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Search for Outbursts in the Narrow 511-keV Line from Compact Sources
Based on INTEGRAL Data: We present the results of a systematic search for outbursts in the narrow
positron annihilation line on various time scales (5x10^4 - 10^6 s) based on
the SPI/INTEGRAL data obtained from 2003 to 2008. We show that no outbursts
were detected with a statistical significance higher than ~6 sigma for any of
the time scales considered over the entire period of observations. We also show
that, given the large number of independent trials, all of the observed spikes
could be associated with purely statistical flux fluctuations and, in part,
with a small systematic prediction error of the telescope's instrumental
background. Based on the exposure achieved in ~6 yr of INTEGRAL operation, we
provide conservative upper limits on the rate of outbursts with a given
duration and flux in different parts of the sky. | astro-ph |
Testing the Frozen-Flow Approximation: We investigate the accuracy of the frozen--flow approximation (FFA), recently
proposed by Matarrese \etal (1992), for following the nonlinear evolution of
cosmological density fluctuations under gravitational instability. We compare a
number of statistics between results of the FFA and nbody simulations,
including those used by Melott, Pellman \& Shandarin (1993) to test the
Zel'dovich approximation. The FFA performs reasonably well in a statistical
sense, e.g. in reproducing the counts--in--cell distribution, at small scales,
but it does poorly in the crosscorrelation with nbody which means it is
generally not moving mass to the right place, especially in models with high
small--scale power. | astro-ph |
Full Monte Carlo simulations of radio emission from extensive air
showers with CoREAS: CoREAS is a Monte Carlo simulation code for the calculation of radio emission
from extensive air showers. It is based on the "endpoint formalism" for
radiation from moving charges implemented directly in CORSIKA. Consequently,
the full complexity of the air-shower physics is taken into account without the
need for approximations or assumptions on the emission mechanism. We present
results of simulations for an unthinned shower performed with CoREAS for both
MHz and GHz frequencies. At MHz frequencies, the simulations predict the
well-known mixture of geomagnetic and charge excess radiation. At GHz
frequencies, the emission is strongly influenced by Cherenkov effects arising
from the varying refractive index in the atmosphere. In addition, a qualitative
difference in the symmetry of the GHz radiation pattern is observed when
compared to the ones at lower frequencies. We also discuss the strong increase
in the ground area subtended by the radio emission when going from
near-vertical to very inclined geometries, making very inclined air showers the
most promising ones for cosmic ray radio detection. | astro-ph |
Rest-frame UV single-epoch black hole mass estimates of low-luminosity
AGN at intermediate redshifts: The ability to accurately derive black hole (BH) masses at progressively
higher redshifts and over a wide range of continuum luminosities has become
indispensable in the era of large-area extragalactic spectroscopic surveys. In
this paper we present an extension of existing comparisons between rest-frame
UV and optical virial BH mass estimators to intermediate redshifts and
luminosities comparable to the local H$\beta$ reverberation mapped active
galactic nuclei (AGN). We focus on the MgII, CIV, and CIII] broad emission
lines and compare them to both H$\alpha$ and H$\beta$. We use newly acquired
near-infrared spectra from the FMOS instrument on the Subaru telescope for 89
broad-lined AGN at redshifts between 0.3 and 3.5, complemented by data from the
AGES survey. We employ two different prescriptions for measuring the emission
line widths and compare the results. We confirm that MgII shows a tight
correlation with H$\alpha$ and H$\beta$, with a scatter of ~0.25 dex. The CIV
and CIII] estimators, while showing larger scatter, are viable virial mass
estimators after accounting for a trend with the UV-to-optical luminosity
ratio. We find an intrinsic scatter of ~0.37 dex between Balmer and carbon
virial estimators by combining our dataset with previous high redshift
measurements. This updated comparison spans a total of 3 decades in BH mass. We
calculate a virial factor for CIV/CIII] logf(CIV/CIII])=0.87 with an estimated
systematic uncertainty of ~0.4 dex and find excellent agreement between the
local reverberation mapped AGN sample and our high-z sample. | astro-ph |
Detrending algorithms in large time-series: Application to TFRM-PSES
data: Certain instrumental effects and data reduction anomalies introduce
systematic errors in photometric time-series. Detrending algorithms such as the
Trend Filtering Algorithm (TFA) (Kov\'{a}cs et al. 2004) have played a key role
in minimizing the effects caused by these systematics. Here we present the
results obtained after applying the TFA, Savitszky-Golay (Savitzky & Golay
1964) detrending algorithms and the Box Least Square phase folding algorithm
(Kov\'{a}cs et al. 2002) to the TFRM-PSES data (Fors et al. 2013). Tests
performed on this data show that by applying these two filtering methods
together, the photometric RMS is on average improved by a factor of 3-4, with
better efficiency towards brighter magnitudes, while applying TFA alone yields
an improvement of a factor 1-2. As a result of this improvement, we are able to
detect and analyze a large number of stars per TFRM-PSES field which present
some kind of variability. Also, after porting these algorithms to Python and
parallelizing them, we have improved, even for large data samples, the
computing performance of the overall detrending+BLS algorithm by a factor of
$\sim$10 with respect to Kov\'{a}cs et al. (2004). | astro-ph |
Proper Motions of H2O Masers in the Water Fountain Source IRAS
19190+1102: We report on the results of two epochs of Very Long Baseline Array (VLBA)
observations of the 22 GHz water masers toward IRAS 19190+1102. The water maser
emission from this object shows two main arc-shaped formations perpendicular to
their NE-SW separation axis. The arcs are separated by ~280 mas in position,
and are expanding outwards at an angular rate of 2.35 mas/yr. We detect maser
emission at velocities between -53.3 km/s to +78.0 km/s and there is a distinct
velocity pattern where the NE masers are blueshifted and the SW masers are
redshifted. The outflow has a three-dimensional outflow velocity of 99.8 km/s
and a dynamical age of about 59 yr. A group of blueshifted masers not located
along the arcs shows a change in velocity of more than 35 km/s between epochs,
and may be indicative of the formation of a new lobe. These observations show
that IRAS 19190+1102 is a member of the class of "water fountain"'
pre-planetary nebulae displaying bipolar structure | astro-ph |
Radio Relics in Clusters of Galaxies: In this paper we review the observational results on Relic radio sources in
clusters of galaxies. We discuss their observational properties, structures and
radio spectra. We will show that Relics can be divided according to their size,
morphology, and location in the galaxy cluster. These differences could be
related to physical properties of Relic sources. The comparison with cluster
conditions suggests that Relics could be related to shock waves originated by
cluster mergers. | astro-ph |
Energy balance and Alfvén Mach numbers in compressible
magnetohydrodynamic turbulence with a large-scale magnetic field: Energy equipartition is a powerful theoretical tool for understanding
astrophysical plasmas. It is invoked, for example, to measure magnetic fields
in the interstellar medium (ISM), as evidence for small-scale turbulent dynamo
action, and, in general, to estimate the energy budget of star-forming
molecular clouds. In this study we motivate and explore the role of the
volume-averaged root-mean-squared (rms) magnetic coupling term between the
turbulent, $\delta\mathbf{B}$ and large-scale, $\mathbf{B}_0$ fields, $\left<
(\delta\mathbf{B}\cdot\mathbf{B}_0)^{2} \right>^{1/2}_{\mathcal{V}}$. By
considering the second moments of the energy balance equations we show that the
rms coupling term is in energy equipartition with the volume-averaged turbulent
kinetic energy for turbulence with a sub-Alfv\'enic large-scale field. Under
the assumption of exact energy equipartition between these terms, we derive
relations for the magnetic and coupling term fluctuations, which provide
excellent, parameter-free agreement with time-averaged data from 280 numerical
simulations of compressible MHD turbulence. Furthermore, we explore the
relation between the turbulent, mean-field and total Alfv\'en Mach numbers, and
demonstrate that sub-Alfv\'enic turbulence can only be developed through a
strong, large-scale magnetic field, which supports an extremely
super-Alfv\'enic turbulent magnetic field. This means that the magnetic field
fluctuations are significantly subdominant to the velocity fluctuations in the
sub-Alfv\'enic large-scale field regime. Throughout our study, we broadly
discuss the implications for observations of magnetic fields and understanding
the dynamics in the magnetised ISM. | astro-ph |
Radio Emission from UV Cet: Auroral Emission from a Stellar
Magnetosphere: The archetypical flare star UV Cet was observed by MeerKAT on 5-6 October
2021. A large radio outburst with a duration of $\sim\!2$ hr was observed
between 886-1682 MHz with a time resolution of 8s and a frequency resolution of
0.84 MHz, enabling sensitive dynamic spectra to be formed. The emission is
characterized by three peaks containing a multitude of broadband arcs or
partial arcs in the time-frequency domain. In general, the arcs are highly
right-hand circularly polarized. During end of the third peak, brief bursts
occur that are significantly elliptically polarized. We present a simple model
that appears to be broadly consistent with the characteristics of the radio
emission from UV Cet. Briefly, the stellar magnetic field is modeled as a
dipole aligned with the rotational axis of the star. The radio emission
mechanism is assumed to be due to the cyclotron maser instability where x-mode
radiation near the electron gyrofrequency is amplified. While the elliptically
polarized bursts may be intrinsic to the source, rather stringent limits are
imposed on the plasma density in the source and along the propagation path. We
suggest that the elliptically polarized radiation may instead be the result of
reflection on an over-dense plasma structure at some distance from the source.
Radio emission from UV~Cet shares both stellar and planetary attributes. | astro-ph |
A multidimensional hydrodynamic code for structure evolution in
cosmology: A cosmological multidimensional hydrodynamic code is described and tested.
This code is based on modern high-resolution shock-capturing techniques. It can
make use of a linear or a parabolic cell reconstruction as well as an
approximate Riemann solver. The code has been specifically designed for
cosmological applications. Two tests including shocks have been considered: the
first one is a standard shock tube and the second test involves a spherically
symmetric shock. Various additional cosmological tests are also presented. In
this way, the performance of the code is proved. The usefulness of the code is
discussed; in particular, this powerful tool is expected to be useful in order
to study the evolution of the hot gas component located inside nonsymmetric
cosmological structures. | astro-ph |
Statistical Analysis of Soft X-Ray Solar Flares During Solar Cycles 21,
22 and 23: This paper presents a statistical analysis of Soft X-ray (SXR) flares during
the period January 1976 to December 2007 covering solar cycles (SCs) 21, 22,
and 23. We have analysed north-south (N-S) and east-west (E-W) asymmetry of SXR
at low (less than equal to 40 degree), high (greater than equal to 50 degree)
and total latitudes and center meridian distances (CMDs) respectively. We have
also presented the N-S and E-W asymmetry of different intensity classes (B, C,
M, and X) during the period of investigation. A slight southern and eastern
excess is found after analysis during SC 21, 22, and 23. We found that the
annual N-S and E-W hemispheric asymmetry at low latitudes and CMDs is the same
as total latitudes and CMDs respectively. E-W asymmetry is different at low and
high CMDs. Our statistical result shows that N-S asymmetry is statistically
more significant than E-W asymmetry. Total SXR flare activity during SC 23 is
high compared to SC 21 and 22. The B class flare activity is higher for SC 23
where as C, M and X class activities are higher for SC 21. We have also
analysied the flare evolution parameters, i.e. duration, rise time, decay time
and event asymmetry for total SXR as well as for different classes for last
three SCs. The duration, rise time and decay time increase with increasing
intensity class. On analysing event asymmetry indices, we found more positive
values during SC 21 (64.86 per cent) and SC 22 (54.31 per cent), but for SC 23
we have more negative values (48.08 per cent). Our study shows that during SC
23 we have more SXR flare events having shorter decay time as compared to SC 21
and SC 22. | astro-ph |
Search for sub-millimeter H2O masers in active galaxies - the detection
of a 321 GHz H2O maser in NGC4945: We present further results of a search for extragalactic submillimeter H2O
masers using the Atacama Large Millimeter/Submillimeter Array (ALMA). The
detection of a 321 GHz H2O maser in the nearby Type 2 Seyfert galaxy, the
Circinus galaxy, has previously been reported, and here the spectral analysis
of four other galaxies is described. We have discovered H2O maser emission at
321 GHz toward the center of NGC 4945, a nearby Type 2 Seyfert. The maser
emission shows Doppler-shifted velocity features with velocity ranges similar
to those of the previously reported 22 GHz H2O masers, however the
non-contemporaneous observations also show differences in velocity offsets. The
sub-parsec-scale distribution of the 22 GHz H2O masers revealed by earlier VLBI
(Very Long Baseline Interferometry) observations suggests that the
submillimeter masers could arise in an edge-on rotating disk. The maser
features remain unresolved by the synthesized beam of ~ 0.54 (~30 pc) and are
located toward the 321 GHz continuum peak within errors. A marginally detected
(3 sigma) high-velocity feature is redshifted by 579 km/s with respect to the
systemic velocity of the galaxy. Assuming that this feature is real and arises
from a Keplerian rotating disk in this galaxy, it is located at a radius of
~0.020 pc (~1.5 x 10^5 Schwarzschild radii), which would enable molecular
material closer to the central engine to be probed than the 22 GHz H2O masers.
This detection confirms that submillimeter H2O masers are a potential tracer of
the circumnuclear regions of active galaxies, which will benefit from higher
angular resolution studies with ALMA. | astro-ph |
The effects of relativistic bulk motion of X-ray flares in the corona on
the iron Kalpha line in Seyfert 1 galaxies: We study the effects of the bulk motion of X-ray flares on the shape and
equivalent width of the iron Kalpha line from an untruncated cold disk around a
Kerr black hole using fully relativistic calculations. The flares are located
above a cold accretion disk -- either on or off the rotation axis. For on- or
off-axis flares, the upward/outward bulk motion causes a reduction of the iron
Kalpha line width. To a distant observer with a low inclination angle (\theta_o
\simlt 30deg.), larger upward/outward bulk velocities decrease the extension of
the red wing, with little change in the location of the blue `edge'. In
contrast, an observer at a large inclination angle (e.g. \theta_o=60deg.) sees
both the red wing and the blue `edge' change with the bulk velocity. The
equivalent width of the iron Kalpha line decreases rapidly with increasing bulk
velocity of flares. However, the `narrower' line profiles observed in some
objects (e.g. IC4329A and NGC4593) are difficult to produce using the
out-flowing magnetic flare model with an appropriate equivalent width unless
the X-ray emission is concentrated in an outer region with a radius of several
tens of r_g=GM/c^2 or more. An important result is that the iron Kalpha line
intensity is found to be constant even though the continuum flux varies
significantly, which is true for out-flowing magnetic flares with different
bulk velocities but similar intrinsic luminosities when located close to the
central black hole. We find that fluctuations in the bulk velocities of
out-flowing low-height flares located at the inner region (r\simlt 15r_g) can
account for a constant iron Kalpha line and significant continuum variation as
observered in MCG-6-30-15 and NGC5548. (Abridged) | astro-ph |
Statistical properties of flares and sunspots over the solar cycle: The present paper reviews results derived from statistical studies on solar
activity indices. The prolonged minimum phase of cycle 23 raised the question
of peculiarities inherent in cycle 23. The most important solar activity index
is the relative sunspot number and though most of other indices are closely
related, shifts are obtained between their peak activity of the order of 1-2
years. These shifts reveal a 22-yr pattern which can be attributed to solar
interior or dynamo related processes. The minimum phase of cycle 23 is not
found to be exceptional. Investigating the relative sunspot numbers over the
past 150 years, solar cycles of more prolonged minima are observed. Since 1920
solar activity is quite high ('modern maximum') and cycle 23 might be the
herald of the end of this phase. | astro-ph |
Radiative and dynamic stability of a dilute plasma: We analyze the linear stability of a dilute, hot plasma, taking into account
the effects of stratification and anisotropic thermal conduction. The work is
motivated by attempts to understand the dynamics of the intracluster medium in
galaxy clusters. We show that magnetic field configurations that nominally
stabilize either the heat-flux driven buoyancy instability (associated with a
positive thermal gradient) or the magnetothermal instability (negative thermal
gradient) can lead to previously unrecognized g-mode overstabilities. The
driving source of the overstability is either radiative cooling (positive
temperature gradient) or the heat flux itself (negative temperature gradient).
