-latency, automated pipelines is therefore\nmission-critical for future multi-messenger efforts. The O2 Low-Latency\nAlgorithm for Multi-messenger Astrophysics (\\pipeline) also served as a\nproof-of-concept for future online GW+HEN searches and led to a codebase that\ncan handle other messengers as well. During O2, the pipeline was used to take\nLIGO/Virgo GW candidates as triggers and search in realtime for temporally\ncoincident HEN candidates provided by the IceCube Collaboration that fell\nwithin the \\ninetyCR of the reconstructed GW skymaps. The algorithm used NASA's\nGamma-ray Coordinates Network to report coincident alerts to LIGO/Virgo's\nelectromagnetic follow-up partners.", "category": "astro-ph_HE" }, { "text": "3D Relativistic MHD simulations of the gamma-ray binaries: In gamma-ray binaries neutron star is orbiting a companion that produces a\nstrong stellar wind. We demonstrate that observed properties of \"stellar\nwind\"-\"pulsar wind\" interaction depend both on the overall wind thrust ratio,\nas well as more subtle geometrical factors: the relative direction of the\npulsar's spin, the plane of the orbit, the direction of motion, and the\ninstantaneous line of sight. Using fully 3D relativistic magnetohydrodynamical\nsimulations we find that the resulting intrinsic morphologies can be\nsignificantly orbital phase-dependent: a given system may change from\ntailward-open to tailward-closed shapes. As a result, the region of unshocked\npulsar wind can change by an order of magnitude over a quarter of the orbit. We\ncalculate radiation maps and synthetic light curves for synchrotron (X-ray) and\nInverse-Compton emission (GeV-TeV), taking into account $\\gamma-\\gamma$\nabsorption. Our modeled light curves are in agreement with the phase-dependent\nobserved light curves of LS5039.", "category": "astro-ph_HE" }, { "text": "An unusual transient following the short GRB 071227: We present X-ray and optical observations of the short duration gamma-ray\nburst GRB 071227 and its host at $z=0.381$, obtained using \\textit{Swift},\nGemini South and the Very Large Telescope. We identify a short-lived and\nmoderately bright optical transient, with flux significantly in excess of that\nexpected from a simple extrapolation of the X-ray spectrum at 0.2-0.3 days\nafter burst. We fit the SED with afterglow models allowing for high extinction\nand thermal emission models that approximate a kilonova to assess the excess'\norigins. While some kilonova contribution is plausible, it is not favoured due\nto the low temperature and high luminosity required, implying superluminal\nexpansion and a large ejecta mass of $\\sim 0.1$ M$_{\\odot}$. We find, instead,\nthat the transient is broadly consistent with power-law spectra with additional\ndust extinction of $E(B-V)\\sim0.4$ mag, although a possibly thermal excess\nremains in the \\textit{z}-band. We investigate the host, a spiral galaxy with\nan edge-on orientation, resolving its spectrum along its major axis to\nconstruct the galaxy rotation curve and analyse the star formation and chemical\nproperties. The integrated host emission shows evidence for high extinction,\nconsistent with the afterglow findings. The metallicity and extinction are\nconsistent with previous studies of this host and indicate the galaxy is a\ntypical, but dusty, late-type SGRB host.", "category": "astro-ph_HE" }, { "text": "Modelling interaction of relativistic and non-relativistic winds in\n binary system PSR B1259-63/SS2883 - II. Impact of magnetization and\n anisotropy of the pulsar wind: In this paper, we present a numerical study of the properties of the flow\nproduced by the collision of a magnetized anisotropic pulsar wind with its\nenvironment in binary system. We compare the impact of both the magnetic field\nand the wind anisotropy to the benchmark case of a purely hydrodynamical (HD)\ninteraction of isotropic winds, which has been studied in detail by Bogovalov\net al. (2008). We consider the interaction in axisymmetric approximation, i.e.\nthe pulsar rotation axis is assumed to be oriented along the line between the\npulsar and the optical star and the effects related to the pulsar orbiting are\nneglected. The impact of the magnetic field is studied for the case of weak\nmagnetization (with magnetization parameter $\\sigma<0.1$), which is consistent\nwith conventional models of pulsar winds. The effects related to anisotropy in\npulsar winds are modeled assuming that the kinetic energy flux in a\nnon-magnetized pulsar wind is strongly anisotropic, with the minimum at the\npulsar rotation axis and the maximum in the perpendicular direction. We show\nthat, although both considered effects change the shape of the region occupied\nby the terminated pulsar wind, their impact appears to be small. In particular,\nfor the magnetization of the pulsar wind below 0.1, the magnetic field pressure\nremains well below the plasma pressure in the post-shock region. Thus, in the\ncase of interaction of a pulsar with the stellar wind environment (opposite to\nthe case of plerions, i.e. the pulsar interaction with the interstellar medium,\nwhen the magnetic field becomes dynamically important independently on the wind\nmagnetization) the HD approach represents a feasible approximation for\nnumerical modelling.", "category": "astro-ph_HE" }, { "text": "Changes in the pulse phase dependence of X-ray emission lines in 4U\n 1626-67 with a torque reversal: We report results from an observation with the XMM-Newton observatory of a\nunique X-ray pulsar 4U 1626-67. EPIC-pn data during the current spin-up phase\nof 4U 1626-67 have been used to study pulse phase dependence of low energy\nemission lines. We found strong variability of low energy emission line at\n0.915 keV with the pulse phase, varying by a factor of 2, much stronger than\nthe continuum variability. Another interesting observation is that behavior of\none of the low energy emission lines across the pulse phase is quite different\nfrom that observed during the spin-down phase. This indicates that the\nstructures in the accretion disk that produce pulse phase dependence of\nemission features have changed from spin-down to spin-up phase. This is well\nsupported by the differences in the timing characteristics (like pulse\nprofiles, QPOs etc) between spin-down and spin-up phases. We have also found\nthat during the current spin-up phase of 4U 1626-67, the X-ray pulse profile\nbelow 2 keV is different compared to the spin-down phase. The X-ray light curve\nalso shows flares which produce a feature around 3 mHz in power density\nspectrum of 4U 1626-67. Since flares are dominant at lower energies, the\nfeature around 3 mHz is prominent at low energies.", "category": "astro-ph_HE" }, { "text": "Long-term periodicity in LSI+61303 as beat frequency between orbital and\n precessional rate: Context: In the binary system LSI+61303 the peak flux density of the radio\noutburst, which is related to the orbital period of 26.4960 +/- 0.0028d,\nexibits a modulation of 1667 +/- 8 d. The radio emission at high spatial\nresolution appears structured in a precessing jet with a precessional period of\n27-28 d. Aims: How close is the precessional period of the radio jet to the\norbital period? Any periodicity in the radio emission should be revealed by\ntiming analysis. The aim of this work is to establish the accurate value of the\nprecessional period. Methods: We analyzed 6.7 years of the Green Bank\nInterferometer database at 2.2 GHz and 8.3 GHz with the Lomb-Scargle and phase\ndispersion minimization (PDM) methods and performed simulations. Results: The\nperiodograms show two periodicities, P1 = 26.49 +/- 0.07 d (\\nu1=0.03775\nd^{-1}) and P2 = 26.92 +/- 0.07 d (\\nu2 = 0.03715 d^{-1}). Whereas radio\noutbursts have been known to have nearly orbital occurrence P1 with timing\nresiduals exhibiting a puzzling sawtooth pattern, we probe in this paper that\nthey are actually periodical outbursts and that their period is Paverage=\n(2/(\\nu1 + \\nu2)= 26.70 +/- 0.05 d. The period Paverage as well as the\nlong-term modulation Pbeat=1/(\\nu1 - \\nu2)=1667 +/- 393 d result from the beat\nof the two close periods, the orbital P1 and the precessional P2 periods.\nConclusions: The precessional period, indicated by the astrometry to be of\n27--28 d, is P2=26.92 d. The system \\lsi seems to be one more case in astronomy\nof beat, i.e., a phenomenon occurring when two physical processes create stable\nvariations of nearly equal frequencies. The very small difference in frequency\ncreates a long-term variation of period 1/(\\nu1-\\nu2). The long-term modulation\nof 1667 d results from the beat of the two close orbital and precessional\nrates.", "category": "astro-ph_HE" }, { "text": "A propeller scenario for the gamma-ray emission of low-mass X-ray\n binaries: The case of XSS J12270-4859: XSS J12270-4859 is the only low mass X-ray binary (LMXB) with a proposed\npersistent gamma-ray counterpart in the Fermi-LAT domain, 2FGL 1227.7-4853.\nHere, we present the results of the analysis of recent INTEGRAL observations,\naimed at assessing the long-term variability of the hard X-ray emission, and\nthus the stability of the accretion state. We confirm that the source behaves\nas a persistent hard X-ray emitter between 2003 and 2012. We propose that XSS\nJ12270-4859 hosts a neutron star in a propeller state, a state we investigate\nin detail, developing a theoretical model to reproduce the associated X-ray and\ngamma-ray properties. This model can be understood as being of a more general\nnature, representing a viable alternative by which LMXBs can appear as\ngamma-ray sources. In particular, this may apply to the case of millisecond\npulsars performing a transition from a state powered by the rotation of their\nmagnetic field, to a state powered by matter in-fall, such as that recently\nobserved from the transitional pulsar PSR J1023+0038. While the surface\nmagnetic field of a typical NS in a LMXB is lower by more than four orders of\nmagnitude than the much more intense fields of neutron stars accompanying\nhigh-mass binaries, the radius at which the matter in-flow is truncated in a\nNS-LMXB system is much lower. The magnetic field at the magnetospheric\ninterface is then orders of magnitude larger at this interface, and as\nconsequence, so is the power to accelerate electrons. We demonstrate that the\ncooling of the accelerated electron population takes place mainly through\nsynchrotron interaction with the magnetic field permeating the interface, and\nthrough inverse Compton losses due to the interaction between the electrons and\nthe synchrotron photons they emit. We found that self-synchrotron Compton\nprocesses can explain the high energy phenomenology of XSS J12270-4859.", "category": "astro-ph_HE" }, { "text": "The impact of resistive electric fields on particle acceleration in\n reconnection layers: In the context of particle acceleration in high-energy astrophysical\nenvironments featuring magnetic reconnection, the importance of the resistive\nterm of the electric field compared to the convective one is still under\ndebate. In this work, we present a quantitative analysis through 2D\nmagnetohydrodynamic numerical simulations of tearing-unstable current sheets\ncoupled to a test-particles approach, performed with the PLUTO code. We find\nthat the resistive field plays a significant role in the early-stage\nenergization of high-energy particles. Indeed, these particles are firstly\naccelerated due to the resistive electric field when they cross an X-point,\ncreated during the fragmentation of the current sheet. If this preliminary\nparticle acceleration mechanism dominated by the resistive field is neglected,\nparticles cannot reach the same high energies. Our results support therefore\nthe conclusion that the resistive field is not only non-negligible but it does\nactually play an important role in the particle acceleration mechanism.", "category": "astro-ph_HE" }, { "text": "Resonant energization of particles by radio AGN: A new mechanism of particle acceleration, based on the resonant interaction\nof a classical electromagnetic wave (EM) with a quantum wave (associated with a\nrelativistic particle), is explored.\n In a model illustrative calculation, we study the fate of a Klein Gordon wave\nsubjected to the intense radio frequency waves generated in the vicinity of an\nactive galactic nuclei (AGN). In the framework of the paper we examine a\nquantum wave associated with a relativistic particle, and it is shown that the\ngroup velocity of the wave approaches the speed of light, implying that the\nparticles resonantly exchange energy with EM waves, eventually leading to\nacceleration of particles to very high energies.\n For typical parameters of under accreting Eddington radio AGN, it is shown\nthat the resonant energization could catapult particles to extreme energies\n$\\sim 10^{16-20}$eV.", "category": "astro-ph_HE" }, { "text": "Non-relativistic perpendicular shocks modeling young supernova remnants:\n nonstationary dynamics and particle acceleration at forward and reverse\n shocks: For parameters that are applicable to the conditions at young supernova\nremnants, we present results of 2D3V particle-in-cell simulations of a\nnon-relativistic plasma shock with a large-scale perpendicular magnetic field\ninclined at 45-deg angle to the simulation plane to approximate 3D physics. We\ndeveloped an improved clean setup that uses the collision of two plasma slabs\nwith different density and velocity, leading to the development of two\ndistinctive shocks and a contact discontinuity. The shock formation is mediated\nby Weibel-type filamentation instabilities that generate magnetic turbulence.\nCyclic reformation is observed in both shocks with similar period, for which we\nnote global variations on account of shock rippling and local variations\narising from turbulent current filaments. The shock rippling occurs on spatial\nand temporal scales given by gyro-motions of shock-reflected ions. The drift\nmotion of electrons and ions is not a gradient drift, but commensurates with E\nx B drift. We observe a stable suprathermal tail in the ion spectra, but no\nelectron acceleration because the amplitude of Buneman modes in the shock foot\nis insufficient for trapping relativistic electrons. We see no evidence of\nturbulent reconnection. A comparison with other 2D simulation results suggests\nthat the plasma beta and the ion-to-electron mass ratio are not decisive for\nefficient electron acceleration, but pre-acceleration efficacy might be reduced\nwith respect to the 2D results once three-dimensional effects are fully\naccounted for. Other microphysical factors may also be at play to limit the\namplitude of Buneman waves or prevent return of electrons to the foot region.", "category": "astro-ph_HE" }, { "text": "Subgrid modeling of neutrino oscillations in astrophysics: Approximating neutrino oscillations as subgrid physics is an appealing\nprospect for simulators of core-collapse supernovae and neutron-star mergers.\nBecause flavor instabilities quickly lead to quasisteady states in oscillation\ncalculations, it is widely believed that flavor mixing can be approximated in\nastrophysical simulations by mapping unstable states onto the appropriate\nasymptotic ones. Subgrid models of this kind, however, are not self-consistent.\nThe miscidynamic theory of quantum-coherent gases furnishes a subgrid model\nthat is.", "category": "astro-ph_HE" }, { "text": "High resolution X-ray spectroscopy of Supergiant HMXB 4U1700$-$37 during\n the compact object eclipse: We present an analysis of the first observation of the iconic High Mass X-ray\nBinary \\so with the \\chandra High Energy Transmission Gratings during an X-ray\neclipse. The goal of the observation was to study the structure/physical\nconditions in the clumpy stellar wind through high resolution spectroscopy. We\nfind that: a) emission line brightness from K shell transitions, corresponding\nto near neutral species, directly correlates with continuum illumination.\nHowever, these lines do not greatly diminish during eclipse. This is readily\nexplained if fluorescence K$\\alpha$ emission comes from the bulk of the wind.\nb) The highly ionised Fexxv and Fexxvi Ly$\\alpha$ diminish during eclipse.\nThus, they must be produced in the vicinity of the compact object where $\\log\n\\xi >3$. c) to describe the emission line spectrum, the sum of two self\nconsistent photo ionisation models with low ionisation ($\\log \\xi\\sim -1$) and\nhigh ionisation ($\\log \\xi\\sim 2.4$) is required. From their emission measures,\nthe clump-to-interclump density ratio can be estimated to be $n_c/n_i\\sim 300$.\nTo fit the complex He-like \\ion{Si}{xiii}{} profile, the plasma requires a\nbroadening with $v_{\\rm bulk}\\sim 840$ km s$^{-1}$. Reproducing the observed\n$r\\approx f$ line fluxes requires the addition of a third collisionally ionised\nplasma. d) Emission lines widths appear unresolved at the \\textsc{hetg}\ngratings resolution with exception of Silicon. There is no clear radial\nsegregation between (quasi)neutral and ionised species, consistent with cold\nwind clumps interspersed in a hot rarefied interclump medium.", "category": "astro-ph_HE" }, { "text": "Probing the evolution of the EAS muon content in the atmosphere with\n KASCADE-Grande: The evolution of the muon content of very high energy air showers (EAS) in\nthe atmosphere is investigated with data of the KASCADE-Grande observatory. For\nthis purpose, the muon attenuation length in the atmosphere is obtained to\n$\\Lambda_\\mu = 1256 \\, \\pm 85 \\, ^{+229}_{-232}(\\mbox{syst})\\, \\mbox{g/cm}^2$\nfrom the experimental data for shower energies between $10^{16.3}$ and\n$10^{17.0} \\, \\mbox{eV}$. Comparison of this quantity with predictions of the\nhigh-energy hadronic interaction models QGSJET-II-02, SIBYLL 2.1, QGSJET-II-04\nand EPOS-LHC reveals that the attenuation of the muon content of measured EAS\nin the atmosphere is lower than predicted. Deviations are, however, less\nsignificant with the post-LHC models. The presence of such deviations seems to\nbe related to a difference between the simulated and the measured zenith angle\nevolutions of the lateral muon density distributions of EAS, which also causes\na discrepancy between the measured absorption lengths of the density of shower\nmuons and the predicted ones at large distances from the EAS core. The studied\ndeficiencies show that all four considered hadronic interaction models fail to\ndescribe consistently the zenith angle evolution of the muon content of EAS in\nthe aforesaid energy regime.", "category": "astro-ph_HE" }, { "text": "Proton acceleration in pulsar magnetospheres: Pulsars have been identified as good candidates for the acceleration of\ncosmic rays, up to ultra-high energies. However, a precise description of the\nacceleration processes at play is still to be established. Using 2D\nparticle-in-cell simulations, we study proton acceleration in axisymmetric\npulsar magnetospheres. Protons and electrons are extracted from the neutron\nstar surface by the strong electric field induced by the rotation of the star,\nand electrons and positrons are produced in the magnetosphere through pair\nproduction process. As pair production has a crucial impact on electromagnetic\nfields, on gaps and thus on particle acceleration, we study its influence on\nthe maximum energy and luminosity of protons escaping the magnetosphere.\nProtons are accelerated and escape in all our simulations. However, the\nacceleration sites are different for the protons and the pairs. As shown in\nprevious studies, pairs are accelerated to their highest energies at the\nY-point and in the equatorial current sheet, where magnetic reconnection plays\nand important role. In contrast, protons gain most of their kinetic energy\nbelow the light-cylinder radius within the separatrix current layers, but they\nare not confined within the equatorial current sheet. Their maximum Lorentz\nfactors can reach $15\\%$ to $75\\%$ of the maximum Lorentz factor obtained by\nacceleration through the full vacuum potential drop from pole to equator, and\nincrease with decreasing pair production. Their luminosity can reach $0.2\\%$ to\n$4\\%$ of the theoretical spin down luminosity of an aligned pulsar, and the\nminimum luminosity is obtained at the transition between the force-free and\nelectrosphere regimes. These estimates support that millisecond pulsars could\naccelerate cosmic rays up to PeV energies and that new born millisecond pulsars\ncould accelerate cosmic rays up to ultra-high energies.", "category": "astro-ph_HE" }, { "text": "The population of X-ray supernova remnants in the Large Magellanic Cloud: We present a comprehensive X-ray study of the population of supernova\nremnants (SNRs) in the LMC. Using primarily XMM-Newton, we conduct a systematic\nspectral analysis of LMC SNRs to gain new insights on their evolution and the\ninterplay with their host galaxy. We combined all the archival XMM observations\nof the LMC with those of our Very Large Programme survey. We produced X-ray\nimages and spectra of 51 SNRs, out of a list of 59. Using a careful modelling\nof the background, we consistently analysed all the X-ray spectra and measure\ntemperatures, luminosities, and chemical compositions. We investigated the\nspatial distribution of SNRs in the LMC and the connection with their\nenvironment, characterised by various SFHs. We tentatively typed all LMC SNRs\nto constrain the ratio of core-collapse to type Ia SN rates in the LMC. We\ncompared the X-ray-derived column densities to HI maps to probe the\nthree-dimensional structure of the LMC. This work provides the first\nhomogeneous catalogue of X-ray spectral properties of LMC SNRs. It offers a\ncomplete census of LMC SNRs exhibiting Fe K lines (13% of the sample), or\nrevealing contribution from hot SN ejecta (39%). Abundances in the LMC ISM are\nfound to be 0.2-0.5 solar, with a lower [$\\alpha$/Fe] than in the Milky Way.\nThe ratio of CC/type Ia SN in the LMC is $N_{\\mathrm{CC}}/N_{\\mathrm{Ia}} =\n1.35(_{-0.24}^{+0.11})$, lower than in local SN surveys and galaxy clusters.\nComparison of X-ray luminosity functions of SNRs in Local Group galaxies\nreveals an intriguing excess of bright objects in the LMC. We confirm that 30\nDoradus and the LMC Bar are offset from the main disc of the LMC, to the far\nand near sides, respectively. (abridged)", "category": "astro-ph_HE" }, { "text": "Detection of Millihertz Quasi-Periodic Oscillations in the X-Ray Binary\n 1RXS J180408.9$-$342058: Millihertz quasi-periodic oscillations (mHz QPOs) observed in neutron-star\nlow-mass X-ray binaries (NS LMXBs) are generally explained as marginally stable\nthermonuclear burning on the neutron star surface. We report the discovery of\nmHz QPOs in an XMM-Newton observation of the transient 1RXS J180408.9$-$342058,\nduring a regular bursting phase of its 2015 outburst. We found significant\nperiodic signals in the March observation, with frequencies in the range\n$5-8\\,\\mathrm{mHz}$, superimposed on a strong $\\sim1/f$ power-law noise\ncontinuum. Neither the QPO signals nor the power-law noise were present during\nthe April observation, which exhibited a $2.5\\times$ higher luminosity and had\ncorrespondingly more frequent bursts. When present, the QPO signal power\ndecreases during bursts and disappears afterwards, similar to the behaviour in\nother sources. 1RXS J180408.9$-$342058 is the eighth source known to date that\nexhibits such QPOs driven by thermonuclear burning. We examine the range of\nproperties of the QPO signals in different sources. Whereas the observed\noscillation profile is similar to that predicted by numerical models, the\namplitudes are significantly higher, challenging their explanation as\noriginating from marginally stable burning.", "category": "astro-ph_HE" }, { "text": "The magnetic field structure in CTA 102 from high resolution mm-VLBI\n observations during the flaring state in 2016-2017: Investigating the magnetic field structure in the innermost regions of\nrelativistic jets is fundamental to shed light on the crucial physical\nprocesses giving rise to the jet formation, as well as to its extraordinary\nradiation output up to gamma-ray energies. We study the magnetic field\nstructure of the quasar CTA 102 with 3 and 7 mm-VLBI polarimetric observations,\nreaching an unprecedented resolution (~50 microarcsec). We also investigate the\nvariability and physical process occurring in the source during the observing\nperiod which coincides with a very active state of the source till\nhigh-energies. The Faraday rotation analysis between 3 and 7mm shows a gradient\nin rotation measure with a maximum value of ~6X10^4 rad/m^2 and intrinsic\nelectric vector position angles (EVPAs) oriented around the centroid of the\ncore, suggesting the presence of large-scale helical magnetic fields. Such a\nmagnetic field structure is also visible in 7 mm images when a new superluminal\ncomponent is crossing the core region. The 7mm EVPAs orientation is different\nwhen the component is exiting the core or crossing a stationary feature at ~0.1\nmas. The interaction between the superluminal component and a recollimation\nshock at ~0.1 mas could have triggered the multi-wavelengths flares. The\nvariability Doppler factor associated with such interaction is large enough to\nexplain the high energy emission, as we infer from the analysis of gamma-ray\nand X-ray data, and it is in agreement with the Doppler factor obtained to\nexplain the extraordinary optical flare by Raiteri et al.(2017).", "category": "astro-ph_HE" }, { "text": "TMRT observations of 26 pulsars at 8.6 GHz: Integrated pulse profiles at 8.6~GHz obtained with the Shanghai Tian Ma Radio\nTelescope (TMRT) are presented for a sample of 26 pulsars. Mean flux densities\nand pulse width parameters of these pulsars are estimated. For eleven pulsars\nthese are the first high-frequency observations and for a further four, our\nobservations have a better signal-to-noise ratio than previous observations.\nFor one (PSR J0742-2822) the 8.6~GHz profiles differs from previously observed\nprofiles. A comparison of 19 profiles with those at other frequencies shows\nthat in nine cases the separation between the outmost leading and trailing\ncomponents decreases with frequency, roughly in agreement with\nradius-to-frequency mapping, whereas in the other ten the separation is nearly\nconstant. Different spectral indices of profile components lead to the\nvariation of integrated pulse profile shapes with frequency. In seven pulsars\nwith multi-component profiles, the spectral indices of the central components\nare steeper than those of the outer components. For the 12 pulsars with\nmulti-component profiles in the high-frequency sample, we estimate the core\nwidth using gaussian fitting and discuss the width-period relationship.", "category": "astro-ph_HE" }, { "text": "X-ray emission from optical novae in M 31: The first supersoft source (SSS) identification with an optical nova in M 31\nwas based on ROSAT observations. Twenty additional X-ray counterparts (mostly\nidentified as SSS by their hardness ratios) were detected using archival ROSAT,\nXMM-Newton and Chandra observations obtained before July 2002. Based on these\nresults optical novae seem to constitute the major class of SSS in M 31. An\nanalysis of archival Chandra HRC-I and ACIS-I observations obtained from July\n2004 to February 2005 demonstrated that M 31 nova SSS states lasted from months\nto about 10 years. Several novae showed short X-ray outbursts starting within\n50 d after the optical outburst and lasting only two to three months. The\nfraction of novae detected in soft X-rays within a year after the optical\noutburst was more than 30%. Ongoing optical nova monitoring programs, optical\nspectral follow-up and an up-to-date nova catalogue are essential for the X-ray\nwork. Re-analysis of archival nova data to improve positions and find\nadditional nova candidates are urgently needed for secure recurrent nova\nidentifications. Dedicated XMM-Newton/Chandra monitoring programs for X-ray\nemission from optical novae covering the center area of M 31 continue to\nprovide interesting new results (e.g. coherent 1105s pulsations in the SSS\ncounterpart of nova M31N 2007-12b). The SSS light curves of novae allow us -\ntogether with optical information - to estimate the mass of the white dwarf, of\nthe ejecta and the burned mass in the outburst. Observations of the central\narea of M 31 allow us - in contrast to observations in the Galaxy - to monitor\nmany novae simultaneously and proved to be prone to find many interesting SSS\nand nova types.", "category": "astro-ph_HE" }, { "text": "Deep Hard X-ray Survey of the Large Magellanic Cloud: Results of the deep survey of the Large Magellanic Cloud (LMC), performed\nwith the INTEGRAL observatory, are presented. The long exposure (~7 Ms) allowed\nus to detect twenty one sources in this sky region: ten belonging to the LMC\nitself (7 HMXBs, 2 PSRs, 1 LMXB), six of extragalactic origin and three\nbelonging to other galaxies from the Local Group - the Milky Way (2 sources)\nand Small Magellanic Cloud (1 source). Four new hard X-ray sources of these 21\nones were discovered during the survey in addition to IGR J05414-6858 reported\nearlier; two of them were identified with extragalactic objects. We report also\nfor the first time the detection of a hard X-ray emission from the Crab-like\npulsar PSR J0537-6910 and identification of the hard X-ray source IGR\nJ05305-6559 with the high-mass X-ray binary EXO 053109-6609.", "category": "astro-ph_HE" }, { "text": "Properties of relativistic hot accretion flow around rotating black hole\n with radially varying viscosity: We examine the effect of variable viscosity parameter ($\\alpha$) in\nrelativistic, low angular momentum advective accretion flow around rotating\nblack holes. Following the recent simulation studies of magnetohydrodynamic\ndisk that reveal the radial variation of $\\alpha(r)$, we theoretically\ninvestigate the properties of the global transonic accretion flow considering a\none-dimensional power law prescription of viscosity parameter as $\\alpha(r)\n\\propto r^{\\theta}$, where the viscosity exponent $\\theta$ is a constant. In\ndoing so, we adopt the relativistic equation of state and solve the fluid\nequations that govern the flow motion inside the disk. We find that depending\non the flow parameters, accretion flow experiences centrifugally supported\nshock transition and such shocked accretion solutions continue to exist for\nwide ranges of the flow energy, angular momentum, accretion rate and viscosity\nexponent, respectively. Due to shock compression, the hot and dense post-shock\nflow (hereafter PSC) can produce the high energy radiations after reprocessing\nthe soft photons from the pre-shock flow via inverse Comptonization. Since PSC\nis usually described using shock radius ($r_s$), compression ratio ($R$) and\nshock strength ($S$), we study the role of $\\theta$ in deciding $r_s$, $R$ and\n$S$, respectively. Moreover, we obtain the parameter space for shock and find\nthat possibility of shock formation diminishes as $\\theta$ is increased.\nFinally, we compute the limiting value of $\\theta$ ($i.e., \\theta^{\\rm max}$)\nthat admits shock and find that flow can sustain more viscosity when it\naccretes onto rapidly rotating ($a_{\\rm k} \\rightarrow 1$) black hole in\ncomparison to weakly rotating ($a_{\\rm k} \\rightarrow 0$) black hole.", "category": "astro-ph_HE" }, { "text": "Fermi-LAT observations of the LIGO/Virgo event GW170817: We present the Fermi Large Area Telescope (LAT) observations of the binary\nneutron star merger event GW170817 and the associated short gamma-ray burst\n(SGRB) GRB\\,170817A detected by the Fermi Gamma-ray Burst Monitor. The LAT was\nentering the South Atlantic Anomaly at the time of the LIGO/Virgo trigger\n($t_{\\rm GW}$) and therefore cannot place constraints on the existence of\nhigh-energy (E $>$ 100 MeV) emission associated with the moment of binary\ncoalescence. We focus instead on constraining high-energy emission on longer\ntimescales. No candidate electromagnetic counterpart was detected by the LAT on\ntimescales of minutes, hours, or days after the LIGO/Virgo detection. The\nresulting flux upper bound (at 95\\% C.L.\\/) from the LAT is\n$4.5\\times$10$^{-10}$ erg cm$^{-2}$ s$^{-1}$ in the 0.1--1 GeV range covering a\nperiod from T0 + 1153 s to T0 + 2027 s. At the distance of GRB\\,170817A, this\nflux upper bound corresponds to a luminosity upper bound of 9.7$\\times10^{43}$\nerg s$^{-1}$, which is 5 orders of magnitude less luminous than the only other\nLAT SGRB with known redshift, GRB\\,090510. We also discuss the prospects for\nLAT detection of electromagnetic counterparts to future gravitational wave\nevents from Advanced LIGO/Virgo in the context of GW170817/GRB\\,170817A.", "category": "astro-ph_HE" }, { "text": "Gamma-ray Emitting Narrow-Line Seyfert 1 Galaxies: Past, Present, and\n Future: This article reviews our current understanding about $\\gamma$-ray detected\nnarrow-line Seyfert 1 ($\\gamma$-NLSy1) galaxies. The detection with the Large\nArea Telescope onboard {\\it Fermi}~Gamma-ray Space Telescope has provided the\nstrongest evidence for the presence of closely aligned relativistic jet in\nthese intriguing active galactic nuclei (AGN) and opened up a realm to explore\nthe physical conditions needed to launch the jet in a different central engine\nand host galaxy environment than that is known for blazars. Promising results\nacquired from various multi-wavelength campaigns are converging to a scenario\nin which the $\\gamma$-NLSy1 galaxies can be considered as `young' blazars.\nThese enigmatic sources hold the key to unravel the jet triggering mechanism\nand evolution of the AGN phase of a galaxy, in general. As such,\n$\\gamma$-NLSy1s should be considered as one of the top priority targets for\nnext generation observational facilities.", "category": "astro-ph_HE" }, { "text": "Gaia's Detectability of Black Hole-Main Sequence Star Binaries Formed in\n Open Clusters: Black hole-main sequence star (BH-MS) binaries are one of the targets of the\nfuture data releases of the astrometric satellite {\\it Gaia}. They are supposed\nto be formed in two main sites: a galactic field and star clusters. However,\nprevious work has never predicted the number of BH-MS binaries originating in\nthe latter site. In this paper, we estimate the number of BH-MS binaries formed\nin open clusters and detectable with {\\it Gaia} based on the results of {\\it\nN}-body simulations. By considering interstellar extinction in the Milky Way\n(MW) and observational constraints, we predict $\\sim 10$ BH-MS binaries are\nobservable. We also find that chemical abundance patterns of companion MSs will\nhelp us to identify the origin of the binaries as star clusters. Such MSs are\nnot polluted by outflows of the BH progenitors, such as stellar winds and\nsupernova ejecta. Chemical anomalies might be a good test to confirm the origin\nof binaries with relatively less massive MSs ($\\lesssim 5M_{\\odot}$), orbital\nperiods ($\\sim 1.5\\;$year) and higher eccentricities ($e \\gtrsim 0.1$).", "category": "astro-ph_HE" }, { "text": "Discovery of a ~5 day characteristic timescale in the Kepler power\n spectrum of Zw 229-15: We present time series analyses of the full Kepler dataset of Zw 229-15. This\nKepler light curve --- with a baseline greater than three years, composed of\nvirtually continuous, evenly sampled 30-minute measurements --- is\nunprecedented in its quality and precision. We utilize two methods of power\nspectral analysis to investigate the optical variability and search for\nevidence of a bend frequency associated with a characteristic optical\nvariability timescale. Each method yields similar results. The first\ninterpolates across data gaps to use the standard Fourier periodogram. The\nsecond, using the CARMA-based time-domain modeling technique of Kelly et al.\n(2014), does not need evenly-sampled data. Both methods find excess power at\nhigh frequencies that may be due to Kepler instrumental effects. More\nimportantly both also show strong bends ({\\Delta}{\\alpha} ~ 2) at timescales of\n~5 days, a feature similar to those seen in the X-ray PSDs of AGN but never\nbefore in the optical. This observed ~5 day timescale may be associated with\none of several physical processes potentially responsible for the variability.\nA plausible association could be made with light-crossing, dynamical or thermal\ntimescales, depending on the assumed value of the accretion disk size and on\nunobserved disk parameters such as {\\alpha} and H/R. This timescale is not\nconsistent with the viscous timescale, which would be years in a ~10^7 Solar\nmass AGN such as Zw 229-15. However there must be a second bend on long (>~1\nyear) timescales, and that feature could be associated with the viscous\ntimescale.", "category": "astro-ph_HE" }, { "text": "The population properties of spinning black holes using\n Gravitational-wave Transient Catalog 3: Binary black holes formed via different pathways are predicted to have\ndistinct spin properties. Measuring these properties with gravitational waves\nprovides an opportunity to unveil the origins of binary black holes. Recent\nwork draws conflicting conclusions regarding the spin distribution observed by\nLIGO--Virgo--KAGRA (LVK). Some analyses suggest that a fraction of the observed\nblack-hole spin vectors are significantly misaligned (by $>90^\\circ$) relative\nto the orbital angular momentum. This has been interpreted to mean that some\nbinaries in the LVK dataset are assembled dynamically in dense stellar\nenvironments. Other analyses find support for a sub-population of binaries with\nnegligible spin and no evidence for significantly misaligned spin -- a result\nconsistent with the field formation scenario. In this work, we study the spin\nproperties of binary black holes in the third LVK gravitational-wave transient\ncatalog. We find that there is insufficient data to resolve the existence of a\nsub-population of binaries with negligible black-hole spin (the presence of\nthis sub-population is supported by a modest Bayes factor of 1.7). We find\nmodest support for the existence of mergers with extreme spin tilt angles $>\n90^\\circ$ (the presence of extreme-tilt binaries is favored by a Bayes factor\nof 10.1). Only one thing is clear: at least some of the LVK binaries formed in\nthe field. At most $89\\%$ of binaries are assembled dynamically (99\\%\ncredibility), though, the true branching fraction could be much lower, even\nnegligible.", "category": "astro-ph_HE" }, { "text": "Taking a break: paused accretion in the symbiotic binary RT Cru: Symbiotic binaries sometimes hide their symbiotic nature for significant\nperiods of time. There is mounting observational evidence that in those\nsymbiotics that are powered solely by accretion of red-giant's wind material\nonto a white dwarf, without any quasi-steady shell burning on the surface of\nthe white dwarf, the characteristic emission lines in the optical spectrum can\nvanish, leaving the semblance of an isolated red giant spectrum. Here we\npresent compelling evidence that this disappearance of optical emission lines\nfrom the spectrum of RT Cru during 2019 was due to a decrease in the accretion\nrate, which we derive by modeling the X-ray spectrum. This drop in accretion\nrate leads to a lower flux of ionizing photons and thus to faint/absent\nphotoionization emission lines in the optical spectrum. We observed the white\ndwarf symbiotic RT Cru with XMM-Newton and Swift in X-rays and UV and collected\nground-based optical spectra and photometry over the last 33 years. This\nlong-term coverage shows that during most of the year 2019, the accretion rate\nonto the white dwarf was so low, $\\dot{M}= (3.2\\pm 0.06)\\, \\times$10$^{-11}$\n$M_{\\odot}$ yr$^{-1}$ (d/2.52 kpc)$^2$, that the historically detected hard\nX-ray emission almost vanished, the UV flux faded by roughly 5 magnitudes, the\n$U$, $B$ and $V$ flickering amplitude decreased, and the Balmer lines virtually\ndisappeared from January through March 2019. Long-lasting low-accretion\nepisodes as the one reported here may hamper the chances of RT Cru experiencing\nnova-type outburst despite the high-mass of the accreting white dwarf.", "category": "astro-ph_HE" }, { "text": "A QPO in NGC 4945 from Archival RXTE Data: We report the discovery of a ~6-week quasi-periodic oscillation (QPO) in\narchival NGC 4945 data observed by the Rossi X-ray Timing Explorer (RXTE)\nsatellite. QPOs are an important observable in accretion disks and have been\nstudied extensively in both neutron star (NS) and black hole (BH) X-ray\nbinaries (XRB). QPOs should be present in Active Galactic Nuclei (AGN) if\ngalactic black holes and supermassive black holes (SMBH) are governed by a\ncommon set of physical processes. The search for QPOs in AGN has proven\ndifficult because the timescales would be much longer, due to their higher\nmass. RXTE AGN light curves spanning 1996 to 2011 provide an excellent and\nperhaps unique opportunity to search for low-frequency QPOs. We investigated\nthe 533 RXTE observations made of the Seyfert-2 AGN, NGC 4945. During a large\ncluster of observations in 2006-2007 (194 observations, spanning 396 days), the\nLomb-Scargle periodogram shows a candidate QPO at 0.274 $\\mu$Hz (period\n$\\approx$ 42.2 days). We estimate the uncertainties using the False Alarm\nProbability (FAP). We discuss the possible identification of this feature with\nthe Lense-Thirring precession period.", "category": "astro-ph_HE" }, { "text": "International X-ray Observatory (IXO) Assessment Study Report for the\n ESA Cosmic Vision 2015-2025: The International X-Ray Observatory (IXO) will address fundamental questions\nin astrophysics, including \"When did the first SMBH form? How does large scale\nstructure evolve? What happens close to a black hole? What is the connection\nbetween these processes? What is the equation of state of matter at\nsupra-nuclear density?\" This report presents an overview of the assessment\nstudy phase of the IXO candidate ESA L-class Cosmic Vision mission. We provide\na description of the IXO science objectives, the mission implementation and the\npayload. The performance will offer more than an order of magnitude improvement\nin capability compared with Chandra and XMM-Newton. This observatory-class\nfacility comprises a telescope with highly nested grazing incidence optics with\na performance requirement of 2.5 sq.m. of effective area at 1.25 keV with a 5\"\nPSF. There is an instrument complement that provides capabilities in imaging,\nspatially resolved spectroscopy, timing, polarimetry and high resolution\ndispersive spectroscopy. Since earlier submissions to the Astro2010 Decadal\nSurvey, substantial technological progress has been made, particularly in the\nmirror development. Risk reduction measures and important programmatic choices\nhave also been identified. An independent internal Technical and Programmatic\nReview has also been carried out by ESA, concluding with positive\nrecommendations. Subject to successful conclusion of agreements between the\npartner space agencies, IXO is fully ready to proceed to further definition,\nmoving towards an eventual launch in 2021-2022.", "category": "astro-ph_HE" }, { "text": "High-energy cosmic particles: A review of the status of the knowledge in the field of High-energy cosmic\nparticles is presented. The spectrum, arrival direction distribution and\ncomposition measurements are summarized, together with some implications for\nthe understanding of the cosmic ray origin and their propagation. Special\nemphasis is put in the ultra-high energy range, corresponding to particles of\nextragalactic origin.", "category": "astro-ph_HE" }, { "text": "Impact of initial mass functions on the dynamical channel of\n gravitational wave sources: Dynamically formed black hole (BH) binaries (BBHs) are important sources of\ngravitational waves (GWs). Globular clusters (GCs) provide a major environment\nto produce such BBHs, but the total mass of the known GCs is small compared to\nthat in the Galaxy; thus, the fraction of BBHs formed in GCs is also small.\nHowever, this assumes that GCs contain a canonical initial mass function (IMF)\nsimilar to that of field stars. This might not be true because several studies\nsuggest that extreme dense and metal-poor environment can result in top-heavy\nIMFs, where GCs may originate. Although GCs with top-heavy IMFs were easily\ndisrupted or have become dark clusters, the contribution to the GW sources can\nbe significant. Using a high-performance and accurate $N$-body code,\n\\textsc{petar}, we investigate the effect of varying IMFs by carrying out four\nstar-by-star simulations of dense GCs with the initial mass of $5\\times10^5\nM_\\odot$ and the half-mass radius of $2$~pc. We find that the BBH merger rate\ndoes not monotonically correlate with the slope of IMFs. Due to a rapid\nexpansion, top-heavy IMFs lead to less efficient formation of merging BBHs. The\nformation rate continuously decreases as the cluster expands because of the\ndynamical heating caused by BHs. However, in star clusters with a top-heavier\nIMF, the total number of BHs is larger, and therefore, the final contribution\nto merging BBHs can still be more than from clusters with the standard IMF, if\nthe initial cluster mass and density is higher than those used in our model.", "category": "astro-ph_HE" }, { "text": "The impact of Faraday effects on polarized black hole images of\n Sagittarius A*: We study model images and polarization maps of Sagittarius A* at 230 GHz. We\npost-process GRMHD simulations and perform a fully relativistic radiative\ntransfer calculation of the emitted synchrotron radiation to obtain polarized\nimages for a range of mass accretion rates and electron temperatures. At low\naccretion rates, the polarization map traces the underlying toroidal magnetic\nfield geometry. At high accretion rates, we find that Faraday rotation internal\nto the emission region can depolarize and scramble the map. We measure the net\nlinear polarization fraction and find that high accretion rate \"jet-disc\"\nmodels are heavily depolarized and are therefore disfavoured. We show how Event\nHorizon Telescope measurements of the polarized \"correlation length\" over the\nimage provide a model-independent upper limit on the strength of these Faraday\neffects, and constrain plasma properties like the electron temperature and\nmagnetic field strength.", "category": "astro-ph_HE" }, { "text": "On radiative acceleration in spine-sheath structured blazar jets: It has been proposed that blazar jets are structured, with a fast spine\nsurrounded by a slower sheath or layer. This structured jet model explains some\nproperties of their emission and morphology. Because of their relative motion,\nthe radiation produced by one component is seen amplified by the other, thus\nenhancing the inverse Compton emission of both. Radiation is emitted\nanisotropically in the comoving frames, and causes the emitting plasma to\nrecoil. As seen in the observer frame, this corresponds to a deceleration of\nthe fastest component (the spine) and an acceleration of the slower one (the\nlayer). While the deceleration of the spine has already been investigated, here\nwe study for the first time the acceleration of the sheath and find\nself-consistent velocity profile solutions for both the spine and the sheath\nwhile accounting for radiative cooling. We find that the sheath can be\naccelerated to the velocities required by the observations if its leptons\nremain energetic in the acceleration region, assumed to be of the order of 100\nSchwarzschild radii, demanding continuous injection of energetic particles in\nthat region.", "category": "astro-ph_HE" }, { "text": "Population Synthesis of Isolated Neutron Stars with magneto-rotational\n evolution II: from radio-pulsars to magnetars: Population synthesis studies constitute a powerful method to reconstruct the\nbirth distribution of periods and magnetic fields of the pulsar population.\nWhen this method is applied to populations in different wavelengths, it can\nbreak the degeneracy in the inferred properties of initial distributions that\narises from single-band studies. In this context, we extend previous works to\ninclude $X$-ray thermal emitting pulsars within the same evolutionary model as\nradio-pulsars. We find that the cumulative distribution of the number of X-ray\npulsars can be well reproduced by several models that, simultaneously,\nreproduce the characteristics of the radio-pulsar distribution. However, even\nconsidering the most favourable magneto-thermal evolution models with fast\nfield decay, log-normal distributions of the initial magnetic field\nover-predict the number of visible sources with periods longer than 12 s. We\nthen show that the problem can be solved with different distributions of\nmagnetic field, such as a truncated log-normal distribution, or a binormal\ndistribution with two distinct populations. We use the observational lack of\nisolated NSs with spin periods P>12 s to establish an upper limit to the\nfraction of magnetars born with B > 10^{15} G (less than 1\\%). As future\ndetections keep increasing the magnetar and high-B pulsar statistics, our\napproach can be used to establish a severe constraint on the maximum magnetic\nfield at birth of NSs.", "category": "astro-ph_HE" }, { "text": "Modelling the disk atmosphere of the low mass X-ray binary EXO 0748-676: Low mass X-ray binaries exhibit ionized emission from an extended disk\natmosphere that surrounds the accretion disk. However, its nature and geometry\nis still unclear. In this work we present a spectral analysis of the extended\natmosphere of EXO 0748-676 using high-resolution spectra from archival\nXMM-Newton observations. We model the RGS spectrum that is obtained during the\neclipses. This enables us to model the emission lines that come only from the\nextended atmosphere of the source, and study its physical structure and\nproperties. The RGS spectrum reveals a series of emission lines consistent with\ntransitions of O VIII, O VII, Ne IX and N VII. We perform both Gaussian line\nfitting and photoionization modelling. Our results suggest that there are two\nphotoionization gas components, out of pressure equilibrium with respect to\neach other. One with ionization parameter of 2.5 and a large opening angle, and\none with 1.3. The second component is possibly covering a smaller fraction of\nthe source. From the density diagnostics of the O vii triplet using\nphotoionization modelling, we detect a rather high density plasma of > 10^13\ncm^-3 for the lower ionization component. This latter component also displays\nan inflow velocity. We propose a scenario where the high ionization component\nconstitutes an extended upper atmosphere of the accretion disk. The lower\nionization component may instead be a clumpy gas created from the impact of the\naccretion stream with the disk.", "category": "astro-ph_HE" }, { "text": "A new possibility of the fast neutrino-flavor conversion in the\n pre-shock region of core-collapse supernova: We make a strong case that the fast neutrino-flavor conversion, one of the\ncollective flavor oscillation modes, commonly occurs in core-collapse\nsupernovae (CCSNe). It is confirmed in the numerical data obtained in realistic\nsimulations of CCSNe but the argument is much more generic and applicable\nuniversally: the coherent neutrino-nucleus scattering makes the electron lepton\nnumber (ELN) change signs at some inward direction and trigger the flavor\nconversion in the outward direction in the pre-shock region. Although the ELN\ncrossing is tiny and that is why it has eluded recognition so far, it is still\nlarge enough to induce the flavor conversion. Our findings will have an\nimportant observational consequences for CCSNe neutrinos.", "category": "astro-ph_HE" }, { "text": "Pre-burst neutrinos of gamma-ray bursters accompanied by high-energy\n photons: Previous researches on high-energy neutrino events from gamma-ray bursters\n(GRBs) suggest a neutrino speed variation $v(E)=c(1\\pm\nE/E^{\\nu}_{\\mathrm{LV}})$ with ${E}^{\\nu}_{\\rm LV}=(6.4\\pm 1.5)\\times10^{17}~{\n\\rm GeV}$, together with an intrinsic time difference ${\\Delta {t}_{\\rm\nin}=(-2.8\\pm 0.7)\\times10^2~{\\rm s}}$, which means that high-energy neutrinos\ncome out about 300~s earlier than low-energy photons in the source reference\nsystem. Considering the possibility that pre-bursts of neutrinos may be\naccompanied by high-energy photons, in this work we search for high-energy\nphoton events with earlier emission time from 100 to 1000~s before low-energy\nphotons at source by analyzing Fermi Gamma-ray Space Telescope (FGST) data. We\nperform the searching of photon events with energies larger than 100~MeV, and\nfind 14 events from 48 GRBs with known redshifts. Combining these events with a\n$1.07~\\rm{TeV}$ photon event observed by the Major Atmospheric Gamma Imaging\nCherenkov telescopes (MAGIC), we suggest a pre-burst stage with a long duration\nperiod of several minutes of high energy neutrino emissions accompanied by high\nenergy photons at the GRB source.", "category": "astro-ph_HE" }, { "text": "Long term study of the light curve of PKS 1510-089 in GeV energies: We have analyzed data from the Flat Spectrum Radio Quasar PKS 1510-089\ncollected over a period of 8 years from August 2008 to December 2016 with the\nFermi-LAT. We have identified several flares of this highly variable source,\nstudied their temporal and spectral properties in detail and compared with\nprevious works on flares of PKS 1510-089. Five major flares and few\nsub-flares/sub-structures have been identified in our study. The fastest\nvariability time is found to be 1.30$\\pm$0.18 hr between MJD 55852.063 and\n55852.188 where we estimate the minimum size of the emission region to be $4.85\n\\times 10^{15}$ cm. In most of the flares the spectral energy distributions are\nbetter fitted with Logparabolic distribution compared to simple Power law or\nPower law with exponential cut-offs. This has strong physics implications\nregarding the nature of the high energy gamma-ray emission region.", "category": "astro-ph_HE" }, { "text": "Radio impulsive events in quiet solar corona and Axion Quark Nugget Dark\n Matter: The Murchison Widefield Array (MWA) recorded \\cite{Mondal-2020} impulsive\nradio events in the quiet solar corona at frequencies 98, 120, 132, and 160\nMHz. We propose that these radio events are the direct manifestation of dark\nmatter annihilation events within the axion quark nugget (AQN) framework. It\nhas been argued \\cite{Zhitnitsky:2017rop,Raza:2018gpb} that the AQN\nannihilation events in the quiet solar corona can be identified with the\nnanoflares conjectured by Parker \\cite{Parker-1983}. We further support this\nclaim by demonstrating that observed impulsive radio events \\cite{Mondal-2020},\nincluding their rate of appearance, their temporal and spatial distributions\nand their energetics, are matching the generic consequences of AQN\nannihilations in the quiet corona. We propose to test this idea by analyzing\nthe correlated clustering of impulsive radio events in different frequency\nbands. These correlations are expressed in terms of the time delays between\nradio events in different frequency bands, measured in seconds. We also make\ngeneric predictions for low (80 and 89 MHz) and high (179, 196, 217 and 240\nMHz) frequency bands, that have been recorded, but not published, by\n\\cite{Mondal-2020}. We finally suggest to test our proposal by studying\npossible cross-correlation between MWA radio signals and Solar Orbiter\nrecording of extreme UV photons (a.k.a. \"campfires\").", "category": "astro-ph_HE" }, { "text": "A high-resolution view of the jets in 3C 465: We present new high-resolution and high-sensitivity studies of the jets in\nthe WAT source 3C 465, using deep transverse-resolved radio observations from\ne-MERLIN, and with complementary observations from the VLA. We derive a lower\nlimit $\\beta_{\\rm j}$ = ($\\nu_{\\rm j}$/$c$) $\\gtrsim$ 0.5 for the jet speed,\nand an upper limit $\\theta_{\\rm j}$ $\\lesssim$ 61$^{\\circ}$ for the jet angle\nto the line of sight. The jet spectral index ($\\alpha$, defined in the sense $S\n\\propto \\nu^{\\alpha}$) is fairly constant (<$\\alpha_{\\rm jet}$> = $-$0.7), and\nspectral flattening within 4.4 kpc of the core coincides with bright knots and\nis consistent with the site of X-ray particle acceleration at the base of the\nradio jet found in previous studies. There is little difference between the\nspectra of the two hotspot components, plausibly indicating that electron\npopulations of the same properties are injected there. The NW and SE plumes are\napproximately homologous structures, with variations in mass injection and\npropagation in external pressure and density gradients in the two regions\nplausibly accounting for the slightly steeper spectrum in the NW plume,\n<$\\alpha_{\\rm NWp}$> = $-$1.43 compared with the SE plume, <$\\alpha_{\\rm SEp}$>\n= $-$1.38. Our synchrotron lifetime model supports plausible reacceleration of\nparticles within the plume materials. Overall, our results show that the\nfirst-order Fermi process at mildly relativistic and non-relativistic shocks is\nthe most likely acceleration mechanism at play in 3C 465 and distinguish\ndifferences between the acceleration at $\\beta_{\\rm j}$ $>$ 0.5 and $\\beta_{\\rm\nj}$ $<$ 0.5. The former case can accelerate electrons to higher Lorentz\nfactors.", "category": "astro-ph_HE" }, { "text": "GRB 090510: a genuine short-GRB from a binary neutron star coalescing\n into a Kerr-Newman black hole: In a new classification of merging binary neutron stars (NSs) we separate\nshort gamma-ray bursts (GRBs) in two sub-classes. The ones with\n$E_{iso}\\lesssim10^{52}$ erg coalesce to form a massive NS and are indicated as\nshort gamma-ray flashes (S-GRFs). The hardest, with $E_{iso}\\gtrsim10^{52}$\nerg, coalesce to form a black hole (BH) and are indicated as genuine short-GRBs\n(S-GRBs). Within the fireshell model, S-GRBs exhibit three different\ncomponents: the P-GRB emission, observed at the transparency of a\nself-accelerating baryon-$e^+e^-$ plasma; the prompt emission, originating from\nthe interaction of the accelerated baryons with the circumburst medium; the\nhigh-energy (GeV) emission, observed after the P-GRB and indicating the\nformation of a BH. GRB 090510 gives the first evidence for the formation of a\nKerr BH or, possibly, a Kerr-Newman BH. Its P-GRB spectrum can be fitted by a\nconvolution of thermal spectra whose origin can be traced back to an axially\nsymmetric dyadotorus. A large value of the angular momentum of the newborn BH\nis consistent with the large energetics of this S-GRB, which reach in the\n1--10000 keV range $E_{iso}=(3.95\\pm0.21)\\times10^{52}$ erg and in the 0.1--100\nGeV range $E_{LAT}=(5.78\\pm0.60)\\times10^{52}$ erg, the most energetic GeV\nemission ever observed in S-GRBs. The theoretical redshift $z_{th}=0.75\\pm0.17$\nthat we derive from the fireshell theory is consistent with the spectroscopic\nmeasurement $z=0.903\\pm0.003$, showing the self-consistency of the theoretical\napproach. All S-GRBs exhibit GeV emission, when inside the Fermi-LAT field of\nview, unlike S-GRFs, which never evidence it. The GeV emission appears to be\nthe discriminant for the formation of a BH in GRBs, confirmed by their observed\noverall energetics.", "category": "astro-ph_HE" }, { "text": "Dependence of the LMXB population on stellar age: We investigate the dependence of the low-mass X-ray binary (LMXB) population\nin early-type galaxies on stellar age, by selecting 20 massive nearby\nearly-type galaxies from the Chandra archive occupying a relatively narrow\nrange of masses and spanning a broad range of ages, from 1.6 Gyr to more than\n10 Gyrs, with the median value of 6 Gyrs. With the ~ 2000 X-ray point sources\ndetected in total, we correlated the specific number of LMXBs in each galaxy\nwith its stellar age and globular cluster (GC) content. We found a correlation\nbetween the LMXB population and stellar age: older galaxies tend to possess\nabout ~50% more LMXBs (per unit stellar mass) than the younger ones. The\ninterpretation of this dependence is complicated by large scatter and a rather\nstrong correlation between stellar age and GC content of galaxies in our\nsample. We present evidence suggesting that the more important factor may be\nthe evolution of the LMXB population with time. Its effect is further amplified\nby the larger GC content of older galaxies and correspondingly, the larger\nnumbers of dynamically formed binaries in them. We also found clear evolution\nof the X-ray luminosity function (XLF) with age, that younger galaxies have\nmore bright sources and fewer faint sources per unit stellar mass. The XLF of\nLMXBs in younger galaxies appears to extend significantly beyond E39 erg/s.\nSuch bright sources seem to be less frequent in older galaxies. We found that 6\nout of ~ 12 (ultra-) luminous sources are located in GCs.", "category": "astro-ph_HE" }, { "text": "Excitation of Trapped Waves in Simulations of Tilted Black Hole\n Accretion Disks with Magnetorotational Turbulence: We analyze the time dependence of fluid variables in general relativistic,\nmagnetohydrodynamic simulations of accretion flows onto a black hole with\ndimensionless spin parameter a/M=0.9. We consider both the case where the\nangular momentum of the accretion material is aligned with the black hole spin\naxis (an untilted flow) and where it is misaligned by 15 degrees (a tilted\nflow). In comparison to the untilted simulation, the tilted simulation exhibits\na clear excess of inertial variability, that is, variability at frequencies\nbelow the local radial epicyclic frequency. We further study the radial\nstructure of this inertial-like power by focusing on a radially extended band\nat 118 (M/10Msol)^-1 Hz found in each of the three analyzed fluid variables.\nThe three dimensional density structure at this frequency suggests that the\npower is a composite oscillation whose dominant components are an over dense\nclump corotating with the background flow, a low order inertial wave, and a low\norder inertial-acoustic wave. Our results provide preliminary confirmation of\nearlier suggestions that disk tilt can be an important excitation mechanism for\ninertial waves.", "category": "astro-ph_HE" }, { "text": "Energy function, formation rate and low-metallicity environment of fast\n radio bursts: In this paper, we investigate the energy function, formation rate and\nenvironment of fast radio bursts (FRBs) using Parkes sample and Australian\nSquare Kilometer Array Pathfinder (ASKAP) sample. For the first time, the\nmetallicity effect on the formation rate is considered. If FRBs are produced by\nthe mergers of compact binaries, the formation rate of FRBs should have a time\ndelay relative to cosmic star formation rate (CSFR). We get the time delay is\nabout 3-5 Gyr and the index of differential energy function $\\gamma$\n($dN/dE\\propto E^{-\\gamma}$) is between 1.6 and 2.0 from redshift cumulative\ndistribution. The value of $\\gamma$ is similar to that of FRB 121102, which\nindicates single bursts may share the same physical mechanism with the\nrepeaters. In another case, if the formation rate of FRB is proportional to the\nSFR without time delay, the index $\\gamma$ is about 2.3. In both cases, we find\nthat FRBs may prefer to occur in low-metallicity environment with $ 12\n+\\log(\\rm{O/H}) \\simeq 8.40$, which is similar to those of long gamma-ray\nbursts (GRBs) and hydrogen-poor superluminous supernovae (SLSNe-I).", "category": "astro-ph_HE" }, { "text": "The discovery of a rotating radio transient J1918$-$0449 with intriguing\n emission properties with the five hundred meter aperture spherical radio\n telescope: In this study, we report on a detailed single pulse analysis of the radio\nemission from a rotating radio transient (RRAT) J1918$-$0449 which is the first\nRRAT discovered with the five hundred meter aperture spherical radio telescope\n(FAST). The sensitive observations were carried out on 30 April 2021 using the\nFAST with a central frequency of 1250 MHz and a short time resolution of 49.152\n$\\mu$s, which forms a reliable basis to probe single pulse emission properties\nin detail. The source was successively observed for around 2 hours. A total of\n83 dispersed bursts with significance above 6$\\sigma$ are detected over 1.8\nhours. The source's DM and rotational period are determined to be 116.1$\\pm$0.4\n\\pcm \\ and 2479.21$\\pm$0.03 ms, respectively. The share of registered pulses\nfrom the total number of observed period is 3.12\\%. No underlying emission is\ndetected in the averaged off pulse profile. For bursts with fluence larger than\n10 Jy ms, the pulse energy follows a power-law distribution with an index of\n$-3.1\\pm0.4$, suggesting the existence of bright pulse emission. We find that\nthe distribution of time between subsequent pulses is consistent with a\nstationary Poisson process and find no evidence of clustering over the 1.8 h\nobservations, giving a mean burst rate of one burst every 66 s. Close\ninspection of the detected bright pulses reveals that 21 pulses exhibit\nwell-defined quasi-periodicities. The subpulse drifting is present in\nnon-successive rotations with periodicity of $2.51\\pm0.06$ periods. Finally,\npossible physical mechanisms are discussed.", "category": "astro-ph_HE" }, { "text": "UHECR: Signatures and Models: The signatures of Ultra High Energy (E >1 EeV) proton propagation through CMB\nradiation are pair-production dip and GZK cutoff. The visible characteristics\nof these two spectral features are ankle, which is intrinsic part of the dip,\nbeginning of GZK cutoff in the differential spectrum and E_{1/2} in integral\nspectrum. Measured by HiRes and Telescope Array (TA) these characteristics\nagree with theoretical predictions. However, directly measured mass composition\nremains a puzzle. While HiRes and TA detectors observe the proton dominated\nmass composition, the data of Auger detector strongly evidence for nuclei mass\ncomposition becoming progressively heavier at energy higher than 4 EeV and\nreaching Iron at energy about 35 EeV. The models based on the Auger and\nHiRes/TA data are considered independently and classified using the transition\nfrom galactic to extragalactic cosmic rays. The ankle cannot provide this\ntransition. since data of all three detector at energy (1 - 3) EeV agree with\npure proton composition (or at least not heavier than Helium). If produced in\nGalaxy these particles result in too high anisotropy. This argument excludes or\nstrongly disfavours all ankle models with ankle energy E_a > 3 EeV. The\ncalculation of elongation curves, X_{\\max}(E), for different ankle models\nstrengthens further this conclusion. Status of other models, the dip, mixed\ncomposition and Auger based models are discussed.", "category": "astro-ph_HE" }, { "text": "Effective potential energy in St\\ormer's problem for an inclined\n rotating magnetic dipole: We discuss the dynamics of a charged nonrelativistic particle in\nelectromagnetic field of a rotating magnetized celestial body. The equations of\nmotion of the particle are obtained and some particular solutions are found.\nEffective potential energy is defined on the base of the first constant of\nmotion. Regions accessible and inaccessible for a charged particle motion are\nstudied and depicted for different values of a constant of motion.", "category": "astro-ph_HE" }, { "text": "XMM-Newton and Swift observations of XTE J1743-363: XTEJ1743-363 is a poorly known hard X-ray transient, that displays short and\nintense flares similar to those observed from Supergiant Fast X-ray Transients.\nThe probable optical counterpart shows spectral properties similar to those of\nan M8 III giant, thus suggesting that XTEJ1743-363 belongs to the class of the\nSymbiotic X-ray Binaries. In this paper we report on the first dedicated\nmonitoring campaign of the source in the soft X-ray range with XMM-Newton and\nSwift/XRT. T hese observations confirmed the association of XTEJ1743-363 with\nthe previously suggested M8 III giant and the classification of the source as a\nmember of the Symbiotic X-ray binaries. In the soft X-ray domain, XTEJ1743-363\ndisplays a high absorption (~6x10^22 cm^-2 ) and variability on time scales of\nhundreds to few thousand seconds, typical of wind accreting systems. A\nrelatively faint flare (peak X-ray flux 3x10^-11 erg/cm^2/s) lasting ~4 ks is\nrecorded during the XMM-Newton observation and interpreted in terms of the wind\naccretion scenario.", "category": "astro-ph_HE" }, { "text": "Evidence for simultaneous jets and disk winds in luminous low-mass X-ray\n binaries: Recent work on jets and disk winds in low-mass X-ray binaries (LMXBs)\nsuggests that they are to a large extent mutually exclusive, with jets observed\nin spectrally hard states and disk winds observed in spectrally soft states. In\nthis paper we use existing literature on jets and disk winds in the luminous\nneutron star (NS) LMXB GX 13+1, in combination with archival Rossi X-ray Timing\nExplorer data, to show that this source is likely able to produce jets and disk\nwinds simultaneously. We find that jets and disk winds occur in the same\nlocation on the source's track in its X-ray color-color diagram. A further\nstudy of literature on other luminous LMXBs reveals that this behavior is more\ncommon, with indications for simultaneous jets and disk winds in the black hole\nLMXBs V404 Cyg and GRS 1915+105 and the NS LMXBs Sco X-1 and Cir X-1. For the\nthree sources for which we have the necessary spectral information, we find\nthat the simultaneous jets/winds all occur in their spectrally hardest states.\nOur findings indicate that in LMXBs with luminosities above a few tens of\npercent of the Eddington luminosity, jets and disk winds are not mutually\nexclusive, and that the presence of disk winds does not necessarily result in\njet suppression.", "category": "astro-ph_HE" }, { "text": "Gamma-Ray Bursts at high and very high energies: Gamma-Ray Bursts (GRBs) are extra-galactic and extremely energetic transient\nemissions of gamma rays, which are thought to be associated with the death of\nmassive stars or the merger of compact objects in binary systems. Their huge\nluminosities involve the presence a newborn stellar-mass black hole emitting a\nrelativistic collimated outflow, which accelerates particles and produces\nnon-thermal emissions from the radio domain to the highest energies. In this\narticle, I review recent progresses in the understanding of GRB jet physics\nabove 100 MeV, based on Fermi observations of bright GRBs. I discuss the\nphysical implications of these observations and their impact on GRB modeling,\nand I present some prospects for GRB observation at very high energies in the\nnear future.", "category": "astro-ph_HE" }, { "text": "Constraints and prospects on gravitational wave and neutrino emission\n using GW150914: The recent LIGO observation of gravitational waves from a binary black hole\nmerger triggered several follow-up searches from both electromagnetic wave as\nwell as neutrino observatories. Since in general, it is expected that all\nmatter has been removed from the binary black hole environment long before the\nmerger, no neutrino emission is expected from such mergers. Still, it remains\ninteresting to test this hypothesis. The ratio of the energy emitted in\nneutrinos with respect to gravitational waves represents a useful parameter to\nconstrain the environment of such astrophysical events. In addition to putting\nconstraints by use of the non-detection of counterpart neutrinos, it is also\npossible to consider the diffuse neutrino flux measured by the IceCube\nobservatory as the maximum contribution from an extrapolated full class of\nBBHs. Both methods currently lead to similar bounds on the fraction of energy\nthat can be emitted in neutrinos. Nevertheless, combining both methods should\nallow to strongly constrain the source population in case of a future neutrino\ncounterpart detection. The proposed approach can and will be applied to\npotential upcoming LIGO events, including binary neutron stars and black\nhole-neutron star mergers, for which a neutrino counterpart is expected.", "category": "astro-ph_HE" }, { "text": "Self Organized Criticality in an one dimensional magnetized grid.\n Application to GRB X-ray afterglows: A simplified one dimensional grid is used to model the evolution of\nmagnetized plasma flow. We implement diffusion laws similar to those so-far\nused to model magnetic reconnection with Cellular Automata. As a novelty, we\nalso explicitly superimpose a background flow. The aim is to numerically\ninvestigate the possibility that Self-Organized Criticality appears in a one\ndimensional magnetized flow. The cellular automaton's cells store information\nabout the parameter relevant to the evolution of the system being modelled.\nUnder the assumption that this parameter stands for the magnetic field, the\nmagnetic energy released by one volume during one individual relaxation event\nis also computed. Our results show that indeed in this system Self-Organized\nCriticality is established. The possible applications of this model to the\nstudy of the X-ray afterglows of GRBs is also briefly considered.", "category": "astro-ph_HE" }, { "text": "Fluid Instabilities of Magnetar-Powered Supernovae: Magnetar-powered supernova explosions are competitive models for explaining\nvery luminous optical transits. However, these explosion models were mainly\ncalculated in 1D. Radiation emitted from the magnetar snowplows into the\nprevious supernovae ejecta and causes a nonphysical dense shell (spike) found\nin previous 1D studies. This suggests that strong fluid instabilities may have\ndeveloped within the magnetar-powered supernovae. Such fluid instabilities\nemerge at the region where luminous transits later occur, so they can affect\nthe consequent observational signatures. We examine the magnetar-powered\nsupernovae with 2D hydrodynamics simulations and find that the 1D dense shell\ntransforms into the development of Rayleigh-Taylor and thin shell instabilities\nin 2D. The resulting mixing is able to fragment the entire shell and break the\nspherical symmetry of supernovae ejecta.", "category": "astro-ph_HE" }, { "text": "High-Energy gamma-ray Astronomy and String Theory: There have been observations, first from the MAGIC Telescope (July 2005) and\nquite recently (September 2008) from the FERMI Satellite Telescope, on\nnon-simultaneous arrival of high-energy photons from distant celestial sources.\nIn each case, the highest energy photons were delayed, as compared to their\nlower-energy counterparts. Although the astrophysics at the source of these\nenergetic photons is still not understood, and such non simultaneous arrival\nmight be due to non simultaneous emission as a result of conventional physics\neffects, nevertheless, rather surprisingly, the observed time delays can also\nfit excellently some scenarios in quantum gravity, predicting Lorentz violating\nspace-time \"foam\" backgrounds with a non-trivial subluminal vacuum refractive\nindex suppressed linearly by a quantum gravity scale of the order of the\nreduced Planck mass. In this pedagogical talk, I discuss the MAGIC and FERMI\nfindings in this context and I argue on a theoretical model of space-time foam\nin string/brane theory that can accommodate the findings of those experiments\nin agreement with all other stringent tests of Lorentz invariance. However, I\nstress the current ambiguities/uncertainties on the source mechanisms, which\nneed to be resolved first before definite conclusions are reached regarding\nquantum gravity foam scenarios.", "category": "astro-ph_HE" }, { "text": "Comptonization by Reconnection Plasmoids in Black Hole Coronae III:\n Dependence on the Guide Field in Pair Plasma: We perform two-dimensional particle-in-cell simulations of magnetic\nreconnection for various strengths of the guide field (perpendicular to the\nreversing field), in magnetically-dominated electron-positron plasmas. Magnetic\nreconnection under such conditions could operate in accretion disk coronae\naround black holes. There, it has been suggested that the trans-relativistic\nbulk motions of reconnection plasmoids containing inverse-Compton-cooled\nelectrons could Compton-upscatter soft photons to produce the observed\nnon-thermal hard X-rays. Our simulations are performed for magnetizations $3\n\\leq \\sigma \\leq 40$ (defined as the ratio of enthalpy density of the reversing\nfield to plasma enthalpy density) and guide field strengths $0 \\leq B_{\\rm\ng}/B_0 \\leq 1$ (normalized to the reversing field strength $B_0$). We find that\nthe mean bulk energy of the reconnected plasma depends only weakly on the flow\nmagnetization but strongly on the guide field strength -- with $B_{\\rm g}/B_0 =\n1$ yielding a mean bulk energy twice smaller than $B_{\\rm g}/B_0 = 0$.\nSimilarly, the dispersion of bulk motions around the mean -- a signature of\nstochasticity in the plasmoid chain's motions -- is weakly dependent on\nmagnetization (for $\\sigma \\gtrsim 10$) but strongly dependent on the guide\nfield strength -- dropping by more than a factor of two from $B_{\\rm g}/B_0 =\n0$ to $B_{\\rm g}/B_0 = 1$. In short, reconnection in strong guide fields\n($B_{\\rm g}/B_0 \\sim 1$) leads to slower and more ordered plasmoid bulk motions\nthan its weak guide field ($B_{\\rm g}/B_0 \\sim 0$) counterpart.", "category": "astro-ph_HE" }, { "text": "NGC 4388: A Test Case for Relativistic Disk Reflection and Fe K\n Fluorescence Features: We present a new analysis of the Suzaku X-ray spectrum of the Compton-thin\nSeyfert 2 galaxy NGC 4388. The spectrum above $\\sim$2 keV can be described by a\nremarkably simple and rather mundane model, consisting of a uniform, neutral\nspherical distribution of matter, with a radial column density of $2.58 \\pm\n0.02 \\times 10^{23}$ cm$^{-2}$, and an Fe abundance of\n$1.102^{+0.024}_{-0.021}$ relative to solar. The model does not require any\nphenomenological adjustments to self-consistently account for the low-energy\nextinction, the Fe K$\\alpha$ and Fe K$\\beta$ fluorescent emission lines, the Fe\nK edge, and the Compton-scattered continuum from the obscuring material. The\nspherical geometry is not a unique description, however, and the\nself-consistent, solar abundance MYTORUS model, applied with toroidal and\nnon-toroidal geometries, gives equally good descriptions of the data. In all\ncases, the key features of the spectrum are so tightly locked together that for\na wide range of parameters, a relativistic disk-reflection component\ncontributes no more than $\\sim$2% to the net spectrum in the 2-20 keV band. We\nshow that the commonly invoked explanations for weak X-ray reflection features,\nnamely a truncated and/or very highly ionized disk, do not work for NGC 4388.