While the implications of these overstabilities have yet to be explored, we
speculate that the cold fronts observed in many relaxed galaxy clusters may be
related to their non-linear evolution. | astro-ph |
Intrinsic Instrumental Polarization and High-Precision Pulsar Timing: Radio telescopes are used to accurately measure the time of arrival (ToA) of
radio pulses in pulsar timing experiments that target mostly millisecond
pulsars (MSPs) due to their high rotational stability. This allows for detailed
study of MSPs and forms the basis of experiments to detect gravitational waves.
Apart from intrinsic and propagation effects, such as pulse-to-pulse jitter and
dispersion variations in the interstellar medium, timing precision is limited
in part by the following: polarization purity of the telescope's orthogonally
polarized receptors, the signal-to-noise ratio (S/N) of the pulsar profile, and
the polarization fidelity of the system. Using simulations, we present how
fundamental limitations in recovering the true polarization reduce the
precision of ToA measurements. Any real system will respond differently to each
source observed depending on the unique pulsar polarization profile. Using the
profiles of known MSPs we quantify the limits of observing system
specifications that yield satisfactory ToA measurements, and we place a
practical design limit beyond which improvement of the system results in
diminishing returns. Our aim is to justify limits for the front-end
polarization characteristics of next generation radio telescopes, leading to
the Square Kilometre Array (SKA). | astro-ph |
WIMP decay as a possible Warm Dark Matter model: The Weakly Interacting Massive Particles(WIMPs) have long been the favored
CDM candidate in the standard $\Lambda$CDM model. However, owing to great
improvement in the experimental sensitivity in the past decade, some parameter
space of the SUSY-based WIMP model is ruled out. In addition, WIMP as the CDM
particle is also at variance with other astrophysical observables at small
scales. We consider a model that addresses both these issues. In the model, the
WIMP decays into a massive particle and radiation. We study the background
evolution and the first order perturbation theory (coupled Einstein-Boltzmann
equations) for this model and show that the dynamics can be captured by a
single parameter $r=m_L/q$, which is the ratio of the lighter mass and the
comoving momentum of the decay particle. We incorporate the relevant equations
in the existing Boltzmann code CLASS to compute the matter power spectra and
CMB angular power spectra. The decaying WIMP model is akin to a non-thermal
Warm Dark Matter(WDM) model and suppresses matter power at small scales, which
could alleviate several issues that plague the CDM model. We compare the
predictions of the model with CMB, galaxy clustering, and high-z HI data. Both
these data sets yield $r\gtrsim 10^6$, which can be translated into the bounds
on other parameters. In particular, we obtain the following lower bounds on the
thermally-averaged self-annihilation cross-section of WIMPs $\langle \sigma v
\rangle$, and the lighter mass $m_L$: $\langle \sigma v \rangle \gtrsim
4.9\times 10^{-34} \, \rm cm^3 \, sec^{-1}$ and $m_L \gtrsim 2.4 \, \rm keV$.
The lower limit on $m_L$ is comparable to constraints on the mass of
thermally-produced WDM particle. The limit on the self-annihilation
cross-section greatly expands the available parameter space as compared to the
stable WIMP scenario. | astro-ph |
WMAP2006: Cosmological Parameters and Large-scale Structure of the
Universe: The parameters of cosmological model with cold dark matter and cosmological
constant (Lambda CDM) have been determined on a basis of three-year cosmic
microwave background observations by space mission WMAP, as well as the data on
the large-scale structure of the Universe. The data cover scales from 1 up to
10000 Mpc. The best-fit values of LambdaCDM model parameters were found by
minimization of chi^2 using the Levenberg-Markquardt approach
(Omega_Lambda=0.736+-0.065, Omega_m=0.238+-0.080, Omega_b=0.05+-0.011, h=0.68+-
0.09, sigma_8=0.73+-0.08 and n_s=0.96+-0.015). It is shown that the LambdaCDM
model with these values of the parameters agrees well with the angular power
spectrum of cosmic microwave background and with power spectra of the density
perturbations, estimated from spatial distributions of galaxies, rich galaxy
clusters and from statistics of Ly_alpha absorption lines in spectra of distant
quasars as well. The accordance of modeled characteristics of the large-scale
structure with observable ones was analyzed, and possible reasons of
significant discrepancies between some of them were considered. | astro-ph |
Numerical and analytical studies on model gravitating systems: In this thesis we study the evolution of systems of concentric shells
interacting gravitationally and in the process (1) propose and implement a
nearly energy-conserving numerical integration scheme for evolving the
concentric spherical shells systems with 1024 particles or less; (2) look at
the possibility of chaos in few shell systems; and (3) study the evolution of
many shell systems in the Vlasov limit.
The proposed numerical integration scheme is a nearly energy conserving
hybrid of the Verlet and modified Euler-Cromer integration schemes.
The rotational 2-shell spherical system is investigated in detail using the
hybrid numerical integration scheme. Plots of time-series, phase space
projections, Poincare sections, power spectra, and Lyapunov exponents are
obtained for the system. These diagnostic tools, taken together, clearly show
the chaotic nature of the rotational 2-shell system. Three types of periodic
orbits are observed: collapsed, one-point, and three-point periodic orbits. We
believe that the three-point periodic orbits result from a rotation-induced
bifurcation. Four types of quasiperiodic orbits are also observed. Three of
these are a result of slight changes in the initial conditions corresponding to
the three types of periodic orbits. The fourth type of quasiperiodic orbit
separates the chaotic region from the non-chaotic regions in phase space.
The short-time evolution of collisionless spherical shells system is studied
using both numerical and analytical methods. Approximate expressions for the
short-time evolution of the collisionless rotational shells system are obtained
using Vlasov-Poisson perturbation theory in the high-virial limit. The
agreement between the analytical results and numerical results for finite
shells systems improves as the number of shells in the system increases. | astro-ph |
Spectral Survey of X-Ray Bright Active Galactic Nuclei from the Rossi
X-ray Timing Explorer: Using long-term monitoring data from the Rossi X-ray Timing Explorer (RXTE),
we have selected 23 active galactic nuclei (AGN) with sufficient brightness and
overall observation time to derive broadband X-ray spectra from 3 to >100 keV.
Our sample includes mainly radio-quiet Seyferts, as well as seven radio-loud
sources. Given the longevity of the RXTE mission, the greater part of our data
is spread out over more than a decade, providing truly long-term average
spectra and eliminating inconsistencies arising from variability. We present
long-term average values of absorption, Fe line parameters, Compton reflection
strengths and photon indices, as well as fluxes and luminosities for the hard
and very hard energy bands, 2-10 keV and 20-100 keV respectively. We find
tentative evidence for high-energy rollovers in three of our objects. We
improve upon previous surveys of the very hard X-ray energy band in terms of
accuracy and sensitivity, particularly with respect to confirming and
quantifying the Compton reflection component. This survey is meant to provide a
baseline for future analysis with respect to the long-term averages for these
sources and to cement the legacy of RXTE, and especially its High Energy X-ray
Timing Experiment, as a contributor to AGN spectral science. | astro-ph |
The Temporal Analysis of Light Pollution in Turkey using VIIRS data: Artificial Light pollution (AL) in Turkey and in Turkish observatories
between 2012--2020 have been studied using the archival data of Visible
Infrared Imaging Radiometer Suite (VIIRS) instrument. The astroGIS database has
been used in processing the data (https://www.astrogis.org)
\cite{2020MNRAS.493.1204A}. The total energy released to space from Turkey
increased by 80\% in 2019 with respect to 2012. In the span of the dataset, a
steady and continuous increase has been observed throughout all cities of the
country. On the other hand, Dark Sky Park locations, East and Southeast
Anatolian regions and mostly rural areas around the cities kept their AL level
constant. Four demographic parameters have been studied and they were found to
be correlated very well with AL: Population ($R \simeq 0.90$); GDP ($R \simeq
0.87$); Total Power Consumption ($R \simeq 0.66$) and Outdoor Lightening ($R
\simeq 0.67$). Contrary to countries acting to prevent AL increases, Turkey
seems to be at the beginning of an era where AL will arithmetically increase
throughout the country and enormous amount of energy will continuously escape
to space and therefore will be wasted. Therefore, a preventive legislation,
especially for invaluable astronomical site locations such as TURAG, TUG, DAG
and \c{C}AAM where each is counted as a truly dark site due to their SQM
values, has to be enacted in Turkey, in very near future. | astro-ph |
The Imprint of Presupernova Winds on Supernova Remnant Evolution:
Towards More Realistic Models for Type Ia Supernova Remnants and their
Spectra: Supernova remnants are usually analysed in the light of hydrodynamical models
of the interaction of supernova ejecta with either a constant density ambient
medium or a circumstellar medium produced by a constant presupernova wind.
However, the ejection of energetic wind during the presupernova phase changes
the ambient medium structure and, consequently, the early supernova remnant
evolution. We have analysed the evolution of young remnants of type Ia
supernovae, focusing on the imprint of the presupernova wind history on the
supernova remnant structure and on the influence of the explosion mechanism. We
have found that the remnant evolution is most sensitive to the explosion
mechanism at ages not larger than a few hundred years, while the presupernova
history shows its influence at later epochs, before the Sedov phase sets in. | astro-ph |
Formation of the Musca filament: Evidence for asymmetries in the
accretion flow due to a cloud-cloud collision: Context. Dense molecular filaments are ubiquituous in the interstellar
medium, yet their internal physical conditions and formation mechanism remain
debated. Aims. We study the kinematics and physical conditions in the Musca
filament and the Chamaeleon-Musca complex to constrain the physics of filament
formation. Methods. We produced CO(2-1) isotopologue maps with the APEX
telescope that cut through the Musca filament. We further study a NANTEN2
$^{12}$CO(1-0) map of the Musca cloud and the HI emission of the
Chamaeleon-Musca complex. Results. The Musca cloud contains multiple velocity
components. Radiative transfer modelling of the CO emission indicates that the
Musca filament consists of a cold ($\sim$10 K), dense (n$_{H_2}\sim$10$^4$
cm$^{-3}$) crest, which is best described with a cylindrical geometry.
Connected to the crest, a separate gas component at T$\sim$15 K and
n$_{H_2}\sim$10$^3$ cm$^{-3}$ is found, the so-called strands. The filament
crest has a transverse velocity gradient that is linked to the kinematics of
the nearby ambient cloud. Studying the large scale kinematics, we find
coherence of the asymmetric kinematics from the 50 pc HI cloud down to the
Musca filament. We also report a strong [C$^{18}$O]/[$^{13}$CO] abundance drop
by an order of magnitude from the filament crest to the strands over a distance
$<$ 0.2 pc in a weak far-ultraviolet (FUV) field. Conclusions. The dense Musca
filament crest is a long-lived (several crossing times), dynamic structure that
can form stars in the near future because of continuous mass accretion. This
mass accretion appears to be triggered by a HI cloud-cloud collision, which
bends the magnetic field around dense filaments. This bending of the magnetic
field is then responsible for the observed asymmetric accretion scenario of the
Musca filament, which is, for instance, seen as a V-shape in the
position-velocity (PV) diagram. | astro-ph |
Joint analysis of EDGES $21$-cm line observations with standard candles
and rulers in $Λ$CDM and non-adiabatic gCg models: A decomposed generalised Chaplygin gas (gCg) with energy flux from dark
energy to dark matter, represented by a negative value for the gas parameter
$\alpha$, is shown to alleviate the tension between EDGES data and the
cosmological standard model. Using EDGES data and employing a Bayesian
statistical analysis, the agreement with the standard model is only marginal.
However, if $\alpha$ is negative enough the gCg fits remarkably well the data,
even in combination with SNe Ia datasets. On the other hand, when the CMB and
BAO acoustic scales are included the preferred value for $\alpha$ is near zero,
implying that a small deviation from $\Lambda$CDM is predicted. | astro-ph |
Bisymmetric normal modes in soft-centred and realistic galactic discs: We test methods for the determination of unstable modes in stellar discs: a
point collocation scheme in the action sub-space, a scheme based on expansion
of the density and potential on the biorthonormal basis, and a finite element
method. Using models of galaxies with low and high mass concentration to the
center, the existence of two different kinds of spectra of unstable modes is
demonstrated. Characteristic features of methods and obtained spectra are
discussed.
Despite ignoring any constraint on the continuity or differentiability of the
perturbed DF, the collocation scheme is reliable for obtaining spectra and
patterns in both kinds of models. The method based on the expansion of the
perturbed potential and surface density over a biorthogonal basis was not
applicable to a model with high mass concentration. The finite element method
successfully used in various fields of science and engineering is currently
sensitive to the presence of resonant orbits due to the choice of interpolation
functions for the distribution function. | astro-ph |
Cosmology with Modified Newtonian Dynamics (MOND): It is well-known that the application of Newtonian dynamics to an expanding
spherical region leads to the correct relativistic expression (the Friedmann
equation) for the evolution of the cosmic scale factor. Here, the cosmological
implications of Milgrom's modified Newtonian dynamics (MOND) are considered by
means of a similar procedure. Earlier work by Felten demonstrated that in a
region dominated by modified dynamics, the expansion cannot be uniform
(separations cannot be expressed in terms of a scale factor) and that any such
region will eventually re-collapse regardless of the initial expansion velocity
and mean density. Here I show that, because of the acceleration threshold for
the MOND phenomenology, a region dominated by MOND will have a finite size
which, in the early Universe ($z > 3$), is smaller than the horizon scale.
Therefore, uniform expansion and homogeneity on horizon scale are consistent
with MOND-dominated non-uniform expansion and the development of
inhomogeneities on smaller scale. In the radiation-dominated era, the amplitude
of MOND-induced inhomogeneities is much smaller than that implied by
observations of the cosmic background radiation, and the thermal and dynamical
history of the Universe is identical to that of the standard Big Bang. When
matter first dominates the energy density of the Universe, the cosmology
diverges from that of the standard model. Objects of galaxy mass are the first
virialized objects to form (by z=10) and larger structure develops rapidly. At
the present epoch, the Universe would be inhomogeneous out to a substantial
fraction of the Hubble radius. | astro-ph |
Multi-wavelength observation of cosmic-ray air-showers with
CODALEMA/EXTASIS: Since 2003, significant efforts have been devoted to the understanding of the
radio emission of extensive air shower in the range [20-200] MHz. Despite some
studies led until the early nineties, the [1-10] MHz band has remained unused
for 20 years. However, it has been measured by some pioneering experiments that
extensive air shower emit a strong electric field in this band and that there
is evidence of a large increase in the amplitude of the radio pulse at lower
frequencies. The EXTASIS experiment, located within the Nan\c{c}ay
Radioastronomy Observatory and supported by the CODALEMA experiment, aims to
reinvestigate the [1-10] MHz band, and especially to study the so-called
"Sudden Death" contribution, the expected electric field emitted by shower
front when hitting the ground level. Currently, EXTASIS has confirmed some
results obtained by the pioneering experiments, and tends to bring explanations
to the other ones, for instance the role of the underlying atmospheric electric
field. Moreover, CODALEMA has demonstrated that in the most commonly used
frequency band ([20-80] MHz) the electric field profile of EAS can be well
sampled, and contains all the information needed for the reconstruction of EAS:
an automatic comparison between the SELFAS3 simulations and data has been
developed, allowing us to reconstruct in an almost real time the primary cosmic
ray characteristics. | astro-ph |
Origin of the long-term modulation of radio emission of LS I +61 303: One of the most unusual aspects of the X-ray binary LSI +61 303 is that at
each orbit (P1=26.4960 \pm 0.0028 d) one radio outburst occurs whose amplitude
is modulated with Plong, a long-term period of more than 4 yr. It is still not
clear whether the compact object of the system or the companion Be star is
responsible for the long-term modulation. We study here the stability of Plong.
Such a stability is expected if Plong is due to periodic (P2) Doppler boosting
of periodic (P1) ejections from the accreting compact object of the system. On
the contrary it is not expected if Plong is related to variations in the mass
loss of the companion Be star. We built a database of 36.8 yr of radio
observations of LSI +61 303 covering more than 8 long-term cycles. We performed
timing and correlation analysis. In addition to the two dominant features at P1
and P2, the timing analysis gives a feature at Plong=1628 \pm 48 days. The
determined value of Plong agrees with the beat of the two dominant features,
i.e. Pbeat=1/(\nu1 -\nu2)=1626 \pm 68 d. The correlation coefficient of the
radio data oscillates at multiples of Pbeat. Cycles in varying Be stars change
in length and disappear after 2-3 cycles following the well-studied case of the
binary system zeta Tau. On the contrary, in LSI +61 303 the long-term period is
quite stable and repeats itself over the available 8 cycles. The long-term
modulation in LSI +61 303 accurately reflects the beat of periodical Doppler
boosting (induced by precession) with the periodicity of the ejecta. The peak
of the long-term modulation occurs at the coincidence of the maximum number of
ejected particles with the maximum Doppler boosting of their emission; this
coincidence occurs every 1/(\nu1 - \nu2) and creates the long-term modulation
observed in LSI +61 303. | astro-ph |
Fossil Signatures of Main-sequence Convective Core Overshoot Estimated
through Asteroseismic Analyses: Some physical processes that occur during a star's main-sequence evolution
also affect its post main-sequence evolution. It is well known that stars with
masses above approximately 1.1 $M_{\odot}$ have well-mixed convective cores on
the main sequence, however, the structure of the star in the neighborhood of
the convective core regions is currently underconstrained. We use
asteroseismology to study the properties of the stellar core, in particular,
convective boundary mixing through convective overshoot, in such intermediate
mass stars. These core regions are poorly constrained by the acoustic (p) mode
oscillations observed for cool main sequence stars. Consequently, we seek
fossil signatures of main sequence core properties during the subgiant and
early first-ascent red giant phases of evolution. During these stages of
stellar evolution, modes of mixed character that sample the deep interior, can
be observed. These modes sample the regions of the stars that are affected by
the main-sequence structure of these regions. We model the global and near-core
properties of 62 subgiant and early first-ascent red giant branch stars
observed by the \textit{Kepler}, K2, and TESS space missions. We find that the
effective overshoot parameter, $\alpha_{\text{ov, eff}}$, increases from $M =
1.0M_{\odot}$ to $M = 1.2 M_{\odot}$ before flattening out, although we note
that the relationship between $\alpha_{\text{ov, eff}}$ and mass will depend on
the incorporated modelling choices of internal physics and nuclear reaction
network. We also situate these results within existing studies of main-sequence
convective core boundaries. | astro-ph |
Generation of High-Energy Photons at Ultra-Relativistic Shock Breakout
in Supernovae: We present theoretical expectations for non-thermal emission due to the bulk
Comptonization at the ultra-relativistic shock breakout. We calculate the
transfer of photons emitted from the shocked matter with a Monte Carlo code
fully taking into account special relativity. As a hydrodynamical model, we use
a self-similar solution of Nakayama & Shigeyama (2005). Our calculations reveal
that the spectral shape exhibits a double peak or a single peak depending on
the shock temperature at the shock breakout. If it is significantly smaller
than the rest energy of an electron, the spectrum has a double peak. We also
display a few example of light curves, and estimate the total radiation energy.
In comparison with observations of gamma-ray bursts, a part of the higher
energy component in the spectra and the total energy can be reproduced by some
parameter sets. Meanwhile, the lower energy counterpart in the Band function is
not reproduced by our results and the duration time seems too short to
represent the entire event of a gamma-ray burst. Therefore the subsequent phase
will constitute the lower energy part in the spectrum. | astro-ph |
A new class of pulsating hot subdwarfs: Using high-cadence observations from the Zwicky Transient Facility at low
Galactic latitudes, we have discovered a new class of pulsating, hot, compact
stars. We have found four candidates, exhibiting blue colors ($g-r\leq-0.1$
mag), pulsation amplitudes of $>5\%$, and pulsation periods of $200 - 475$ sec.
Fourier transforms of the lightcurves show only one dominant frequency.
Phase-resolved spectroscopy for three objects reveals significant radial
velocity, T$_{\rm eff}$ and log(g) variations over the pulsation cycle,
consistent with large amplitude radial oscillations. The mean T$_{\rm eff}$ and
log(g) for these stars are consistent with hot subdwarf B (sdB) effective
temperatures and surface gravities. We calculate evolutionary tracks using MESA
and adiabatic pulsations using GYRE for low-mass helium-core pre-white dwarfs
and low mass helium-burning stars. Comparison of low-order radial oscillation
mode periods with the observed pulsation periods show better agreement with the
pre-white dwarf models. Therefore, we suggest that these new pulsators and Blue
Large-Amplitude Pulsators (BLAPs) could be members of the same class of
pulsators, composed of young $\approx0.25-0.35$ M$_\odot$ helium-core pre-white
dwarfs. | astro-ph |
Understanding Radio-Selected Thermal Sources in M 33: Ultraviolet,
Optical, Near-Infrared, Spitzer Mid-Infrared, and Radio Observations: We present ultraviolet, optical, near-infrared, Spitzer mid-infrared, and
radio images of 14 radio-selected objects in M 33. These objects are thought to
represent the youngest phase of star cluster formation. We have detected the
majority of cluster candidates in M 33 at all wavelengths. From the near-IR
images, we derived ages 2-10 Myr, K_S-band extinctions (A_K_S) of 0-1 mag, and
stellar masses of 10^3-10^4 M_solar. We have generated spectral energy
distributions (SEDs) of each cluster from 0.1 micron to 160 microns. From these
SEDs, we have modeled the dust emission around these star clusters to determine
the dust masses (1-10^3 M_solar) and temperatures (40-90 K) of the clusters'
local interstellar medium. Extinctions derived from the JHK_S, Halpha, and UV
images are similar to within a factor of 2 or 3. These results suggest that
eleven of the fourteen radio-selected objects are optically-visible young star
clusters with a surrounding H II region, that two are background objects,
possibly AGN, and that one is a Wolf-Rayet star with a surrounding H II region. | astro-ph |
A star disrupted by a stellar black hole as the origin of the cloud
falling toward the Galactic center: We propose that the cloud moving on a highly eccentric orbit near the central
black hole in our Galaxy, reported by Gillessen et al., is formed by a
photoevaporation wind originating in a disk around a star that is tidally
perturbed and shocked at every peribothron passage. The disk is proposed to
have formed when a stellar black hole flew by the star, tidally disrupted its
envelope, and placed the star on its present orbit with some of the tidal
debris forming a disk. A disrupting encounter at the location of the observed
cloud is most likely to be caused by a stellar black hole because of the
expected dynamical mass segregation; the rate of these disk-forming encounters
may be as high as $\sim 10^{-6}$ per year. The star should also be spun up by
the encounter, so the disk may subsequently expand by absorbing angular
momentum from the star. Once the disk expands up to the tidal truncation
radius, the tidal perturbation of the outer disk edge at every peribothron may
place gas streams on larger orbits which can give rise to a photoevaporation
wind that forms the cloud at every orbit. This model predicts that, after the
cloud is disrupted at the next peribothron passage in 2013, a smaller
unresolved cloud will gradually grow around the star on the same present orbit.
An increased infrared luminosity from the disk may also be detectable when the
peribothron is reached. We also note that this model revives the encounter
theory for planet formation. | astro-ph |
Coronal Polarization: We present an overview of the physical mechanisms responsible for the coronal
polarization at different wavelength regimes. We also review different
techniques using coronal polarization to determine various quantities necessary
for understanding the thermodynamic properties of the solar coronal plasma.
This includes the coronal magnetic field, electronic densities, temperatures,
velocities, etc. The future needs to acquire better information on the solar
corona using polarization will be outlined. | astro-ph |
Improving convergence in smoothed particle hydrodynamics simulations
without pairing instability: The numerical convergence of smoothed particle hydrodynamics (SPH) can be
severely restricted by random force errors induced by particle disorder,
especially in shear flows, which are ubiquitous in astrophysics. The increase
in the number NH of neighbours when switching to more extended smoothing
kernels at fixed resolution (using an appropriate definition for the SPH
resolution scale) is insufficient to combat these errors. Consequently, trading
resolution for better convergence is necessary, but for traditional smoothing
kernels this option is limited by the pairing (or clumping) instability.
Therefore, we investigate the suitability of the Wendland functions as
smoothing kernels and compare them with the traditional B-splines. Linear
stability analysis in three dimensions and test simulations demonstrate that
the Wendland kernels avoid the pairing instability for all NH, despite having
vanishing derivative at the origin (disproving traditional ideas about the
origin of this instability; instead, we uncover a relation with the kernel
Fourier transform and give an explanation in terms of the SPH density
estimator). The Wendland kernels are computationally more convenient than the
higher-order B-splines, allowing large NH and hence better numerical
convergence (note that computational costs rise sub-linear with NH). Our
analysis also shows that at low NH the quartic spline kernel with NH ~= 60
obtains much better convergence then the standard cubic spline. | astro-ph |
Large Inverse Transient Phase Response of Titanium-nitride-based
Microwave Kinetic Inductance Detectors: Following optical pulses ($\lambda=405~\text{nm}$) on titanium nitride (TiN)
Microwave Kinetic Inductance Detectors (MKIDs) cooled down at temperatures $T
\le T_c / 20$ ($T_c \simeq 4.6~\text{K}$), we observe a large phase-response
highlighting two different modes simultaneously that are nevertheless related.
The first corresponds to the well-known transition of cooper-pair breaking into
quasi-particles which produces a known phase response. This is immediately
followed by a large inverse response lasting several hundreds of microseconds
to several milliseconds depending on the temperature. We propose to model this
inverse pulse as the thermal perturbation of the superconductor and interaction
with two level system (TLS) that reduces the dielectric constant which in turns
modify the capacitance and therefore the resonance frequency. The ratio of the
TLS responding to the illumination is on the order of that of the area of the
inductor to the whole resonator | astro-ph |
Hydrodynamics of Embedded Planets' First Atmospheres. I. A Centrifugal
Growth Barrier for 2D Flows: In the core accretion paradigm of planet formation, gas giants only form a
massive atmosphere after their progenitors exceeded a threshold mass: the
critical core mass. Most (exo)planets, being smaller and rock/ice-dominated,
never crossed this line. Nevertheless, they were massive enough to attract
substantial amounts of gas from the disc, while their atmospheres remained in
pressure-equilibrium with the disc. Our goal is to characterise the
hydrodynamical properties of the atmospheres of such embedded planets and their
implication for their (long-term) evolution. In this paper -- the first in
series -- we start to investigate the properties of an isothermal and inviscid
flow past a small, embedded planet by conducting local, 2D hydrodynamical
simulations. Using the PLUTO code we confirm that the flow is steady and bound.
This steady outcome is most apparent for the log-polar grid (with the grid
spacing proportional to the distance from the planet). For low-mass planets,
Cartesian grids are somewhat less efficient as they have difficulty to follow
the circular, large speeds in the deep atmosphere. Relating the amount of
rotation to the gas fraction of the atmosphere, we find that more massive
atmospheres rotate faster -- a finding consistent with Kelvin's circulation
theorem. Rotation therefore limits the amount of gas that planets can acquire
from the nebula. Dependent on the Toomre-Q parameter of the circumstellar disc,
the planet's atmosphere will reach Keplerian rotation before self-gravity
starts to become important. | astro-ph |
Stellar collisions in galactic centers: black hole growth and color
gradients: We study the effects of stellar collisions, particularly on feeding massive
black holes (BHs) and color gradients, in realistic galactic centers. We find
that the mass released by stellar collisions is not sufficient to account for
the present BH mass in galactic centers, especially in bright galaxies. This
study, together with the study by Magorrian & Tremaine (1999) on tidal
disruption of stars by massive BHs, implies that the material for BH growth
(especially in galaxies brighter than ~10^9 Lsun) can only come from other
sources, for example, the mass released by stellar evolution in the initial ~1
Gyr of the galaxy's lifetime, or the gas that sinks to the galactic center in a
galaxy merger. We also analyze how the color of a stellar system is affected by
collisions of stars. We find that collisions between main-sequence stars cannot
cause observable color gradients in the visible bands at projected radius
R>0.1" in M31, M32 and other nearby galactic centers. This result is consistent
with the lack of an observable color gradient in M32 at R>0.1". At even smaller
radii, the color differences caused by collisions between main-sequence stars
are at most 0.08 mag at R=0.02". The averaged blueing due to stellar collisions
in the region R<0.1" of M32 should not be larger than 0.06 mag in color index
U-V and 0.02 mag in V-I. The observed blueing in the center of the galaxy M31
(in a 0.14"x0.14" box) must be caused by some mechanism other than collisions
between main-sequence stars. | astro-ph |
Decaying dark matter search with NuSTAR deep sky observations: We present the results of the search for decaying dark matter with particle
mass in the 6-40 keV range with NuSTAR deep observations of COSMOS and ECDFS
empty sky fields. We show that main contribution to the decaying dark matter
signal from the Milky Way galaxy comes through the aperture of the NuSTAR
detector, rather than through the focusing optics. High sensitivity of the
NuSTAR detector, combined with the large aperture and large exposure times of
the two observation fields allow us to improve previously existing constraints
on the dark matter decay time by up to an order of magnitude in the mass range
10-30 keV. In the particular case of the nuMSM sterile neutrino dark matter,
our constraints impose an upper bound m<20 keV on the dark matter particle
mass. We report detection of four unidentified spectral lines in our data set.