\nIf relativistically-broadened Fe K$\\alpha$ lines and reflection are ubiquitous\nin Seyfert 1 galaxies, they should also be ubiquitous in Compton-thin Seyfert 2\ngalaxies. The case of NGC 4388 shows the need for similar studies of more\nCompton-thin AGN to ascertain whether this is true.", "category": "astro-ph_HE" }, { "text": "Optical photometry and spectroscopy of the low-luminosity, broad-lined\n Ic supernova iPTF15dld: Core-collapse stripped-envelope supernova (SN) explosions reflect the\ndiversity of physical parameters and evolutionary paths of their massive star\nprogenitors. We have observed the type Ic SN iPTF15dld (z = 0.047), reported by\nthe Palomar Transient Factory. Spectra were taken starting 20 rest-frame days\nafter maximum luminosity and are affected by a young stellar population\nbackground. Broad spectral absorption lines associated with the SN are detected\nover the continuum, similar to those measured for broad-lined, highly energetic\nSNe Ic. The light curve and maximum luminosity are instead more similar to\nthose of low luminosity, narrow-lined Ic SNe. This suggests a behavior whereby\ncertain highly-stripped-envelope SNe do not produce a large amount of Ni56, but\nthe explosion is sufficiently energetic that a large fraction of the ejecta is\naccelerated to higher-than-usual velocities. We estimate SN iPTF15dld had a\nmain sequence progenitor of 20-25 Msun, produced a Ni56 mass of ~0.1-0.2 Msun,\nhad an ejecta mass of [2-10] Msun, and a kinetic energy of [1-18] e51 erg.", "category": "astro-ph_HE" }, { "text": "Spin down during quiescence of the fastest known accretion-powered\n pulsar: We present a timing solution for the 598.89 Hz accreting millisecond pulsar,\nIGR J00291+5934, using Rossi X-ray Timing Explorer data taken during the two\noutbursts exhibited by the source on 2008 August and September. We estimate the\nneutron star spin frequency and we refine the system orbital solution. To\nachieve the highest possible accuracy in the measurement of the spin frequency\nvariation experienced by the source in-between the 2008 August outburst and the\nlast outburst exhibited in 2004, we re-analysed the latter considering the\nwhole data set available. We find that the source spins down during quiescence\nat an average rate of {\\nu}dot_{sd}=(-4.1 +/- 1.2)E-15 Hz/s. We discuss\npossible scenarios that can account for the long-term neutron star spin-down in\nterms of either magneto-dipole emission, emission of gravitational waves, and a\npropeller effect. If interpreted in terms of magneto-dipole emission, the\nmeasured spin down translates into an upper limit to the neutron star magnetic\nfield, B<=3E+08 G, while an upper limit to the average neutron star mass\nquadrupole moment of Q<=2E+36 g cm^2 is set if the spin down is interpreted in\nterms of the emission of gravitational waves.", "category": "astro-ph_HE" }, { "text": "Multi-wavelength Photometry and Progenitor Analysis of the Nova V906 Car: We present optical and infrared photometry of the classical nova V906 Car,\nalso known as Nova Car 2018 and ASASSN-18fv, discovered by ASASS-SN survey on\n16.32 March 2018 UT (MJD 58193.0). The nova reached its maximum on MJD 58222.56\nat $V_{\\rm{max}} = 5.84 \\pm 0.09$ mag and had decline times of $t_{2,V} = 26.2\n$ d and $t_{3,V} = 33.0 $ d. The data from Evryscope shows that the nova had\nalready brightened to $g'\\simeq 13$\\,mag five days before discovery, as\ncompared to its quiescent magnitude of $g=$20.13$\\pm$0.03. The extinction\ntowards the nova, as derived from high resolution spectroscopy, shows an\nestimate consistent with foreground extinction to the Carina Nebula of $A_V =\n1.11_{-0.39}^{+0.54}$. The light curve resembles a rare C (cusp) class nova\nwith a steep decline slope of $\\alpha=-3.94$ post cusp flare. From the\nlightcurve decline rate, we estimate the mass of white dwarf to be $M_{WD}$ = $\n< 0.8$M\\textsubscript{\\(\\odot\\)}, consistent with $M_{WD}=0.71^{+0.23}_{-0.19}$\nderived from modelling the accretion disk of the system in quiescence. The\ndonor star is likely a K-M dwarf of 0.23-0.43\\,\\Msun, which is being heated by\nits companion.", "category": "astro-ph_HE" }, { "text": "Swift monitoring of the central X-ray source in RCW 103: The X-ray source 1E 161348-5055 lies at the centre of the 2-kyr-old supernova\nremnant RCW 103. Owing to its 24-ks modulation, orders-of-magnitude flux\nvariability over a few months/years, and lack of an obvious optical\ncounterpart, 1E 161348-5055 defies assignment to any known class of X-ray\nsources. Starting from April 2006, Swift observed 1E 161348-5055 with its X-ray\ntelescope for 2 ks approximately once per month. During the five years covered,\nthe source has remained in a quiescent state, with an average observed flux of\n1.7e-12 erg/cm^2/s (1-10 keV), about 20 times lower than the historical maximum\nattained in its 1999-2000 outburst. The long time-span of the Swift data allows\nus to obtain an accurate measure of the period of 1E 161348-5055 [P =\n24030.42(2) s] and to derive the first upper limit on its period derivative\n(|dP/dt| < 1.6e-9 s/s at 3 sigma).", "category": "astro-ph_HE" }, { "text": "Between the cosmic-ray `knee' and the `ankle': Contribution from star\n clusters: We show that massive young star clusters may be possible candidates that can\naccelerate Galactic cosmic rays (CRs) in the range of $10^7\\hbox{--}10^9$ GeV\n(between the `knee' and `ankle'). Various plausible scenarios such as\nacceleration at the wind termination shock (WTS), supernova shocks inside these\nyoung star clusters, etc. have been proposed,since it is difficult to\naccelerate particles up to the $10^7\\hbox{--}10^9$ GeV range in the standard\nparadigm of CR acceleration in supernova remnants. We consider a model for the\nproduction of different nuclei in CRs from massive stellar winds using the\nobserved distribution of young star clusters in the Galactic plane. We present\na detailed calculation of CR transport in the Galaxy, taking into account the\neffect of diffusion, interaction losses during propagation, and particle\nre-acceleration by old supernova remnants to determine the all-particle CR\nspectrum. Using the maximum energy estimate from the Hillas criterion, we argue\nthat a young massive star cluster can accelerate protons up to a few tens of\nPeV. Upon comparison with the observed data, our model requires a CR source\nspectrum with an exponential cutoff of $5\\times 10^7 Z$ GeV ($50\\,Z$~PeV) from\nthese clusters together with a cosmic-ray injection fraction of $\\sim 5\\%$ of\nthe wind kinetic energy. We discuss the possibility of achieving these\nrequirements in star clusters, and the associated uncertainties, in the context\nof considering star clusters as the natural accelerator of the `second\ncomponent' of Galactic cosmic rays.", "category": "astro-ph_HE" }, { "text": "Detection of X-ray flares from AX J1714.1-3912, the unidentified source\n near RX J1713.7-3946: Molecular clouds are predicted to emit nonthermal X-rays when they are close\nto particle-accelerating supernova remnants (SNRs), and the hard X-ray source\nAX J1714.1-3912, near the SNR RX J1713.7-3946, has long been considered a\ncandidate for diffuse nonthermal emission associated with cosmic rays diffusing\nfrom the remnant to a closeby molecular cloud. We aim at ascertaining the\nnature of this source by analyzing two dedicated X-ray observations performed\nwith Suzaku and Chandra. We extracted images from the data in various energy\nbands, spectra, and light curves and studied the long-term evolution of the\nX-ray emission on the basis of the ~4.5 yr time separation between the two\nobservations. We found that there is no diffuse emission associated with AX\nJ1714.1-3912, which is instead the point-like source CXOU J171343.9-391205. We\ndiscovered rapid time variability (timescale ~1 ks), together with a high\nintrinsic absorption and a hard nonthermal spectrum (power law with photon\nindex Gamma~1.4). We also found that the X-ray flux of the source drops down by\n1-2 orders of magnitude on a timescale of a few years. Our results suggest a\npossible association between AX J1714.1-3912 and a previously unknown\nsupergiant fast X-ray transient, although further follow-up observations are\nnecessary to prove this association definitively.", "category": "astro-ph_HE" }, { "text": "Neutrino-Driven Outflows and the Elemental Abundance Patterns of Very\n Metal-Poor Stars: The elemental abundances between strontium and silver ($Z = 38-47$) observed\nin the atmospheres of very metal-poor stars (VMP) in the Galaxy may contain the\nfingerprint of the weak $r$-process and $\\nu p$-process occurring in early\ncore-collapse supernovae explosions. In this work, we combine various\nastrophysical conditions based on a steady-state model to cover the richness of\nthe supernova ejecta in terms of entropy, expansion timescale, and electron\nfraction. The calculated abundances based on different combinations of\nconditions are compared with stellar observations with the aim of constraining\nsupernova ejecta conditions. We find that some conditions of the\nneutrino-driven outflows consistently reproduce the observed abundances of our\nsample. In addition, from the successful combinations, the neutron-rich\ntrajectories better reproduce the observed abundances of Sr-Zr ($Z= 38-40$),\nwhile the proton-rich ones, Mo-Pd ($Z= 42-47$).", "category": "astro-ph_HE" }, { "text": "Progenitors and Explosion Properties of Supernova Remnants Hosting\n Central Compact Objects: I. RCW 103 Associated with the Peculiar Source 1E\n 161348-5055: We present a Chandra and XMM-Newton imaging and spectroscopic study of the\nsupernova remnant (SNR) RCW 103 (G332.4-00.4) containing the Central Compact\nObject 1E 161348-5055. The high resolution Chandra X-ray images reveal enhanced\nemission in the south-eastern and north-western regions. Equivalent width line\nimages of Fe L, Mg, Si, and S using XMM-Newton data were used to map the\ndistribution of ejecta. The SNR was sectioned into 56 regions best\ncharacterized by two-component thermal models. The harder component (kT~0.6\nkeV) is adequately fitted by the VPSHOCK non-equilibrium ionization model with\nan ionization timescale ~ 1E11-1E12 cm^-3 s, and slightly enhanced abundances\nover solar values. The soft component (kT~0.2 keV), fitted by the APEC model,\nis well described by plasma in collisional ionization equilibrium with\nabundances consistent with solar values. Assuming a distance of 3.1 kpc and a\nSedov phase of expansion into a uniform medium, we estimate an SNR age of 4.4\nkyr, a swept-up mass M_sw=16 f_s^{-1/2} D_{3.1}^{5/2} solar masses, and a low\nexplosion energy E=3.7E49 f_s^{-1/2} D_{3.1}^{5/2} erg. This energy could be an\norder of magnitude higher if we relax the Sedov assumption, the plasma has a\nlow filling factor, the plasma temperature is under-estimated, or if the SNR is\nexpanding into the progenitor's wind-blown bubble. Standard explosion models\ndid not match the ejecta yields. By comparing the fitted abundances to the most\nrecent core-collapse nucleosynthesis models, our best estimate yields a\nlow-mass progenitor around 12-13 solar masses, lower than previously reported.\nWe discuss degeneracies in the model fitting, particularly the effect of\naltering the explosion energy on the progenitor mass estimate.", "category": "astro-ph_HE" }, { "text": "Inflows, Outflows, and a Giant Donor in the Remarkable Recurrent Nova\n M31N 2008-12a? - Hubble Space Telescope Photometry of the 2015 Eruption: The recurrent nova M31N 2008-12a experiences annual eruptions, contains a\nnear-Chandrasekhar mass white dwarf, and has the largest mass accretion rate in\nany nova system. In this paper, we present Hubble Space Telescope (HST)\nWFC3/UVIS photometry of the late decline of the 2015 eruption. We couple these\nnew data with archival HST observations of the quiescent system and Keck\nspectroscopy of the 2014 eruption. The late-time photometry reveals a rapid\ndecline to a minimum luminosity state, before a possible recovery /\nre-brightening in the run-up to the next eruption. Comparison with accretion\ndisk models supports the survival of the accretion disk during the eruptions,\nand uncovers a quiescent disk mass accretion rate of the order of\n$10^{-6}\\,M_\\odot\\,\\mathrm{yr}^{-1}$, which may rise beyond\n$10^{-5}\\,M_\\odot\\,\\mathrm{yr}^{-1}$ during the super-soft source phase - both\nof which could be problematic for a number of well-established nova eruption\nmodels. Such large accretion rates, close to the Eddington limit, might be\nexpected to be accompanied by additional mass loss from the disk through a wind\nand even collimated outflows. The archival HST observations, combined with the\ndisk modeling, provide the first constraints on the mass donor;\n$L_\\mathrm{donor}=103^{+12}_{-11}\\,L_\\odot$,\n$R_\\mathrm{donor}=14.14^{+0.46}_{-0.47}\\,R_\\odot$, and $T_\\mathrm{eff,\ndonor}=4890\\pm110$ K, which may be consistent with an irradiated M31 red-clump\nstar. Such a donor would require a system orbital period $\\gtrsim5$ days. Our\nupdated analysis predicts that the M31N 2008-12a WD could reach the\nChandrasekhar mass in < 20 kyr.", "category": "astro-ph_HE" }, { "text": "Simultaneous X-ray, gamma-ray, and Radio Observations of the repeating\n Fast Radio Burst FRB 121102: We undertook coordinated campaigns with the Green Bank, Effelsberg, and\nArecibo radio telescopes during Chandra X-ray Observatory and XMM-Newton\nobservations of the repeating fast radio burst FRB 121102 to search for\nsimultaneous radio and X-ray bursts. We find 12 radio bursts from FRB 121102\nduring 70 ks total of X-ray observations. We detect no X-ray photons at the\ntimes of radio bursts from FRB 121102 and further detect no X-ray bursts above\nthe measured background at any time. We place a 5$\\sigma$ upper limit of\n$3\\times10^{-11}$ erg cm$^{-2}$ on the 0.5--10 keV fluence for X-ray bursts at\nthe time of radio bursts for durations $<700$ ms, which corresponds to a burst\nenergy of $4\\times10^{45}$ erg at the measured distance of FRB 121102. We also\nplace limits on the 0.5--10 keV fluence of $5\\times10^{-10}$ erg cm$^{-2}$ and\n$1\\times10^{-9}$ erg cm$^{-2}$ for bursts emitted at any time during the\nXMM-Newton and Chandra observations, respectively, assuming a typical X-ray\nburst duration of 5 ms. We analyze data from the Fermi Gamma-ray Space\nTelescope Gamma-ray Burst Monitor and place a 5$\\sigma$ upper limit on the\n10--100 keV fluence of $4\\times10^{-9}$ erg cm$^{-2}$ ($5\\times10^{47}$ erg at\nthe distance of FRB 121102) for gamma-ray bursts at the time of radio bursts.\nWe also present a deep search for a persistent X-ray source using all of the\nX-ray observations taken to date and place a 5$\\sigma$ upper limit on the\n0.5--10 keV flux of $4\\times10^{-15}$ erg s$^{-1}$ cm$^{-2}$ ($3\\times10^{41}$\nerg~s$^{-1}$ at the distance of FRB 121102). We discuss these non-detections in\nthe context of the host environment of FRB 121102 and of possible sources of\nfast radio bursts in general.", "category": "astro-ph_HE" }, { "text": "MAGIC observations provide compelling evidence of the hadronic multi-TeV\n emission from the putative PeVatron SNR G106.3+2.7: The SNR G106.3+2.7, detected at 1--100 TeV energies by different $\\gamma$-ray\nfacilities, is one of the most promising PeVatron candidates. This SNR has a\ncometary shape which can be divided into a head and a tail region with\ndifferent physical conditions. However, it is not identified in which region\nthe 100 TeV emission is produced due to the limited position accuracy and/or\nangular resolution of existing observational data. Additionally, it remains\nunclear whether the origin of the $\\gamma$-ray emission is leptonic or\nhadronic. With the better angular resolution provided by these new MAGIC data\ncompared to earlier $\\gamma$-ray datasets, we aim to reveal the acceleration\nsite of PeV particles and the emission mechanism by resolving the SNR\nG106.3+2.7 with 0.1$^\\circ$ resolution at TeV energies. We detected extended\n$\\gamma$-ray emission spatially coincident with the radio continuum emission at\nthe head and tail of SNR G106.3+2.7. The fact that we detected a significant\n$\\gamma$-ray emission with energies above 6.0 TeV from the tail region only\nsuggests that the emissions above 10 TeV, detected with air shower experiments\n(Milagro, HAWC, Tibet AS$\\gamma$ and LHAASO), are emitted only from the SNR\ntail. Under this assumption, the multi-wavelength spectrum of the head region\ncan be explained with either hadronic or leptonic models, while the leptonic\nmodel for the tail region is in contradiction with the emission above 10 TeV\nand X-rays. In contrast, the hadronic model could reproduce the observed\nspectrum at the tail by assuming a proton spectrum with a cutoff energy of\n$\\sim 1$ PeV for the tail region. Such a high energy emission in this\nmiddle-aged SNR (4--10 kyr) can be explained by considering the scenario that\nprotons escaping from the SNR in the past interact with surrounding dense gases\nat present.", "category": "astro-ph_HE" }, { "text": "A JWST Near- and Mid-Infrared Nebular Spectrum of the Type Ia Supernova\n 2021aefx: We present JWST near- and mid-infrared spectroscopic observations of the\nnearby normal Type Ia supernova SN 2021aefx in the nebular phase at $+255$ days\npast maximum light. Our Near Infrared Spectrograph (NIRSpec) and Mid Infrared\nInstrument (MIRI) observations, combined with ground-based optical data from\nthe South African Large Telescope (SALT), constitute the first complete optical\n$+$ NIR $+$ MIR nebular SN Ia spectrum covering 0.3$-$14 $\\mu$m. This spectrum\nunveils the previously unobserved 2.5$-$5 $\\mu$m region, revealing strong\nnebular iron and stable nickel emission, indicative of high-density burning\nthat can constrain the progenitor mass. The data show a significant improvement\nin sensitivity and resolution compared to previous Spitzer MIR data. We\nidentify numerous NIR and MIR nebular emission lines from iron-group elements\nand as well as lines from the intermediate-mass element argon. The argon lines\nextend to higher velocities than the iron-group elements, suggesting stratified\nejecta that are a hallmark of delayed-detonation or double-detonation SN Ia\nmodels. We present fits to simple geometric line profiles to features beyond\n1.2 $\\mu$m and find that most lines are consistent with Gaussian or spherical\nemission distributions, while the [Ar III] 8.99 $\\mu$m line has a distinctively\nflat-topped profile indicating a thick spherical shell of emission. Using our\nline profile fits, we investigate the emissivity structure of SN 2021aefx and\nmeasure kinematic properties. Continued observations of SN 2021aefx and other\nSNe Ia with JWST will be transformative to the study of SN Ia composition,\nionization structure, density, and temperature, and will provide important\nconstraints on SN Ia progenitor and explosion models.", "category": "astro-ph_HE" }, { "text": "The Varying Kinematics of Multiple Ejecta from the Black Hole X-ray\n Binary MAXI J1820+070: During a 2018 outburst, the black hole X-ray binary MAXI J1820+070 was\ncomprehensively monitored at multiple wavelengths as it underwent a hard to\nsoft state transition. During this transition a rapid evolution in X-ray timing\nproperties and a short-lived radio flare were observed, both of which were\nlinked to the launching of bi-polar, long-lived relativistic ejecta. We provide\ndetailed analysis of two Very Long Baseline Array observations, using both time\nbinning and a new dynamic phase centre tracking technique to mitigate the\neffects of smearing when observing fast-moving ejecta at high angular\nresolution. We identify a second, earlier ejection, with a lower proper motion\nof $18.0\\pm1.1$ mas day$^{-1}$. This new jet knot was ejected $4\\pm1$ hours\nbefore the beginning of the rise of the radio flare, and $2\\pm1$ hours before a\nswitch from type-C to type-B X-ray quasi-periodic oscillations (QPOs). We show\nthat this jet was ejected over a period of $\\sim6$ hours and thus its ejection\nwas contemporaneous with the QPO transition. Our new technique locates the\noriginal, faster ejection in an observation in which it was previously\nundetected. With this detection we revised the fits to the proper motions of\nthe ejecta and calculated a jet inclination angle of $(64\\pm5)^\\circ$, and jet\nvelocities of $0.97_{-0.09}^{+0.03}c$ for the fast-moving ejecta ($\\Gamma>2.1$)\nand $(0.30\\pm0.05)c$ for the newly-identified slow-moving ejection\n($\\Gamma=1.05\\pm0.02$). We show that the approaching slow-moving component is\npredominantly responsible for the radio flare, and is likely linked to the\nswitch from type-C to type-B QPOs, while no definitive signature of ejection\nwas identified for the fast-moving ejecta.", "category": "astro-ph_HE" }, { "text": "Predicting Short-duration GRB Rates in the Advanced LIGO Volume: Starting with models for the compact object merger event rate, the\nshort-duration Gamma-ray Burst (sGRB) luminosity function, and the Swift/BAT\ndetector, we calculate the observed Swift sGRB rate and its uncertainty. Our\nprobabilistic sGRB world model reproduces the observed number distributions in\nredshift and flux for 123 Swift/BAT detected sGRBs and can be used to predict\njoint sGRB/LIGO detection rates. We discuss the dependence of the rate\npredictions on the model parameters and explore how they vary with increasing\nexperimental sensitivity. In particular, the number of bursts in the LIGO\nvolume depends strongly on the parameters that govern sGRB beaming. Our results\nsuggest that nearby sGRBs should be observed to have broader jets on average\n($\\theta_{\\rm jet}\\gtrsim 30$ degrees), as compared to the narrowly-beamed\nappearance of cosmological sGRBs due to detection selection effect driving\nobserved jet angle. Assuming all sGRBs are due to compact object mergers,\nwithin a $D < 200$ Mpc aLIGO volume, we predict $0.18^{+0.19}_{-0.08}$ sGRB/GW\nassociations all-sky per year for on-axis events at Swift sensitivities,\nincreasing to $1.2^{+1.9}_{-0.6}$ with the inclusion of off-axis events. We\nexplore the consistency of our model with GW170817/GRB~170817A in the context\nof structured jets. Predictions for future experiments are made.", "category": "astro-ph_HE" }, { "text": "The unreasonable weakness of r-process cosmic rays in the\n neutron-star-merger nucleosynthesis scenario: We reach the robust conclusion that, by combining the observed cosmic rays of\nr-process elements with the fact that the velocity of the neutron-star-merger\nejecta is much higher than that of the supernova ejecta, either (1) the reverse\nshock in the neutron-star-merger ejecta is a very inefficient accelerator that\nconverts less than 0.003% of the ejecta kinetic energy to the cosmic-ray energy\nor (2) the neutron star merger is not the origin of the Galactic r-process\nelements. We also find that the acceleration efficiency should be less than\n0.1% for the reverse shock of the supernova ejecta with the observed cosmic\nrays lighter than the iron.", "category": "astro-ph_HE" }, { "text": "Formation of Lower Mass-gap Black Hole--Neutron Star Binary Mergers\n through Super-Eddington Stable Mass Transfer: Super-Eddington accretion of neutron stars (NSs) has been suggested both\nobservationally and theoretically. In this paper, we propose that NSs in\nclose-orbit binary systems with companions of helium (He) stars, most of which\nsystems form after the common-envelope phase, could experience super-Eddington\nstable Case BB/BC mass transfer (MT), and can sometimes occur accretion-induced\ncollapses (AICs) to form lower mass-gap black holes (mgBHs). Our detailed\nbinary evolution simulations reveal that AIC events tend to happen if the\nprimaries NS have an initial mass $\\gtrsim1.7\\,M_\\odot$ with an accretion rate\nof $\\gtrsim300$ times the Eddington limit. These mgBHs would have a mass nearly\nequal to or slightly higher than the NS maximum mass. The remnant mgBH--NS\nbinaries after the core collapses of He stars are potential progenitors of\ngravitational-wave (GW) source. Multimessenger observation between GW and\nkilonova signals from a population of high-mass binary NS and mgBH--NS mergers\nformed through super-Eddington stable MT are helpful in constraining the\nmaximum mass and equation of state of NSs. S230529ay, a mgBH--NS merger\ncandidate recently detected in the fourth observing run of the LIGO-Virgo-KAGRA\nCollaboration, could possibly originate from this formation scenario.", "category": "astro-ph_HE" }, { "text": "Long-term multi-wavelength study of 1ES 0647+250: The BL Lac object 1ES 0647+250 is one of the few distant $\\gamma$-ray\nemitting blazars detected at very high energies (VHE, $\\gtrsim$100 GeV) during\na non-flaring state. It was detected with the MAGIC telescopes during its low\nactivity in the years 2009-2011, as well as during three flaring activities in\nthe years 2014, 2019 and 2020, with the highest VHE flux in the latter epoch.\nAn extensive multi-instrument data set was collected within several coordinated\nobserving campaigns throughout these years. We aim to characterise the\nlong-term multi-band flux variability of 1ES 0647+250, as well as its broadband\nspectral energy distribution (SED) during four distinct activity states\nselected in four different epochs, in order to constrain the physical\nparameters of the blazar emission region under certain assumptions. We evaluate\nthe variability and correlation of the emission in the different energy bands\nwith the fractional variability and the Z-transformed Discrete Correlation\nFunction, as well as its spectral evolution in X-rays and $\\gamma$ rays. Owing\nto the controversy in the redshift measurements of 1ES 0647+250 reported in the\nliterature, we also estimate its distance in an indirect manner through the\ncomparison of the GeV and TeV spectra from simultaneous observations with\nFermi-LAT and MAGIC during the strongest flaring activity detected to date.\nMoreover, we interpret the SEDs from the four distinct activity states within\nthe framework of one-component and two-component leptonic models, proposing\nspecific scenarios that are able to reproduce the available multi-instrument\ndata.", "category": "astro-ph_HE" }, { "text": "Energetic Gamma Radiation from Rapidly Rotating Black Holes: Supermassive black holes are believed to be the central power house of active\ngalactic nuclei. Applying the pulsar outer-magnetospheric particle accelerator\ntheory to black-hole magnetospheres, we demonstrate that an electric field is\nexerted along the magnetic field lines near the event horizon of a rotating\nblack hole. In this particle accelerator (or a gap), electrons and positrons\nare created by photon-photon collisions and accelerated in the opposite\ndirections by this electric field, efficiently emitting gamma-rays via\ncurvature and inverse-Compton processes. It is shown that a gap arises around\nthe null charge surface formed by the frame-dragging effect, provided that\nthere is no current injection across the gap boundaries. The gap is dissipating\na part of the hole's rotational energy, and the resultant gamma-ray luminosity\nincreases with decreasing plasma accretion from the surroundings. Considering\nan extremely rotating supermassive black hole, we show that such a gap\nreproduces the significant very-high-energy (VHE) gamma-ray flux observed from\nthe radio galaxy IC 310, provided that the accretion rate becomes much less\nthan the Eddington rate particularly during its flare phase. It is found that\nthe curvature process dominates the inverse-Compton process in the\nmagnetosphere of IC~310, and that the observed power-law-like spectrum in VHE\ngamma-rays can be explained to some extent by a superposition of the curvature\nemissions with varying curvature radius. It is predicted that the VHE spectrum\nextends into higher energies with increasing VHE photon flux.", "category": "astro-ph_HE" }, { "text": "Magnetar as the Central Engine of AT2018cow: Optical, Soft X-Ray, and\n Hard X-Ray Emission: AT2018cow is the most extensively observed and widely studied fast blue\noptical transient to date; its unique observational properties challenge all\nexisting standard models. In this paper, we model the luminosity evolution of\nthe optical, soft X-ray, and hard X-ray emission, as well as the X-ray spectrum\nof AT2018cow with a magnetar-centered engine model. We consider a two-zone\nmodel with a striped magnetar wind in the interior and an expanding ejecta\noutside. The soft and hard X-ray emission of AT2018cow can be explained by the\nleakage of high-energy photons produced by internal gradual magnetic\ndissipation in the striped magnetar wind, while the luminous thermal UV/optical\nemission results from the thermalization of the ejecta by the captured photons.\nThe two-component energy spectrum yielded by our model with a quasi-thermal\ncomponent from the optically thick region of the wind superimposed on an\noptically thin synchrotron component well reproduces the X-ray spectral shape\nof AT2018cow. The Markov Chain Monte Carlo fitting results suggest that in\norder to explain the very short rise time to peak of the thermal optical\nemission, a low ejecta mass $M_{\\rm ej}\\approx0.1~M_\\odot$ and high ejecta\nvelocity $v_{\\rm SN}\\approx0.17c$ are required. A millisecond magnetar with\n$P_0\\approx3.7~\\rm ms$ and $B_p\\approx2.4\\times10^{14}~\\rm G$ is needed to\nserve as the central engine of AT2018cow.", "category": "astro-ph_HE" }, { "text": "K-shell photoionization of Nickel ions using R-matrix: We present R-matrix calculations of photoabsorption and photoionization cross\nsections across the K edge of the Li-like to Ca-like ions stages of Ni.\nLevel-resolved, Breit-Pauli calculations were performed for the Li-like to\nNa-like stages. Term-resolved calculations, which include the mass-velocity and\nDarwin relativistic corrections, were performed for the Mg-like to Ca-like ion\nstages. This data set is extended up to Fe-like Ni using the distorted wave\napproximation as implemented by AUTOSTRUCTURE. The R-matrix calculations\ninclude the effects of radiative and Auger dampings by means of an optical\npotential. The damping processes affect the absorption resonances converging to\nthe K thresholds causing them to display symmetric profiles of constant width\nthat smear the otherwise sharp edge at the K-shell photoionization threshold.\nThese data are important for the modeling of features found in photoionized\nplasmas.", "category": "astro-ph_HE" }, { "text": "Chandra observations of NGC4342, an optically faint, X-ray gas-rich\n early-type galaxy: Chandra X-ray observations of NGC4342, a low stellar mass (M_K=-22.79 mag)\nearly-type galaxy, show luminous, diffuse X-ray emission originating from hot\ngas with temperature of kT~0.6 keV. The observed 0.5-2 keV band luminosity of\nthe diffuse X-ray emission within the D_25 ellipse is L_0.5-2keV = 2.7 x 10^39\nerg/s. The hot gas has a significantly broader distribution than the stellar\nlight, and shows strong hydrodynamic disturbances with a sharp surface\nbrightness edge to the northeast and a trailing tail. We identify the edge as a\ncold front and conclude that the distorted morphology of the hot gas is\nproduced by ram pressure as NGC4342 moves through external gas. From the\nthermal pressure ratios inside and outside the cold front, we estimate the\nvelocity of NGC4342 and find that it moves supersonically (M~2.6) towards the\nnortheast. Outside the optical extent of the galaxy we detect ~17 bright\n(L_0.5-8keV > 3 x 10^37 erg/s) excess X-ray point sources. The excess sources\nare presumably low-mass X-ray binaries (LMXBs) located in metal-poor globular\nclusters (GCs) in the extended dark matter halo of NGC4342. Based on the number\nof excess sources and the average frequency of bright LMXBs in GCs, we estimate\nthat NGC4342 may host roughly 850-1700 GCs. In good agreement with this,\noptical observations hint that NGC4342 may harbor 1200 +/- 500 GCs. This number\ncorresponds to a GC specific frequency of S_N = 19.9 +/- 8.3, which is among\nthe largest values observed in full-size galaxies.", "category": "astro-ph_HE" }, { "text": "Gamma-Ray Burst long lasting X-ray flaring activity: In this paper we shed light on late time (i.e. with peak time t_{pk} \\gtrsim\n1000 s) flaring activity. We address the morphology and energetic of flares in\nthe window \\sim 10^3-10^6 s to put constraints on the temporal evolution of the\nflare properties and to identify possible differences in the mechanism\nproducing the early and late time flaring emission, if any. This requires the\ncomplete understanding of the observational biases affecting the detection of\nX-ray flares superimposed on a fading continuum at t > 1000 s. We consider all\nthe Swift GRBs that exhibit late time flares. Our sample consists of 36 flares,\n14 with redshift measurements. We inherit the strategy of data analysis from\nChincarini et al. (2010) in order to make a direct comparison with the early\ntime flare properties. The morphology of the flare light curve is the same for\nboth early time and late time flares, while they differ energetically. The\nwidth of late time flares increases with time similarly to the early time\nflares. Simulations confirmed that the increase of the width with time is not\ndue to the decaying statistics, at least up to 10^4 s. The energy output of\nlate time flares is one order of magnitude lower than the early time flare one,\nbeing \\sim 1% E_{prompt}. The evolution of the peak luminosity as well as the\ndistribution of the peak flux-to-continuum ratio for late time flares indicate\nthat the flaring emission is decoupled from the underlying continuum,\ndifferently from early time flares/steep decay. A sizable fraction of late time\nflares are compatible with afterglow variability. The internal shock origin\nseems the most promising explanation for flares. However, some differences that\nemerge between late and early time flares suggest that there could be no unique\nexplanation about the nature of late time flares.", "category": "astro-ph_HE" }, { "text": "On the Geometry of Curvature Radiation and Implications for Subpulse\n Drifting: The phenomenon of subpulse drifting offers unique insights into the emission\ngeometry of pulsars, and is commonly interpreted in terms of a rotating\ncarousel of \"spark\" events near the stellar surface. We develop a detailed\ngeometric model for the emission columns above a carousel of sparks that is\nentirely calculated in the observer's inertial frame, and which is consistent\nwith the well-understood rotational effects of aberration and retardation. We\nexplore the observational consequences of the model, including (1) the\nappearance of the reconstructed beam pattern via the cartographic transform and\n(2) the morphology of drift bands and how they might evolve as a function of\nfrequency. The model, which is implemented in the software package PSRGEOM, is\napplicable to a wide range of viewing geometries, and we illustrate its\nimplications using PSRs B0809+74 and B2034+19 as examples. Some specific\npredictions are made with respect to the difference between subpulse evolution\nand microstructure evolution, which provides a way to further test our model.", "category": "astro-ph_HE" }, { "text": "Low Mass X-ray Binary Simulation Data Release: We present DABS (Database of Accreting Binary Simulations), an open-access\ndatabase of modelled Low Mass X-ray Binaries (LMXBs). DABS has been created\nusing evolutionary tracks of neutron star and black hole LMXBs, spanning a\nlarge set of initial conditions for the accretor mass, donor mass, and orbital\nperiod. The LMXBs are evolved with the Convection and Rotation Boosted Magnetic\nBraking prescription. The most important asset of this online database is the\ntool PEAS (Progenitor Extractor for Accreting Systems). This tool can be used\nto predict the progenitors of any user-entered LMXB system and view their\nproperties before the start of mass transfer. This prediction can facilitate\npreliminary searches for the progenitors of observed LMXBs, which can help in\nstreamlining further detailed analyses. The PEAS tool can also be used to\nconstrain population synthesis techniques that specialize in supernova kicks in\nbinaries and common envelope outcomes.", "category": "astro-ph_HE" }, { "text": "Electromagnetic Helicity in Classical Physics: This pedagogical note revisits the concept of electromagnetic helicity in\nclassical systems. In particular, magnetic helicity and its role in mean field\ndynamo theories is briefly discussed highlighting the major mathematical\ninconsistency in most of these theories---violation of magnetic helicity\nconservation. A short review of kinematic dynamo theory and its classic\ntheorems is also presented in the Appendix.", "category": "astro-ph_HE" }, { "text": "On the relation between hard X-ray photon index versus accretion rate\n for super-Eddington accreting quasars: We investigate whether the hard X-ray photon index (${\\Gamma}$) versus\naccretion rate correlation for super-Eddington accreting quasars is different\nfrom that for sub-Eddington accreting quasars. We construct a sample of 113\nbright quasars from the Sloan Digital Sky Survey Data Release 14 quasar\ncatalog, including 38 quasars as the super-Eddington subsample and 75 quasars\nas the sub-Eddington subsample. We derive black-hole masses using a\nsimple-epoch virial mass formula based on the ${\\rm H\\beta}$ lines, and we use\nthe standard thin disk model to derive the dimensionless accretion rates\n($\\dot{\\mathscr{M}}$) for our sample. The X-ray data for these quasars are\ncollected from the Chandra and XMM-Newton archives. We fit the hard X-ray\nspectra using a single power-law model to obtain ${\\Gamma}$ values. We find a\nstatistically significant ($R_{\\rm S}=0.43$, $p=7.75\\times{10}^{-3}$)\ncorrelation between ${\\Gamma}$ and $\\dot{\\mathscr{M}}$ for the super-Eddington\nsubsample. The ${\\Gamma}$-$\\dot{\\mathscr{M}}$ correlation for the sub-Eddington\nsubsample is also significant, but weaker ($R_{\\rm S}=0.30$,\n$p=9.98\\times{10}^{-3}$). Linear regression analysis shows that ${\\rm\n\\Gamma}=(0.34\\pm0.11){\\rm log}{\\dot{\\mathscr{M}}}+(1.71\\pm0.17)$ and ${\\rm\n\\Gamma}=(0.09\\pm0.04){\\rm log}{\\dot{\\mathscr{M}}}+(1.93\\pm0.04)$ for the super-\nand sub-Eddington subsamples, respectively. The ${\\Gamma}$-$\\dot{\\mathscr{M}}$\ncorrelations of the two subsamples are different, suggesting different\ndisk-corona connections in these two types of systems. We propose one\nqualitative explanation of the steeper ${\\Gamma}$-$\\dot{\\mathscr{M}}$\ncorrelation in the super-Eddington regime that involves larger seed photon\nfluxes received by the compact coronae from the thick disks in super-Eddington\naccreting quasars.", "category": "astro-ph_HE" }, { "text": "XMM-Newton and Swift Observations of the Seyfert 1 AGN NGC 5940: Probing the physics of the accretion flow around active galactic nuclei (AGN)\nis crucial to understanding their emission mechanisms as well as being able to\nconstrain the geometrical and variability properties of the different regions\naround them. The soft X-ray excess -- usually observed below\n$\\sim2\\,\\mathrm{keV}$ in excess of the dominant X-ray powerlaw continuum -- is\none prominent feature that is commonly seen in type 1 Seyfert AGN and therefore\nreadily provides a useful diagnostic of the accretion flow mechanism around\nthese systems. NGC 5940 is a Seyfert 1 AGN which reveals strong, prominent soft\nX-ray excess below $\\sim2\\,\\mathrm{keV}$ as seen in both its XMM-Newton and\nSwift observations. Model fit to the data revealed that this feature could be\nequally well explained by the ionised partial covering, the thermal\nComptonisation and the blurred reflection models. Although the other models\ncannot be decisively ruled out with the data at hand, the lack of significant\nbroad iron $K_{\\alpha}$ as well as any significant emission/absorption line\nfeatures in the reflection grating spectrometer (RGS) data tend to favour the\nthermal Comptonisation origin for the soft X-ray excess in NGC 5940.", "category": "astro-ph_HE" }, { "text": "First observation of MeV gamma-ray universe with bijective imaging\n spectroscopy using the Electron-Tracking Compton Telescope aboard SMILE-2+: MeV gamma-rays provide a unique window for the direct measurement of line\nemissions from radioisotopes, but observations have made little significant\nprogress after COMPTEL/{\\it CGRO}. To observe celestial objects in this band,\nwe are developing an electron-tracking Compton camera (ETCC), which realizes\nboth bijective imaging spectroscopy and efficient background reduction gleaned\nfrom the recoil electron track information. The energy spectrum of the\nobservation target can then be obtained by a simple ON-OFF method using a\ncorrectly defined point spread function on the celestial sphere. The\nperformance of celestial object observations was validated on the second\nballoon SMILE-2+ installed with an ETCC having a gaseous electron tracker with\na volume of 30$\\times$30$\\times$30 cm$^3$. Gamma-rays from the Crab nebula were\ndetected with a significance of 4.0$\\sigma$ in the energy range 0.15--2.1 MeV\nwith a live time of 5.1 h, as expected before launching. Additionally, the\nlight curve clarified an enhancement of gamma-ray events generated in the\nGalactic center region, indicating that a significant proportion of the final\nremaining events are cosmic gamma rays. Independently, the observed intensity\nand time variation were consistent with the pre-launch estimates except in the\nGalactic center region. The estimates were based on the total background of\nextragalactic diffuse, atmospheric, and instrumental gamma-rays after\naccounting for the variations in the atmospheric depth and rigidity during the\nlevel flight. The Crab results and light curve strongly support our\nunderstanding of both the detection sensitivity and the background in real\nobservations. This work promises significant advances in MeV gamma-ray\nastronomy.", "category": "astro-ph_HE" }, { "text": "Feature Extraction on Synthetic Black Hole Images: The Event Horizon Telescope (EHT) recently released the first horizon-scale\nimages of the black hole in M87. Combined with other astronomical data, these\nimages constrain the mass and spin of the hole as well as the accretion rate\nand magnetic flux trapped on the hole. An important question for EHT is how\nwell key parameters such as spin and trapped magnetic flux can be extracted\nfrom present and future EHT data alone. Here we explore parameter extraction\nusing a neural network trained on high resolution synthetic images drawn from\nstate-of-the-art simulations. We find that the neural network is able to\nrecover spin and flux with high accuracy. We are particularly interested in\ninterpreting the neural network output and understanding which features are\nused to identify, e.g., black hole spin. Using feature maps, we find that the\nnetwork keys on low surface brightness features in particular.", "category": "astro-ph_HE" }, { "text": "The High Energy X-ray Probe (HEX-P): A New Window into Neutron Star\n Accretion: Accreting neutron stars (NSs) represent a unique laboratory for probing the\nphysics of accretion in the presence of strong magnetic fields ($B\\gtrsim 10^8$\nG). Additionally, the matter inside the NS itself exists in an ultra-dense,\ncold state that cannot be reproduced in Earth-based laboratories. Hence,\nobservational studies of these objects are a way to probe the most extreme\nphysical regimes. Here we present an overview of the field and discuss the most\nimportant outstanding problems related to NS accretion. We show how these open\nquestions regarding accreting NSs in both low-mass and high-mass X-ray binary\nsystems can be addressed with the High-Energy X-ray Probe (HEX-P) via simulated\ndata. In particular, with the broad X-ray passband and improved sensitivity\nafforded by a low X-ray background, HEX-P will be able to 1) distinguish\nbetween competing continuum emission models; 2) provide tighter upper limits on\nNS radii via reflection modeling techniques that are independent and\ncomplementary to other existing methods; 3) constrain magnetic field geometry,\nplasma parameters, and accretion column emission patterns by characterizing\nfundamental and harmonic cyclotron lines and exploring their behavior with\npulse phase; 4) directly measure the surface magnetic field strength of highly\nmagnetized NSs at the lowest accretion luminosities; as well as 5) detect\ncyclotron line features in extragalactic sources and probe their dependence on\nluminosity in the super-Eddington regime in order to distinguish between\ngeometrical evolution and accretion-induced decay of the magnetic field. In\nthese ways HEX-P will provide an essential new tool for exploring the physics\nof NSs, their magnetic fields, and the physics of extreme accretion.", "category": "astro-ph_HE" }, { "text": "Cooling of neutron stars and hybrid stars with a stiff hadronic EoS: Within the \"nuclear medium cooling\" scenario of neutron stars all reliably\nknown temperature - age data, including those of the central compact objects in\nthe supernova remnants of Cassiopeia A and XMMU-J1732, can be comfortably\nexplained by a set of cooling curves obtained by variation of the star mass\nwithin the range of typical observed masses. The recent measurements of the\nhigh masses of the pulsars PSR J1614-2230 and PSR J0348-0432 on the one hand,\nand of the low masses for PSR J0737-3039B and the companion of PSR J1756-2251\non the other, provide independent proof for the existence of neutron stars with\nmasses in a broad range from $\\sim 1.2$ to 2 $M_\\odot$. The values $M>2\nM_{\\odot}$ call for sufficiently stiff equations of state for neutron star\nmatter. We investigate the response of the set of neutron star cooling curves\nto a stiffening of the nuclear equation of state so that maximum masses of\nabout $2.4 M_\\odot$ would be accessible and to a deconfinement phase transition\nfrom such stiff nuclear matter in the outer core to color superconducting quark\nmatter in the inner core. Without readjustment of cooling inputs the mass range\nrequired to cover all cooling data for the stiff DD2 equation of state should\ninclude masses of $2.426 M_\\odot$ for describing the fast cooling of CasA while\nthe existence of a quark matter core accelerates the cooling so that CasA\ncooling data are described with a hybrid star of mass $1.674 M_\\odot$.", "category": "astro-ph_HE" }, { "text": "Delving Into X-ray Obscuration of Type 2 AGN, Near and Far: Using self-consistent, physically motivated models, we investigate the X-ray\nobscuration in 19 Type 2 [OIII] 5007 \\AA\\ selected AGN, 9 of which are local\nSeyfert 2 galaxies and 10 of which are Type 2 quasar candidates. We derive\nreliable line-of-sight and global column densities for these objects, which is\nthe first time this has been reported for an AGN sample; 4 AGN have\nsignificantly different global and line-of-sight column densities. Five sources\nare heavily obscured to Compton-thick. We comment on interesting sources\nrevealed by our spectral modeling, including a candidate ``naked'' Sy2. After\ncorrecting for absorption, we find that the ratio of the rest-frame, 2-10 keV\nluminosity (L$_{\\rm 2-10keV,in}$) to L$_{\\rm [OIII]}$ is 1.54 $\\pm$ 0.49 dex\nwhich is essentially identical to the mean Type 1 AGN value. The Fe K$\\alpha$\nluminosity is significantly correlated with L$_{\\rm [OIII]}$, but with\nsubstantial scatter. Finally, we do not find a trend between L$_{\\rm\n2-10keV,in}$ and global or line-of-sight column density, between column density\nand redshift, between column density and scattering fraction or between\nscattering fraction and redshift.", "category": "astro-ph_HE" }, { "text": "Giant Primeval Magnetic Dipoles: Macroscopic magnetic dipoles are considered as cosmic dark matter. Permanent\nmagnetism in relativistic field structures can involve some form of\nsuperconductivity, one example being current-carrying string loops (`springs')\nwith vanishing net tension. We derive the cross section for free classical\ndipoles to collide, finding it depends weakly on orientation when mutual\nprecession is rapid. The collision rate of `spring' loops with tension ${\\cal\nT} \\sim 10^{-8}c^4/G$ in galactic halos approaches the measured rate of fast\nradio bursts (FRBs) if the loops comprise most of the dark matter. A large\nsuperconducting dipole (LSD) with mass $\\sim 10^{20}$ g and size $\\sim 1$ mm\nwill form a $\\sim 100$ km magnetosphere moving through interstellar plasma.\nAlthough hydromagnetic drag is generally weak, it is strong enough to capture\nsome LSDs into long-lived rings orbiting supermassive black holes (SMBHs) that\nform by the direct collapse of massive gas clouds. Repeated collisions near\nyoung SMBHs could dominate the global collision rate, thereby broadening the\ndipole mass spectrum. Colliding LSDs produce tiny, hot electromagnetic\nexplosions. The accompanying paper shows that these explosions couple\neffectively to propagating low-frequency electromagnetic modes, with output\npeaking at 0.01-1 THz. We describe several constraints on, and predictions of,\nLSDs as cosmic dark matter. The shock formed by an infalling LSD triggers\nself-sustained thermonuclear burning in a C/O (ONeMg) white dwarf (WD) of mass\n$\\gtrsim 1\\,M_\\odot$ ($1.3\\,M_\\odot$). The spark is generally located well off\nthe center of the WD. The rate of LSD-induced explosions matches the observed\nrate of Type Ia supernovae.", "category": "astro-ph_HE" }, { "text": "Penetration of cosmic rays into dense molecular clouds: role of diffuse\n envelope: A flux of cosmic rays (CRs) propagating through a diffuse ionized gas can\nexcite MHD waves, thus generating magnetic disturbances. We propose a generic\nmodel of CR penetration into molecular clouds through their diffuse envelopes,\nand identify the leading physical processes controlling their transport on the\nway from a highly ionized interstellar medium to a dense interior of the cloud.\nThe model allows us to describe a transition between a free streaming of CRs\nand their diffusive propagation, determined by the scattering on the\nself-generated disturbances. A self consistent set of equations, governing the\ndiffusive transport regime in an envelope and the MHD turbulence generated by\nthe modulated CR flux, is essentially characterized by two dimensionless\nnumbers. We demonstrate a remarkable mutual complementarity of different\nmechanisms leading to the onset of the diffusive regime, which results in a\nuniversal energy spectrum of the modulated CRs. In conclusion, we briefly\ndiscuss implications of our results for several fundamental astrophysical\nproblems, such as the spatial distribution of CRs in the Galaxy as well as the\nionization, heating, and chemistry in dense molecular clouds.", "category": "astro-ph_HE" }, { "text": "Testing the millisecond pulsar scenario of the Galactic center gamma-ray\n excess with very high energy gamma-rays: The recent analyses of the Fermi Large Area Telescope data show an extended\nGeV $\\gamma$-ray excess on top of the expected diffuse background in the\nGalactic center region, which can be explained with annihilating dark matter or\na population of millisecond pulsars (MSPs). We propose to observe the very high\nenergy $\\gamma$-rays for distinguishing the MSP scenario from the dark matter\nscenario. The GeV $\\gamma$-ray MSPs should release most energy to the\nrelativistic $e^{\\pm}$ wind, which will diffuse in the Galaxy and radiate TeV\n$\\gamma$-rays through inverse Compton scattering and bremsstrahlung processes.\nBy calculating the spectrum and spatial distribution, we show that such\nemission is detectable with the next generation very high energy $\\gamma$-ray\nobservatory, the Cherenkov Telescope Array (CTA), under reasonable model\nparameters. It is essential to search for the multi-wavelength counterparts to\nthe GeV $\\gamma$-ray excess for solving this mystery in the high energy\nuniverse.", "category": "astro-ph_HE" }, { "text": "Study of general relativistic magnetohydrodynamic accretion flow around\n black holes: We present a novel approach to study the global structure of steady,\naxisymmetric, advective, geometrically thin, magnetohydrodynamic (MHD)\naccretion flow around black holes in full general relativity (GR). Considering\nideal MHD conditions and relativistic equation of state (REoS), we solve the\ngoverning equations to obtain all possible smooth global accretion solutions.\nWe examine the dynamical and thermodynamical properties of accreting matter in\nterms of the flow parameters, namely energy (${\\cal E}$), angular momentum\n(${\\cal L}$), and local magnetic fields. For a thin GRMHD flow, we observe that\ntoroidal component ($b^\\phi$) of the magnetic fields generally dominates over\nradial component ($b^r$) at the disk equatorial plane. This evidently suggests\nthat toroidal magnetic field indeed plays important role in regulating the disk\ndynamics. We further notice that the disk remains mostly gas pressure ($p_{\\rm\ngas}$) dominated ($\\beta = p_{\\rm gas}/p_{\\rm mag} > 1$, $p_{\\rm mag}$ refers\nmagnetic pressure) except at the near horizon region, where magnetic fields\nbecome dynamically important ($\\beta \\sim 1$). We observe that Maxwell stress\nis developed that eventually yields angular momentum transport inside the disk.\nTowards this, we calculate the viscosity parameter ($\\alpha$) that appears to\nbe radially varying. In addition, we examine the underlying scaling relation\nbetween $\\alpha$ and $\\beta$, which clearly distinguishes two domains coexisted\nalong the radial extent of the disk. Finally, we discuss the utility of the\npresent formalism in the realm of GRMHD simulation studies.", "category": "astro-ph_HE" }, { "text": "Supernovae Powered by Collapsar Accretion in Gamma-Ray Burst Sources: The association of long-duration gamma-ray bursts (LGRBs) with Type Ic\nsupernovae presents a challenge to supernova explosion models. In the collapsar\nmodel for LGRBs, gamma rays are produced in an ultrarelativistic jet launching\nfrom the magnetosphere of the black hole that forms in the aftermath of the\ncollapse of a rotating progenitor star. The jet is collimated along the star's\nrotation axis, but the concomitant luminous supernova should be\nrelatively--though certainly not entirely--spherical, and should synthesize a\nsubstantial mass of 56Ni. Our goal is to provide a qualitative assessment of\nthe possibility that accretion of the progenitor envelope onto the black hole,\nwhich powers the LGRB, could also deposit sufficient energy and nickel mass in\nthe envelope to produce a luminous supernova. For this, the energy dissipated\nnear the black hole during accretion must be transported outward, where it can\ndrive a supernova-like shockwave. Here we suggest that the energy is\ntransported by convection and develop an analytical toy model, relying on\nglobal mass and energy conservation, for the dynamics of stellar collapse. The\nmodel suggests that a ~10,000 km/s shock can be driven into the envelope and\nthat ~10^51 erg explosions are possible. The efficiency with which the\naccretion energy is being transferred to the envelope is governed by the\ncompetition of advection and convection at distances ~100-1,000 km from the\nblack hole and is sensitive to the values of the convective mixing length, the\nmagnitude of the effective viscous stress, and the specific angular momentum of\nthe infalling envelope. Substantial masses of 56Ni may be synthesized in the\nconvective accretion flow over the course of tens of seconds from the initial\ncircularization of the infalling envelope around the black hole. The\nsynthesized nickel is convectively mixed with a much larger mass of unburned\nejecta.", "category": "astro-ph_HE" }, { "text": "Superbroad Component in Emission Lines of SS 433: We have detected new components in stationary emission lines of SS 433; these\nare the superbroad components that are low-contrast substrates with a width of\n2000--2500 km s-1 in He I $\\lambda4922$ and H$\\beta$ and 4000--5000 km s-1 in\nHe II $\\lambda4686$. Based on 44 spectra taken during four years of\nobservations from 2003 to 2007, we have found that these components in the He\nII and He I lines are eclipsed by the donor star; their behavior with\nprecessional and orbital phases is regular and similar to the behavior of the\noptical brightness of SS 433. The same component in H$\\beta$ shows neither\neclipses nor precessional variability. We conclude that the superbroad\ncomponents in the helium and hydrogen lines are different in origin. Electron\nscattering is shown to reproduce well the superbroad component of H$\\beta$ at a\ngas temperature of 20--35 kK and an optical depth for Thomson scattering $\\tau\n\\approx$ 0.25--0.35. The superbroad components of the helium lines are probably\nformed in the wind from the supercritical accretion disk. We have computed a\nwind model based on the concept of Shakura-Sunyaev supercritical disk\naccretion. The main patterns of the He II line profiles are well reproduced in\nthis model: not only the appearance of the superbroad component but also the\nevolution of the central two-component part of the profile of this line during\nits eclipse by the donor star can be explained.", "category": "astro-ph_HE" }, { "text": "Statistics of X-ray flares of Sagittarius A*: evidence for solar-like\n self-organized criticality phenomenon: X-ray flares have routinely been observed from the supermassive black hole,\nSagittarius A$^\\star$ (Sgr A$^\\star$), at our Galactic center. The nature of\nthese flares remains largely unclear, despite of many theoretical models. In\nthis paper, we study the statistical properties of the Sgr A$^\\star$ X-ray\nflares, by fitting the count rate (CR) distribution and the structure function\n(SF) of the light curve with a Markov Chain Monte Carlo (MCMC) method. With the\n3 million second \\textit{Chandra} observations accumulated in the Sgr A$^\\star$\nX-ray Visionary Project, we construct the theoretical light curves through\nMonte Carlo simulations. We find that the $2-8$ keV X-ray light curve can be\ndecomposed into a quiescent component with a constant count rate of\n$\\sim6\\times10^{-3}~$count s$^{-1}$ and a flare component with a power-law\nfluence distribution $dN/dE\\propto E^{-\\alpha_{\\rm E}}$ with $\\alpha_{\\rm\nE}=1.65\\pm0.17$. The duration-fluence correlation can also be modelled as a\npower-law $T\\propto E^{\\alpha_{\\rm ET}}$ with $\\alpha_{\\rm ET} < 0.55$ ($95\\%$\nconfidence). These statistical properties are consistent with the theoretical\nprediction of the self-organized criticality (SOC) system with the spatial\ndimension $S = 3$. We suggest that the X-ray flares represent plasmoid\nejections driven by magnetic reconnection (similar to solar flares) in the\naccretion flow onto the black hole.", "category": "astro-ph_HE" }, { "text": "Measuring the delay time distribution of binary neutron stars. II. Using\n the redshift distribution from third-generation gravitational wave detectors\n network: We investigate the ability of current and third-generation gravitational wave\n(GW) detectors to determine the delay time distribution (DTD) of binary neutron\nstars (BNS) through a direct measurement of the BNS merger rate as a function\nof redshift. We assume that the DTD follows a power law distribution with a\nslope $\\Gamma$ and a minimum merger time $t_{\\rm min}$, and also allow the\noverall BNS formation efficiency per unit stellar mass to vary. By convolving\nthe DTD and mass efficiency with the cosmic star formation history, and then\nwith the GW detector capabilities, we explore two relevant regimes. First, for\nthe current generation of GW detectors, which are only sensitive to the local\nuniverse, but can lead to precise redshift determinations via the\nidentification of electromagnetic counterparts and host galaxies, we show that\nthe DTD parameters are strongly degenerate with the unknown mass efficiency and\ntherefore cannot be determined uniquely. Second, for third-generation detectors\nsuch as Einstein Telescope (ET) and Cosmic Explorer (CE), which will detect BNS\nmergers at cosmological distances, but with a redshift uncertainty inherent to\nGW-only detections ($\\delta(z)/z\\approx 0.1z$), we show that the DTD and mass\nefficiency can be well-constrained to better than 10\\% with a year of\nobservations. This long-term approach to determining the DTD through a direct\nmapping of the BNS merger redshift distribution will be supplemented by more\nnear term studies of the DTD through the properties of BNS merger host galaxies\nat $z\\approx 0$ (Safarzadeh & Berger 2019).", "category": "astro-ph_HE" }, { "text": "On the origin of supergiant fast X-ray transients: A fraction of high-mass X-ray binaries are supergiant fast X-ray transients.\nThese systems have on average low X-ray luminosities, but display short flares\nduring which their X-ray luminosity rises by a few orders of magnitude. The\nleading model for the physics governing this X-ray behaviour suggests that the\nwinds of the donor OB supergiants are magnetized. In agreement with this model,\nthe first spectropolarimetric observations of the SFXT IGR J11215-5952 using\nthe FORS2 instrument at the Very Large Telescope indicate the presence of a kG\nlongitudinal magnetic field. Based on these results, it seems possible that the\nkey difference between supergiant fast X-ray transients and other high-mass\nX-ray binaries are the properties of the supergiant's stellar wind and the\nphysics of the wind's interaction with the neutron star magnetosphere.", "category": "astro-ph_HE" }, { "text": "Cosmological effects on the observed flux and fluence distributions of\n gamma-ray bursts: Several claims have been put forward that an essential fraction of\nlong-duration BATSE gamma-ray bursts should lie at redshifts larger than 5.\nThis point-of-view follows from the natural assumption that fainter objects\nshould, on average, lie at larger redshifts. However, redshifts larger than 5\nare rare for bursts observed by Swift. The purpose of this article is to show\nthat the most distant bursts in general need not be the faintest ones. We\nderive the cosmological relationships between the observed and emitted\nquantities, and arrive at a prediction that is tested on the ensembles of\nBATSE, Swift and Fermi bursts. This analysis is independent on the assumed\ncosmology, on the observational biases, as well as on any gamma-ray burst\nmodel. We arrive to the conclusion that apparently fainter bursts need not, in\ngeneral, lie at large redshifts. Such a behaviour is possible, when the\nluminosities (or emitted energies) in a sample of bursts increase more than the\ndimming of the observed values with redshift. In such a case dP(z)/dz > 0 can\nhold, where P(z) is either the peak-flux or the fluence. This also means that\nthe hundreds of faint, long-duration BATSE bursts need not lie at high\nredshifts, and that the observed redshift distribution of long Swift bursts\nmight actually represent the actual distribution.", "category": "astro-ph_HE" }, { "text": "Deep Neural Networks for Estimation of Gamma-Ray Burst Redshifts: While the available set of Gamma-ray Burst (GRB) data with known redshift is\ncurrently limited, a much larger set of GRB data without redshift is available\nfrom different instruments. This data includes well-measured prompt gamma-ray\nflux and spectral information. We estimate the redshift of a selection of these\nGRBs detected by Fermi-GBM and Konus-Wind using Machine Learning techniques\nthat are based on spectral parameters. We find that Deep Neural Networks with\nRandom Forest models employing non-linear relations among input parameters can\nreasonably reproduce the pseudo-redshift distribution of GRBs, mimicking the\ndistribution of GRBs with spectroscopic redshift. Furthermore, we find that the\npseudo-redshift samples of GRBs satisfy (i) Amati relation between the peak\nphoton energy of the time-averaged energy spectrum in the cosmological rest\nframe of the GRB ${E}_{\\rm i, p}$ and the isotropic-equivalent radiated energy\n${E}_{\\rm iso}$ during the prompt phase; and (ii) Yonetoku relation between\n${E}_{\\rm i, p}$ and isotropic-equivalent luminosity ${L}_{\\rm iso}$, both\nmeasured during the peak flux interval.", "category": "astro-ph_HE" }, { "text": "Search for photon line-like signatures from Dark Matter annihilations\n with H.E.S.S: Gamma-ray line signatures can be expected in the very-high-energy (VHE;\nE_\\gamma > 100 GeV) domain due to self-annihilation or decay of dark matter\n(DM) particles in space. Such a signal would be readily distinguishable from\nastrophysical \\gamma-ray sources that in most cases produce continuous spectra\nwhich span over several orders of magnitude in energy. Using data collected\nwith the H.E.S.S. \\gamma-ray instrument, upper limits on line-like emission are\nobtained in the energy range between ~500 GeV and ~25 TeV for the central part\nof the Milky Way halo and for extragalactic observations, complementing recent\nlimits obtained with the Fermi-LAT instrument at lower energies. No\nstatistically significant signal could be found. For monochromatic \\gamma-ray\nline emission, flux limits of (2x10^-7 - 2x10^-5) m^-2 s^-1 sr^-1 and (1x10^-8\n- 2x10^-6) m^-2 s^-1 sr^-1 are obtained for the central part of the Milky Way\nhalo and extragalactic observations, respectively. For a DM particle mass of 1\nTeV, limits on the velocity-averaged DM annihilation cross section < \\sigma v\n>(\\chi\\chi -> \\gamma\\gamma) reach ~10^-27 cm^3 s^-1, based on the Einasto\nparametrization of the Galactic DM halo density profile.", "category": "astro-ph_HE" }, { "text": "Evidence for two neutrinos bursts from SN1987A: The SN1987A in the Giant Magellanic Cloud was an amazing and extraordinary\nevent because it was detected in real time for different neutrinos experiments\n($\\nu$s) around the world. Approximate $\\sim25$ events were observed in three\ndifferent experiments: Kamiokande II (KII) $\\sim 12$,\nIrvine-Michigan-Brookhaven (IMB) $\\sim 8$ e Baksan $\\sim 5$, plus a contrived\nburst at Mont Blanc (Liquid Scintillator Detector - LSD) later dismissed\nbecause of energetic requirements (Aglietta et al. 1988). The neutrinos have an\nimportant play role into the neutron star newborn: at the moment when the\nsupernova explodes the compact object remnant is freezing by neutrinos\n($\\sim99\\%$ energy is lost in the few seconds of the explosion). The work is\nmotivated by neutrinos' event in relation arrival times where there is a\ntemporal gap between set of events ($\\sim6\\mbox{s}$). The first part of dataset\ncame from the ordinary mechanism of freezing and the second part suggests\ndifferent mechanism of neutrinos production. We tested two models of cooling\nfor neutrinos from SN1987A: 1st an exponential cooling is an ordinary model of\ncooling and 2nd a two-step temperature model that it considers two bursts\nseparated with temporal gap. Our analysis was done with Bayesian tools ({\\it\nBayesian Information Criterion} - BIC) The result showed strong evidence in\nfavor of a two-step model against one single exponential cooling\n($\\ln\\mbox{B}_{ij} > 5.0$), and suggests the existence of two neutrino bursts\nat the moment the neutron star was born.", "category": "astro-ph_HE" }, { "text": "Combined analyses of the antiproton production from cosmic-ray\n interactions and its possible dark matter origin: Recent cosmic-ray (CR) studies have claimed the possibility of an excess on\nthe antiproton flux over the predicted models at around $10$ GeV, which can be\nthe signature of dark matter annihilating into hadronic final states that\nsubsequently form antiprotons. However, this excess is subject to many\nuncertainties related to the evaluation of the antiproton spectrum produced\nfrom spallation interactions of CRs. In this work, we implement a combined\nMarkov-Chain Monte Carlo analysis of the secondary ratios of B, Be and Li and\nthe antiproton-to-proton ratio ($\\bar{p}/p$), while also including nuisance\nparameters to consider the uncertainties related to the spallation cross\nsections. This study allows us to constrain the Galactic halo height and the\nrest of propagation parameters, evaluate the impact of cross sections\nuncertainties in the determination of the antiproton spectrum and test the\norigin of the excess of antiprotons. In this way, we provide a set of\npropagation parameters and scale factors for renormalizing the cross sections\nparametrizations that allow us to reproduce all the ratios of B, Be, Li and\n$\\bar{p}$ simultaneously. We show that the energy dependence of the $\\bar{p}/p$\nratio is compatible with a pure secondary origin. We find that the energy\ndependence of the evaluated $\\bar{p}/p$ spectrum matches the AMS-02 data at\nenergies above $\\sim3$GeV, although there is still a nearly constant $\\sim10\\%$\nexcess of $\\bar{p}$ over our prediction. We discuss that this discrepancy is\nmore likely explained from a $\\sim10\\%$ scaling in the cross sections of\nantiproton production, rather than a component of dark matter leading to\nantiprotons. In particular, we find that the best-fit WIMP mass ($\\sim 300$\nGeV) needed to explain the discrepancy lies above the constraints from most\nindirect searches of dark matter and the resultant fit is poorer than with a\ncross sections scaling.", "category": "astro-ph_HE" }, { "text": "Testing astrophysical models for the PAMELA positron excess with cosmic\n ray nuclei: The excess in the positron fraction reported by the PAMELA collaboration has\nbeen interpreted as due to annihilation or decay of dark matter in the Galaxy.\nMore prosaically, it has been ascribed to direct production of positrons by\nnearby pulsars, or due to pion production during stochastic acceleration of\nhadronic cosmic rays in nearby sources. We point out that measurements of\nsecondary nuclei produced by cosmic ray spallation can discriminate between\nthese possibilities. New data on the titanium-to-iron ratio from the ATIC-2\nexperiment support the hadronic source model above and enable a prediction to\nbe made for the boron-to-carbon ratio at energies above 100 GeV. Presently, all\ncosmic ray data are consistent with the positron excess being astrophysical in\norigin.", "category": "astro-ph_HE" }, { "text": "Construction of a Multidimensional Parallel Adaptive Mesh Refinement\n Special Relativistic Hydrodynamics Code for Astrophysical Applications: We have developed a new computer code, RELDAFNA, to solve the conservative\nequations of special relativistic hydrodynamics (SRHD) using adaptive mesh\nrefinement (AMR) on parallel computers. We have implemented a\ncharacteristic-wise, finite volume Godunov scheme using the full characteristic\ndecomposition of the SRHD equations, to achieve second and third order accuracy\nin space (both PLM and PPM reconstruction). For time integration, we use the\nmethod of directional splitting with symmetrization, which is second order\naccurate in time. We have also implemented second and third order Runge-Kutta\ntime integration scheme for comparison. In addition to the hydrodynamics\nsolvers we have implemented approximate Riemann solvers along with an exact\nRiemann solver. We examine the ability of RELDAFNA to accurately simulate\nspecial relativistic flows efficiently in number of processors, computer memory\nand over all integration time. We show that a wide variety of test problems can\nbe solved as accurately as solved by higher order programs, such as RAM,\nGENESIS, or PLUTO, but with a less number of variables kept in memory and\ncomputer calculations than most schemes, an advantage which is crucial for 3D\nhigh resolution simulations to be of practical use for scientific research in\ncomputational astrophysics. RELDAFNA has been tested in one, two and three\ndimensions and in Cartesian, cylindrical and spherical geometries. We present\nthe ability of RELDAFNA to assist with the understanding of open questions in\nhigh energy astrophysics which involve relativistic flows.", "category": "astro-ph_HE" }, { "text": "Dwarf (Twin) Neutron Stars I: Did GW170817 Involve One?: Dwarf neutron stars are stable twins of neutron stars but with a maximum mass\nless than that of neutron stars. Their existence brings into concordance the\nseemingly conflicting information on the size of neutron stars inferred from\ngravitational waves from GW170817, from the NICER mission, and from the PREX-II\nexperiment. Their distinctive characteristics lead to rich and falsifiable\npredictions that are expected to be tested in the near future. If corroborated,\nthe existence of dwarf neutron stars would substantially improve our\nunderstanding of the QCD phase diagram and offer valuable insights into the\ndark sector.", "category": "astro-ph_HE" }, { "text": "Test of the superdiffusion model in the interstellar medium around the\n Geminga pulsar: The TeV $\\gamma$-ray halo around the Geminga pulsar is an important indicator\nof cosmic-ray (CR) propagation in the local zone of the Galaxy as it reveals\nthe spatial distribution of the electrons and positrons escaping from the\npulsar. Considering the intricate magnetic field in the interstellar medium\n(ISM), it is proposed that superdiffusion model could be more realistic to\ndescribe the CR propagation than the commonly used normal diffusion model. In\nthis work, we test the superdiffusion model in the ISM around the Geminga\npulsar by fitting to the surface brightness profile of the Geminga halo\nmeasured by HAWC. Our results show that the chi-square statistic monotonously\nincreases as $\\alpha$ decreases from 2 to 1, where $\\alpha$ is the\ncharacteristic index of superdiffusion describing the degree of fractality of\nthe ISM and $\\alpha=2$ corresponds to the normal diffusion model. We find that\nmodel with $\\alpha<1.32$ (or $<1.4$, depending on the data used in fit) is\ndisfavored at 95\\% confidence level. Superdiffusion model with $\\alpha$ close\nto 2 can well explain the morphology of the Geminga halo, while it predicts\nmuch higher positron flux on the Earth than the normal diffusion model. This\nhas important implication for the interpretation of the CR positron excess.", "category": "astro-ph_HE" }, { "text": "On the triple pulsar profiles generated by ordinary mode: A detailed study of the refraction of an ordinary wave in the magnetosphere\nof radio pulsars was carried out. For this, a consistent theory of the\ngeneration of secondary particles was constructed, which essentially takes into\naccount the dependence of the number density and the energy spectrum of\nsecondary particles on the distance from the magnetic axis. This made it\npossible to determine with high accuracy the refraction of the ordinary O-mode\nin the central region of the outflowing plasma, which makes it possible to\nexplain the central peak of three-humped mean radio profiles. As shown by\ndetailed numerical calculations, in most cases it is possible to reproduce\nquite well the observed mean profiles of radio pulsars.", "category": "astro-ph_HE" }, { "text": "Are Superluminous Supernovae Powered By Collision Or By Millisecond\n Magnetars?: Using our previously derived simple analytic expression for the bolometric\nlight curves of supernovae, we demonstrate that the collision of the fast\ndebris of ordinary supernova explosions with relatively slow-moving shells from\npre-supernova eruptions can produce the observed bolometric light curves of\nsuperluminous supernovae (SLSNe) of all types. These include both, those which\ncan be explained as powered by spinning-down millisecond magnetars and those\nwhich cannot. That and the observed close similarity between the bolometric\nlight-curves of SLSNe and ordinary interacting SNe suggest that SLSNe are\npowered mainly by collisions with relatively slow moving circumstellar shells\nfrom pre-supernova eruptions rather than by the spin-down of millisecond\nmagnetars born in core collapse supernova explosions.", "category": "astro-ph_HE" }, { "text": "Dark Matter Annihilation in the Universe: The astronomical dark matter is an essential component of the Universe and\nyet its nature is still unresolved. It could be made of neutral and massive\nelementary particles which are their own antimatter partners. These dark matter\nspecies undergo mutual annihilations whose effects are briefly reviewed in this\narticle. Dark matter annihilation plays a key role at early times as it sets\nthe relic abundance of the particles once they have decoupled from the\nprimordial plasma. A weak annihilation cross section naturally leads to a\ncosmological abundance in agreement with observations. Dark matter species\nsubsequently annihilate - or decay - during Big Bang nucleosynthesis and could\nplay havoc with the light element abundances unless they offer a possible\nsolution to the $^{7}$Li problem. They could also reionize the intergalactic\nmedium after recombination and leave visible imprints in the cosmic microwave\nbackground. But one of the most exciting aspects of the question lies in the\npossibility to indirectly detect the dark matter species through the rare\nantimatter particles - antiprotons, positrons and anti-deuterons - which they\nproduce as they currently annihilate inside the galactic halo. Finally, the\neffects of dark matter annihilation on stars is discussed.", "category": "astro-ph_HE" }, { "text": "Determining the Origin of Very-high-energy Gamma Rays from Galactic\n Sources by Future Neutrino Observations: Recently, the Large High Altitude Air Shower Observatory (LHAASO) identified\n12 $\\gamma$-ray sources emitting gamma rays with energies above 100 TeV, making\nthem potential PeV cosmic-ray accelerators (PeVatrons). Neutrino observations\nare crucial in determining whether the gamma-ray radiation process is of\nhadronic or leptonic origin. In this paper, we study three detected sources,\nLHAASO J1908+0621, LHAASO J2018+3651, and LHAASO J2032+4102, which are also the\nmost promising galactic high-energy neutrino candidate sources with the lowest\npre-trial p-value based on the stacking searches testing for excess neutrino\nemission by IceCube Neutrino Observatory. We study the lepto-hadronic scenario\nfor the observed multiband spectra of these LHAASO sources considering the\npossible counterpart source of the LHAASO sources. The very-high-energy gamma\nrays are entirely attributed to the hadronic contribution, therefore the most\noptimistic neutrino flux can be derived. Then, we evaluate the statistical\nsignificance (p-value) as a function of the observation time of IceCube and the\nnext-generation IceCube-Gen2 neutrino observatory respectively. Our results\ntend to disfavor that all gamma rays above $100\\,\\rm GeV$ from LHAASO\nJ1908+0621 are of purely hadronic origin based on current IceCube observations,\nbut the purely hadronic origin of gamma rays above $100\\,\\rm TeV$ is still\npossible. By IceCube-Gen2, the origin of gamma rays above $100\\,\\rm TeV$ from\nLHAASO J1908+0621 can be further determined at a $5\\sigma$ significance level\nwithin a running time of $\\sim 3$ years. For LHAASO J2018+3651 and LHAASO\nJ2032+4102, the required running time of IceCube-Gen2 is $\\sim 10$ years\n($3\\sigma$) and $\\sim 10$ years ($5\\sigma$), respectively. Future observations\nby the next-generation neutrino telescope will be crucial to understanding the\nparticle acceleration and radiation processes inside the sources.", "category": "astro-ph_HE" }, { "text": "A multi-wavelength study of the gamma-ray binary candidate HESS\n J1832-093: We investigate the nature of the unidentified very-high-energy (VHE)\ngamma-ray object, HESS J1832-093, in a multi-wavelength context. Based on X-ray\nvariability and spectral index ($\\Gamma_X\\sim\\,1.5$), and its broad-band\nspectrum (which was remarkably similar to HESS J0632+057, a confirmed gamma-ray\nbinary), HESS J1832-093 has been considered to be a strong gamma-ray binary\ncandidate in previous works. In this work, we provide further evidence for this\nscenario. We obtained a spectrum of its IR counterpart using Gemini/Flamingo,\nfinding absorption lines that are usually seen in massive stars, in particular\nO stars. We also obtained a rather steep ATCA spectrum\n($\\alpha=-1.18^{+1.04}_{-0.88}$) which prefers a gamma-ray binary over an AGN\nscenario. Based on spatial-spectral analysis and variability search, we found\nthat 4FGL J1832.9-0913 is possible to be associated with SNR G22.7-0.2 rather\nthan with HESS J1832-093 only.", "category": "astro-ph_HE" }, { "text": "Jets, Disks and Winds from Spinning Black Holes: Nature or Nurture?: A brief summary is given of an alternative interpretation of the Event\nHorizon Telescope observations of the massive black hole in the nucleus of the\nnearby galaxy M87. It is proposed that the flow is primarily powered by the\nblack hole rotation, not the release of gravitational energy by the infalling\ngas. Consequently, the observed millimetre emission is produced by an\n\"ergomagnetosphere\" that connects the black hole horizon to an \"ejection disk\"\nfrom which most of the gas supplied at a remote \"magnetopause\" is lost through\na magnetocentrifugal wind. It is argued that the boundary conditions at high\nlatitude on the magnetopause play a crucial role in the collimation of the\nrelativistic jets. The application of these ideas to other types of source is\nbriefly discussed.", "category": "astro-ph_HE" }, { "text": "Detecting Kozai-Lidov imprints on the gravitational waves of\n intermediate-mass black holes in galactic nuclei: A third object in the vicinity of a binary system causes variations in the\neccentricity and the inclination of the binary through the Kozai-Lidov effect.