These line detections are either due to the systematic effects (uncertainties
of calibrations of the NuSTAR detectors) or have an astrophysical origin. We
discuss different possibilities for testing the nature of the detected lines. | astro-ph |
HST Imaging in the Chandra Deep Field South: I. Multiple AGN Populations: We present preliminary results from imaging three HST/WFPC2 fields in V and I
within the Chandra Deep Field South (CDFS). HST's sensitivity and resolution
are sufficient to reveal optical counterparts for 24 of the 26 CDFS X-ray
sources detected in the 300 ksec X-ray catalog and to determine the
morphologies of most of these. We find that the X-ray sources comprise two
apparently distinct populations of optical candidates: one optically faint (I >
24) with V - I colors consistent with the I > 24 field population; the other
significantly brighter (I < 22) with colors redder than the I < 22 field
population. More than 2/3 of the X-ray source counterparts are resolved
galaxies. The brighter sources are mostly AGN, based on their high X-ray
luminosity. The optically resolved sources in the brighter population have a
very narrow range of V - I color and appear to be a mix of both late and early
type morphologies at low to moderate redshift. We show that the second
population, with fainter optical counterparts, can be explained as higher
redshift Type 2 AGN. | astro-ph |
Modelling extragalactic extinction through gamma-ray burst afterglows: We analyze extragalactic extinction profiles derived through gamma-ray burst
afterglows, using a dust model specifically constructed on the assumption that
dust grains are not immutable but respond time-dependently to the local
physics. Such a model includes core-mantle spherical particles of mixed
chemical composition (silicate core, sp2 and sp3 carbonaceous layers), and an
additional molecular component, in the form of free-flying polycyclic aromatic
hydrocarbons. We fit most of the observed extinction profiles. Failures occur
for lines of sight presenting remarkable rises blueward the bump. We find a
tendency in the carbon chemical structure to become more aliphatic with the
galactic activity, and to some extent with increasing redshifts. Moreover, the
contribution of the moleclar component to the total extinction is more
important in younger objects. The results of the fitting procedure (either
successes and failures) may be naturally interpreted through an evolutionary
prescription based on the carbon cycle in the interstellar medium of galaxies. | astro-ph |
Likelihood Methods for the Detection and Characterization of Gamma-ray
Pulsars with the Fermi Large Area Telescope: The sensitivity of the Large Area Telescope (LAT) aboard the Fermi Gamma-ray
Space Telescope allows detection of thousands of new gamma-ray sources and
detailed characterization of the spectra and variability of bright sources.
Unsurprisingly, this increased capability leads to increased complexity in data
analysis. Likelihood methods are ideal for connecting models with data, but the
computational cost of folding the model input through the multi-scale
instrument response function is appreciable. Both interactive analysis and
large projects---such as analysis of the full gamma-ray sky---can be
prohibitive or impossible, reducing the scope of the science possible with the
LAT. To improve on this situation, we have developed pointlike, a software
package for fast maximum likelihood analysis of LAT data. It is interactive by
design and its rapid evaluation of the likelihood facilitates exploratory and
large-scale, all-sky analysis. We detail its implementation and validate its
performance on simulated data. We demonstrate its capability for interactive
analysis and present several all-sky analyses. These include a search for new
gamma-ray sources and the selection of LAT sources with pulsar-like
characteristics for targeted radio pulsation searches. We conclude by
developing sensitive periodicity tests incorporating spectral information
obtained from pointlike. | astro-ph |
Astroinformatics based search for globular clusters in the Fornax Deep
Survey: In the last years, Astroinformatics has become a well defined paradigm for
many fields of Astronomy. In this work we demonstrate the potential of a
multidisciplinary approach to identify globular clusters (GCs) in the Fornax
cluster of galaxies taking advantage of multi-band photometry produced by the
VLT Survey Telescope using automatic self-adaptive methodologies. The data
analyzed in this work consist of deep, multi-band, partially overlapping images
centered on the core of the Fornax cluster. In this work we use a Neural-Gas
model, a pure clustering machine learning methodology, to approach the GC
detection, while a novel feature selection method ($\Phi$LAB) is exploited to
perform the parameter space analysis and optimization. We demonstrate that the
use of an Astroinformatics based methodology is able to provide GC samples that
are comparable, in terms of purity and completeness with those obtained using
single band HST data (Brescia et al. 2012) and two approaches based
respectively on a morpho-photometric (Cantiello et al. 2018b) and a PCA
analysis (D'Abrusco et al. 2015) using the same data discussed in this work. | astro-ph |
Significance for signal changes in gamma-ray astronomy: We describe a straightforward modification of frequently invoked methods for
the determination of the statistical significance of a gamma-ray signal
observed in a counting process. A simple criterion is proposed to decide
whether a set of measurements of the numbers of photons registered in the
source and background regions is consistent with the assumption of a constant
source activity. This method is particularly suitable for immediate evaluation
of the stability of the observed gamma-ray signal. It is independent of the
exposure estimates, reducing thus the impact of systematic inaccuracies, and
properly accounts for the fluctuations in the number of detected photons. The
usefulness of the method is demonstrated on several examples. We discuss
intensity changes for gamma-ray emitters detected at very high energies by the
current gamma-ray telescopes (e.g. 1ES 0229+200, 1ES 1959+650 and PG 1553+113).
Some of the measurements are quantified to be exceptional with large
statistical significances. | astro-ph |
Evidence of residual Doppler shift on three pulsars, PSR B1259-63,
4U1627-67 and PSR J2051-0827: The huge derivative of orbital period observed in binary pulsar PSR B1259-63,
the torque reversal displaying on low mass X-ray binary, 4U1627-67 and the long
term change of orbital period of PSR J2051-0827, seem totally unrelated
phenomena occurring at totally different pulsar systems. In this paper, they
are simply interpreted by the same mechanism, residual Doppler shift. In a
binary system with periodic signals sending to an observer, the drift of the
signal frequency actually changes with the varying orbital velocity, projected
to line of sight at different phases of orbit. And it has been taken for
granted that the net red-shift and blue-shift of an full orbit circle be
cancelled out, so that the effect of Doppler shift to the signal in binary
motion cannot be accumulated over the orbital period. However, taking the
propagation time at each velocity state into account, the symmetry of the
velocity distribution over the orbital phase is broken. Consequently, the net
Doppler shift left in an orbit is non-zero. Understanding this Newtonian second
Doppler effect not only makes pulsars better laboratory in the test of
gravitational effects, but also allows us to extract the angular momentum of
the pulsar of PSR J2051-0827, $\leq 2\times 10^{43}gcm^2$; and the accretion
disc of 4U 1627-67, $7\times 10^{50} gcm^2/s$, respectively, which are of
importance in the study of structure of neutron stars and the physics of
accretion disc of X-ray binaries. | astro-ph |
The small observed scale of AGN--driven outflows, and inside--out disc
quenching: Observations of massive outflows with detectable central AGN typically find
them within radii $\lesssim 10$ kpc. We show that this apparent size
restriction is a natural result of AGN driving if this process injects total
energy only of order the gas binding energy to the outflow, and the AGN varies
over time (`flickers') as suggested in recent work. After the end of all AGN
activity the outflow continues to expand to larger radii, powered by the
thermal expansion of the remnant shocked AGN wind. We suggest that on average,
outflows should be detected further from the nucleus in more massive galaxies.
In massive gas--rich galaxies these could be several tens of kpc in radius. We
also consider the effect that pressure of such outflows has on a galaxy disc.
In moderately gas--rich discs, with gas-to-baryon fraction $< 0.2$, the outflow
may induce star formation significant enough to be distinguished from quiescent
by an apparently different normalisation of the Kennicutt-Schmidt law. The star
formation enhancement is probably stronger in the outskirts of galaxy discs, so
coasting outflows might be detected by their effects upon the disc even after
the driving AGN has shut off. We compare our results to the recent inference of
inside--out quenching of star formation in galaxy discs. | astro-ph |
The state of globular clusters at birth II: primordial binaries: (abridged) In this paper, we constrain the properties of primordial binary
populations in Galactic globular clusters using the MOCCA Monte Carlo code for
cluster evolution. Our results are compared to the observations of Milone et
al. (2012) using the photometric binary populations as proxies for the true
underlying distributions, in order to test the hypothesis that the data are
consistent with an universal initial binary fraction near unity and the binary
orbital parameter distributions of Kroupa (1995). With the exception of a few
possible outliers, we find that the data are to first-order consistent with the
universality hypothesis. Specifically, the present-day binary fractions inside
the half-mass radius r$_{\rm h}$ can be reproduced assuming either high initial
binary fractions near unity with a dominant soft binary component as in the
Kroupa distribution combined with high initial densities (10$^4$-10$^6$
M$_{\odot}$ pc$^{-3}$), or low initial binary fractions ($\sim$ 5-10%) with a
dominant hard binary component combined with moderate initial densities near
their present-day values (10$^2$-10$^3$ M$_{\odot}$ pc$^{-3}$). This apparent
degeneracy can be broken using the binary fractions outside r$_{\rm h}$- only
high initial binary fractions with a significant soft component combined with
high initial densities can contribute to reproducing the observed
anti-correlation between the binary fractions outside r$_{\rm h}$ and the total
cluster mass. We further illustrate using the simulated present-day binary
orbital parameter distributions and the technique introduced in Leigh et al.
(2012) that the relative fractions of hard and soft binaries can be used to
further constrain the initial cluster density and mass-density relation. Our
results favour an initial mass-density relation of the form r$_{\rm h} \propto$
M$_{\rm clus}^{\alpha}$ with $\alpha <$ 1/3. | astro-ph |
Kinematic evolution of the young local associations and the Sco-Cen
complex: In this work we propose a scenario for the history of the recent star
formation (during the last 20-30 Myr) in the nearest solar neighbourhood (~150
pc), from the study of the spatial and kinematic properties of the members of
the so-called young local associations, the Sco-Cen complex and the Local
Bubble, the most important structure observed in the local interstellar medium
(ISM). | astro-ph |
Science Highlights from VERITAS: The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is a
ground-based array located at the Fred Lawrence Whipple Observatory in southern
Arizona and is one of the world's most sensitive gamma-ray instruments at
energies of 85 GeV to $>$30 TeV. VERITAS has a wide scientific reach that
includes the study of extragalactic and Galactic objects as well as the search
for astrophysical signatures of dark matter and the measurement of cosmic rays.
In this paper, we will summarize the current status of the VERITAS observatory
and present some of the scientific highlights from the last two years, focusing
in particular on those results shown at the 2015 ICRC in The Hague,
Netherlands. | astro-ph |
CMB Polarization can constrain cosmology better than CMB temperature: We demonstrate that for a cosmic variance limited experiment, CMB E
polarization alone places stronger constraints on cosmological parameters than
CMB temperature. For example, we show that EE can constrain parameters better
than TT by up to a factor 2.8 when a multipole range of l=30-2500 is
considered. We expose the physical effects at play behind this remarkable
result and study how it depends on the multipole range included in the
analysis. In most relevant cases, TE or EE surpass the TT based cosmological
constraints. This result is important as the small scale astrophysical
foregrounds are expected to have a much reduced impact on polarization, thus
opening the possibility of building cleaner and more stringent constraints of
the LCDM model. This is relevant specially for proposed future CMB satellite
missions, such as CORE or PRISM, that are designed to be cosmic variance
limited in polarization till very large multipoles. We perform the same
analysis for a Planck-like experiment, and conclude that even in this case TE
alone should determine the constraint on $\Omega_ch^2$ better than TT by 15%,
while determining $\Omega_bh^2$, $n_s$ and $\theta$ with comparable accuracy.
Finally, we explore a few classical extensions of the LCDM model and show again
that CMB polarization alone provides more stringent constraints than CMB
temperature in case of a cosmic variance limited experiment. | astro-ph |
Intra Cluster Light properties in the CLASH-VLT cluster MACS
J1206.2-0847: We aim at constraining the assembly history of clusters by studying the intra
cluster light (ICL) properties, estimating its contribution to the fraction of
baryons in stars, f*, and understanding possible systematics/bias using
different ICL detection techniques. We developed an automated method, GALtoICL,
based on the software GALAPAGOS to obtain a refined version of typical BCG+ICL
maps. We applied this method to our test case MACS J1206.2-0847, a massive
cluster located at z=0.44, that is part of the CLASH sample. Using deep
multi-band SUBARU images, we extracted the surface brightness (SB) profile of
the BCG+ICL and we studied the ICL morphology, color, and contribution to f*
out to R500. We repeated the same analysis using a different definition of the
ICL, SBlimit method, i.e. a SB cut-off level, to compare the results. The most
peculiar feature of the ICL in MACS1206 is its asymmetric radial distribution,
with an excess in the SE direction and extending towards the 2nd brightest
cluster galaxy which is a Post Starburst galaxy. This suggests an interaction
between the BCG and this galaxy that dates back to t <= 1.5 Gyr. The BCG+ICL
stellar content is 8% of M_(*,500) and the (de-) projected baryon fraction in
stars is f*=0.0177 (0.0116), in excellent agreement with recent results. The
SBlimit method provides systematically higher ICL fractions and this effect is
larger at lower SB limits. This is due to the light from the outer envelopes of
member galaxies that contaminate the ICL. Though more time consuming, the
GALtoICL method provides safer ICL detections that are almost free of this
contamination. This is one of the few ICL study at redshift z > 0.3. At
completion, the CLASH/VLT program will allow us to extend this analysis to a
statistically significant cluster sample spanning a wide redshift range:
0.2<z<0.6. | astro-ph |
Planetary Nebulae in the Magellanic Clouds: Probing Stellar Evolution
and Populations: This review contains: (1) the scientific motivations for studying Planetary
Nebulae in the Magellanic Clouds; (2) a review of this field of study, from the
origins to the most recent results, focusing on the papers that have been
published since the last IAU Symposium on Planetary Nebulae; (3) a review of
the Hubble contribution to the field, from the early results to our own
Magellanic Cloud Planetary Nebula program. | astro-ph |
Constraints on Helium Enhancement in the Globular Cluster M3 (NGC 5272):
The Horizontal Branch Test: It has recently been suggested that the presence of multiple populations
showing various amounts of helium enhancement is the rule, rather than the
exception, among globular star clusters. An important prediction of this helium
enhancement scenario is that the helium-enhanced blue horizontal branch (HB)
stars should be brighter than the red HB stars which are not helium-enhanced.
In this Letter, we test this prediction in the case of the Galactic globular
cluster M3 (NGC 5272), for which the helium-enhancement scenario predicts
helium enhancements of > 0.02 in virtually all blue HB stars. Using
high-precision Stroemgren photometry and spectroscopic gravities for blue HB
stars, we find that any helium enhancement among most of the cluster's blue HB
stars is very likely less than 0.01, thus ruling out the much higher helium
enhancements that have been proposed in the literature. | astro-ph |
Is there a polarization horizon?: Modern radio spectrometers make measurement of polarized intensity as a
function of Faraday depth possible. I investigate the effect of depolarization
along a model line of sight. I model sightlines with two components informed by
observations: a diffuse interstellar medium with a lognormal electron density
distribution and a narrow, denser component simulating a spiral arm or H~{\sc
ii} region, all with synchrotron-emitting gas mixed in. I then calculate the
polarized intensity from 300-1800~MHz and calculate the resulting Faraday depth
spectrum. The idealized synthetic observations show far more Faraday complexity
than is observed in Global Magneto-Ionic Medium Survey observations. In a model
with a very nearby H~{\sc ii} region observed at low frequencies, most of the
effects of a "depolarization wall" are evident: the H~{\sc ii} region
depolarizes background emission and less (but not zero) information from beyond
the H~{\sc ii} region reaches the observer. In other cases, the effects are not
so clear, as significant amounts of information reach the observer even through
significant depolarization, and it is not clear that low-frequency observations
sample largely different volumes of the interstellar medium than high-frequency
observations. The observed Faraday depth can be randomized such that it does
not always have any correlation with the true Faraday depth. | astro-ph |
Asteroseismic modeling of gravity modes in slowly rotating A/F stars
with radiative levitation: It has been known for several decades that transport of chemical elements is
induced by the process of microscopic atomic diffusion. Yet, the effect of
atomic diffusion, including radiative levitation, has hardly been studied in
the context of gravity mode pulsations of core-hydrogen burning stars. In this
paper, we study the difference in the properties of such modes for models with
and without atomic diffusion. We perform asteroseismic modeling of two slowly
rotating A- and F-type pulsators, KIC11145123 ($f_{\rm rot} \approx0.010~{\rm
d}^{-1}$) and KIC9751996 ($f_{\rm rot} \approx0.0696~{\rm d}^{-1}$),
respectively, based on the periods of individual gravity modes. For both stars,
we find models whose g-mode periods are in very good agreement with the {\it
Kepler\/} asteroseismic data, keeping in mind that the theoretical/numerical
precision of present-day stellar evolution models is typically about two orders
of magnitude lower than the measurement errors. Using the Akaike Information
Criterion (AIC) we have made a comparison between our best models with and
without diffusion, and found very strong evidence for signatures of atomic
diffusion in the pulsations of KIC11145123. In the case of KIC9751996 the
models with atomic diffusion are not able to explain the data as well as the
models without it. Furthermore, we compare the observed surface abundances with
those predicted by the best fitting models. The observed abundances are
inconclusive for KIC9751996, while those of KIC11145123 from the literature can
better be explained by a model with atomic diffusion. | astro-ph |
The Mexican Million Models Database: a virtual observatory for gaseous
nebulae: The 3MdB (Mexican Million Models database) is a large database of
photoionization models for H II regions. The number of free parameters for the
models is close to 15, including the description of the ionizing Spectral
Energy Distribution (effective temperature, luminosity, surface gravity, for
different type of stellar atmosphere models) and the description of the ionized
gas (distance to the ionizing source, density, abundances of the most common
elements, dust). The outputs of the models are more than 70 emission line
intensities, the ionic fractions and temperatures. All the parameters and
outputs are included in the MySQL database, giving the possibility to the user
to search into the database for example for all the models that reproduce a
given set of observations. | astro-ph |
Probing the magnetosphere of the M8.5 dwarf TVLM513-46546 by modelling
its auroral radio emission. Hint of star exoplanet interaction?: In this paper we simulate the cyclic circularly-polarised pulses of the
ultra-cool dwarf TVLM513-46546, observed with the VLA at 4.88 and 8.44 GHz on
May 2006, by using a 3D model of the auroral radio emission from the stellar
magnetosphere. During this epoch, the radio light curves are characterised by
two pulses left-hand polarised at 4.88 GHz, and one doubly-peaked (of opposite
polarisations) pulse at 8.44 GHz. To take into account the possible deviation
from the dipolar symmetry of the stellar magnetic field topology, the model
described in this paper is also able to simulate the auroral radio emission
from a magnetosphere shaped like an offset-dipole. To reproduce the timing and
pattern of the observed pulses, we explored the space of parameters controlling
the auroral beaming pattern and the geometry of the magnetosphere. Through the
analysis of the TVLM513-46546 auroral radio emission, we derive some
indications on the magnetospheric field topology that is able to simultaneously
reproduce the timing and patterns of the auroral pulses measured at 4.88 and
8.44 GHz. Each set of model solutions simulates two auroral pulses (singly or
doubly peaked) per period. To explain the presence of only one 8.44 GHz pulse
per period, we analyse the case of auroral radio emission limited only to a
magnetospheric sector activated by an external body, like the case of the
interaction of Jupiter with its moons. | astro-ph |
Magnetic flux emergence in fast rotating stars: Fast rotating cool stars are characterised by high magnetic activity levels
and frequently show dark spots up to polar latitudes. Their distinctive surface
distributions of magnetic flux are investigated in the context of the
solar-stellar connection by applying the solar flux eruption and surface flux
transport models to stars with different rotation rates, mass, and evolutionary
stage. The rise of magnetic flux tubes through the convection zone is primarily
buoyancy-driven, though their evolution can be strongly affected by the
Coriolis force. The poleward deflection of the tube's trajectory increases with
the stellar rotation rate, which provides an explanation for magnetic flux
eruption at high latitudes. The formation of proper polar spots likely requires
the assistance of meridional flows both before and after the eruption of
magnetic flux on the stellar surface. Since small radiative cores support the
eruption of flux tubes at high latitudes, low-mass pre-main sequence stars are
predicted to show high mean latitudes of flux emergence. In addition to flux
eruption at high latitudes, main sequence components of close binary systems
show spot distributions which are non-uniform in longitude. Yet these
`preferred longitudes' of flux eruption are expected to vanish beyond a certain
post-main sequence evolutionary stage. | astro-ph |
Gemini Near-Infrared Field Spectrograph Observations of the Seyfert 2
Galaxy Mrk 3: Feeding and Feedback on Galactic and Nuclear Scales: We explore the kinematics of the stars, ionized gas, and warm molecular gas
in the Seyfert 2 galaxy Mrk~3 (UGC~3426) on nuclear and galactic scales with
{\it Gemini} Near-Infrared Field Spectrograph (NIFS) observations, previous
{\it Hubble Space Telescope} data, and new long-slit spectra from the {\it
Apache Point Observatory} ({\it APO}) 3.5 m telescope. The {\it APO} spectra
are consistent with our previous suggestion that a galactic-scale gas/dust disk
at PA $=$ 129\arcdeg, offset from the major axis of the host S0 galaxy at PA
$=$ 28\arcdeg, is responsible for the orientation of the extended narrow-line
region (ENLR). The disk is fed by an H~I tidal stream from a gas-rich spiral
galaxy (UGC~3422) $\sim$100 kpc to the NW of Mrk 3, and is ionized by the AGN
to a distance of at least $\sim$20\arcsec\ ($\sim$5.4 kpc) from the central
supermassive black hole (SMBH). The kinematics within at least 320 pc of the
SMBH are dominated by outflows with radial (line of sight) velocities up to
1500 km s$^{-1}$ in the ionized gas and 500 km s$^{-1}$ in the warm molecular
gas, consistent with in situ heating, ionization, and acceleration of ambient
gas to produce the narrow-line region (NLR) outflows. There is a disk of
ionized and warm molecular gas within $\sim$400 pc of the SMBH that has
re-oriented close to the stellar major axis but is counter-rotating, consistent
with claims of external fueling of AGN in S0 galaxies. | astro-ph |
Inverse Compton cooling in Klein-Nishina regime and GRB prompt spectrum: Synchrotron radiation mechanism, when electrons are accelerated in a
relativistic shock, is known to have serious problems to explain the observed
gamma-ray spectrum below the peak for most Gamma-Ray Bursts (GRBs); the
synchrotron spectrum below the peak is much softer than observed spectra.
Recently, the possibility that electrons responsible for the radiation cool via
Inverse Compton, but in the Klein-Nishina regime, has been proposed as a
solution to this problem. We provide an analytical study of this effect and
show that it leads to a hardening of the low energy spectrum but not by enough
to make it consistent with the observed spectra for most GRBs (this is assuming
that electrons are injected continuously over a time scale comparable to the
dynamical time scale, as is expected for internal shocks of GRBs). In
particular, we find that it is not possible to obtain a spectrum with
\alpha>-0.1 (f_{\nu} \propto \nu^{\alpha}) whereas the typical observed value
is \alpha\sim0. Moreover, extreme values for a number of parameters are
required in order that \alpha\sim-0.1: the energy fraction in magnetic field
needs to be less than about 10^{-4}, the thermal Lorentz factor of electrons
should be larger than 10^6, and the radius where gamma-rays are produced should
be not too far away from the deceleration radius. These difficulties suggest
that the synchrotron radiation mechanism in internal shocks does not provide a
self-consistent solution when \alpha>-0.2. | astro-ph |
Photometric Study on Stellar Magnetic Activity: I. Flare Variability of
Red Dwarf Stars in the Open Cluster M37: Based on one-month long MMT time-series observations of the open cluster M37,
we monitored light variations of nearly 2500 red dwarfs and successfully
identified 420 flare events from 312 cluster M dwarf stars. For each flare
light curve, we derived observational and physical parameters, such as flare
shape, peak amplitude, duration, energy, and peak luminosity. We show that cool
stars produce serendipitous flares energetic enough to be observed in the
$r$-band, and their temporal and peak characteristics are almost the same as
those in traditional $U$-band observations. We also found many large-amplitude
flares with inferred $\Delta u > 6$ mag in the cluster sample which had been
rarely reported in previous ground-based observations. Following the ergodic
hypothesis, we investigate in detail statistical properties of flare parameters
over a range of energy ($E_{r}$ $\simeq$ $10^{31}-10^{34}$ erg). As expected,
there are no statistical differences in the distributions of flare timescales,
energies, and frequencies among stars of the same age and mass group. We note
that our sample tend to have longer rise and decay timescales compared to those
seen in field flare stars of the same spectral type and be more energetic.
Flare frequency distributions follow power-law distributions with slopes $\beta
\sim0.62-1.21$ for all flare stars and $\beta \sim0.52-0.97$ for stars with
membership information ($P_{mem} \geq 0.2$). These are in general agreement
with previous works on flare statistics of young open clusters and nearby field
stars. Our results give further support to the classical age-activity
relations. | astro-ph |
On the early chemical evolution of the Milky Way: A few topics concerning the early chemical evolution of the Milky Way are
critically discussed. In particular, it is argued that: 1) Observed abundance
patterns of extremely metal poor stars (of Pop. II) do not constrain the mass
range of the first generation (Pop. III) stars; the latter may well be normal
massive stars (10-50 Msun) or very massive ones (140-1000 Msun) or a
combination of the two classes. 2) The discrepancy between primordial Li
abundance (after WMAP) and the observed ``Spite plateau'' cannot be due to
astration by a generation of massive Pop. III stars, as recently suggested,
unless if such stars eject negligible amounts of metals. 3) The observed halo
metallicity disribution may well be understood in the framework of hierarchical
galaxy formation, as shown here with a simple semi-analytical model. 4)
Formation of the Milky Way's halo from a myriad of smaller sub-haloes may have
important implications for our understanding of the abundance patterns of
r-elements, the origin of which remains still unclear. | astro-ph |
Shedding Light on the Isolation of Luminous Blue Variables: In the standard view of massive star evolution, luminous blue variables
(LBVs) are transitional objects between the most massive O-type stars and
Wolf-Rayet (WR) stars. With short lifetimes, these stars should all be found
near one another. A recent study of LBVs in the Large Magellanic Cloud (LMC)
found instead that LBVs are considerably more isolated than either O-type stars
or WRs, with a distribution intermediate between that of the WRs and red
supergiants (RSGs). A similar study, using a more restricted sample of LBVs,
reached the opposite conclusion. Both studies relied upon the distance to the
nearest spectroscopically identified O-type star to define the degree of
isolation. However, our knowledge of the spectroscopic content of the LMC is
quite spotty. Here we re-examine the issue using carefully defined photometric
criteria to select the highest mass unevolved stars ("bright blue stars," or
BBSs), using spatially complete photometric catalogs of the LMC, M31, and M33.
Our study finds that the LBVs are no more isolated than BBSs or WRs. This
result holds no matter which sample of LBVs we employ. A statistical test shows
that we can rule out the LBVs having the same distribution as the RSGs, which
are about 2x more isolated. We demonstrate the robustness of our results using
the second-closest neighbor. Furthermore, the majority of LBVs in the LMC are
found in or near OB associations as are the BBS and WRs; the RSGs are not. We
conclude that the spatial distribution of LBVs therefore is consistent with the
standard picture of massive star evolution. | astro-ph |
Luminosity effect of O I 7771-5 triplet and atmospheric microturbulence
in evolved A-, F-, and G-type stars: It is known that the strength of neutral oxygen triplet lines at 7771-5 A
shows a luminosity effect in evolved A through G stars. However, its general
behavior across the HR diagram is not yet well understood, since the
applicability limit of the relations proposed by various previous work (tending
to be biased toward supergiants) still remains unclear. Besides, our
understanding on the nature of atmospheric micro-scale turbulence, which is
considered to play a significant role (along with the non-LTE line
intensification) for the cause of this effect, is still insufficient. Towards
clarifying these problems, we carried out an extensive non-LTE spectrum-fitting
analysis of O I 7771--5 lines for unbiased sample of 75 evolved A-, F,- and
G-type stars over wide luminosity classes (from subgiants through supergiants)
including rapid rotators, from which the total equivalent width (W_77) was
derived and the microturbulence (xi) was determined by two different (profile-
and abundance-based) methods for each star. While we confirmed that W_77 tends
to increase in the global sense as a star's absolute magnitude (M_V) becomes
more luminous, distinctly different trends were found between lower-gravity
(log g <~ 2.5) and higher-gravity (log g >~ 2.5) stars, in the sense that the
M_V vs. W_77 formulas proposed by past studies are applicable only to the
former supergiant group. In case of using W_77 for empirical M_V evaluation by
such simple formulas, it is recommended to confine only to supergiants of -5 >
M_V > -10. Regarding the microturbulence significantly controlling W_77, it
roughly shows an increasing tendency with a decrease in surface gravity.
However, the trend is not monotonic but rather intricate (e.g., hump,
stagnation, or discontinuously large increase) depending on the stellar type
and evolutionary stage. | astro-ph |
HST/ACS color-magnitude diagrams of candidate intermediate-age M 31
globular clusters. The role of blue horizontal branches: We present deep (V ~ 28.0) BV photometry obtained with the wide field channel
of the Advanced Camera for Surveys on board HST for four M31 globular clusters
that were identified as candidate intermediate-age (age ~ 1-9 Gyr) by various
authors, based on their integrated spectra and/or broad/intermediate-band
colors. Two of them (B292 and B350) display an obvious blue horizontal branch,
indicating that they are as old as the oldest Galactic globulars. On the other
hand, for the other two (B058 and B337), which display red horizontal branches,
it was not possible either to confirm or disconfirm the age estimate from
integrated spectra. The analysis of the distribution in the spectral indices
Mg2 and H_beta of the M31 and Milky Way clusters whose horizontal branch can be
classified as red or blue based on existing CMDs, strongly suggests that
classical age diagnostics from integrated spectra may be significantly
influenced by the HB morphology of the clusters and can lead to erroneous
age-classifications. We also provide the CMD for another two clusters that fall
into the field of the main targets, B336, an old and metal-poor globular with a
significant population of RR-Lyrae variables, and the newly discovered B531, a
cluster with a very red red giant branch. | astro-ph |
Dissipative dark matter explains rotation curves: Dissipative dark matter, where dark matter particles interact with a massless
(or very light) boson, is studied. Such dark matter can arise in simple hidden
sector gauge models, including those featuring an unbroken $U(1)'$ gauge
symmetry, leading to a dark photon. Previous work has shown that such models
can not only explain the LSS and CMB, but potentially also dark matter
phenomena on small scales, such as the inferred cored structure of dark matter
halos. In this picture, dark matter halos of disk galaxies not only cool via
dissipative interactions but are also heated via ordinary supernovae
(facilitated by an assumed photon - dark photon kinetic mixing interaction).