\nWe examine if such variations leave a detectable imprint on the gravitational\nwaves of a binary consisting of intermediate mass black holes and stellar mass\nobjects. As a proof of concept, we present an example where LISA may detect the\nKozai-Lidov modulated gravitational wave signals of such sources from at least\na distance of 1Mpc if the perturbation is caused by a supermassive black hole\ntertiary. Although the quick pericenter precession induced by general\nrelativity significantly reduces the appropriate parameter space for this\neffect by quenching the Kozai-Lidov oscillations, we still find reasonable\nparameters where the Kozai-Lidov effect may be detected with high\nsignal-to-noise ratios.", "category": "astro-ph_HE" }, { "text": "Model on pulsed GeV radiation from magnetars: We discuss a possible scenario for radiation mechanism of pulsed GeV\ngamma-rays from magnetars. The magnetars have shown frequent X-ray bursts,\nwhich would be triggered by crust fractures and could release the energy of\norder of ~10^{41-42}erg. If the location of the crust cracking of the magnetic\nfield is close to the magnetic pole, part of the released energy may excite the\nAlfevn wave that can propagate into outer magnetosphere. The oscillation of the\nmagnetic field induces the available potential drop ~10^{15}Volts, which can\naccelerate the electrons and/or positrons to the Lorentz factor ~10^{7} in the\nouter magnetosphere. The curvature radiation process at outer magnetosphere can\nproduce GeV gamma-rays. If the radiation process is occurred above r~5x 10^7cm\nfrom the stellar surface, the emitted GeV gamma-rays can escape from the\npair-creation process with the X-rays and/or the magnetic field. The expected\nluminosity of the GeV emissions is order of L_{\\gamma}< 10^{35} erg/s, and the\nradiation process will last for a temporal scale of years. The expected pulse\nprofiles have a broad shape with sometimes sharp peaks. We apply the model to\nAXP 1E~2259+586.", "category": "astro-ph_HE" }, { "text": "Cosmic Frontier Indirect Dark Matter Detection Working Group Summary: As part of the Snowmass process, the Cosmic Frontier Indirect-Detection\nsubgroup (CF2) has drawn on input from the Cosmic Frontier and the broader\nParticle Physics community to produce this document. The purposes of this\nreport are to identify opportunities for dark matter science through indirect\ndetection, to give an overview of the primary scientific drivers for indirect\nsearches for dark matter, and to survey current and planned experiments that\nhave, as a large part of their scientific program, the goal of searching for\nindirect (or astrophysical) signatures of dark matter. We primarily address\nexisting experiments with a large U.S. role, or future experiments where a U.S.\ncontribution is sought. We also address the limitations of this technique, and\nanswer the tough questions relevant to this subgroup posed by the HEP community\nthrough the Snowmass process.", "category": "astro-ph_HE" }, { "text": "Stripped-envelope core-collapse supernova $^{56}$Ni masses: Persistently\n larger values than supernovae type II: The mass of synthesised radioactive material is an important power source for\nall supernova (SN) types. Anderson 2019 recently compiled literature values and\nobtained $^{56}$Ni distributions for different core-collapse supernovae\n(CC-SNe), showing that the $^{56}$Ni distribution of stripped envelope CC-SNe\n(SE-SNe: types IIb, Ib, and Ic) is highly incompatible with that of hydrogen\nrich type II SNe (SNe-II). This motivates questions on differences in\nprogenitors, explosion mechanisms, and $^{56}$Ni estimation methods. Here, we\nre-estimate the nucleosynthetic yields of $^{56}$Ni for a well-observed and\nwell-defined sample of SE-SNe in a uniform manner. This allows us to\ninvestigate whether the observed SN-II--SE-SN $^{56}$Ni separation is due to\nreal differences between these SN types, or because of systematic errors in the\nestimation methods. We compiled a sample of well observed SE-SNe and measured\n$^{56}$Ni masses through three different methods proposed in the literature.\nArnett's rule -as previously shown - gives $^{56}$Ni masses for SE-SNe that are\nconsiderably higher than SNe-II. While for the distributions calculated using\nboth the Khatami&Kasen prescription and Tail $^{56}$Ni masses are offset to\nlower values than `Arnett values', their $^{56}$Ni distributions are still\nstatistically higher than that of SNe II. Our results are strongly driven by a\nlack of SE-SN with low $^{56}$Ni masses (that are in addition strictly lower\nlimits). The lowest SE-SN $^{56}$Ni mass in our sample is of 0.015M$_\\odot$,\nbelow which are more than 25$\\%$ of SNe II. We conclude that there exists real,\nintrinsic differences in the mass of synthesised radioactive material between\nSNe II and SE-SNe . Any proposed current or future CCSN progenitor scenario and\nexplosion mechanism must be able to explain why and how such differences arise,\nor outline a yet to be fully explored bias in current SN samples.", "category": "astro-ph_HE" }, { "text": "GRB jet structure and the jet break: We investigate the shape of the jet break in within-beam gamma-ray burst\n(GRB) optical afterglows for various lateral jet structure profiles. We\nconsider cases with and without lateral spreading and a range of inclinations\nwithin the jet core half-opening angle, $\\theta_c$. We fit model and observed\nafterglow lightcurves with a smoothly-broken power-law function with a\nfree-parameter $\\kappa$ that describes the sharpness of the break. We find that\nthe jet break is sharper ($\\kappa$ is greater) when lateral spreading is\nincluded than in the absence of lateral spreading. For profiles with a\nsharp-edged core, the sharpness parameter has a broad range of\n$0.1\\lesssim\\kappa\\lesssim4.6$, whereas profiles with a smooth-edged core have\na narrower range of $0.1\\lesssim\\kappa\\lesssim2.2$ when models both with and\nwithout lateral spreading are included. For sharp-edged jets, the jet break\nsharpness depends strongly on the inclination of the system within $\\theta_c$,\nwhereas for smooth-edged jets, $\\kappa$ is more strongly dependent on the size\nof $\\theta_c$. Using a sample of 20 GRBs we find nine candidate smooth-edged\njet structures and eight candidate sharp-edged jet structures, while the\nremaining three are consistent with either. The shape of the jet break, as\nmeasured by the sharpness parameter $\\kappa$, can be used as an initial check\nfor the presence of lateral structure in within-beam GRBs where the afterglow\nis well-sampled at and around the jet-break time.", "category": "astro-ph_HE" }, { "text": "A 350-MHz Green Bank Telescope Survey of Unassociated Fermi LAT Sources:\n Discovery and Timing of Ten Millisecond Pulsars: We have searched for radio pulsations towards 49 Fermi Large Area Telescope\n(LAT) 1FGL Catalog $\\gamma$-ray sources using the Green Bank Telescope at 350\nMHz. We detected 18 millisecond pulsars (MSPs) in blind searches of the data;\n10 of these were discoveries unique to our survey. Sixteen are binaries, with\neight having short orbital periods $P_B < 1$ day. No radio pulsations from\nyoung pulsars were detected, although three targets are coincident with\napparently radio-quiet $\\gamma$-ray pulsars discovered in LAT data. Here, we\ngive an overview of the survey and present radio and $\\gamma$-ray timing\nresults for the 10 MSPs discovered. These include the only isolated MSP\ndiscovered in our survey and six short-$P_B$ binary MSPs. Of these, three have\nvery low-mass companions ($M_c$ $\\ll$ 0.1M$_{\\odot}$) and hence belong to the\nclass of black widow pulsars. Two have more massive, non-degenerate companions\nwith extensive radio eclipses and orbitally modulated X-ray emission consistent\nwith the redback class. Significant $\\gamma$-ray pulsations have been detected\nfrom nine of the discoveries. This survey and similar efforts suggest that the\nmajority of Galactic $\\gamma$-ray sources at high Galactic latitudes are either\nMSPs or relatively nearby non-recycled pulsars, with the latter having on\naverage a much smaller radio/$\\gamma$-ray beaming ratio as compared to MSPs. It\nalso confirms that past surveys suffered from an observational bias against\nfinding short-$P_B$ MSP systems.", "category": "astro-ph_HE" }, { "text": "High-energy emission of variable objects in the OMC--VAR catalogue: OMC-VAR is the first catalogue of variable sources observed by the Optical\nMonitoring Camera (OMC) on board INTEGRAL. It includes photometry and\nvariability data for more than 5000 sources of very different nature. In order\nto study the multi-wavelength similarities and differences of the engines\npowering AGN and X-ray binaries, especially those powered by a black hole, we\nhave searched for counterparts in several high-energy and infrared catalogues,\nincluding the 4th IBIS/ISGRI soft gamma-ray survey catalogue, the XMM-Newton\nSerendipitous Source catalogue, the 2MASS All-Sky catalogue of Point Sources,\nthe 2MASS Extended Source Catalog and the WISE All-Sky Data Release.\nPreliminary multiwavelength results for the IBIS counterparts are presented and\ndiscussed.", "category": "astro-ph_HE" }, { "text": "Evidence for Astrophysical Muon Neutrinos from the Northern Sky with\n IceCube: Results from the IceCube Neutrino Observatory have recently provided\ncompelling evidence for the existence of a high energy astrophysical neutrino\nflux utilizing a dominantly Southern Hemisphere dataset consisting primarily of\nnu_e and nu_tau charged current and neutral current (cascade) neutrino\ninteractions. In the analysis presented here, a data sample of approximately\n35,000 muon neutrinos from the Northern sky was extracted from data taken\nduring 659.5 days of livetime recorded between May 2010 and May 2012. While\nthis sample is composed primarily of neutrinos produced by cosmic ray\ninteractions in the Earth's atmosphere, the highest energy events are\ninconsistent with a hypothesis of solely terrestrial origin at 3.7 sigma\nsignificance. These neutrinos can, however, be explained by an astrophysical\nflux per neutrino flavor at a level of Phi(E_nu) = 9.9^{+3.9}_{-3.4} times\n10^{-19} GeV^{-1} cm^{-2} sr^{-1} s^{-1} ({E_nu / 100 TeV})^{-2}, consistent\nwith IceCube's Southern Hemisphere dominated result. Additionally, a fit for an\nastrophysical flux with an arbitrary spectral index was performed. We find a\nspectral index of 2.2^{+0.2}_{-0.2}, which is also in good agreement with the\nSouthern Hemisphere result.", "category": "astro-ph_HE" }, { "text": "Absolute timing of the Crab pulsar with the INTEGRAL/SPI telescope: We have investigated the pulse shape evolution of the Crab pulsar emission in\nthe hard X-ray domain of the electromagnetic spectrum. In particular, we have\nstudied the alignment of the Crab pulsar phase profiles measured in the hard\nX-rays and in other wavebands. To obtain the hard X-ray pulse profiles, we have\nused six year (2003-2009, with a total exposure of about 4 Ms) of publicly\navailable data of the SPI telescope on-board of the INTEGRAL observatory,\nfolded with the pulsar time solution derived from the Jodrell Bank Crab Pulsar\nMonthly Ephemeris. We found that the main pulse in the hard X-ray 20-100 keV\nenergy band is leading the radio one by $8.18\\pm0.46$ milliperiods in phase, or\n$275\\pm15 \\mu s$ in time. Quoted errors represent only statistical\nuncertainties.Our systematic error is estimated to be $\\sim 40 \\mu s$ and is\nmainly caused by the radio measurement uncertainties. In hard X-rays, the\naverage distance between the main pulse and interpulse on the phase plane is\n$0.3989\\pm0.0009$. To compare our findings in hard X-rays with the soft 2-20\nkeV X-ray band, we have used data of quasi-simultaneous Crab observations with\nthe PCA monitor on-board the Rossi X-Ray Timing Explorer (RXTE) mission. The\ntime lag and the pulses separation values measured in the 3-20 keV band are\n$0.00933\\pm0.00016$ (corresponding to $310\\pm6 \\mu s$) and $0.40016\\pm0.00028$\nparts of the cycle, respectively. While the pulse separation values measured in\nsoft X-rays and hard X-rays agree, the time lags are statistically different.\nAdditional analysis show that the delay between the radio and X-ray signals\nvaries with energy in the 2 - 300 keV energy range. We explain such a behaviour\nas due to the superposition of two independent components responsible for the\nCrab pulsed emission in this energy band.", "category": "astro-ph_HE" }, { "text": "The spin of the black hole GRS 1716-249 determined from the hard\n intermediate state: We present three simultaneous/quasi-simultaneous NuSTAR and Swift datasets of\nthe black hole GRS 1716-249 in its hard intermediate state. The accretion disk\nin this state may have reached the innermost stable circular orbit, and the\nNuSTAR spectra show a broad relativistic iron line and a strong Compton hump.\nTo measure the black hole spin, we construct a joint model consisting of a\nrelativistic disk model kerrbb and a reflection model relxill, to fit the\ncontinuum and the reflection components, respectively. By applying this model\nto each dataset independently, a consistent result is obtained on the black\nhole spin and the disk inclination. The black hole spin is a* >~ 0.92, and the\ninclination angle (i) is around 40-50 degree, based on the measurements of all\ndatasets. In the third dataset, a high black hole mass (M_BH) is strongly\ndisfavored by the spectral fits. By unfreezing the black hole mass, we find\na*>0.92, i=49.9^{+1.0}_{-1.3} degree and M_BH<8.0 Msun, at a 90% confidence\nlevel. Considering the lower limit derived from a previous optical constraint,\nM_BH is in a range of 4.9-8.0 Msun.", "category": "astro-ph_HE" }, { "text": "Uncovering Red and Dusty Ultraluminous X-ray Sources with Spitzer: We present a mid-infrared (IR) sample study of nearby ultraluminous X-ray\nsources (ULXs) using multi-epoch observations with the Infrared Array Camera\n(IRAC) on the Spitzer Space Telescope. Spitzer/IRAC observations taken after\n2014 were obtained as part of the Spitzer Infrared Intensive Transients Survey\n(SPIRITS). Our sample includes 96 ULXs located within 10 Mpc. Of the 96~ULXs,\n12 have candidate counterparts consistent with absolute mid-IR magnitudes of\nsupergiants, and 16 counterparts exceeded the mid-IR brightness of single\nsupergiants and are thus more consistent with star clusters or non-ULX\nbackground active galactic nuclei (AGN). The supergiant candidate counterparts\nexhibit a bi-modal color distribution in a Spitzer/IRAC color-magnitude\ndiagram, where \"red\" and \"blue\" ULXs fall in IRAC colors $[3.6] - [4.5]\\sim0.7$\nand $[3.6] - [4.5]\\sim0.0$, respectively. The mid-IR colors and absolute\nmagnitudes of 4 \"red\" and 5 \"blue\" ULXs are consistent with that of supergiant\nB[e] (sgB[e]) and red supergiant (RSG) stars, respectively. While \"blue\",\nRSG-like mid-IR ULX counterparts likely host RSG mass donors, we propose the\n\"red\" counterparts are ULXs exhibiting the \"B[e] phenomenon'' rather than hosts\nof sgB[e] mass donors. We show that the mid-IR excess from the \"red\" ULXs is\nlikely due to thermal emission from circumstellar or circumbinary dust. Using\ndust as a probe for total mass, we estimate mass-loss rates of\n$\\dot{M}\\sim1\\times10^{-4}$ M$_\\odot$ yr$^{-1}$ in dust-forming outflows of red\nULXs. Based on the transient mid-IR behavior and its relatively flat spectral\nindex, $\\alpha=-0.19\\pm0.1$, we suggest that the mid-IR emission from Holmberg\nIX X-1 originates from a variable jet.", "category": "astro-ph_HE" }, { "text": "Gamma-Ray Burst Central Engines: Black Hole Vs. Magnetar: Discovered over forty years ago, Gamma-Ray Bursts (GRBs) remain a forefront\ntopic in modern astrophysics. Perhaps the most fundamental question associated\nwith GRBs is the nature of the astrophysical agent (or agents) that ultimately\npowers them: the central engine. In this review, I focus on the possible\ncentral engines of long-duration GRBs, and the constraints that present\nobservations place on these models. Long GRBs are definitively associated with\nthe deaths of massive stars, but whether the central engine is an accreting\nblack hole or a rapidly-spinning, highly-magnetized neutron star (a\n\"proto-magnetar\") remains unsettled. This distinction has been brought into\nparticular focus by recent MHD simulations of the core-collapse of massive,\nrotating \"collapsar progenitors,\" which suggest that powerful\nmagneto-centrifugal outflows from the proto-neutron star may stave off black\nhole formation entirely. Although both black hole and magnetar GRB models\nremain viable, I argue that the magnetar model is more mature in the sense that\nit provides quantitative explanations for the durations, energies, Lorentz\nfactors, and collimation of long GRB outflows. Given these virtues, one\npromising strategy to break the present stalemate is to further develop the\nmagnetar model until inescapable (and falsifiable) predictions emerge. This\ncourse of action signals a renewed challenge to translate time-dependent jet\nproperties (power, magnetization, and Lorentz factor) into observables\n(gamma-ray light curves and spectra).", "category": "astro-ph_HE" }, { "text": "The fast radio burst population evolves, consistent with the\n star-formation rate: Fast radio bursts (FRBs) are extremely powerful sources of radio waves\nobserved at cosmological distances. We use a sophisticated model of FRB\nobservations -- presented in detail in a companion paper -- to fit FRB\npopulation parameters using large samples of FRBs detected by ASKAP and Parkes,\nincluding seven sources with confirmed host galaxies. Our fitted parameters\ndemonstrate that the FRB population evolves with redshift in a manner\nconsistent with, or faster than, the star-formation rate (SFR), ruling out a\nnon-evolving population at 99.9\\% C.L. Our estimated maximum FRB energy is\n$\\log_{10} E_{\\rm max} [{\\rm erg}] = 41.84_{-0.18}^{+0.49}$ (68\\% C.L.)\nassuming a 1\\,GHz emission bandwidth, with slope of the cumulative luminosity\ndistribution $\\gamma=-1.16_{-0.12}^{+0.11}$. We find a log-mean host DM\ncontribution of $145_{-60}^{+64}$\\,pc\\,cm$^{-3}$ on top of a typical local (ISM\nand halo) contribution of $\\sim80$\\,pc\\,cm$^{-3}$, which is higher than most\nliterature values. These results are consistent with the model of FRBs arising\nas the high-energy limit of magnetar bursts, but allow for FRB progenitors that\nevolve faster than the SFR.", "category": "astro-ph_HE" }, { "text": "Anomalous bremsstrahlung and the structure of cosmic ray\n electron-positron fluxes at the GeV-TeV energy range: We reveal that the energy spectra of electrons-positrons in primary cosmic\nrays measured at atmosphere top have double structures: an excess component\n$\\Phi^s_{e^+}(E)=\\Phi^s_{e^-}(E)$ around $400 GeV$, which origins from a strong\n$e^+e^-$-source and the distorted background $\\Phi^0_{e^-}(E)$. We supposed\nthat the difference between AMS-CALET and Fermi-LAT-DAMPE data origins from the\nenergy loss of the fluxes due to the anomalous bremsstrahlung effect at a\nspecial window. The evolution of spectra under anomalous bremsstrahlung effect\nsatisfies an improved electromagnetic cascade equation. The above spectra are\nparameterized and they can be regarded as the subjects exploring new physics.\nWe suggest to check the previous applications of the Bethe-Heitler formula in\nthe study of the propagation of high energy electrons and photons.", "category": "astro-ph_HE" }, { "text": "Magnetic field reversal in the turbulent environment around a repeating\n fast radio burst: Fast radio bursts (FRBs) are brief, intense flashes of radio waves from\nunidentified extragalactic sources. Polarized FRBs originate in highly\nmagnetized environments. We report observations of the repeating FRB 20190520B\nspanning seventeen months , which show its amount of Faraday rotation is highly\nvariable and twice changes its sign. The FRB also depolarizes below radio\nfrequencies around 1 to 3 GHz. We interpret these properties as due to change\nin the parallel component of the integrated magnetic field along the\nline-of-sight, including reversals. This could result from propagation through\na turbulent, magnetized screen of plasma located between $10^{-5}$ to 100\nparsecs of the FRB source. This is consistent with the bursts passing through\nthe stellar wind of a binary companion of the FRB source.", "category": "astro-ph_HE" }, { "text": "Non-linear diffusive shock acceleration: A recipe for injection of\n electrons: Prescriptions for electron injection into the diffusive shock acceleration\nprocess are required in many practical considerations of cosmic-ray\nastrophysics, particularly in modeling of the synchrotron emission of\nastrophysical sources. In particle-in-cell simulations of quasi-parallel\nmagnetized collisionless shocks, we analyse the evolution of particle spectra.\nWe find that in the later stages of shock evolution, the initially strong\nsuprathermal part in the ion spectra fades, thus leaving the spectra composed\nof a Maxwellian and a power law. Once the electron and ion spectra flatten and\nbecome parallel, we find that the amounts of cosmic ray ions and electrons\nbecome similar. We make the step towards relating the micro and macro-scale\nphysics by applying this injection rule to Blasi's semi-analytical model of\nnon-linear diffusive shock acceleration, in order to obtain the particle\nspectra and electron-to-proton ratio $K_{\\mathrm{ep}}$ at high energies. By\nusing shock jump conditions that include the electron heating, we find\n$K_{\\mathrm{ep}}$ as a function of Mach number. For Mach number $\\sim$ 100, our\nmodel finely reproduces the typically observed ratio for Galactic cosmic-rays\n$K_{\\mathrm{ep}} \\sim$ 1:100 in the test particle regime.", "category": "astro-ph_HE" }, { "text": "3D simulations of microquasar jets in clumpy stellar winds: High-mass microquasars consist of a massive star and a compact object, the\nlatter producing jets that will interact with the stellar wind. The evolution\nof the jets, and ultimately their radiative outcome, could depend strongly on\nthe inhomogeneity of the wind, which calls for a detailed study. The\nhydrodynamics of the interaction between a jet and a clumpy wind is studied,\nfocusing on the global wind and single clump-jet interplay. We have performed,\nusing the code \\textit{Ratpenat}, three-dimensional numerical simulations of a\nclumpy wind interacting with a mildly relativistic jet, and of individual\nclumps penetrating into a jet. For typical wind and jet velocities, filling\nfactors of about > 0.1 are already enough for the wind to be considered as\nclumpy. An inhomogeneous wind makes the jet more unstable when crossing the\nsystem. Kinetic luminosities of the order 1.e37 erg/s allow the jet to reach\nthe borders of a compact binary with an O star, as in the smooth wind case,\nalthough with a substantially higher degree of disruption. When able to enter\ninto the jet, clumps are compressed and heated during a time of about their\nsize divided by the sound speed in the shocked clump. Then, clumps quickly\ndisrupt, mass-loading and slowing down the jet. We conclude that moderate wind\nclumpiness makes already a strong difference with the homogeneous wind case,\nenhancing jet disruption, mass-loading, bending, and likely energy dissipation\nin the form of emission. All this can have observational consequences at\nhigh-energies and also in the large scale radio jets.", "category": "astro-ph_HE" }, { "text": "XMM-Newton and Chandra observations of the unidentified Fermi-LAT source\n 3FGL J1016.5-6034: A Young Pulsar with a Nebula?: We report the discovery of a bright X-ray source in the XMM-Newton and\nChandra X-ray Observatory (CXO) images of the unidentified Fermi-LAT source\n3FGL J1016.5-6034. The XMM-Newton spectrum of the source is well fit by an\nabsorbed blackbody+power-law model with a temperature, $kT=0.20\\pm0.02$ keV,\nand photon index $\\Gamma=1.8\\pm0.1$. The CXO resolves the same source into a\npoint source (CXOU J101546.0-602939) and a surrounding compact nebula seen up\nto about $30''$ from the point source. The CXO spectrum of the nebula can be\ndescribed by an absorbed power-law with $\\Gamma=1.7\\pm0.3$ and is partly\nresponsible for the non-thermal emission observed in the XMM-Newton spectrum.\nThe XMM-Newton images also reveal faint extended emission on arcminute scales.\nThese properties strongly suggest that the X-ray source and the accompanying\nextended emission are a newly discovered young pulsar with a pulsar wind\nnebula. We also analyze $\\sim10$ years of Fermi-LAT data and find that the\nimproved LAT source localization is consistent with the position of CXOU\nJ101546.0-602939.", "category": "astro-ph_HE" }, { "text": "Have Cherenkov telescopes detected a new light boson?: Recent observations by H.E.S.S. and MAGIC strongly suggest that the Universe\nis more transparent to very-high-energy gamma rays than previously thought. We\nshow that this fact can be reconciled with standard blazar emission models\nprovided that photon oscillations into a very light Axion-Like Particle occur\nin extragalactic magnetic fields. A quantitative estimate of this effect indeed\nexplains the observed data and in particular the spectrum of blazar 3C279.", "category": "astro-ph_HE" }, { "text": "Nonparametric Representation of Neutron Star Equation of State Using\n Variational Autoencoder: We introduce a new nonparametric representation of the neutron star (NS)\nequation of state (EoS) by using the variational autoencoder (VAE). As a deep\nneural network, the VAE is frequently used for dimensionality reduction since\nit can compress input data to a low-dimensional latent space using the encoder\ncomponent and then reconstruct the data using the decoder component. Once a VAE\nis trained, one can take the decoder of the VAE as a generator. We employ\n100,000 EoSs that are generated using the nonparametric representation method\nbased on \\citet{2021ApJ...919...11H} as the training set and try different\nsettings of the neural network, then we get an EoS generator (trained VAE's\ndecoder) with four parameters. We use the mass\\textendash{}tidal-deformability\ndata of binary neutron star (BNS) merger event GW170817, the\nmass\\textendash{}radius data of PSR J0030+0451, PSR J0740+6620, PSR J0437-4715,\nand 4U 1702-429, and the nuclear constraints to perform the joint Bayesian\ninference. The overall results of the analysis that includes all the\nobservations are $R_{1.4}=12.59^{+0.36}_{-0.42}\\,\\rm km$,\n$\\Lambda_{1.4}=489^{+114}_{-110}$, and $M_{\\rm max}=2.20^{+0.37}_{-0.19}\\,\\rm\nM_\\odot$ ($90\\%$ credible levels), where $R_{1.4}$/$\\Lambda_{1.4}$ are the\nradius/tidal-deformability of a canonical $1.4\\,\\rm M_\\odot$ NS, and $M_{\\rm\nmax}$ is the maximum mass of a non-rotating NS. The results indicate that the\nimplementation of the VAE techniques can obtain the reasonable results, while\naccelerate calculation by a factor of $\\sim$ 3\\textendash10 or more, compared\nwith the original method.", "category": "astro-ph_HE" }, { "text": "Brightest Fermi-LAT Flares of PKS 1222+216: Implications on Emission and\n Acceleration Processes: We present a high time resolution study of the two brightest $\\gamma$-ray\noutbursts from a blazar PKS 1222+216 observed by the \\textit{Fermi} Large Area\nTelescope (LAT) in 2010. The $\\gamma$-ray light-curves obtained in four\ndifferent energy bands: 0.1--3, 0.1--0.3, 0.3--1 and 1--3 GeV, with time bin of\n6 hr, show asymmetric profiles with a similar rise time in all the bands but a\nrapid decline during the April flare and a gradual one during the June. The\nlight-curves during the April flare show $\\sim 2$ days long plateau in 0.1--0.3\nGeV emission, erratic variations in 0.3--1 GeV emission, and a daily recurring\nfeature in 1--3 GeV emission until the rapid rise and decline within a day. The\nJune flare shows a monotonic rise until the peak, followed by a gradual decline\npowered mainly by the multi-peak 0.1--0.3 GeV emission. The peak fluxes during\nboth the flares are similar except in the 1--3 GeV band in April which is twice\nthe corresponding flux during the June flare. Hardness ratios during the April\nflare indicate spectral hardening in the rising phase followed by softening\nduring the decay. We attribute this behavior to the development of a shock\nassociated with an increase in acceleration efficiency followed by its decay\nleading to spectral softening. The June flare suggests hardening during the\nrise followed by a complicated energy dependent behavior during the decay.\nObserved features during the June flare favor multiple emission regions while\nthe overall flaring episode can be related to jet dynamics.", "category": "astro-ph_HE" }, { "text": "Mechanism of Outflows in Accretion System: Advective Cooling Cannot\n Balance Viscous Heating?: Based on no-outflow assumption, we investigate steady state, axisymmetric,\noptically thin accretion flows in spherical coordinates. By comparing the\nvertically integrated advective cooling rate with the viscous heating rate, we\nfind that the former is generally less than 30% of the latter, which indicates\nthat the advective cooling itself cannot balance the viscous heating. As a\nconsequence, for radiatively inefficient flows with low accretion rates such as\n$\\dot M \\la 10^{-3} \\dot M_{Edd}$, where $\\dot M_{Edd}$ is the Eddington\naccretion rate, the viscous heating rate will be larger than the sum of the\nadvective cooling rate and the radiative cooling one. Thus, no thermal\nequilibrium can be established under the no-outflow assumption. We therefore\nargue that in such case outflows ought to occur and take away more than 70% of\nthe thermal energy generated by viscous dissipation. Similarly, for optically\nthick flows with extremely large accretion rates such as $\\dot M \\ga 10 \\dot\nM_{Edd}$, outflows should also occur due to the limited advection and the low\nefficiency of radiative cooling. Our results may help to understand the\nmechanism of outflows found in observations and numerical simulations.", "category": "astro-ph_HE" }, { "text": "Multi-wavelength temporal and spectral study of PKS 0402-362: We study the long-term behavior of the bright gamma-ray blazar PKS 0402-362.\nOver a span of approximately 12.5 years, from August 2008 to January 2021, we\ngathered Fermi-LAT temporal data and identified three distinct periods of\nintense $\\gamma$-ray activity. Notably, the second period exhibited the highest\nbrightness ever observed in this particular source. We observed most of the\n$\\gamma$-ray flare peaks to be asymmetric in profile suggesting a slow cooling\ntime of particles or the varying Doppler factor as the main cause of these\nflares. The $\\gamma$-ray spectrum is fitted with power-law and log-parabola\nmodels, and in both cases, the spectral index is very steep. The lack of time\nlags between optical-IR and $\\gamma$-ray emissions indicates the presence of a\nsingle-zone emission model. Using this information, we modeled the broadband\nSEDs with a simple one-zone leptonic model using the publicly available code\n`GAMERA'. The particle distribution index is found to be the same as expected\nin diffusive shock acceleration suggesting it as the main mechanism of particle\nacceleration to very high energy up to 4 - 6 GeV. During the different flux\nphases, we observed that the thermal disk dominates the optical emission,\nindicating that this source presents a valuable opportunity to investigate the\nconnection between the disk and the jet.", "category": "astro-ph_HE" }, { "text": "Search for correlations between the arrival directions of IceCube\n neutrino events and ultrahigh-energy cosmic rays detected by the Pierre Auger\n Observatory and the Telescope Array: This paper presents the results of different searches for correlations\nbetween very high-energy neutrino candidates detected by IceCube and the\nhighest-energy cosmic rays measured by the Pierre Auger Observatory and the\nTelescope Array. We first consider samples of cascade neutrino events and of\nhigh-energy neutrino-induced muon tracks, which provided evidence for a\nneutrino flux of astrophysical origin, and study their cross-correlation with\nthe ultrahigh-energy cosmic ray (UHECR) samples as a function of angular\nseparation. We also study their possible directional correlations using a\nlikelihood method stacking the neutrino arrival directions and adopting\ndifferent assumptions on the size of the UHECR magnetic deflections. Finally,\nwe perform another likelihood analysis stacking the UHECR directions and using\na sample of through-going muon tracks optimized for neutrino point-source\nsearches with sub-degree angular resolution. No indications of correlations at\ndiscovery level are obtained for any of the searches performed. The smallest of\nthe p-values comes from the search for correlation between UHECRs with IceCube\nhigh-energy cascades, a result that should continue to be monitored.", "category": "astro-ph_HE" }, { "text": "A Deep Observation of Gamma-ray Emission from Cassiopeia A using VERITAS: Supernova remnants (SNRs) have long been considered the leading candidates\nfor the accelerators of cosmic rays within the Galaxy through the process of\ndiffusive shock acceleration. The connection between SNRs and cosmic rays is\nsupported by the detection of high energy (HE; 100 MeV to 100 GeV) and very\nhigh energy (VHE; 100 GeV to 100 TeV) gamma rays from young and middle-aged\nSNRs. However, the interpretation of the gamma-ray observations is not unique.\nThis is because gamma rays can be produced both by electrons through\nnon-thermal Bremsstrahlung and inverse Compton scattering, and by protons\nthrough proton-proton collisions and subsequent neutral-pion decay. To\ndisentangle and quantify the contributions of electrons and protons to the\ngamma-ray flux, it is necessary to measure precisely the spectra and morphology\nof SNRs over a broad range of gamma-ray energies. Cassiopeia A (Cas A) is one\nsuch young SNR (~ 350 years) which is bright in radio and X-rays. It has been\ndetected as a bright point source in HE gamma rays by Fermi-LAT and in VHE\ngamma rays by HEGRA, MAGIC and VERITAS. Cas A has been observed with VERITAS\nfor more than 60 hours, tripling the published exposure. The observations span\n2007-2013, and half of the data were taken at large zenith angles to boost the\neffective area above few TeV. We will present the detailed spectral and\nmorphological results from the complete dataset.", "category": "astro-ph_HE" }, { "text": "Gamma-ray Observation of the Cygnus Region in the 100 TeV Energy Region: We report observations of gamma-ray emissions with energies in the 100 TeV\nenergy region from the Cygnus region in our Galaxy. Two sources are\nsignificantly detected in the directions of the Cygnus OB1 and OB2\nassociations. Based on their positional coincidences, we associate one with a\npulsar PSR J2032+4127 and the other mainly with a pulsar wind nebula PWN\nG75.2+0.1 with the pulsar moving away from its original birthplace situated\naround the centroid of the observed gamma-ray emission. This work would\nstimulate further studies of particle acceleration mechanisms at these\ngamma-ray sources.", "category": "astro-ph_HE" }, { "text": "Dead zone in the polar-cap accelerator of pulsars: We study plasma flows above pulsar polar caps using time-dependent\nsimulations of plasma particles in the self-consistent electric field. The flow\nbehavior is controlled by the dimensionless parameter alpha=(j/c rho_GJ) where\nj is the electric current density and rho_GJ is the Goldreich-Julian charge\ndensity. The region of the polar cap where 00). In\nthe latter case, we consider the possibility of a mixed flow consisting of\ndifferent ion species, and observe the development of two-stream instability.\nThe dead zone at the polar cap is essential for the development of an outer gap\nnear the null surface rho_GJ=0.", "category": "astro-ph_HE" }, { "text": "Towards an Understanding of Changing-Look Quasars: An Archival\n Spectroscopic Search in SDSS: The uncertain origin of the recently-discovered `changing-looking' quasar\nphenomenon -- in which a luminous quasar dims significantly to a quiescent\nstate in repeat spectroscopy over ~10 year timescales -- may present unexpected\nchallenges to our understanding of quasar accretion. To better understand this\nphenomenon, we take a first step to building a sample of changing-look quasars\nwith a systematic but simple archival search for these objects in the Sloan\nDigital Sky Survey Data Release 12. By leveraging the >10 year baselines for\nobjects with repeat spectroscopy, we uncover two new changing-look quasars, and\na third discovered previously. Decomposition of the multi-epoch spectra and\nanalysis of the broad emission lines suggest that the quasar accretion disk\nemission dims due to rapidly decreasing accretion rates (by factors of >2.5),\nwhile disfavoring changes in intrinsic dust extinction for the two objects\nwhere these analyses are possible. Broad emission line energetics also support\nintrinsic dimming of quasar emission as the origin for this phenomenon rather\nthan transient tidal disruption events or supernovae. Although our search\ncriteria included quasars at all redshifts and transitions from either\nquasar-like to galaxy-like states or the reverse, all of the clear cases of\nchanging-look quasars discovered were at relatively low-redshift (z ~ 0.2 -\n0.3) and only exhibit quasar-like to galaxy-like transitions.", "category": "astro-ph_HE" }, { "text": "Linear Analysis of Fast-Pairwise Collective Neutrino Oscillations in\n Core-Collapse Supernovae based on the Results of Boltzmann Simulations: Neutrinos are densely populated deep inside the core of massive stars after\ntheir gravitational collapse to produce supernova explosions and form compact\nstars such as neutron stars (NS) and black holes (BH). It has been considered\nthat they may change their flavor identities through so-called fast-pairwise\nconversions induced by mutual forward scatterings. If that is really the case,\nthe dynamics of supernova explosion will be influenced, since the conversion\nmay occur near the neutrino sphere, from which neutrinos are effectively\nemitted. In this paper, we conduct a pilot study of such possibilities based on\nthe results of fully self-consistent, realistic simulations of a core-collapse\nsupernova explosion in two spatial dimensions under axisymmetry. As we solved\nthe Boltzmann equations for neutrino transfer in the simulation not as a\npost-process but in real time, the angular distributions of neutrinos in\nmomentum space for all points in the core at all times are available, a\ndistinct feature of our simulations. We employ some of these distributions\nextracted at a few selected points and times from the numerical data and apply\nlinear analysis to assess the possibility of the conversion. We focus on the\nvicinity of the neutrino sphere, where different species of neutrinos move in\ndifferent directions and have different angular distributions as a result. This\nis a pilot study for a more thorough survey that will follow soon. We find no\npositive sign of conversion unfortunately at least for the spatial points and\ntimes we studied in this particular model. We hence investigate rather in\ndetail the condition for the conversion by modifying the neutrino distributions\nrather arbitrarily by hand.", "category": "astro-ph_HE" }, { "text": "Constraining the size of the Comptonizing medium by modeling the energy\n dependent time-lags of kHz QPOs of Neutron star system: In earlier works, we had shown that the observed soft lags and r.m.s versus\nenergy of the lower kHz QPO of neutron star binaries can be explained in the\nframework of a thermal Comptonization model. It was also shown that such an\ninterpretation can provide estimates of the size and geometry of the\nComptonizing medium. Here we study the dependence of these estimates on the\ntime-averaged spectral model assumed and on the frequency of the QPO. We use\nthe high quality time lag and r.m.s obtained during March 3rd 1996 observation\nof 4U 1608-52 by RXTE as well as other observations of the source at different\nQPO frequencies where a single time-lag between two broad energy bands have\nbeen reported. We compare the results obtained when assuming that the\ntime-averaged spectra are represented by the spectrally degenerate \"hot\" and\n\"cold\" seed photon spectral models. We find that for the \"hot\" seed photon\nmodel the medium size is in the range of 0.3-2.0 kms and the size decreases\nwith increasing QPO frequency. On the other hand for the \"cold\" seed photon\nmodel the range for the sizes are much larger 0.5-20 kms and hence perhaps show\nno variation with QPO frequency. Our results emphasis the need for broad band\nspectral information combined with high frequency timing to lift this\ndegeneracy. We further show that the r.m.s as a function of energy for the\nupper kHz QPO is similar to the lower one and indeed we find that the driver\nfor this QPO should be temperature variations of the corona identical to the\nlower kHz QPO. However, the time lag reported for the upper kHz QPO is hard,\nwhich if confirmed, would challenge the simple Comptonization model presented\nhere. It would perhaps imply that reverberation lags are also important and/or\nthe dominate spectral component is not a single temperature medium but a\nmulti-temperature complex one.", "category": "astro-ph_HE" }, { "text": "Chasing candidate Supergiant Fast X-ray Transients in the 1,000 orbits\n INTEGRAL/IBIS catalog: We report results from an investigation at hard X-rays (above 18 keV) and\nsoft X-rays (below 10 keV) of a sample of X-ray transients located on the\nGalactic plane and detected with the bursticity method, as reported in the\nlatest 1,000 orbits INTEGRAL/IBIS catalog. Our main aim has been to individuate\nthose with X-rays characteristics strongly resembling Supergiant Fast X-ray\nTransients (SFXTs). As a result, we found four unidentified fast X-ray\ntransients which now can be considered good SFXT candidates. In particular,\nthree transients (IGR J16374-5043, IGR J17375-3022 and IGR J12341-6143) were\nvery poorly studied in the literature before the current work, and our findings\nlargely improved the knowledge of their X-ray characteristics. The other\ntransient (XTE J1829-098) was previously studied in detail only below 10 keV,\nconversely the current work provides the first detailed study in outburst above\n18 keV. In addition we used archival infrared observations of the transients to\npinpoint, among the field objects, their best candidate counterpart. We found\nthat their photometric properties are compatible with an early type spectral\nclassification, further supporting our proposed nature of SFXTs. Infrared\nspectroscopy is advised to confirm or disprove our interpretation. The reported\nfindings allowed a significant increase of the sample of candidate SFXTs known\nto date, effectively doubling their number.", "category": "astro-ph_HE" }, { "text": "Infrared study of H1743-322 in outburst: a radio-quiet and NIR-dim\n microquasar: The X-ray binary, black hole candidate, and microquasar H1743-322 exhibited a\nseries of X-ray outbursts between 2003 and 2008. We took optical/infrared (OIR)\nobservations with the ESO/NTT telescope during 3 of these outbursts (2003,\n2004, and 2008), to study its spectral energy distribution (SED).\n We detect rapid flares of duration ~5 mn in the high time-resolution IR\nlightcurve. We identify H and He emission lines in the IR spectra, coming from\nthe accretion disk. The IR SED exhibits the spectral index typically associated\nwith the X-ray high, soft state in our observations taken during the 2003 and\n2004 outbursts, while the index changes to one that is typical of the X-ray\nlow, hard state during the 2008 outburst. During this last outburst, we\ndetected a change of slope in the NIR spectrum between the J and Ks bands,\nwhere the JH part is characteristic of an optically thick disk emission, while\nthe HKs part is typical of optically thin synchrotron emission. Furthermore,\nthe comparison of our IR data with radio and X-ray data shows that H1743-322\nexhibits a faint jet both in radio and NIR domains. Finally, we suggest that\nthe companion star is a late-type main sequence star located in the Galactic\nbulge.\n These OIR photometric and spectroscopic observations of the microquasar\nH1743-322, the first of this source to be published in a broad multiwavelength\ncontext, allow us to unambiguously identify two spectra of different origins in\nthe OIR domain, evolving from optically thick thermal emission to optically\nthin synchrotron emission toward longer wavelengths. Comparing these OIR\nobservations with other black hole candidates suggests that H1743-322 behaves\nlike a radio-quiet and NIR-dim black hole in the low, hard state. This study\nwill be useful when quantitatively comparing the overall contribution of the\ncompact jet and accretion flow in the energy budget of microquasars.", "category": "astro-ph_HE" }, { "text": "Simulations of coalescing black holes: We describe the methods and results of numerical simulations of coalescing\nblack holes. The simulation in dynamical spacetime covers the inspiral, merger,\nand ringdown phases. We analyze the emission of gravitational waves and\nproperties of a black hole being the merger product. We discuss the results in\nthe context of astrophysical environment of black holes that exist in the\nUniverse.", "category": "astro-ph_HE" }, { "text": "Massive BH Binaries as Periodically-Variable AGN: Massive black-hole (MBH) binaries, which are expected to form following the\nmerger of their parent galaxies, produce gravitational waves which will be\ndetectable by Pulsar Timing Arrays at nanohertz frequencies (year periods).\nWhile no confirmed, compact MBH binary systems have been seen in\nelectromagnetic observations, a large number of candidates have recently been\nidentified in optical surveys of AGN variability. Using a combination of\ncosmological, hydrodynamic simulations; comprehensive, semi-analytic binary\nmerger models; and analytic AGN spectra and variability prescriptions; we\ncalculate the expected electromagnetic detection rates of MBH binaries as\nperiodically variable AGN. In particular, we consider two independent\nvariability models: (i) Doppler boosting due to large orbital velocities, and\n(ii) hydrodynamic variability in which the fueling of MBH accretion disks is\nperiodically modulated by the companion. Our models predict that numerous MBH\nbinaries should be present and distinguishable in the existing data. In\nparticular, our fiducial models produce an expectation value of $0.2$ (Doppler)\nand $5$ (hydrodynamic) binaries to be identifiable in CRTS, while $20$ and\n$100$ are expected after five years of LSST observations. The brightness\nvariations in most systems are too small to be distinguishable, but almost\n$1\\%$ of AGN at redshifts $z \\lesssim 0.6$ could be in massive binaries. We\nanalyze the predicted binary parameters of observable systems and their\nselection biases, and include an extensive discussion of our model parameters\nand uncertainties.", "category": "astro-ph_HE" }, { "text": "Evolution of Accretion Disc Geometry of GRS~1915+105 during its $\u03c7$\n state as revealed by TCAF solution: The evolution of the C-type low frequency quasi-periodic oscillations\n(LFQPOs) and associated time lag in transient black hole sources as a function\nof time can be explained by variation of the Compton cloud size in a Two\nComponent Advective Flow solution (TCAF). A similar study of a persistent\nsource, GRS~1915+105, has not been attempted. We fit the evolution of QPOs with\npropagatory oscillating shock (POS) solution for two sets of so-called\n$\\chi$-state observations and find that the shock steadily recedes with almost\nconstant velocity when QPO frequency is decreasing and the spectrum is\nhardening. The shock moves inward with a constant velocity $v_0=473.0$ cm\ns$^{-1}$ and $v_0=400.0$ cm s$^{-1}$ respectively in these two cases, when the\nQPO frequency is increasing and the spectrum softens. This behavior is similar\nto what was observed in XTE~J1550-564 during the 1998 outburst. The time lag\nmeasured at the QPO frequency varies in a similar way as the size of the\nCompton cloud. Most interestingly, in both the cases, the lag switches sign\n(hard lag to soft lag) at a QPO frequency of $\\sim 2.3 - 2.5$ Hz irrespective\nof the energy of photons. We find, at very low frequencies $< 1$ Hz, the\nComptonizing Efficiency (CE) increases with QPO frequency and at higher QPO\nfrequencies the trend is opposite. The time lags become mostly positive at all\nenergies when CE is larger than $\\sim 0.85\\%$ for both the sources.", "category": "astro-ph_HE" }, { "text": "Potential for Precision Measurement of Low-Energy Antiprotons with GAPS\n for Dark Matter and Primordial Black Hole Physics: The general antiparticle spectrometer (GAPS) experiment is a proposed\nindirect dark matter search focusing on antiparticles produced by WIMP (weakly\ninteracting massive particle) annihilation and decay in the Galactic halo. In\naddition to the very powerful search channel provided by antideuterons, GAPS\nhas a strong capability to measure low-energy antiprotons (0.07 $\\le$ E $\\le$\n0.25 GeV) as dark matter signatures. This is an especially effective means for\nprobing light dark matter, whose existence has been hinted at in the direct\ndark matter searches, including the recent result from the CDMS-II experiment.\nWhile severely constrained by LUX and other direct dark matter searches, light\ndark matter candidates are still viable in an isospin-violating dark matter\nscenario and halo-independent analysis. Along with the excellent antideuteron\nsensitivity, GAPS will be able to detect an order of magnitude more low-energy\nantiprotons, compared to BESS, PAMELA and AMS-02, providing a precision\nmeasurement of low-energy antiproton flux and a unique channel for probing\nlight dark matter models. Additionally, dark matter signatures from gravitinos\nand Kaluza-Klein right-handed neutrinos as well as evidence of primordial black\nhole evaporation can be observed through low-energy antiproton search.", "category": "astro-ph_HE" }, { "text": "Effects of nuclear matter and composition in core-collapse supernovae\n and long-term proto-neutron star cooling: We study the influence of hot and dense matter in core-collapse supernovae by\nadopting up-to-date nuclear equation of state (EOS) based on the microscopic\nnuclear many-body frameworks. We explore effects of EOS based on the Dirac\nBrueckner Hartree-Fock theory through comparisons with those based on the\nvariational method. We also examine effects of the differences in the\ncomposition of nuclei and nucleons by using the same EOS by the variational\nmethod but employing two different treatments in computations of nuclear\nabundances. We perform numerical simulations of core-collapse supernovae\nadopting the three EOSs. We also perform numerical simulations of the long-term\nevolution over 70 s of the proto-neutron star cooling. We show that impacts by\ndifferent modeling of composition are remarkable as in those by different\ntreatments of uniform matter in the gravitational collapse, bounce, and shock\npropagation. The cooling of proto-neutron star and the resulting neutrino\nemission are also affected by the compositional difference even if the same\ntreatment in computing uniform matter of EOS.", "category": "astro-ph_HE" }, { "text": "Phase-Resolved Spectroscopy of the Low-Mass X-ray Binary V801 Ara: We present phase-resolved optical spectra of the low mass X-ray binary system\nV801 Ara. The spectra, obtained in 2014 with IMACS on the Magellan/Baade\ntelescope at Las Campanas Observatory, cover the full binary orbit of 3.8\nhours. They contain strong emission features allowing us to map the emission of\nHAlpha, HBeta, He II 4686, and the Bowen blend at 4640. The radial velocity\ncurves of the Bowen blend shows significantly stronger modulation at the\norbital period than HAlpha as expected for the former originating on the\nsecondary with the latter consistent with emission dominated by the disk. Our\ntomograms of HAlpha and HBeta are the most detailed studies of these lines for\nV801 to date and they clearly detect the accretion disk. The HBeta emission\nextends to higher velocities than HAlpha, suggesting emission from closer to\nthe neutron star and differentiating temperature variance in the accretion disk\nfor the first time. The center of the accretion disk appears offset from the\ncenter-of-mass of the neutron star as has been seen in several other X-ray\nbinaries. This is often interpreted to imply disk eccentricity. Our tomograms\ndo not show strong evidence for a hot spot at the point where the accretion\nstream hits the disk. This could imply a reduced accretion rate or could be due\nto the spot being drowned out by bright accretion flow around it. There is\nenhanced emission further along the disk, however, which implies gas stream\ninteraction downstream of the hot spot.", "category": "astro-ph_HE" }, { "text": "Pulse Profile Modeling of Thermonuclear Burst Oscillations I: The Effect\n of Neglecting Variability: We study the effects of the time-variable properties of thermonuclear X-ray\nbursts on modeling their millisecond-period burst oscillations. We apply the\npulse profile modeling technique that is being used in the analysis of\nrotation-powered millisecond pulsars by the Neutron Star Interior Composition\nExplorer (NICER) to infer masses, radii, and geometric parameters of neutron\nstars. By simulating and analyzing a large set of models, we show that\noverlooking burst time-scale variability in temperatures and sizes of the hot\nemitting regions can result in substantial bias in the inferred mass and\nradius. To adequately infer neutron star properties, it is essential to develop\na model for the time variable properties or invest a substantial amount of\ncomputational time in segmenting the data into non-varying pieces. We discuss\nprospects for constraints from proposed future X-ray telescopes.", "category": "astro-ph_HE" }, { "text": "A low-mass binary neutron star: long-term ejecta evolution and kilonovae\n with weak blue emission: We study the long-term evolution of ejecta formed in a binary neutron star\n(BNS) merger that results in a long-lived remnant NS by performing a\nhydrodynamics simulation with the outflow data of a numerical relativity\nsimulation as the initial condition. At the homologously expanding phase, the\ntotal ejecta mass reaches $\\approx0.1\\,M_\\odot$ with an average velocity of\n$\\approx0.1\\,c$ and lanthanide fraction of $\\approx 0.005$. We further perform\nthe radiative transfer simulation employing the obtained ejecta profile. We\nfind that, contrary to a naive expectation from the large ejecta mass and low\nlanthanide fraction, the optical emission is not as bright as that in\nGW170817/AT2017gfo, while the infrared emission can be brighter. This light\ncurve property is attributed to preferential diffusion of photons toward the\nequatorial direction due to the prolate ejecta morphology, large opacity\ncontribution of Zr, Y, and lanthanides, and low specific heating rate of the\nejecta. Our results suggest that these light curve features could be used as an\nindicator for the presence of a long-lived remnant NS. We also found that the\nbright optical emission broadly consistent with GW170817/AT2017gfo is realized\nfor the case that the high-velocity ejecta components in the polar region are\nsuppressed. These results suggest that the remnant in GW170817/AT2017gfo is\nunlikely to be a long-lived NS, but might have collapsed to a black hole within\n${\\cal O}(0.1)$ s.", "category": "astro-ph_HE" }, { "text": "A VLA search for 5 GHz radio transients and variables at low Galactic\n latitudes: We present the results of a 5 GHz survey with the Very Large Array, designed\nto search for short-lived (<1 day) transients and to characterize the\nvariability of radio sources at milli-Jansky levels. A total sky area of 2.66\ndeg^2, spread over 141 fields at low Galactic latitudes was observed 16 times\nwith a cadence sampling timescales of days, months and years. Most of the data\nwere searched for transients in near real time. Candidates were followed up\nusing visible light telescopes (1-2 hr delays) and the X-Ray Telescope on board\nthe Swift satellite. The final processing of the data revealed a single\npossible transient with a flux density of 2.4 mJy. This implies a transients,\n>1.8 mJy, sky surface density of 0.039 (-0.032/+0.13) deg^-2. This areal\ndensity is consistent with the sky surface density of transients from the Bower\net al. survey extrapolated to 1.8 mJy. Our observed transient areal density is\nconsistent with a Neutron Stars (NSs) origin for these events. Furthermore, we\nuse the data to measure the sources variability on days to years time scales,\nand we present the variability structure function of 5 GHz sources. The mean\nstructure function shows a fast increase on ~1 day time scale, followed by a\nslower increase on time scales of up to 10 days. On time scales between 10-60\ndays the structure function is roughly constant. We find that >30% of the\nunresolved sources brighter than 1.8 mJy are variable at the >4 sigma\nconfidence level, presumably due mainly to refractive scintillation.", "category": "astro-ph_HE" }, { "text": "Dependence of inner accretion disk stress on parameters: the\n Schwarzschild case: We explore the parameter dependence of inner disk stress in black hole\naccretion by contrasting the results of a number of simulations, all employing\n3-d general relativistic MHD in a Schwarzschild spacetime. Five of these\nsimulations were performed with the intrinsically conservative code HARM3D,\nwhich allows careful regulation of the disk aspect ratio, H/R; our simulations\nspan a range in H/R from 0.06 to 0.17. We contrast these simulations with two\npreviously reported simulations in a Schwarzschild spacetime in order to\ninvestigate possible dependence of the inner disk stress on magnetic topology.\nIn all cases, much care was devoted to technical issues: ensuring adequate\nresolution and azimuthal extent, and averaging only over those time-periods\nwhen the accretion flow is in approximate inflow equilibrium. We find that the\ntime-averaged radial-dependence of fluid-frame electromagnetic stress is almost\ncompletely independent of both disk thickness and poloidal magnetic topology.\nIt rises smoothly inward at all radii (exhibiting no feature associated with\nthe ISCO) until just outside the event horizon, where the stress plummets to\nzero. Reynolds stress can also be significant near the ISCO and in the plunging\nregion; the magnitude of this stress, however, depends on both disk thickness\nand magnetic topology. The two stresses combine to make the net angular\nmomentum accreted per unit rest-mass 7-15% less than the angular momentum of\nthe ISCO.", "category": "astro-ph_HE" }, { "text": "Hydrodynamics of Core-Collapse Supernovae at the Transition to\n Explosion. I. Spherical Symmetry: We study the transition to runaway expansion of an initially stalled\ncore-collapse supernova shock. The neutrino luminosity, mass accretion rate,\nand neutrinospheric radius are all treated as free parameters. In spherical\nsymmetry, this transition is mediated by a global non-adiabatic instability\nthat develops on the advection time and reaches non-linear amplitude. Here we\nperform high-resolution, time-dependent hydrodynamic simulations of stalled\nsupernova shocks with realistic microphysics to analyze this transition. We\nfind that radial instability is a sufficient condition for runaway expansion if\nthe neutrinospheric parameters do not vary with time and if heating by the\naccretion luminosity is neglected. For a given unstable mode, transition to\nrunaway occurs when fluid in the gain region reaches positive specific energy.\nWe find approximate instability criteria that accurately describe the behavior\nof the system over a wide region of parameter space. The threshold neutrino\nluminosities are in general different than the limiting value for a\nsteady-state solution. We hypothesize that multidimensional explosions arise\nfrom the excitation of unstable large-scale modes of the turbulent background\nflow, at threshold luminosities that are lower than in the laminar case.", "category": "astro-ph_HE" }, { "text": "Constraining the lensing of binary neutron stars from their stochastic\n background: Gravitational wave (GW) transients from binary neutron star (BNS)\ncoalescences can, in principle, be subject to gravitational lensing thereby\nincreasing the amplitude and signal-to-noise ratio. We estimate the rate of\nlensed BNS events resolvable by LIGO and Virgo and find that it is constrained\nby the current non-detection of a stochastic GW background. Following closely\nthe formalism we developed (10.1103/PhysRevLett.125.141102) in the context of\nbinary black hole lensing, we show that at current sensitivities the fraction\nof BNS coalescences with lensing magnifications $\\mu> 1.02$ is less than $\\sim\n7\\times 10^{-8}$ and therefore such events should not be expected in the near\nfuture. We also make predictions for projected future sensitivities.", "category": "astro-ph_HE" }, { "text": "Inferences from the Distributions of Fast Radio Burst Pulse Widths,\n Dispersion Measures and Fluences: The widths, dispersion measures, dispersion indices and fluences of Fast\nRadio Bursts (FRB) impose coupled constraints that all models must satisfy.\nObservation of dispersion indices close to their low density limit of $-2$ sets\na model-independent upper bound on the electron density and a lower bound on\nthe size of any dispersive plasma cloud. The non-monotonic dependence of burst\nwidths (after deconvolution of instrumental effects) on dispersion measure\nexcludes the intergalactic medium as the location of scattering that broadens\nthe FRB in time. Temporal broadening far greater than that of pulsars at\nsimilar high Galactic latitudes implies that scattering occurs close to the\nsources, where high densities and strong turbulence are plausible. FRB\nenergetics are consistent with supergiant pulses from young, fast, high-field\npulsars at cosmological distances. The distribution of FRB dispersion measures\nis inconsistent with expanding clouds (such as SNR). It excludes space-limited\ndistributions (such as the local supercluster), but agrees with a homogeneous\ncosmological distribution with intergalactic dispersion. The FRB\n$\\log{N}$--$\\log{S}$ relation also indicates a cosmological distribution, aside\nfrom the anomalously bright Lorimer burst.", "category": "astro-ph_HE" }, { "text": "Electromagnetic Window into the Dawn of Black Holes: Massive 10^6-10^10 Msun black holes (BHs) are ubiquitous in local galactic\nnuclei. They were common by the time the Universe is several Gyr old, and many\nof them were in place within the first 1~Gyr after the Big Bang. Their quick\nassembly has been attributed to mechanisms such as the rapid collapse of gas\ninto the nuclei of early protogalaxies, accretion and mergers of stellar-mass\nBHs accompanying structure formation at early times, and the runaway collapse\nof early, ultra-dense stellar clusters. The origin of the early massive BHs\nremains an intriguing and long-standing unsolved puzzle in astrophysics. Here\nwe discuss strategies for discerning between BH seeding models using\nelectromagnetic observations. We argue that the most direct answers will be\nobtained through detection of BHs with masses M<10^5 Msun at redshifts z>10,\nwhere we expect them to first form. Reaching out to these redshifts and down to\nthese masses is crucial, because BHs are expected to lose the memory of their\ninitial assembly by the time they grow well above 10^5 Msun and are\nincorporated into higher-mass galaxies. The best way to detect 10^4-10^5 Msun\nBHs at high redshifts is by a sensitive X-ray survey. Critical constraining\npower is augmented by establishing the properties and the environments of their\nhost galaxies in deep optical/IR imaging surveys. Required OIR data can be\nobtained with the JWST and WFIRST missions. The required X-ray flux limits\n(down to 10^{-19} erg/s/cm^2) are accessible only with a next-generation X-ray\nobservatory which has both high (sub-1\") angular resolution and high\nthroughput. A combination of deep X-ray and OIR surveys will be capable of\nprobing several generic markers of the BH seed scenarios, and resolving the\nlong-stanging puzzle of their origin. These electromagnetic observations are\nalso highly synergistic with the information from LISA on high-z BH mergers.", "category": "astro-ph_HE" }, { "text": "Dynamical Formation Scenarios for GW190521 and Prospects for Decihertz\n Gravitational-Wave Astronomy with GW190521-Like Binaries: The gravitational-wave (GW) detection of GW190521 has provided new insights\non the mass distribution of black holes and new constraints for astrophysical\nformation channels. With independent claims of GW190521 having significant\npre-merger eccentricity, we investigate what this implies for GW190521-like\nbinaries that form dynamically. The Laser Interferometer Space Antenna (LISA)\nwill also be sensitive to GW190521-like binaries if they are circular from an\nisolated formation channel. We show, however, that GW190521-like binaries that\nform dynamically may skip the LISA band entirely. To this end, we simulate\nGW190521 analogues that dynamically form via post-Newtonian binary-single\nscattering. From these scattering experiments, we find that GW190521-like\nbinaries may enter the LIGO-Virgo band with significant eccentricity as\nsuggested by recent studies, though well below an eccentricity of $e_{\\rm 10Hz}\n\\lesssim 0.7$. Eccentric GW190521-like binaries further motivate the\nastrophysical science case for a decihertz GW observatory, such as the\nkilometer-scale version of the Midband Atomic Gravitational-wave\nInterferometric Sensor (MAGIS). Pre-merger observations of GW190521-like\nbinaries with such a decihertz GW detector would be able to constrain the\neccentricity of GW190521-like binaries to greater precision than with just\nLIGO-Virgo alone. These eccentricity constraints would also provide additional\ninsights into the possible environments that GW190521-like binaries form in.", "category": "astro-ph_HE" }, { "text": "Evidence for GeV Pair Halos around Low Redshift Blazars: We report on the results of a search for $\\gamma$-ray pair halos with a\nstacking analysis of low-redshift blazars using data from the Fermi Large Area\nTelescope. For this analysis we used a number of a-priori selection criteria,\nincluding the spatial and spectral properties of the Fermi sources. The angular\ndistribution of $\\sim$ 1GeV photons around 24 stacked isolated\nhigh-synchrotron-peaked BL Lacs with redshift $z<0.5$ shows an excess over that\nof point-like sources. A statistical analysis yields a Bayes factor of\n$\\mathrm{log}_{10}B_{10}>2$, providing evidence in favor of extended emission\nagainst the point-source hypothesis, consistent with expectations for pair\nhalos produced in the IGMF with strength $B_{\\mathrm{IGMF}}\\sim\n10^{-17}-10^{-15}\\mathrm{G}$.", "category": "astro-ph_HE" }, { "text": "The XMM-Newton survey of the Small Magellanic Cloud:\n XMMUJ010633.1-731543 and XMMUJ010743.1-715953, two new Be/X-ray binary\n systems: In the course of the XMM-Newton survey of the Small Magellanic Cloud (SMC),\ntwo new bright X-ray sources were discovered exhibiting the spectral\ncharacteris- tics of High Mass X-ray Binaries - but revealing only weak\nevidence for pulsations in just one of the objects(at 153s in\nXMMUJ010743.1-715953). The accurate X- ray source locations permit the\nidentification of these X-ray source with Be stars, thereby strongly suggesting\nthese systems are new Be/X-ray binaries. From blue spectra the proposed\nclassification for XMMUJ010633.1-731543 is B0.5-1Ve and for\nXMMUJ010743.1-715953 it is B2IV-Ve.", "category": "astro-ph_HE" }, { "text": "Lorentz Violation: Motivation and new constraints: We review the main theoretical motivations and observational constraints on\nPlanck scale suppressed violations of Lorentz invariance. After introducing the\nproblems related to the phenomenological study of quantum gravitational\neffects, we discuss the main theoretical frameworks within which possible\ndepartures from Lorentz invariance can be described. In particular, we focus on\nthe framework of Effective Field Theory, describing several possible ways of\nincluding Lorentz violation therein and discussing their theoretical viability.\nWe review the main low energy effects that are expected in this framework. We\ndiscuss the current observational constraints on such a framework, focusing on\nthose achievable through high-energy astrophysics observations. In this context\nwe present a summary of the most recent and strongest constraints on QED with\nLorentz violating non-renormalizable operators. Finally, we discuss the present\nstatus of the field and its future perspectives.", "category": "astro-ph_HE" }, { "text": "Investigating evidence for different black hole accretion modes since\n redshift z~1: Chandra data in the COSMOS, AEGIS-XD and 4Ms CDFS are combined with\noptical/near-IR photometry to determine the rest-frame U-V vs V-J colours of\nX-ray AGN hosts at mean redshifts 0.40 and 0.85. This combination of colours\n(UVJ) provides an efficient means of separating quiescent from star-forming,\nincluding dust reddened, galaxies. Morphological information emphasises\ndifferences between AGN split by their UVJ colours. AGN in quiescent galaxies\nare dominated by spheroids, while star-forming hosts are split between bulges\nand disks. The UVJ diagram of AGN hosts is then used to set limits on the\naccretion density associated with evolved and star-forming systems. Most of the\nblack hole growth since z~1 is associated with star-forming hosts.\nNevertheless, ~15-20% of the X-ray luminosity density since z~1, is taking\nplace in the quiescent region of the UVJ diagram. For the z~0.40 subsample,\nthere is tentative evidence (2sigma significance), that AGN split by their UVJ\ncolours differ in Eddington ratio. AGN in star-forming hosts dominate at high\nEddington ratios, while AGN in quiescent hosts become increasingly important as\na fraction of the total population toward low Eddington ratios. At higher\nredshift, z~0.8, such differences are significant at the 2sigma level only at\nEddington ratios >1e-3. These findings are consistent with scenarios in which\ndiverse accretion modes are responsible for the build-up of SMBHs at the\ncentres of galaxies. We compare our results with the GALFORM semi-analytic\nmodel, which postulates two black hole fuelling modes, the first linked to\nstar-formation and the second occuring in passive galaxies. GALFORM predicts a\nlarger fraction of black hole growth in quiescent galaxies at z<1, compared to\nthe data. Relaxing the strong assumption of the model that passive AGN hosts\nhave zero star-formation rate could reconcile this disagreement.", "category": "astro-ph_HE" }, { "text": "Can very massive Population III stars produce a super-collapsar?: A fraction of the first generation of stars in the early Universe may be very\nmassive ($\\gtrsim 300~\\mathrm{M_\\odot}$) as they form in metal-free\nenvironments. Formation of black holes from these stars can be accompanied by\nsupermassive collapsars to produce long gamma-ray bursts of a unique type\nhaving a very high total energy ($\\sim 10^{54}~\\mathrm{erg}$) as recently\nsuggested by several authors. We present new stellar evolution models of very\nmassive Population III stars including the effect of rotation to provide\ntheoretical constraints on super-collapsar progenitors. We find that the\nangular momentum condition for super-collapsar can be fulfilled if magnetic\ntorques are ignored, in which case Eddington-Sweet circulations play the\ndominant role for the transport of angular momentum. We further find that the\ninitial mass range for super-collapsar progenitors would be limited to\n$300~\\mathrm{M_\\odot} \\lesssim M \\lesssim 700~\\mathrm{M_\\odot}$. However, all\nof our very massive star models of this mass range end their lives as red\nsupergiants rather than blue supergiants, in good agreement with most of the\nprevious studies. The predicted final fate of these stars is either a\njet-powered type IIP supernova or an ultra-long, relatively faint gamma-ray\ntransient, depending on the initial amount of angular momentum.", "category": "astro-ph_HE" }, { "text": "Observation of Anisotropy in the Arrival Direction Distribution of TeV\n Cosmic Rays with HAWC: The High-Altitude Water Cherenkov (HAWC) Observatory, located 4100 m above\nsea level near Sierra Negra (19$^\\circ$ N) in Mexico, is sensitive to gamma\nrays and cosmic rays at TeV energies. The arrival direction distribution of\ncosmic rays at these energies shows significant anisotropy on several angular\nscales, with a relative intensity ranging between 10$^{-3}$ and 10$^{-4}$. We\npresent the results of a study of cosmic-ray anisotropy based on more than 86\nbillion cosmic-ray air showers recorded with HAWC since June 2013. The HAWC\ncosmic-ray sky map, which has a median energy of 2 TeV, exhibits several\nregions of significantly enhanced cosmic-ray flux. We present the energy\ndependence of the anisotropy and the cosmic-ray spectrum in the regions of\nsignificant excess.", "category": "astro-ph_HE" }, { "text": "A systematic study of photoionized emission and warm absorption\n signatures of the NLS1 Mrk 335: We present an analysis of all the archival high resolution spectra of the\nNarrow-line Seyfert 1 galaxy Mrk~335 obtained with Reflection Grating\nSpectrometer (RGS) on board \\textit{XMM-Newton}. The spectra show rich emission\nand absorption features in low and intermediate flux intervals. We model the\nemission lines with the \\textsc{pion\\_xs} grid and try to find any possible\ncorrelation between the properties of the emitting gas and the source flux.