This interaction between the dark matter halo and ordinary baryons, a very
special feature of these types of models, plays a critical role in governing
the physical properties of the dark matter halo. Here, we further study the
implications of this type of dissipative dark matter for disk galaxies.
Building on earlier work, we develop a simple formalism which aims to describe
the effects of dissipative dark matter in a fairly model independent way. This
formalism is then applied to generic disk galaxies. We also consider specific
examples, including NGC 1560 and a sample of dwarf galaxies from the LITTLE
THINGS survey. We find that dissipative dark matter, as developed here, does a
fairly good job accounting for the rotation curves of the galaxies considered.
Not only does dissipative dark matter explain the linear rise of the rotational
velocity of dwarf galaxies at small radii, but it can also explain the observed
wiggles in rotation curves which are known to be correlated with corresponding
features in the disk gas distribution. | astro-ph |
Investigating the Effects of Finite Resolution on Observed Transverse
Jet Profiles: Both the emission properties and evolution of Active Galactic Nuclei (AGN)
radio jets are dependent on the magnetic fields that thread them. Faraday
Rotation gradients are a very important way of investigating these magnetic
fields, and can provide information on the orientation and structure of the
magnetic field in the immediate vicinity of the jet; for example, a toroidal or
helical field component should give rise to a systematic gradient in the
observed Faraday rotation across the jet, as well as characteristic intensity
and polarization profiles. However, real observed radio images have finite
resolution, usually expressed via convolution with a Gaussian beam whose size
corresponds to the central lobe of the point source response function. This
will tend to blur transverse structure in the jet profile, raising the question
of how well resolved a jet must be in the transverse direction in order to
reliably detect transverse structure associated with a helical jet magnetic
field. We present results of simulated intensity, polarization and Faraday
rotation images designed to directly and empirically investigate the effect of
finite resolution on observed transverse jet structures. | astro-ph |
The Bright Rim Prominences according to 2.5D Radiative Transfer: Solar prominences observed close to the limb commonly include a bright
feature that, from the perspective of the observer, runs along the interface
between itself and the underlying chromosphere. Despite several idealised
models being proposed to explain the underlying physics, a more general
approach remains outstanding. In this manuscript we demonstrate as a
proof-of-concept the first steps in applying the Lightweaver radiative transfer
framework's 2.5D extension to a `toy' model prominence + VAL3C chromosphere,
inspired by recent 1.5D experiments that demonstrated a significant radiative
chromosphere--prominence interaction. We find the radiative connection to be
significant enough to enhance both the electron number density within the
chromosphere, as well as its emergent intensity across a range of spectral
lines in the vicinity of the filament absorption signature. Inclining the
viewing angle from the vertical, we find these enhancements to become
increasingly asymmetric and merge with a larger secondary enhancement sourced
directly from the prominence underside. In wavelength, the enhancements are
then found to be the largest in both magnitude and horizontal extent for the
spectral line cores, decreasing into the line wings. Similar behaviour is found
within new Chinese H$\alpha$ Solar Explorer (CHASE)/H$\alpha$ Imaging
Spectrograph (HIS) observations, opening the door for subsequent statistical
confirmations of the theoretical basis we develop here. | astro-ph |
Methoxymethanol Formation Starting from CO-Hydrogenation: Methoxymethanol (CH3OCH2OH, MM) has been identified through gas-phase
signatures in both high- and low-mass star-forming regions. This molecule is
expected to form upon hydrogen addition and abstraction reactions in CO-rich
ice through radical recombination of CO hydrogenation products. The goal of
this work is to investigate experimentally and theoretically the most likely
solid-state MM reaction channel -- the recombination of CH2OH and CH3O radicals
-- for dark interstellar cloud conditions and to compare the formation
efficiency with that of other species that were shown to form along the
CO-hydrogenation line. Hydrogen atoms and CO or H2CO molecules are co-deposited
on top of the predeposited H2O ice to mimic the conditions associated with the
beginning of 'rapid' CO freeze-out. Quadrupole mass spectrometry is used to
analyze the gas-phase COM composition following a temperature programmed
desorption. Monte Carlo simulations are used for an astrochemical model
comparing the MM formation efficiency with that of other COMs. Unambiguous
detection of newly formed MM has been possible both in CO+H and H2CO+H
experiments. The resulting abundance of MM with respect to CH3OH is about 0.05,
which is about 6 times less than the value observed toward NGC 6334I and about
3 times less than the value reported for IRAS 16293B. The results of
astrochemical simulations predict a similar value for the MM abundance with
respect to CH3OH factors ranging between 0.06 to 0.03. We find that MM is
formed by co-deposition of CO and H2CO with H atoms through the recombination
of CH2OH and CH3O radicals. In both the experimental and modeling studies, the
efficiency of this channel alone is not sufficient to explain the observed
abundance of MM. These results indicate an incomplete knowledge of the reaction
network or the presence of alternative solid-state or gas-phase formation
mechanisms. | astro-ph |
Ram Pressure Stripping in the Low Luminosity Virgo Cluster Elliptical
Galaxy NGC 4476: We present a deep VLA search for HI emission from the low-luminosity Virgo
Cluster elliptical galaxy NGC 4476, which contains 1.1 x 10^8 M_sun of
molecular gas in an undisturbed disk in regular rotation. No HI was detected.
The rms noise in the final image corresponds to a 3 sigma column density
sensitivity of 1.2 x 10^20 cm^{-2} at the position of NGC 4476, averaged over
the 4 kpc beam. The total HI mass is less than 1.5 x 10^7 M_sun. If we compare
our HI upper limit to the H_2 content, we find that NGC 4476 is extremely
deficient in HI compared to other galaxies detected in these two species. The
H_2/HI mass ratio for NGC 4476 is > 7, whereas typical H_2/HI ratios for
elliptical galaxies detected in both HI and H_2 are <~2. Based on this extreme
HI deficiency and the intra-cluster medium (ICM) density at the projected
distance from M87 we argue that either NGC 4476 has undergone ram-pressure
stripping while traveling through the Virgo cluster core or its average
molecular gas density is larger and its interstellar UV field is smaller than
in typical spiral galaxies. NGC 4476 is located 12' in projection from M87,
which causes extreme continuum confusion problems. We also discuss in detail
the techniques used for continuum subtraction. The spectral dynamic range of
our final image is 50,000 to 1. | astro-ph |
The Crab Pulsar at Centimeter Wavelengths II: Single Pulses: We have carried out new, high-frequency, high-time-resolution observations of
the Crab pulsar. Combining these with our previous data, we characterize bright
single pulses associated with the Main Pulse, both the Low-Frequency and
High-Frequency Interpulses, and the two High-Frequency Components. Our data
include observations at frequencies ranging from 1 to 43 GHz with time
resolution down to a fraction of a nanosecond. We find at least two types of
emission physics are operating in this pulsar. Both Main Pulses and
Low-Frequency Interpulses, up to about 10 GHz, are characterized by nanoshot
emission - overlapping clumps of narrow-band nanoshots, each with its own
polarization signature. High-Frequency Interpulses, between 5 and 30 GHz, are
characterized by spectral band emission - linearly polarized emission
containing about 30 proportionately spaced spectral bands. We cannot say
whether the longer-duration High-Frequency Component pulses are due to a
scattering process, or if they come from yet another type of emission physics. | astro-ph |
The Role for the Inner Disk in Mass Accretion to the Star in the Early
Phase of Star Formation: A physical mechanism that drives FU Orionis-type outbursts is reconsidered.
We study the effect of inner part of a circumstellar disk covering a region
from near the central star to the radius of approximately $5$ AU (hereafter,
the inner disk). Using the fluctuated mass accretion rate onto the inner disk
$\dot{M}_{\rm out}$, we consider the viscous evolution of the inner disk and
the time variability of the mass accretion rate onto the central star
$\dot{M}_{\rm in}$ by means of numerical calculation of an unsteady viscous
accretion disk in a one-dimensional axisymmetric model. First, we calculate the
evolution of the inner disk assuming an oscillating $\dot{M}_{\rm out}$. It is
shown that the time variability of $\dot{M}_{\rm in}$ does not coincide with
$\dot{M}_{\rm out}$ due to viscous diffusion. Second, we investigate the
properties of spontaneous outbursts with temporally constant $\dot{M}_{\rm
out}$. Outburst occur only in a limited range of mass accretion rates onto the
inner disk $10^{-10}<\dot{M}_{\rm out}< 3\times 10^{-6}~{\rm M}_{\odot} {\rm
yr}^{-1}$ due to gravo-magneto limit cycle (GML). Finally, we discuss the case
with a combination of episodic $\dot{M}_{\rm out}$ and accretion outbursts
cause by the GML in the inner disk. The GML can drive accretion outbursts onto
the star even for the case of fluctuating $\dot{M}_{\rm out}$, although
fluctuations of $\dot{M}$ decay during transmitting the inner disk inwards. We
newly identified two modes of outburst which are spontaneous one and stimulated
one. In a stimulated mode of outburst, $\dot{M}_{\rm out}$ does appear directly
in $\dot{M}_{\rm in}$ (the latter defining the stellar accretion luminosity).
In a spontaneous mode of outburst, $\dot{M}_{\rm out}$ appears as the interval
between outbursts. | astro-ph |
Kinetic "jets" from fast moving pulsars: Some fast-moving pulsars, like the Guitar and the Lighthouse, exhibit
asymmetric non-thermal emission features that extend well beyond their ram
pressure confined pulsar wind nebulae (PWNe). Using 3D relativistic simulations
we explain these features as kinetically streaming pulsar wind particles that
escaped into the interstellar medium (ISM) due to reconnection between the PWN
and ISM magnetic fields. The structure of the reconnecting magnetic fields at
the incoming and outgoing regions produce highly asymmetric magnetic bottles,
and result in asymmetric extended features. For the features to become visible,
the ISM magnetic field should be sufficiently high, $B_{\rm ISM}>10$~$\mu$G. We
also discuss archival observations of PWNe displaying evidence of kinetic jets:
the Dragonfly PWN (PSR J2021+3651), G327.1--1.1, and MSH 11--62, the latter two
of which exhibit "snail eyes" morphologies. We suggest that in those cases the
pulsar is moving along the ambient magnetic field in a frisbee-type
configuration. | astro-ph |
The Spitzer ice legacy: Ice evolution from cores to protostars: Ices regulate much of the chemistry during star formation and account for up
to 80% of the available oxygen and carbon. In this paper, we use the Spitzer
c2d ice survey, complimented with data sets on ices in cloud cores and
high-mass protostars, to determine standard ice abundances and to present a
coherent picture of the evolution of ices during low- and high-mass star
formation. The median ice composition H2O:CO:CO2:CH3OH:NH3:CH4:XCN is
100:29:29:3:5:5:0.3 and 100:13:13:4:5:2:0.6 toward low- and high-mass
protostars, respectively, and 100:31:38:4:-:-:- in cloud cores. In the low-mass
sample, the ice abundances with respect to H2O of CH4, NH3, and the component
of CO2 mixed with H2O typically vary by <25%, indicative of co-formation with
H2O. In contrast, some CO and CO2 ice components, XCN and CH3OH vary by factors
2-10 between the lower and upper quartile. The XCN band correlates with CO,
consistent with its OCN- identification. The origin(s) of the different levels
of ice abundance variations are constrained by comparing ice inventories toward
different types of protostars and background stars, through ice mapping,
analysis of cloud-to-cloud variations, and ice (anti-)correlations. Based on
the analysis, the first ice formation phase is driven by hydrogenation of
atoms, which results in a H2O-dominated ice. At later prestellar times, CO
freezes out and variations in CO freeze-out levels and the subsequent CO-based
chemistry can explain most of the observed ice abundance variations. The last
important ice evolution stage is thermal and UV processing around protostars,
resulting in CO desorption, ice segregation and formation of complex organic
molecules. The distribution of cometary ice abundances are consistent with with
the idea that most cometary ices have a protostellar origin. | astro-ph |
X-ray properties of early-type stars in the Tarantula Nebula from T-ReX: We reassess the historical $L_{X}/L_{Bol}$ relation for early-type stars from
a comparison between T-ReX, the Chandra ACIS X-ray survey of the Tarantula
Nebula in the LMC, and contemporary spectroscopic analysis of massive stars
obtained primarily from VLT/FLAMES, VLT/MUSE and HST/STIS surveys. For 107
sources in common (some host to multiple stars), the majority of which are
bolometrically luminous (40% exceed $10^6 L_{\odot}$), we find an average $\log
L_{X} /L_{Bol} = -6.90 \pm 0.65$. Excluding extreme systems Mk 34 (WN5h+WN5h),
R140a (WC4+WN6+) and VFTS 399 (O9 IIIn+?), plus four WR sources with
anomalously hard X-ray components (R130, R134, R135, Mk 53) and 10 multiple
sources within the spatially crowded core of R136a, $\log L_{X}/L_{Bol} = -7.00
\pm 0.49$, in good agreement with Galactic OB stars. No difference is found
between single and binary systems, nor between O, Of/WN and WR stars, although
there does appear to be a trend towards harder X-ray emission from O dwarfs,
through O (super)giants, Of/WN stars and WR stars. The majority of known OB
stars in the Tarantula are not detected in the T-ReX point source catalogue, so
we have derived upper limits for all undetected OB stars for which log
$L_{Bol}/L_{\odot} \geq 5.0$. A survival analysis using detected and
upper-limit log $L_{X}/L_{Bol}$ values indicates no significant difference
between luminous O stars in the LMC and the Carina Nebula. This analysis
suggests that metallicity does not strongly influence $L_{X}/L_{Bol}$. Plasma
temperatures for single, luminous O stars in the Tarantula
($\overline{kT_{m}}=1.0$ keV) are higher than counterparts in Carina
($\overline{kT_{m}}=0.5$ keV). | astro-ph |
Extreme chemical abundance ratio suggesting an exotic origin for an
ultra-diffuse galaxy: Ultra diffuse galaxies are a population of extended galaxies but with
relatively low luminosities. The origin of these objects remains unclear,
largely due to the observational challenges of the low surface brightness
Universe. We present here a detailed stellar population analysis of a
relatively isolated UDG, DGSAT I, based on spectroscopic data from the Keck
Cosmic Web Imager integral field unit. The star formation history of DGSAT I
seems to be extended, with a mean luminosity-weighted age of ~3 Gyr, in
agreement with previous photometric studies. However, we find a very high
[Mg/Fe] abundance ratio, which is extreme even in the context of the highly
alpha-enhanced massive ellipticals and ultra-faint dwarfs. The
[Mg/Fe]-enhancement of DGSAT I appears to be 10 times higher than the most
magnesium-enhanced stellar systems discovered to date, and suggests that the
chemical enrichment of this object was dominated by core-collapse supernovae.