\nCurrent data does not allow detailed trace of the response of the line emitting\ngas to the X-ray flux of Mrk~335, but the flux of the X-ray lines is\nsignificantly less variable than the X-ray continuum. We also find that the\nwarm absorber's properties are not correlated with the flux variability. From\nthe latest \\textit{XMM-Newton} observation in 2019 December, we find that the\nphotoionized emission and distant reflection components have not responded to\nthe flux drop of Mrk~335 from 2018 July. However, the possible existence of\npartial covering absorber in the 2018--2019 low state of Mrk~335 makes it\ndifficult to constrain the scale of the emitting gas using this lack of\nresponse.", "category": "astro-ph_HE" }, { "text": "Growing evidence that SNe Iax are not a one-parameter family: the case\n of PS1-12bwh: In this study, we present observations of a type Iax supernova, PS1-12bwh,\ndiscovered during the Pan-STARRS1 3$\\pi$-survey. Our analysis was driven by\npreviously unseen pre-maximum, spectroscopic heterogeneity. While the light\ncurve and post-maximum spectra of PS1-12bwh are virtually identical to those of\nthe well-studied type Iax supernova, SN 2005hk, the $-$2 day spectrum of\nPS1-12bwh does not resemble SN 2005hk at a comparable epoch; instead, we found\nit to match a spectrum of SN 2005hk taken over a week earlier ($-$12 day). We\nare able to rule out the cause as being incorrect phasing, and argue that it is\nnot consistent with orientation effects predicted by existing explosion\nsimulations. To investigate the potential source of this difference, we\nperformed radiative transfer modelling of both supernovae. We found that the\npre-maximum spectrum of PS1-12bwh is well matched by a synthetic spectrum\ngenerated from a model with a lower density in the high velocity ($\\gtrsim$6000\nkm~s$^{-1}$) ejecta than SN 2005hk. The observed differences between SN 2005hk\nand PS1-12bwh may therefore be attributed primarily to differences in the high\nvelocity ejecta alone, while comparable densities for the lower velocity ejecta\nwould explain the nearly identical post-maximum spectra. These two supernovae\nfurther highlight the diversity within the SNe Iax class, as well as the\nchallenges in spectroscopically identifying and phasing these objects,\nespecially at early epochs.", "category": "astro-ph_HE" }, { "text": "Superorbital modulation at GeV energies in the gamma-ray binary LS I +61\n 303: We report the results from our analysis of 8 years of the data for the\ngamma-ray binary LS I +61 303, obtained with the Large Area Telescope onboard\nthe Fermi Gamma-Ray Space Telescope. We find a significant dip around the\nbinary's periastron in the superorbital light curves, and by fitting the light\ncurves with a sinusoidal function, clear phase shifts are obtained. The\nsuperorbital modulation seen in the binary has been long known and different\nscenarios have been proposed. Based on our results, we suggest that the\ncircumstellar disk around the Be companion of this binary has an elliptical\nshape and the major axis of the disk rotates at the superorbital period of 1667\ndays. As a result, the density of the ambient material around the compact star\nof the binary changes along the binary orbit over the superorbital period,\ncausing the phase shifts in the modulation, and around periastron, the compact\nstar probably enters the disk, causing the appearance of the dip. Numerical\nsimulations may be conducted in order to study the detailed physical processes\nand verify our suggestion.", "category": "astro-ph_HE" }, { "text": "The Einstein@Home Gamma-ray Pulsar Survey. I. Search Methods,\n Sensitivity and Discovery of New Young Gamma-ray Pulsars: We report on the results of a recent blind search survey for gamma-ray\npulsars in Fermi Large Area Telescope (LAT) data being carried out on the\ndistributed volunteer computing system, Einstein@Home. The survey has searched\nfor pulsations in 118 unidentified pulsar-like sources, requiring about 10,000\nyears of CPU core time. In total, this survey has resulted in the discovery of\n17 new gamma-ray pulsars, of which 13 are newly reported in this work, and an\naccompanying paper. These pulsars are all young, isolated pulsars with\ncharacteristic ages between 12 kyr and 2 Myr, and spin-down powers between\n$10^{34}$ and $4\\times10^{36}$ erg/s. Two of these are the slowest spinning\ngamma-ray pulsars yet known. One pulsar experienced a very large glitch $\\Delta\nf/f \\approx 3.5\\times10^{-6}$ during the Fermi mission. In this, the first of\ntwo associated papers, we describe the search scheme used in this survey, and\nestimate the sensitivity of our search to pulsations in unidentified Fermi-LAT\nsources. One such estimate results in an upper limit of 57% for the fraction of\npulsed emission from the gamma-ray source associated with the Cas A supernova\nremnant, constraining the pulsed gamma-ray photon flux that can be produced by\nthe neutron star at its center. We also present the results of precise timing\nanalyses for each of the newly detected pulsars.", "category": "astro-ph_HE" }, { "text": "A tale of two mergers: constraints on kilonova detection in two short\n GRBs at z$\\sim$0.5: We present a detailed multi-wavelength analysis of two short Gamma-Ray Bursts\n(sGRBs) detected by the Neil Gehrels Swift Observatory: GRB 160624A at\n$z=0.483$ and GRB 200522A at $z=0.554$. These sGRBs demonstrate very different\nproperties in their observed emission and environment. GRB 160624A is\nassociated to a late-type galaxy with an old stellar population ($\\approx$3\nGyr) and moderate on-going star formation ($\\approx$1 $M_{\\odot}$ yr$^{-1}$).\nHubble and Gemini limits on optical/nIR emission from GRB 160624A are among the\nmost stringent for sGRBs, leading to tight constraints on the allowed kilonova\nproperties. In particular, we rule out any kilonova brighter than AT2017gfo,\ndisfavoring large masses of wind ejecta ($\\lesssim$0.03 $M_\\odot$). In\ncontrast, observations of GRB 200522A uncovered a luminous\n($L_\\textrm{F125W}\\approx 10^{42}$ erg s$^{-1}$ at 2.3~d) and red ($r-H\\approx\n1.3$ mag) counterpart. The red color can be explained either by bright kilonova\nemission powered by the radioactive decay of a large amount of wind ejecta\n(0.03 $M_\\odot$ $\\lesssim$ $M$ $\\lesssim$ 0.1 $M_\\odot$) or moderate\nextinction, $E(B-V)\\approx0.1-0.2$ mag, along the line of sight. The location\nof this sGRB in the inner regions of a young ($\\approx$0.1 Gyr) star-forming\n($\\approx$2-6 $M_{\\odot}$ yr$^{-1}$) galaxy and the limited sampling of its\ncounterpart do not allow us to rule out dust effects as contributing, at least\nin part, to the red color.", "category": "astro-ph_HE" }, { "text": "Variety of disk wind-driven explosions in massive rotating stars: We perform a set of two-dimensional, non-relativistic, hydrodynamics\nsimulations for supernova-like explosion associated with stellar core collapse\nof rotating massive stars to a system of a black hole and a disk connected by\nthe transfer of matter and angular momentum. Our model of the central engine\nalso includes the contribution of the disk wind. In this work, we specifically\ninvestigate the wind-driven explosion of rotating, large-mass progenitor stars\nwith the zero-age main-sequence mass of $M_\\mathrm{ZAMS}=20\\,M_\\odot$ from\narXiv:2008.09132 . This study is carried out using the open-source hydrodynamic\ncode Athena++, for which we implement a method to calculate self-gravity for\naxially symmetric density distributions. We, then, investigate the explosion\nproperties and the $^{56}$Ni production as a function of (varying) some\nfeatures of the wind injection. We find a large variety of explosion energy\nwith $E_\\mathrm{expl}$ ranging from $\\sim 0.049\\times10^{51}$~erg to $\\sim\n34\\times10^{51}$~erg and ejecta mass $M_\\mathrm{ej}$ from 0.58 to 6 $M_\\odot$,\nwhich shows a bimodal distribution in high- and low-energy branches. We\ndemonstrate that the resulting outcome of a highly- or sub-energetic explosion\nfor a certain stellar structure is mainly determined by the competition between\nthe ram pressure of the injected matter and that of the infalling envelope. In\nthe nucleosynthesis analysis the $^{56}$Ni mass produced in our models goes\nfrom $< 0.2~M_\\odot$ in the sub-energetic explosions to $2.1~M_\\odot$ in the\nhighly-energetic ones. These results are consistent with the observational data\nof stripped-envelope and high-energy SNe such as broad-lined type Ic SNe.\nHowever, we find a tighter correlation between the explosion energy and the\nejecta mass than that observationally measured.", "category": "astro-ph_HE" }, { "text": "The Spectral Sharpness Angle of Gamma-ray Bursts: We explain the results of Yu et al. (2015b) of the novel sharpness angle\nmeasurement to a large number of spectra obtained from the Fermi gamma-ray\nburst monitor. The sharpness angle is compared to the values obtained from\nvarious representative emission models: blackbody, single-electron synchrotron,\nsynchrotron emission from a Maxwellian or power-law electron distribution. It\nis found that more than 91% of the high temporally and spectrally resolved\nspectra are inconsistent with any kind of optically thin synchrotron emission\nmodel alone. It is also found that the limiting case, a single temperature\nMaxwellian synchrotron function, can only contribute up to 58+23 -18% of the\npeak flux. These results show that even the sharpest but non-realistic case,\nthe single-electron synchrotron function, cannot explain a large fraction of\nthe observed spectra. Since any combination of physically possible synchrotron\nspectra added together will always further broaden the spectrum, emission\nmechanisms other than optically thin synchrotron radiation are likely required\nin a full explanation of the spectral peaks or breaks of the GRB prompt\nemission phase.", "category": "astro-ph_HE" }, { "text": "r-Process Lanthanide Production and Heating Rates in Kilonovae: r-Process nucleosynthesis in material ejected during neutron star mergers may\nlead to radioactively powered transients called kilonovae. The timescale and\npeak luminosity of these transients depend on the composition of the ejecta,\nwhich determines the local heating rate from nuclear decays and the opacity.\nKasen et al. (2013, ApJ, 774, 25) and Tanaka & Hotokezaka (2013, ApJ, 775, 113)\npointed out that lanthanides can drastically increase the opacity in these\noutflows. We use the new general-purpose nuclear reaction network SkyNet to\ncarry out a parameter study of r-process nucleosynthesis for a range of initial\nelectron fractions $Y_e$, initial specific entropies $s$, and expansion\ntimescales $\\tau$. We find that the ejecta is lanthanide-free for $Y_e \\gtrsim\n0.22 - 0.30$, depending on $s$ and $\\tau$. The heating rate is insensitive to\n$s$ and $\\tau$, but certain, larger values of $Y_e$ lead to reduced heating\nrates, due to individual nuclides dominating the heating. We calculate\napproximate light curves with a simplified gray radiative transport scheme. The\nlight curves peak at about a day (week) in the lanthanide-free (-rich) cases.\nThe heating rate does not change much as the ejecta becomes lanthanide-free\nwith increasing $Y_e$, but the light curve peak becomes about an order of\nmagnitude brighter because it peaks much earlier when the heating rate is\nlarger. We also provide parametric fits for the heating rates between 0.1 and\n$100\\,\\text{days}$, and we provide a simple fit in $Y_e$, $s$, and $\\tau$ to\nestimate whether the ejecta is lanthanide-rich or not.", "category": "astro-ph_HE" }, { "text": "Sub-arcsecond radio and optical observations of the likely counterpart\n to the gamma-ray source 2FGL J2056.7+4939: We have searched and reviewed all multi- wavelength data available for the\nregion towards the gamma-ray source 2FGL J2056.7+4939 in order to con- strain\nits possible counterpart at lower energies. As a result, only a point-like\noptical/infrared source with flat-spectrum radio emission is found to be\nconsistent with all X-ray and gamma-ray error circles. Its struc- ture is\nmarginally resolved at radio wavelengths at the sub-arcsecond level. An\nextragalactic scenario appears to be the most likely interpretation for this\nobject.", "category": "astro-ph_HE" }, { "text": "Investigating the true nature of three hard X-ray sources: Many of the new high energy sources discovered both by INTEGRAL/IBIS and\nSwift/BAT have been characterised thanks to extensive, multi-band follow-up\ncampaigns, but there are still objects whose nature remains to be asserted. In\nthis paper we investigate the true nature of three high energy sources, IGR\nJ12134-6015, IGR J16058-7253 and Swift J2037.2+4151, employing multiwavelength\ndata from the NIR to the X-rays. Through Gaia and ESO-VLT measurements and\nthrough Swift/XRT X-ray spectral analysis, we re-evaluate the classification\nfor IGR J12134-6015, arguing that the source is a Galactic object and in\nparticular a Cataclysmic Variable. We were able to confirm, thanks to NuSTAR\nobservations, that the hard X-ray emission detected by INTEGRAL/IBIS and\nSwift/BAT from IGR J16058-7253 is coming from two Seyfert 2 galaxies which are\nboth counterparts for this source. Through optical and X-ray spectral analysis\nof Swift J2037.2+4151 we find that this source is likely part of the rare and\npeculiar class of Symbiotic X-ray binaries and displays flux and spectral\nvariability as well as interesting spectral features, such as a blending of\nseveral emission lines around the iron line complex.", "category": "astro-ph_HE" }, { "text": "Exploring properties of high-density matter through remnants of\n neutron-star mergers: Remnants of neutron-star mergers are essentially massive, hot, differentially\nrotating neutron stars, which are initially strongly oscillating. They\nrepresent a unique probe for high-density matter because the oscillations are\ndetectable via gravitational-wave measurements and are strongly dependent on\nthe equation of state. The impact of the equation of state is apparent in the\nfrequency of the dominant oscillation mode of the remnant. For a fixed total\nbinary mass a tight relation between the dominant postmerger frequency and the\nradii of nonrotating neutron stars exists. Inferring observationally the\ndominant postmerger frequency thus determines neutron star radii with high\naccuracy of the order of a few hundred meters. By considering symmetric and\nasymmetric binaries of the same chirp mass, we show that the knowledge of the\nbinary mass ratio is not critical for this kind of radius measurements. We\nsummarize different possibilities to deduce the maximum mass of nonrotating\nneutron stars. We clarify the nature of the three most prominent features of\nthe postmerger gravitational-wave spectrum and argue that the merger remnant\ncan be considered to be a single, isolated, self-gravitating object that can be\ndescribed by concepts of asteroseismology. The understanding of the different\nmechanisms shaping the gravitational-wave signal yields a physically motivated\nanalytic model of the gravitational-wave emission, which may form the basis for\ntemplate-based gravitational-wave data analysis. We explore the observational\nconsequences of a scenario of two families of compact stars including hadronic\nand quark stars. We find that this scenario leaves a distinctive imprint on the\npostmerger gravitational-wave signal. In particular, a strong discontinuity in\nthe dominant postmerger frequency as function of the total mass will be a\nstrong indication for two families of compact stars. (abridged)", "category": "astro-ph_HE" }, { "text": "Intrinsic disc emission and the Soft X-ray Excess in AGN: (Abridged) Narrow Line Seyfert 1 (NLS1) galaxies have low mass black holes\nand mass accretion rates close to (or exceeding) Eddington, so a standard\nblackbody accretion disc should peak in the EUV. However, the lack of true\nabsorption opacity in the disc means that the emission is better approximated\nby a colour temperature corrected blackbody, and this colour temperature\ncorrection is large enough ($\\sim 2.4$) that the bare disc emission from a zero\nspin black hole can extend into the soft X-ray bandpass. Part of the soft X-ray\nexcess seen in these objects must be intrinsic emission from the disc unless\nthe vertical structure is very different to that predicted.\n However, the soft excess is much broader than predicted by a bare disc\nspectrum, indicating some Compton upscattering by cool, optically thick\nmaterial. We associate this with the disc itself, so it must ultimately be\npowered by mass accretion. We build an energetically self consistent model\nassuming that the emission thermalises at large radii, but that at smaller\nradii the gravitational energy is split between powering optically thick\nComptonised disc emission (forming the soft X-ray excess) and an optically thin\ncorona above the disc (forming the tail to higher energies). We show examples\nof this model fit to the extreme NLS1 REJ1034+396, and to the much lower\nEddington fraction Broad Line Seyfert 1 PG1048+231. We use these to guide our\nfits and interpretations of three template spectra made from co-adding multiple\nsources to track out a sequence of AGN spectra as a function of $L/L_{Edd}$.\n The new model is publically available within the {\\sc xspec} spectral fitting\npackage.", "category": "astro-ph_HE" }, { "text": "Magnetic-distortion-induced ellipticity and gravitational wave radiation\n of neutron stars: millisecond magnetars in short GRBs, Galactic pulsars, and\n magnetars: Neutron stars may sustain a non-axisymmetric deformation due to magnetic\ndistortion and are potential sources of continuous gravitational waves (GWs)\nfor ground-based interferometric detectors. With decades of searches using\navailable GW detectors, no evidence of a GW signal from any pulsar has been\nobserved. Progressively stringent upper limits of ellipticity have been placed\non Galactic pulsars. In this work, we use the ellipticity inferred from the\nputative millisecond magnetars in short gamma-ray bursts (SGRBs) to estimate\ntheir detectability by current and future GW detectors. For $\\sim 1$ ms\nmagnetars inferred from the SGRB data, the detection horizon is $\\sim 30$ Mpc\nand $\\sim 600$ Mpc for advanced LIGO (aLIGO) and Einstein Telescope (ET),\nrespectively. Using the ellipticity of SGRB millisecond magnetars as\ncalibration, we estimate the ellipticity and gravitational wave strain of\nGalactic pulsars and magnetars assuming that the ellipticity is\nmagnetic-distortion-induced. We find that the results are consistent with the\nnull detection results of Galactic pulsars and magnetars with the aLIGO O1. We\nfurther predict that the GW signals from these pulsars/magnetars may not be\ndetectable by the currently designed aLIGO detector. The ET detector may be\nable to detect some relatively low frequency signals ($<50$ Hz) from some of\nthese pulsars. Limited by its design sensitivity, the eLISA detector seems not\nsuitable for detecting the signals from Galactic pulsars and magnetars.", "category": "astro-ph_HE" }, { "text": "Implications on the origin of cosmic rays in light of 10 TV spectral\n softenings: Precise measurements of the energy spectra of cosmic rays (CRs) show various\nkinds of features deviating from single power-laws, which give very interesting\nand important implications on their origin and propagation. Previous\nmeasurements from a few balloon and space experiments indicate the existence of\nspectral softenings around 10 TV for protons (and probably also for Helium\nnuclei). Very recently, the DArk Matter Particle Explorer (DAMPE) measurement\nabout the proton spectrum clearly reveals such a softening with a high\nsignificance. Here we study the implications of these new measurements, as well\nas the groundbased indirect measurements, on the origin of CRs. We find that a\nsingle component of CRs fails to fit the spectral softening and the air shower\nexperiment data simultaneously. In the framework of multiple components, we\ndiscuss two possible scenarios, the multiple source population scenario and the\nbackground plus nearby source scenario. Both scenarios give reasonable fits to\nthe wide-band data from TeV to 100 PeV energies. Considering the anisotropy\nobservations, the nearby source model is favored.", "category": "astro-ph_HE" }, { "text": "Maximum mass of a hybrid star having a mixed phase region in the light\n of pulsar PSR J1614-2230: Recent observation of pulsar PSR J1614-2230 with mass about 2 solar masses\nposes a severe constraint on the equations of state (EOS) of matter describing\nstars under extreme conditions. Neutron stars (NS) can reach the mass limits\nset by PSR J1614-2230. But stars having hyperons or quark stars (QS) having\nboson condensates, with softer EOS can barely reach such limits and are ruled\nout. QS with pure strange matter also cannot have such high mass unless the\neffect of strong coupling constant or colour superconductivity are taken into\naccount. In this work I try to calculate the upper mass limit for a hybrid\nstars (HS) having a quark-hadron mixed phase. The hadronic matter (having\nhyperons) EOS is described by relativistic mean field theory and for the quark\nphase I use the simple MIT bag model. I construct the intermediate mixed phase\nusing Glendenning construction. HS with a mixed phase cannot reach the mass\nlimit set by PSR J1614-2230 unless I assume a density dependent bag constant.\nFor such case the mixed phase region is small. The maximum mass of a mixed\nhybrid star obtained with such mixed phase region is $2.01 M_{\\odot}$.", "category": "astro-ph_HE" }, { "text": "Mass and radius relations of quarkyonic stars using an excluded volume\n model: Inspired by the excluded volume model for isospin symmetric quarkyonic\nmatter, we construct an `excluded volume' model for a charge neutral quarkyonic\nphase whose hadronic sector contains only neutrons. We refer to this model as\nquarkyonic neutron matter. We compute the equation of state for this model and\nsolve the Tolman-Oppenhermer-Volkoff equations to obtain mass and radius\nrelations relevant for neutron stars. The most straightforward extension of the\nmodel for symmetric quarkyonic matter to quarkyonic neutron matter does not\nsatisfy the mass radius constraints from neutron star measurements. However, we\nshow that by incorporating appropriate nuclear interactions in the excluded\nvolume model one can produce mass-radius relations that lie within the\nconstraints obtained from gravitational waves of binary neutron star mergers\nand maximum mass measurements of neutron stars.", "category": "astro-ph_HE" }, { "text": "Stellar and AGN feedback in isolated early-type galaxies: the role in\n regulating star formation and ISM properties: Understanding how galaxies maintain the inefficiency of star formation with\nphysically self-consistent models is a central problem for galaxy evolution.\nAlthough numerous theoretical models have been proposed in recent decades, the\ndebate still exists. By means of high-resolution two-dimensional hydrodynamical\nsimulations, we study the three feedback effects (the stellar wind heating, SNe\nfeedback, and AGN feedback) in suppressing star formation activities on the\nevolution of early-type galaxies with different stellar masses. AGN feedback\nmodels are updated based on \\citet{Yuan2018}. The gas sources comes exclusively\nfrom the mass losses of dying low-mass stars for most of our models. We find\nthat SNe feedback can keep star formation at a significantly low level for low\nmass elliptical galaxies for a cosmological evolution time. For the high mass\ngalaxies, AGN feedback can efficiently offset the radiative cooling and thus\nregulate the star formation activities. Such a suppression of star formation is\nextremely efficient in the inner region of the galaxies. AGB heating cannot\naccount for this suppression for low and high mass galaxies. The X-ray\ntemperature $T_{\\rm X}$ and luminosity $L_{\\rm X}$ of hot plasma can be in\nagreement with the observed data with the inclusion of effective feedback\nprocesses. These results thus suggest that we can use $T_{\\rm X}$ and $L_{\\rm\nX}$ to probe the role of different feedback processes. The inclusion of\nadditional gas sources can make the mass scale between SNe and AGN feedback\ndominating in suppressing star formation decrease to an observationally\ninferred value of a few $10^{10}~M_{\\odot}$.", "category": "astro-ph_HE" }, { "text": "The polarized Gamma-Ray Burst GRB 061122: We report on the polarization measure, obtained with IBIS on board INTEGRAL,\nof the prompt emission of GRB 061122. Over an 8 s interval containing the\nbrightest part of the Gamma-Ray Burst we put a lower limit on its polarization\nfraction of 60% at 68% c.l. and of 33% at 90% c.l. on the 250-800 keV energy\nrange.\n We performed late time optical and near infra-red imaging observations of the\nGRB field using the Telescopio Nazionale Galileo (TNG), and the\nCanada-France-Hawaii Telescope (CFHT). Our multi-band (ugrizYJHK) photometry\nallowed us to identify the host galaxy of GRB 061122 and to build its SED.\nUsing a photometric redshift code we fitted these data, and derived the basic\nproperties of the galaxy, including its type and redshift, that we could\nconstrain to the interval [0.57, 2.10] at a 90% c.l., with a best fit value of\nz=1.33.\n The polarization measurement in different energy bands, together with the\ndistance determination, allowed us to put the most stringent limit (xi <\n3.4x10-16) to date to a possible Lorentz Invariance Violation based on the\nvacuum birefringence effect, predicted by some quantum-gravity theories.", "category": "astro-ph_HE" }, { "text": "Assessing the Observability of Hypernovae and Pair-Instability\n Supernovae in the Early Universe: The era of the universe's first (Population III) stars is essentially\nunconstrained by observation. Ultra-luminous and massive stars from this time\naltered the chemistry of the cosmos, provided the radiative scaffolding to\nsupport the formation of the first protogalaxies, and facilitated the creation\nand growth of now-supermassive black holes. Unfortunately, because these stars\nlie literally at the edge of the observable universe, they will remain beyond\nthe reach of even the next generation of telescopes such as the James Webb\nSpace Telescope and the Thirty-Meter Telescope. In this paper, we provide a\nprimer to supernovae modeling and the first stars to make our discussion\naccessible to those new to or outside our field. We review recent work of the\nLos Alamos Supernova Light Curve Project and Brigham Young University to\nexplore the possibility of probing this era through observations of the\nspectacular deaths of the first stars. We find that many such brilliant\nsupernova explosions will be observable as far back as $\\sim 99$% of the\nuniverse's current age, tracing primordial star formation rates and the\nlocations of their protogalaxies on the sky. The observation of Population III\nsupernovae will be among the most spectacular discoveries in observational\nastronomy in the coming decade.", "category": "astro-ph_HE" }, { "text": "Binary Black Hole Accretion From a Circumbinary Disk: Gas Dynamics\n Inside the Central Cavity: We present the results of 2D hydrodynamical simulations of circumbinary disk\naccretion using the finite-volume code DISCO. This code solves the 2D viscous\nNavier-Stokes equations on a high-resolution moving mesh which shears with the\nfluid flow, greatly reducing advection errors in comparison with a fixed grid.\nWe perform a series of simulations for binary mass ratios in the range 0.026 <\nq < 1.0, each lasting longer than a viscous time so that we reach a\nquasi-steady accretion state. In each case, we find that gas is efficiently\nstripped from the inner edge of the circumbinary disk and enters the cavity\nalong accretion streams, which feed persistent \"mini-disks\" surrounding each\nblack hole. We find that for q > 0.1, the binary excites eccentricity in the\ninner region of the circumbinary disk, creating an overdense lump which gives\nrise to enhanced periodicity in the accretion rate. The dependence of the\nperiodicity on mass ratio may provide a method for observationally inferring\nmass ratios from measurements of the accretion rate. We also find that for all\nmass ratios studied, the magnitude of the accretion onto the secondary is\nsufficient to drive the binary toward larger mass ratio. This suggests a\nmechanism for biasing mass ratio distributions toward equal mass.", "category": "astro-ph_HE" }, { "text": "Nucleosynthesis in magnetorotational supernovae: impact of the magnetic\n field configuration: The production of heavy elements is one of the main by-products of the\nexplosive end of massive stars. A long sought goal is finding differentiated\npatterns in the nucleosynthesis yields, which could permit identifying a number\nof properties of the explosive core. Among them, the traces of the magnetic\nfield topology are particularly important for \\emph{extreme} supernova\nexplosions, most likely hosted by magnetorotational effects. We investigate the\nnucleosynthesis of five state-of-the-art magnetohydrodynamic models with fast\nrotation that have been previously calculated in full 3D and that involve an\naccurate neutrino transport (M1). One of the models does not contain any\nmagnetic field and synthesizes elements around the iron group, in agreement\nwith other CC-SNe models in literature. All other models host a strong magnetic\nfield of the same intensity, but with different topology. For the first time,\nwe investigate the nucleosynthesis of MR-SNe models with a quadrupolar magnetic\nfield and a 90 degree tilted dipole. We obtain a large variety of ejecta\ncompositions reaching from iron nuclei to nuclei up to the third r-process\npeak. We assess the robustness of our results by considering the impact of\ndifferent nuclear physics uncertainties such as different nuclear masses,\n$\\beta^{-}$-decays and $\\beta^{-}$-delayed neutron emission probabilities,\nneutrino reactions, fission, and a feedback of nuclear energy on the\ntemperature. We find that the qualitative results do not change with different\nnuclear physics input. The properties of the explosion dynamics and the\nmagnetic field configuration are the dominant factors determining the ejecta\ncomposition.", "category": "astro-ph_HE" }, { "text": "ThunderKAT: The MeerKAT Large Survey Project for Image-Plane Radio\n Transients: ThunderKAT is the image-plane transients programme for MeerKAT. The goal as\noutlined in 2010, and still today, is to find, identify and understand\nhigh-energy astrophysical processes via their radio emission (often in concert\nwith observations at other wavelengths). Through a comprehensive and\ncomplementary programme of surveying and monitoring Galactic synchrotron\ntransients (across a range of compact accretors and a range of other explosive\nphenomena) and exploring distinct populations of extragalactic synchrotron\ntransients (microquasars, supernovae and possibly yet unknown transient\nphenomena) - both from direct surveys and commensal observations - we will\nrevolutionise our understanding of the dynamic and explosive transient radio\nsky. As well as performing targeted programmes of our own, we have made\nagreements with the other MeerKAT large survey projects (LSPs) that we will\nalso search their data for transients. This commensal use of the other surveys,\nwhich remains one of our key programme goals in 2016, means that the combined\nMeerKAT LSPs will produce by far the largest GHz-frequency radio transient\nprogramme to date.", "category": "astro-ph_HE" }, { "text": "The Luminosity Distribution of Short Gamma-Ray Bursts under a Structured\n Jet Scenario: Luminosity of GRB 170817A is much lower than that of other sGRBs. The\nmeasurement of the superluminal movement of the radio afterglow emission\nconfirms the presence of the relativistic jet, and the emission features can be\nwell explained by the structured jet model. In this paper, we calculate the\nluminosity distribution of sGRBs and its evolution with redshift based on the\nstructured (Gaussian) jet model, and find that the typical luminosity increase\nwith redshift, for nearby sGRBs (such as for luminosity distance less than 200\nMpc) the typical gamma-ray luminosity is just around 10^47-10^48 erg s-1, which\nnaturally explains the very low radiation luminosity of GRB 170817A. We derived\nthe detection probability of sGRBs by Fermi-GBM and found that the expected\ndetection rate of sGRBs is only about 1 yr-1 within the distance of several\nhundred Mpc. We explored the effect of the power-law index {\\alpha} of the\nmerger time distribution on the observed characteristics and found that it had\nlittle effect on the observed luminosity and viewing-angle distributions.\nHowever, it is very interesting that, for different values of {\\alpha}, the\ndistributions of the number of observed sGRBs are quite different, so it is\npossible to determine the value of {\\alpha} through observed distributions of\nthe number of sGRBs. We used the Bayesian method to make a quantitative\nanalysis and found that the value of {\\alpha} may be identified when the number\nof observed sGRBs with known redshifts is more than 200. Finally, we compare\nour results of gamma-ray luminosity distribution with sGRBs with known\nredshifts, and found that our results are consistent with the observation,\nwhich implies that our simulation results can reproduce the observed luminosity\ndistribution well.", "category": "astro-ph_HE" }, { "text": "Multi-waveform cross-correlation search method for intermediate-duration\n gravitational waves from gamma-ray bursts: Gamma-ray Bursts (GRBs) are flashes of gamma-rays thought to originate from\nrare forms of massive star collapse (long GRBs), or from mergers of compact\nbinaries (short GRBs) containing at least one neutron star (NS). The nature of\nthe post-explosion / post-merger remnant (NS versus black hole, BH) remains\nhighly debated. In ~50% of both long and short GRBs, the temporal evolution of\nthe X-ray afterglow that follows the flash of gamma-rays is observed to\n\"plateau\" on timescales of 100-10000 s since explosion, possibly signaling the\npresence of energy injection from a long-lived, highly magnetized NS\n(magnetar). The Cross-Correlation Algorithm (CoCoA) proposed by [R. Coyne et.\nal., (2016)] aims to optimize searches for intermediate-duration (100-10000s)\ngravitational waves (GWs) from GRB remnants. In this work, we test CoCoA on\nreal data collected with ground-based GW detectors. We further develop the\ndetection statistics on which CoCoA is based to allow for multi-waveform\nsearches spanning a physically-motivated parameter space, so as to account for\nuncertainties in the physical properties of GRB remnants.", "category": "astro-ph_HE" }, { "text": "Enhanced X-ray emission from Lyman Break Analogues and a Possible\n $L_{\\rm X}$--SFR--Metallicity Plane: The source of energetic photons that heated and reionized the early Universe\nremains uncertain. Early galaxies had low metallicity and recent population\nsynthesis calculations suggest that the number and luminosity of high-mass\nX-ray binaries are enhanced in star-forming galaxies with low metallicity,\noffering a potentially important and previously overlooked source of heating\nand reionization. Lyman break analogue (LBA) galaxies are local galaxies that\nstrongly resemble the high-redshift, star-forming Lyman Break Galaxies and have\nbeen suggested as local analogues to these metal-deficient galaxies found in\nthe early Universe. We studied a sample of ten LBAs in order to measure the\nrelation between star formation rate and X-ray luminosity. We found that for\nLBAs with metallicities in the range $12+\\log_{10}({\\rm O/H}) = 8.15-8.80$, the\n$L_X-$SFR relation was $\\log_{10} (L_X/{\\rm SFR}\\, {[\\rm erg\\ s^{-1}\\\nM_{\\odot}^{-1}\\ yr]}) = 39.85(\\pm 0.10)$ in the $0.5-8$~keV band with a\ndispersion of $\\sigma = 0.25$~dex. This is an enhancement of nearly a factor of\n$2$ in the $L_{0.5-8\\text{keV}}$--SFR relation relative to results for nearby,\nnear-solar metallicity galaxies. The enhancement is significant at the 98.2\\%\nlevel ($2.4\\sigma$). Our enhanced $L_X/{\\rm SFR}$ relation is consistent with\nthe metallicity-dependent predicted value from population synthesis models. We\ndiscuss the possibility of a $L_X$--SFR--Metallicity plane for star-forming\ngalaxies. These results are important to our understanding of reionization and\nthe formation of early galaxies.", "category": "astro-ph_HE" }, { "text": "Detectability of Sub-Solar Mass Neutron Stars Through a Template Bank\n Search: We study the detectability of gravitational-wave signals from sub-solar mass\nbinary neutron star systems by the current generation of ground-based\ngravitational-wave detectors. We find that finite size effects from large tidal\ndeformabilities of the neutron stars and lower merger frequencies can\nsignificantly impact the sensitivity of the detectors to these sources. By\nsimulating a matched-filter based search using injected binary neutron star\nsignals with tidal deformabilities derived from physically motivated equations\nof state, we calculate the reduction in sensitivity of the detectors. We\nconclude that the loss in sensitive volume can be as high as $78.4 \\%$ for an\nequal mass binary system of chirp mass $0.17 \\, \\textrm{M}_{\\odot}$, in a\nsearch conducted using binary black hole template banks. We use this loss in\nsensitive volume, in combination with the results from the search for sub-solar\nmass binaries conducted on data collected by the LIGO-Virgo observatories\nduring their first three observing runs, to obtain a conservative upper limit\non the merger rate of sub-solar mass binary neutron stars. Since the discovery\nof a low-mass neutron star would provide new insight into formation mechanisms\nof neutron stars and further constrain the equation of state of dense nuclear\nmatter, our result merits a dedicated search for sub-solar mass binary neutron\nstar signals.", "category": "astro-ph_HE" }, { "text": "The long-term optical evolution of the black hole candidate MAXI\n J1659-152: We present 5 years of optical and infrared data of the black hole candidate\nMAXI J1659-152 covering its 2010 outburst, decay and quiescence. Combining\noptical data taken during the outburst decay, we obtain an orbital period of\n2.414 $\\pm$ 0.005 h, in perfect agreement with the value previously measured\nfrom X-ray dips. In addition, we detect a clear H$\\alpha$ excess in MAXI\nJ1659-152 with data taken during the outburst decay. We also detect a single\nhump modulation most likely produced by irradiation. Assuming that the maximum\noccurs at orbital phase 0.5, we constrain the phase of the X-ray dips to be ~\n0.65. We also detect the quiescent optical counterpart at r' = 24.20 $\\pm$\n0.08, I = 23.32 $\\pm$ 0.02 and H = 20.7 $\\pm$ 0.1. These magnitudes provide\ncolour indices implying an M2-M5 donor star assuming 60% contribution from a\ndisc component in the r'-band.", "category": "astro-ph_HE" }, { "text": "Glitches: the exact quantum signatures of pulsars metamorphosis: The observed recurrence of glitches in pulsars and neutron stars carry rich\ninformation about the evolution of their internal structures. In this article I\nshow that the glitch-events observed in pulsars are exact quantum signatures\nfor their metamorphosis into dark super-baryons (SBs), whose interiors are made\nof purely incompressible superconducting gluon-quark superfluids. Here the\nquantum nuclear shell model is adopted to describe the permitted energy levels\nof the SB, which are assumed to be identical to the discrete spinning rates\n$\\Omega_{SB},$ that SBs are allowed to rotate with. Accordingly, a glitch-event\ncorresponds to a prompt spin-down of the superconducting SB from one energy\nlevel to the next, thereby expelling a certain number of vortices, which in\nturn spins-up the ambient medium. The process is provoked mainly by the\nnegative torque of the ambient dissipative nuclear fluid and by a universal\nscalar field $\\phi$ at the background of a supranuclear dense matter. As\ndictated by the Onsager-Feynman equation, the prompt spin-down must be\nassociated with increase of the dimensions of the embryonic SB to finally\nconvert the entire pulsar into SB-Objects on the scale of Gyrs. Based on our\ncalculations, a Vela-like pulsar should display billions of glitches during its\nlifetime, before it metamorphoses entirely into a maximally compact SB-object\nand disappears from our observational windows. The present model predicts the\nmass of SBs and $\\Delta \\Omega/\\Omega$ in young pulsars to be relatively lower\nthan their older counterparts.", "category": "astro-ph_HE" }, { "text": "Quantifying How Density Gradients and Front Curvature Affect Carbon\n Detonation Strength During Type Ia Supernovae: Accurately reproducing the physics behind the detonations of Type Ia\nsupernovae and the resultant nucleosynthetic yields is important for\ninterpreting observations of spectra and remnants. The scales of the processes\ninvolved span orders of magnitudes, making the problem computationally\nimpossible to ever fully resolve in full star simulations in the present and\nnear future. In the lower density regions of the star, the curvature of the\ndetonation front will slow the detonation, affecting the production of\nintermediate mass elements. We find that shock strengthening due to the density\ngradient present in the outer layers of the progenitor is essential for\nunderstanding the nucleosynthesis there, with burning extending well below the\ndensity at which a steady-state detonation is extinct. We show that a complete\nreaction network is not sufficient to obtain physical detonations at high\ndensities and modest resolution due to numerical mixing at the unresolved\nreaction front. At low densities, below 6$\\times$10$^{5}$ g cm$^{-3}$, it is\npossible to achieve high enough resolution to separate the shock and the\nreaction region,and the abundance structure predicted by fully resolved\nquasi-steady-state calculations is obtained. For our best current benchmark\nyields, we utilize a method in which the unresolved portion of Lagrangian\nhistories are reconstructed based on fully resolved quasi-steady-state\ndetonation calculations. These computations demonstrate that under-resolved\nsimulations agree approximately, $\\sim$10\\% in post-shock values of\ntemperature, pressure, density, and abundances, with expected detonation\nstructures sufficiently far from the under-resolved region, but that there is\nstill room for some improvement in the treatment of subgrid reactions in the\nhydrodynamics to before better than 1$\\%$ can be achieved at all densities.", "category": "astro-ph_HE" }, { "text": "Using machine learning for transient classification in searches for\n gravitational-wave counterparts: The large sky localization regions offered by the gravitational-wave\ninterferometers require efficient follow-up of the many counterpart candidates\nidentified by the wide field-of-view telescopes. Given the restricted telescope\ntime, the creation of prioritized lists of the many identified candidates\nbecomes mandatory. Towards this end, we use \\text{\\astrorapid}, a multi-band\nphotometric lightcurve classifier, to differentiate between kilonovae,\nsupernovae, and other possible transients. We demonstrate our method on the\nphotometric observations of real events. In addtion, the classification\nperformance is tested on simulated lightcurves, both ideally and realistically\nsampled. We show that after only a few days of observations of an astronomical\nobject, it is possible to rule out candidates as supernovae and other known\ntransients.", "category": "astro-ph_HE" }, { "text": "Search for gravitational waves associated with gamma-ray bursts during\n LIGO science run 6 and Virgo science runs 2 and 3: We present the results of a search for gravitational waves associated with\n154 gamma-ray bursts (GRBs) that were detected by satellite-based gamma-ray\nexperiments in 2009-2010, during the sixth LIGO science run and the second and\nthird Virgo science runs. We perform two distinct searches: a modeled search\nfor coalescences of either two neutron stars or a neutron star and black hole;\nand a search for generic, unmodeled gravitational-wave bursts. We find no\nevidence for gravitational-wave counterparts, either with any individual GRB in\nthis sample or with the population as a whole. For all GRBs we place lower\nbounds on the distance to the progenitor, under the optimistic assumption of a\ngravitational-wave emission energy of 10^-2 M c^2 at 150 Hz, with a median\nlimit of 17 Mpc. For short hard GRBs we place exclusion distances on binary\nneutron star and neutron star-black hole progenitors, using astrophysically\nmotivated priors on the source parameters, with median values of 16 Mpc and 28\nMpc respectively. These distance limits, while significantly larger than for a\nsearch that is not aided by GRB satellite observations, are not large enough to\nexpect a coincidence with a GRB. However, projecting these exclusions to the\nsensitivities of Advanced LIGO and Virgo, which should begin operation in 2015,\nwe find that the detection of gravitational waves associated with GRBs will\nbecome quite possible.", "category": "astro-ph_HE" }, { "text": "A New Spin on an Old Black Hole: NuSTAR Spectroscopy of EXO 1846-031: The black hole candidate EXO 1846-031 underwent an outburst in 2019, after at\nleast 25 years in quiescence. We observed the system using \\textit{NuSTAR} on\nAugust 3rd, 2019. The 3--79 keV spectrum shows strong relativistic reflection\nfeatures. Our baseline model gives a nearly maximal black hole spin value of\n$a=0.997_{-0.002}^{+0.001}$ ($1\\sigma$ statistical errors). This high value\nnominally excludes the possibility of the central engine harboring a neutron\nstar. Using several models, we test the robustness of our measurement to\nassumptions about the density of the accretion disk, the nature of the corona,\nthe choice of disk continuum model, and addition of reflection from the outer\nregions of the accretion disk. All tested models agree on a very high black\nhole spin value and a high value for the inclination of the inner accretion\ndisk of $\\theta\\approx73^\\circ$. We discuss the implications of this spin\nmeasurement in the population of stellar mass black holes with known spins,\nincluding LIGO events.", "category": "astro-ph_HE" }, { "text": "Jet Suppression by Accretion Disk Winds in the Microquasar GRS 1915+105: Stellar-mass black holes with relativistic jets, also known as microquasars,\nmimic the behavior of quasars and active galactic nuclei. Because timescales\naround stellar-mass black holes are orders of magnitude smaller than those\naround more distant supermassive black holes, microquasars are ideal nearby\n`laboratories' for studying the evolution of accretion disks and jet formation\nin black-hole systems. Although studies of black holes have revealed a complex\narray of accretion activity, the mechanisms that trigger and suppress jet\nformation remain a mystery. Here we report the discovery of a broad emission\nline during periods of intense hard X-ray flux in the microquasar GRS 1915+105,\nand highly ionized narrow absorption lines during softer states. We argue that\nthe broad emission line arises when the inner accretion disk is illuminated by\nhard X-rays, possibly from the jet. In contrast, during softer states, when the\njet is weak or absent, absorption lines appear as the powerful radiation field\naround the black hole drives a hot wind off the accretion disk. Our analysis\nstrongly suggests that this wind carries enough mass away from the disk to halt\nthe flow of matter into the radio jet.", "category": "astro-ph_HE" }, { "text": "Supergiant Fast X-ray Transients with Swift: spectroscopic and temporal\n properties: Supergiant fast X-ray transients (SFXTs) are a class of high-mass X-ray\nbinaries with possible counterparts in the high energy gamma rays. The Swift\nSFXT Project has conducted a systematic investigation of the properties of\nSFTXs on timescales ranging from minutes to years and in several intensity\nstates (from bright flares, to intermediate intensity states, and down to\nalmost quiescence). We also performed broad-band spectroscopy of outbursts, and\nintensity-selected spectroscopy outside of outbursts. We demonstrated that\nwhile the brightest phase of the outburst only lasts a few hours, further\nactivity is observed at lower fluxes for a remarkably longer time, up to weeks.\nFurthermore, we assessed the fraction of the time these sources spend in each\nphase, and their duty cycle of inactivity. We present the most recent results\nfrom our investigation. The spectroscopic and, most importantly, timing\nproperties of SFXTs we have uncovered with Swift will serve as a guide in\nsearch for the high energy emission from these enigmatic objects.", "category": "astro-ph_HE" }, { "text": "Improved early warning of compact binary mergers using higher modes of\n gravitational radiation: A population study: A gravitational-wave (GW) early-warning of a compact-binary coalescence\nevent, with a sufficiently tight localisation skymap, would allow telescopes to\npoint in the direction of the potential electromagnetic counterpart before its\nonset. This will enable astronomers to extract valuable information of the\ncomplex astrophysical phenomena triggered around the time of the merger. Use of\nhigher-modes of gravitational radiation, in addition to the dominant mode\ntypically used in templated real-time searches, was recently shown to produce\nsignificant improvements in early-warning times and skyarea localisations for a\nrange of asymmetric-mass binaries. In this work, we perform a large-scale study\nto assess the benefits of this method for a population of compact binary merger\nobservations. In particular, we inject 100,000 such signals in Gaussian noise,\nwith component masses $m_1 \\in \\left[1, 60 \\right] M_{\\odot}$ and $m_2 \\in\n\\left [1, 3 \\right] M_{\\odot}$. We consider three scenarios involving\nground-based detectors: the fifth (O5) observing run of the Advanced\nLIGO-Virgo-KAGRA network, its projected Voyager upgrade, as well as a proposed\nthird generation (3G) network. We find that for fixed early warning times of\n$20-60$ seconds, the inclusion of the higher modes can provide localisation\nimprovements of a factor of $\\gtrsim 2$ for up to $\\sim 60\\%$ ($70 \\%$) of the\nneutron star-black hole systems in the O5 (Voyager) scenario. Considering only\nthose neutron star-black hole systems which can produce potential\nelectromagnetic counterparts, such improvements in the localisation can be\nexpected for $\\sim 5-35\\%$ $(20-50\\%)$ binaries in O5 (Voyager), although the\nlocalisation areas themselves depend on the distances. For the 3G scenario, a\nsignificant fraction of the events have time gains of a minute to several\nminutes, assuming fiducial target localisation areas of 100 to 1000 sq. deg.", "category": "astro-ph_HE" }, { "text": "DW Cancri in x-rays: We report on the $XMM$-Newton observation of DW Cnc, a candidate intermediate\npolar candidate whose historical optical light curve shows the existence of\nperiods at $\\simeq 38$, $\\simeq 86$ and $\\simeq 69$ minutes which were\ninterpreted as the white dwarf spin, the orbital and the spin-orbit beat\nperiodicities. By studying the $0.3-10$ keV light curves, we confirm the\nexistence of a period at $\\simeq 38$ minutes and find in the OM light curve a\nsignature for a period at $75\\pm 21$ minutes which is consistent with both the\norbital and spin-orbit beat. { These findings allow us to unveil without any\ndoubt, the nature of DW Cnc as an accreting intermediate polar. The EPIC and\nRGS source spectra were analyzed and a best fit model, consisting of a\nmulti-temperature plasma, was found. The maximum temperature found when fitting\nthe data is $kT_{max}\\simeq 31$ keV which can be interpreted as an upper limit\nto the temperature of the shock.", "category": "astro-ph_HE" }, { "text": "Evidence for crust cooling in the transiently accreting 11-Hz X-ray\n pulsar in the globular cluster Terzan 5: The temporal heating and subsequent cooling of the crusts of transiently\naccreting neutron stars carries unique information about their structure and a\nvariety of nuclear reaction processes. We report on a new Chandra Director's\nDiscretionary Time observation of the globular cluster Terzan 5, aimed to\nmonitor the transiently accreting 11-Hz X-ray pulsar IGR J17480-2446 after the\ncessation of its recent 10-week long accretion outburst. During the\nobservation, which was performed ~125 days into quiescence, the source displays\na thermal spectrum that fits to a neutron star atmosphere model with a\ntemperature for an observer at infinity of kT~92 eV. This is ~10% lower than\nfound ~75 days earlier, yet ~20% higher than the quiescent base level measured\nprior to the recent outburst. This can be interpreted as cooling of the\naccretion-heated neutron star crust, and implies that crust cooling is\nobservable after short accretion episodes. Comparison with neutron star thermal\nevolution simulations indicates that substantial heat must be released at\nshallow depth inside the neutron star, which is not accounted for in current\nnuclear heating models.", "category": "astro-ph_HE" }, { "text": "Extreme Variability in a Long Duration Gamma-ray Burst Associated with a\n Kilonova: The recent discovery of a kilonova from the long duration gamma-ray burst,\nGRB 211211A, challenges classification schemes based on temporal information\nalone. Gamma-ray properties of GRB 211211A reveal an extreme event, which\nstands out among both short and long GRBs. We find very short variations (few\nms) in the lightcurve of GRB 211211A and estimate ~1000 for the Lorentz factor\nof the outflow. We discuss the relevance of the short variations in identifying\nsimilar long GRBs resulting from compact mergers. Our findings indicate that in\nfuture gravitational wave follow-up campaigns, some long duration GRBs should\nbe treated as possible strong gravitational wave counterparts.", "category": "astro-ph_HE" }, { "text": "Correlations between radio and bolometric fluxes in GX 339-4 and\n H1743-322: Compact radio jets are ubiquitous in stellar-mass black-hole binaries in\ntheir hard spectral state. Empirical relations between the radio and\nnarrow-band X-ray fluxes have been used to understand the connection between\ntheir accretion discs and jets. However, a narrow-band (e.g., 1--10 or 3--9\nkeV) X-ray flux can be a poor proxy for either the bolometric luminosity or the\nmass accretion rate. Here, we study correlations between the radio and\nunabsorbed broad-band X-ray fluxes, the latter providing good estimates of the\nbolometric flux. We consider GX 339--4, the benchmark object for the main\nbranch of the correlation, and H1743--322, the first source found to be an\noutlier of the correlation. The obtained power-law dependencies of the radio\nflux on the bolometric flux have significantly different indices from those\nfound for the narrow X-ray bands. Also, the radio/bolometric flux correlations\nfor the rise of the outbursts are found to be significantly different from\nthose for the outburst decline. This points to a possible existence of a jet\nhysteresis in the radio/X-ray source evolution, in addition to that seen in the\nhardness/flux diagram of low-mass X-ray binaries. The correlation during the\nrise of the outbursts is similar for both GX 339--4 and H1743--322. The\ncorrelation for the decline of the outbursts for H1743--322 lies below that of\nGX 339--4 at intermediate X-ray fluxes, whereas it approaches the standard\ncorrelation at lower X-ray luminosities. We also compare these correlations to\nthose for the high-mass X-ray binaries Cyg X-1 and Cyg X-3.", "category": "astro-ph_HE" }, { "text": "Constraints on Blazar Jet Conditions During Gamma-Ray Flaring from\n Radiative Transfer Modeling: As part of a program to investigate jet flow conditions during GeV gamma-ray\nflares detected by Fermi, we are using UMRAO multi-frequency, centimeter-band\ntotal flux density and linear polarization monitoring observations to constrain\nradiative transfer models incorporating propagating shocks orientated at an\narbitrary angle to the flow direction. We describe the characteristics of the\nmodel, illustrate how the data are used to constrain the models, and present\nresults for three program sources with diverse characteristics: PKS 0420-01, OJ\n287, and 1156+295. The modeling of the observed spectral behavior yields\ninformation on the sense, strength and orientation of the shocks producing the\nradio-band flaring; on the energy distribution of the radiating particles; and\non the observer's viewing angle with respect to the jet independent of VLBI\ndata. We present evidence that, while a random component dominates the jet\nmagnetic field, a distinguishing feature of those radio events with an\nassociated gamma-ray flare is the presence of a weak but non-negligible ordered\nmagnetic field component along the jet axis.", "category": "astro-ph_HE" }, { "text": "X-Ray Luminous Binaries, Metallicity, and the Early Universe: High mass X-ray binaries (HMXBs) may have had a significant impact on the\nheating of the intergalactic medium in the early universe. Study of HMXBs in\nnearby, low metallicity galaxies that are local analogues to early galaxies can\nhelp us understand early HMXBs. The total luminosity of HMXB populations is\ndominated by sources at high luminosities. These sources exhibit X-ray spectra\nthat show curvature above 2 keV and the same is likely true of HMXB populations\nat high redshifts. The spectral curvature changes the K-correction for X-rays\nfrom HMXBs in a manner that weakens the constraints on X-ray emission of early\nHMXBs obtained from the soft X-ray background. Applied to deep X-ray surveys of\nstar forming galaxies, the modified K-correction suggests a moderate increase\nin the ratio of X-ray luminosity to star formation rate at intermediate\nredshifts, z=3-5, and is consistent with a large enhancement at high redshifts,\nz=6-7.", "category": "astro-ph_HE" }, { "text": "New unidentified H.E.S.S. Galactic sources: H.E.S.S. is one of the most sensitive instruments in the very high energy\n(VHE; > 100 GeV) gamma-ray domain and has revealed many new sources along the\nGalactic Plane. After the successful first VHE Galactic Plane Survey of 2004,\nH.E.S.S. has continued and extended that survey in 2005-2008, discovering a\nnumber of new sources, many of which are unidentified. Some of the unidentified\nH.E.S.S. sources have several positional counterparts and hence several\ndifferent possible scenarios for the origin of the VHE gamma-ray emission;\ntheir identification remains unclear. Others have so far no counterparts at any\nother wavelength. Particularly, the lack of an X-ray counterpart puts serious\nconstraints on emission models. Several newly discovered and still unidentified\nVHE sources are reported here.", "category": "astro-ph_HE" }, { "text": "Modeling pulsar time noise with long term power law decay modulated by\n short term oscillations of the magnetic fields of neutron stars: We model the evolution of the magnetic fields of neutron stars as consisting\nof a long term power-law decay modulated by short term small amplitude\noscillations. Our model predictions on the timing noise $\\ddot\\nu$ of neutron\nstars agree well with the observed statistical properties and correlations of\nnormal radio pulsars. Fitting the model predictions to the observed data, we\nfound that their initial parameter implies their initial surface magnetic\ndipole magnetic field strength ~ 5E14 G at ~0.4 year old and that the\noscillations have amplitude between E-8 to E-5 and period on the order of\nyears. For individual pulsars our model can effectively reduce their timing\nresiduals, thus offering the potential of more sensitive detections of\ngravitational waves with pulsar timing arrays. Finally our model can also\nre-produce their observed correlation and oscillations of the second derivative\nof spin frequency, as well as the \"slow glitch\" phenomenon.", "category": "astro-ph_HE" }, { "text": "On the Tidal Capture of White Dwarfs by Intermediate-mass Black Holes in\n Dense Stellar Environments: Intermediate-mass black holes (IMBHs) are the missing link between\nstellar-mass and supermassive black holes, widely believed to reside in at\nleast some dense star clusters, but not yet observed directly. Tidal\ndisruptions of white dwarfs (WDs) are luminous only for black holes less\nmassive than $\\sim 10^5\\,M_{\\odot}$, therefore providing a unique smoking gun\nthat could finally prove the existence of IMBHs beyond any reasonable doubt.\nHere, we investigate the tidal captures of WDs by IMBHs in dense star clusters,\nand estimate a typical rate of $\\sim 1\\,{\\rm Myr}^{-1}$ for galactic nuclei and\n$\\sim 0.01\\,{\\rm Myr}^{-1}$ for globular clusters. Following the capture, the\nWD inspirals onto the IMBH producing gravitational waves detectable out to\n$\\sim100$ Mpc by LISA for $\\sim 10^4\\,M_{\\odot}$ IMBHs. The subsequent tidal\nstripping/disruption of the WD can also release bright X-ray and gamma-ray\nemission with luminosities of at least $\\gtrsim10^{40}\\,\\rm{erg\\,s^{-1}}$,\ndetectable by \\textit{Chandra}, \\textit{Swift}, and upcoming telescopes, such\nas the \\textit{Einstein Probe}.", "category": "astro-ph_HE" }, { "text": "Radio Evolution of Supernova Remnants Including Non-linear Particle\n Acceleration: Insights from Hydrodynamic Simulations: We present a model for the radio evolution of supernova remnants (SNRs)\nobtained by using three-dimensional (3D) hydrodynamic simulations, coupled with\nnonlinear kinetic theory of cosmic ray (CR) acceleration in SNRs. We model the\nradio evolution of SNRs on a global level, by performing simulations for wide\nrange of the relevant physical parameters, such as the ambient density, the\nsupernova (SN) explosion energy, the acceleration efficiency and the magnetic\nfield amplification (MFA) efficiency. We attribute the observed spread of radio\nsurface brightnesses for corresponding SNR diameters to the spread of these\nparameters. In addition to our simulations of type Ia SNRs, we also considered\nSNR radio evolution in denser, nonuniform circumstellar environment, modified\nby the progenitor star wind. These simulations start with the mass of the\nejecta substantially higher than in the case of a type Ia SN and presumably\nlower shock speed. The magnetic field is understandably seen as very important\nfor the radio evolution of SNRs. In terms of MFA, we include both resonant and\nnon-resonant modes in our large scale simulations, by implementing models\nobtained from first-principles, particle-in-cell (PIC) simulations and\nnon-linear magnetohydrodynamical (MHD) simulations. We test the quality and\nreliability of our models on a sample consisting of Galactic and extragalactic\nSNRs. Our simulations give $\\Sigma-D$ slopes between -4 and -6 for the full\nSedov regime. Recent empirical slopes obtained for the Galactic samples are\naround -5, while for the extragalactic samples are around -4.", "category": "astro-ph_HE" }, { "text": "Collective excitations in neutron-star crusts: We explore the spectrum of low-energy collective excitations in the crust of\na neutron star, especially in the inner region where neutron-proton clusters\nare immersed in a sea of superfluid neutrons. The speeds of the different modes\nare calculated systematically from the nuclear energy density functional theory\nusing a Skyrme functional fitted to essentially all experimental atomic mass\ndata.", "category": "astro-ph_HE" }, { "text": "Meeting the Challenge from Bright and Fast Gamma-Ray Flares of 3C 279: Bright and fast gamma-ray flares with hard spectra have been recently\ndetected from the blazar 3C 279, with apparent GeV luminosities up to $10^{49}$\nerg/s. The source is observed to flicker on timescales of minutes with no\ncomparable optical-UV counterparts. Such observations challenge current models\nof high-energy emissions from 3C 279 and similar blazar sources that are\ndominated by relativistic jets along our line of sight with bulk Lorentz\nfactors up to $ \\Gamma \\sim 20$ launched by supermassive black holes. We\ncompute and discuss a model based on a clumpy jet comprising strings of compact\nplasmoids as indicated by radio observations. We follow the path of the\nsynchrotron radiations emitted in the optical - UV bands by relativistic\nelectrons accelerated around the plasmoids to isotropic Lorentz factors $\\gamma\n\\sim 1000$. These primary emissions are partly reflected back by a leading\nmember in the string that acts as a moving mirror for the approaching\ncompanions. Around the plasmoids, shrinking \\emph{gap} transient overdensities\nof seed photons build up. These are upscattered into the GeV range by inverse\nCompton interactions with the relativistic electrons accelerated in situ. We\nshow that such a combined process produces bright gamma-ray flares with minor\noptical to X-ray enhancements. Main features of our model include: bright\ngamma-ray flares with risetimes as short as a few minutes, occurring at\ndistances of order $10^{18} $ cm from the central black hole; Compton dominance\nat GeV energies by factors up to some $10^2$; little reabsorption from local\nphoton-photon interactions.", "category": "astro-ph_HE" }, { "text": "A report on the type II X-ray burst from SMC X-1: We study RXTE PCA data for the high mass X-ray binary source SMC X-1 between\n2003-10 and 2003-12 when the source was in high states. The source is found to\nbe frequently bursting which can be seen as flares in lightcurves on an average\nof one in every 800 s, with an average of 4-5 X-ray burst per hour of type II.\nWe note that typically burst was short lasting for few tens of seconds in\naddition few long bursts of more than hundred seconds were also observed. The\nflares apparently occupied 2.5$\\%$ of the total observing time of 225.5 ks. We\nnote a total of 272 flares with mean FWHM of the flare $\\backsim$21 s. The rms\nvariability and the aperiodic variability are independent of flares. As\nobserved the pulse profiles of the lightcurves do not change its shape implying\nthat there is no change in the geometry of accretion disk due to burst. The\nhardness ratio and the rms variability of lightcurves show no correlation with\nthe flares. The flare-fraction shows a positive correlation with the\npeak-to-peak ratio of the primary and secondary peaks of the pulse profile. The\nobserved hardening or the softening of the spectrum cannot be correlated with\nthe flaring rate but may be due to the interstellar absorption of X-rays as\nevident from the change in the hydrogen column density ($n_{H}$). It is found\nthat the luminosity of the source increases with the flaring rate. Considering\nthe viscous timescale equal to mean recurrence time of flares we fixed the\nviscosity parameter $\\alpha$ $\\backsim$ 0.16.", "category": "astro-ph_HE" }, { "text": "Optical Spectral Variations of a Large Sample of Fermi Blazars: We have investigated the optical spectral behavior of a large sample of Fermi\nblazars (40 FSRQs and 13 BL Lacs), and found two new universal optical spectral\nbehaviors. In the low state the optical spectrum gradually becomes softer\n(steeper) or harder (flatter) but more and more slowly when the brightness\nincreases, and then tends to stable in the high state, which are briefly named\nthe redder-stable-when-brighter (RSWB) and bluer-stable-when-brighter (BSWB)\nbehaviors, respectively. 34 FSRQs and 7 BL Lacs exhibit clear RSWB behavior,\nand 2 FSRQs and 5 BL Lacs show distinct BSWB behavior, which mean that FSRQs\nfavor more RSWB than BSWB behavior, while BL Lacs have no clear preference\namong both behaviors. We have put forward a unified nonlinear formula to\nquantitatively characterize the optical spectral behaviors of FSRQs and BL\nLacs, which can fit both kinds of behaviors very well. We argue that the RSWB\nand BSWB behaviors originate from the same mechanism, and they are the\nuniversal optical spectral behaviors for blazars. The frequently observed\nredder-when-brighter (RWB) and bluer-when-brighter (BWB) trends can be\nconsidered to be the approximations of the behaviors of RSWB and BSWB,\nrespectively. The rarely observed stable-when-brighter (SWB) trend can also be\nviewed as an approximation or a special case of the RSWB or BSWB behavior. We\nhave developed a model with two constant-spectral-index components which can\nnot only explain well both two kinds of optical spectral behaviors, but also\nsuccessfully interpret the differential behaviors between FSRQs and BL Lacs.", "category": "astro-ph_HE" }, { "text": "Radio surface fluctuations in radio relics: Recent observations have revealed detailed structures of radio relics in a\nwide range of frequencies. In this work, we perform three-dimensional\nmagnetohydrodynamical simulations of merger shocks propagating through a\nturbulent magnetized intracluster medium, and employ on-the-fly Lagrangian\nparticles to explore the physical processes originating radio substructures and\ntheir appearances in high and low-frequency observations. We employ two\ncosmic-ray (CR) electron acceleration models: the fresh injection of electrons\nfrom the thermal pool and the re-acceleration of mildly relativistic electrons.\nWe use the relative surface brightness fluctuations, $\\delta S_{\\nu}$, to\ndefine a \"degree of patchiness''. We find that: 1) Patchiness is produced if\nthe shock's surface has a distribution of Mach numbers, rather than a single\nMach number; 2) Radio relics appear patchier if the Mach number distribution\nconsists of a large percentage of low Mach numbers ($\\mathcal{M}\\lesssim2.5$);\n3) As the frequency increases, the patchiness also becomes larger.\nNevertheless, if radio relics are patchy at high frequencies (e.g., 18.6 GHz),\nthey are necessarily also at low frequencies (e.g., 150 MHz); 4) To produce\nnoticeable differences in the patchiness at low and high frequencies, the shock\nfront should have a Mach number spread of $\\sigma_{\\mathcal{M}}\\gtrsim0.3$-0.4;\n5) The amount of the patchiness depends on the Mach number distribution as well\nas the CR acceleration model. We propose $\\delta S_{\\nu}$ as a potential tool\nfor extracting merger shock properties and information about particle\nacceleration processes at shocks in radio observations.", "category": "astro-ph_HE" }, { "text": "Cosmic-ray electron transport in the galaxy M 51: Context. Indirect observations of the cosmic-ray electron (CRE) distribution\nvia synchrotron emission is crucial for deepening the understanding of the CRE\ntransport in the interstellar medium, and in investigating the role of galactic\noutflows.\n Aims. In this paper, we quantify the contribution of diffusion- and\nadvection-dominated transport of CREs in the galaxy M51 considering relevant\nenergy loss processes.\n Methods. We used recent measurement from M 51 that allow for the derivation\nof the diffusion coefficient, the star formation rate, and the magnetic field\nstrength. With this input, we solved the 3D transport equation numerically\nincluding the spatial dependence as provided by the measurements, using the\nopen-source transport framework CRPropa (v3.1). We included 3D transport\n(diffusion and advection), and the relevant loss processes.\n Results. We find that the data can be described well with the parameters from\nrecent measurements. For the best fit, it is required that the wind velocity,\nfollowing from the observed star formation rate, must be decreased by a factor\nof 5. We find a model in which the inner galaxy is dominated by advective\nescape and the outer galaxy is composed by both diffusion and advection.\n Conclusions. Three-dimensional modelling of cosmic-ray transport in the\nface-on galaxy M51 allows for conclusions about the strength of the outflow of\nsuch galaxies by quantifying the need for a wind in the description of the\ncosmic-ray signatures. This opens up the possibility of investigating galactic\nwinds in face-on galaxies in general.", "category": "astro-ph_HE" }, { "text": "When and where did GW150914 form?: The recent LIGO detection of gravitational waves (GW150914), likely\noriginating from the merger of two $\\sim 30 M_\\odot$ black holes suggests\nprogenitor stars of low metallicity ($[Z/Z_\\odot] \\lesssim 0.3$), constraining\nwhen and where the progenitor of GW150914 may have formed. We combine estimates\nof galaxy properties (metallicity, star formation rate and merger rate) across\ncosmic time to predict the low redshift black hole - black hole merger rate as\na function of present day host galaxy mass, $M_\\mathrm{gal}$, and the formation\nredshift of the progenitor system $z_\\mathrm{form}$ for different progenitor\nmetallicities $Z_\\mathrm{c}$. At $Z_\\mathrm{c}=0.1 Z_\\odot$, the signal is\ndominated by binaries in massive galaxies with $z_\\mathrm{form}\\simeq 2$, with\na small contribution from binaries formed around $z_\\mathrm{form}\\simeq 0.5$ in\ndwarf galaxies. For $Z_\\mathrm{c}=0.01Z_\\odot$, fast mergers are possible and\nvery recent star formation in dwarfs likely dominates. Additional gravitational\nwave detections from merging massive black holes will provide constraints on\nthe mass-metallicity relation and massive star formation at high redshifts.", "category": "astro-ph_HE" }, { "text": "Thermal synchrotron radiation from RRMHD simulations of the double\n tearing mode reconnection - Application to the Crab flares: We study the magneto-hydrodynamic tearing instability occurring in a double\ncurrent sheet configuration when a guide field is present. This is investigated\nby means of resistive relativistic magneto-hydrodynamic (RRMHD) simulations.\nFollowing the dynamics of the double tearing mode (DTM), we are able to compute\nsynthetic synchrotron spectra in the explosive reconnection phase. The pulsar\nstriped wind model represents a site where such current sheets are formed,\nincluding a guide field. The variability of the Crab nebula/pulsar system, seen\nas flares, can be therefore naturally explained by the DTM explosive phase in\nthe striped wind. Our results indicate that the Crab GeV flare can be explained\nby the double tearing mode in the striped wind region if the magnetization\nparameter $\\sigma$ is around $10^5$.", "category": "astro-ph_HE" }, { "text": "Gamma Rays From Blazars: Blazars are high-energy engines providing us natural laboratories to study\nparticle acceleration, relativistic plasma processes, magnetic field dynamics,\nblack hole physics. Key informations are provided by observations at\nhigh-energy (in particular by Fermi/LAT) and very-high energy (by Cherenkov\ntelescopes). I give a short account of the current status of the field, with\nparticular emphasis on the theoretical challenges connected to the observed\nultra-fast variability events and to the emission of flat spectrum radio\nquasars in the very high energy band.", "category": "astro-ph_HE" }, { "text": "Effects of nuclear matter and composition in core-collapse supernovae\n and long-term proto-neutron star cooling: We study the influence of hot and dense matter in core-collapse supernovae by\nadopting up-to-date nuclear equation of state (EOS) based on the microscopic\nnuclear many-body frameworks. We explore effects of EOS based on the Dirac\nBrueckner Hartree-Fock theory through comparisons with those based on the\nvariational method. We also examine effects of the differences in the\ncomposition of nuclei and nucleons by using the same EOS by the variational\nmethod but employing two different treatments in computations of nuclear\nabundances. We perform numerical simulations of core-collapse supernovae\nadopting the three EOSs. We also perform numerical simulations of the long-term\nevolution over 70 s of the proto-neutron star cooling. We show that impacts by\ndifferent modeling of composition are remarkable as in those by different\ntreatments of uniform matter in the gravitational collapse, bounce, and shock\npropagation. The cooling of proto-neutron star and the resulting neutrino\nemission are also affected by the compositional difference even if the same\ntreatment in computing uniform matter of EOS.", "category": "astro-ph_HE" }, { "text": "The 999th Swift gamma-ray burst: Some like it thermal: We present a multiwavelength study of GRB 151027A. This is the 999th GRB\ndetected by the Swift satellite and it has a densely sampled emission in the\nX-ray and optical band and has been observed and detected in the radio up to\n140 days after the prompt. The multiwavelength light curve from 500 s to 140\ndays can be modelled through a standard forward shock afterglow but requires an\nadditional component to reproduce the early X-ray and optical emission. We\npresent TNG and LBT optical observations performed 19.6, 33.9 and 92.3 days\nafter the trigger which show a bump with respect to a standard afterglow flux\ndecay and are possibly interpreted as due to the underlying SN and host galaxy\n(of 0.4 uJy in the R band). Radio observations, performed with SRT, Medicina,\nEVN and VLBA between day 4 and 140, suggest that the burst exploded in an\nenvironment characterised by a density profile scaling with the distance from\nthe source (wind profile). A remarkable feature of the prompt emission is the\npresence of a bright flare 100 s after the trigger, lasting 70 seconds in the\nsoft X-ray band, which was simultaneously detected from the optical band up to\nthe MeV energy range. By combining Swift-BAT/XRT and Fermi-GBM data, the\nbroadband (0.3-1000 keV) time resolved spectral analysis of the flare reveals\nthe coexistence of a non-thermal (power law) and thermal blackbody components.\nThe BB component contributes up to 35% of the luminosity in the 0.3-1000 keV\nband. The gamma-ray emission observed in Swift-BAT and Fermi-GBM anticipates\nand lasts less than the soft X-ray emission as observed by Swift-XRT, arguing\nagainst a Comptonization origin. The BB component could either be produced by\nan outflow becoming transparent or by the collision of a fast shell with a\nslow, heavy and optically thick fireball ejected during the quiescent time\ninterval between the initial and later flares of the burst.", "category": "astro-ph_HE" }, { "text": "Time-resolved Spectral Study Of Fermi GRBs Having Single Pulses: We analyze gamma-ray bursts (GRBs) detected by \\textit{Fermi}/Gamma-Ray Burst\nMonitor (GBM) and having single pulse. We fit the light curves with a model\nhaving exponential rise and decay parts. We perform a detailed time-resolved\nspectroscopy using four models: Band, blackbody with a power-law (BBPL),\nmulticolour blackbody with a power-law (mBBPL) and two blackbodies with a\npower-law (2BBPL). We find that models other than the BBPL give better\n$\\chi_{red}^2$ for the \"hard-to-soft\" (HTS) pulses, while for the \"intensity\ntracking\" (IT) pulses, the BBPL model is statistically as good as the other\nmodels. Interestingly, the energy at the peak of the spectrum resulting from\nthe BBPL model ($\\sim3kT$), is always lower than that of the $\\nu F_{\\nu}$\nspectrum of the Band function. The values of the low energy photon index\n($\\alpha$) of the Band function are often higher than the fundamental single\nparticle synchrotron limit, especially for the HTS pulses. Specifically we find\ntwo extreme cases --- for GRB~110817A (HTS GRB) $\\alpha$ is always higher,\nwhile for GRB~100528A (IT GRB) $\\alpha$ is always within the synchrotron\nregime. The PL component of the BBPL model always starts with a delay compared\nto the BB component, and it lingers at the later part of the prompt emission\nphase. For three HTS GRBs, namely, GRB~081224, GRB~100707A and GRB~110721A this\nbehaviour is particularly significant and interestingly there are reported LAT\ndetections for them. Finally, we argue that various evidences hint that neither\nBBPL nor Band model is acceptable, while 2BBPL and mBBPL are the most\nacceptable models for the set of GRBs we have analyzed.", "category": "astro-ph_HE" }, { "text": "Deep Chandra observations of Pictor A: We report on deep Chandra observations of the nearby broad-line radio galaxy\nPictor A, which we combine with new Australia Telescope Compact Array (ATCA)\nobservations. The new X-ray data have a factor 4 more exposure than\nobservations previously presented and span a 15-year time baseline, allowing a\ndetailed study of the spatial, temporal and spectral properties of the AGN,\njet, hotspot and lobes. We present evidence for further time variation of the\njet, though the flare that we reported in previous work remains the most\nsignificantly detected time-varying feature. We also confirm previous tentative\nevidence for a faint counterjet. Based on the radio through X-ray spectrum of\nthe jet and its detailed spatial structure, and on the properties of the\ncounterjet, we argue that inverse-Compton models can be conclusively rejected,\nand propose that the X-ray emission from the jet is synchrotron emission from\nparticles accelerated in the boundary layer of a relativistic jet. For the\nfirst time, we find evidence that the bright western hotspot is also\ntime-varying in X-rays, and we connect this to the small-scale structure in the\nhotspot seen in high-resolution radio observations. The new data allow us to\nconfirm that the spectrum of the lobes is in good agreement with the\npredictions of an inverse-Compton model and we show that the data favour models\nin which the filaments seen in the radio images are predominantly the result of\nspatial variation of magnetic fields in the presence of a relatively uniform\nelectron distribution.", "category": "astro-ph_HE" }, { "text": "Observations of V404 Cygni during the 2015 outburst by the Nasu\n telescope array at 1.4 GHz: Waseda University Nasu telescope array is a spatial fast Fourier transform\n(FFT) interferometer consisting of eight linearly aligned antennas with 20 m\nspherical dishes. This type of interferometer was developed to survey transient\nradio sources with an angular resolution as high as that of a 160 m dish with a\nfield of view as wide as that of a 20 m dish. We have been performing\ndrift-scan-mode observations, in which the telescope scans the sky around a\nselected declination as the earth rotates. The black hole X-ray binary V404\nCygni underwent a new outburst in 2015 June after a quiescent period of 26\nyears. Because of the interest in black hole binaries, a considerable amount of\ndata on this outburst at all wavelengths was accumulated. Using the above\ntelescope, we had been monitoring V404 Cygni daily from one month before the\nX-ray outburst, and two radio flares at 1.4 GHz were detected on June 21.73 and\nJune 26.71. The flux density and time-scale of each flare were 313+/-30 mJy and\n1.50+/-0.49 days, 364+/-30 mJy and 1.70+/-0.16 days, respectively. We have also\nconfirmed the extreme variation of radio spectra within a short period by\ncollecting other radio data observed with several radio telescopes. Such\nspectral behaviors are considered to reflect the change in the opacity of the\nejected blobs associated with these extreme activities in radio and X-ray. Our\n1.4 GHz radio data are expected to be helpful for studying the physics of the\naccretion and ejection phenomena around black holes.", "category": "astro-ph_HE" }, { "text": "Merging White Dwarfs and Thermonuclear Supernovae: Thermonuclear supernovae result when interaction with a companion reignites\nnuclear fusion in a carbon-oxygen white dwarf, causing a thermonuclear runaway,\na catastrophic gain in pressure, and the disintegration of the whole white\ndwarf. It is usually thought that fusion is reignited in near-pycnonuclear\nconditions when the white dwarf approaches the Chandrasekhar mass. I briefly\ndescribe two long-standing problems faced by this scenario, and our suggestion\nthat these supernovae instead result from mergers of carbon-oxygen white\ndwarfs, including those that produce sub-Chandrasekhar mass remnants. I then\nturn to possible observational tests, in particular those that test the absence\nor presence of electron captures during the burning.", "category": "astro-ph_HE" }, { "text": "Vortex Pinning in Neutron Stars, Slip-stick Dynamics, and the Origin of\n Spin Glitches: We study pinning and unpinning of superfluid vortices in the inner crust of a\nneutron star using 3-dimensional dynamical simulations. Strong pinning occurs\nfor certain lattice orientations of an idealized, body-centered cubic lattice,\nand occurs generally in an amorphous or impure nuclear lattice. The pinning\nforce per unit length is $\\sim 10^{16}$ dyn cm$^{-1}$ for a vortex-nucleus\ninteraction that is repulsive, and $\\sim 10^{17}$ dyn cm$^{-1}$ for an\nattractive interaction. The pinning force is strong enough to account for\nobserved spin jumps (glitches). Vortices forced through the lattice move with a\nslip-stick character; for a range of superfluid velocities, the vortex can be\nin either a cold, pinned state or a hot unpinned state, with strong excitation\nof Kelvin waves on the vortex. This two-state nature of vortex motion sets the\nstage for large-scale vortex movement that creates an observable spin glitch.\nWe argue that the vortex array is likely to become tangled as a result of\nrepeated unpinnings and repinnings. We conjecture that during a glitch, the\nKelvin-wave excitation spreads rapidly along the direction of the mean\nsuperfluid vorticity and slower in the direction perpendicular to it, akin to\nan anisotropic deflagration.", "category": "astro-ph_HE" }, { "text": "Searching for energy-resolved quasi-periodic oscillations in AGN: X-ray quasi-periodic oscillations (QPOs) in AGN allow us to probe and\nunderstand the nature of accretion in highly curved space-time, yet the most\nrobust form of detection (i.e. repeat detections over multiple observations)\nhas been limited to a single source to-date, with only tentative claims of\nsingle observation detections in several others. The association of those\nestablished AGN QPOs with a specific spectral component has motivated us to\nsearch the XMM-Newton archive and analyse the energy-resolved lightcurves of 38\nbright AGN. We apply a conservative false alarm testing routine folding in the\nuncertainty and covariance of the underlying broad-band noise. We also explore\nthe impact of red-noise leak and the assumption of various different forms\n(power-law, broken power-law and lorentzians) for the underlying broad-band\nnoise. In this initial study, we report QPO candidates in 6 AGN (7 including\none tentative detection in MRK~766) from our sample of 38, which tend to be\nfound at characteristic energies and, in four cases, at the same frequency\nacross at least two observations, indicating they are highly unlikely to be\nspurious in nature.", "category": "astro-ph_HE" }, { "text": "Searching for TeV emission from GRBs: the status of the H.E.S.S. GRB\n programme: H.E.S.S. is an array of five Imaging Atmospheric Cherenkov Telescopes (IACTs)\nlocated 1800 m above sea level in the Khomas Highland of Namibia and is\nsensitive to very-high-energy (VHE) gamma rays between tens of GeV to tens of\nTeV. The very-high background rejection capabilities of IACTs provide excellent\nsensitivity of H.E.S.S. to GRBs. In this contribution the status of the\nH.E.S.S. GRB programme, already started in 2003, is reviewed. A highlight is\nthe recent addition of the fifth telescope, which is the world's largest IACT.\nIts 600 square metre mirror lowers the energy threshold to tens of GeV and\nprovides an effective area that is ten thousands of times larger than Fermi-LAT\nat these energies. The higher performance drive system will reduce the response\ntime to a GRB alert, which will significantly enhance the chances of a H.E.S.S.\nGRB detection. Recent results on selected GRBs will be shown.", "category": "astro-ph_HE" }, { "text": "EAS optical Cherenkov signatures of tau neutrinos for space and\n suborbital detectors: Multi-messenger observations of transient astrophysical sources have the\npotential to characterize the highest energy accelerators and the most extreme\nenvironments in the Universe. Detection of neutrinos, in particular tau\nneutrinos generated by neutrino oscillations in transit from their sources to\nEarth, is possible for neutrino energies above 10 PeV using optical Cherenkov\ndetectors imaging upward-moving extensive air showers (EAS). These EAS are\nproduced from Earth interacting tau neutrinos leading to tau leptons that\nsubsequently decay in the atmosphere. We compare neutrino detection\nsensitivities for generic short- and long-burst transient neutrino sources and\nsensitivities to the diffuse neutrino flux for the second generation Extreme\nUniverse Space Observatory on a Super Pressure Balloon (EUSO-SPB2)\nballoon-borne mission and the proposed space-based Probe of Extreme\nMulti-Messenger Astrophysics (POEMMA) mission.", "category": "astro-ph_HE" }, { "text": "Kink-driven magnetic reconnection in relativistic jets: consequences for\n X-ray polarimetry of BL Lacs: We investigate with relativistic MHD simulations the dissipation physics of\nBL Lac jets, by studying the synchrotron polarization signatures of particles\naccelerated by the kink instability in a magnetically-dominated plasma column.\nThe nonlinear stage of the kink instability generates current sheets, where\nparticles can be efficiently accelerated via magnetic reconnection. We identify\ncurrent sheets as regions where s = J d/B is above some predefined threshold\n(where B is the field strength, J the current density and d the grid scale),\nand assume that the particle injection efficiency scales as proportional to the\nsquare of the current. X-ray emitting particles have short cooling times, so\nthey only probe the field geometry of their injection sites. In contrast,\nparticles emitting in the optical band, which we follow self-consistently as\nthey propagate away from their injection sites while cooling, sample a larger\nvolume, and so they may be expected to produce different polarimetric\nsignatures. We find that the degree of polarization is roughly the same between\nX-ray and optical bands, because even the optical-emitting particles do not\ntravel far from the current sheet where they were injected, due to lack of\nsufficient kink-generated turbulence. The polarization angle shows a different\ntemporal evolution between the two bands, due to the different regions probed\nby X-ray and optical emitting particles. In view of the upcoming IXPE\nsatellite, our results can help constrain whether kink-induced reconnection (as\nopposed to shocks) can be the source of multi-wavelength emission from BL Lacs.", "category": "astro-ph_HE" }, { "text": "The inner knot of the Crab nebula: We model the inner knot of the Crab Nebula as a synchrotron emission coming\nfrom the non-spherical MHD termination shock of relativistic pulsar wind. The\npost-shock flow is mildly relativistic; as a result the Doppler-beaming has a\nstrong impact on the shock appearance. The model can reproduce the knot\nlocation, size, elongation, brightness distribution, luminosity and\npolarization provided the effective magnetization of the section of the pulsar\nwind producing the knot is low, $\\sigma \\leq 1$. In the striped wind model,\nthis implies that the striped zone is rather wide, with the magnetic\ninclination angle of the Crab pulsar $\\ge 45^\\circ$; this agrees with the\nprevious model-dependent estimate based on the gamma-ray emission of the\npulsar. We conclude that the tiny knot is indeed a bright spot on the surface\nof a quasi-stationary magnetic relativistic shock and that this shock is a site\nof efficient particle acceleration. On the other hand, the deduced low\nmagnetization of the knot plasma implies that this is an unlikely site for the\nCrab's gamma-ray flares, if they are related to the fast relativistic magnetic\nreconnection events.", "category": "astro-ph_HE" }, { "text": "Revealing x-ray and gamma ray temporal and spectral similarities in the\n GRB 190829A afterglow: Gamma-ray bursts (GRBs), which are bright flashes of gamma rays from\nextragalactic sources followed by fading afterglow emission, are associated\nwith stellar core collapse events. We report the detection of very-high-energy\n(VHE) gamma rays from the afterglow of GRB 190829A, between 4 and 56 hours\nafter the trigger, using the High Energy Stereoscopic System (H.E.S.S.). The\nlow luminosity and redshift of GRB 190829A reduce both internal and external\nabsorption, allowing determination of its intrinsic energy spectrum. Between\nenergies of 0.18 and 3.3 tera-electron volts, this spectrum is described by a\npower law with photon index of 2.07 $\\pm$ 0.09, similar to the x-ray spectrum.\nThe x-ray and VHE gamma-ray light curves also show similar decay profiles.\nThese similar characteristics in the x-ray and gamma-ray bands challenge GRB\nafterglow emission scenarios.", "category": "astro-ph_HE" }, { "text": "Associated molecular and atomic clouds with X-ray shell of superbubble\n 30 Doradus C in the LMC: 30 Doradus C is a superbubble which emits the brightest nonthermal X- and TeV\ngamma-rays in the Local Group. In order to explore detailed connection between\nthe high energy radiation and the interstellar medium, we have carried out new\nCO and HI observations using the Atacama Large Millimeter$/$Submillimeter Array\n(ALMA), Atacama Submillimeter Telescope Experiment, and the Australia Telescope\nCompact Array with resolutions of up to 3 pc. The ALMA data of $^{12}$CO($J$ =\n1-0) emission revealed 23 molecular clouds with the typical diameters of\n$\\sim$6-12 pc and masses of $\\sim$600-10000 $M_{\\odot}$. The comparison with\nthe X-rays of $XMM$-$Newton$ at $\\sim$3 pc resolution shows that X-rays are\nenhanced toward these clouds. The CO data were combined with the HI to estimate\nthe total interstellar protons. Comparison of the interstellar proton column\ndensity and the X-rays revealed that the X-rays are enhanced with the total\nproton. These are most likely due to the shock-cloud interaction modeled by the\nmagnetohydrodynamical simulations (Inoue et al. 2012, ApJ, 744, 71). Further,\nwe note a trend that the X-ray photon index varies with distance from the\ncenter of the high-mass star cluster, suggesting that the cosmic-ray electrons\nare accelerated by one or multiple supernovae in the cluster. Based on these\nresults we discuss the role of the interstellar medium in cosmic-ray particle\nacceleration.", "category": "astro-ph_HE" }, { "text": "Effect of dark matter on the shadow of a distorted and deformed compact\n object: This work investigates observational properties, namely the shadow and photon\nring structure, of emission profiles originated near compact objects. In\nparticular, we consider a distorted and deformed compact object characterized\nby quadrupoles and surrounded by an optically thin and geometrically thin\naccretion disk with different emission profiles modeled by the Johnson's\nStandard-Unbound (SU) distribution in the reference frame of the emitter. Under\nthese assumptions, we produce the observed intensity profiles and shadow images\nfor a face-on observer. Our results indicate that, due to the fact that\nmodifications of the quadrupole parameters affect the radius of the innermost\nstable circular orbit (ISCO) and the photon sphere (PS), the observed shadow\nimages and their properties are significantly influenced by the quadrupole\nparameters and emission profiles. Furthermore, we analyze the impact of the\npresence of a dark matter halo in the observational imprints considered and\nverify that both the increase in the matter contained in the halo or decrease\nin the length-scale of the halo lead to an increase in the size of the observed\nshadow. Our results indicate potential degeneracies between the observational\nfeatures of distorded and deformed compact objects with those of spherically\nsymmetric blackholes, which could be assessed by a comparison with the current\nand future generation of optical experiments in gravitational physics.", "category": "astro-ph_HE" }, { "text": "Progenitor, environment, and modelling of the interacting transient, AT\n 2016jbu (Gaia16cfr): We present the bolometric lightcurve, identification and analysis of the\nprogenitor candidate, and preliminary modelling of AT2016jbu (Gaia16cfr). We\nfind a progenitor consistent with a $\\sim$22--25~$M_{\\odot}$ yellow hypergiant\nsurrounded by a dusty circumstellar shell, in agreement with what has been\npreviously reported. We see evidence for significant photometric variability in\nthe progenitor, as well as strong H$\\alpha$ emission consistent with\npre-existing circumstellar material. The age of the environment as well as the\nresolved stellar population surrounding AT2016jbu, support a progenitor age of\n$>$10 Myr, consistent with a progenitor mass of $\\sim$22~$M_{\\odot}$. A joint\nanalysis of the velocity evolution of AT2016jbu, and the photospheric radius\ninferred from the bolometric lightcurve shows the transient is consistent with\ntwo successive outbursts/explosions. The first outburst ejected material with\nvelocity $\\sim$650$kms^{-1}$, while the second, more energetic event, ejected\nmaterial at $\\sim$4500$kms^{-1}$. Whether the latter is the core-collapse of\nthe progenitor remains uncertain. We place a limit on the ejected $^{56}$Ni\nmass of $<$0.016$M_{\\odot}$. Using the BPASS code, we explore a wide range of\npossible progenitor systems, and find that the majority of these are in\nbinaries, some of which are undergoing mass transfer or common envelope\nevolution immediately prior to explosion. Finally, we use the SNEC code to\ndemonstrate that the low-energy explosion within some of these binary systems,\ntogether with sufficient CSM, can reproduce the overall morphology of the\nlightcurve of AT2016jbu.", "category": "astro-ph_HE" }, { "text": "Supernova Search with the AMANDA / IceCube Detectors: Since 1997 the neutrino telescope AMANDA at the geographic South Pole has\nbeen monitoring our Galaxy for neutrino bursts from supernovae. Triggers were\nintroduced in 2004 to submit burst candidates to the Supernova Early Warning\nSystem SNEWS. From 2007 the burst search was extended to the much larger\nIceCube telescope, which now supersedes AMANDA. By exploiting the low\nphotomultiplier noise in the antarctic ice (on average 280Hz for IceCube),\nneutrino bursts from nearby supernovae can be identified by the induced\ncollective rise in the pulse rates. Although only a counting experiment,\nIceCube will provide the world's most precise measurement of the time profile\nof a neutrino burst near the galactic center. The sensitivity to neutrino\nproperties such as the theta13 mixing angle and the neutrino hierarchy are\ndiscussed as well as the possibility to detect the deleptonization burst.", "category": "astro-ph_HE" }, { "text": "The glitches and rotational history of the highly energetic young pulsar\n PSR J0537$-$6910: We present a timing and glitch analysis of the young X-ray pulsar PSR\nJ0537$-$6910, located within the Large Magellanic Cloud, using 13 years of data\nfrom the now decommissioned Rossi X-ray Timing Explorer. Rotating with a spin\nperiod of 16 ms, PSR J0537$-$6910 is the fastest spinning and most energetic\nyoung pulsar known. It also displays the highest glitch activity of any known\npulsar. We have found 42 glitches over the data span, corresponding to a glitch\nrate of 3.2 yr$^{-1}$, with an overall glitch activity rate of $8.8\\times\n10^{-7}\\,$yr$^{-1}$. The high glitch frequency has allowed us to study the\nglitch behavior in ways that are inaccessible in other pulsars. We observe a\nstrong linear correlation between spin frequency glitch magnitude and wait time\nto the following glitch. We also find that the post-glitch spin-down recovery\nis well described by a single two-component model fit to all glitches for which\nwe have adequate input data. This consists of an exponential amplitude $A =\n(7.6 \\pm 1.0)\\times 10^{-14}\\,$s$^{-2}$ and decay timescale $\\tau =\n27_{-6}^{+7}\\,$d, and linear slope $m = (4.1\\pm 0.4)\\times\n10^{-16}\\,$s$^{-2}\\,$d$^{-1}$. The latter slope corresponds to a second\nfrequency derivative $\\ddot{\\nu} = (4.7\\pm 0.5) \\times 10^{-22}\\,$s$^{-3}$,\nfrom which we find an implied braking index $n=7.4 \\pm 0.8$. We also present a\nmaximum-likelihood technique for searching for periods in event-time data,\nwhich we used to both confirm previously published values and determine\nrotation frequencies in later observations. We discuss the implied constraints\non glitch models from the observed behavior of this system, which we argue\ncannot be fully explained in the context of existing theories.", "category": "astro-ph_HE" }, { "text": "X-ray emission of contact binary variables within 1 kpc: By assembling the largest sample to date of X-ray emitting EW-type binaries\n(EWXs), we carried out correlation analyses for the X-ray luminosity\nlog$L_{\\textrm{X}}$, and X-ray activity level\nlog($L_{\\textrm{X}}$/$L_{\\textrm{bol}}$) versus the orbital period $P$ and\neffective temperature $T_{\\rm eff}$. We find strong $P$-log$L_{\\textrm{X}}$ and\n$P$-log($L_{\\textrm{X}}$/$L_{\\textrm{bol}}$) correlations for EWXs with $P$ <\n0.44 days and we provide the linear parametrizations for these relations, on\nthe basis of which the orbital period can be treated as a good predictor for\nlog$L_{\\textrm{X}}$ and log($L_{\\textrm{X}}$/$L_{\\textrm{bol}}$). The\naforementioned binary stellar parameters are all correlated with\nlog$L_{\\textrm{X}}$, while only $T_{\\rm eff}$ exhibits a strong correlation\nwith log($L_{\\textrm{X}}$/$L_{\\textrm{bol}}$). Then, EWXs with higher\ntemperature show lower X-ray activity level, which could indicate the thinning\nof the convective area related to the magnetic dynamo mechanism. The total\nX-ray luminosity of an EWX is essentially consistent with that of an X-ray\nsaturated main sequence star with the same mass as its primary, which may imply\nthat the primary star dominates the X-ray emission. The monotonically\ndecreasing $P$-log($L_{\\textrm{X}}$/$L_{\\textrm{bol}}$) relation and the short\norbital periods indicate that EWXs could all be in the X-ray saturated state,\nand they may inherit the changing trend of the saturated X-ray luminosities\nalong with the mass shown by single stars. For EWXs, the orbital period, mass,\nand effective temperature increase in concordance. We demonstrate that the\nperiod $P=0.44$ days corresponds to the primary mass of $\\sim1.1 \\rm M_\\odot$,\nbeyond which the saturated X-ray luminosity of single stars will not continue\nto increase with mass. This explains the break in the positive\n$P$-log$L_{\\textrm{X}}$ relation for EWXs with $P>0.44$ days.", "category": "astro-ph_HE" }, { "text": "High Resolution X-ray Spectroscopy of the Seyfert 1, Mrk 1040. Revealing\n the Failed Nuclear Wind with Chandra: High resolution X-ray spectroscopy of the warm absorber in the nearby X-ray\nbright Seyfert 1 galaxy, Mrk 1040 is presented. The observations were carried\nout in the 2013-2014 timeframe using the Chandra High Energy Transmission\nGrating with a total exposure of 200 ks. A multitude of absorption lines from\nNe, Mg and Si are detected from a wide variety of ionization states. In\nparticular, the detection of inner K-shell absorption lines from Ne, Mg and Si,\nfrom charge states ranging from F-like to Li-like ions, suggests the presence\nof a substantial amount of low ionization absorbing gas, illuminated by a steep\nsoft X-ray continuum. The observations reveal at least 3 warm absorbing\ncomponents ranging in ionization parameter from $\\log\\xi = 0-2$ and with column\ndensities of $N_{\\rm H} =1.5-4.0 \\times 10^{21}$cm$^{-2}$. The velocity\nprofiles imply that the outflow velocities of the absorbing gas are low and\nwithin $\\pm100$ km s$^{-1}$ of the systemic velocity of Mrk 1040, which\nsuggests any outflowing gas may have stalled in this AGN on large enough\nscales. The warm absorber is likely located far from the black hole, within 300\npc of the nucleus and is spatially coincident with emission from an extended\nNarrow Line Region as seen in the HST images. The iron K band spectrum reveals\nonly narrow emission lines, with Fe K$\\alpha$ at 6.4 keV consistent with\noriginating from reflection off Compton thick pc-scale reprocessing gas.", "category": "astro-ph_HE" }, { "text": "Extremal energy shifts of radiation from a ring near a rotating black\n hole: Radiation from a narrow circular ring shows a characteristic double-horn\nprofile dominated by photons having energy around the maximum or minimum of the\nallowed range, i.e. near the extremal values of the energy shift. The energy\nspan of a spectral line is a function of the ring radius, black hole spin, and\nobserver's view angle. We describe a useful approach to calculate the extremal\nenergy shifts in the regime of strong gravity. Then we consider an accretion\ndisk consisting of a number of separate nested annuli in the equatorial plane\nof Kerr black hole, above the innermost stable circular orbit (ISCO). We\nsuggest that the radial structure of the disk emission could be reconstructed\nusing the extremal energy shifts of the individual rings deduced from the broad\nwings of a relativistic spectral line.", "category": "astro-ph_HE" }, { "text": "Gamma-Ray and X-Ray Observations of the Periodic-Repeater FRB 180916\n During Active Phases: FRB 180916 is a most intriguing source at 150 Mpc distance capable of\nproducing repeating fast radio bursts with a periodic 16.35 day temporal\npattern. We report on the X-ray and $\\gamma$-ray observations of FRB 180916\nobtained by AGILE and Swift. We focused on the recurrent 5-day time intervals\nof active radio bursting and present results obtained on Feb. 3 - 8; Feb. 25;\nMar. 5 - 10; Mar. 22 - 28, 2020 during a multiwavelength campaign involving\nhigh-energy and radio observations. We also searched for temporal coincidences\nat millisecond timescales between all known radio bursts of FRB 180916 and\nX-ray and MeV events detectable by AGILE. We do not detect any simultaneous\nevent or any extended X-ray and $\\gamma$-ray emission on timescales of\nhours/days/weeks. Our cumulative X-ray (0.3-10 keV) flux upper limit of $5\n\\times\\,10^{-14} \\rm \\, erg \\, cm^{-2} s^{-1}$ (obtained during 5-day active\nintervals) translates into an isotropic luminosity upper limit of $L_{X,UL}\n\\sim 1.5 \\times\\, 10^{41} \\rm erg \\, s^{-1}$. Observations above 100 MeV over a\nmany-year timescale provide an average luminosity upper limit one order of\nmagnitude larger. These results provide the so-far most stringent limits on\nhigh-energy emission from FRB 180916 and constrain the dissipation of magnetic\nenergy from a magnetar-like source of radius $R_m$, internal magnetic field\n$B_m$ and dissipation timescale $\\tau_d$ to satisfy the relation $R_{m,6}^3\nB_{m,16}^2 \\tau_{d,8}^{-1} \\lesssim 1$, where $R_{m,6}$ is $R_m$ in units of\n$10^6$ cm, $B_{m,16}$ is $B_m$ in units of $10^{16}$ G, and $\\tau_{d,8}$ in\nunits of $10^8$ s.", "category": "astro-ph_HE" }, { "text": "Equations of state for supernovae and compact stars: We review various theoretical approaches for the equation of state (EoS) of\ndense matter, relevant for the description of core-collapse supernovae, compact\nstars and compact star mergers. The emphasis is put on models that are\napplicable to all of these scenarios. Such EoS models have to cover large\nranges in baryon number density, temperature and isospin asymmetry. The\ncharacteristics of matter change dramatically within these ranges, from a\nmixture of nucleons, nuclei, and electrons to uniform, strongly interacting\nmatter containing nucleons, and possibly other particles such as hyperons or\nquarks. As the development of an EoS requires joint efforts from many\ndirections we consider different theoretical approaches and discuss relevant\nexperimental and observational constraints which provide insights for future\nresearch. Finally, results from applications of the discussed EoS models are\nsummarized.", "category": "astro-ph_HE" }, { "text": "Neural Simulation-Based Inference of the Neutron Star Equation of State\n directly from Telescope Spectra: Neutron stars provide a unique opportunity to study strongly interacting\nmatter under extreme density conditions. The intricacies of matter inside\nneutron stars and their equation of state are not directly visible, but\ndetermine bulk properties, such as mass and radius, which affect the star's\nthermal X-ray emissions. However, the telescope spectra of these emissions are\nalso affected by the stellar distance, hydrogen column, and effective surface\ntemperature, which are not always well-constrained. Uncertainties on these\nnuisance parameters must be accounted for when making a robust estimation of\nthe equation of state. In this study, we develop a novel methodology that, for\nthe first time, can infer the full posterior distribution of both the equation\nof state and nuisance parameters directly from telescope observations. This\nmethod relies on the use of neural likelihood estimation, in which normalizing\nflows use samples of simulated telescope data to learn the likelihood of the\nneutron star spectra as a function of these parameters, coupled with\nHamiltonian Monte Carlo methods to efficiently sample from the corresponding\nposterior distribution. Our approach surpasses the accuracy of previous\nmethods, improves the interpretability of the results by providing access to\nthe full posterior distribution, and naturally scales to a growing number of\nneutron star observations expected in the coming years.", "category": "astro-ph_HE" }, { "text": "Cosmic-ray spectral anomaly at GeV-TeV energies as due to\n re-acceleration by weak shocks in the Galaxy: Recent cosmic-ray measurements by the ATIC, CREAM and PAMELA experiments have\nfound an apparent hardening of the energy spectrum at TeV energies. Although\nthe origin of the hardening is not clearly understood, possible explanations\ninclude hardening in the cosmic-ray source spectrum, changes in the cosmic-ray\npropagation properties in the Galaxy and the effect of nearby sources. In this\ncontribution, we propose that the spectral anomaly might be an effect of\nre-acceleration of cosmic rays by weak shocks in the Galaxy. After acceleration\nby strong supernova remnant shock waves, cosmic rays undergo diffusive\npropagation through the Galaxy. During the propagation, cosmic rays may again\nencounter expanding supernova remnant shock waves, and get re-accelerated. As\nthe probability of encountering old supernova remnants is expected to be larger\nthan the young ones due to their bigger size, re-acceleration is expected to be\nproduced mainly by weaker shocks. Since weaker shocks generate a softer\nparticle spectrum, the resulting re-accelerated component will have a spectrum\nsteeper than the initial cosmic-ray source spectrum produced by strong shocks.\nFor a reasonable set of model parameters, it is shown that such re-accelerated\ncomponent can dominate the GeV energy region while the non-reaccelerated\ncomponent dominates at higher energies, explaining the observed GeV-TeV\nspectral anomaly.", "category": "astro-ph_HE" }, { "text": "A radio, optical, UV and X-ray view of the enigmatic changing look\n Active Galactic Nucleus 1ES~1927+654 from its pre- to post-flare states: The nearby type-II AGN 1ES1927+654 went through a violent changing-look (CL)\nevent beginning December 2017 during which the optical and UV fluxes increased\nby four magnitudes over a few months, and broad emission lines newly appeared\nin the optical/UV. By July 2018 the X-ray coronal emission had completely\nvanished, only to reappear a few months later. In this work we report the\nevolution of the radio, optical, UV and X-rays from the pre-flare state through\nmid-2021 with new and archival data from the Very Long Baseline Array (VLBA),\nthe European VLBI Network, the Very Large Array (VLA), the Telescopio Nazionale\nGalileo (TNG), Gran Telescopio Canarias (GTC), The Neil Gehrels Swift\nobservatory and XMM-Newton. The main results from our work are: (1) The source\nhas returned to its pre-CL state in optical, UV, and X-ray; the disk-corona\nrelation has been re-established as has been in the pre-CL state, with an\n$\\alpha_{\\rm OX}\\sim 1.02$. The optical spectra are dominated by narrow\nemission lines. (2) The UV light curve follows a shallower slope of $\\propto\nt^{-0.91\\pm 0.04}$ compared to that predicted by a tidal disruption event. We\nconjecture that a magnetic flux inversion event is the possible cause for this\nenigmatic event. (3) The compact radio emission which we tracked in the pre-CL\n(2014), during CL (2018) and post-CL(2021) at spatial scales $<1$ pc was at its\nlowest level during the changing look event in 2018, nearly contemporaneous\nwith a low $2-10$ keV emission. The radio to X-ray ratio of the compact source\n$L_{\\rm Radio}/L_{\\rm X-ray}\\sim 10^{-5.5}$, follows the Gudel-Benz relation,\ntypically found in coronally active stars, and several AGN. (4) We do not\ndetect any presence of nascent jets at the spatial scales of $\\sim 5-10$ pc.", "category": "astro-ph_HE" }, { "text": "Fermi Large Area Telescope observations of Local Group galaxies:\n Detection of M31 and search for M33: Cosmic rays (CRs) can be studied through the galaxy-wide gamma-ray emission\nthat they generate when propagating in the interstellar medium. The comparison\nof the diffuse signals from different systems may inform us about the key\nparameters in CR acceleration and transport. We aim to determine and compare\nthe properties of the CR-induced gamma-ray emission of several Local Group\ngalaxies. We use 2 years of nearly continuous sky-survey observations obtained\nwith the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope to\nsearch for gamma-ray emission from M31 and M33. We compare the results with\nthose for the Large Magellanic Cloud, the Small Magellanic Cloud, the Milky\nWay, and the starburst galaxies M82 and NGC253. We detect a gamma-ray signal at\n5sigma significance in the energy range 200 MeV-20 GeV that is consistent with\noriginating from M31. The integral photon flux above 100MeV amounts to 9.1 +/-\n1.9 (stat) +/- 1.0 (sys) x10e-9 ph/cm2/s. We find no evidence for emission from\nM33 and derive an upper limit on the photon flux >100MeV of 5.1 x10e-9 ph/cm2/s\n(2sigma). Comparing these results to the properties of other Local Group\ngalaxies, we find indications for a correlation between star formation rate and\ngamma-ray luminosity that also holds for the starburst galaxies. The gamma-ray\nluminosity of M31 is about half that of the Milky Way, which implies that the\nratio between the average CR densities in M31 and the Milky Way amounts to 0.35\n+/- 0.25. The observed correlation between gamma-ray luminosity and star\nformation rate suggests that the flux of M33 is not far below the current upper\nlimit from the LAT observations.", "category": "astro-ph_HE" }, { "text": "Variations of the physical parameters of the blazar Mrk 421 based on the\n analysis of the spectral energy distributions: We report on the variations of the physical parameters of the jet observed in\nthe blazar Mrk 421, and discuss the origin of X-ray flares in the jet, based on\nthe analysis of the several spectral energy distributions (SEDs). The SEDs are\nmodeled using the one-zone synchrotron self-Compton (SSC) model and its\nparameters determined using a Markov chain Monte Carlo method. The lack of data\nat TeV energies means many of the parameters cannot be uniquely determined and\nare correlated. These are studied in detail. We found that the optimal solution\ncan be uniquely determined only when we apply a constraint to one of four\nparameters: the magnetic field (B), Doppler factor, size of the emitting\nregion, and normalization factor of the electron energy distribution. We used\n31 sets of SED from 2009 to 2014 with optical-UV data observed with UVOT/Swift\nand the Kanata telescope, X-ray data with XRT/Swift, and gamma-ray data with\nthe Fermi Large Area Telescope (LAT). The result of our SED analysis suggests\nthat, in the X-ray faint state, the emission occurs in a relatively small area\n(~ 10^16 cm) with relatively strong magnetic field (B~10^-1 G). The X-ray\nbright state shows a tendency opposite to that of the faint state, that is, a\nlarge emitting area (~10^18 cm), probably in the downstream of the jet and weak\nmagnetic field (B~10^-3 G). The high X-ray flux was due to an increase in the\nmaximum energy of electrons. On the other hand, the presence of two kinds of\nemitting areas implies that the one-zone model is unsuitable to reproduce, at\nleast a part of the observed SEDs.", "category": "astro-ph_HE" }, { "text": "Accretion-powered pulsations in an apparently quiescent neutron star\n binary: Accreting millisecond X-ray pulsars are an important subset of low-mass X-ray\nbinaries in which coherent X-ray pulsations can be observed during occasional,\nbright outbursts (X-ray luminosity $L_X\\sim 10^{36}$ erg s$^{-1}$). These\npulsations show that matter is being channeled onto the neutron star's magnetic\npoles. However, such sources spend most of their time in a low-luminosity,\nquiescent state ($L_X\\lesssim 10^{34}$ erg s$^{-1}$), where the nature of the\naccretion flow onto the neutron star (if any) is not well understood. Here we\nreport that the millisecond pulsar/low-mass X-ray binary transition object PSR\nJ1023+0038 intermittently shows coherent X-ray pulsations at luminosities\nnearly 100 times fainter than observed in any other accreting millisecond X-ray\npulsar. We conclude that in spite of its low luminosity PSR J1023+0038\nexperiences episodes of channeled accretion, a discovery that challenges\nexisting models for accretion onto magnetized neutron stars.", "category": "astro-ph_HE" }, { "text": "Indications of Negative Evolution for the Sources of the Highest Energy\n Cosmic Rays: Using recent measurements of the spectrum and chemical composition of the\nhighest energy cosmic rays, we consider the sources of these particles. We find\nthat the data strongly prefers models in which the sources of the ultra-high\nenergy cosmic rays inject predominantly intermediate mass nuclei, with\ncomparatively few protons or heavy nuclei, such as iron or silicon. If the\nnumber density of sources per comoving volume does not evolve with redshift,\nthe injected spectrum must be very hard ($\\alpha\\simeq 1$) in order to fit the\nspectrum observed at Earth. Such a hard spectral index would be surprising and\ndifficult to accommodate theoretically. In contrast, much softer spectral\nindices, consistent with the predictions of Fermi acceleration ($\\alpha\\simeq\n2$), are favored in models with negative source evolution. With this\ntheoretical bias, these observations thus favor models in which the sources of\nthe highest energy cosmic rays are preferentially located within the\nlow-redshift universe.", "category": "astro-ph_HE" }, { "text": "Stellar wind accretion and accretion disk formation: applications to\n neutron star high mass X-ray binaries: Recent X-ray observations have revealed the complexity and diversity of\nhigh-mass X-ray binaries (HMXBs). This diversity challenges a classical\nunderstanding of the accretion process onto the compact objects. In this study,\nwe reinforce the conventional concept of the nature of wind-fed accretion onto\na neutron star considering the geometrical effect of radiatively accelerated\nwind, and re-evaluate the transported angular momentum by using a simple wind\nmodel. Our results suggest that even in an OB-type HMXB fed by stellar wind, a\nlarge amount of angular momentum could be transported to form an accretion disk\ndue to the wind-inhomogeneity, if the binary separation is tight enough and/or\nstellar wind is slow. We apply our model into actual systems such as LMC X-4\nand OAO 1657-415, and discuss the possibility of disk formations in these\nsystems.", "category": "astro-ph_HE" }, { "text": "Stability of three-dimensional relativistic jets: implications for jet\n collimation: The stable propagation of jets in FRII sources is remarkable if one takes\ninto account that large-scale jets are subjected to potentially highly\ndisruptive three-dimensional (3D) Kelvin-Helmholtz instabilities. Numerical\nsimulations can address this problem and help clarify the causes of this\nremarkable stability. Following previous studies of the stability of\nrelativistic flows in two dimensions (2D), it is our aim to test and extend the\nconclusions of such works to three dimensions. We present numerical simulations\nfor the study of the stability properties of 3D, sheared, relativistic flows.\nThis work uses a fully parallelized code Ratpenat that solves equations of\nrelativistic hydrodynamics in 3D. The results of the present simulations\nconfirm those in 2D. We conclude that the growth of resonant modes in sheared\nrelativistic flows could be important in explaining the long-term collimation\nof extragalactic jets.", "category": "astro-ph_HE" }, { "text": "Non-parametric Bayesian reconstruction of Galactic magnetic fields using\n Information Field Theory: The inclusion of line-of-sight information in\n ultra-high energy cosmic ray backtracking: (abridged) Ultra-high energy cosmic rays (UHECRs) are extremely energetic\ncharged particles with energies surpassing $10^{18}$ eV. Their sources remain\nelusive, obscured by deflections caused by the Galactic magnetic field (GMF).\nThis challenge is further complicated by our limited understanding of the\nthree-dimensional structure of the GMF, as current GMF observations consist\nprimarily of quantities integrated along the line-of-sight (LOS). Nevertheless,\ndata from upcoming stellar polarisation surveys along with Gaia's stellar\nparallax data are expected to yield local GMF measurements.. In this work, we\nemploy methods of Bayesian statistical inference in order to sample the\nposterior distribution of the GMF within part of the Galaxy. By assuming a\nknown rigidity and arrival direction of an UHECR, we backtrack its trajectory\nthrough various GMF configurations drawn from the posterior distribution. Our\nobjective is to rigorously evaluate our algorithm's performance in scenarios\nthat closely mirror the setting of expected future applications. In pursuit of\nthis, we condition the posterior to synthetic integrated LOS measurements of\nthe GMF, in addition to synthetic local POS-component measurements. In this\nproof of concept work, we assume the ground truth to be a magnetic field\nproduced by a dynamo simulation of the Galactic ISM. Our results demonstrate\nthat for all locations of the observed arrival direction on the POS, our\nalgorithm is able to substantially update our knowledge on the original arrival\ndirection of UHECRs with rigidity $E/Z = 5 \\times 10^{19}$ eV, even in the case\nof complete absence of LOS information. If integrated data is included in the\ninference, then the regions of the celestial sphere where the maximum error\noccurs diminishes greatly. Even in those regions the maximum error is\ndiminished by a factor of about $3$ in the specific setting studied.", "category": "astro-ph_HE" }, { "text": "Mass of intermediate black hole in the source M82 X-1 restricted by\n models of twin high-frequency quasi-periodic oscillations: We apply the relativistic precession (RP) model with its variants and the\nresonance epicyclic model with its variants, based on the frequencies of the\ngeodesic epicyclic motion in the field of a Kerr black hole, to put limits on\nthe mass of the black hole in the ultraluminous X-ray source M82 X-1\ndemonstrating twin high-frequency quasi-periodic oscillations (HF QPOs) with\nthe frequency ratio near 3:2. The mass limits implied by the geodesic HF QPO\nmodels are compared to those obtained due to the model of string loop\noscillations around a stable equilibrium position. Assuming the whole range of\nthe black hole dimensionless spin, 0 < a < 1, the restrictions on the black\nhole mass related to the twin HF QPOs are widely extended and strongly model\ndependent; nevertheless, they give the lower limit M_{M82 X-1} > 130 M_{sun}\nconfirming existence of an intermediate black hole in the M82 X-1 source. The\nupper limit given by one of the variants of the geodesic twin HF QPO models\ngoes up to M_{M82 X-1}<1500 M_{sun}. The range 37-210 mHz of the low-frequency\nQPOs observed in the M82 X-1 source introduces additional restrictive limits on\nthe black hole mass, if we model the low-frequency QPOs by nodal precession of\nthe epicyclic motion. The nodal precession model restrictions combined with\nthose implied by the geodesic models of the twin HF QPOs give allowed ranges of\nthe M82 X-1 black hole parameters, namely 140 M_{sun} 8 to the ground. Such high-n states can only be populated\nby the charge exchange process. In this scenario, the observed 3.5 keV line\nflux in clusters can be naturally explained by an interaction in an effective\nvolume of ~1 kpc^3 between a ~3 keV temperature plasma and cold dense clouds\nmoving at a few hundred km/s. The S XVI lines at ~3.5 keV also provide a unique\ndiagnostic of the charge exchange phenomenon in hot cosmic plasmas.", "category": "astro-ph_HE" }, { "text": "The Peculiar Bursting Nature of CP Pup: The classical nova CP Puppis has been observed to have particularly puzzling\nand peculiar properties. In particular, this classical nova displays occasional\nbursts in its long-term ASAS-SN light curve. Here we report on 5 sectors of\nTESS data displaying 2 of these rapid bursts, lasting ~1 day. Based on the\nestimated lower energy limits of the bursts we discuss whether the bursts may\nbe examples of micronovae resulting from localised thermonuclear explosion.\nFurthermore, its orbital period remains uncertain, with several inconsistent\nperiodic signals appearing in spectroscopic and photometric observations at\nvarious wavelengths. Although we cannot unambiguously unravel the physical\norigin of the signals, the previously suggested nature of CP Puppis as a long\norbital period system may be a viable explanation. The recurrence time of the\nbursts in CP Puppis, together with the unexplained variable modulations make it\na prime candidate for intense monitoring.", "category": "astro-ph_HE" }, { "text": "Redback: A Bayesian inference software package for electromagnetic\n transients: Fulfilling the rich promise of rapid advances in time-domain astronomy is\nonly possible through confronting our observations with physical models and\nextracting the parameters that best describe what we see. Here, we introduce\n{\\sc Redback}; a Bayesian inference software package for electromagnetic\ntransients. {\\sc Redback} provides an object-orientated {\\sc python} interface\nto over 12 different samplers and over 100 different models for kilonovae,\nsupernovae, gamma-ray burst afterglows, tidal disruption events, engine-driven\ntransients, X-ray afterglows of gamma-ray bursts driven by millisecond\nmagnetars among other explosive transients. The models range in complexity from\nsimple analytical and semi-analytical models to surrogates built upon numerical\nsimulations accelerated via machine learning. {\\sc Redback} also provides a\nsimple interface for downloading and processing data from Swift, Fink, Lasair,\nthe open-access catalogues, and BATSE and fit this or private data. {\\sc\nRedback} can also be used as an engine to simulate transients for telescopes\nsuch as the Zwicky Transient Facility and Vera Rubin with realistic cadences,\nlimiting magnitudes, and sky-coverage or a hypothetical user-constructed survey\nwith arbitrary settings. We also provide a more general simulation interface\nsuitable for target of opportunity observations with different telescopes. We\ndemonstrate through a series of examples how {\\sc Redback} can be used as a\ntool to simulate a large population of transients for realistic surveys, fit\nmodels to real, simulated, or private data, multi-messenger inference and serve\nas an end-to-end software toolkit for parameter estimation and interpreting the\nnature of electromagnetic transients.", "category": "astro-ph_HE" }, { "text": "A theoretical color-velocity correlation for supernovae associated with\n gamma-ray bursts: We carry out the first multi-dimensional radiative transfer calculations to\nsimultaneously compute synthetic spectra and light curves for models of\nsupernovae driven by fast bipolar outflows. These allow us to make\nself-consistent predictions for the orientation dependence of both colour\nevolution and spectral features. We compare models with different degrees of\nasphericity and metallicity and find significant observable consequences of\nboth. In aspherical models, we find spectral and light curve features that vary\nsystematically with observer orientation. In particular, we find that the early\nphase light curves are brighter and bluer when viewed close to the polar axis\nbut that the peak flux is highest for equatorial (off-axis) inclinations.\nSpectral line features also depend systematically on observer orientation,\nincluding the velocity of the SiII 6355A line. Consequently, our models predict\na correlation between line velocity and color that could assist the\nidentification of supernovae associated with off-axis jet-driven explosions.\nThe amplitude and range of this correlation depends on the degree of\nasphericity, the metallicity and the epoch of observation but we find that it\nis always present and acts in the same direction.", "category": "astro-ph_HE" }, { "text": "Evidence of jet induced optical microvariability in radio-loud Narrow\n Line Seyfert 1 Galaxies: To quantify the role of radio jets for Intra-Night Optical Variability (INOV)\nin Radio-Loud Narrow-Line Seyfert 1 (RLNLSy1) galaxies, we report the first\nsystematic comparative INOV study of 23 RLNLSy1 galaxies, with 15 RLNLSy1s\nhaving confirmed detection of jets (jetted) and the remaining 8 RLNLSy1s having\nno detection of jets (non-jetted) based on their Very Long Baseline Array\nobservations. We have monitored these two samples, respectively, in 37 and 16\nsessions of a minimum 3-hour duration each. Based upon F$^{\\eta}$-test at 99\\%\nconfidence level with a typical INOV amplitude ($\\psi$) detection threshold of\n$>$ 3\\%, we find the INOV duty cycles of 12\\% for the sample of jetted\nRLNLSy1s, however, none of the sources showed INOV in the sample of non-jetted\nRLNLSy1s. Among the jetted RLNLSy1s, we find that the Duty Cycle (DC) for\njetted $\\gamma$-ray detected ($\\gamma$-ray) RLNLSy1s is found to be 34\\% in\ncontrast to null INOV detection in the case of non-$\\gamma$-ray RLNLSy1s. It\nsuggests that instead of the mere presence of a jet, relativistic beaming plays\na significant role for INOV in the case of low-luminous high accreting AGNs\nsuch as NLSy1s in which dilution of the AGN's non-thermal optical emission by\nthe (much steadier) optical emission contributed by the nuclear accretion disc\nis quite likely. Our study of jetted $\\gamma$-ray RLNLSy1s shows more frequent\nINOV detection for sources with higher apparent jet speed. Further, our results\nalso suggest that among the NLSy1s, only jetted $\\gamma$-ray RNLSy1 galaxies DC\napproaches blazar like DC.", "category": "astro-ph_HE" }, { "text": "On the paucity of Fast Radio Bursts at low Galactic latitudes: We examine the effect of Galactic diffractive interstellar scintillation as a\nmeans of explaining the reported deficit of Fast Radio Burst (FRB) detections\nat low Galactic latitude. We model the unknown underlying FRB flux density\ndistribution as a power law with a rate scaling as $S_\\nu^{-5/2+\\delta}$ and\naccount for the fact that the FRBs are detected at unknown positions within the\ntelescope beam. We find that the event rate of FRBs located off the Galactic\nplane may be enhanced by a factor ~30-300% relative to objects near the\nGalactic plane without necessarily affecting the slope of the distribution. For\nFRBs whose flux densities are subject to relatively weak diffractive\nscintillation, as is typical for events detected at high Galactic latitudes, we\ndemonstrate that an effect associated with Eddington bias is responsible for\nthe enhancement. The magnitude of the enhancement increases with the steepness\nof the underlying flux density distribution, so that existing limits on the\ndisparity in event rates between high and low Galactic latitudes suggest that\nthe FRB population has a steep differential flux density distribution, scaling\nas $S_\\nu^{-3.5}$ or steeper. Existing estimates of the event rate in the flux\ndensity range probed by the High Time Resolution Universe (HTRU) survey\noverestimate the true rate by a factor of ~3.", "category": "astro-ph_HE" }, { "text": "The smooth cyclotron line in Her X-1 as seen with NuSTAR: Her X-1, one of the brightest and best studied X-ray binaries, shows a\ncyclotron resonant scattering feature (CRSF) near 37 keV. This makes it an\nideal target for detailed study with the Nuclear Spectroscopic Telescope Array\n(NuSTAR), taking advantage of its excellent hard X-ray spectral resolution. We\nobserved Her X-1 three times, coordinated with Suzaku, during one of the high\nflux intervals of its 35d super-orbital period. This paper focuses on the shape\nand evolution of the hard X-ray spectrum. The broad-band spectra can be fitted\nwith a powerlaw with a high-energy cutoff, an iron line, and a CRSF. We find\nthat the CRSF has a very smooth and symmetric shape, in all observations and at\nall pulse-phases. We compare the residuals of a line with a Gaussian optical\ndepth profile to a Lorentzian optical depth profile and find no significant\ndifferences, strongly constraining the very smooth shape of the line. Even\nthough the line energy changes dramatically with pulse phase, we find that its\nsmooth shape does not. Additionally, our data show that the continuum is only\nchanging marginally between the three observations. These changes can be\nexplained with varying amounts of Thomson scattering in the hot corona of the\naccretion disk. The average, luminosity-corrected CRSF energy is lower than in\npast observations and follows a secular decline. The excellent data quality of\nNuSTAR provides the best constraint on the CRSF energy to date.", "category": "astro-ph_HE" }, { "text": "Constraints on Axion-like Particles from a Hard $X$-ray Observation of\n Betelgeuse: We use the first observation of Betelgeuse in hard $X$-rays to perform a\nnovel search for axion-like particles (ALPs). Betelgeuse is not expected to be\na standard source of $X$-rays, but light ALPs produced in the stellar core\ncould be converted back into photons in the Galactic magnetic field, producing\na detectable flux that peaks in the hard $X$-ray band\n($E_\\gamma>10\\mathrm{\\,keV}$). Using a 50 ks observation of Betelgeuse by the\n$NuSTAR$ satellite telescope, we find no significant excess of events above the\nexpected background. Using models of the regular Galactic magnetic field in the\ndirection of Betelgeuse, we set a 95% C.L. upper limit on the ALP-photon\ncoupling of ${g_{a\\gamma}<(0.5-1.8)\\times10^{-11}}$ GeV$^{-1}$ (depending on\nmagnetic field model) for ALP masses ${m_{a}<(5.5-3.5) \\times10^{-11}}$ eV.", "category": "astro-ph_HE" }, { "text": "Using radial velocities to reveal black holes in binaries: a test case: Aims. Large radial velocity variations in the LAMOST spectra of giant stars\nhave been used to infer the presence of unseen companions. Some of them have\nbeen proposed as possible black hole candidates. We test this selection by\ninvestigating the classification of the one candidate having a known X-ray\ncounterpart (UCAC4 721-037069). Methods. We obtained time-resolved spectra from\nthe Liverpool Telescope and a 5ks observation from the Chandra observatory to\nfully constrain the orbital parameters and the X-ray emission of this system.\nResults. We find the source to be an eclipsing stellar binary that can be\nclassified as a RS CVn. The giant star fills its Roche Lobe and the binary mass\nratio is greater than one. The system may be an example of stable mass transfer\nfrom an intermediate-mass star with a convective envelope. Conclusions. Using\nonly radial velocity to identify black hole candidates can lead to many false\npositives. The presence of an optical orbital modulation, such as observed for\nall LAMOST candidates, will in most cases indicate that this is a stellar\nbinary.", "category": "astro-ph_HE" }, { "text": "X-ray counterpart candidates for six new $\u03b3$-ray pulsars: Using archival X-ray data we have found point-like X-ray counterpart\ncandidates positionally coincident with six $\\gamma$-ray pulsars discovered\nrecently in the Fermi Gamma-ray Space Telescope data by the Einstein@Home\nproject. The candidates for PSRs J0002$+$6216, J0554$+$3107, J1844$-$0346 and\nJ1105$-$6037 are detected with Swift, and those for PSRs J0359$+$5414 and\nJ2017$+$3625 are detected with Chandra. Despite a low count statistics for some\ncandidates, assuming plausible constraints on the absorbing column density\ntowards the pulsars, we show that X-ray spectral properties for all of them are\nconsistent with those observed for other pulsars. J0359$+$5414 is the most\nreliably identified object. We detect a nebula around it, whose spectrum and\nextent suggest that this is a pulsar wind nebula powered by the pulsar.\nAssociations of J0002$+$6216 and J1844$-$0346 with supernova remnants CTB 1 and\nG28.6$-$0.1 are proposed.", "category": "astro-ph_HE" }, { "text": "AMS02 results support the secondary origin of cosmic ray positrons: We show that the recent AMS02 positron fraction measurement is consistent\nwith a secondary origin for positrons, and does not require additional primary\nsources such as pulsars or dark matter. The measured positron fraction at high\nenergy saturates the previously predicted upper bound for secondary production\n(Katz et al 2009), obtained by neglecting radiative losses. This coincidence,\nwhich will be further tested by upcoming AMS02 data at higher energy, is a\ncompelling indication for a secondary source. Within the secondary model the\nAMS02 data imply a cosmic ray propagation time in the Galaxy of < Myr and an\naverage traversed interstellar matter density of order 1/cc, comparable to the\ndensity of the Milky Way gaseous disk, at a rigidity of 300 GV.", "category": "astro-ph_HE" }, { "text": "An in-depth investigation of 11 pulsars discovered by FAST: We present timing solutions and analyses of 11 pulsars discovered by the\nFive-hundred-meter Aperture Spherical radio Telescope (FAST). These pulsars\nwere discovered using an ultra-wide bandwidth receiver in drift-scan\nobservations made during the commissioning phase of FAST, and were then\nconfirmed and timed using the 64-m Parkes Radio Telescope. Each pulsar has been\nobserved over a span of at least one year. Highlighted discoveries include PSR\nJ0344-0901, which displays mode-changing behaviour and may belong to the class\nof so-called `swooshing' pulsars (alongside PSRs B0919+06 and B1859+07); PSR\nJ0803-0942, whose emission is almost completely linearly polarised; and PSRs\nJ1900-0134 and J1945+1211, whose well defined polarisation angle curves place\nstringent constraints on their emission geometry. We further discuss the\ndetectability of these pulsars by earlier surveys, and highlight lessons\nlearned from our work in carrying out confirmation and monitoring observations\nof pulsars discovered by a highly sensitive telescope, many of which may be\napplicable to next-generation pulsar surveys. This paper marks one of the first\nmajor releases of FAST-discovered pulsars, and paves the way for future\ndiscoveries anticipated from the Commensal Radio Astronomy FAST Survey\n(CRAFTS).", "category": "astro-ph_HE" }, { "text": "Disentangling Instrumental Features of the 130 GeV Fermi Line: We study the instrumental features of photons from the peak observed at\n$E_\\gamma=130$ GeV in the spectrum of Fermi-LAT data. We use the {\\sc sPlots}\nalgorithm to reconstruct -- seperately for the photons in the peak and for\nbackground photons -- the distributions of incident angles, the recorded time,\nfeatures of the spacecraft position, the zenith angles, the conversion type and\ndetails of the energy and direction reconstruction. The presence of a striking\nfeature or cluster in such a variable would suggest an instrumental cause for\nthe peak. In the publically available data, we find several suggestive features\nwhich may inform further studies by instrumental experts, though the size of\nthe signal sample is too small to draw statistically significant conclusions.", "category": "astro-ph_HE" }, { "text": "Self-consistent neutrino and UHE cosmic ray spectra from Mrk 421: We examine the neutrino and cosmic ray spectra resulting from two models of\nfitting the spectral energy distribution (SED) of the blazar Mrk~421 using a\nself-consistent leptohadronic code. The $\\gamma$-ray emission is attributed to\neither synchrotron radiation of ultra-high energy protons (LHs model) or to\nsynchrotron radiation from electrons that result from photopion interactions of\nlower energy protons (LH$\\pi$ model). Although both models succeed in fitting\nsatisfactorily the SED, the parameter values that they use result in\nsignificantly different neutrino and cosmic-ray spectra. For the LH$\\pi$ model,\nwhich requires high proton energy density, we find that the neutrino spectrum\npeaks at an energy $E_{\\rm \\nu,peak}= 3.3$ PeV which falls well within the\nenergy range of recent neutrino observations. While at the same time its peak\nflux is just under the sensitivity limit of IC-40 observations, it cannot\nproduce ultra-high energy cosmic rays. In the LHs model, on the other hand,\nneutrinos are far from being detectable because of their low flux and peak\nenergy at $E_{{\\rm \\nu,peak}} \\simeq 100$ PeV. However, the propagation of\nprotons produced by the decay of escaping neutrons results in an ultra-high\nenergy cosmic ray flux close to that observed by Pierre Augere, HiRes and\nTelescope Array at energies $E_p \\simeq 30$ EeV.", "category": "astro-ph_HE" }, { "text": "Neutrino-driven massive stellar explosions in 3D fostered by magnetic\n fields via turbulent $\u03b1$-effect: We investigate the influence of magnetic field amplification on the\ncore-collapse supernovae in highly magnetized progenitors through\nthree-dimensional simulations. By considering rotating models, we observe a\nstrong correlation between the exponential growth of the magnetic field in the\ngain region and the initiation of shock revival, with a faster onset compared\nto the non-rotating model. We highlight that the mean magnetic field\nexperiences exponential amplification as a result of $\\alpha$-effect in the\ndynamo process, which works efficiently with the increasing kinetic helicity of\nthe turbulence within the gain region. Our findings indicate that the\nsignificant amplification of the mean magnetic fields leads to the development\nof locally intense turbulent magnetic fields, particularly in the vicinity of\nthe poles, thereby promoting the revival of the shock by neutrino heating.", "category": "astro-ph_HE" }, { "text": "A systematic study of magnetar-powered hydrogen-rich supernovae: Context: It has been suggested that some supernovae (SNe) may be powered by a\nmagnetar formed at the moment of the explosion. While this scenario has mostly\nbeen applied to hydrogen-free events, it may be possible also for hydrogen-rich\nobjects. Aims: We explore the effect of including a magnetar on the light\ncurves of supernovae with H-rich progenitors. Methods: We have applied a\nversion of our one-dimensional LTE radiation hydrodynamics code that takes into\naccount the relativistic motion of the ejecta caused by the extra energy\nprovided by the magnetar. For a fixed red supergiant (RSG) progenitor, we have\nobtained a set of light curves that corresponds to different values of the\nmagnetar initial rotation energy and the spin-down timescale. The model is\napplied to SN~2004em and OGLE-2014-SN-073, two peculiar Type II SNe with\nlong-rising SN1987A-like light curves, although with much larger luminosities.\nResults: The presence of a plateau phase in either normal or superluminous\nsupernovae is one possible outcome, even if a magnetar is continuously\ninjecting energy into the ejecta.In other cases, the light curve shows a peak\nbut not a plateau. Also, there are intermediate events with a first peak\nfollowed by a slow decline and a late break of the declining slope. Our models\nshow that bright and long rising morphologies are possible even assuming RSG\nstructures. Conclusions: A large number of supernova discoveries per year\nreveal unexpected new types of explosions. According to our results, SLSNe II-P\nare to be expected, as well as a variety of light curve morphologies that can\nall be possible signs of a newly born magnetar.", "category": "astro-ph_HE" }, { "text": "Self-similar structure of a hot magnetized flow with thermal conduction: We have explored the structure of hot magnetized accretion flow with thermal\nconduction. The importance of thermal conduction in hot accretion flows has\nbeen confirmed by observations of the hot gas surrounding Sgr $A^*$ and a few\nother nearby galactic nuclei. For a steady state structure of such accretion\nflows a set of self similar solutions are presented. In this paper, we have\nactually tried to re-check the solution presented by Abbassi et al. (2008)\nusing a physical constrain. In this study we find that Eq 29 places a new\nconstrain that limits answers presented by Abbassi et al. 2008. In that paper\nthe parameter space in which it is established in the new constrain was\nplotted. However, the new requirement makes up only a small parameter space\nwith physically acceptable solutions. And now in this manuscript we have\nfollowed the idea with more effort, and tried to find out how thermal\nconduction influences the structur of the disks in a physical parameter space.\nWe have found out that the existence of thermal conduction will lead to\nreduction of accretion and radial and azimuthal velocities as well as the\nvertical thickness of the disk, which is slightly reduced. Moreover, the\nsurface density of the disk will increase when the thermal conduction becomes\nimportant in the hot magnetized flow.", "category": "astro-ph_HE" }, { "text": "A combined timing/spectral study of IRAS 13224-3809 using XMM-Newton\n data: We present the results from an X-ray variability study of IRAS 13224-3809.\nThis is probably the best source for X-ray reverberation studies since it is\nX-ray bright, extremely variable, and it has been extensively observed with\nXMM-Newton. We used all the archival XMM-Newton data from the three EPIC\ncameras (to increase the signal-to-noise) and, given the many observations of\nthe source, we were able to compute the time-lags spectra in three different\nflux levels/periods. We fitted the time-lags and energy spectra,\nsimultaneously, using a new X-ray reverberation code which computes the time\ndependent reflection spectra of the disc as a response to an X-ray flash from a\npoint source located on the axis of the black-hole (BH) accretion disc\n(lamp-post geometry). To the best of our knowledge, this is the first time for\nan AGN that both time-lags and energy spectra are fitted by a model\nsimultaneously in different flux periods. The model fits in the case when the\nBH is rapidly rotating are significantly better than the model fits in the case\nof a Schwarzschild BH. This result strongly favours the hypothesis of a\nrotating central BH in this source. We also detect significant variations in\nthe height of the X-ray corona. The X-ray height appears to increase from 3-5\ngravitational radii when the X-ray luminosity is of the order of 1.5-3 percent\nof the Eddington limit, up to 10 gravitational radii, when the luminosity\ndoubles.", "category": "astro-ph_HE" }, { "text": "Time-dependent Modeling of Pulsar Wind Nebulae: A spatially independent model that calculates the time evolution of the\nelectron spectrum in a spherically expanding pulsar wind nebula (PWN) is\npresented, allowing one to make broadband predictions for the PWN's non-thermal\nradiation. The source spectrum of electrons injected at the termination shock\nof the PWN is chosen to be a broken power law. In contrast to previous PWN\nmodels of a similar nature, the source spectrum has a discontinuity in\nintensity at the transition between the low and high-energy components. To test\nthe model, it is applied to the young PWN G21.5--0.9, where it is found that a\ndiscontinuous source spectrum can model the emission at all wavelengths better\nthan a continuous one. The model is also applied to the unidentified sources\nHESS J1427--608 and HESS J1507--622. Parameters are derived for these two\ncandidate nebulae that are consistent with the values predicted for other PWNe.\nFor HESS J1427--608 a present-day magnetic field of $B\\s{age}=0.4\\,\\mu\\text{G}$\nis derived. As a result of the small present-day magnetic field, this source\nhas a low synchrotron luminosity, while remaining bright at GeV/TeV energies.\nIt is therefore possible to interpret HESS J1427--608 within the ancient PWN\nscenario. For the second candidate PWN HESS J1507--622, a present-day magnetic\nfield of $B\\s{age}=1.7\\,\\mu\\text{G}$ is derived. Furthermore, for this\ncandidate PWN a scenario is favoured in the present paper in which HESS\nJ1507--622 has been compressed by the reverse shock of the supernova remnant.", "category": "astro-ph_HE" }, { "text": "A search for ultra high energy neutrinos from TXS 0506+056 using the\n Pierre Auger Observatory: Results of a search for ultra-high-energy neutrinos with the Pierre Auger\nObservatory from the direction of the blazar TXS 0506+056 are presented. They\nwere obtained as part of the follow-up that stemmed from the detection of\nhigh-energy neutrinos and gamma rays with IceCube, \\textit{Fermi}-LAT, MAGIC,\nand other detectors of electromagnetic radiation in several bands. The Pierre\nAuger Observatory is sensitive to neutrinos in the energy range from 100 PeV to\n100 EeV and in the zenith angle range from $\\theta=60^\\circ$ to\n$\\theta=95^\\circ$, where the zenith angle is measured from the vertical\ndirection. No neutrinos from the direction of TXS 0506+056 have been found. The\nresults were analyzed in three periods: One of 6 months around the detection of\nIceCube-170922A, coinciding with a flare period of TXS 0506+056, a second one\nof 110 days during which the IceCube collaboration found an excess of 13\nneutrinos from a direction compatible with TXS 0506+056, and a third one from\n2004 January 1 up to 2018 August 31, over which the Pierre Auger Observatory\nhas been taking data. The sensitivity of the Observatory is addressed for\ndifferent spectral indices by considering the fluxes that would induce a single\nexpected event during the observation period. For indices compatible with those\nmeasured by the IceCube collaboration the expected number of neutrinos at the\nObservatory is well-below one. Spectral indices as hard as 1.5 would have to\napply in this energy range to expect a single event to have been detected.", "category": "astro-ph_HE" }, { "text": "Observable signatures of stellar-mass black holes in active galactic\n nuclei: Stellar-mass black holes (BHs) are predicted to be embedded in the disks of\nactive galactic nuclei (AGN) due to gravitational drag and in-situ star\nformation. However, clear evidence for AGN disk-embedded BHs is currently\nlacking. Here, as possible electromagnetic signatures of these BHs, we\ninvestigate breakout emission from shocks emerging around Blandford-Znajek jets\nlaunched from accreting BHs in AGN disks. We assume that the majority of the\nhighly super-Eddington flow reaches the BH, produces a strong jet, and the jet\nproduces feedback that shuts off accretion and thus leads to episodic flaring.\nWhile these assumptions are highly uncertain at present, they predict a\nbreakout emission characterized by luminous thermal emission in the X-ray\nbands, and bright, broadband non-thermal emission from the infrared to the\ngamma-ray bands. The flare duration depends on the BH's distance $r$ from the\ncentral supermassive BH, varying between $10^3-10^6$ s for $r \\sim 0.01-1$ pc.\nThis emission can be discovered by current and future infrared, optical, and\nX-ray wide-field surveys and monitoring campaigns of nearby AGNs.", "category": "astro-ph_HE" }, { "text": "HAFFET: Hybrid Analytic Flux FittEr for Transients: The progenitors for many types of supernovae (SNe) are still unknown, and an\napproach to diagnose their physical origins is to investigate the light curve\nbrightness and shape of a large set of SNe. However, it is often difficult to\ncompare and contrast the existing sample studies due to differences in their\napproaches and assumptions, for example in how to eliminate host galaxy\nextinction, and this might lead to systematic errors when comparing the\nresults. We therefore introduce the Hybrid Analytic Flux FittEr for Transients\n(haffet), a Python-based software package that can be applied to download\nphotometric and spectroscopic data for transients from open online sources,\nderive bolometric light curves, and fit them to semi-analytical models for\nestimation of their physical parameters. In a companion study, we have\ninvestigated a large collection of SNe Ib and Ic observed with the Zwicky\nTransient Facility (ZTF) with haffet, and here we detail the methodology and\nthe software package to encourage more users. As large-scale surveys such as\nZTF and LSST continue to discover increasing numbers of transients, tools such\nas haffet will be critical for enabling rapid comparison of models against data\nin statistically consistent, comparable and reproducable ways. Additionally,\nhaffet is created with a Graphical User Interface mode, which we hope will\nboost the efficiency and make the usage much easier.", "category": "astro-ph_HE" }, { "text": "A millisecond pulsar discovery in a survey of unidentified Fermi\n $\u03b3$-ray sources with LOFAR: Using LOFAR, we have performed a very-low-frequency (115-155 MHz) radio\nsurvey for millisecond pulsars (MSPs). The survey targeted 52 unidentified\nFermi $\\gamma$-ray sources. Employing a combination of coherent and incoherent\ndedispersion, we have mitigated the dispersive effects of the interstellar\nmedium while maintaining sensitivity to fast-spinning pulsars. Toward 3FGL\nJ1553.1+5437 we have found PSR J1552+5437, the first MSP to be discovered\n(through its pulsations) at a radio frequency $<$ 200 MHz. PSR J1552+5437 is an\nisolated MSP with a 2.43 ms spin period and a dispersion measure of 22.9 pc\ncm$^{-3}$. The pulsar has a very steep radio spectral index ($\\alpha < -$2.8\n$\\pm$ 0.4). We obtain a phase-connected timing solution combining the 0.74\nyears of radio observations with $\\gamma$-ray photon arrival times covering 7.5\nyears of Fermi observations. We find that the radio and $\\gamma$-ray pulse\nprofiles of PSR J1552+5437 appear to be nearly aligned. The very steep spectrum\nof PSR J1552+5437, along with other recent discoveries, hints at a population\nof radio MSPs that have been missed in surveys using higher observing\nfrequencies. Detecting such steep spectrum sources is important for mapping the\npopulation of MSPs down to the shortest spin periods, understanding their\nemission in comparison to slow pulsars, and quantifying the prospects for\nfuture surveys with low-frequency radio telescopes like SKA-Low and its\nprecursors.", "category": "astro-ph_HE" }, { "text": "An XMM-Newton Study of the Mixed-Morphology Supernova Remnant G346.6-0.2: We present an X-ray imaging and spectroscopic study of the molecular cloud\ninteracting mixed-morphology (MM) supernova remnant (SNR) G346.6-0.2 using\nXMM-Newton. The X-ray spectrum of the remnant is well described by a\nrecombining plasma that most likely arises from adiabatic cooling, and has\nsub-solar abundances of Mg, Si, and S. Our fits also suggest the presence of\neither an additional power-law component with a photon index of $\\sim$2, or an\nadditional thermal component with a temperature of $\\sim$2.0 keV. We\ninvestigate the possible origin of this component and suggest that it could\narise from either the Galactic ridge X-ray emission, an unidentified pulsar\nwind nebula or X-ray synchrotron emission from high-energy particles\naccelerated at the shock. However, deeper, high resolution observations of this\nobject are needed to shed light on the presence and origin of this feature.