Intriguingly, this breaks the canonical relation between [Mg/Fe] and star
formation time-scale. With a measured velocity dispersion of 56 +/- 10 km/s,
DGSAT I also shows a high mass-to-light ratio, which indicates that it is
highly dark matter-dominated. The metal-poor conditions of DGSAT I may have
enhanced the formation of massive stars, while at the same time, additional
mechanisms are needed to prevent iron-rich yields from being recycled into
stars. These results suggest that some ultra-diffuse galaxies could have
experienced chemical enrichment episodes similar to the first building blocks
of galaxies. | astro-ph |
Discerning Exoplanet Migration Models Using Spin-Orbit Measurements: We investigate the current sample of exoplanet spin-orbit measurements to
determine whether a dominant planet migration channel can be identified, and at
what confidence. We use the predictions of Kozai migration plus tidal friction
(Fabrycky and Tremaine 2007) and planet-planet scattering (Nagasawa et al.
2008) as our misalignment models, and we allow for a fraction of intrinsically
aligned systems, explainable by disk migration. Bayesian model comparison
demonstrates that the current sample of 32 spin-orbit measurements strongly
favors a two-mode migration scenario combining planet-planet scattering and
disk migration over a single-mode Kozai migration scenario. Our analysis
indicates that between 34% and 76% of close-in planets (95% confidence)
migrated via planet-planet scattering. Separately analyzing the subsample of 12
stars with T_eff > 6250 K---which Winn et al. (2010) predict to be the only
type of stars to maintain their primordial misalignments---we find that the
data favor a single-mode scattering model over Kozai with 81% confidence. We
also assess the number of additional hot star spin-orbit measurements that will
likely be necessary to provide a more confident model selection, finding that
an additional 20-30 measurements has a >50% chance of resulting in a
95%-confident model selection, if the current model selection is correct. While
we test only the predictions of particular Kozai and scattering migration
models in this work, our methods may be used to test the predictions of any
other spin-orbit misaligning mechanism. | astro-ph |
Laboratory Observations and Simulations of Phase Reddening: The visible reflectance spectrum of many solar system bodies changes with
changing viewing geometry for reasons not fully understood. It is often
observed to redden (increasing spectral slope) with increasing solar phase
angle, an effect known as phase reddening. Only once, in an observation of the
Martian surface by the Viking 1 lander, was reddening observed up to a certain
phase angle with bluing beyond, making the reflectance ratio as a function of
phase angle shaped like an arch. However, in laboratory experiments this
arch-shape is frequently encountered. To investigate this, we measured the
bidirectional reflectance of particulate samples of several common rock types
in the 400-1000 nm wavelength range and performed ray-tracing simulations. We
confirm the occurrence of the arch for surfaces that are forward scattering,
i.e. are composed of semi-transparent particles and are smooth on the scale of
the particles, and for which the reflectance increases from the lower to the
higher wavelength in the reflectance ratio. The arch shape is reproduced by the
simulations, which assume a smooth surface. However, surface roughness on the
scale of the particles, such as the Hapke and van Horn (1963) fairy castles
that can spontaneously form when sprinkling a fine powder, leads to monotonic
reddening. A further consequence of this form of microscopic roughness (being
indistinct without the use of a microscope) is a flattening of the disk
function at visible wavelengths, i.e. Lommel-Seeliger-type scattering. The
experiments further reveal monotonic reddening for reflectance ratios at
near-IR wavelengths. The simulations fail to reproduce this particular
reddening, and we suspect that it results from roughness on the surface of the
particles. Given that the regolith of atmosphereless solar system bodies is
composed of small particles, our results indicate that the prevalence of
monotonic (...) | astro-ph |
The COS CGM Compendium. III: Metallicity and Physical Properties of the
Cool Circumgalactic Medium at z<1: We characterize the metallicities and physical properties of cool,
photoionized gas in a sample of 152 z<1 strong Lya forest systems (SLFSs,
absorbers with 15<log N(HI)<16.2). The sample is drawn from our COS
circumgalactic medium (CGM) compendium (CCC), an ultraviolet survey of
HI-selected circumgalactic gas around z<1 galaxies that targets 262 absorbers
with 15<log N(HI)<19. We show that the metallicity probability distribution
function of the SLFSs at z<1 is unimodal, skewed to low metallicities with a
mean and median of [X/H]=-1.47$ and -1.18 dex. Very metal-poor gas with
[X/H]<-1.4 represents about half of the population of absorbers with 15<log
N(HI)<18. Thus, there are important reservoirs of primitive (though not
pristine) gas around z<1 galaxies. The photoionized gas around z<1 galaxies is
highly inhomogeneous based on the wide range of metallicities observed
(-3<[X/H]<+0.4) and that there are large metallicity variations (factors of 2
to 25) for most of the closely-spaced absorbers (Dv<300 km/s) along the same
sightlines. These absorbers show a complex evolution with redshift and HI
column density, and we identify subtle cosmic evolution effects that affect the
interpretation of metallicity distributions and comparison with other of
absorbers samples. We discuss the physical conditions and cosmic baryon and
metal budgets of the CCC absorbers. Finally, we compare the CCC results to
recent cosmological zoom simulations and explore the origins of the 15<log
N(HI)<19 absorbers within the EAGLE high-resolution simulations. | astro-ph |
Particle Acceleration in Relativistic Shearing Flows: Energy Spectrum: We consider the acceleration of charged particles in relativistic shearing
flows, with Lorentz factor up to $\Gamma_0 \sim 20$. We present numerical
solutions to the particle transport equation and compare these with results
from analytical calculations. We show that in the highly relativistic limit the
particle energy spectrum that results from acceleration approaches a power law,
$N(E)\propto E^{-\tilde{q}}$, with a universal value $\tilde{q}=(1+\alpha)$ for
the slope of this power law, where $\alpha$ parameterizes the power-law
momentum dependence of the particle mean free path. At mildly relativistic flow
speeds, the energy spectrum becomes softer and sensitive to the underlying flow
profile. We explore different flow examples, including Gaussian and
power-law-type velocity profiles, showing that the latter yield comparatively
harder spectra, producing $\tilde{q}\simeq 2$ for $\Gamma_0 \simeq 3$ and
Kolmogorov turbulence. We provide a comparison with a simplified leaky-box
approach and derive an approximate relation for estimating the spectral index
as a function of the maximum shear flow speed. These results are of relevance
for jetted, high-energy astrophysical sources such as active galactic nuclei,
since shear acceleration is a promising mechanism for the acceleration of
charged particles to relativistic energies and is likely to contribute to the
high-energy radiation observed. | astro-ph |
Relativistic outflows from remnants of compact object mergers and their
viability for short gamma-ray bursts: We present the first general relativistic hydrodynamic models of the launch
and evolution of relativistic jets and winds, driven by thermal energy
deposition, possibly due to neutrino-antineutrino annihilation, in the close
vicinity of black hole-accretion torus systems. The latter are considered to be
the remnants of compact object mergers. Our two-dimensional simulations
establish the link between such mergers and future observations of short
gamma-ray bursts (GRBs) by the SWIFT satellite. They show that
ultrarelativistic outflow with maximum terminal Lorentz factors (Gamma) around
1000 develops for polar energy deposition rates above some 1e48 erg/s per
steradian, provided the merger environment has a sufficiently low baryon
density. Due to the collimation by the dense accretion torus the typical
semi-opening angles of the Gamma > 100 cone are 5-10 degrees, corresponding to
about 0.4-1.5% of the hemisphere and apparent isotropized energies (kinetic
plus internal) up to ~1e51 erg. 10-30% of the deposited energy are transferred
to the outflow with Gamma > 100. Our models confirm the viability of
post-merger BH-torus systems as engines of short, hard GRBs and can explain the
durations of all observed short GRBs, because different propagation velocities
of the front and rear ends lead to a radial stretching of the ultrarelativistic
fireball before transparency is reached. The ultrarelativistic flow reveals a
highly non-uniform structure with Lorentz factor variations up to factors of a
few, caused by the action of Kelvin-Helmholtz instabilities that originate at
the fireball-torus interface (abbreviated). | astro-ph |
Distance measurements as a probe of cosmic acceleration: A major recent evelopment in observational cosmology has been an accurate
measurement of the luminosity distance-redshift relation out to redshifts z=0.8
from Type Ia supernova standard candles. The results have been argued as
evidence for cosmic acceleration. It is well known that this assertion depends
on the assumption that we know the equation of state for all mass-energy other
than normal pressureless matter; popular models are based on either the
cosmological constant or on the more general quintessence formulation. But this
assertion also depends on a number of other assumptions, implicit in the
derivation of the standard cosmological field equations: large-scale isotropy
and homogeneity, the flatness of the Universe, and the validity of general
relativity on cosmological scales (where it has not been tested). A detailed
examination of the effects of these assumptions on the interplay between the
luminosity distance-redshift relation and the acceleration of the Universe is
not possible unless one can define the precise nature of the failure of any
particular assumption. However a simple quantitative investigation is possible
and reveals a number of considerations about the relative importance of the
different assumptions. In this paper we present such an investigation. We find
that the relationship between the distant-redshift relation and the sign of the
deceleration parameter is fairly robust and is unaffected if only one of the
assumptions that we investigate is invalid so long as the deceleration
parameter is not close to zero (it would not be close to zero in the
currently-favored Omega_Lambda = 1 - Omega_matter = 0.7 or 0.8 Universe, for
example). Failures of two or more assumptions in concordance may have stronger
effects. | astro-ph |
Light Nuclei solving Auger puzzles. The Cen-A imprint: Ultra High Energy Cosmic Rays (UHECR) map at 60 EeV have been found recently
by AUGER group spreading anisotropy signatures in the sky. The result have been
interpreted as a manifestation of AGN sources ejecting protons at GZK edges
mostly from Super-galactic Plane. The result is surprising due to the absence
of much nearer Virgo cluster. Moreover, early GZK cut off in the spectra may be
better reconcile with light nuclei (than with protons). In addition a large
group (nearly a dozen) of events cluster suspiciously along Cen-A. Finally,
proton UHECR composition nature is in sharp disagreement with earlier AUGER
claim of a heavy nuclei dominance at 40 EeV. Therefore we interpret here the
signals as mostly UHECR light nuclei (He, Be, B, C, O), very possibly mostly
the lightest (He,Be) ones, ejected from nearest AGN Cen-A, UHECR smeared by
galactic magnetic fields, whose random vertical bending is overlapping with
super-galactic arm. The eventual AUGER misunderstanding took place because of
such a rare coincidence between the Super Galactic Plane (arm) and the smeared
(randomized) signals from Cen-A, bent orthogonally to the Galactic fields. Our
derivation verify the consistence of the random smearing angles for He, Be and
B, C, O, in reasonable agreement with the AUGER main group events around Cen-A.
Only few other rare events are spread elsewhere. The most collimated from Cen-A
are the lightest. The most spread the heavier. Consequently Cen-A is the best
candidate UHE neutrino tau observable by HEAT and AMIGA as enhanced AUGER array
at tens-hundred PeV energy. This model maybe soon tested by new events
clustering around the Cen-A and by composition imprint study. | astro-ph |
Selecting Quasar Candidates by a SVM Classification System: We develop and demonstrate a classification system constituted by several
Support Vector Machines (SVM) classifiers, which can be applied to select
quasar candidates from large sky survey projects, such as SDSS, UKIDSS, GALEX.
How to construct this SVM classification system is presented in detail. When
the SVM classification system works on the test set to predict quasar
candidates, it acquires the efficiency of 93.21% and the completeness of
97.49%. In order to further prove the reliability and feasibility of this
system, two chunks are randomly chosen to compare its performance with that of
the XDQSO method used for SDSS-III's BOSS. The experimental results show that
the high faction of overlap exists between the quasar candidates selected by
this system and those extracted by the XDQSO technique in the dereddened i-band
magnitude range between 17.75 and 22.45, especially in the interval of
dereddened i-band magnitude < 20.0. In the two test areas, 57.38% and 87.15% of
the quasar candidates predicted by the system are also targeted by the XDQSO
method. Similarly, the prediction of subcategories of quasars according to
redshift achieves a high level of overlap with these two approaches. Depending
on the effectiveness of this system, the SVM classification system can be used
to create the input catalog of quasars for the GuoShouJing Telescope (LAMOST)
or other spectroscopic sky survey projects. In order to get higher confidence
of quasar candidates, cross-result from the candidates selected by this SVM
system with that by XDQSO method is applicable. | astro-ph |
Broadband Observations of the Compton-thick Nucleus of NGC 3393: We present new NuSTAR and Chandra observations of NGC 3393, a galaxy reported
to host the smallest separation dual AGN resolved in the X-rays. While past
results suggested a 150 pc separation dual AGN, three times deeper Chandra
imaging, combined with adaptive optics and radio imaging suggest a single,
heavily obscured, radio-bright AGN. Using VLA and VLBA data, we find an AGN
with a two-sided jet rather than a dual AGN and that the hard X-ray, UV,
optical, NIR, and radio emission are all from a single point source with a
radius <0.2". We find that the previously reported dual AGN is most likely a
spurious detection resulting from the low number of X-ray counts (<160) at 6-7
keV and Gaussian smoothing of the data on scales much smaller than the PSF
(0.25" vs. 0.80" FWHM). We show that statistical noise in a single Chandra PSF
generates spurious dual peaks of the same separation (0.55$\pm$0.07" vs. 0.6")
and flux ratio (39$\pm$9% vs. 32% of counts) as the purported dual AGN. With
NuSTAR, we measure a Compton-thick source (NH=$2.2\pm0.4\times10^{24}$
cm$^{-2}$) with a large torus half-opening angle, {\theta}=79 which we
postulate results from feedback from strong radio jets. This AGN shows a 2-10
keV intrinsic to observed flux ratio of 150. Using simulations, we find that
even the deepest Chandra observations would severely underestimate the
intrinsic luminosity of NGC 3393 above z>0.2, but would detect an unobscured
AGN of this luminosity out to high redshift (z=5). | astro-ph |
Estimation of the XUV radiation onto close planets and their evaporation: Context: The current distribution of planet mass vs. incident stellar X-ray
flux supports the idea that photoevaporation of the atmosphere may take place
in close-in planets. Integrated effects have to be accounted for. A proper
calculation of the mass loss rate due to photoevaporation requires to estimate
the total irradiation from the whole XUV range. Aims: The purpose of this paper
is to extend the analysis of the photoevaporation in planetary atmospheres from
the accessible X-rays to the mostly unobserved EUV range by using the coronal
models of stars to calculate the EUV contribution to the stellar spectra. The
mass evolution of planets can be traced assuming that thermal losses dominate
the mass loss of their atmospheres. Methods: We determine coronal models for 82
stars with exoplanets that have X-ray observations available. Then a synthetic
spectrum is produced for the whole XUV range (~1-912 {\AA}). The determination
of the EUV stellar flux, calibrated with real EUV data, allows us to calculate
the accumulated effects of the XUV irradiation on the planet atmosphere with
time, as well as the mass evolution for planets with known density. Results: We
calibrate for the first time a relation of the EUV luminosity with stellar age
valid for late-type stars. In a sample of 109 exoplanets, few planets with
masses larger than ~1.5 Mj receive high XUV flux, suggesting that intense
photoevaporation takes place in a short period of time, as previously found in
X-rays. The scenario is also consistent with the observed distribution of
planet masses with density. The accumulated effects of photoevaporation over
time indicate that HD 209458b may have lost 0.2 Mj since an age of 20 Myr.