\nBased on its morphology, its Galactic latitude, the density of the surrounding\nenvironment and its association with a dense molecular cloud, G346.6-0.2 most\nlikely arises from a massive progenitor that underwent core-collapse.", "category": "astro-ph_HE" }, { "text": "A search for VHE counterparts of Galactic Fermi bright sources and MeV\n to TeV spectral characterization: Very high-energy (VHE; E>100 GeV) gamma-rays have been detected from a wide\nrange of astronomical objects, such as pulsar wind nebulae (PWNe), supernova\nremnants (SNRs), giant molecular clouds, gamma-ray binaries, the Galactic\nCenter, active galactic nuclei (AGN), radio galaxies, starburst galaxies, and\npossibly star-forming regions as well. At lower energies, observations using\nthe Large Area Telescope (LAT) onboard Fermi provide a rich set of data which\ncan be used to study the behavior of cosmic accelerators in the MeV to TeV\nenergy bands. In particular, the improved angular resolution of current\ntelescopes in both bands compared to previous instruments significantly reduces\nsource confusion and facilitates the identification of associated counterparts\nat lower energies. In this paper, a comprehensive search for VHE gamma-ray\nsources which are spatially coincident with Galactic Fermi/LAT bright sources\nis performed, and the available MeV to TeV spectra of coincident sources are\ncompared. It is found that bright LAT GeV sources are correlated with TeV\nsources, in contrast to previous studies using EGRET data. Moreover, a single\nspectral component seems unable to describe the MeV to TeV spectra of many\ncoincident GeV/TeV sources. It has been suggested that gamma-ray pulsars may be\naccompanied by VHE gamma-ray emitting nebulae, a hypothesis that can be tested\nwith VHE observations of these pulsars.", "category": "astro-ph_HE" }, { "text": "Hard X-ray selected giant radio galaxies -- III. The LOFAR view: Giant radio galaxies (GRGs), with extended structures reaching hundreds of\nkpc, are among the most spectacular examples of ejection of relativistic plasma\nfrom super-massive black holes. In this work, third of a series, we present LOw\nFrequency ARray (LOFAR) images at 144 MHz, collected in the framework of the\nLOFAR Two-metre Sky Survey Data Release 2 (LoTSS DR2), for nine sources\nextracted from our sample of hard X-ray selected GRGs (HXGRG, i.e. from\nINTEGRAL/IBIS and Swift/BAT catalogues at >20 keV). Thanks to the resolution\nand sensitivity of LoTSS, we could probe the complex morphology of these GRGs,\nunveiling cases with diffuse (Mpc-scale) remnant emission, presence of faint\noff-axis wings, or a misaligned inner jet. In particular, for one source\n(B21144+35B), we could clearly detect a $\\sim$300 kpc wide off-axis emission,\nin addition to an inner jet which orientation is not aligned with the lobes\naxis. For another source (J1153.9+5848) a structure consistent with jet\nprecession was revealed, appearing as an X-shaped morphology with relic lobes\nhaving an extension larger than the present ones, and with a different axis\norientation. From an environment analysis, we found 2 sources showing an\noverdensity of cosmological neighbours, and a correspondent association with a\ngalaxy cluster from catalogues. Finally, a comparison with radio-selected GRGs\nfrom LoTSS DR1 suggested that, on average, HXGRG can grow to larger extents.\nThese results highlight the importance of deep low-frequency observations to\nprobe the evolution of radio galaxies, and ultimately estimate the duty cycle\nof their jets.", "category": "astro-ph_HE" }, { "text": "Structures in the cosmic ray energy spectra: All the components of cosmic rays have 'structure' in their energy spectra at\nsome level, ie deviations from a simple power law, and their examination is\nrelevant to the origin of the particles. Emphasis, here, is placed on the\nlarge-scale structures in the spectra of nuclei (the 'knee' at about 3 PeV),\nthat of electrons and positrons (a shallow 'upturn' at about 100 GeV) and the\npositron to electron plus positron ratio (an upturn starting at about 5 GeV).\n Fine structure is defined as deviations from the smooth spectra which already\nallow for the large-scale structure. Search for the fine structure has been\nperformed in the precise data on positron to electron plus positron ratio\nmeasured by the AMS-02 experiment. Although no fine structure is indicated, it\ncould in fact be present at the few percent level.", "category": "astro-ph_HE" }, { "text": "Pair-Instability Supernovae in the Local Universe: The discovery of 150 - 300 M$_{\\odot}$ stars in the Local Group and\npair-instability supernova candidates at low redshifts has excited interest in\nthis exotic explosion mechanism. Realistic light curves for pair-instability\nsupernovae at near-solar metallicities are key to identifying and properly\ninterpreting these events as more are found. We have modeled pair-instability\nsupernovae of 150 - 500 M$_{\\odot}$ Z $\\sim$ 0.1 - 0.4 Z$_{\\odot}$ stars. These\nstars lose up to 80% of their mass to strong line-driven winds and explode as\nbare He cores. We find that their light curves and spectra are quite different\nfrom those of Population III pair-instability explosions, which therefore\ncannot be used as templates for low-redshift events. Although non-zero\nmetallicity pair-instability supernovae are generally dimmer than their\nPopulation III counterparts, in some cases they will be bright enough to be\ndetected at the earliest epochs at which they can occur, the formation of the\nfirst galaxies at $z \\sim$ 10 - 15. Others can masquerade as dim, short\nduration supernovae that are only visible in the local universe and that under\nthe right conditions could be hidden in a wide variety of supernova classes. We\nalso report for the first time that some pair-instability explosions can create\nblack holes with masses of $\\sim$ 100 M$_{\\odot}$.", "category": "astro-ph_HE" }, { "text": "Ultraviolet Diversity of Type Ia Supernovae: Ultraviolet (UV) observations of Type Ia supernovae (SNe Ia) probe the\noutermost layers of the explosion, and UV spectra of SNe Ia are expected to be\nextremely sensitive to differences in progenitor composition and the details of\nthe explosion. Here we present the first study of a sample of high\nsignal-to-noise ratio SN Ia spectra that extend blueward of 2900 A. We focus on\nspectra taken within 5 days of maximum brightness. Our sample of ten SNe Ia\nspans the majority of the parameter space of SN Ia optical diversity. We find\nthat SNe Ia have significantly more diversity in the UV than in the optical,\nwith the spectral variance continuing to increase with decreasing wavelengths\nuntil at least 1800 A (the limit of our data). The majority of the UV variance\ncorrelates with optical light-curve shape, while there are no obvious and\nunique correlations between spectral shape and either ejecta velocity or\nhost-galaxy morphology. Using light-curve shape as the primary variable, we\ncreate a UV spectral model for SNe Ia at peak brightness. With the model, we\ncan examine how individual SNe vary relative to expectations based on only\ntheir light-curve shape. Doing this, we confirm an excess of flux for SN 2011fe\nat short wavelengths, consistent with its progenitor having a subsolar\nmetallicity. While most other SNe Ia do not show large deviations from the\nmodel, ASASSN-14lp has a deficit of flux at short wavelengths, suggesting that\nits progenitor was relatively metal rich.", "category": "astro-ph_HE" }, { "text": "Combined Rastall and Rainbow theories of gravity with applications to\n neutron stars: The possibility of modifications on general relativity is investigated. We\npropose an alternative theory of gravity constructed with the combination of\nRastall and Rainbow theories. The hydrostatic equilibrium equations are\nobtained in order to test the new theory in neutron stars, whose mass-radius\ndiagrams are obtained using modern equations of state of nuclear matter derived\nfrom relativistic mean field models and compared with the ones computed by the\nTolman-Oppenheimer-Volkoff equations. We conclude that substantial\nmodifications are obtained even for very small alterations on the two free\nparameters, making the reproduction of astrophysical observations an easy task.", "category": "astro-ph_HE" }, { "text": "The broad band spectral variability of MCG-6-30-15 observed by NuSTAR\n and XMM-Newton: MCG-6-30-15, at a distance of 37 Mpc (z=0.008), is the archetypical Seyfert 1\ngalaxy showing very broad Fe K$\\alpha$ emission. We present results from a\njoint NuSTAR and XMM-Newton observational campaign that, for the first time,\nallows a sensitive, time-resolved spectral analysis from 0.35 keV up to 80 keV.\nThe strong variability of the source is best explained in terms of intrinsic\nX-ray flux variations and in the context of the light bending model: the\nprimary, variable emission is reprocessed by the accretion disk, which produces\nsecondary, less variable, reflected emission. The broad Fe K$\\alpha$ profile\nis, as usual for this source, well explained by relativistic effects occurring\nin the innermost regions of the accretion disk around a rapidly rotating black\nhole. We also discuss the alternative model in which the broadening of the Fe\nK$\\alpha$ is due to the complex nature of the circumnuclear absorbing\nstructure. Even if this model cannot be ruled out, it is disfavored on\nstatistical grounds. We also detected an occultation event likely caused by BLR\nclouds crossing the line of sight.", "category": "astro-ph_HE" }, { "text": "Gamma-ray emission from PSR J0007+7303 using 7 years of Fermi Large Area\n Telescope observations: Based on more than seven years of Fermi Large Area Telescope (LAT) Pass 8\ndata, we report on a detailed analysis of the bright gamma-ray pulsar (PSR)\nJ0007+7303. We confirm that PSR J0007+7303 is significantly detected as a point\nsource also during the off-peak phases with a TS value of 262 ($\\sim$ 16\n$\\sigma$). In the description of PSR J0007+7303 off-peak spectrum, a power law\nwith an exponential cutoff at 2.7$\\pm$1.2$\\pm$1.3 GeV (the first/second\nuncertainties correspond to statistical/systematic errors) is preferred over a\nsingle power law at a level of 3.5 $\\sigma$. The possible existence of a cutoff\nhints at a magnetospheric origin of the emission. In addition, no extended\ngamma-ray emission is detected compatible with either the supernova remnant\n(CTA 1) or the very high energy (> 100 GeV) pulsar wind nebula. A flux upper\nlimit of 6.5$\\times$10$^{-12}$ erg cm$^{-2}$ s$^{-1}$ in the 10-300 GeV energy\nrange is reported, for an extended source assuming the morphology of the\nVERITAS detection. During on-peak phases, a sub-exponential cutoff is\nsignificantly preferred ($\\sim$11 $\\sigma$) for representing the spectral\nenergy distribution, both in the phase-averaged and in the phase-resolved\nspectra. Three glitches are detected during the observation period and we found\nno flux variability at the time of the glitches or in the long-term behavior.\nWe also report the discovery of a previously unknown gamma-ray source in the\nvicinity of PSR J0007+7303, Fermi J0020+7328, which we associate with the z =\n1.781 quasar S5 0016+73. A concurrent analysis of this source is needed to\ncorrectly characterize the behavior of CTA 1 and it is also presented in the\npaper.", "category": "astro-ph_HE" }, { "text": "Formation of the First Two Black Hole-Neutron Star Mergers (GW200115 and\n GW200105) from Isolated Binary Evolution: In this work we study the formation of the first two black hole-neutron star\n(BHNS) mergers detected in gravitational waves (GW200115 and GW200105) from\nmassive stars in wide isolated binary systems - the isolated binary evolution\nchannel. We use 560 BHNS binary population synthesis model realizations from\nBroekgaarden et al. (2021a) and show that the system properties (chirp mass,\ncomponent masses and mass ratios) of both GW200115 and GW200105 match\npredictions from the isolated binary evolution channel. We also show that most\nmodel realizations can account for the local BHNS merger rate densities\ninferred by LIGO-Virgo. However, to simultaneously also match the inferred\nlocal merger rate densities for BHBH and NSNS systems we find we need models\nwith moderate kick velocities ($\\sigma\\lesssim 10^2\\,\\rm{km}\\,\\rm{s}^{-1}$) or\nhigh common-envelope efficiencies ($\\alpha_{\\rm{CE}}\\gtrsim 2$) within our\nmodel explorations. We conclude that the first two observed BHNS mergers can be\nexplained from the isolated binary evolution channel for reasonable model\nrealizations.", "category": "astro-ph_HE" }, { "text": "Evidence for Declination Dependence of the Ultrahigh Energy Cosmic Ray\n Spectrum in the Northern Hemisphere: Telescope Array (TA) is the largest experiment in the Northern Hemisphere\nstudying ultrahigh energy cosmic rays. TA measurements of the cosmic ray\nspectrum using the surface detector have the best statistical power in the\nexperiment, and observe the ankle of the spectrum and the high energy cutoff.\nWhen the data are divided into two declination bands, above and below 24.8\ndegrees, the cutoff appears at $10^{19.64 \\pm 0.04}$ ($10^{19.84 \\pm 0.02}$) eV\nin the lower (higher) band, an energy difference of 58\\%. The global\nsignificance of the difference is 4.3 standard deviations. The lack of an\ninstrumental cause of this difference implies it is astrophysical in nature.", "category": "astro-ph_HE" }, { "text": "High Metallicity LGRB Hosts: We present our imaging and spectroscopic observations of the host galaxies of\ntwo dark long bursts with anomalously high metallicities, LGRB 051022 and LGRB\n020819B, which in conjunction with another LGRB event with an optical afterglow\ncomprise the three LGRBs with high metallicity host galaxies in the Graham &\nFruchter (2013) sample. In Graham & Fruchter (2013), we showed that LGRBs\nexhibit a strong and apparently intrinsic preference for low metallicity\nenvironments (12+log(O/H) < 8.4 in the KK04 scale) in spite of these three\ncases with abundances of about solar and above. These exceptions however are\nconsistent with the general star-forming galaxy population of comparable\nbrightness & redshift. This is surprising: even among a preselected sample of\nhigh metallicity LGRBs, were the metal aversion to remain in effect for these\nobjects, we would expect their metallicity to still be lower than the typical\nmetallicity for the galaxies at that luminosity and redshift. Therefore we\ndeduce that it is possible to form an LGRB in a high metallicity environment\nalthough with greater rarity.\n From this we conclude that there are three possible explanations for the\npresence of the LGRBs observed in high metallicity hosts as seen to date: (1)\nLGRBs do not occur in high metallicity environments and those seen in high\nmetallicity hosts are in fact occurring in low metallicity environments that\nhave become associated with otherwise high metallicity hosts but remain\nunenriched. (2) The LGRB formation mechanism while preferring low metallicity\nenvironments does not strictly require it resulting in a gradual decline in\nburst formation with increasing metallicity. (3) The typical low metallicity\nLGRBs and the few high metallicity cases are the result of physically different\nburst formation pathways with only the former affected by the metallicity and\nthe later occurring much more infrequently.", "category": "astro-ph_HE" }, { "text": "Unexpected emission pattern adds to the enigma of fast radio bursts: Observations of millisecond-long radio bursts from beyond the Milky Way have\nrevealed a repeat pattern with a roughly 16-day period -- a finding that adds\nto the enigma of the origin of these bursts.", "category": "astro-ph_HE" }, { "text": "A variable ionized disk wind in MAXI J1803-298 revealed by NICER: We present the results from the NICER observation data of MAXI J1803-298\nacross the entire 2021 outburst. In the intermediate and soft state, we detect\nsignificant absorption lines at $\\sim 7.0$ keV and $\\sim 6.7$ keV, arising from\nthe X-ray disk wind outflowing with a velocity of hundreds of km per second\nalong our line of sight. The fitting results from photoionized model suggest\nthat the wind is driven by thermal pressure and the mass-loss rate is low. We\nfind a clear transition for iron from predominantly H-like to predominantly\nHe-like during the intermediate-to-soft state transition. Our results indicate\nthis transition for iron is caused by the evolution of the illuminating\nspectrum and the slow change of the geometric properties of the disk wind\ntogether. The coexistence of disk wind and QPOs features in intermediate state\nis also reported. Our study makes MAXI J1803-298 the first source in which a\ntransition from optical wind to X-ray wind is detected, offering new insights\ninto the evolution of disk winds across an entire outburst and long-term\ncoupling of accretion disks and mass outflows around accreting black holes.", "category": "astro-ph_HE" }, { "text": "Afterglow Observations of Fermi-LAT Gamma-Ray Bursts and the Emerging\n Class of Hyper-Energetic Events: We present broadband (radio, optical, and X-ray) light curves and spectra of\nthe afterglows of four long-duration gamma-ray bursts (GRBs 090323, 090328,\n090902B, and 090926A) detected by the Gamma-Ray Burst Monitor (GBM) and Large\nArea Telescope (LAT) instruments on the Fermi satellite. With its wide spectral\nbandpass, extending to GeV energies, Fermi is sensitive to GRBs with very large\nisotropic energy releases (10e54 erg). Although rare, these events are\nparticularly important for testing GRB central-engine models. When combined\nwith spectroscopic redshifts, our afterglow data for these four events are able\nto constrain jet collimation angles, the density structure of the circumburst\nmedium, and both the true radiated energy release and the kinetic energy of the\noutflows. In agreement with our earlier work, we find that the relativistic\nenergy budget of at least one of these events (GRB 090926A) exceeds the\ncanonical value of 10e51 erg by an order of magnitude. Such energies pose a\nsevere challenge for models in which the GRB is powered by a magnetar or\nneutrino-driven collapsar, but remain compatible with theoretical expectations\nfor magneto-hydrodynamical (MHD) collapsar models. Our jet opening angles\n(theta) are similar to those found for pre-Fermi GRBs, but the large initial\nLorentz factors (Gamma_0) inferred from the detection of GeV photons imply\ntheta Gamma_0 ~ 70-90, values which are above those predicted in MHD models of\njet acceleration. Finally, we find that these Fermi-LAT events preferentially\noccur in a low-density circumburst environment, and we speculate that this\nmight result from the lower mass-loss rates of their lower-metallicity\nprogenitor stars. Future studies of Fermi-LAT afterglows in the radio with the\norder-of-magnitude improvement in sensitivity offered by the EVLA should\ndefinitively establish the relativistic energy budgets of these events.", "category": "astro-ph_HE" }, { "text": "Discovery of the source HESS J1356-645 associated with the young and\n energetic PSR J1357-6429: Several newly discovered very-high-energy (VHE; E > 100 GeV) gamma-ray\nsources in the Galaxy are thought to be associated with energetic pulsars.\nAmong them, middle-aged (> 1E+4 yr) systems exhibit large centre-filled VHE\nnebulae, offset from the pulsar position, which result from the complex\nrelationship between the pulsar wind and the surrounding medium, and reflect\nthe past evolution of the pulsar. Imaging Atmospheric Cherenkov Telescopes\n(IACTs) have been successful in revealing extended emission from these sources\nin the VHE regime. Together with radio and X-ray observations, this\nobservational window allows one to probe the energetics and magnetic field\ninside these large-scale nebulae. H.E.S.S., with its large field of view,\nangular resolution of < 0.1deg and unprecedented sensitivity, has been used to\ndiscover a large population of such VHE sources. In this paper, the H.E.S.S.\ndata from the continuation of the Galactic Plane Survey (-80deg < l < 60deg,\n|b| < 3deg), together with the existing multi-wavelength observations, are\nused. A new VHE gamma-ray source was discovered at R.A. (J2000) = 13h56m00s,\nDec. (J2000) = -64d30m00s with a 2' statistical error in each coordinate,\nnamely HESS J1356-645. The source is extended, with an intrinsic Gaussian width\nof (0.20 +/- 0.02)deg. Its integrated energy flux between 1 and 10 TeV of 8E-12\nerg cm-2 s-1 represents ~ 11% of the Crab Nebula flux in the same energy band.\nThe energy spectrum between 1 and 20 TeV is well described by a power law dN/dE\n~ E-Gamma with photon index Gamma = 2.2 +/- 0.2stat +/- 0.2sys. The inspection\nof archival radio images at three frequencies and the analysis of X-ray data\nfrom ROSAT/PSPC and XMM-Newton/MOS reveal the presence of faint non-thermal\ndiffuse emission coincident with HESS J1356-645. HESS J1356-645 is most likely\nassociated with the young and energetic pulsar PSR J1357-6429 (Abridged)", "category": "astro-ph_HE" }, { "text": "General relativistic treatment of $f$-mode oscillations of hyperonic\n stars: We present a systematic study of $f$-mode oscillations in neutron stars\ncontaining hyperons, extending recent results obtained within the Cowling\napproximation to linearized General Relativity. Employing a relativistic mean\nfield model, we find that the Cowling approximation can overestimate the\nquadrupolar $f$-mode frequency of neutron stars by up to 30\\% compared to the\nfrequency obtained in the linearized general relativistic formalism. Imposing\ncurrent astrophysical constraints, we derive updated empirical relations for\ngravitational wave asteroseismology. The frequency and damping time of\nquadrupole $f$-mode oscillations of hyperonic stars are found to be in the\nrange of 1.47 - 2.45kHz and 0.13 - 0.51 sec respectively. Our correlation\nstudies demonstrate that among the various parameters of the nucleonic and\nhyperonic sectors of the model, the nucleon effective mass shows the strongest\ncorrelation with mode characteristics and neutron star observables. Estimates\nfor the detectability of $f$-modes in a transient burst of gravitational waves\nfrom isolated hyperonic stars is also provided.", "category": "astro-ph_HE" }, { "text": "Timing and spectral variability of high mass X-ray pulsar GX 301--2 over\n orbital phases observed by Insight-HXMT: We report the orbital X-ray variability of high mass X-ray binary (HMXB)\nGX301--2. GX301--2 undergone a spin up process in 2018--2020 with the period\nevolving from $\\sim$ 685 s to 670 s. The energy resolved pulse-profiles of the\npulsar in 1--60 keV varied from single peaked and sinusoidal shapes to\nmulti-peaked across different orbital phases. Pulse fractions evolving over\norbit had negative correlations with the X-ray flux. The broad-band X-ray\nenergy spectrum of the pulsar can be described with a partial covering negative\npositive cutoff power-law continuum model. Near the periastron passage of the\npulsar we found a strong variation in the additional column density ($NH_{2}$),\nwhich correlated with variation of the flux. Curves of growth for both Fe\nK$\\alpha$ and Fe K$\\beta$ lines were plotted to investigate the distribution of\nmatter around neutron star. We have also found the evidence of two cyclotron\nabsorption lines in the phase-averaged spectra in GX301--2, with one line of\n30--42 keV and the other line varying in 48--56 keV. Both two line's centroid\nenergies show the similar relationship with X-ray luminosity: positive\ncorrelation in lower luminosity range, and negative relation above a critical\nluminosity of $10^{37}$ erg s$^{-1}$. We estimated the surface magnetic field\nof the neutron star in GX301--2 at ~$(0.5-2)\\times 10^{13}$ G. Two cyclotron\nline energies have a nearly fixed ratio of ~1.63 while having a low strength\nratio (~0.05), suggesting that these two features may actually be one line.", "category": "astro-ph_HE" }, { "text": "Exploring the role of X-ray reprocessing and irradiation in the\n anomalous bright optical outbursts of A0538-66: In 1981, the Be/X-ray binary (Be/XRB) A0538-66 showed outbursts characterized\nby high peak luminosities in the X-ray and optical bands. The optical outbursts\nwere qualitatively explained as X-ray reprocessing in a gas cloud surrounding\nthe binary system. Since then, further important information about A0538-66\nhave been obtained, and sophisticated photoionization codes have been developed\nto calculate the radiation emerging from a gas nebula illuminated by a central\nX-ray source. In the light of the new information and tools available, we\nstudied again the enhanced optical emission displayed by A0538-66 to understand\nthe mechanisms responsible for these unique events among the class of Be/XRBs.\nWe performed about 10^5 simulations of a gas envelope photoionized by an X-ray\nsource. We assumed for the shape of the gas cloud either a sphere or a\ncircumstellar disc observed edge-on. We studied the effects of varying the main\nproperties of the envelope and the influence of different input X-ray spectra\non the optical/UV emission emerging from the photoionized cloud. We compared\nthe computed spectra with the IUE spectrum and photometric UBV measurements\nobtained during the outburst of 29 April 1981. We also explored the role played\nby the X-ray heating of the surface of the donor star irradiated by the X-ray\nemission of the neutron star (NS). We found that reprocessing in a spherical\ncloud with a shallow radial density distribution can reproduce the optical/UV\nemission. To our knowledge, this configuration has never been observed either\nin A0538-66 during other epochs or in other Be/XRBs. We found, contrary to the\ncase of most other Be/XRBs, that the optical/UV radiation produced by the X-ray\nheating of the surface of the donor star irradiated by the NS is\nnon-negligible, due to the particular orbital parameters of this system that\nbring the NS very close to its companion.", "category": "astro-ph_HE" }, { "text": "Type Ia supernovae from violent mergers of carbon-oxygen white dwarfs:\n polarisation signatures: The violent merger of two carbon-oxygen white dwarfs has been proposed as a\nviable progenitor for some Type Ia supernovae. However, it has been argued that\nthe strong ejecta asymmetries produced by this model might be inconsistent with\nthe low degree of polarisation typically observed in Type Ia supernova\nexplosions. Here, we test this claim by carrying out a spectropolarimetric\nanalysis for the model proposed by Pakmor et al. (2012) for an explosion\ntriggered during the merger of a 1.1 M$_{\\odot}$ and 0.9 M$_{\\odot}$\ncarbon-oxygen white dwarf binary system. Owing to the asymmetries of the\nejecta, the polarisation signal varies significantly with viewing angle. We\nfind that polarisation levels for observers in the equatorial plane are modest\n($\\lesssim$ 1 per cent) and show clear evidence for a dominant axis, as a\nconsequence of the ejecta symmetry about the orbital plane. In contrast,\norientations out of the plane are associated with higher degrees of\npolarisation and departures from a dominant axis. While the particular model\nstudied here gives a good match to highly-polarised events such as SN 2004dt,\nit has difficulties in reproducing the low polarisation levels commonly\nobserved in normal Type Ia supernovae. Specifically, we find that significant\nasymmetries in the element distribution result in a wealth of strong\npolarisation features that are not observed in the majority of currently\navailable spectropolarimetric data of Type Ia supernovae. Future studies will\nmap out the parameter space of the merger scenario to investigate if\nalternative models can provide better agreement with observations.", "category": "astro-ph_HE" }, { "text": "Optical polarization variations in the blazar PKS 1749+096: We report on the variation in the optical polarization of the blazar PKS\n1749+096 observed in 2008--2015. The degree of polarization (PD) tends to\nincrease in short flares having a time-scale of a few days. The object favors a\npolarization angle (PA) of $40^\\circ$--$50^\\circ$ at the flare maxima, which is\nclose to the position angle of the jet ($20^\\circ$--$40^\\circ$). Three clear\npolarization rotations were detected in the negative PA direction associated\nwith flares. In addition, a rapid and large decrease in the PA was observed in\nthe other two flares, while another two flares showed no large PA variation.\nThe light curve maxima of the flares possibly tend to lag behind the PD maxima\nand color-index minima. The PA became $-50^\\circ$ to $-20^\\circ$ in the decay\nphase of active states, which is almost perpendicular to the jet position\nangle. We propose a scenario to explain these observational features, where\ntransverse shocks propagate along curved trajectories. The favored PA at the\nflare maxima suggests that the observed variations were governed by the\nvariations in the Doppler factor, $\\delta$. Based on this scenario, the minimum\nviewing angle of the source, $\\theta_\\mathrm{min}=4.8^\\circ$--$6.6^\\circ$, and\nthe location of the source, $\\Delta r\\gtrsim 0.1$pc, from the central black\nhole were estimated. In addition, the acceleration of electrons by the shock\nand synchrotron cooling would have a time-scale similar to that of the change\nin $\\delta$. The combined effect of the variation in $\\delta$ and\nacceleration/cooling of electrons is probably responsible for the observed\ndiversity of the polarization variations in the flares.", "category": "astro-ph_HE" }, { "text": "A new universal cosmic-ray knee near the magnetic rigidity 10 TV with\n the NUCLEON space observatory: Data from the NUCLEON space observatory give a strong indication of the\nexistence of a new universal cosmic ray \"knee\", which is observed in all groups\nof nuclei, including heavy nuclei, near a magnetic rigidity of about 10\\,TV.\nUniversality means the same position of the knee in the magnetic rigidity scale\nfor all groups of nuclei. The knee is observed by both methods of measurement\nof particles energy implemented in the NUCLEON observatory---the calorimetric\nmethod and the kinematic method KLEM. This new cosmic ray \"knee\" is probably\nconnected with the limit of acceleration of cosmic rays by some generic or\nnearby source of cosmic rays.", "category": "astro-ph_HE" }, { "text": "How, where and when do cosmic rays reach ultrahigh energies?: Understanding the origins of ultrahigh energy cosmic rays (UHECRs) - which\nreach energies in excess of $10^{20}~{\\rm eV}$ - stretches particle\nacceleration physics to its very limits. In this review, we discuss how such\nenergies can be reached, using general arguments that can often be derived on\nthe back of an envelope. We explore possible particle acceleration mechanisms,\nwith special attention paid to shock acceleration. Informed by the arguments\nderived, we discuss where UHECRs might come from and which classes of powerful\nastrophysical objects could be UHECR sources; generally, we favour radio\ngalaxies, GRB afterglows and other sources which are not too compact and\ndissipate prodigious amounts of energy on large scales, allowing them to\ngenerate large products $\\beta B R$ without the CRs undergoing restrictive\nlosses. Finally, we discuss when UHECRs are accelerated by highlighting the\nimportance of source variability, and explore the intriguing possibility that\nthe UHECR arrival directions are partly a result of \"echoes\" from magnetic\nstructures in the local Universe.", "category": "astro-ph_HE" }, { "text": "eROSITA Science Book: Mapping the Structure of the Energetic Universe: eROSITA is the primary instrument on the Russian SRG mission. In the first\nfour years of scientific operation after its launch, foreseen for 2014, it will\nperform a deep survey of the entire X-ray sky. In the soft X-ray band (0.5-2\nkeV), this will be about 20 times more sensitive than the ROSAT all sky survey,\nwhile in the hard band (2-10 keV) it will provide the first ever true imaging\nsurvey of the sky at those energies. Such a sensitive all-sky survey will\nrevolutionize our view of the high-energy sky, and calls for major efforts in\nsynergic, multi-wavelength wide area surveys in order to fully exploit the\nscientific potential of the X-ray data. The design-driving science of eROSITA\nis the detection of very large samples (~10^5 objects) of galaxy clusters out\nto redshifts z>1, in order to study the large scale structure in the Universe,\ntest and characterize cosmological models including Dark Energy. eROSITA is\nalso expected to yield a sample of around 3 millions Active Galactic Nuclei,\nincluding both obscured and un-obscured objects, providing a unique view of the\nevolution of supermassive black holes within the emerging cosmic structure. The\nsurvey will also provide new insights into a wide range of astrophysical\nphenomena, including accreting binaries, active stars and diffuse emission\nwithin the Galaxy, as well as solar system bodies that emit X-rays via the\ncharge exchange process. Finally, such a deep imaging survey at high spectral\nresolution, with its scanning strategy sensitive to a range of variability\ntimescales from tens of seconds to years, will undoubtedly open up a vast\ndiscovery space for the study of rare, unpredicted, or unpredictable\nhigh-energy astrophysical phenomena. In this living document we present a\ncomprehensive description of the main scientific goals of the mission, with\nstrong emphasis on the early survey phases.", "category": "astro-ph_HE" }, { "text": "Investigation of Dense Gas Towards Relativistic Outflow Sources: We probe the interstellar medium towards the objects Circinus X-1, a low-mass\nX-ray binary with relativistic jets; and the highly energetic Westerlund 2\nstellar cluster, which is located towards TeV gamma-ray emission and\ninteresting arc- and jet-like features seen in Nanten 12CO data. We have mapped\nboth regions with the Mopra radio telescope, in 7 mm and 12 mm wavebands,\nlooking for evidence of disrupted/dense gas caused by the interaction between\nhigh energy outflows and the ISM. Towards Westerlund 2, peaks in CS(J=1-0)\nemission indicate high density gas towards the middle of the arc and the\nendpoint of the jet; and radio recombination line emission is seen overlapping\nthe coincident HII region RCW49. Towards Circinus X-1, 12CO(J=1-0) Nanten data\nreveals three molecular clouds that lie in the region of Cir X-1. Gas\nparameters for each cloud are presented here.", "category": "astro-ph_HE" }, { "text": "Dark matter in the solar system I: The distribution function of WIMPs at\n the Earth from solar capture: The next generation of dark matter (DM) direct detection experiments and\nneutrino telescopes will probe large swaths of dark matter parameter space. In\norder to interpret the signals in these experiments, it is necessary to have\ngood models of both the halo DM streaming through the solar system and the\npopulation of DM bound to the solar system. In this paper, the first in a\nseries of three on DM in the solar system, we present simulations of orbits of\nDM bound to the solar system by solar capture in a toy solar system consisting\nof only the Sun and Jupiter, assuming that DM consists of a single species of\nweakly interacting massive particle (WIMP). We describe how the size of the\nbound WIMP population depends on the WIMP mass, spin-independent cross section,\nand spin-dependent cross section. Using a standard description of the Galactic\nDM halo, we find that the maximum enhancement to the direct detection event\nrate, consistent with current experimental constraints on the WIMP-nucleon\ncross section, is < 1% relative to the event rate from halo WIMPs, while the\nevent rate from neutrinos from WIMP annihilation in the center of the Earth is\nunlikely to meet the threshold of next-generation, km^3-sized (IceCube, KM3NeT)\nneutrino telescopes.", "category": "astro-ph_HE" }, { "text": "Predictions for the Rates of Compact Binary Coalescences Observable by\n Ground-based Gravitational-wave Detectors: We present an up-to-date, comprehensive summary of the rates for all types of\ncompact binary coalescence sources detectable by the Initial and Advanced\nversions of the ground-based gravitational-wave detectors LIGO and Virgo.\nAstrophysical estimates for compact-binary coalescence rates depend on a number\nof assumptions and unknown model parameters, and are still uncertain. The most\nconfident among these estimates are the rate predictions for coalescing binary\nneutron stars which are based on extrapolations from observed binary pulsars in\nour Galaxy. These yield a likely coalescence rate of 100 per Myr per Milky Way\nEquivalent Galaxy (MWEG), although the rate could plausibly range from 1 per\nMyr per MWEG to 1000 per Myr per MWEG. We convert coalescence rates into\ndetection rates based on data from the LIGO S5 and Virgo VSR2 science runs and\nprojected sensitivities for our Advanced detectors. Using the detector\nsensitivities derived from these data, we find a likely detection rate of 0.02\nper year for Initial LIGO-Virgo interferometers, with a plausible range between\n0.0002 and 0.2 per year. The likely binary neutron-star detection rate for the\nAdvanced LIGO-Virgo network increases to 40 events per year, with a range\nbetween 0.4 and 400 per year.", "category": "astro-ph_HE" }, { "text": "Tilted black hole accretion disc models of Sagittarius A*: time-variable\n millimetre to near-infrared emission: High-resolution, multi-wavelength, and time-domain observations of the\nGalactic centre black hole candidate, Sgr A*, allow for a direct test of\ncontemporary accretion theory. To date, all models have assumed alignment\nbetween the accretion disc and black hole angular momentum axes, but this is\nunjustified for geometrically thick accretion flows like that onto Sgr A*.\nInstead, we calculate images and spectra from a set of simulations of accretion\nflows misaligned ('tilted') by 15 degrees from the black hole spin axis and\ncompare them with millimetre (mm) to near-infrared (NIR) observations.\nNon-axisymmetric standing shocks from eccentric fluid orbits dominate the\nemission, leading to a wide range of possible image morphologies. These effects\ninvalidate previous parameter estimates from model fitting, including estimates\nof the dimensionless black hole spin, except possibly at low values of spin or\ntilt. At 1.3mm, the images have crescent morphologies, and the black hole\nshadow may still be accessible to future mm-VLBI observations. Shock heating\nleads to high energy electrons (T > 10^12 K), which can naturally produce the\nobserved NIR flux, spectral index, and rapid variability ('flaring'). This NIR\nemission is uncorrelated with that in the mm, which also agrees with\nobservations.\n These are the first models to self-consistently explain the time-variable mm\nto NIR emission of Sgr A*. Predictions of the model include significant\nstructural changes observable with mm-VLBI on both the dynamical (hour) and\nLense-Thirring precession (day-year) timescales; and ~30-50 microarcsecond\nchanges in centroid position from extreme gravitational lensing events during\nNIR flares, detectable with the future VLT instrument GRAVITY. If the observed\nNIR emission is caused by shock heating in a tilted accretion disc, then the\nGalactic centre black hole has a positive, non-zero spin parameter (a > 0).", "category": "astro-ph_HE" }, { "text": "Relations Between Neutron-Star Parameters in the Hartle-Thorne\n Approximation: Using stellar structure calculations in the Hartle-Thorne approximation, we\nderive analytic expressions connecting the ellipticity of the stellar surface\nto the compactness, the spin angular momentum, and the quadrupole moment of the\nspacetime. We also obtain empirical relations between the compactness, the spin\nangular momentum, and the spacetime quadrupole. Our formulae reproduce the\nresults of numerical calculations to within a few percent and help reduce the\nnumber of parameters necessary to model the observational appearance of\nmoderately spinning neutron stars. This is sufficient for comparing theoretical\nspectroscopic and timing models to observations that aim to measure the masses\nand radii of neutron stars and to determine the equation of state prevailing in\ntheir interiors.", "category": "astro-ph_HE" }, { "text": "Free motion around black holes with discs or rings: between\n integrability and chaos -- V: Complete integrability of geodesic motion, the well known feature of the\nfields of isolated stationary black holes, can easily be \"spoilt\" by the\npresence of some additional source (even if highly symmetric). In previous\npapers, we used various methods to show how free time-like motion becomes\nchaotic if the gravitational field of the Schwarzschild black hole is perturbed\nby that of a circular disc or ring, considering specifically the inverted first\ndisc of the Morgan-Morgan counter-rotating family and the Bach-Weyl ring as the\nadditional sources. The present paper focuses on two new points. First, since\nthe Bach-Weyl thin ring is physically quite unsatisfactory, we now repeat some\nof the analysis for a different, Majumdar-Papapetrou--type (extremally charged)\nring around an extreme Reissner-Nordstr\\\"om black hole, and compare the results\nwith those obtained before. We also argue that such a system is in fact more\nrelevant astrophysically than it may seem. Second, we check numerically, for\nthe latter system as well as for the Schwarzschild black hole encircled by the\ninverted Morgan-Morgan disc, how indicative is the geometric (curvature)\ncriterion for chaos suggested by Sota, Suzuki & Maeda (1996). We also add a\nreview of the literature where the relevance of geometric criteria in general\nrelativity (as well as elsewhere) has been discussed for decades.", "category": "astro-ph_HE" }, { "text": "Kinematics of the jet in M87 on scales of 100 -- 1000 Schwarzschild\n radii: Very long baseline interferometry (VLBI) imaging of radio emission from\nextragalactic jets provides a unique probe of physical mechanisms governing the\nlaunching, acceleration, and collimation of relativistic outflows. The\ntwo-dimensional structure and kinematics of the jet in M\\,87 (NGC\\,4486) have\nbeen studied by applying the Wavelet-based Image Segmentation and Evaluation\n(WISE) method to 11 images obtained from multi-epoch Very Long Baseline Array\n(VLBA) observations made in January-August 2007 at 43 GHz ($\\lambda = 7$ mm).\nThe WISE analysis recovers a detailed two-dimensional velocity field in the jet\nin M\\,87 at sub-parsec scales. The observed evolution of the flow velocity with\ndistance from the jet base can be explained in the framework of MHD jet\nacceleration and Poynting flux conversion. A linear acceleration regime is\nobserved up to $z_{obs} \\sim 2$\\,mas. The acceleration is reduced at larger\nscales, which is consistent with saturation of Poynting flux conversion.\nStacked cross correlation analysis of the images reveals a pronounced\nstratification of the flow. The flow consists of a slow, mildly relativistic\nlayer (moving at $\\beta \\sim 0.5\\,c$), associated either with instability\npattern speed or an outer wind, and a fast, accelerating stream line (with\n$\\beta \\sim 0.92$, corresponding to a bulk Lorentz factor $\\gamma \\sim 2.5$). A\nsystematic difference of the apparent speeds in the northern and southern limbs\nof the jet is detected, providing evidence for jet rotation. The angular\nvelocity of the magnetic field line associated with this rotation suggests that\nthe jet in M87 is launched in the inner part of the disk, at a distance $r_0\n\\sim 5\\, R_\\mathrm{s}$ from the central engine. The combined results of the\nanalysis imply that MHD acceleration and conversion of Poynting flux to kinetic\nenergy play the dominant roles in collimation and acceleration of the flow in\nM\\,87.", "category": "astro-ph_HE" }, { "text": "Radial diffusion in corotating magnetosphere of Pulsar PSR J0737-3039B: Rich observational phenomenology associated with Pulsar B in PSR\nJ0737$-$3039A/B system resembles in many respects phenomena observed in the\nEarth and Jupiter magnetospheres, originating due to the wind-magnetosphere\ninteraction. We consider particle dynamics in the fast corotating magnetosphere\nof Pulsar B, when the spin period is shorter than the third adiabatic period.\nWe demonstrate that trapped particles occasionally experience large radial\nvariations of the L-parameter (effective radial distance) due to the parametric\ninteraction of the gyration motion with the large scale electric fields induced\nby the deformations of the magnetosphere, in what could be called a\nbetatron-induced diffusion. The dynamics of particles from the wind of Pulsar A\ntrapped inside Pulsar B magnetosphere is governed by Mathieu's equation, so\nthat the parametrically unstable orbits are occasionally activated; particle\ndynamics is not diffusive per se. The model explains the high plasma density on\nthe closed field lines of Pulsar B, and the fact that the observed eclipsing\nregion is several times smaller than predicted by the hydrodynamic models.", "category": "astro-ph_HE" }, { "text": "News from the very-high-energy sky seen with H.E.S.S: The H.E.S.S. experiment, the largest Cherenkov telescope array to date, has\nbeen observing the sky at TeV energies for the past 16 years. Its location in\nthe Southern hemisphere provides H.E.S.S. with equally good access to Galactic\nand extra-galactic sources. The focus of observations is now gradually shifting\nfrom discoveries of new TeV emitters to studies of transients and the detailed\nexploration of known sources. A few recent highlights from these observations\nare presented here, with a focus on extragalactic observations.", "category": "astro-ph_HE" }, { "text": "Detecting Gravitational-waves from Extreme Mass Ratio Inspirals using\n Convolutional Neural Networks: Extreme mass ratio inspirals (EMRIs) are among the most interesting\ngravitational wave (GW) sources for space-borne GW detectors. However,\nsuccessful GW data analysis remains challenging due to many issues, ranging\nfrom the difficulty of modeling accurate waveforms, to the impractically large\ntemplate bank required by the traditional matched filtering search method. In\nthis work, we introduce a proof-of-principle approach for EMRI detection based\non convolutional neural networks (CNNs). We demonstrate the performance with\nsimulated EMRI signals buried in Gaussian noise. We show that over a wide range\nof physical parameters, the network is effective for EMRI systems with a\nsignal-to-noise ratio larger than 50, and the performance is most strongly\nrelated to the signal-to-noise ratio. The method also shows good generalization\nability towards different waveform models. Our study reveals the potential\napplicability of machine learning technology like CNNs towards more realistic\nEMRI data analysis.", "category": "astro-ph_HE" }, { "text": "A numerical approach to the non-uniqueness problem of cosmic ray\n two-fluid equations at shocks: Cosmic rays (CRs) are frequently modeled as an additional fluid in\nhydrodynamic (HD) and magnetohydrodynamic (MHD) simulations of astrophysical\nflows. The standard CR two-fluid model is described in terms of three\nconservation laws (expressing conservation of mass, momentum and total energy)\nand one additional equation (for the CR pressure) that cannot be cast in a\nsatisfactory conservative form. The presence of non-conservative terms with\nspatial derivatives in the model equations prevents a unique weak solution\nbehind a shock. We investigate a number of methods for the numerical solution\nof the two-fluid equations and find that, in the presence of shock waves, the\nresults generally depend on the numerical details (spatial reconstruction, time\nstepping, the CFL number, and the adopted discretization). All methods converge\nto a unique result if the energy partition between the thermal and non-thermal\nfluids at the shock is prescribed using a subgrid prescription. This highlights\nthe non-uniqueness problem of the two-fluid equations at shocks. From our\nnumerical investigations, we report a robust method for which the solutions are\ninsensitive to the numerical details even in absence of a subgrid prescription,\nalthough we recommend a subgrid closure at shocks using results from kinetic\ntheory. The subgrid closure is crucial for a reliable post-shock solution and\nalso its impact on large scale flows because the shock microphysics that\ndetermines CR acceleration is not accurately captured in a fluid approximation.\nCritical test problems, limitations of fluid modeling, and future directions\nare discussed.", "category": "astro-ph_HE" }, { "text": "A Luminous Be+White Dwarf Supersoft Source in the Wing of the SMC: MAXI\n J0158-744: We present a multi-wavelength analysis of the very fast X-ray transient MAXI\nJ0158-744, which was detected by MAXI/GSC on 2011 November 11. The subsequent\nexponential decline of the X-ray flux was followed with Swift observations, all\nof which revealed spectra with low temperatures (~100eV) indicating that MAXI\nJ0158-744 is a new Supersoft Source (SSS). The Swift X-ray spectra near maximum\nshow features around 0.8 keV that we interpret as possible absorption from\nOVIII, and emission from O, Fe, and Ne lines. We obtained SAAO and ESO optical\nspectra of the counterpart early in the outburst and several weeks later. The\nearly spectrum is dominated by strong Balmer and HeI emission, together with\nweaker HeII emission. The later spectrum reveals absorption features that\nindicate a B1/2IIIe spectral type, and all spectral features are at velocities\nconsistent with the Small Magellanic Cloud. At this distance, it is a luminous\nSSS (>10^37 erg/s) but whose brief peak luminosity of >10^39 erg/s in the 2-4\nkeV band makes it the brightest SSS yet seen at \"hard\" X-rays. We propose that\nMAXI J0158-744 is a Be-WD binary, and the first example to possibly enter ULX\nterritory. The brief hard X-ray flash could possibly be a result of the\ninteraction of the ejected nova shell with the B star wind in which the white\ndwarf (WD) is embedded. This makes MAXI J0158-744 only the third Be/WD system\nin the Magellanic Clouds, but it is by far the most luminous. The properties of\nMAXI J0158-744 give weight to previous suggestions that SSS in nearby galaxies\nare associated with early-type stellar systems.", "category": "astro-ph_HE" }, { "text": "Collective baryon decay and gravitational collapse: While it is widely believed that the gravitational collapse of a sufficiently\nlarge mass will lead to a density singularity and an event horizon, we propose\nthat this never happens when quantum effects are taken into account. In\nparticular, we propose that when the conditions become ripe for a trapped\nsurface to form, a quantum critical surface sweeps over the collapsing body,\ntransforming the nucleons in the collapsing matter into a lepton/photon gas\ntogether with a positive vacuum energy. This will happen regardless of the\nmatter density at the time a trapped surface starts to form, and as a result we\npredict that at least in all cases of gravitational collapse involving ordinary\nmatter, a large fraction of the rest mass of the collapsing matter will be\nconverted into a burst of neutrinos, and {\\gamma}-rays. We predict that the\npeak luminosity of these bursts is only weakly dependent on the mass of the\ncollapsing object, and is on the order of ({\\epsilon}_q/m_Pc^2)^1/4c^5/G, where\n{\\epsilon}_q is the mean energy of a nucleon parton and m_P is the Planck mass.\nThe duration of the bursts will depend the mass of the collapsing objects; in\nthe case of stellar core collapse we predict that the duration of both the\nneutrino and {\\gamma}-ray bursts will be on the order of 10 seconds.", "category": "astro-ph_HE" }, { "text": "Updating the orbital ephemeris of the dipping source XB 1254-690 and the\n distance to the source: XB 1254-690 is a dipping low mass X-ray binary system hosting a neutron star\nand showing type I X-ray bursts. We aim at obtaining more accurate orbital\nephemeris and at constraining the orbital period derivative of the system for\nthe first time. In addition, we want to better constrain the distance to the\nsource in order to locate the system in a well defined evolutive scenario. We\napply for the first time an orbital timing technique to XB 1254-690, using the\narrival times of the dips present in the light curves that have been collected\nduring 26 years of X-ray pointed observations performed from different space\nmissions. We estimate the dip arrival times using a statistical method that\nweights the count-rate inside the dip with respect to the level of the\npersistent emission outside the dip. We fit the obtained delays as a function\nof the orbital cycles both with a linear and a quadratic function. We infer the\norbital ephemeris of XB 1254-690 improving the accuracy of the orbital period\nwith respect to previous estimates. We infer a mass of M$_{2}=0.42\\pm 0.04$\nM$_{\\odot}$ for the donor star, in agreement with the estimations already\npresent in literature, assuming that the star is in thermal equilibrium while\nit transfers part of its mass via the inner Lagrangian point, and assuming a\nneutron star mass of 1.4 M$_{\\odot}$. Using these assumptions, we also\nconstrain the distance to the source, finding a value of 7.6$\\pm 0.8$ kpc.\nFinally, we discuss the evolution of the system suggesting that it is\ncompatible with a conservative mass transfer driven by magnetic braking.", "category": "astro-ph_HE" }, { "text": "Hidden Hearts of Neutrino Active Galaxies: Recent multimessenger studies have provided evidence for high-energy neutrino\nsources that are opaque to GeV-TeV gamma rays. We present model-independent\nstudies on the connection between neutrinos and gamma rays in the active galaxy\nNGC 1068, and find that the neutrinos most likely come from regions within\nabout 30-100 Schwarzschild radii. This is especially the case if neutrinos are\nproduced via the photomeson production process, although the constraints could\nbe alleviated if hadronuclear interactions are dominant. We consider the most\nfavorable neutrino production regions, and discuss coronae, jets, winds, and\ntheir interactions with dense material. The results strengthen the importance\nof understanding dissipation mechanisms near the coronal region and the outflow\nbase. There could be a connection between active galactic nuclei with\nnear-Eddington accretion and tidal disruptions events, in that neutrinos are\nproduced in the obscured vicinity of supermassive black holes.", "category": "astro-ph_HE" }, { "text": "Accreting Millisecond X-Ray Pulsars: Accreting Millisecond X-Ray Pulsars (AMXPs) are astrophysical laboratories\nwithout parallel in the study of extreme physics. In this chapter we review the\npast fifteen years of discoveries in the field. We summarize the observations\nof the fifteen known AMXPs, with a particular emphasis on the multi-wavelength\nobservations that have been carried out since the discovery of the first AMXP\nin 1998. We review accretion torque theory, the pulse formation process, and\nhow AMXP observations have changed our view on the interaction of plasma and\nmagnetic fields in strong gravity. We also explain how the AMXPs have deepened\nour understanding of the thermonuclear burst process, in particular the\nphenomenon of burst oscillations. We conclude with a discussion of the open\nproblems that remain to be addressed in the future.", "category": "astro-ph_HE" }, { "text": "The progenitor of binary millisecond radio pulsar PSR J1713+0747\n (Research Note): PSR J1713+0747 is a binary system comprising millisecond radio pulsar with a\nspin period of 4.57 ms, and a low-mass white dwarf (WD) companion orbiting the\npulsar with a period of 67.8 days. Using the general relativistic Shapiro\ndelay, the masses of the WD and pulsar components were previously found to be\n$0.28\\pm 0.03 M_{\\odot}$ and $1.3\\pm 0.2 M_{\\odot}$ (68% confidence),\nrespectively. Standard binary evolution theory suggests that PSR J1713+0747\nevolved from a low-mass X-ray binary (LMXB). Here, we test this hypothesis. We\nused a binary evolution code and a WD evolution code to calculate evolutionary\nsequences of LMXBs that could result in binary millisecond radio pulsars such\nas PSR J1713+0747. During the mass exchange, the mass transfer is\nnonconservative. Because of the thermal and viscous instabilities developing in\nthe accretion disk, the neutron star accretes only a small part of the incoming\nmaterial. We find that the progenitor of PSR J1713+0747 can be modelled as an\nLMXB including a donor star with mass $1.3-1.6 M_{\\odot}$ and an initial\norbital period ranging from 2.40 to 4.15 days. If the cooling timescale of the\nWD is 8 Gyr, its present effective temperature is between 3870 and 4120 K,\nslightly higher than the observed value. We estimate a surface gravity of ${\\rm\nLog} (g) \\approx 7.38 - 7.40$.", "category": "astro-ph_HE" }, { "text": "Deriving fundamental parameters of millisecond pulsars via AIC in white\n dwarfs: We present a study of the observational properties of Millisecond Pulsars\n(MSPs) by way of their magnetic fields, spin periods and masses. These\nmeasurements are derived through the scenario of Accretion Induced Collapse\n(AIC) of white dwarfs (WDs) in stellar binary systems, in order to provide a\ngreater understanding of the characteristics of MSP populations. In addition,\nwe demonstrate a strong evolutionary connection between neutron stars and WDs\nwith binary companions from a stellar binary evolution perspective via the AIC\nprocess.", "category": "astro-ph_HE" }, { "text": "PS1-14bj: A Hydrogen-Poor Superluminous Supernova With a Long Rise and\n Slow Decay: We present photometry and spectroscopy of PS1-14bj, a hydrogen-poor\nsuperluminous supernova (SLSN) at redshift $z=0.5215$ discovered in the last\nmonths of the Pan-STARRS1 Medium Deep Survey. PS1-14bj stands out by its\nextremely slow evolution, with an observed rise of $\\gtrsim 125$ rest-frame\ndays, and exponential decline out to $\\sim 250$ days past peak at a measured\nrate of $0.01~{\\rm mag~day}^{-1}$, consistent with fully-trapped $^{56}$Co\ndecay. This is the longest rise time measured in a SLSN to date, and the first\nSLSN to show a rise time consistent with pair-instability supernova (PISN)\nmodels. Compared to other slowly-evolving SLSNe, it is spectroscopically\nsimilar to the prototype SN2007bi at maximum light, though lower in luminosity\n($L_{\\rm peak} \\simeq 4.6 \\times 10^{43} {\\rm erg s}^{-1}$) and with a flatter\npeak than previous events. PS1-14bj shows a number of peculiar properties,\nincluding a near-constant color temperature for $>200$ days past peak, and\nstrong emission lines from [O III] $\\lambda$5007 and [O III] $\\lambda$4363 with\na velocity width of $\\sim$3400 km/s, in its late-time spectra. These both\nsuggest there is a sustained source of heating over very long timescales, and\nare incompatible with a simple $^{56}$Ni-powered/PISN interpretation. A\nmodified magnetar model including emission leakage at late times can reproduce\nthe light curve, in which case the blue continuum and [O III] features are\ninterpreted as material heated and ionized by the inner pulsar wind nebula\nbecoming visible at late times. Alternatively, the late-time heating could be\ndue to interaction with a shell of H-poor circumstellar material.", "category": "astro-ph_HE" }, { "text": "Particle acceleration in relativistic turbulence: A theoretical\n appraisal: We discuss the physics of stochastic particle acceleration in relativistic\nmagnetohydrodynamic (MHD) turbulence, combining numerical simulations of\ntest-particle acceleration in synthetic wave turbulence spectra with detailed\nanalytical estimates. In particular, we study particle acceleration in wavelike\nisotropic fast mode turbulence, in Alfv\\'en and slow Goldreich-Sridhar type\nwave turbulence (properly accounting for anisotropy effects), including\nresonance broadening due to wave decay and pitch-angle randomization. At high\nparticle rigidities, the contributions of those three modes to acceleration are\ncomparable to within an order of magnitude, as a combination of several effects\n(partial disappearance of transit-time damping for fast modes, increased\nscattering rate for Alfv\\'en and slow modes due to resonance broadening).\nAdditionally, we provide analytical arguments regarding acceleration beyond the\nregime of MHD wave turbulence, addressing the issue of nonresonant acceleration\nin a turbulence comprised of structures rather than waves, as well as the issue\nof acceleration in small-scale parallel electric fields. Finally, we compare\nour results to the existing literature and provide ready-to-use formulas for\napplications to high-energy astrophysical phenomenology.", "category": "astro-ph_HE" }, { "text": "About the physical nature of some peculiarities of the primary cosmic\n radiation nuclei and gamma quanta spectra: About 20 years ago we published the data concerning some peculiarities of the\nbehavior of cosmic radiation EAS hadron component spectra. The results pointed\nto the possible existence in the interstellar space of the background of weakly\ninteracting objects of the mass (the energy of the resonance oscillations) of\nthe order of 37 eV. The absence of such background particle creation in\naccelerator experiments may mean that it is not the elementary particle but the\nobject of some different nature.\n On the other hand, the experimental data of the last years are pointing to\nthe existence of cosmic gamma radiation with the specific spectrum having the\nsteep right front again in the region of the order of 37 eV and the left front\nfalling down to the energies of the order less than 10-6 eV. Obviously, no\nelementary object may possess such spectrum of frequencies or a decay spectrum.\nSuch spectrum may have for example the topological defect of the space.\n Further the possibility will be presented of exactly such explanation of the\ncosmic rays primary radiation spectra peculiarities experimentally observed. It\nis based on the hypothesis about the discreteness of the space and existence in\nit of the topological defects distributed with sufficient density. In the\nframes of the proposed model some essential experimental peculiarities of the\nprimary cosmic radiation nuclei and gamma quanta spectra find the unified\nexplanation.", "category": "astro-ph_HE" }, { "text": "Intra-Night Optical Variability Monitoring of Fermi Blazars: First\n Results from 1.3 m J C Bhattacharya Telescope: We report the first results obtained from our campaign to characterize the\nintranight-optical variability (INOV) properties of {\\it Fermi}~detected\nblazars, using the observations from the recently commissioned 1.3 m J C\nBhattacharya telescope (JCBT). During the first run, we were able to observe 17\nblazars in the Bessel $R$ filter for $\\sim$137 hrs. Using $C$ and scaled\n$F$-statistics, we quantify the extent of INOV and derive the duty cycle (DC)\nwhich is the fraction of time during which a source exhibits a substantial flux\nvariability. We find a high DC of 40\\% for BL Lac objects and the flat spectrum\nradio quasars are relatively less variable (DC $\\sim15$\\%). However, when\nestimated for blazars sub-classes, a high DC of $\\sim$59\\% is found in low\nsynchrotron peaked (LSP) blazars, whereas, intermediate and high synchrotron\npeaked objects have a low DC of $\\sim$11\\% and 13\\%, respectively. We find\nevidences about the association of the high amplitude INOV with the\n$\\gamma$-ray flaring state. We also notice a high polarization during the\nelevated INOV states (for the sources that have polarimetric data available),\nthus supporting the jet based origin of the observed variability. We plan to\nenlarge the sample and utilize the time availability from the small telescopes,\nsuch as 1.3 m JCBT, to strengthen/verify the results obtained in this work and\nthose existed in the literature.", "category": "astro-ph_HE" }, { "text": "First Multi-wavelength Campaign on the Gamma-ray-loud Active Galaxy IC\n 310: The extragalactic VHE gamma-ray sky is rich in blazars. These are jetted\nactive galactic nuclei viewed at a small angle to the line-of-sight. Only a\nhandful of objects viewed at a larger angle are known so far to emit above 100\nGeV. Multi-wavelength studies of such objects up to the highest energies\nprovide new insights into the particle and radiation processes of active\ngalactic nuclei. We report the results from the first multi-wavelength campaign\nobserving the TeV detected nucleus of the active galaxy IC 310, whose jet is\nobserved at a moderate viewing angle of 10 deg - 20 deg. The multi-instrument\ncampaign was conducted between 2012 Nov. and 2013 Jan., and involved\nobservations with MAGIC, Fermi, INTEGRAL, Swift, OVRO, MOJAVE and EVN. These\nobservations were complemented with archival data from the AllWISE and 2MASS\ncatalogs. A one-zone synchrotron self-Compton model was applied to describe the\nbroad-band spectral energy distribution. IC 310 showed an extraordinary TeV\nflare at the beginning of the campaign, followed by a low, but still detectable\nTeV flux. Compared to previous measurements, the spectral shape was found to be\nsteeper during the low emission state. Simultaneous observations in the soft\nX-ray band showed an enhanced energy flux state and a harder-when-brighter\nspectral shape behaviour. No strong correlated flux variability was found in\nother frequency regimes. The broad-band spectral energy distribution obtained\nfrom these observations supports the hypothesis of a double-hump structure. The\nharder-when-brighter trend in the X-ray and VHE emission is consistent with the\nbehaviour expected from a synchrotron self-Compton scenario. The\ncontemporaneous broad-band spectral energy distribution is well described with\na one-zone synchrotron self-Compton model using parameters that are comparable\nto those found for other gamma-ray-emitting misaligned blazars.", "category": "astro-ph_HE" }, { "text": "Pevatron at the Galactic Center: Multi-Wavelength Signatures from\n Millisecond Pulsars: Diffuse TeV emission has been observed by H.E.S.S. in the Galactic Center\nregion, in addition to the GeV gamma rays observed by Fermi. We propose that a\npopulation of unresolved millisecond pulsars located around the Galactic\nCenter, suggested as possible candidates for the diffuse Galactic Center excess\nobserved by Fermi, accelerate cosmic rays up to very high energies, and are\nthus also responsible for the TeV excess. We model analytically the diffusion\nof these accelerated protons and their interaction with the molecular clouds,\nproducing gamma rays. The spatial and spectral dependences of the gamma rays\nproduced can reproduce the H.E.S.S. observations, for a population of $\\sim\n10^4-10^5$ millisecond pulsars above the cosmic-ray luminosity $10^{34}\\,{\\rm\nerg\\,s}^{-1}$, with moderate acceleration efficiency. More precise measurements\nat the highest energies would allow us to constrain the properties of the\npulsar population, such as the magnetic field or initial spin distributions.", "category": "astro-ph_HE" }, { "text": "Ne X X-ray Emission due to Charge Exchange in M82: Recent X-ray observations of star-forming galaxies such as M82 have shown the\nLy beta/Ly alpha line ratio of Ne X to be in excess of predictions for thermal\nelectron impact excitation. Here we demonstrate that the observed line ratio\nmay be due to charge exchange and can be used to constrain the ion kinetic\nenergy to be <500 eV/u. This is accomplished by computing spectra and line\nratios via a range of theoretical methods and comparing these to experiments\nwith He over astrophysically relevant collision energies. The charge exchange\nemission spectra calculations were performed for Ne[10+] +H and Ne[10+] +He\nusing widely applied approaches including the atomic orbital close coupling,\nclassical trajectory Monte Carlo, and multichannel Landau- Zener (MCLZ)\nmethods. A comparison of the results from these methods indicates that for the\nconsidered energy range and neutrals (H, He) the so-called \"low-energy\nl-distribution\" MCLZ method provides the most likely reliable predictions.", "category": "astro-ph_HE" }, { "text": "Neutrinos from Choked Jets Accompanied by Type-II Supernovae: The origin of the IceCube neutrinos is still an open question. Upper limits\nfrom diffuse gamma-ray observations suggest that the neutrino sources are\neither distant or hidden from gamma-ray observations. It is possible that the\nneutrinos are produced in jets that are formed in the core-collapsing massive\nstars and fail to break out, the so-called choked jets. We study neutrinos from\nthe jets choked in the hydrogen envelopes of red supergiant stars. Fast\nphoto-meson cooling softens the neutrino spectrum, making it difficult to\nexplain the PeV neutrinos observed by IceCube in a one-component scenario, but\na two-component model can explain the spectrum. Furthermore, we predict that a\nnewly born jet-driven type-II supernova may be observed to be associated with a\nneutrino burst detected by IceCube.", "category": "astro-ph_HE" }, { "text": "Recent multi-wavelength campaigns in the Fermi-GST era: Since 2008 the Fermi/LAT instrument has delivered highly time-resolved\ngamma-ray spectra and detailed variability curves for a steadily increasing\nnumber of AGN. For detailed AGN studies the Fermi/LAT data have to be combined\nwith, and accompanied by, dedicated ground- and space-based multi-frequency\nobservations. In this framework, the Fermi AGN team has realized a detailed\nplan for multi-wavelength campaigns including a large suite of cm/mm/sub-mm\nband instruments. Many of those campaigns have been triggered, often for\nsources detected in flaring states. We review here a few interesting results\nrecently obtained during three such campaigns, namely for the flat-spectrum\nradio quasar 3C 279, the Narrow Line Seyfert 1 PMN J0948+0022 and quasar 3C\n454.3.", "category": "astro-ph_HE" }, { "text": "Pair Production in Low Luminosity Galactic Nuclei: Electron-positron pairs may be produced near accreting black holes by a\nvariety of physical processes, and the resulting pair plasma may be accelerated\nand collimated into a relativistic jet. Here we use a self-consistent dynamical\nand radiative model to investigate pair production by \\gamma\\gamma collisions\nin weakly radiative accretion flows around a black hole of mass M and accretion\nrate \\dot{M}. Our flow model is drawn from general relativistic\nmagnetohydrodynamic simulations, and our radiation field is computed by a Monte\nCarlo transport scheme assuming the electron distribution function is thermal.\nWe argue that the pair production rate scales as r^{-6} M^{-1} \\dot{M}^{6}. We\nconfirm this numerically and calibrate the scaling relation. This relation is\nself-consistent in a wedge in M, \\dot{M} parameter space. If \\dot{M} is too low\nthe implied pair density over the poles of the black hole is below the\nGoldreich-Julian density and \\gamma\\gamma pair production is relatively\nunimportant; if \\dot{M} is too high the models are radiatively efficient. We\nalso argue that for a power-law spectrum the pair production rate should scale\nwith the observables L_X \\equiv X-ray luminosity and M as L_X^2 M^{-4}. We\nconfirm this numerically and argue that this relation likely holds even for\nradiatively efficient flows. The pair production rates are sensitive to black\nhole spin and to the ion-electron temperature ratio which are fixed in this\nexploratory calculation. We finish with a brief discussion of the implications\nfor Sgr A* and M87.", "category": "astro-ph_HE" }, { "text": "The blazar sequence 2.0: I discuss the spectral energy distribution (SED) of all blazars with redshift\ndetected by the {\\it Fermi} satellite and listed in the 3LAC catalog. I will\nupdate the so called \"blazar sequence\" from the phenomenological point of view,\nwith no theory or modelling. I will show that: i) pure data show that jet and\naccretion power are related; ii) the updated blazar sequence maintains the\nproperties of the old version, albeit with a less pronounced dominance of the\n$\\gamma$--ray emission; iii) at low bolometric luminosities, two different type\nof objects have the same high energy power: low black hole mass flat spectrum\nradio quasars and high mass BL Lacs. Therefore, at low luminosities, there is a\nvery large dispersion of SED shapes; iv) in low power BL Lacs, the contribution\nof the host galaxy is important. Remarkably, the luminosity distribution of the\nhost galaxies of BL Lacs are spread in a very narrow range; v) a simple sum of\ntwo smoothly joining power laws can describe the blazar SEDs very well.", "category": "astro-ph_HE" }, { "text": "Measuring the Spins of Accreting Black Holes: A typical galaxy is thought to contain tens of millions of stellar-mass black\nholes, the collapsed remnants of once massive stars, and a single nuclear\nsupermassive black hole. Both classes of black holes accrete gas from their\nenvironments. The accreting gas forms a flattened orbiting structure known as\nan accretion disk. During the past several years, it has become possible to\nobtain measurements of the spins of the two classes of black holes by modeling\nthe X-ray emission from their accretion disks. Two methods are employed, both\nof which depend upon identifying the inner radius of the accretion disk with\nthe innermost stable circular orbit (ISCO), whose radius depends only on the\nmass and spin of the black hole. In the Fe K method, which applies to both\nclasses of black holes, one models the profile of the\nrelativistically-broadened iron line with a special focus on the\ngravitationally redshifted red wing of the line. In the continuum-fitting\nmethod, which has so far only been applied to stellar-mass black holes, one\nmodels the thermal X-ray continuum spectrum of the accretion disk. We discuss\nboth methods, with a strong emphasis on the continuum-fitting method and its\napplication to stellar-mass black holes. Spin results for eight stellar-mass\nblack holes are summarized. These data are used to argue that the high spins of\nat least some of these black holes are natal, and that the presence or absence\nof relativistic jets in accreting black holes is not entirely determined by the\nspin of the black hole.", "category": "astro-ph_HE" }, { "text": "Inferring the core-collapse supernova explosion mechanism with\n three-dimensional gravitational-wave simulations: A detection of a core-collapse supernova signal with an Advanced LIGO and\nVirgo gravitational-wave detector network will allow us to measure\nastrophysical parameters of the source. In real advanced gravitational-wave\ndetector data there are transient noise artifacts that may mimic a true\ngravitational-wave signal. In this paper, we outline a procedure implemented in\nthe Supernova Model Evidence Extractor (SMEE) that determines if a\ncore-collapse supernova signal candidate is a noise artefact, a\nrapidly-rotating core-collapse supernova signal, or a neutrino explosion\nmechanism core-collapse supernova signal. Further to this, we use the latest\navailable three-dimensional gravitational-wave core-collapse supernova\nsimulations, and we outline a new procedure for the rejection of background\nnoise transients when only one detector is operational. We find the minimum SNR\nneeded to detect all waveforms is reduced when using three-dimensional\nwaveforms as signal models.", "category": "astro-ph_HE" }, { "text": "A New Transient Ultraluminous X-ray Source in NGC 7090: We report on the discovery of a new, transient ultraluminous X-ray source\n(ULX) in the galaxy NGC 7090. This new ULX, which we refer to as NGC 7090 ULX3,\nwas discovered via monitoring with $Swift$ during 2019-20, and to date has\nexhibited a peak luminosity of $L_{\\rm{X}} \\sim 6 \\times 10^{39}$ erg s$^{-1}$.\nArchival searches show that, prior to its recent transition into the ULX\nregime, ULX3 appeared to exhibit a fairly stable luminosity of $L_{\\rm{X}} \\sim\n10^{38}$ erg s$^{-1}$. Such strong long-timescale variability may be\nreminiscent of the small population of known ULX pulsars, although deep\nfollow-up observations with $XMM$-$Newton$ and $NuSTAR$ do not reveal any\nrobust X-ray pulsation signals. Pulsations similar to those seen from known ULX\npulsars cannot be completely excluded, however, as the limit on the pulsed\nfraction of any signal that remains undetected in these data is $\\lesssim$20\\%.\nThe broadband spectrum from these observations is well modelled with a simple\nthin disc model, consistent with sub-Eddington accretion, which may instead\nimply a moderately large black hole accretor ($M_{\\rm{BH}} \\sim 40 ~\nM_{\\odot}$). Similarly, though, more complex models consistent with the\nsuper-Eddington spectra seen in other ULXs (and the known ULX pulsars) cannot\nbe excluded given the limited signal-to-noise of the available broadband data.\nThe nature of the accretor powering this new ULX therefore remains uncertain.", "category": "astro-ph_HE" }, { "text": "Jet Launching from Merging Magnetized Binary Neutron Stars with\n Realistic Equations of State: We perform general relativistic, magnetohydrodynamic (GRMHD) simulations of\nbinary neutron stars in quasi-circular orbit that merge and undergo delayed or\nprompt collapse to a black hole (BH). The stars are irrotational and modeled\nusing an SLy or an H4 nuclear equation of state. To assess the impact of the\ninitial magnetic field configuration on jet launching, we endow the stars with\na purely poloidal magnetic field that is initially unimportant dynamically and\nis either confined to the stellar interior or extends from the interior into\nthe exterior as in typical pulsars. Consistent with our previous results, we\nfind that only the BH + disk remnants originating from binaries that form\nhypermassive neutron stars (HMNSs) and undergo delayed collapse can drive\nmagnetically-powered jets. We find that the closer the total mass of the binary\nis to the threshold value for prompt collapse, the shorter is the time delay\nbetween the gravitational wave peak amplitude and jet launching. This time\ndelay also strongly depends on the initial magnetic field configuration. We\nalso find that seed magnetic fields confined to the stellar interior can launch\na jet over $\\sim 25\\,\\rm ms$ later than those with pulsar-like magnetic fields.\nThe lifetime of the jet [$\\Delta t\\lesssim 150\\,\\rm ms$] and its outgoing\nPoynting luminosity [$L_{\\rm EM}\\sim 10^{52\\pm 1}\\rm erg/s$] are consistent\nwith typical short gamma-ray burst central engine lifetimes, as well as with\nthe Blandford--Znajek mechanism for launching jets and their associated\nPoynting luminosities. Our numerical results also suggest that the dynamical\nejection of matter can be enhanced by the magnetic field. Therefore, GRMHD\nstudies are required to fully understand kilonova signals from GW170818-like\nevents.", "category": "astro-ph_HE" }, { "text": "Black hole spin of Cyg X-1 determined from the softest state ever\n observed: We show the softest ever spectrum from Cyg~X-1, detected in 2013 with Suzaku.\nThis has the weakest high energy Compton tail ever seen from this object, so\nshould give the cleanest view of the underlying disk spectrum, and hence the\nbest determination of black hole spin from disk continuum fitting. Using the\nstandard model of a disk with simple non-thermal Comptonisation to produce the\nweak high energy tail gives a high spin black hole. However, we get a\nsignificantly better fit by including an additional, low temperature thermal\nComptonisation component, which allows a much lower black hole spin.\nCorroboration of the existence of an additional Compton component comes from\nthe frequency dependent hard lags seen in the rapid variability in archival\nhigh/soft state data. These can not be explained if the continuum is a single\nnon-thermal Comptonisation component, but are instead consistent with a\nradially stratified, multi zone Comptonisation spectrum, where the spectrum is\nsofter further from the black hole. A complex multi-zone Comptonisation\ncontinuum is required to explain both spectra and timing together, and this has\nan impact on the derived black hole spin.", "category": "astro-ph_HE" }, { "text": "A general relativistic model of accretion disks with coronae surrounding\n Kerr black holes: We calculate the structure of a standard accretion disk with corona\nsurrounding a massive Kerr black hole in general relativistic frame, in which\nthe corona is assumed to be heated by the reconnection of the strongly buoyant\nmagnetic fields generated in the cold accretion disk. The emergent spectra of\nthe accretion disk-corona systems are calculated by using the relativistic\nray-tracing method. We propose a new method to calculate the emergent\nComptonized spectra from the coronae. The spectra of the disk-corona systems\nwith a modified $\\alpha$-magnetic stress show that both the hard X-ray spectral\nindex and the hard X-ray bolometric correction factor $L_{\\rm bol}/L_{\\rm\nX,2-10keV}$ increase with the dimensionless mass accretion rate, which are\nqualitatively consistent with the observations of active galactic nuclei\n(AGNs). The fraction of the power dissipated in the corona decreases with\nincreasing black hole spin parameter $a$, which leads to lower electron\ntemperatures of the coronas for rapidly spinning black holes. The X-ray\nemission from the coronas surrounding rapidly spinning black holes becomes weak\nand soft. The ratio of the X-ray luminosity to the optical/UV luminosity\nincreases with the viewing angle, while the spectral shape in the X-ray band is\ninsensitive with the viewing angle. We find that the spectral index in the\ninfrared waveband depends on the mass accretion rate and the black hole spin\n$a$, which deviates from $f_\\nu\\propto\\nu^{1/3}$ expected by the standard thin\ndisk model.", "category": "astro-ph_HE" }, { "text": "Multifrequency Variability and Correlations from Extensive Observing\n Campaigns of Mkn 421 and Mkn 501 in 2009: We are performing an unprecedentedly long and dense monitoring of the\nmulti-frequency (radio to TeV) emission from the classical TeV blazars Mkn 421\nand Mkn 501. These objects are among the brightest X-ray/TeV blazars in the sky\nand among the few sources whose spectral energy distributions (SED) can be\ncompletely characterised by the current instruments. This is a multi-year and\nmulti-instrument programme which includes the participation of VLBA, Swift,\nRXTE, MAGIC, VERITAS, Whipple, the Fermi/LAT Gamma-ray Observatory, GASP-WEBT,\namong other collaborations and instruments which combined provide the most\ndetailed temporal and energy coverage of these sources to date. In this\nproceedings we will focus mostly on the results obtained with the\nmulti-frequency data from 2009, for which the SEDs of Mkn 421 and Mkn 501 are\nvery similar and can be described by a one-zone synchrotron self-Compton\nscenario. We will report on the multi-frequency variability derived from these\ndata.", "category": "astro-ph_HE" }, { "text": "Magnetically Advected Winds: Observations of X-ray absorption lines in magnetically driven disk winds\naround black hole binaries and active galactic nuclei yield a universal radial\ndensity profile rho proportional to r^{-1.2} in the wind. This is in\ndisagreement with the standard Blandford & Payne profile rho_BP proportional to\nr^{-1.5} expected when the magnetic field is neither advected nor diffusing\nthrough the accretion disk. In order to account for this discrepancy, we\nestablish a new paradigm for magnetically driven astrophysical winds according\nto which the large scale ordered magnetic field that threads the disk is\ncontinuously generated by the Cosmic Battery around the inner edge of the disk\nand continuously diffuses outward. We obtain self-similar solutions of such\nmagnetically advected winds (MAW) and discuss their observational\nramifications.", "category": "astro-ph_HE" }, { "text": "5.5 years multi-wavelength variability of Mrk 421: evidences of leptonic\n emission from the radio to TeV: Mrk 421 is a high-synchrotron-peaked blazar featuring bright and persistent\nGeV and TeV emission. We use the longest and densest ongoing unbiased observing\ncampaign obtained at TeV and GeV energies during 5.5 years with the FACT\ntelescope and the Fermi-LAT detector. The contemporaneous multi-wavelength\nobservations were used to characterize the variability of the source and to\nconstrain the underlying physical mechanisms. We study and correlate light\ncurves obtained by nine different instruments from radio to gamma rays and\nfound two significant results. The TeV and X-ray light curves are very well\ncorrelated with lag, if any, shorter than a day. The GeV light curve varies\nindependently and accurately leads the variations observed at long wavelengths,\nin particular in the radio band. We find that the observations match the\npredictions of leptonic models and suggest that the physical conditions vary\nalong the jet, when the emitting region moves outwards.", "category": "astro-ph_HE" }, { "text": "Investigating High Mass X-ray Binaries at hard X-rays with INTEGRAL: The INTEGRAL archive developed at INAF-IASF Milano with the available public\nobservations from late 2002 to 2016 is investigated to extract the X-ray\nproperties of 58 High Mass X-ray Binaries (HMXBs). This sample consists of\nsources hosting either a Be star (Be/XRBs) or an early-type supergiant\ncompanion (SgHMXBs), including the Supergiant Fast X-ray Transients (SFXTs).\nINTEGRAL light curves (sampled at 2 ks) are used to build their hard X-ray\nluminosity distributions, returning the source duty cycles, the range of\nvariability of the X-ray luminosity and the time spent in each luminosity\nstate. The phenomenology observed with INTEGRAL, together with the source\nvariability at soft X-rays taken from the literature, allows us to obtain a\nquantitative overview of the main sub-classes of massive binaries in accretion\n(Be/XRBs, SgHMXBs and SFXTs). Although some criteria can be derived to\ndistinguish them, some SgHMXBs exist with intermediate properties, bridging\ntogether persistent SgHMXBs and SFXTs.", "category": "astro-ph_HE" }, { "text": "PHEMTO : Polarimetric High Energy Modular Telescope Observatory: With the opening of the X and gamma--ray windows in the sixties, thanks to to\nsounding rockets and satellite-borne instruments, extremely energetic and\nviolent phenomena were discovered and subsequently found to be ubiquitous in\nthe Universe. Observations in the high energy domain are fundamental for\nunderstanding how matter is organized and behaves around black holes;\nunravelling how these extreme objects influence their environments on a very\nlarge scale; and finding the still elusive obscured massive objects in the\ncentre of galaxies. Other major problems in contemporary astrophysics, such as\nthe understanding of acceleration processes at shocks of all sizes (those of\npulsar wind nebulae, supernova remnants, but also at larger scales those of\nActive Galactic Nuclei radio lobes) in relation to the origin of cosmic-rays,\nor the definitive characterization of the debated non-thermal X-ray energy\ncontent of clusters of galaxies, also requires observations at very high\nenergies. An observatory type medium mission operating from around 1 keV to\nabout 600 keV can provide direct insights into these major questions. The\nessential characteristics will be coverage of the full energy range by\ntelescopes featuring a large throughput and arc-second resolution optics,\ncoupled to a compact focal plane assembly, with excellent imaging resolution\nand spectroscopy. In addition, the mission will provide unique polarimetry\nmeasurements in the hard X-ray domain, an important new diagnostic tool at\nenergies for which the non-thermal processes dominate. The Polarimetric\nHigh-Energy Modular Telescope Observatory (PHEMTO) is designed to have\nperformance several orders of magnitude better than the present hard X-ray\ninstruments. This gives to PHEMTO the improvements in scientific performance\nneeded for a mission in the 2050 era.", "category": "astro-ph_HE" }, { "text": "Variable jet properties in GRB110721A: Time resolved observations of the\n jet photosphere: {\\it Fermi Gamma-ray Space Telescope} observations of GRB110721A have\nrevealed two emission components from the relativistic jet: emission from the\nphotosphere, peaking at $\\sim 100$ keV and a non-thermal component, which peaks\nat $\\sim 1000$ keV. We use the photospheric component to calculate the\nproperties of the relativistic outflow. We find a strong evolution in the flow\nproperties: the Lorentz factor decreases with time during the bursts from\n$\\Gamma \\sim 1000$ to $\\sim 150$ (assuming a redshift $z=2$; the values are\nonly weakly dependent on unknown efficiency parameters). Such a decrease is\ncontrary to the expectations from the internal shocks and the isolated magnetar\nbirth models. Moreover, the position of the flow nozzle measured from the\ncentral engine, $r_0$, increases by more than two orders of magnitude. Assuming\na moderately magnetised outflow we estimate that $r_0$ varies from $10^6$ cm to\n$\\sim 10^9$ cm during the burst. We suggest that the maximal value reflects the\nsize of the progenitor core. Finally, we show that these jet properties\nnaturally explain the observed broken power-law decay of the temperature which\nhas been reported as a characteristic for GRB pulses.", "category": "astro-ph_HE" }, { "text": "LOFAR Detection of 110-188 MHz Emission and Frequency-Dependent Activity\n from FRB 20180916B: FRB 20180916B is a well-studied repeating fast radio burst source. Its\nproximity (~150 Mpc), along with detailed studies of the bursts, have revealed\nmany clues about its nature -- including a 16.3-day periodicity in its\nactivity. Here we report on the detection of 18 bursts using LOFAR at 110-188\nMHz, by far the lowest-frequency detections of any FRB to date. Some bursts are\nseen down to the lowest-observed frequency of 110 MHz, suggesting that their\nspectra extend even lower. These observations provide an order-of-magnitude\nstronger constraint on the optical depth due to free-free absorption in the\nsource's local environment. The absence of circular polarization and nearly\nflat polarization angle curves are consistent with burst properties seen at\n300-1700 MHz. Compared with higher frequencies, the larger burst widths\n(~40-160 ms at 150 MHz) and lower linear polarization fractions are likely due\nto scattering. We find ~2-3 rad/m^2 variations in the Faraday rotation measure\nthat may be correlated with the activity cycle of the source. We compare the\nLOFAR burst arrival times to those of 38 previously published and 22 newly\ndetected bursts from the uGMRT (200-450 MHz) and CHIME/FRB (400-800 MHz).\nSimultaneous observations show 5 CHIME/FRB bursts when no emission is detected\nby LOFAR. We find that the burst activity is systematically delayed towards\nlower frequencies by ~3 days from 600 MHz to 150 MHz. We discuss these results\nin the context of a model in which FRB 20180916B is an interacting binary\nsystem featuring a neutron star and high-mass stellar companion.", "category": "astro-ph_HE" }, { "text": "Dense matter equation of state for neutron star mergers: In simulations of binary neutron star mergers, the dense matter equation of\nstate (EOS) is required over wide ranges of density and temperature as well as\nunder conditions in which neutrinos are trapped, and the effects of magnetic\nfields and rotation prevail. Here we assess the status of dense matter theory\nand point out the successes and limitations of approaches currently in use. A\ncomparative study of the excluded volume (EV) and virial approaches for the\n$np\\alpha$ system using the equation of state of Akmal, Pandharipande and\nRavenhall for interacting nucleons is presented in the sub-nuclear density\nregime. Owing to the excluded volume of the $\\alpha$-particles, their mass\nfraction vanishes in the EV approach below the baryon density 0.1 fm$^{-3}$,\nwhereas it continues to rise due to the predominantly attractive interactions\nin the virial approach. The EV approach of Lattimer et al. is extended here to\ninclude clusters of light nuclei such as d, $^3$H and $^3$He in addition to\n$\\alpha$-particles. Results of the relevant state variables from this\ndevelopment are presented and enable comparisons with related but slightly\ndifferent approaches in the literature. We also comment on some of the sweet\nand sour aspects of the supra-nuclear EOS. The extent to which the neutron star\ngravitational and baryon masses vary due to thermal effects, neutrino trapping,\nmagnetic fields and rotation are summarized from earlier studies in which the\neffects from each of these sources were considered separately. Increases of\nabout $20\\% (\\gtrsim 50\\%)$ occur for rigid (differential) rotation with\ncomparable increases occurring in the presence of magnetic fields only for\nfields in excess of $10^{18}$ Gauss. Comparatively smaller changes occur due to\nthermal effects and neutrino trapping. Some future studies to gain further\ninsight into the outcome of dynamical simulations are suggested.", "category": "astro-ph_HE" }, { "text": "Hall cascades versus instabilities in neutron star magnetic fields: The Hall effect is an important nonlinear mechanism affecting the evolution\nof magnetic fields in neutron stars. Studies of the governing equation, both\ntheoretical and numerical, have shown that the Hall effect proceeds in a\nturbulent cascade of energy from large to small scales. We investigate the\nsmall-scale Hall instability conjectured to exist from the linear stability\nanalysis of Rheinhardt and Geppert. Identical linear stability analyses are\nperformed to find a suitable background field to model Rheinhardt and Geppert's\nideas. The nonlinear evolution of this field is then modelled using a\nthree-dimensional pseudospectral numerical MHD code. Combined with the\nbackground field, energy was injected at the ten specific eigenmodes with the\ngreatest positive eigenvalues as inferred by the linear stability analysis.\nEnergy is transferred to different scales in the system, but not into small\nscales to any extent that could be interpreted as a Hall instability. Any\ninstabilities are overwhelmed by a late-onset turbulent Hall cascade, initially\navoided by the choice of background field, but soon generated by nonlinear\ninteractions between the growing eigenmodes. The Hall cascade is shown here,\nand by several authors elsewhere, to be the dominant mechanism in this system.", "category": "astro-ph_HE" }, { "text": "Suzaku Observation of X-ray Variability in Soft State LMC X-1: This paper reports the results of Suzaku observation of the spectral\nvariation of the black hole binary LMCX-1 in the soft state. The observationwas\ncarried out in 2009 from July 21 to 24. the obtained net count rate was\n$\\sim$30 counts s$^{-1}$ in the 0.5--50 keV band with $\\sim$10% peak-to-peak\nflux variation. The time-averaged X-ray spectrum cannot be described by a\nmulti-color disk and single Compton component with its reflection, but requires\nadditional Comptonized emissions. This double Compton component model allows a\nslightly larger inner radius of the multi-color disk, implying a lower spin\nparameter. Significant spectral evolution was observed above 8 keV along with a\nflux decrease on a timescale of $\\sim$10$^4$--10$^5$ s. By spectral fitting, we\nshow that this behavior is well explained by changes in the hard Comptonized\nemission component in contrast to the maintained disk and soft Comptonized\nemission.", "category": "astro-ph_HE" }, { "text": "Observation of TeV Gamma Rays from the Fermi Bright Galactic Sources\n with the Tibet Air Shower Array: Using the Tibet-III air shower array, we search for TeV gamma-rays from 27\npotential Galactic sources in the early list of bright sources obtained by the\nFermi Large Area Telescope at energies above 100 MeV. Among them, we observe 7\nsources instead of the expected 0.61 sources at a significance of 2 sigma or\nmore excess. The chance probability from Poisson statistics would be estimated\nto be 3.8 x 10^-6. If the excess distribution observed by the Tibet-III array\nhas a density gradient toward the Galactic plane, the expected number of\nsources may be enhanced in chance association. Then, the chance probability\nrises slightly, to 1.2 x 10^-5, based on a simple Monte Carlo simulation. These\nlow chance probabilities clearly show that the Fermi bright Galactic sources\nhave statistically significant correlations with TeV gamma-ray excesses. We\nalso find that all 7 sources are associated with pulsars, and 6 of them are\ncoincident with sources detected by the Milagro experiment at a significance of\n3 sigma or more at the representative energy of 35 TeV. The significance maps\nobserved by the Tibet-III air shower array around the Fermi sources, which are\ncoincident with the Milagro >=3sigma sources, are consistent with the Milagro\nobservations. This is the first result of the northern sky survey of the Fermi\nbright Galactic sources in the TeV region.", "category": "astro-ph_HE" }, { "text": "Is the 130 GeV Line Real? A Search for Systematics in the Fermi-LAT Data: Our recent claims of a Galactic center feature in Fermi-LAT data at\napproximately 130 GeV have prompted an avalanche of papers proposing\nexplanations ranging from dark matter annihilation to exotic pulsar winds.\nBecause of the importance of such interpretations for physics and astrophysics,\na discovery will require not only additional data, but a thorough investigation\nof possible LAT systematics. While we do not have access to the details of each\nevent reconstruction, we do have information about each event from the public\nevent lists and spacecraft parameter files. These data allow us to search for\nsuspicious trends that could indicate a spurious signal. We consider several\nhypotheses that might make an instrumental artifact more apparent at the\nGalactic center, and find them implausible. We also search for an instrumental\nsignature in the Earth limb photons, which provide a smooth reference spectrum\nfor null tests. We find no significant 130 GeV feature in the Earth limb\nsample. However, we do find a marginally significant 130 GeV feature in Earth\nlimb photons with a limited range of detector incidence angles. This raises\nconcerns about the 130 GeV Galactic center feature, even though we can think of\nno plausible model of instrumental behavior that connects the two. A modest\namount of additional limb data would tell us if the limb feature is a\nstatistical fluke. If the limb feature persists, it would raise doubts about\nthe Pass 7 processing of E > 100 GeV events. At present we find no instrumental\nsystematics that could plausibly explain the excess Galactic center emission at\n130 GeV.", "category": "astro-ph_HE" }, { "text": "Cooling and Instabilities in Colliding Flows: Collisional self-interactions occurring in protostellar jets give rise to\nstrong shocks, the structure of which can be affected by radiative cooling\nwithin the flow. To study such colliding flows, we use the AstroBEAR AMR code\nto conduct hydrodynamic simulations in both one and three dimensions with a\npower law cooling function. The characteristic length and time scales for\ncooling are temperature dependent and thus may vary as shocked gas cools. When\nthe cooling length decreases sufficiently rapidly the system becomes unstable\nto the radiative shock instability, which produces oscillations in the position\nof the shock front; these oscillations can be seen in both the one and three\ndimensional cases. Our simulations show no evidence of the density clumping\ncharacteristic of a thermal instability, even when the cooling function meets\nthe expected criteria. In the three-dimensional case, the nonlinear thin shell\ninstability (NTSI) is found to dominate when the cooling length is sufficiently\nsmall. When the flows are subjected to the radiative shock instability,\noscillations in the size of the cooling region allow NTSI to occur at larger\ncooling lengths, though larger cooling lengths delay the onset of NTSI by\nincreasing the oscillation period.", "category": "astro-ph_HE" }, { "text": "Multiwavelength Study of an X-ray Tidal Disruption Event Candidate in\n NGC 5092: We present multiwavelength studies of a transient X-ray source, XMMSL1\nJ131952.3+225958, associated with the galaxy NGC 5092 at $z=0.023$ detected in\nthe XMM-Newton SLew survey (XMMSL). The source brightened in the 0.2--2 keV\nband by a factor of $>20$ in 2005 as compared with previous flux limits and\nthen faded by a factor of $>200$ as observed with it XMM-Newton in 2013 and\nwith it Swift in 2018. At the flaring state, the X-ray spectrum can be modeled\nwith a blackbody at a temperature of $\\sim$ 60 eV and an overall luminosity of\n$\\sim$ $1.5 \\times 10^{43}$ erg s$^{-1}$. A UV flare and optical flare were\nalso detected with the Galaxy Evolution Explorer and the Sloan Digital Sky\nSurvey, respectively, within several months of the X-ray flare, whose\nnonstellar UV--optical spectrum can be described with a blackbody at a\ntemperature of $\\sim$ $(1-2) \\times 10^4$ K and a luminosity of $\\sim$ $(2-6)\n\\times 10^{43}$ erg s$^{-1}$. Interestingly, mid-infrared monitoring\nobservations of NGC 5092 with the Wide-field Infrared Survey Explorer 5--13 yr\nlater show a continuous flux decline. These dramatic variability properties,\nfrom the X-ray through UV and optical to infrared, appear to be orderly,\nsuggestive of a stellar tidal disruption event (TDE) by a massive black hole,\nconfirming the postulation by Kanner et al.(2013). This TDE candidate belongs\nto a rare sample with contemporaneous bright emission detected in the X-ray,\nUV, and optical, which are later echoed by dust-reprocessed light in the\nmid-infrared. The black hole has a mass of $\\sim$ $5 \\times 10^{7} \\rm\nM_{\\odot}$, residing in a galaxy that is dominated by a middle-aged stellar\npopulation of 2.5 Gyr.", "category": "astro-ph_HE" }, { "text": "A Search for Fast Optical Transients in the Pan-STARRS1 Medium-Deep\n Survey: M Dwarf Flares, Asteroids, Limits on Extragalactic Rates, and\n Implications for LSST: [Abridged] We present a search for fast optical transients (~0.5 hr-1 day)\nusing repeated observations of the Pan-STARRS1 Medium-Deep Survey (PS1/MDS)\nfields. Our search takes advantage of the consecutive g/r-band observations\n(16.5 min in each filter), by requiring detections in both bands, with\nnon-detections on preceding and subsequent nights. We identify 19 transients\nbrighter than 22.5 AB mag (S/N>10). Of these, 11 events exhibit quiescent\ncounterparts in the deep PS1/MDS templates that we identify as M4-M9 dwarfs.\nThe remaining 8 transients exhibit a range of properties indicative of\nmain-belt asteroids near the stationary point of their orbits. With\nidentifications for all 19 transients, we place an upper limit of\nR_FOT(0.5hr)<0.12 deg^-2 d^-1 (95% confidence level) on the sky-projected rate\nof extragalactic fast transients at <22.5 mag, a factor of 30-50 times lower\nthan previous limits; the limit for a timescale of ~day is R_FOT<2.4e-3 deg^-2\nd^-1. To convert these sky-projected rates to volumetric rates, we explore the\nexpected peak luminosities of fast optical transients powered by various\nmechanisms, and find that non-relativistic events are limited to M~-10 mag\n(M~-14 mag) for a timescale of ~0.5 hr (~day), while relativistic sources\n(e.g., GRBs, magnetar-powered transients) can reach much larger luminosities.\nThe resulting volumetric rates are <13 (M~-10 mag), <0.05 (M~-14 mag) and <1e-6\nMpc^-3 yr^-1 (M~-24 mag), significantly above the nova, supernova, and GRB\nrates, respectively, indicating that much larger surveys are required to\nprovide meaningful constraints. Motivated by the results of our search we\ndiscuss strategies for identifying fast optical transients in the LSST main\nsurvey, and reach the optimistic conclusion that the veil of foreground\ncontaminants can be lifted with the survey data, without the need for expensive\nfollow-up observations.", "category": "astro-ph_HE" }, { "text": "The Swift/BAT Hard X-ray Transient Monitor: The Swift/Burst Alert Telescope (BAT) hard X-ray transient monitor provides\nnear real-time coverage of the X-ray sky in the energy range 15-50 keV. The BAT\nobserves 88% of the sky each day with a detection sensitivity of 5.3 mCrab for\na full-day observation and a time resolution as fine as 64 seconds. The three\nmain purposes of the monitor are (1) the discovery of new transient X-ray\nsources, (2) the detection of outbursts or other changes in the flux of known\nX-ray sources, and (3) the generation of light curves of more than 900 sources\nspanning over eight years. The primary interface for the BAT transient monitor\nis a public web page. Between 2005 February 12 and 2013 April 30, 245 sources\nhave been detected in the monitor, 146 of them persistent and 99 detected only\nin outburst. Among these sources, 17 were previously unknown and were\ndiscovered in the transient monitor. In this paper, we discuss the methodology\nand the data processing and filtering for the BAT transient monitor and review\nits sensitivity and exposure. We provide a summary of the source detections and\nclassify them according to the variability of their light curves. Finally, we\nreview all new BAT monitor discoveries; for the new sources that are previously\nunpublished, we present basic data analysis and interpretations.", "category": "astro-ph_HE" }, { "text": "The High Time Resolution Universe Pulsar Survey - VII: discovery of five\n millisecond pulsars and the different luminosity properties of binary and\n isolated recycled pulsars: This paper presents the discovery and timing parameters for five millisecond\npulsars (MSPs), four in binary systems with probable white dwarf companions and\none isolated, found in ongoing processing of the High Time Resolution Universe\nPulsar Survey (HTRU). We also present high quality polarimetric data on four of\nthem. These further discoveries confirm the high potential of our survey in\nfinding pulsars with very short spin periods. At least two of these five MSPs\nare excellent candidates to be included in the Pulsar Timing Array projects.\nThanks to the wealth of MSP discoveries in the HTRU survey, we revisit the\nquestion of whether the luminosity distributions of isolated and binary MSPs\nare different. Using the Cordes and Lazio distance model and our new and\ncatalogue flux density measurements, we find that 41 of the 42 most luminous\nMSPs in the Galactic disk are in binaries and a statistical analysis suggests\nthat the luminosity functions differ with 99.9% significance. We conclude that\nthe formation process that leads to solitary MSPs affects their luminosities,\ndespite their period and period derivatives being similar to those of pulsars\nin binary systems.", "category": "astro-ph_HE" }, { "text": "Constraints on Proton Synchrotron Origin of Very High Energy Gamma Rays\n from the Extended Jet of AP Librae: The multi-wavelength photon spectrum from the BL Lac object AP Librae extends\nfrom radio to TeV gamma rays. The X-ray to very high energy gamma ray emission\nfrom the extended jet of this source has been modeled earlier with inverse\nCompton (IC) scattering of relativistic electrons off the CMB photons. The\nIC/CMB model requires the kpc scale extended jet to be highly collimated with\nbulk Lorentz factor close to 10. Here we discuss the possibility of proton\nsynchrotron origin of X-rays and gamma-rays from the extended jet with bulk\nLorentz factor 3. This scenario requires extreme energy of protons and high\nmagnetic field 1 mG of the extended jet with very high jet power in particles\nand magnetic field (which is more than 100 times the Eddington luminosity of AP\nLibrae) to explain the very high energy gamma ray emission. Moreover, we have\nshown that X-ray emission from the extended jets of 3C 273 and PKS 0637-752\ncould be possible by proton synchrotron emission with jet powers comparable to\ntheir Eddington luminosities.", "category": "astro-ph_HE" }, { "text": "A falling Corona model for the anomalous behavior of the broad emission\n lines in NGC 5548: NGC 5548 has been intensively monitored by the AGN Space Telescope and\nOptical Reverberation Mapping collaboration. Approximately after half of the\nlight curves, the correlation between the broad emission lines and the\nlag-corrected ultraviolet continua becomes weak. This anomalous behavior is\naccompanied by an increase of soft X-ray emission. We propose a simple model to\nunderstand this anomalous behavior, i.e., the corona might fall down, thereby\nincreasing the covering fraction of the inner disk. Therefore, X-ray and\nextreme ultraviolet emission suffer from spectral variations. The ultraviolet\ncontinua variations are driven by both X-ray and extreme ultraviolet\nvariations. Consequently, the spectral variability induced by the falling\ncorona would dilute the correlation between the broad emission lines and the\nultraviolet continua. Our model can explain many additional observational\nfacts, including the dependence of the anomalous behavior on velocity and\nionization energy. We also show that the time lag and correlation between the\nX-ray and the ultraviolet variations change as NGC 5548 displays the anomalous\nbehavior. The time lag is dramatically longer than the expectation from disk\nreprocessing if the anomalous behavior is properly excluded. During the\nanomalous state, the time lag approaches the light-travel timescale of disk\nreprocessing albeit with a much weaker correlation. We speculate that the time\nlag in the normal state is caused by reprocessing of the broad line region gas.\nAs NGC 5548 enters the abnormal state, the contribution of the broad line\nregion gas is smaller; the time lag reflects disk reprocessing. We also discuss\nalternative scenarios.", "category": "astro-ph_HE" }, { "text": "Swift X-ray and ultraviolet observations of the shortest orbital period\n double-degenerate system RX J0806.3+1527 (HM Cnc): The system RX J0806.3+1527 (HM Cnc) is a pulsating X-ray source with 100 per\ncent modulation on a period of 321.5 s (5.4 min). This period reflects the\norbital motion of a close binary consisting of two interacting white dwarfs.\nHere we present a series of simultaneous X-ray (0.2-10 keV) and\nnear-ultraviolet (2600 angstrom and 1928 angstrom) observations that were\ncarried out with the Swift satellite. In the near-ultraviolet, the counterpart\nof RX J0806.3+1527 was detected at flux densities consistent with a blackbody\nwith temperature 27E+3 K. We found that the emission at 2600 angstrom is\nmodulated at the 321.5-s period with the peak ahead of the X-ray one by 0.28\ncycles and is coincident within 0.05 cycles with the optical. This phase-shift\nmeasurement confirms that the X-ray hot spot (located on the primary white\ndwarf) is at about 80-100 degrees from the direction that connects the two\nwhite dwarfs. Albeit at lower significance, the 321.5-s signature is present\nalso in the 1928-angstrom data; at this wavelength, however, the pulse peak is\nbetter aligned with that observed at X-rays. We use the constraints on the\nsource luminosity and the geometry of the emitting regions to discuss the\nmerits and limits of the main models for RX J0806.3+1527.", "category": "astro-ph_HE" }, { "text": "X-ray Spectroscopy of the gamma-ray Brightest Nova V906 Car\n (ASASSN-18fv): Shocks in gamma-ray emitting classical novae are expected to produce bright\nthermal and non-thermal X-rays. We test this prediction with simultaneous\nNuSTAR and Fermi/LAT observations of nova V906 Car, which exhibited the\nbrightest GeV gamma-ray emission to date. The nova is detected in hard X-rays\nwhile it is still gamma-ray bright, but contrary to simple theoretical\nexpectations, the detected 3.5-78 keV emission of V906 Car is much weaker than\nthe simultaneously observed >100 MeV emission. No non-thermal X-ray emission is\ndetected, and our deep limits imply that the gamma-rays are likely hadronic.\nAfter correcting for substantial absorption (N_H ~ 2 x 10^23 cm^-2), the\nthermal X-ray luminosity (from a 9 keV optically-thin plasma) is just ~2% of\nthe gamma-ray luminosity. We consider possible explanations for the low thermal\nX-ray luminosity, including the X-rays being suppressed by corrugated,\nradiative shock fronts or the X-rays from the gamma-ray producing shock are\nhidden behind an even larger absorbing column (N_H >10^25 cm^-2). Adding\nXMM-Newton and Swift/XRT observations to our analysis, we find that the\nevolution of the intrinsic X-ray absorption requires the nova shell to be\nexpelled 24 days after the outburst onset. The X-ray spectra show that the\nejecta are enhanced in nitrogen and oxygen, and the nova occurred on the\nsurface of a CO-type white dwarf. We see no indication of a distinct super-soft\nphase in the X-ray lightcurve, which, after considering the absorption effects,\nmay point to a low mass of the white dwarf hosting the nova.", "category": "astro-ph_HE" }, { "text": "Accretion disc by Roche lobe overflow in the supergiant fast X-ray\n transient IGR J08408-4503: Supergiant fast X-ray transients (SFXTs) are X-ray binary systems with a\nsupergiant companion and likely a neutron star, which show a fast ($\\sim 10^3$\ns) and high variability with a dynamic range up to $10^{5-6}$. Given their\nextreme properties, they are considered among the most valuable laboratories to\ntest accretion models. Recently, the orbital parameters of a member of this\nclass, IGR J08408-4503, were obtained from optical observations. We used this\ninformation, together with X-ray observations from previous publications and\nnew results from X-ray and optical data collected by INTEGRAL and presented in\nthis work, to study the accretion mechanisms at work in IGR J08408-4503. We\nfound that the high eccentricity of the compact object orbit and the large size\nof the donor star imply Roche lobe overflow (RLO) around the periastron. It is\nalso likely that a fraction of the outer layers of the photosphere of the donor\nstar are lost from the Lagrangian point $L_2$ during the periastron passages.\nOn the basis of these findings, we discuss the flaring variability of IGR\nJ08408-4503 assuming the presence of an accretion disc. We point out that IGR\nJ08408-4503 may not be the only SFXT with an accretion disc fueled by RLO.\nThese findings open a new scenario for accretion mechanisms in SFXTs, since\nmost of them have so far been based on the assumption of spherically symmetric\naccretion.", "category": "astro-ph_HE" }, { "text": "What Powered the Kilonova-Like Emission After GRB 230307A in the\n Framework of a Neutron Star-White Dwarf Merger?: The second brightest gamma-ray burst, GRB 230307A (with a duration $T_{90}$ ~\n40 s), exhibited characteristics indicative of a magnetar engine during the\nprompt emission phase. Notably, a suspected kilonova was identified in its\nfollow-up optical and infrared observations. Here we propose that the origin of\nGRB 230307A is a neutron star-white dwarf (NS-WD) merger, as this could\nnaturally interpret the long duration and the large physical offset from the\ncenter of its host galaxy. In the framework of such a NS-WD merger event, the\nlate-time kilonova-like emission is very likely to be powered by the spin-down\nof the magnetar and the radioactive decay of $^{56}$Ni, rather than by the\ndecay of r-process elements as these heavy elements may not be easy to be\nsynthesized in a NS-WD merger. It is demonstrated that the above scenario can\nbe supported by our fit to the late-time observational data, where a mass of ~\n$10^{-3} \\ \\rm M_{\\odot}$ $^{56}$Ni is involved in the ejecta of a mass of ~\n$0.1 \\ \\rm M_{\\odot}$. Particularly, the magnetar parameters required by the\nfit are consistent with those derived from the early X-ray observation.", "category": "astro-ph_HE" }, { "text": "The Knee and the Second Knee of the Cosmic-Ray Energy Spectrum: The cosmic ray flux measured by the Telescope Array Low Energy Extension\n(TALE) exhibits three spectral features: the knee, the dip in the $10^{16}$ eV\ndecade, and the second knee. Here the spectrum has been measured for the first\ntime using fluorescence telescopes, which provide a calorimetric,\nmodel-independent result. The spectrum appears to be a rigidity-dependent\ncutoff sequence, where the knee is made by the hydrogen and helium portions of\nthe composition, the dip comes from the reduction in composition from helium to\nmetals, the rise to the second knee occurs due to intermediate range nuclei,\nand the second knee is the iron knee.", "category": "astro-ph_HE" }, { "text": "Revisit of Local X-ray Luminosity Function of Active Galactic Nuclei\n with the MAXI Extragalactic Survey: We construct a new X-ray (2--10 keV) luminosity function of Compton-thin\nactive galactic nuclei (AGNs) in the local universe, using the first MAXI/GSC\nsource catalog surveyed in the 4--10 keV band. The sample consists of 37\nnon-blazar AGNs at $z=0.002-0.2$, whose identification is highly ($>97%$)\ncomplete. We confirm the trend that the fraction of absorbed AGNs with $N_{\\rm\nH} > 10^{22}$ cm$^{-2}$ rapidly decreases against luminosity ($L_{\\rm X}$),\nfrom 0.73$\\pm$0.25 at $L_{\\rm X} = 10^{42-43.5}$ erg s$^{-1}$ to 0.12$\\pm0.09$\nat $L_{\\rm X} = 10^{43.5-45.5}$ erg s$^{-1}$. The obtained luminosity function\nis well fitted with a smoothly connected double power-law model whose indices\nare $\\gamma_1 = 0.84$ (fixed) and $\\gamma_2 = 2.0\\pm0.2$ below and above the\nbreak luminosity, $L_{*} = 10^{43.3\\pm0.4}$ ergs s$^{-1}$, respectively. While\nthe result of the MAXI/GSC agrees well with that of HEAO-1 at $L_{\\rm X} \\gtsim\n10^{43.5}$ erg s$^{-1}$, it gives a larger number density at the lower\nluminosity range. Comparison between our luminosity function in the 2--10 keV\nband and that in the 14--195 keV band obtained from the Swift/BAT survey\nindicates that the averaged broad band spectra in the 2--200 keV band should\ndepend on luminosity, approximated by $\\Gamma\\sim1.7$ for $L_{\\rm X} \\ltsim\n10^{44}$ erg s$^{-1}$ while $\\Gamma\\sim 2.0$ for $L_{\\rm X} \\gtsim 10^{44}$ erg\ns$^{-1}$. This trend is confirmed by the correlation between the luminosities\nin the 2--10 keV and 14--195 keV bands in our sample. We argue that there is no\ncontradiction in the luminosity functions between above and below 10 keV once\nthis effect is taken into account.", "category": "astro-ph_HE" }, { "text": "The influence of outflows on the 1/f-like luminosity fluctuations: In accretion systems, outflows may have significant influence on the\nluminosity fluctuations. In this paper, following the Lyubarskii's general\nscheme, we revisit the power spectral density of luminosity fluctuations by\ntaking into account the role of outflows. Our analysis is based on the\nassumption that the coupling between the local outflow and inflow is weak on\nthe accretion rate fluctuations. We find that, for the inflow mass accretion\nrate $\\dot M \\propto r^{s}$, the power spectrum of flicker noise component will\npresent a power-law distribution $p(f) \\propto f^{-(1+4s/3)}$ for\nadvection-dominated flows. We also obtain descriptions of $p(f)$ for both\nstandard thin discs and neutrino-cooled discs, which show that the power-law\nindex of a neutrino-cooled disc is generally larger than that of a\nphoton-cooled disc. Furthermore, the obtained relationship between $p(f)$ and\n$s$ indicates the possibility of evaluating the strength of outflows by the\npower spectrum in X-ray binaries and gamma-ray bursts. In addition, we discuss\nthe possible influence of the outflow-inflow coupling on our results.", "category": "astro-ph_HE" }, { "text": "Evolution of the X-ray Properties of the Transient Magnetar XTE\n J1810-197: We report on X-ray observations of the 5.54 s transient magnetar XTE\nJ1810-197 using the XMM-Newton and Chandra observatories, analyzing new data\nfrom 2008 through 2014, and re-analyzing data from 2003 through 2007 with the\nbenefit of these six years of new data. From the discovery of XTE J1810-197\nduring its 2003 outburst to the most recent 2014 observations, its 0.3-10 keV\nX-ray flux has declined by a factor of about 50 from 4.1E-11 to 8.1E-13\nerg/cm^2/s. Its X-ray spectrum has now reached a steady state. Pulsations\ncontinue to be detected from a 0.3 keV thermal hot-spot that remains on the\nneutron star surface. The luminosity of this hot-spot exceeds XTE J1810-197's\nspin down luminosity, indicating continuing magnetar activity. We find that XTE\nJ1810-197's X-ray spectrum is best described by a multiple component blackbody\nmodel in which the coldest 0.14 keV component likely originates from the entire\nneutron star surface, and the thermal hot-spot is, at different epochs, well\ndescribed by an either one or two-component blackbody model. A 1.2 keV\nabsorption line, possibly due to resonant proton scattering, is detected at all\nepochs. The X-ray flux of the hot spot decreased by approximately 20% between\n2008 March and 2009 March, the same period during which XTE J1810-197 became\nradio quiet.", "category": "astro-ph_HE" }, { "text": "Resonant scattering in the Perseus Cluster: spectral model for\n constraining gas motions with Astro-H: X-ray spectra from cores of galaxy clusters can be strongly distorted by\nresonant scattering of line photons, affecting metal abundance and gas velocity\nmeasurements. We introduce simulated spectral models that take into account the\nresonant scattering effect, radial variations of thermodynamic properties of\nthe hot gas, projection effects and small-scale isotropic gas motions. The key\nfeature of the models is that all these effects are treated self-consistently\nfor the whole spectrum, rather than for individual lines. The model spectra are\npublicly available and can be used for direct comparison with observed\nprojected spectra. Comparison with the existing XMM-Newton and Chandra data of\nthe Perseus Cluster shows that even though there is no strong evidence for the\nresonant scattering in Perseus, the low energy resolution of the X-ray CCDs is\nnot sufficient to robustly distinguish spectral distortions due to the resonant\nscattering, different metal abundance profiles and different levels of gas\nturbulence. Future Astro-H data will resolve most of the problems we are facing\nwith CCDs. With the help of our models, the resonant scattering analysis can be\ndone self-consistently using the whole spectral information, constraining the\nlevel of gas turbulence already with a 100 ks observation with Astro-H.", "category": "astro-ph_HE" }, { "text": "Constraints on MeV dark matter and primordial black holes: Inverse\n Compton signals at the SKA: We investigate the possibilities for probing MeV dark matter (DM) particles\nand primordial black holes (PBHs) (for masses $\\sim 10^{15}$--$10^{17}$ g) at\nthe upcoming radio telescope SKA, using photon signals from the Inverse Compton\n(IC) effect within a galactic halo. Pair-annihilation or decay of MeV DM\nparticles (into $e^+ e^-$ pairs) or Hawking radiation from a population of PBHs\ngenerates mildly relativistic $e^{\\pm}$ which can lead to radio signals through\nthe IC scattering on low energy cosmic microwave background (CMB) photons. We\nstudy the ability of SKA to detect such signals coming from nearby ultra-faint\ndwarf galaxies Segue I and Ursa Major II as well as the globular cluster\n$\\omega$-cen and the Coma cluster. We find that with $\\sim 100$ hours of\nobservation, the SKA improves the Planck constraints on the DM\nannihilation/decay rate and the PBH abundance for masses in the range $\\sim 1$\nto few tens of MeV and above $10^{15}$ to $10^{17}$ g, respectively.\nImportantly, the SKA limits are independent of the assumed magnetic fields\nwithin the galaxies. Previously allowed regions of diffusion parameters of MeV\nelectrons inside a dwarf galaxy that give rise to observable signals at the SKA\nare also excluded. For objects like dwarf galaxies, predicted SKA constraints\ndepend on both the DM and diffusion parameters. Independent observations in\ndifferent frequency bands, e.g., radio and $\\gamma$-ray frequencies, may break\nthis degeneracy and thus enable one to constrain the combined parameter space\nof DM and diffusion. However, the constraints are independent of diffusion\nparameters for galaxy clusters such as Coma.", "category": "astro-ph_HE" }, { "text": "Three-flavor collective neutrino conversions with multi-azimuthal-angle\n instability in an electron-capture supernova model: We investigate the multi-azimuthal angle (MAA) effect on collective neutrino\noscillation by considering the three-dimensional neutrino momentum distribution\nin a realistic electron-capture supernova model with an $8.8 M_{\\odot}$\nprogenitor. We find that the MAA effect induces collective flavor conversions\nat epochs when it is completely suppressed under the axial-symmetric\napproximation. This novel activity is switched on/off by the growth of the MAA\ninstability and imprints additional time evolution in the expected neutrino\nevent rate. We validate our results by extending the linear stability analysis\ninto the three-flavor scheme including mixing angles, and confirm that the\nonset of collective neutrino oscillation matches the steep growth of flavor\ninstability. We discuss how the MAA effect alters neutrino detection at\nSuper-Kamiokande and DUNE.", "category": "astro-ph_HE" }, { "text": "Possible role of magnetic reconnection in the electromagnetic\n counterpart of binary black hole merger: We propose a qualitative scenario to interpret the argued association between\nthe direct measurement of the gravitational wave event GW150914 by Laser\nInterferometer Gravitational Wave Observatory (LIGO)-Virgo collaborations and\nthe hard $X$-ray transient detected by Fermi-Gamma-ray Burst Monitor (GBM)\n$0.4$ sec after. In a binary system of two gravitationally collapsing objects\nwith a non-vanishing electric charge, the compenetration of the two\nmagnetospheres occurring during the coalescence, through magnetic reconnection,\nproduces a highly collimated relativistic outflow that becomes optically thin\nand shines in the GBM field of view. We propose that this process should be\nexpected as a commonplace in the future joint gravitational/electromagnetic\ndetections and, in case of neutron star-neutron star merger event, might lead\nto detectable $X$- or $\\gamma$-ray precursors to, or transients associated\nwith, the gravitational bursts.", "category": "astro-ph_HE" }, { "text": "The Ultra-Fast Outflow of the Quasar PG 1211+143 as Viewed by\n Time-Averaged Chandra Grating Spectroscopy: We present a detailed X-ray spectral study of the quasar PG 1211+143 based on\nChandra High Energy Transmission Grating Spectrometer (HETGS) observations\ncollected in a multi-wavelength campaign with UV data using the Hubble Space\nTelescope Cosmic Origins Spectrograph (HST-COS) and radio bands using the\nJansky Very Large Array (VLA). We constructed a multi-wavelength ionizing\nspectral energy distribution using these observations and archival infrared\ndata to create XSTAR photoionization models specific to the PG 1211+143 flux\nbehavior during the epoch of our observations. Our analysis of the\nChandra-HETGS spectra yields complex absorption lines from H-like and He-like\nions of Ne, Mg and Si which confirm the presence of an ultra-fast outflow (UFO)\nwith a velocity ~ $-$17,300 km s$^{-1}$ (outflow redshift $z_{\\rm out}$ ~\n$-$0.0561) in the rest frame of PG 1211+143. This absorber is well described by\nan ionization parameter $\\log \\xi$ ~ 2.9 erg s$^{-1}$ cm and column density\n$\\log N_{\\rm H}$ ~ 21.5 cm$^{-2}$. This corresponds to a stable region of the\nabsorber's thermal stability curve, and furthermore its implied neutral\nhydrogen column is broadly consistent with a broad Ly$\\alpha$ absorption line\nat a mean outflow velocity of ~ $-$16,980 km s$^{-1}$ detected by our HST-COS\nobservations. Our findings represent the first simultaneous detection of a UFO\nin both X-ray and UV observations. Our VLA observations provide evidence for an\nactive jet in PG 1211+143, which may be connected to the X-ray and UV outflows;\nthis possibility can be evaluated using very-long-baseline interferometric\n(VLBI) observations.", "category": "astro-ph_HE" }, { "text": "AKARI Near-infrared Spectral Observations of Shocked H2 Gas of the\n Supernova Remnant IC 443: We present near-infrared (2.5 - 5.0 um) spectra of shocked H2 gas in the\nsupernova remnant IC 443, obtained with the satellite AKARI. Three shocked\nclumps-known as B, C, and G-and one background region were observed, and only\nH2 emission lines were detected. Except the clump B, the extinctioncorrected\nlevel population shows the ortho-to-para ratio of ~ 3.0. From the level\npopulation of the clumps C and G-both AKARI's only and the one extended with\nprevious mid-infrared observations-we found that the v = 0 levels are more\npopulated than the v > 0 levels at a fixed level energy, which cannot be\nreproduced by any combination of H2 gas in Local Thermodynamic Equilibrium. The\npopulations are described by the two-density power-law thermal admixture model,\nrevised to include the collisions with H atoms. We attributed the lower\n(n(H2)=10^(2.8-3.8) cm-3) and higher (n(H2)=10^(5.4-5.8) cm-3) density gases to\nthe shocked H2 gas behind C-type and J-type shocks, respectively, based on\nseveral arguments including the obtained high H I abundance n(H I)/n(H2)=0.01.\nUnder the hierarchical picture of molecular clouds, the C-type and J-type\nshocks likely propagate into \"clumps\" and \"clouds\" (interclump media),\nrespectively. The power-law index b of 1.6 and 3.5, mainly determined by the\nlower density gas, is attributed to the shock-velocity diversity, which may be\na natural result during shock-cloud interactions. According to our results, H2\nv = 1 - 0 S(1) emission is mainly from J-shocks propagating into interclump\nmedia. The H2 emission was also detected at the background region, and this\ndiffuse H2 emission may originate from collisional process in addition to the\nultraviolet photon pumping.", "category": "astro-ph_HE" }, { "text": "Magnetic-Field Amplification in the Thin X-ray Rims of SN1006: Several young supernova remnants (SNRs), including SN1006, emit synchrotron\nX-rays in narrow filaments, hereafter thin rims, along their periphery. The\nwidths of these rims imply 50 to 100 $\\mu$G fields in the region immediately\nbehind the shock, far larger than expected for the interstellar medium\ncompressed by unmodified shocks, assuming electron radiative losses limit rim\nwidths. However, magnetic-field damping could also produce thin rims. Here we\nreview the literature on rim width calculations, summarizing the case for\nmagnetic-field amplification. We extend these calculations to include an\narbitrary power-law dependence of the diffusion coefficient on energy, $D\n\\propto E^{\\mu}$. Loss-limited rim widths should shrink with increasing photon\nenergy, while magnetic-damping models predict widths almost independent of\nphoton energy. We use these results to analyze Chandra observations of SN 1006,\nin particular the southwest limb. We parameterize the full widths at half\nmaximum (FWHM) in terms of energy as FWHM $\\propto E^{m_E}_{\\gamma}$. Filament\nwidths in SN1006 decrease with energy; $m_E \\sim -0.3$ to $-0.8$, implying\nmagnetic field amplification by factors of 10 to 50, above the factor of 4\nexpected in strong unmodified shocks. For SN 1006, the rapid shrinkage rules\nout magnetic damping models. It also favors short mean free paths (small\ndiffusion coefficients) and strong dependence of $D$ on energy ($\\mu \\ge 1$).", "category": "astro-ph_HE" }, { "text": "Supernova Shock Breakout Through a Wind: The breakout of a supernova shock wave through the progenitor star's outer\nenvelope is expected to appear as an X-ray flash. However, if the supernova\nexplodes inside an optically-thick wind, the breakout flash is delayed. We\npresent a simple model for estimating the conditions at shock breakout in a\nwind based on the general observable quantities in the X-ray flash lightcurve:\nthe total energy E_X, and the diffusion time after the peak, t_diff. We base\nthe derivation on the self-similar solution for the forward-reverse shock\nstructure expected for an ejecta plowing through a pre-existing wind at large\ndistances from the progenitor's surface. We find simple quantitative relations\nfor the shock radius and velocity at breakout. By relating the ejecta density\nprofile to the pre-explosion structure of the progenitor, the model can also be\nextended to constrain the combination of explosion energy and ejecta mass. For\nthe observed case of XRO08109/SN2008D, our model provides reasonable\nconstraints on the breakout radius, explosion energy, and ejecta mass, and\npredicts a high shock velocity which naturally accounts for the observed\nnon-thermal spectrum.", "category": "astro-ph_HE" }, { "text": "Observations of High-Energy Gamma-Ray Emission Toward the Galactic\n Centre with the Fermi Large Area Telescope: The inner region of the Milky Way is one of the most interesting and complex\nregions of the gamma-ray sky. The intense interstellar emission and resolved\npoint sources, as well as potential contributions by other sources such as\nunresolved source populations and dark matter, complicate the interpretation of\nthe data. In this paper the Fermi LAT team analysis of a 15x15 degree region\nabout the Galactic centre is described. The methodology for point-source\ndetection and treatment of the interstellar emission is given. In general, the\nbulk of the gamma-ray emission from this region is attributable to a\ncombination of these two contributions. However, low-intensity residual\nemission remains and its characterisation is discussed.", "category": "astro-ph_HE" }, { "text": "Constraining black hole spins with low-frequency quasi-periodic\n oscillations in soft states: Black hole X-ray transients show a variety of state transitions during their\noutburst phases, characterized by changes in their spectral and timing\nproperties. In particular, power density spectra (PDS) show quasi periodic\noscillations (QPOs) that can be related to the accretion regime of the source.\nWe looked for type-C QPOs in the disc-dominated state (i.e. the high soft\nstate) and in the ultra-luminous state in the RXTE archival data of 12\ntransient black hole X-ray binaries known to show QPOs during their outbursts.\nWe detected 6 significant QPOs in the soft state that can be classified as\ntype-C QPOs. Under the assumption that the accretion disc in disc-dominated\nstates extends down or close to the innermost stable circular orbit (ISCO) and\nthat type-C QPOs would arise at the inner edge of the accretion flow, we use\nthe relativistic precession model (RPM) to place constraints on the black hole\nspin. We were able to place lower limits on the spin value for all the 12\nsources of our sample while we could place also an upper limit on the spin for\n5 sources.", "category": "astro-ph_HE" }, { "text": "Why not any tau double bang in Icecube, yet?: High Energy Neutrino Astronomy has been revealed by a sudden change in the\nflavor composition around maximal energies since three years of recording in\nice km detector. However these discover didn' t led to any clear promised\nNeutrino Astronomy land yet. No correlation with hard gamma sources was found.\nMoreover the astrophysical spectra expected at Fermi value, seem to converge to\na softer value, also required to avoid any Glashow resonant neutrino signal. We\nsuggest a main solution within a composite flux ruled by prompt atmospheric\nneutrinos. Nevertheless the recent discover of twentyone crossing muons at\nhundreds TeVs had shown a first narrow overlapped doublet and a correlated\ntrack with a peculiar hard UHECR event clustering pointing to a well known\nmicrojet in bynary precession; these first connections hint for a non\nnegligible astrophysical component making neutrino astronomy anyway already\nmore than a hope.", "category": "astro-ph_HE" }, { "text": "Vertical Structure of Neutrino Dominated Accretion Disks and Neutrino\n Transport in the disks: We investigate the vertical structure of neutrino dominated accretion disks\nby self-consistently considering the detailed microphysics, such as the\nneutrino transport, vertical hydrostatic equilibrium, the conservation of\nlepton number, as well as the balance between neutrino cooling, advection\ncooling and viscosity heating. After obtaining the emitting spectra of\nneutrinos and antineutrinos by solving the one dimensional Boltzmann equation\nof neutrino and antineutrino transport in the disk, we calculate the\nneutrino/antineutrino luminosity and their annihilation luminosity. We find\nthat the total neutrino and antineutrino luminosity is about $10^{54}$ ergs/s\nand their annihilation luminosity is about $5\\times10^{51}$ ergs/s with an\nextreme accretion rate $10 M_{\\rm {sun}}$/s and an alpha viscosity\n$\\alpha=0.1$. In addition, we find that the annihilation luminosity is\nsensitive to the accretion rate and will not exceed $10^{50}$ ergs/s which is\nnot sufficient to power the most fireball of GRBs, if the accretion rate is\nlower than $1 M_{\\rm {sun}}$/s. Therefore, the effects of the spin of black\nhole or/and the magnetic field in the accretion flow might be introduced to\npower the central engine of GRBs.", "category": "astro-ph_HE" }, { "text": "Optical Afterglows and IGM Attenuation: Because of their cosmological origin, gamma-ray burst (GRB) optical\nafterglows are attenuated when they pass intergalactic absorbers in the GRB\nline-of-sight. Without the knowledge of the number of absorbers and their\nphysical properties, the effect of absorption on the observed magnitudes can\nnot be determined precisely. Different methods have been applied in order to\ncorrect for this effect statistically, either using semi-analytical\ncalculations or numerical simulations. We follow these works and present the\nexpected magnitude corrections as a function of redshift for a set of filters\nmost commonly used in the scientific community. The results are publically\navailable on the web (http://igmac.fmf.uni-lj.si).", "category": "astro-ph_HE" }, { "text": "First combined studies on Lorentz Invariance Violation from observations\n of astrophysical sources: Imaging Atmospheric Cherenkov Telescopes study the highest energy (up to tens\nof TeV) photon emission coming from nearby and distant astrophysical sources,\nthus providing valuable results from searches for Lorentz Invariance Violation\n(LIV) effects. Highly variable, energetic and distant sources such as Pulsars\nand AGNs are the best targets for the Time-of-Flight LIV studies. However, the\nlimited number of observations of AGN flares or of high-energy pulsed emission\ngreatly restricts the potential of such studies, especially any potential LIV\neffects as a function of redshift. To address these issues, an inter-experiment\nworking group has been established by the three major collaborations taking\ndata with Imaging Atmospheric Cherenkov Telescopes (H.E.S.S., MAGIC and\nVERITAS) with the aim to increase sensitivity to any effects of LIV, together\nwith an improved control of systematic uncertainties, by sharing data samples\nand developing joint analysis methods. This will allow an increase in the\nnumber of available sources and to perform a sensitive search for redshift\ndependencies. This presentation reviews the first combined maximum likelihood\nmethod analyses using simu- lations of published source observations done in\nthe past with H.E.S.S., MAGIC and VERITAS. The results from analyses based on\ncombined maximum likelihood methods, the strategies to deal with data from\ndifferent types of sources and instruments, as well as future plans will be\npresented.", "category": "astro-ph_HE" }, { "text": "Interpreting the relation between the gamma-ray and infrared\n luminosities of star-forming galaxies: It has been found that there is a quasi-linear scaling relationship between\nthe gamma-ray luminosity in GeV energies and the total infrared luminosity of\nstar-forming galaxies, i.e. $L_{\\gamma}\\propto L_{\\rm IR}^{\\alpha}$ with\n$\\alpha\\simeq 1$. However, the origin of this linear slope is not well\nunderstood. Although extreme starburst galaxies can be regarded as calorimeters\nfor hadronic cosmic ray interaction and thus a quasi-linear scaling may hold,\nit may not be the case for low star-formation-rate (SFR) galaxies, as the\nmajority of cosmic rays in these galaxies are expected to escape. We calculate\nthe gamma-ray production efficiency in star-forming galaxies by considering\nrealistic galaxy properties, such as the gas density and galactic wind velocity\nin star-forming galaxies. We find that the slope for the relation between\ngamma-ray luminosity and the infrared luminosity gets steeper for low infrared\nluminosity galaxies, i.e. $\\alpha\\rightarrow 1.6$, due to increasingly lower\nefficiency for the production of gamma-ray emission. We further find that the\nmeasured data of the gamma-ray luminosity is compatible with such a steepening.\nThe steepening in the slope suggests that cosmic-ray escape is very important\nin low-SFR galaxies.", "category": "astro-ph_HE" }, { "text": "A search for fast radio burst-like emission from Fermi gamma-ray bursts: We report the results of the rapid follow-up observations of gamma-ray bursts\n(GRBs) detected by the Fermi satellite to search for associated fast radio\nbursts. The observations were conducted with the Australian Square Kilometre\nArray Pathfinder at frequencies from 1.2-1.4 GHz. A set of 20 bursts, of which\nfour were short GRBs, were followed up with a typical latency of about one\nminute, for a duration of up to 11 hours after the burst. The data was searched\nusing 4096 dispersion measure trials up to a maximum dispersion measure of 3763\npc cm$^{-3}$, and for pulse widths $w$ over a range of duration from 1.256 to\n40.48 ms. No associated pulsed radio emission was observed above $26 {\\rm Jy\nms} (w/1 {\\rm ms})^{-1/2}$ for any of the 20 GRBs.", "category": "astro-ph_HE" }, { "text": "Observational signatures of stellar explosions driven by relativistic\n jets: The role of relativistic jets in unbinding the stellar envelope during a\nsupernova (SN) associated with a gamma-ray burst (GRB) is unclear. To study\nthat, we explore observational signatures of stellar explosions that are driven\nby jets. We focus on the final velocity distribution of the outflow in such\nexplosions and compare its observational imprints to SN/GRB data. We find that\njet driven explosions produce an outflow with a flat distribution of energy per\nlogarithmic scale of proper velocity. The flat distribution seems to be\nuniversal as it is independent of the jet and the progenitor properties that we\nexplored. The velocity range of the flat distribution for typical GRB\nparameters is $\\gamma\\beta \\approx 0.03-3$, where $\\gamma$ is the outflow\nLorentz factor and $\\beta$ is its dimensionless velocity. A flat distribution\nis seen also for collimated choked jets where the highest outflow velocity\ndecreases with the depth at which the jet is choked. Comparison to observations\nof SN/GRBs rules out jets as the sole explosion source in these events.\nInstead, in SN/GRB the collapsing star must deposit its energy into two\nchannels - a quasi-spherical (or wide angle) channel and a narrowly collimated\none. The former carries most of the energy and is responsible for the SN\nsub-relativistic ejecta while the latter carries 0.01-0.1 of the total outflow\nenergy and is the source of the GRB. Intriguingly, the same two channels, with\na similar energy ratio, were seen in the binary neutron star merger GW170817,\nsuggesting that similar engines are at work in both phenomena.", "category": "astro-ph_HE" }, { "text": "The luminosity function and the rate of Swift's Gamma Ray Bursts: We invert directly the redshift - luminosity distribution of observed long\nSwift GRBs to obtain their rate and luminosity function. Our best fit rate is\ndescribed by a broken power law that rises like (1+z)^2.1{+0.5-0.6} for 03. The local rate is\n1.3^{+0.6-0.7} [Gpc^-3 yr^-1]. The luminosity function is well described by a\nbroken power law with a break at L* = 10^52.5{+-0.2}[erg/sec] and with indices\nalpha = 0.2^{+0.2-0.1} and beta = 1.4^{+0.3-0.6}. The recently detected GRB\n090423, with redshift ~8, fits nicely into the model's prediction, verifying\nthat we are allowed to extend our results to high redshifts. While there is a\npossible agreement with the star formation rate (SFR) for z<3, the high\nredshift slope is shallower than the steep decline in the SFR for 4 5 sigma confidence level. Combining\ndata from the XRT and BAT detectors, we find that the 0.3-150 keV spectra of RT\nCru, T CrB, and CD -57 3057 are well described by emission from a\nsingle-temperature, optically thin thermal plasma, plus an unresolved 6.4-6.9\nkeV Fe line complex. The X-ray spectrum of CH Cyg contains an additional bright\nsoft component. For all 4 systems, the spectra suffer high levels of absorption\nfrom material that both fully and partially covers the source of hard X-rays.\nThe XRT data did not show any of the rapid, periodic variations that one would\nexpect if the X-ray emission were due to accretion onto a rotating, highly\nmagnetized WD. The X-rays were thus more likely from the accretion-disk\nboundary layer around a massive, non-magnetic WD in each binary. The X-ray\nemission from RT Cru varied on timescales of a few days. This variability is\nconsistent with being due to changes in the absorber that partially covers the\nsource, suggesting localized absorption from a clumpy medium moving into the\nline of sight. The X-ray emission from CD -57 3057 and T CrB also varied during\nthe 9 months of Swift observations, in a manner that was also consistent with\nvariable absorption.", "category": "astro-ph_HE" }, { "text": "Supernova 2013by: A Type IIL Supernova with a IIP-like light curve drop: We present multi-band ultraviolet and optical light curves, as well as\nvisual-wavelength and near-infrared spectroscopy of the Type II linear (IIL)\nsupernova (SN) 2013by. We show that SN 2013by and other SNe IIL in the\nliterature, after their linear decline phase that start after maximum, have a\nsharp light curve decline similar to that seen in Type II plateau (IIP)\nsupernovae. This light curve feature has rarely been observed in other SNe IIL\ndue to their relative rarity and the intrinsic faintness of this particular\nphase of the light curve. We suggest that the presence of this drop could be\nused as a physical parameter to distinguish between subclasses of SNe II,\nrather than their light curve decline rate shortly after peak. Close inspection\nof the spectra of SN 2013by indicate asymmetric line profiles and signatures of\nhigh-velocity hydrogen. Late (less than 90 days after explosion) near-infrared\nspectra of SN 2013by exhibit oxygen lines, indicating significant mixing within\nthe ejecta. From the late-time light curve, we estimate that 0.029 solar mass\nof 56Ni was synthesized during the explosion. It is also shown that the V -band\nlight curve slope is responsible for part of the scatter in the luminosity (V\nmagnitude 50 days after explosion) vs. 56Ni relation. Our observations of SN\n2013by and other SNe IIL through the onset of the nebular phase indicate that\ntheir progenitors are similar to those of SNe IIP.", "category": "astro-ph_HE" }, { "text": "Signatures of the disk-jet coupling in the Broad-line Radio Quasar\n 4C+74.26: Here we explore the disk-jet connection in the broad-line radio quasar\n4C+74.26, utilizing the results of the multiwavelength monitoring of the\nsource. The target is unique in that its radiative output at radio wavelengths\nis dominated by a moderately-beamed nuclear jet, at optical frequencies by the\naccretion disk, and in the hard X-ray range by the disk corona. Our analysis\nreveals a correlation (local and global significance of 96\\% and 98\\%,\nrespectively) between the optical and radio bands, with the disk lagging behind\nthe jet by $250 \\pm 42$ days. We discuss the possible explanation for this,\nspeculating that the observed disk and the jet flux changes are generated by\nmagnetic fluctuations originating within the innermost parts of a truncated\ndisk, and that the lag is related to a delayed radiative response of the disk\nwhen compared with the propagation timescale of magnetic perturbations along\nrelativistic outflow. This scenario is supported by the re-analysis of the\nNuSTAR data, modelled in terms of a relativistic reflection from the disk\nilluminated by the coronal emission, which returns the inner disk radius\n$R_{\\rm in}/R_{\\rm ISCO} =35^{+40}_{-16}$. We discuss the global energetics in\nthe system, arguing that while the accretion proceeds at the Eddington rate,\nwith the accretion-related bolometric luminosity $L_{\\rm bol} \\sim 9 \\times\n10^{46}$ erg s$^{-1}$ $\\sim 0.2 L_{\\rm Edd}$, the jet total kinetic energy\n$L_\\textrm{j} \\sim 4 \\times 10^{44}$ erg s$^{-1}$, inferred from the dynamical\nmodelling of the giant radio lobes in the source, constitutes only a small\nfraction of the available accretion power.", "category": "astro-ph_HE" }, { "text": "Status of the Lunar Detection Mode for Cosmic Particles of LOFAR: Cosmic particles hitting Earth's moon produce radio emission via the Askaryan\neffect. If the resulting radio ns-pulse can be detected by radio telescopes,\nthis technique potentially increases the available collective area for ZeV\nscale particles by several orders of magnitude compared to current experiments.\nThe LOw Frequency ARray (LOFAR) is the largest radio telescope operating in the\noptimum frequency regime for this technique. In this contribution, we report on\nthe status of the implementation of the lunar detection mode at LOFAR.", "category": "astro-ph_HE" }, { "text": "On the Polarized Absorption Lines in Gamma-ray Burst Optical Afterglows: Spectropolarimetric measurements of gamma-ray burst (GRB) optical afterglows\ncontain polarization information for both continuum and absorption lines. Based\non the Zeeman effect, an absorption line in a strong magnetic field is\npolarized and split into a triplet. In this paper, we solve the polarization\nradiative transfer equations of the absorption lines, and obtain the degree of\nlinear polarization of the absorption lines as a function of the optical depth.\nIn order to effectively measure the degree of linear polarization for the\nabsorption lines, a magnetic field strength of at least $10^3$ G is required.\nThe metal elements that produce the polarized absorption lines should be\nsufficiently abundant and have large oscillation strengths or Einstein\nabsorption coefficients. We encourage both polarization measurements and\nhigh-dispersion observations of the absorption lines in order to detect the\ntriplet structure in early GRB optical afterglows.", "category": "astro-ph_HE" }, { "text": "Observation of the Crab Nebula with the HAWC Gamma-Ray Observatory: The Crab Nebula is the brightest TeV gamma-ray source in the sky and has been\nused for the past 25 years as a reference source in TeV astronomy, for\ncalibration and verification of new TeV instruments. The High Altitude Water\nCherenkov Observatory (HAWC), completed in early 2015, has been used to observe\nthe Crab Nebula at high significance across nearly the full spectrum of\nenergies to which HAWC is sensitive. HAWC is unique for its wide field-of-view,\nnearly 2 sr at any instant, and its high-energy reach, up to 100 TeV. HAWC's\nsensitivity improves with the gamma-ray energy. Above $\\sim$1 TeV the\nsensitivity is driven by the best background rejection and angular resolution\never achieved for a wide-field ground array.\n We present a time-integrated analysis of the Crab using 507 live days of HAWC\ndata from 2014 November to 2016 June. The spectrum of the Crab is fit to a\nfunction of the form $\\phi(E) = \\phi_0 (E/E_{0})^{-\\alpha\n-\\beta\\cdot{\\rm{ln}}(E/E_{0})}$. The data is well-fit with values of\n$\\alpha=2.63\\pm0.03$, $\\beta=0.15\\pm0.03$, and\nlog$_{10}(\\phi_0~{\\rm{cm}^2}~{\\rm{s}}~{\\rm{TeV}})=-12.60\\pm0.02$ when $E_{0}$\nis fixed at 7 TeV and the fit applies between 1 and 37 TeV. Study of the\nsystematic errors in this HAWC measurement is discussed and estimated to be\n$\\pm$50\\% in the photon flux between 1 and 37 TeV.\n Confirmation of the Crab flux serves to establish the HAWC instrument's\nsensitivity for surveys of the sky. The HAWC survey will exceed sensitivity of\ncurrent-generation observatories and open a new view of 2/3 of the sky above 10\nTeV.", "category": "astro-ph_HE" }, { "text": "The Intrinsic Properties of Multiwavelength Energy Spectra for Fermi\n Teraelectronvolt Blazars: In this paper, we have selected a sample of 64 teraelectronvolt blazars, with\nredshift, from those classified in the fourth Fermi Large Area Telescope source\ncatalog\\footnote{\\url{https://fermi.gsfc.nasa.gov/ssc/data/access/lat/8yr_catalog/}}.\nWe have obtained the values of the relevant physical parameters by performing a\nlog-parabolic fitting of the average-state multiwavelength spectral energy\ndistributions. We estimate the range of the radiation zone parameters, such as\nthe Doppler factor (${D}$), the magnetic field strength ($B$), the radiative\nzone radius ($R$) and the peak Lorentz factor (${\\gamma _{\\rm p}}$) of\nnonthermal electrons. Here, we show that (1) there is a strong linear positive\ncorrelation between the intrinsic synchrotron peak frequency and the intrinsic\ninverse Compton scattering (ICs) peak frequency among different types of\nblazars; (2) if radio bands are excluded, the spectral index of each band is\nnegatively correlated with the intrinsic peak frequency; (3) there is a strong\nlinear negative correlation between the curvature at the peak and the intrinsic\npeak frequency of the synchrotron bump, and a weak positive correlation between\nthe curvature at the peak and the intrinsic peak frequency of the ICs bump; (4)\nthere is a strong linear positive correlation between the intrinsic ICs peak\nluminosity and intrinsic $\\gamma$-ray luminosity and between the intrinsic ICs\npeak frequency and peak Lorentz factor; (5) there is a strong negative linear\ncorrelation between $\\rm log{\\;B}$ and $\\rm log{\\;\\gamma_{p}}$; and (6) there\nis no correlation between $\\rm log{\\;R}$ and $\\rm log{\\;\\gamma_{p}}$.", "category": "astro-ph_HE" }, { "text": "Bayesian Analysis of Hybrid EoS based on Astrophysical Observational\n Data: We perform a Bayesian analysis of probability measures for compact star\nequations of state using new, disjunct constraints for mass and radius. The\nanalysis uses a simple parametrization for hybrid equations of state to\ninvestigate the possibility of a first order deconfinement transition in\ncompact stars. The latter question is relevant for the possible existence of a\ncritical endpoint in the QCD phase diagram under scrutiny in heavy-ion\ncollisions.", "category": "astro-ph_HE" }, { "text": "A multi-wavelength study of the gamma-ray binary candidate HESS\n J1832-093: We investigate the nature of the unidentified very-high-energy (VHE)\ngamma-ray object, HESS J1832-093, in a multi-wavelength context. Based on X-ray\nvariability and spectral index ($\\Gamma_X\\sim\\,1.5$), and its broad-band\nspectrum (which was remarkably similar to HESS J0632+057, a confirmed gamma-ray\nbinary), HESS J1832-093 has been considered to be a strong gamma-ray binary\ncandidate in previous works. In this work, we provide further evidence for this\nscenario. We obtained a spectrum of its IR counterpart using Gemini/Flamingo,\nfinding absorption lines that are usually seen in massive stars, in particular\nO stars. We also obtained a rather steep ATCA spectrum\n($\\alpha=-1.18^{+1.04}_{-0.88}$) which prefers a gamma-ray binary over an AGN\nscenario. Based on spatial-spectral analysis and variability search, we found\nthat 4FGL J1832.9-0913 is possible to be associated with SNR G22.7-0.2 rather\nthan with HESS J1832-093 only.", "category": "astro-ph_HE" }, { "text": "X-ray nova MAXI J1828-249. Evolution of the broadband spectrum during\n its 2013-2014 outburst: Based on data from the SWIFT, INTEGRAL, MAXI/ISS orbital observatories, and\nthe ground-based RTT-150 telescope, we have investigated the broadband (from\nthe optical to the hard X-ray bands) spectrum of the X-ray nova MAXI J1828-249\nand its evolution during the outburst of the source in 2013-2014. The optical\nand infrared emissions from the nova are shown to be largely determined by the\nextension of the power-law component responsible for the hard X-ray emission.