Conclusions: Coronal radiation produces rapid photoevaporation of the
atmospheres of planets close to young late-type stars. More complex models are
needed to explain fully the observations. | astro-ph |
Twenty Years of Searching for (and Finding) Globular Cluster Pulsars: Globular clusters produce orders of magnitude more millisecond pulsars per
unit mass than the Galactic disk. Since the first cluster pulsar was uncovered
twenty years ago, at least 138 have been identified - most of which are binary
millisecond pulsars. Because of their origins involving stellar encounters,
many of these systems are exotic objects that would never be observed in the
Galactic disk. Examples include pulsar-main sequence binaries, extremely rapid
rotators (including the current record holder), and millisecond pulsars in
highly eccentric orbits. These systems are allowing new probes of the
interstellar medium, the equation of state of material at supra-nuclear
density, the mass distribution of neutron stars, and the dynamics of globular
clusters. | astro-ph |
Survey of Li-rich giants among Kepler and LAMOST fields: Determination
of Li-rich giants Evolutionary Phase: In this letter, we report the discovery of 24 new super Li-rich (A(Li) $\ge$
3.2) giants of He-core burning phase at red clump region. Results are based on
systematic search of a large sample of about 12,500 giants common to the LAMOST
spectroscopic and Kepler time resolved photometric surveys. The two key
parameters derived from Kepler data; average period spacing ($\Delta p$)
between $l=1$ mixed gravity dominated g-modes and average large frequency
separation ($\Delta \nu$) $l=0$ acoustic p-modes, suggest all the Li-rich
giants are in He-core burning phase. This is the first unbiased survey
subjected to a robust technique of asteroseismic analysis to unambiguously
determine evolutionary phase of Li-rich giants. The results provide a strong
evidence that Li enhancement phenomenon is associated with giants of He-core
burning phase, post He-flash, rather than any other phase on RGB with inert
He-core surrounded by H-burning shell. | astro-ph |
Planet formation via pebble accretion in externally photoevaporating
discs: We demonstrate that planet formation via pebble accretion is sensitive to
external photoevaporation of the outer disc. In pebble accretion, planets grow
by accreting from a flux of solids (pebbles) that radially drift inwards from
the pebble production front. If external photoevaporation truncates the outer
disc fast enough, it can shorten the time before the pebble production front
reaches the disc outer edge, cutting off the supply of pebble flux for
accretion, hence limiting the pebble mass reservoir for planet growth.
Conversely, cloud shielding can protect the disc from strong external
photoevaporation and preserve the pebble reservoir. Because grain growth and
drift can occur quickly, shielding even on a short time-scale (<1 Myr) can have
a non-linear impact on the properties of planets growing by pebble accretion.
For example a $10^{-3} M_\oplus$ planetary seed at 25 au stays at 25 au with a
lunar mass if the disc is immediately irradiated by a $10^3$ G$_0$ field, but
grows and migrates to be approximately Earth-like in both mass and orbital
radius if the disc is shielded for just 1 Myr. In NGC 2024, external
photoevaporation is thought to happen to discs that are <0.5 Myr old, which
coupled with the results here suggests that the exact planetary parameters can
be very sensitive to the star forming environment. Universal shielding for
time-scales of at least $\sim1.5$ Myr would be required to completely nullify
the environmental impact on planetary architectures. | astro-ph |
Galaxies behind the Milky Way and the Great Attractor: Dust and stars in the plane of the Milky Way create a "Zone of Avoidance" in
the extragalactic sky. Galaxies are distributed in gigantic labyrinth
formations, filaments and great walls with occasional dense clusters. They can
be traced all over the sky, except where the dust within our own galaxy becomes
too thick - leaving about 25% of the extragalactic sky unaccounted for. Our
Galaxy is a natural barrier which constrains the studies of large-scale
structures in the Universe, the peculiar motion of our Local Group of galaxies
and other streaming motions (cosmic flows) which are important for
understanding formation processes in the Early Universe and for cosmological
models. Only in recent years have astronomers developed the techniques to peer
through the disk and uncover the galaxy distribution in the Zone of Avoidance.
I present the various observational multi-wavelength procedures (optical, far
infrared, near infrared, radio and X-ray) that are currently being pursued to
map the galaxy distribution behind our Milky Way. Particular emphasis is given
to discoveries in the Great Attractor region -- a from streaming motions
predicted huge overdensity centered behind the Galactic Plane. The recently
unveiled massive rich cluster A3627 seems to constitute the previously
unidentified core of the Great Attractor. | astro-ph |
Dark energy with rigid voids versus relativistic voids alone: The standard model of cosmology is dominated - at the present epoch - by dark
energy. Its voids are rigid and Newtonian within a relativistic background. The
model prevents them from becoming hyperbolic. Observations of rapid velocity
flows out of voids are normally interpreted within the standard model that is
rigid in comoving coordinates, instead of allowing the voids' density parameter
to drop below critical and their curvature to become negative. Isn't it time to
advance beyond nineteenth century physics and relegate dark energy back to the
"no significant evidence" box? | astro-ph |
Serendipitous discovery of a cluster of galaxies with a peculiar central
galaxy: We report the serendipitous discovery of a cluster of galaxies at z=0.369.
Thirty-eight candidate members were identified based on rough broad-band
photometric redshifts, and three members were confirmed spectroscopically. The
brightest cluster galaxy (BCG) is exceptionally blue, with B-V=0.12 and
V-I=1.02. The surface-brightness profile of the BCG follows an r^(1/4)-law
profile out to 3" in all three bands. The effective radius is significantly
smaller in bluer bandpasses, resulting in a blue core and a color gradient
opposite to the metallicity-induced color gradient observed in typical
elliptical galaxies. Beyond 3" an extended envelope of emission in excess of
the r^(1/4)-law profile is observed, the position angle of which coincides with
the major axis of the galaxy cluster. The spectrum of the BCG contains strong
Balmer absorption, a minimal 4000 A break, and a broad Mg II emission line,
suggesting that the galaxy has undergone recent star formation and may harbor a
central AGN. The presence of numerous nearby bright stars makes this cluster an
interesting target for next-generation adaptive optics using natural guide
stars. | astro-ph |
A deterministic model for forecasting long-term solar activity: A phenomenological model is presented for the quantitative description of
individual solar cycles' features, such as onset, intensity, evolution, in
terms of the number of M and X-class solar flares. The main elements of the
model are the relative ecliptic motion of the planets Jupiter and Saturn, and
its synergy with a quasi-periodic component of solar activity. Using as input
the temporal distribution of flares during cycle 21, the general evolution of
cycles 22-24 is reproduced in notable agreement with the observations,
including the resurgence of activity in the last months of 2017, and further
predictions are provided for cycle 25. This deterministic description could
contribute to elucidating the responsible physical mechanisms and forecasting
space weather. | astro-ph |
NGC 5128 globular cluster candidates out to 150 kpc: a comprehensive
catalog from Gaia and ground based data: We present a new catalog of 40502 globular cluster (GC) candidates in NGC
5128 out to a projected radius of $\sim$150 kpc, based on data from the
Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS), Gaia Data Release
2, and the NOAO Source Catalog. Ranking these candidates based on the
likelihood that they are true GCs, we find that approximately 1900 belong to
our top two ranking categories and should be the highest priority for
spectroscopic follow-up for confirmation. Taking into account our new data and
a vetting of previous GC catalogs, we estimate a total GC population of $1450
\pm 160$ GCs. We show that a substantial number of sources previously argued to
be low-velocity GCs are instead foreground stars, reducing the inferred GC
velocity dispersion. This work showcases the power of Gaia to identify slightly
extended sources at the $\sim 4$ Mpc distance of NGC 5128, enabling accurate
identification of GCs throughout the entire extended halo, not just the inner
regions that have been the focus of most previous work. | astro-ph |
Probing Flavor Structure of Cosmic Ray $e^\mp$ Spectrum and Implications
for Dark Matter Indirect Searches: Measuring high energy cosmic ray electrons/positrons (CRE) provides important
means for the dark matter (DM) indirect detection and for probing the nearby
galactic sources. In this work, we perform a systematic analysis of the flavor
structure of DM annihilations into charged leptons based on the cosmic ray CRE
spectra measured by DAMPE, Fermi-LAT, AMS-02, and CALET experiments. We study
the annihilations of possible TeV scale DM particles in a nearby subhalo, which
is proposed to explain the possible peak-like structure of the DAMPE CRE data.
We pay special attention to the possible non-resonant excess (besides the
possible peak-like structure) and demonstrate that such non-resonant excess can
mainly arise from the decay of muons produced by the DM annihilations in the
subhalo. With these we study the flavor composition of the lepton final states
from DM annihilations $\chi\chi\to e^+e^-, \mu^+\mu^-, \tau^+\tau^-$ by fitting
the CRE data. We demonstrate that decays of the final states $\mu^+ \mu^-$ and
$\tau^+ \tau^-$ can provide the non-resonant excess, while the peak excess
arises from the $e^+ e^-$ final state. We further analyze the constraints on
the lepton flavor composition using the Fermi-LAT $\gamma$-ray measurements. We
find that the flavor composition is consistent with the Fermi-LAT data at
relatively low Galactic latitudes, while the fraction of the final state
$\tau^\pm$ is severely bounded. | astro-ph |
Constraining the equation of state of neutron stars using multimessenger
observations: Neutron stars are the densest objects known in our visible universe.
Properties of matter inside a neutron star are encoded in its equation of
state, which has wide-ranging uncertainty from a theoretical perspective. With
the current understanding of quantum chromodynamics, it is hard to determine
the interactions of neutron star matter at such high densities. Also performing
many body calculations is computationally intractable. Besides the constitution
of the neutron star core is highly speculative -- it is not ruled out that it
contains exotic matter like strange baryons, meson condensates, quark matter,
etc. Although the matter inside the neutron star is extremely dense, but the
temperature of this object is very cold in most of its life span. We cannot
produce such dense but rather cold material in our laboratory. Since probing
the physics of neutron star matter is inaccessible by our earth based
experiments, we look for astrophysical observations of neutron stars. This
thesis deals with the theoretical and computational techniques required to
translate neutron star observables from astrophysical observations to its
equation of state. | astro-ph |
Timing calibration of the APOLLO experiment: The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) began
millimeter-precision ranging to the Moon in 2006. Until now, a comprehensive
validation of APOLLO system range accuracy has not been possible because of
centimeter-scale deficiencies in computational models of the Earth-Moon range,
and because APOLLO lacked an internal timing calibration system. Here, we
report on the development of a system that enables in-situ calibration of the
timing response of the APOLLO apparatus, simultaneous with lunar range
measurements. The system was installed in August 2016. Preliminary results show
that the APOLLO system can provide lunar range measurements with millimeter
accuracy. | astro-ph |
Mass measurements and 3D orbital geometry of PSR J1933$-$6211: PSR J1933$-$6211 is a 3.5-ms pulsar in a 12.8-d orbit with a white dwarf
(WD). Its high proper motion and low dispersion measure result in such
significant interstellar scintillation that high signal-to-noise detections
require long observing durations or fortuitous timing. We turn to the sensitive
MeerKAT telescope and, combined with historic Parkes data, leverage PSR
J1933$-$6211's kinematic and relativistic effects to constrain its 3D orbital
geometry and the component masses. We obtain precise proper motion and parallax
estimates, and measure their effects as secular changes in the Keplerian
orbital parameters: a variation in orbital period of $7(1) \times 10^{-13}$ s
s$^{-1}$ and a change in projected semi-major axis of $1.60(5) \times 10^{-14}$
s s$^{-1}$. A self-consistent analysis of all kinematic and relativistic
effects yields a distance of $1.6^{+0.2}_{-0.3}$ kpc, an orbital inclination,
$i = 55(1)$ deg and a longitude of the ascending node, $\Omega =
255^{+8}_{-14}$ deg. The probability densities for $\Omega$ and $i$ and their
symmetric counterparts, ($180-i$, $360-\Omega$), are seen to depend on the
fiducial orbit used to measure the time of periastron passage. We investigate
this unexpected dependence and rule out software-related causes using
simulations. Nevertheless, we constrain the pulsar and WD masses to
$1.4^{+0.3}_{-0.2}$ M$_\odot$ and $0.43(5)$ M$_\odot$ respectively. These
strongly disfavour a helium-dominated WD. The orbital similarities between PSRs
J1933$-$6211 and J1614$-$2230 suggest they underwent Case A Roche lobe
overflow, an extended evolution while the companion star is still on the Main
Sequence. However, with a mass of $\sim 1.4$ M$_\odot$, PSR J1933$-$6211 has
not accreted significant matter. This highlights the low accretion efficiency
of the spin-up process and suggests that observed neutron star masses are
mostly a result of supernova physics. | astro-ph |
Searching for Low-mass Population III Stars Disguised as White Dwarfs: It is uncertain whether or not low-mass Population III stars ever existed.
While limits on the number density of Population III stars with $M_{\ast}
\approx 0.8~M_{\odot}$ have been derived using Sloan Digital Sky Survey (SDSS)
data, little is known about the occurrence of Population III stars at lower
masses. In the absence of reliable parallaxes, the spectra of metal-poor main
sequence (MPMS) stars with $M_{\ast} \lesssim 0.8~M_{\odot}$ can easily be
confused with cool white dwarfs. To resolve this ambiguity, we present a
classifier that differentiates between MPMS stars and white dwarfs based on
photometry and/or spectroscopy without the use of parallax information. We
build and train our classifier using state-of-the-art theoretical spectra and
evaluate it on existing SDSS-based classifications for objects with reliable
Gaia DR2 parallaxes. We then apply our classifier to a large catalog of objects
with SDSS photometry and spectroscopy to search for MPMS candidates. We
discover several previously unknown candidate extremely metal-poor (EMP) stars
and recover numerous confirmed EMP stars already in the literature. We conclude
that archival SDSS spectroscopy has already been exhaustively searched for EMP
stars. We predict that the lowest-mass primordial-composition stars will have
redder optical-to-infrared colors than cool white dwarfs at constant effective
temperature due to surface gravity-dependent collision-induced absorption from
molecular hydrogen. We suggest that the application of our classifier to data
produced by next-generation spectroscopic surveys will set stronger constraints
on the number density of low-mass Population III stars in the Milky Way. | astro-ph |
Applicability of the linearly perturbed FRW metric and Newtonian
cosmology: It has been argued that the effect of cosmological structure formation on the
average expansion rate is negligible, because the linear approximation to the
metric remains applicable in the regime of non-linear density perturbations. We
discuss why the arguments based on the linear theory are not valid. We
emphasise the difference between Newtonian gravity and the weak field, small
velocity limit of general relativity in the cosmological setting. | astro-ph |