\nThe contribution from the outer cold regions of the accretion disk, even if the\nX-ray heating of its surface is taken into account, turns out to be moderate\nduring the source's \"high\" state (when a soft blackbody emission component is\nobserved in the X-ray spectrum) and is virtually absent during its \"low\"\n(\"hard\") state. This result suggests that much of the optical and infrared\nemissions from such systems originates in the same region of main energy\nrelease where their hard X-ray emission is formed. This can be the Compton or\nsynchro-Compton radiation from a high-temperature plasma in the central\naccretion disk region puffed up by instabilities, the synchrotron radiation\nfrom a hot corona above the disk, or the synchrotron radiation from its\nrelativistic jets.", "category": "astro-ph_HE" }, { "text": "X-rays from the episodic dust maker WR 137: We present an analysis of the XMM-Newton observation of the episodic dust\nmaker WR 137. Global spectral fits show that its X-ray spectrum is well matched\nby a two-temperature optically-thin plasma emission (kT_1 ~ 0.4 keV and kT_2 ~\n2.2 keV). If we adopt the colliding stellar wind (CSW) picture for this wide\nWR+O binary, the theoretical CSW spectra match well the shape of the observed\nX-ray spectrum of WR 137 but they overestimate the observed flux (emission\nmeasure) by about two orders of magnitude. To reconcile the model predictions\nwith observations, the mass loss of \\WR must be reduced considerably (by about\nan order of magnitude) with respect to its currently accepted value. In all the\nspectral fits, the derived X-ray absorption is consistent with the optical\nextinction to WR 137.", "category": "astro-ph_HE" }, { "text": "The evaporating primordial black hole fraction in cool-core galaxy\n clusters: Recent studies of gamma-ray, cosmic-ray and radio data put stringent\nconstraints on the fraction of primordial black holes (PBHs) in our universe.\nIn this article, we propose a new indirect method in using the X-ray luminosity\ndata of cool-core clusters to constrain the evaporating PBH fraction for the\nmonochromatic, log-normal and power-law mass distributions. The present results\nshow that the amount of evaporating PBHs only constitutes a minor component of\ndark matter for a large parameter space. The constraints are consistent with\nand close to that obtained from other cosmic-ray and multi-wavelength\nobservations.", "category": "astro-ph_HE" }, { "text": "The GMRT High Resolution Southern Sky Survey for pulsars and transients\n -- VI: Discovery of nulling, localisation and timing of PSR J1244-4708: Many pulsars in the known population exhibit nulling, which is characterised\nby a sudden cessation and subsequent restoration of radio emission. In this\nwork, we present the localization, timing, and emission properties of a GHRSS\ndiscovered pulsar J1244-4708. Moreover, we find that this pulsar shows nulling\nwith a nulling fraction close to 60%. A quasi-periodicity is also seen in the\nnulling from this pulsar with two timescales. We demonstrate the broadband\nnature of nulling in this pulsar using simultaneous observations in band-3\n(300-500 MHz) and band-4 (550-750 MHz) with the uGMRT. We also present a\ncomparison of the efficiency of various search approaches such as single pulse\nsearch, Fast Folding Algorithm (FFA) based search, and Fast Fourier Transform\n(FFT) based search to search for nulling pulsars. We demonstrated that the FFA\nsearch is advantageous for detecting extreme nulling pulsars, which is also\nconfirmed with multiple epochs of observations for the nulling pulsars using\nthe GMRT.", "category": "astro-ph_HE" }, { "text": "Constraints on UHECR sources and extragalactic magnetic fields from\n directional anisotropies: A dipole anisotropy in ultra-high-energy cosmic ray (UHECR) arrival\ndirections, of extragalactic origin, is now firmly established at energies E >\n8 EeV. Furthermore, the UHECR angular power spectrum shows no power at smaller\nangular scales than the dipole, apart from hints of possible individual hot or\nwarm spots for energy thresholds $\\gtrsim$40 EeV. Here, we exploit the\nmagnitude of the dipole and the limits on smaller-scale anisotropies to place\nconstraints on two quantities: the extragalactic magnetic field (EGMF) and the\nnumber density of UHECR sources or the volumetric event rate if UHECR sources\nare transient. We also vary the bias between the extragalactic matter and the\nUHECR source densities, reflecting whether UHECR sources are preferentially\nfound in over- or under-dense regions, and find that little or no bias is\nfavored. We follow Ding et al. (2021) in using the Cosmic Flows 2 density\ndistribution of the local universe as our baseline distribution of UHECR\nsources, but we improve and extend that work by employing an accurate and\nself-consistent treatment of interactions and energy losses during propagation.\nDeflections in the Galactic magnetic field are treated using both the full JF12\nmagnetic field model, with random as well as coherent components, or just the\ncoherent part, to bracket the impact of the GMF on the dipole anisotropy. This\nLarge Scale Structure (LSS) model gives good agreement with both the direction\nand magnitude of the measured dipole anisotropy and forms the basis for\nsimulations of discrete sources and the inclusion of EGMF effects.", "category": "astro-ph_HE" }, { "text": "Data-driven extrapolation schemes of Fermi-LAT spectra to the VHE: After 10 years of operations of the Large Area Telescope (LAT), a high-energy\npair-creation telescope onboard the Fermi satellite, the Fermi Collaboration\nhas produced two major catalogs: the 4FGL and the 3FHL. These catalogs\nrepresent the best sample of potential very high energy (VHE) emitters that may\nbe studied by Imaging Atmospheric Cherenkov Telescopes (IACTs). Several methods\nare used to extrapolate the Fermi-LAT spectra to TeV energies, generally using\nsimple analytical functions. The recent success of IACTs has motivated the\ncreation of catalogs listing the discoveries of these experiments. Among these\ninitiatives, gamma-cat excels as an open-access tool to archive high-level\nresults in the VHE field, such as catalogs, spectra and light curves. By using\nthese resources, we present a data-driven methodology to test the reliability\nof different VHE extrapolation schemes used in the literature and evaluate\ntheir accuracy reproducing real VHE observations.", "category": "astro-ph_HE" }, { "text": "Evidence of a Hadronic Origin for the TeV Source J1834-087: We report on the discovery of compact, narrow OH line emission from the\nhydroxl molecule at 1720 MHz toward the extended TeV source J1834-087. The\norigin of this high energy emission is unknown; it could be powered by one or\nmore candidate neutron stars (leptonic) or by cosmic rays interacting with\ndense gas (hadronic). The OH emission is detected near the center of J1834-087,\ncoincident with the radio continuum of the supernova remnant W41, and the\nradial velocity of the line is the same velocity as a giant molecular cloud\nalong the line of sight. We argue that the OH is maser emission stimulated by\nthe interaction of the W41 shock with the molecular cloud. The known\ncorrelation between gamma-ray bright supernova remnants and OH masers favors a\nhadronic interpretation for this high energy emission.", "category": "astro-ph_HE" }, { "text": "An XMM-Newton view of the dipping low-mass X-ray binary XTE J1710-281: (abridged) We analyzed the archived XMM-Newton observation of the poorly\nstudied low-mass X-ray binary XTE J1710-281 performed in 2004 that covered one\norbital period of the system (3.8 hr). The source shows dips as well as\neclipses, hence it is viewed close to edge-on. We modeled the spectral changes\nbetween persistent and dips in the framework of the partial covering model and\nthe ionized absorber approach. The persistent spectrum can be fit by a power\nlaw with a photon index of 1.94(+-0.02) affected by absorption from cool\nmaterial with a hydrogen column density of 0.401(+-0.007)*10^22 cm^-2. The\nspectral changes from persistent to deep-dipping intervals are consistent with\nthe partial covering of the power-law emission, with the covering fraction\nincreasing from 26% during shallow dipping to 78% during deep dipping. We do\nnot detect any absorption lines from highly ionized species such as FeXXV. The\nupper-limits we derive on their equivalent width (EW) are not constraining.\nDespite not detecting any signatures of a warm absorber, we show that the\nspectral changes are consistent with an increase in column density\n(4.3(-0.5;+0.4)*10^22 cm^-2 during shallow dipping to 11.6(-0.6;+0.4)*10^22\ncm^-2 during deep dipping) and a decrease in ionization state of a\nhighly-ionized absorber (10^2.52 during shallow dipping to 10^2.29 erg.s^-1.cm\nduring deep dipping), associated with a slight increase in the column density\nof a neutral absorber. The parameters of the ionized absorber are not\nconstrained during persistent emission. The warm absorber model better accounts\nfor the ~1 keV depression visible in the pn dipping spectra, and naturally\nexplains it as a blend of lines and edges unresolved by pn. A deeper\nobservation of XTE J1710-281 would enable this interpretation to be confirmed.", "category": "astro-ph_HE" }, { "text": "The Morphology and Dynamics of Relativistic Jets with Relativistic\n Equation of State: We study the effect of plasma composition on the dynamics and morphology of\nthe relativistic astrophysical jets. Our work is based on a relativistic total\nvariation diminishing (TVD) simulation code. We use a relativistic equation of\nstate in the simulation code which accounts for the thermodynamics of a\nmultispecies plasma which is a mixture of electrons, positrons, and protons. To\nstudy the effect of plasma composition we consider various jet models. These\nmodels are characterized by the same injection parameters, same jet kinetic\nluminosity, and the same Mach numbers. The evolution of these models shows that\nthe plasma composition affects the jet head propagation speed, the structure of\nthe jet head, and the morphology despite fixing the initial parameters. We\nconclude that the electron-positron jets are the slowest and show more\npronounced turbulent structures in comparison to other plasma compositions. The\narea and locations of the hot-spots also depend on the composition of jet\nplasma. Our results also show that boosting mechanisms are also an important\naspect of multi-dimensional simulations which are also influenced by the change\nin composition.", "category": "astro-ph_HE" }, { "text": "Sensitivity of the correlation between the depth of shower maximum and\n the muon shower size to the cosmic ray composition: The composition of ultra-high energy cosmic rays is an important issue in\nastroparticle physics research, and additional experimental results are\nrequired for further progress. Here we investigate what can be learned from the\nstatistical correlation factor r between the depth of shower maximum and the\nmuon shower size, when these observables are measured simultaneously for a set\nof air showers. The correlation factor r contains the lowest-order moment of a\ntwo-dimensional distribution taking both observables into account, and it is\nindependent of systematic uncertainties of the absolute scales of the two\nobservables. We find that, assuming realistic measurement uncertainties, the\nvalue of r can provide a measure of the spread of masses in the primary beam.\nParticularly, one can differentiate between a well-mixed composition (i.e., a\nbeam that contains large fractions of both light and heavy primaries) and a\nrelatively pure composition (i.e., a beam that contains species all of a\nsimilar mass). The number of events required for a statistically significant\ndifferentiation is ~ 200. This differentiation, though diluted, is maintained\nto a significant extent in the presence of uncertainties in the phenomenology\nof high energy hadronic interactions. Testing whether the beam is pure or\nwell-mixed is well motivated by recent measurements of the depth of shower\nmaximum.", "category": "astro-ph_HE" }, { "text": "Kinematics of Parsec-Scale Jets of Gamma-Ray Bright Blazars at 43 GHz\n during Ten Years of the VLBA-BU-BLAZAR Program: We analyze the parsec-scale jet kinematics from 2007 June to 2018 December of\na sample of $\\gamma$-ray bright blazars monitored roughly monthly with the Very\nLong Baseline Array at 43 GHz under the VLBA-BU-BLAZAR program. We implement a\nnovel piece-wise linear fitting method to derive the kinematics of 521 distinct\nemission knots from a total of 3705 total intensity images in 22 quasars, 13 BL\nLacertae objects, and 3 radio galaxies. Apparent speeds of these components\nrange from $0.01c$ to $78c$, and 18.6\\% of knots (other than the \"core\") are\nquasi-stationary. One-fifth of moving knots exhibit non-ballistic motion, with\nacceleration along the jet within 5 pc of the core (projected) and deceleration\nfarther out. These accelerations occur mainly at locations coincident with\nquasi-stationary features. We calculate the physical parameters of 273 knots\nwith statistically significant motion, including their Doppler factors, Lorentz\nfactors, and viewing angles. We determine the typical values of these\nparameters for each jet and the average for each subclass of active galactic\nnuclei. We investigate the variability of the position angle of each jet over\nthe ten years of monitoring. The fluctuations in position of the\nquasi-stationary components in radio galaxies tend to be parallel to the jet,\nwhile no directional preference is seen in the components of quasars and BL\nLacertae objects. We find a connection between $\\gamma$-ray states of blazars\nand their parsec-scale jet properties, with blazars with brighter 43 GHz cores\ntypically reaching higher $\\gamma$-ray maxima during flares.", "category": "astro-ph_HE" }, { "text": "Late Afterglow Bump/Plateau around the Jet Break: Signature of a\n free-to-shocked wind Environment in Gamma-ray Burst: A number of gamma-ray bursts (GRBs) exhibit the late simultaneous bumps in\ntheir optical and Xray afterglows around the jet break. Its origin is unclear.\nBased on the following two facts, we suggest that this feature may sound a\ntransition of circum-burst environment from a free-wind medium to a homogeneous\nmedium. (I) The late bump followed by a steep decay is strongly reminiscent of\nthe afterglows of GRB 170817A, which is attributed to an off-axis observed\nexternal-forward shock (eFS) propagating in an interstellar medium. (II)\nObservations seem to feature a long shallow decay before the late optical bump,\nwhich is different from the afterglow of GRB 170817A. In this paper, we study\nthe emission of an eFS propagating in a free-to-shocked wind for on/off-axis\nobservers, where the mass density in the shocked-wind is almost constant. The\nlate simultaneous bumps/plateaux in the optical and X-ray afterglows are really\nfound around the jet break for high-viewing-angle observers. Moreover, there is\na long plateau or shallow decay before the late bump in the theoretical\nlight-curves, which is formed during the eFS propagating in the free-wind. For\nlow-viewing-angle observers, the above bumps appear only in the situation that\nthe structured jet has a low characteristic angle and the deceleration radius\nof the on-axis jet flow is at around or beyond the free-wind boundary. As\nexamples, the X-ray and optical afterglows of GRBs 120326A, 120404A, and\n100814A are fitted. We find that an off-axis observed eFS in a free-to-shocked\nwind can well explain the afterglows in these bursts.", "category": "astro-ph_HE" }, { "text": "An Occultation Event in Centaurus A and the Clumpy Torus Model: We have analyzed 16 months of sustained monitoring observations of Cen A from\nthe Rossi X-ray Timing Explorer to search for changes in the absorbing column\nin the line of sight to the central nucleus. We present time-resolved\nspectroscopy which indicates that a discrete clump of material transited the\nline of sight to the central illuminating source over the course of ~170 days\nbetween 2010 August and 2011 February with a maximum increase in the column\ndensity of about 8.4 x 10^22 cm^-2. This is the best quality data of such an\nevent that has ever been analyzed with the shape of the ingress and egress\nclearly seen. Modeling the clump of material as roughly spherical with a\nlinearly decreasing density profile and assuming a distance from the central\nnucleus commensurate with the dusty torus we found that the clump would have a\ndiameter of 1.4-2.4 x 10^15 cm with a central number density of n_H = 1.8-3.0 x\n10^7 cm^-3. This is consistent with previous results for a similar (though\npossibly much longer) occultation event inferred in this source in 2003-2004\nand supports models of the molecular torus as a clumpy medium.", "category": "astro-ph_HE" }, { "text": "Cosmic Evolution of Stellar-mass Black Hole Merger Rate in Active\n Galactic Nuclei: Binary black hole mergers encode information about their environment and the\nastrophysical processes that led to their formation. Measuring the redshift\ndependence of their merger rate will help probe the formation and evolution of\ngalaxies and the evolution of the star formation rate. Here we compute the\ncosmic evolution of the merger rate for stellar-mass binaries in the disks of\nActive Galactic Nuclei (AGNs). We focus on recent evolution out to redshift\n$z=2$, covering the accessible range of current Earth-based gravitational-wave\nobservatories. On this scale, the AGN population density is the main\ncontributor to redshift-dependence. We find that the AGN-assisted merger rate\ndoes not meaningfully evolve with redshift, differentiating this channel from\nfield binaries and some other dynamical formation scenarios.", "category": "astro-ph_HE" }, { "text": "Reapproaching the Spin Estimate of GX 339-4: We systematically reanalyze two previous observations of the black hole (BH)\nGX 339-4 in the very high and intermediate state taken with $\\emph{XMM-Newton}$\nand $\\emph{Suzaku}$. We utilize up-to-date data reduction procedures and\nimplement the recently developed, self-consistent model for X-ray reflection\nand relativistic ray tracing, {\\sc relxill}. In the very high and intermediate\nstate, the rate of accretion is high and thus the disk remains close to the\ninnermost stable circular orbit (ISCO). We require a common spin parameter and\ninclination when fitting the two observations since these parameters should\nremain constant across all states. This allows for the most accurate\ndetermination of the spin parameter of this galactic black hole binary from\nfitting the Fe K$\\alpha$ emission line and provides a chance to test previous\nestimates. We find GX 339-4 to be consistent with a near maximally spinning\nblack hole with a spin parameter $a_{*}$ $>0.97$ with an inclination of $36 \\pm\n4$ degrees. This spin value is consistent with previous high estimates for this\nobject. Further, if the inner disk is aligned with the binary inclination, this\nmodest inclination returns a high black hole mass, but they need not be\naligned. Additionally, we explore how the spin is correlated with the power of\nthe jet emitted but find no correlation between the two.", "category": "astro-ph_HE" }, { "text": "Unraveling the Geometry of the Crab Nebula's \"Inner Ring\": Chandra images of the Crab Nebula resolve the detailed structure of its\n\"inner ring\", possibly a termination shock where pulsar-accelerated\nrelativistic particles begin to emit X radiation. Analysis of these images\nfinds that the center of the ellipse-presumably a circular ring in\nprojection-lies about 0.9\" (10 light-days at 2 kpc) from the pulsar's image, at\na position angle of about 300{\\deg} (East of North). This analysis also\nmeasures properties of the ellipse: The position angle of the semi-major axis\nis about 210{\\deg} (East of North); the aspect ratio, 0.49.\n In a simple-albeit, not unique-de-projection of the observed geometry, a\ncircular ring is centered on the axis of symmetry of the pulsar wind nebula.\nThis ring is not equatorial but rather lies near +4.5{\\deg} latitude in\npulsar-centered coordinates. Alternative geometries are briefly discussed.", "category": "astro-ph_HE" }, { "text": "Low metallicity natal environments and black hole masses in\n Ultraluminous X-ray Sources: We review the available estimates of the masses of the compact object in\nUltraluminous X-ray Sources (ULXs) and critically reconsider the stellar-mass\nversus intermediate-mass black hole interpretations. Black holes of several\nhundreds to thousands of $M_\\odot$ are not required for the majority of ULXs,\nalthough they might be present in the handful of known hyper-luminous ($\\sim\n10^{41}$ erg s$^{-1}$) objects and/or some sources showing timing features in\ntheir power density spectra. At the same time, however, stellar mass BHs may be\nquite a reasonable explanation for ULXs below $\\sim 10^{40}$ erg s$^{-1}$, but\nthey need super-Eddington accretion and some suitable dependence of the beaming\nfactor on the accretion rate in order to account for ULXs above this\n(isotropic) luminosity. We investigate in detail a 'third way' in which a\nproportion of ULXs contain $\\approx 30-90 M_\\odot$ black holes formed in a low\nmetallicity environment and accreting in a slightly critical regime and find\nthat it can consistently account for the properties of bright ULXs. Surveys of\nULX locations looking for a statistically meaningful relationship between ULX\nposition, average luminosity and local metallicity will provide a definitive\ntest of our proposal.", "category": "astro-ph_HE" }, { "text": "Overview of MHz air shower radio experiments and results: In this paper, I present a review of the main results obtained in the last 10\nyears in the field of radio-detection of cosmic-ray air showers in the MHz\nrange. All results from all experiments cannot be reported here so that I will\nfocus on the results more than on the experiments themselves. Modern\nexperiments started in 2003 with CODALEMA and LOPES. In 2006, small-size\nautonomous prototypes setup were installed at the Pierre Auger Observatory\nsite, to help the design of the Auger Engineering Radio Array (AERA). We will\ndiscuss the principal aspects of the radio data analysis and the determination\nof the primary cosmic ray characteristics: the arrival direction, the lateral\ndistribution of the electric field, the correlation with the primary energy,\nthe emission mechanisms and the sensitivity to the composition of the cosmic\nrays.", "category": "astro-ph_HE" }, { "text": "Neutron Stars with realistic EoS in f(R) theories of gravity: In this paper, we examine neutron star structure in perturbative $f(R)$\ngravity models with realistic equation of state. We obtain mass-radius\nrelations in two gravity models of the form $f_{1}(R)=R+ \\alpha\nR(e^{-R/R_0}-1)$ and $f_{2}(R)=R+\\alpha R^2$. For this purpose, we consider NS\nwith several nucleonic as well as strange EoSs generated in the framework of\nrelativistic mean field models. The strange particles in the core of NS are in\nthe form of $\\Lambda$ hyperons and quarks, in addition to the nucleons and\nleptons. The M-R relation of the chosen EoSs lies well within the observational\nlimit in the case of GR. We show that these EoSs provide the most stringent\nconstraint on the perturbative parameter $\\alpha$ and therefore can be\nconsidered as important experimental probe for modified gravity at\nastrophysical level.", "category": "astro-ph_HE" }, { "text": "Light Curve Modeling of Superluminous Supernova 2006gy: Collision\n between Supernova Ejecta and Dense Circumstellar Medium: We show model light curves of superluminous supernova 2006gy on the\nassumption that the supernova is powered by the collision of supernova ejecta\nand its dense circumstellar medium. The initial conditions are constructed\nbased on the shock breakout condition, assuming that the circumstellar medium\nis dense enough to cause the shock breakout within it. We perform a set of\nnumerical light curve calculations by using a one-dimensional multigroup\nradiation hydrodynamics code STELLA. We succeeded in reproducing the overall\nfeatures of the early light curve of SN 2006gy with the circumstellar medium\nwhose mass is about 15 Msun (the average mass-loss rate ~ 0.1 Msun/yr). Thus,\nthe progenitor of SN 2006gy is likely a very massive star. The density profile\nof the circumstellar medium is not well constrained by the light curve modeling\nonly, but our modeling disfavors the circumstellar medium formed by steady mass\nloss. The ejecta mass is estimated to be comparable to or less than 15 Msun and\nthe explosion energy is expected to be more than 4e51 erg. No 56Ni is required\nto explain the early light curve. We find that the multidimensional effect,\ne.g., the Rayleigh-Taylor instability, which is expected to take place in the\ncool dense shell between the supernova ejecta and the dense circumstellar\nmedium, is important in understanding supernovae powered by the shock\ninteraction. We also show the evolution of the optical and near-infrared model\nlight curves of high-redshift superluminous supernovae. They can be potentially\nused to identify SN 2006gy-like superluminous supernovae in the future optical\nand near-infrared transient surveys.", "category": "astro-ph_HE" }, { "text": "diffSph: a Python tool to compute diffuse signals from dwarf spheroidal\n galaxies: So far no diffuse emissions in dwarf spheroidal satellites of the Milky Way\nhave ever been observed. Given that dwarf galaxies are predominantly composed\nof Dark Matter, the discovery of these signals could offer valuable insights\ninto understanding the nature of Dark Matter. We present \"diffSph\", a Python\ntool which in its present version provides fast predictions of such diffuse\nsignals in radio frequencies. It also features a very comprehensive module for\nthe computation of \"J\" and \"D\" factors that are relevant for indirect Dark\nMatter detection using gamma rays. Routines are coupled to parton-shower\nalgorithms and Dark Matter halo mass functions from state-of-the-art kinematic\nfits. This code is also useful for testing generic hypotheses (not necessarily\nassociated with any Dark Matter candidate) about the cosmic-ray\nelectron/positron sources in the dwarf galaxies. The diffSph tool has already\nbeen employed in searches for diffuse signals from dwarf spheroidal galaxies\nusing the LOw Frequency ARray (LOFAR).", "category": "astro-ph_HE" }, { "text": "A Comparative Study of Long and Short GRBs. I. Overlapping Properties: Gamma ray bursts (GRBs) are classified into long and short categories based\non their durations. Broad band studies suggest that these two categories of\nobjects roughly correspond to two different classes of progenitor systems, i.e.\ncompact star mergers (Type I) vs. massive star core collapse (Type II).\nHowever, the duration criterion sometimes leads to mis-identification of the\nprogenitor systems. We perform a comprehensive multi-wavelength comparative\nstudy between duration-defined long GRBs and short GRBs as well as the\nso-called \"consensus\" long GRBs and short GRBs (which are believed to be more\nclosely related to the two types of progenitor systems). The parameters we\nstudy include two parts: the prompt emission properties including duration\n($T_{90}$), spectral peak energy ($E_{\\rm p}$), low energy photon index\n($\\alpha$), isotropic $\\gamma$-ray energy ($E_{\\rm \\gamma, iso}$), isotropic\npeak luminosity ($L_{\\rm p,iso}$), and the amplitude parameters ($f$ and\n$f_{\\rm eff}$); and the host galaxy properties including stellar mass ($M_*$),\nstar formation rate (SFR), metallicity ([X/H]), half light radius ($R_{50}$),\nangular and physical ($R_{\\rm off}$) offset of the afterglow from the center of\nthe host galaxy, the normalized offset ($r_{\\rm off}=R_{\\rm off}/R_{50}$), and\nthe brightness fraction $F_{\\rm light}$. For most parameters, we find\ninteresting overlapping properties between the two populations in both 1D and\n2D distribution plots. The three best parameters for the classification purpose\nare $T_{90}$, $f_{\\rm eff}$, and $F_{\\rm light}$. However, no single parameter\nalone is good enough to place a particular burst into the right physical\ncategory, suggesting a need of multiple criteria for physical classification.", "category": "astro-ph_HE" }, { "text": "A Bayesian test for periodic signals in red noise: Many astrophysical sources, especially compact accreting sources, show\nstrong, random brightness fluctuations with broad power spectra in addition to\nperiodic or quasi-periodic oscillations (QPOs) that have narrower spectra. The\nrandom nature of the dominant source of variance greatly complicates the\nprocess of searching for possible weak periodic signals. We have addressed this\nproblem using the tools of Bayesian statistics; in particular using Markov\nchain Monte Carlo techniques to approximate the posterior distribution of model\nparameters, and posterior predictive model checking to assess model fits and\nsearch for periodogram outliers that may represent periodic signals. The\nmethods developed are applied to two example datasets, both long XMM-Newton\nobservations of highly variable Seyfert 1 galaxies: RE J1034+396 and Mrk 766.\nIn both cases a bend (or break) in the power spectrum is evident. In the case\nof RE J1034+396 the previously reported QPO is found but with somewhat weaker\nstatistical significance than reported in previous analyses. The difference is\ndue partly to the improved continuum modelling, better treatment of nuisance\nparameters, and partly to different data selection methods.", "category": "astro-ph_HE" }, { "text": "The puzzling symbiotic X-ray system 4U1700+24: Symbiotic X-ray binaries form a subclass of low-mass X-ray binary systems\nconsisting of a neutron star accreting material from a red giant donor star via\nstellar wind or Roche lobe overflow. Only a few confirmed members are currently\nknown; 4U 1700+24 is a good candidate as it is a relatively bright X-ray\nobject, possibly associated with the late-type star V934 Her. We analysed the\narchive {\\it XMM}-Newton and Swift/XRT observations of 4U 1700+24 in order to\nhave a uniform high-energy ($0.3-10$ keV) view of the source. We confirmed the\nexistence of a red-shifted O VIII Ly-$\\alpha$ transition (already observed in\nthe 2002 {\\it XMM}-Newton data) in the high-resolution spectra collected via\nthe RGS instruments. The red-shift of the line is found in all the analysed\nobservations and, on average, it was estimated to be $\\simeq 0.009$. We also\nobserved a modulation of the centroid energy of the line on short time scales\n(a few days) and discuss the observations in the framework of different\nscenarios. If the modulation is due to the gravitational red-shift of the\nneutron star, it might arise from a sudden re-organization of the emitting\n$X$-ray matter on the scale of a few hundreds of km. Alternatively, we are\nwitnessing a uni-polar jet of matter (with typical velocity of $1000-4000$ km\ns$^{-1}$) possibly emitted by the neutron star in an almost face-on system. The\nsecond possibility seems to be required by the apparent lack of any modulation\nin the observed $X$-ray light curve. We also note also that the low-resolution\nspectra (both {\\it XMM}-Newton and Swift/XRT in the $0.3-10$ keV band) show the\nexistence of a black body radiation emitted by a region (possibly associated\nwith the neutron star polar cap) with typical size from a few tens to hundreds\nof meters. The size of this spot-like region reduces as the overall luminosity\nof 4U 1700+24 decreases.", "category": "astro-ph_HE" }, { "text": "The Swift Serendipitous Survey in deep XRT GRB fields (SwiftFT) I. The\n X-ray catalog and number counts: We searched for X-ray serendipitous sources in over 370 Swift-XRT fields\ncentered on gamma ray bursts detected between 2004 and 2008 and observed with\ntotal exposures ranging from 10 ks to over 1 Ms. This defines the Swift\nSerendipitous Survey in deep XRT GRB fields, which is quite broad compared to\nexisting surveys (~33 square degrees) and medium depth, with a faintest flux\nlimit of 7.2e-16 erg cm^-2 s^-1 in the 0.5 to 2 keV energy range. The survey\nhas a high degree of uniformity thanks to the stable point spread function and\nsmall vignetting correction factors of the XRT, moreover is completely random\non the sky as GRBs explode in totally unrelated parts of the sky. In this paper\nwe present the sample and the X-ray number counts of the high Galactic-latitude\nsample, estimated with high statistics over a wide flux range (i.e., 7.2e-16 to\n~5e-13 erg cm^-2 s^-1 in the 0.5-2 keV band and 3.4e-15 to ~6e-13 erg cm^-2\ns^-1 in the 2-10 keV band). We detect 9387 point-like sources, while 7071\npoint-like sources are found at high Galactic-latitudes (i.e. >=20 deg). The\nlarge number of detected sources resulting from the combination of large area\nand deep flux limits make this survey a new important tool for investigating\nthe evolution of AGN. In particular, the large area permits finding rare\nhigh-luminosity objects like QSO2, which are poorly sampled by other surveys,\nadding precious information for the luminosity function bright end. The high\nGalactic-latitude logN-logS relation is well determined over all the flux\ncoverage, and it is nicely consistent with previous results at 1 sigma\nconfidence level. By the hard X-ray color analysis, we find that the Swift\nSerendipitous Survey in deep XRT GRB fields samples relatively unobscured and\nmildly obscured AGN, with a fraction of obscured sources of ~37% (~15%) in the\n2-10 (0.3-3 keV) band.", "category": "astro-ph_HE" }, { "text": "Solar Wind Charge Exchange contribution to the ROSAT All Sky Survey Maps: DXL (Diffuse X-ray emission from the Local Galaxy) is a sounding rocket\nmission designed to estimate the contribution of Solar Wind Charge eXchange\n(SWCX) to the Diffuse X-ray Background (DXB) and to help determine the\nproperties of the Local Hot Bubble (LHB). The detectors are large-area\nthin-window proportional counters with a spectral response similar to that of\nthe PSPC used in the ROSAT All Sky Survey (RASS). A direct comparison of DXL\nand RASS data for the same part of the sky viewed from quite different vantage\npoints in the Solar system and the assumption of approximate isotropy for the\nSolar wind allowed us to quantify the SWCX contribution to all 6 RASS bands\n(R1-R7, excepting R3). We find that the SWCX contribution at l=140 deg, b=0\ndeg, where the DXL path crosses the Galactic plane is 33%+-6%\n(statistical)+-12%(systematic) for R1, 44%+-\\%+-5% for R2, 18%+-12%+-11% for\nR4, 14%+-11%+-9% for R5, and negligible for R6 and R7 bands. Reliable models\nfor the distribution of neutral H and He in the Solar system permit estimation\nof the contribution of interplanetary SWCX emission over the the whole sky and\ncorrection of the RASS maps. We find that the average SWCX contribution in the\nwhole sky is 26%+-6%+-13% for R1, 30%+-4%+-4% for R2, 8%+-5%+-5% for R4,\n6%+-4%+-4% for R5, and negligible for R6 and R7.", "category": "astro-ph_HE" }, { "text": "Modeling the oxygen K absorption in the interstellar medium: an {\\it\n XMM}-Newton view of Sco X-1: We investigate the absorption structure of the oxygen in the interstellar\nmedium by analyzing {\\it XMM}-Newton observations of the low mass X-ray binary\nSco X-1. We use simple models based on the O {\\sc i} atomic cross section from\ndifferent sources to fit the data and evaluate the impact of the atomic data in\nthe interpretation of astrophysical observations. We show that relatively small\ndifferences in the atomic calculations can yield spurious results. We also show\nthat the most complete and accurate set of atomic cross sections successfully\nreproduce the observed data in the $21-24.5$ {\\AA} wavelength region of the\nspectrum. Our fits indicate that the absorption is mainly due to neutral gas\nwith an ionization parameter of $\\xi=10^{-4}$ erg cm s$^{-1}$, and an oxygen\ncolumn density of $N_{\\mathrm{O}}\\approx 8-10\\times 10^{17}$ cm$^{-2}$. Our\nmodels are able to reproduce both the K edge and the K$\\alpha$ absorption line\nfrom O {\\sc i}, which are the two main features in this region. We find no\nconclusive evidence for absorption by other than atomic oxygen.", "category": "astro-ph_HE" }, { "text": "VERITAS Observations of the Unidentified Point Source HESS J1943+213: The H.E.S.S. Galactic plane scan has revealed a large population of Galactic\nvery high energy (VHE; E > 100 GeV) emitters. The majority of the galactic\nsources are extended and can typically be associated with pulsar wind nebulae\n(35%) and supernova remnants (21%), while some of the sources remain\nunidentified (31%). A much smaller fraction of point-like sources (5 in total,\ncorresponding to 4%) are identified as gamma-ray binaries. Active galactic\nnuclei located behind the Galactic plane are also a potential source class. An\nactive galaxy could be identified in the VHE regime by a point-like appearance,\na high variability amplitude (up to a factor of 100) and a typically soft\nspectrum (due to absorption by the extra-galactic background light). Here we\nreport on VERITAS observations of HESS J1943+213, an unidentified point source\ndiscovered to emit above 470 GeV during the extended H.E.S.S. Galactic plane\nscan. This source is thought to be a distant BL Lac object behind the Galactic\nplane and, though it exhibits a steep spectrum it is a weak GeV source, only\nrecently detected using 5 years of Fermi-LAT data. Deep VERITAS observations at\nhigh elevations result in the most significant VHE detection of this object so\nfar, with an excess above 200 GeV of more than 18 standard deviations. We use\nvariability and spectral analyses of VERITAS data on HESS J1943+213 in a\nmulti-wavelength context to address the source classification.", "category": "astro-ph_HE" }, { "text": "Gravitational waves from the Papaloizou-Pringle instability in black\n hole-torus systems: Black hole (BH)--torus systems are promising candidates for the central\nengine of gamma-ray bursts (GRBs), and also possible outcomes of the collapse\nof supermassive stars to supermassive black holes (SMBHs). By three-dimensional\ngeneral relativistic numerical simulations, we show that an $m=1$\nnonaxisymmetric instability grows for a wide range of self-gravitating tori\norbiting BHs. The resulting nonaxisymmetric structure persists for a timescale\nmuch longer than the dynamical one, becoming a strong emitter of large\namplitude, quasiperiodic gravitational waves. Our results indicate that both,\nthe central engine of GRBs and newly formed SMBHs, can be strong gravitational\nwave sources observable by forthcoming ground-based and spacecraft detectors.", "category": "astro-ph_HE" }, { "text": "2D radiation-hydrodynamic simulations of supernova ejecta with a central\n power source: We present the results of two-dimensional radiation-hydrodynamic simulations\nof expanding supernova ejecta with a central energy source. As suggested in\nprevious multi-dimensional hydrodynamic simulations, a sufficiently powerful\ncentral energy source can blow away the expanding supernova ejecta, leading to\nefficient mixing of stratified layers in the ejecta. We assume that the energy\ninjection is realized in the form of non-thermal radiation from the wind nebula\nembedded at the center of the ejecta. We found that the multi-dimensional\nmixing in the ejecta assists the injected non-thermal radiation escaping from\nthe ejecta. When the non-thermal radiation is absorbed by the ejecta, it is\nconverted into bright thermal radiation or is consumed as the kinetic energy of\nthe supernova ejecta. We found that central energy sources with the injection\ntimescale similar to the photon diffusion timescale realize an efficient\nconversion of the injected energy into thermal radiation. On the other hand, a\nrapid energy injection ends up accelerating the ejecta rather than giving rise\nto bright thermal emission. This remarkable difference potentially explains the\ndiversity of energetic supernovae including broad-lined Ic and superluminous\nsupernovae.", "category": "astro-ph_HE" }, { "text": "Neutrino and cosmic-ray emission from multiple internal shocks in\n gamma-ray bursts: Gamma-ray bursts are short-lived, luminous explosions at cosmological\ndistances, thought to originate from relativistic jets launched at the deaths\nof massive stars. They are among the prime candidates to produce the observed\ncosmic rays at the highest energies. Recent neutrino data have, however,\nstarted to constrain this possibility in the simplest models with only one\nemission zone. In the classical theory of gamma-ray bursts, it is expected that\nparticles are accelerated at mildly relativistic shocks generated by the\ncollisions of material ejected from a central engine. We consider neutrino and\ncosmic-ray emission from multiple emission regions since these internal\ncollisions must occur at very different radii, from below the photosphere all\nthe way out to the circumburst medium, as a consequence of the efficient\ndissipation of kinetic energy. We demonstrate that the different messengers\noriginate from different collision radii, which means that multi-messenger\nobservations open windows for revealing the evolving GRB outflows.", "category": "astro-ph_HE" }, { "text": "Uncloaking hidden repeating fast radio bursts with unsupervised machine\n learning: The origins of fast radio bursts (FRBs), astronomical transients with\nmillisecond timescales, remain unknown. One of the difficulties stems from the\npossibility that observed FRBs could be heterogeneous in origin; as some of\nthem have been observed to repeat, and others have not. Due to limited\nobserving periods and telescope sensitivities, some bursts may be misclassified\nas non-repeaters. Therefore, it is important to clearly distinguish FRBs into\nrepeaters and non-repeaters, to better understand their origins. In this work,\nwe classify repeaters and non-repeaters using unsupervised machine learning,\nwithout relying on expensive monitoring observations. We present a repeating\nFRB recognition method based on the Uniform Manifold Approximation and\nProjection (UMAP). The main goals of this work are to: (i) show that the\nunsupervised UMAP can classify repeating FRB population without any prior\nknowledge about their repetition, (ii) evaluate the assumption that\nnon-repeating FRBs are contaminated by repeating FRBs, and (iii) recognise the\nFRB repeater candidates without monitoring observations and release a\ncorresponding catalogue. We apply our method to the Canadian Hydrogen Intensity\nMapping Experiment Fast Radio Burst (CHIME/FRB) database. We found that the\nunsupervised UMAP classification provides a repeating FRB completeness of 95\nper cent and identifies 188 FRB repeater source candidates from 474\nnon-repeater sources. This work paves the way to a new classification of\nrepeaters and non-repeaters based on a single epoch observation of FRBs.", "category": "astro-ph_HE" }, { "text": "A statistical analysis of the \"heartbeat\" behaviour of GRS 1915+105: GRS 1915+105 has been active for more than 26 years since it was discovered\nin 1992. There are hundreds of RXTE pointed observations on this source, and\nthe quasi-regular flares with a slow rise and a sharp decrease (i.e. the\n\"heartbeat\" state) were recorded in more than 200 observations. The connections\namong the disk/corona, jet, and the disk wind at the heartbeat state have been\nextensively studied. In this work, we firstly perform a statistical analysis of\nthe light curves and the X-ray spectra to investigate this peculiar state. We\ncalculate the parameters for heartbeat cycles, including the recurrence time,\nthe maximum and the minimum count rate, the flare amplitude, and the cumulative\nradiation for each cycle. The recurrence time has a bimodal distribution\nranging from $\\sim 20$ to $\\sim 200$ s. The minimum count rate increases with\nincreasing recurrence time; while the maximum count rate remains nearly\nconstant around 2 Crab. Fitting the averaged spectrum for each observation, we\nfind the strong correlations among the recurrence time, the apparent inner\nradius of the accretion disk (or the color correction factor), and the\n(nonthermal) X-ray luminosity. We suggest that the true inner edge of the\naccretion disk might always extend to the marginally stable orbit, while the\nchange in corona size should result in the observed correlations.", "category": "astro-ph_HE" }, { "text": "Subphotospheric emission from short gamma-ray bursts: Protons mold the\n multi-messenger signals: The origin of the observed Band-like photon spectrum in short gamma-ray\nbursts (sGRBs) is a long-standing mystery. We carry out the first general\nrelativistic magnetohydrodynamic simulation of a sGRB jet with initial\nmagnetization $\\sigma_0 = 150$ in dynamical ejecta from a binary merger. From\nthis simulation, we identify regions along the jet of efficient energy\ndissipation due to magnetic reconnection and collisionless sub-shocks. Taking\ninto account electron and proton acceleration processes, we solve for the first\ntime the coupled transport equations for photons, electrons, protons,\nneutrinos, and intermediate particles species up to close to the photosphere\n(i.e., up to $1 \\times 10^{12}$ cm), accounting for all relevant radiative and\ncooling processes. We find that the subphotospheric multi-messenger signals\ncarry strong signatures of the hadronic interactions and their resulting\nparticle cascades. Importantly, the spectral energy distribution of photons is\nsignificantly distorted with respect to the Wien one, commonly assumed below\nthe photosphere. Our findings suggest that the bulk of the non-thermal photon\nspectrum observed in sGRBs can stem from hadronic processes, occurring below\nthe photosphere and previously neglected, with an accompanying energy flux of\nneutrinos peaking in the GeV energy range.", "category": "astro-ph_HE" }, { "text": "TRAPUM discovery of thirteen new pulsars in NGC 1851 using MeerKAT: We report the discovery of 13 new pulsars in the globular cluster NGC 1851 by\nthe TRAPUM Large Survey Project using the MeerKAT radio telescope. The\ndiscoveries consist of six isolated millisecond pulsars (MSPs) and seven binary\npulsars, of which six are MSPs and one is mildly recycled. For all the pulsars,\nwe present the basic kinematic, astrometric, and orbital parameters, where\napplicable, as well as their polarimetric properties, when these are\nmeasurable. Two of the binary MSPs (PSR J0514-4002D and PSR J0514-4002E) are in\nwide and extremely eccentric (e > 0.7) orbits with a heavy white dwarf and a\nneutron star as their companion, respectively. With these discoveries, NGC 1851\nis now tied with M28 as the cluster with the third largest number of known\npulsars (14). Its pulsar population shows remarkable similarities with that of\nM28, Terzan 5 and other clusters with comparable structural parameters. The\nnewly-found pulsars are all located in the innermost regions of NGC 1851 and\nwill likely enable, among other things, detailed studies of the cluster\nstructure and dynamics.", "category": "astro-ph_HE" }, { "text": "Disc-corona interaction in the heartbeat state of GRS 1915+105: Timing analysis provides information about the dynamics of matter accreting\non to neutron stars and black holes, and hence is crucial for studying the\nphysics of the accretion flow around these objects. It is difficult, however,\nto associate the different variability components with each of the spectral\ncomponents of the accretion flow. We apply several new methods to two Rossi\nX-ray Timing Explorer observations of the black hole binary GRS 1915+105 during\nits heartbeat state to explore the origin of the X-ray variability and the\ninteractions of the accretion-flow components. We offer a promising window into\nthe disc--corona interaction through analysing the formation regions of the\ndisc aperiodic variabilities with different time-scales via comparing the\ncorresponding transition energies of the amplitude-ratio spectra. In a previous\npaper, we analysed the Fourier power density as a function of energy and\nfrequency to study the origin of the aperiodic variability, and combined that\nanalysis with the phase lag as a function of frequency to derive a picture of\nthe disc--corona interaction in this source. We here, for the first time,\ninvestigate the phase lag as a function of energy and frequency, and display\nsome interesting details of the disc--corona interaction. Besides, the results\nfrom the shape of amplitude-ratio spectrum and from several other aspects\nsuggest that the quasi-periodic oscillation originates from the corona.", "category": "astro-ph_HE" }, { "text": "Evidence for gravitational lensing of GRB 200716C: Observationally, there is a small fraction of Gamma-ray bursts (GRBs) with\nprompt emission observed by Fermi/GBM that are composed of two pulses.\nOccasionally, the distance to a GRB may be lensed when a high mass\nastrophysical object resides in the path between the GRB source and observer.\nIn this paper, we describe GRB 200716C, which has a two-pulse emission and\nduration of a few seconds. We present a Bayesian analysis identifying\ngravitational lensing in both temporal and spectral properties, and calculate\nthe time delay ($\\Delta t\\sim 1.92$ s) and magnification ($\\gamma\\sim 1.5$)\nbetween those two pulses based on the temporal fits. One can roughly estimate\nthe lens mass to be about $2.4\\times 10^{5}~M_{\\odot}$ in the rest frame. We\nalso calculate the false alarm probability for this detection to be about\n0.07\\% with trial factors, and a present-day number density of about $808\n\\rm~Mpc^{-3}$ with an energy density $\\Omega\\sim 1.4\\times 10^{-3}$. If the\nfirst pulse of this GRB near the trigger time is indeed gravitationally echoed\nby a second pulse, GRB 200716C may be a short GRB candidate with extended\nemission.", "category": "astro-ph_HE" }, { "text": "An outflow powers the optical rise of the nearby, fast-evolving tidal\n disruption event AT2019qiz: At 66 Mpc, AT2019qiz is the closest optical tidal disruption event (TDE) to\ndate, with a luminosity intermediate between the bulk of the population and\niPTF16fnl. Its proximity allowed a very early detection and triggering of\nmultiwavelength and spectroscopic follow-up well before maximum light. The\nvelocity dispersion of the host galaxy and fits to the TDE light curve indicate\na black hole mass $\\approx 10^6$ M$_\\odot$, disrupting a star of $\\approx 1$\nM$_\\odot$. Comprehensive UV, optical and X-ray data shows that the early\noptical emission is dominated by an outflow, with a luminosity evolution $L\n\\propto t^2$, consistent with a photosphere expanding at constant velocity\n($\\gtrsim 2000$ km s$^{-1}$), and a line-forming region producing initially\nblueshifted H and He II profiles with $v=3000-10000$ km s$^{-1}$. The fastest\noptical ejecta approach the velocity inferred from radio detections (modelled\nin a forthcoming companion paper from K.~D.~Alexander et al.), thus the same\noutflow may be responsible for both the fast optical rise and the radio\nemission -- the first time this connection has been observed in a TDE. The\nlight curve rise begins $29 \\pm 2$ days before maximum light, peaking when the\nphotosphere reaches the radius where optical photons can escape. The\nphotosphere then undergoes a sudden transition, first cooling at constant\nradius then contracting at constant temperature. At the same time, the\nblueshifts disappear from the spectrum and Bowen fluorescence lines (N III)\nbecome prominent, implying a source of far-UV photons, while the X-ray light\ncurve peaks at $\\approx 10^{41}$ erg s$^{-1}$. Assuming that these X-rays are\nfrom prompt accretion, the size and mass of the outflow are consistent with the\nreprocessing layer needed to explain the large optical to X-ray ratio in this\nand other optical TDEs, possibly favouring accretion-powered over\ncollision-powered outflow models.", "category": "astro-ph_HE" }, { "text": "Light curves and spectra from off-axis gamma-ray bursts: If gamma-ray burst prompt emission originates at a typical radius, and if\nmaterial producing the emission moves at relativistic speed, then the\nvariability of the resulting light curve depends on the viewing angle. This is\ndue to the fact that the pulse evolution time scale is Doppler contracted,\nwhile the pulse separation is not. For off-axis viewing angles $\\theta_{\\rm\nview} \\gtrsim \\theta_{\\rm jet} + \\Gamma^{-1}$, the pulse broadening\nsignificantly smears out the light curve variability. This is largely\nindependent of geometry and emission processes. To explore a specific case, we\nset up a simple model of a single pulse under the assumption that the pulse\nrise and decay are dominated by the shell curvature effect. We show that such a\npulse observed off-axis is (i) broader, (ii) softer and (iii) displays a\ndifferent hardness-intensity correlation with respect to the same pulse seen\non-axis. For each of these effects, we provide an intuitive physical\nexplanation. We then show how a synthetic light curve made by a superposition\nof pulses changes with increasing viewing angle. We find that a highly variable\nlight curve, (as seen on-axis) becomes smooth and apparently single-pulsed\n(when seen off-axis) because of pulse overlap. To test the relevance of this\nfact, we estimate the fraction of off-axis gamma-ray bursts detectable by\n\\textit{Swift} as a function of redshift, finding that a sizable fraction\n(between 10\\% and 80\\%) of nearby ($z<0.1$) bursts are observed with\n$\\theta_{\\rm view} \\gtrsim \\theta_{\\rm jet} + \\Gamma^{-1}$. Based on these\nresults, we argue that low luminosity gamma-ray bursts are consistent with\nbeing ordinary bursts seen off-axis.", "category": "astro-ph_HE" }, { "text": "Seven Years with the Swift Supergiant Fast X-ray Transients Project: Supergiant Fast X-ray Transients (SFXTs) are HMXBs with OB supergiant\ncompanions. I review the results of the Swift SFXT Project, which since 2007\nhas been exploiting Swift's capabilities in a systematic study of SFXTs and\nsupergiant X-ray binaries (SGXBs) by combining follow-ups of outbursts, when\ndetailed broad-band spectroscopy is possible, with long-term monitoring\ncampaigns, when the out-of-outburst fainter states can be observed. This\nstrategy has led us to measure their duty cycles as a function of luminosity,\nto extract their differential luminosity distributions in the soft X-ray\ndomain, and to compare, with unprecedented detail, the X-ray variability in\nthese different classes of sources. I also discuss the \"seventh year crisis\",\nthe challenges that the recent Swift observations are making to the prevailing\nmodels attempting to explain the SFXT behaviour.", "category": "astro-ph_HE" }, { "text": "A Genetic Algorithm for Astroparticle Physics Studies: Precision measurements of charged cosmic rays have recently been carried out\nby space-born (e.g. AMS-02), or ground experiments (e.g. HESS). These measured\ndata are important for the studies of astro-physical phenomena, including\nsupernova remnants, cosmic ray propagation, solar physics and dark matter.\nThose scenarios usually contain a number of free parameters that need to be\nadjusted by observed data. Some techniques, such as Markov Chain Monte Carlo\nand MultiNest, are developed in order to solve the above problem. However, it\nis usually required a computing farm to apply those tools. In this paper, a\ngenetic algorithm for finding the optimum parameters for cosmic ray injection\nand propagation is presented. We find that this algorithm gives us the same\nbest fit results as the Markov Chain Monte Carlo but consuming less computing\npower by nearly 2 orders of magnitudes.", "category": "astro-ph_HE" }, { "text": "Optical Counterparts of ULXs and Their Host Environments in NGC\n 4490/4485: We report the identification of the possible optical counterparts of five out\nof seven Ultraluminous X-ray Sources (ULXs) in NGC 4490/4485 galaxy pair. Using\narchival Hubble Space Telescope ({\\it HST}) imaging data, we identified a\nsingle optical candidate for two ULXs (X-4 and X-7) and multiple optical\ncandidates for the other three ULXs (X-2, X-3 and X-6) within $\\sim$ $0\\farcs2$\nerror radius at the 90\\% confidence level. Of the two remaining ULXs, X-1 has\nno {\\it HST} imaging data and photometry could not be performed due to the\nposition of X-5 in NGC4490. Absolute magnitudes ($M_{V}$) of the optical\ncandidates lie between $-5.7$ and $-3.8$. Color-Magnitude Diagrams (CMDs) have\nbeen used to investigate the properties of counterparts and their environments.\nThe locations of the counterparts of X-2, X-4, and X-6 suggest possible\nassociation with nearby group of stars while others have no association with a\nstar cluster or group of stars. For comparison purposes, we analyzed previously\nunused three archival XMM-Newton observations. The long-term X-ray light curves\nof the sources (except transient X-7) show variability by a factor of three in\na time scale more than a decade. The use of disk blackbody model for the mass\nof the compact objects indicates that these objects might have masses most\nlikely in the range 10$-$15 $M_{\\sun}$.", "category": "astro-ph_HE" }, { "text": "Radio WISSH: tuning on the most luminous quasars in the Universe: In the past years, the results obtained by the WISSH quasar project provided\na novel general picture on the distinctive multi-band properties of\nhyper-luminous ($L_{bol}>10^{47}$ erg/s) quasars at high redshift (z$\\sim$2-4),\nunveiling interesting relations among active galactic nuclei, winds and\ninterstellar medium, in these powerful sources at cosmic noon. Since 2022, we\nare performing a systematic and statistically-significant VLA study of the\nradio properties of WISSH. We carried out high-resolution VLA observations\naiming at: 1) identifying young radio source from the broad-band spectral shape\nof these objects; 2) sample an unexplored high redshift/high luminosity regime,\ntracking possible evolutionary effects on the radio-loud/radio-quiet dichotomy;\n3) quantifying orientation effects on the observed winds/outflows properties.", "category": "astro-ph_HE" }, { "text": "Contribution to the Extragalactic Gamma-ray Background from the Cascades\n of Very-high Energy Gamma Rays: As very-high--energy photons propagate through the extragalactic background\nlight (EBL), they interact with the soft photons and initiate electromagnetic\ncascades of lower energy photons and electrons. The collective intensity of a\ncosmological population emitting at very-high energies (VHE) will be attenuated\nat the highest energies through interactions with the EBL and enhanced at lower\nenergies by the resulting cascade. We calculate the cascade radiation created\nby VHE photons produced by blazars and investigate the effects of cascades on\nthe collective intensity of blazars and the resulting effects on the\nextragalactic gamma-ray background. We find that cascade radiation greatly\nenhances the collective intensity from blazars at high energies before turning\nover due to attenuation. The prominence of the resulting features depends on\nthe blazar gamma-ray luminosity function, spectral index distribution, and the\nmodel of the EBL. We additionally calculate the cascade radiation from the\ndistinct spectral sub-populations of blazars, BL Lacertae objects (BL Lacs) and\nflat-spectrum radio quasars (FSRQs), finding that the collective intensity of\nBL Lacs is considerably more enhanced by cascade radiation than that of the\nFSRQs due to their harder spectra. As such, studies of the blazar contribution\nto the EGRB by Fermi will have profound implications for the nature of the EBL,\nthe evolution of blazars, and blazar spectra.", "category": "astro-ph_HE" }, { "text": "A young SNR illuminating nearby Molecular Clouds with cosmic rays: The Supernova Remnant (SNR) HESS J1731-347 displays strong non-thermal TeV\ngamma-ray and X-ray emission, thus the object is at present time accelerating\nparticles to very high energies. A distinctive feature of this young SNR is the\nnearby (~30 pc in projection) extended source HESS J1729-345, which is\ncurrently unidentified but is in spatial projection coinciding with known\nmolecular clouds (MC). We model the SNR evolution to explore if the TeV\nemission from HESS J1729-345 can be explained as emission from runaway hadronic\ncosmic rays (CRs) that are illuminating these MCs. The observational data of\nHESS J1729-345 and HESS J1731-347 can be reproduced using core-collapse SN\nmodels for HESS J1731-347. Starting with different progenitor stars and their\npre-supernova environment, we model potential SNR evolution histories along\nwith the CR acceleration in the SNR and the diffusion of the CRs. A simplified\n3-dimensional structure of the MCs is introduced based on 12CO data, adopting a\ndistance of 3.2 kpc to the source. A Monte Carlo-based diffusion model for the\nescaping CRs is developed to deal with the inhomogeneous environment. The fast\nSNR forward shock speed as implied from the X-ray data can easily be explained\nwhen employing scenarios with progenitor star masses between 20 and 25 solar\nmasses, where the SNR shock is still expanding inside the main sequence\n(MS)-bubble at present time. The TeV spectrum of HESS J1729-345 is\nsatisfactorily fitted by the emission from the highest-energy CRs that have\nescaped the SNR, using a standard galactic CR diffusion coefficient in the\ninter-clump medium. The TeV image of HESS J1729-345 can be explained with a\nreasonable 3-dimensional structure of MCs. The TeV emission from the SNR itself\nis dominated by leptonic emission in this model. We also explore scenarios\nwhere the shock is starting to encounter the dense MS progenitor wind bubble\nshell.", "category": "astro-ph_HE" }, { "text": "SN 2015an: a normal luminosity type II supernova with low expansion\n velocity at early phases: We present the photometry and spectroscopy of SN 2015an, a Type II Supernova\n(SN) in IC 2367. The recombination phase of the SN lasts up to $\\sim$120 d,\nwith a decline rate of 1.24 mag/100d, higher than the typical SNe IIP. The SN\nexhibits bluer colours than most SNe II, indicating higher ejecta temperatures.\nThe absolute $V$-band magnitude of SN 2015an at 50 d is $-$16.83$\\pm$0.04 mag,\npretty typical for SNe II. However, the $^{56}$Ni mass yield, estimated from\nthe tail $V$-band light curve to be 0.021$\\pm$0.010 M$_\\odot$, is comparatively\nlow. The spectral properties of SN 2015an are atypical, with low H$\\alpha$\nexpansion velocity and presence of high velocity component of H$\\alpha$ at\nearly phases. Moreover, the continuum exhibits excess blue flux up to $\\sim$50\nd, which is interpreted as a progenitor metallicity effect. The high velocity\nfeature indicates ejecta-circumstellar material interaction at early phases.\nThe semi-analytical modelling of the bolometric light curve yields a total\nejected mass of $\\sim$12 M$_\\odot$, a pre-supernova radius of\n$\\sim$388~R$_\\odot$ and explosion energy of $\\sim$1.8 foe.", "category": "astro-ph_HE" }, { "text": "Early-time VLA observations and broad-band afterglow analysis of the\n Fermi-LAT detected GRB 130907A: We present multi-wavelength observations of the hyper-energetic gamma-ray\nburst (GRB) 130907A, a Swift-discovered burst with early radio observations\nstarting at $\\approx 4$ hr after the $\\gamma$-ray trigger. GRB 130907A was also\ndetected by the Fermi/LAT instrument and, at late times, showed a strong\nspectral evolution in X-rays. We focus on the early-time radio observations,\nespecially at $>10 $ GHz, to attempt identifying reverse shock signatures.\nWhile our radio follow-up of GRB 130907A ranks among the earliest observations\nof a GRB with the Karl G. Jansky Very Large Array (VLA), we did not see an\nunambiguous signature of a reverse shock. While a model with both reverse and\nforward-shock can correctly describe the observations, the data is not\nconstraining enough to decide upon the presence of the reverse-shock component.\nWe model the broad-band data using a simple forward-shock synchrotron scenario\nwith a transition from a wind environment to a constant density interstellar\nmedium (ISM) in order to account for the observed features. Within the confines\nof this model, we also derive the underlying physical parameters of the\nfireball, which are within typical ranges except for the wind density parameter\n($A_*$), which is higher than those for bursts with wind-ISM transition, but\ntypical for the general population of bursts. We note the importance of\nearly-time radio observations of the afterglow (and of well sampled light\ncurves) to unambiguously identify the potential contribution of the reverse\nshock.", "category": "astro-ph_HE" }, { "text": "Fornax A, Centaurus A and other radio galaxies as sources of ultra-high\n energy cosmic rays: The origin of ultra-high energy cosmic rays (UHECRs) is still unknown. It has\nrecently been proposed that UHECR anisotropies can be attributed to starburst\ngalaxies or active galactic nuclei. We suggest that the latter is more likely\nand that giant-lobed radio galaxies such as Centaurus A and Fornax A can\nexplain the data.", "category": "astro-ph_HE" }, { "text": "Gamma Ray Signal from the Pulsar Wind in the Binary Pulsar system PSR\n B1259-63/LS2883: Binary pulsar systems emit potentially detectable components of gamma ray\nemission due to Comptonization of the optical radiation of the companion star\nby relativistic electrons of the pulsar wind, both before and after termination\nof the wind. The recent optical observations of binary pulsar system PSR\nB1259-63/LS 2883 revealed radiation properties of the companion star which\ndiffer significantly from previous measurements. In this paper we study the\nimplications of these observations for the interaction rate of the unshocked\npulsar wind with the stellar photons and the related consequences for fluxes of\nhigh energy (HE) and very high energy (VHE) gamma rays. We show that the signal\nshould be strong enough to be detected with Fermi close to the periastron\npassage, unless the pulsar wind is strongly anisotropic or the Lorentz factor\nof the wind is smaller than $10^3$ or larger that $10^5$. The higher luminosity\nof the optical star also has two important implications: (i) attenuation of\ngamma rays due to photon-photon pair production, and (ii) Compton drag of the\nunshocked wind. While the first effect has an impact on the lightcurve of VHE\ngamma rays, the second effect may significantly decrease the energy available\nfor particle acceleration after termination of the wind.", "category": "astro-ph_HE" }, { "text": "First observation of PeV-energy neutrinos with IceCube: We report on the observation of two neutrino-induced events which have an\nestimated deposited energy in the IceCube detector of 1.04 $\\pm$ 0.16 and 1.14\n$\\pm$ 0.17 PeV, respectively, the highest neutrino energies observed so far.\nThese events are consistent with fully contained particle showers induced by\nneutral-current $\\nu_{e,\\mu,\\tau}$ ($\\bar\\nu_{e,\\mu,\\tau}$) or charged-current\n$\\nu_{e}$ ($\\bar\\nu_{e}$) interactions within the IceCube detector. The events\nwere discovered in a search for ultra-high energy neutrinos using data\ncorresponding to 615.9 days effective livetime. The expected number of\natmospheric background is $0.082 \\pm 0.004 \\text{(stat)}^{+0.041}_{-0.057}\n\\text{(syst)}$. The probability to observe two or more candidate events under\nthe atmospheric background-only hypothesis is $2.9\\times10^{-3}$ ($2.8\\sigma$)\ntaking into account the uncertainty on the expected number of background\nevents. These two events could be a first indication of an astrophysical\nneutrino flux, the moderate significance, however, does not permit a definitive\nconclusion at this time.", "category": "astro-ph_HE" }, { "text": "Properties of neutrino transfer in a deformed remnant of neutron star\n merger: We study properties of neutrino transfer in a remnant of neutron star merger,\nconsisting of a massive neutron star and a surrounding torus. We perform\nnumerical simulations of the neutrino transfer by solving the Boltzmann\nequation with momentum-space angles and energies of neutrinos for snapshots of\nthe merger remnant having elongated shapes. The evaluation of the neutrino\ndistributions in the multi-dimensions enable us to provide the detailed\ninformation of angle and energy spectra and neutrino reaction rates. We\ndemonstrate features of asymmetric neutrino fluxes from the deformed remnant\nand investigate the neutrino emission region by determining the neutrinosphere\nfor each energy. We examine the emission and absorption of neutrinos to\nidentify important ingredients of heating rates through neutrino irradiation.\nWe show that the contributions of $\\mu$- and $\\tau$-types neutrinos are\nimportant for the heating in the region above the massive neutron star. We also\nexamine the angle moments and the Eddington tensor calculated directly by the\nneutrino distribution functions and compare them with those obtained by a\nmoment closure approach, which is often used in the study of neutrino-radiation\nhydrodynamics. We show that the components of the Eddington tensor have\nnon-monotonic behaviors and the approximation of the closure relation may\nbecome inaccurate for high energy neutrinos, whose fluxes are highly aspherical\ndue to the extended merger remnant.", "category": "astro-ph_HE" }, { "text": "Investigation of the particle-particle interaction effects in the cosmic\n Zevatron based on cyclotron auto-resonance by particle-in-cell simulations: Cyclotron autoresonance acceleration has been recently advanced as a\npotential mechanism for accelerating nuclei to ZeV energies (1 ZeV = $10^{21}$\neV). All results have been based on single- and many-particle calculations\nemploying analytic solutions to the relativistic equations of motion in the\ncombined magnetic and radiation fields, excluding effects related to the\nparticle-particle interactions. Here, results from many-particle calculations\nand Particle-In-Cell (PIC) simulations, are presented which lend support to the\nsingle-particle investigations. Each single-particle result is found to lie\nwell within one standard deviation about the ensemble average obtained from the\ncorresponding many-particle calculation. The PIC simulations show that, even\nfor number densities far exceeding those employed in the non-interacting case,\nthe energy gain drops markedly due to the particle-particle interactions, over\nthe first $\\sim 8~ mm$ of the acceleration length. Together with the\nsubstantial attenuation, this finding supports the conclusion that the\nparticle-particle interaction effects can be negligibly small over acceleration\nlengths of typically many kilometers.", "category": "astro-ph_HE" }, { "text": "Search for Tens of MeV Neutrinos associated with Gamma-Ray Bursts in\n Super-Kamiokande: A search for neutrinos produced in coincidence with Gamma-Ray Bursts(GRB) was\nconducted with the Super-Kamiokande (SK) detector. Between December 2008 and\nMarch 2017, the Gamma-ray Coordinates Network recorded 2208 GRBs that occurred\nduring normal SK operation. Several time windows around each GRB were used to\nsearch for coincident neutrino events. No statistically significant signal in\nexcess of the estimated backgrounds was detected. The $\\bar\\nu_e$ fluence in\nthe range from 8 MeV to 100 MeV in positron total energy for\n$\\bar\\nu_e+p\\rightarrow e^{+}+n$ was found to be less than $\\rm 5.07\\times10^5$\ncm$^{-2}$ per GRB in 90\\% C.L. Upper bounds on the fluence as a function of\nneutrino energy were also obtained.", "category": "astro-ph_HE" }, { "text": "On the Particle Acceleration Mechanisms in a Double Radio Relic Galaxy\n Cluster, Abell 1240: We present a 368 ks deep Chandra observation of Abell~1240, a binary merging\ngalaxy cluster at a redshift of 0.195 with two Brightest Cluster Galaxies\n(BCGs) may have passed each other 0.3 Gyr ago. Building upon previous\ninvestigations involving GMRT, VLA, and LOFAR data, our study focuses on two\nprominent extended radio relics at the north-west (NW) and south-east (SE) of\nthe cluster core. By leveraging the high-resolution Chandra imaging, we have\nidentified two distinct surface brightness edges at $\\sim$ 1 Mpc and 1.2 Mpc NW\nand SE of the cluster center, respectively, coinciding with the outer edges of\nboth relics. Our temperature measurements hint the edges to be shock front\nedges. The Mach numbers, derived from the gas density jumps, yield\n$\\cal{M}_{\\rm SE}$ = 1.49$^{+0.22}_{-0.24}$ for the South Eastern shock and\n$\\cal{M}_{\\rm NW}$ = 1.41$^{+0.17}_{-0.19}$ for the North Western shock. Our\nestimated Mach numbers are remarkably smaller compared to those derived from\nradio observations ($\\cal{M}_{\\rm SE}$ = 2.3 and $\\cal{M}_{\\rm NW}$ = 2.4),\nhighlighting the prevalence of a re-acceleration scenario over direct\nacceleration of electrons from the thermal pool. Furthermore, we compare the\nobserved temperature profiles across both shocks with that of predictions from\ncollisional vs. collisionless models. Both shocks favor the Coulomb collisional\nmodel, but we could not rule out a purely collisionless model due to pre-shock\ntemperature uncertainties.", "category": "astro-ph_HE" }, { "text": "Different behaviors of wavelet results for type-B and type-C QPOs of\n MAXI J1535-571 based on NICER data: Wavelet analysis, in addition to power density spectra, is another method to\nstudy the quasi-periodic signals in the light curves, but has been rarely used\nin black hole X-ray transients. We performed wavelet analysis of X-ray timing\nfeatures and quasi-periodic oscillations (QPOs) based on NICER observations of\nthe black hole candidate MAXI J1535-571 in this paper. Separating the light\ncurves by the confidence level of wavelet results, we find significant\ndifferences exist in the PDS, hardness ratio and mean count between light curve\nsegments above and below the confidence level. The S-factor, which is defined\nas the ratio of the effective oscillation time and the total time, demonstrates\ndistinct values between type-C and type-B QPOs. Based on our results, the\nS-factor for type-B QPO is very close or equal to 0, no matter the confidence\nlevel is set as 95\\% or 68\\%, while the S-factor of type-C QPO is significantly\nhigher, especially in the 68\\% confidence level case. We discuss the\nimplications of the wavelet results on resolving type-B and type-C QPOs in\nblack hole X-ray binaries.", "category": "astro-ph_HE" }, { "text": "Non-Maxwellian electron distributions in clusters of galaxies: Thermal X-ray spectra of clusters of galaxies and other sources are commonly\ncalculated assuming Maxwellian electron distributions. There are situations\nwhere this approximation is not valid, for instance near interfaces of hot and\ncold gas and near shocks. The presence of non-thermal electrons affects the\nX-ray spectrum. To study the role of these electrons in clusters and other\nenvironments, an efficient algorithm to calculate the X-ray spectra is needed.\nWe approximate an arbitrary electron distribution by the sum of Maxwellian\ncomponents. The decomposition is done using either a genetic algorithm or an\nanalytical approximation. The X-ray spectrum is then evaluated using a linear\ncombination of those Maxwellian components. Our method is fast and leads to an\naccurate evaluation of the spectrum. The use of Maxwellian components allows to\nuse the standard collisional rates that are available in plasma codes such as\nSPEX. We give an example of a spectrum for the supra-thermal electron\ndistribution behind a shock in a cluster of galaxies. The relative intensities\nof the satellite lines in such a spectrum are sensitive to the presence of the\nsupra-thermal electrons. These lines can only be investigated with high\nspectral resolution. We show that the instruments on future missions like\nAstro-H and IXO will be able to demonstrate the presence or absence of these\nsupra-thermal electrons.", "category": "astro-ph_HE" }, { "text": "A Compton-thin Solution for the Suzaku X-ray Spectrum of the Seyfert 2\n Galaxy Mkn 3: Mkn 3 is a Seyfert 2 galaxy that is widely regarded as an exemplary\nCompton-thick AGN. We study the Suzaku X-ray spectrum using models of the X-ray\nreprocessor that self-consistently account for the Fe K$\\alpha$ fluorescent\nemission line and the associated Compton-scattered, or reflection, continuum.\nWe find a solution in which the average global column density,\n$0.234^{+0.012}_{-0.010} \\times 10^{24} \\ \\rm cm^{-2}$, is very different to\nthe line-of-sight column density, $0.902^{+0.012}_{-0.013} \\times 10^{24} \\ \\rm\ncm^{-2}$. The global column density is $\\sim 5$ times smaller than that\nrequired for the matter distribution to be Compton-thick. Our model accounts\nfor the profiles of the Fe K$\\alpha$ and Fe K$\\beta$ lines, and the Fe K edge\nremarkably well, with a solar abundance of Fe. The matter distribution could\nconsist of a clumpy medium with a line-of-sight column density higher than the\nglobal average. A uniform, spherically-symmetric distribution alone cannot\nsimultaneously produce the correct fluorescent line spectrum and reflection\ncontinuum. Previous works on Mkn 3, and other AGN, that assumed a reflection\ncontinuum from matter with an infinite column density could therefore lead to\nerroneous or \"puzzling\" conclusions if the matter out of the line-of-sight is\nreally Compton-thin. Whereas studies of samples of AGN have generally only\nprobed the line-of-sight column density, with simplistic, one-dimensional\nmodels, it is important now to establish the global column densities in AGN. It\nis the global properties that affect the energy budget in terms of reprocessing\nof X-rays into infrared emission, and that constrain population synthesis\nmodels of the cosmic X-ray background.", "category": "astro-ph_HE" } ]