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Hydrodynamics of Core-Collapse Supernovae at the Transition to
Explosion. I. Spherical Symmetry: We study the transition to runaway expansion of an initially stalled
core-collapse supernova shock. The neutrino luminosity, mass accretion rate,
and neutrinospheric radius are all treated as free parameters. In spherical
symmetry, this transition is mediated by a global non-adiabatic instability
that develops on the advection time and reaches non-linear amplitude. Here we
perform high-resolution, time-dependent hydrodynamic simulations of stalled
supernova shocks with realistic microphysics to analyze this transition. We
find that radial instability is a sufficient condition for runaway expansion if
the neutrinospheric parameters do not vary with time and if heating by the
accretion luminosity is neglected. For a given unstable mode, transition to
runaway occurs when fluid in the gain region reaches positive specific energy.
We find approximate instability criteria that accurately describe the behavior
of the system over a wide region of parameter space. The threshold neutrino
luminosities are in general different than the limiting value for a
steady-state solution. We hypothesize that multidimensional explosions arise
from the excitation of unstable large-scale modes of the turbulent background
flow, at threshold luminosities that are lower than in the laminar case. | astro-ph_HE |
Constraining the lensing of binary neutron stars from their stochastic
background: Gravitational wave (GW) transients from binary neutron star (BNS)
coalescences can, in principle, be subject to gravitational lensing thereby
increasing the amplitude and signal-to-noise ratio. We estimate the rate of
lensed BNS events resolvable by LIGO and Virgo and find that it is constrained
by the current non-detection of a stochastic GW background. Following closely
the formalism we developed (10.1103/PhysRevLett.125.141102) in the context of
binary black hole lensing, we show that at current sensitivities the fraction
of BNS coalescences with lensing magnifications $\mu> 1.02$ is less than $\sim
7\times 10^{-8}$ and therefore such events should not be expected in the near
future. We also make predictions for projected future sensitivities. | astro-ph_HE |
Inferences from the Distributions of Fast Radio Burst Pulse Widths,
Dispersion Measures and Fluences: The widths, dispersion measures, dispersion indices and fluences of Fast
Radio Bursts (FRB) impose coupled constraints that all models must satisfy.
Observation of dispersion indices close to their low density limit of $-2$ sets
a model-independent upper bound on the electron density and a lower bound on
the size of any dispersive plasma cloud. The non-monotonic dependence of burst
widths (after deconvolution of instrumental effects) on dispersion measure
excludes the intergalactic medium as the location of scattering that broadens
the FRB in time. Temporal broadening far greater than that of pulsars at
similar high Galactic latitudes implies that scattering occurs close to the
sources, where high densities and strong turbulence are plausible. FRB
energetics are consistent with supergiant pulses from young, fast, high-field
pulsars at cosmological distances. The distribution of FRB dispersion measures
is inconsistent with expanding clouds (such as SNR). It excludes space-limited
distributions (such as the local supercluster), but agrees with a homogeneous
cosmological distribution with intergalactic dispersion. The FRB
$\log{N}$--$\log{S}$ relation also indicates a cosmological distribution, aside
from the anomalously bright Lorimer burst. | astro-ph_HE |
Electromagnetic Window into the Dawn of Black Holes: Massive 10^6-10^10 Msun black holes (BHs) are ubiquitous in local galactic
nuclei. They were common by the time the Universe is several Gyr old, and many
of them were in place within the first 1~Gyr after the Big Bang. Their quick
assembly has been attributed to mechanisms such as the rapid collapse of gas
into the nuclei of early protogalaxies, accretion and mergers of stellar-mass
BHs accompanying structure formation at early times, and the runaway collapse
of early, ultra-dense stellar clusters. The origin of the early massive BHs
remains an intriguing and long-standing unsolved puzzle in astrophysics. Here
we discuss strategies for discerning between BH seeding models using
electromagnetic observations. We argue that the most direct answers will be
obtained through detection of BHs with masses M<10^5 Msun at redshifts z>10,
where we expect them to first form. Reaching out to these redshifts and down to
these masses is crucial, because BHs are expected to lose the memory of their
initial assembly by the time they grow well above 10^5 Msun and are
incorporated into higher-mass galaxies. The best way to detect 10^4-10^5 Msun
BHs at high redshifts is by a sensitive X-ray survey. Critical constraining
power is augmented by establishing the properties and the environments of their
host galaxies in deep optical/IR imaging surveys. Required OIR data can be
obtained with the JWST and WFIRST missions. The required X-ray flux limits
(down to 10^{-19} erg/s/cm^2) are accessible only with a next-generation X-ray
observatory which has both high (sub-1") angular resolution and high
throughput. A combination of deep X-ray and OIR surveys will be capable of
probing several generic markers of the BH seed scenarios, and resolving the
long-stanging puzzle of their origin. These electromagnetic observations are
also highly synergistic with the information from LISA on high-z BH mergers. | astro-ph_HE |
Dynamical Formation Scenarios for GW190521 and Prospects for Decihertz
Gravitational-Wave Astronomy with GW190521-Like Binaries: The gravitational-wave (GW) detection of GW190521 has provided new insights
on the mass distribution of black holes and new constraints for astrophysical
formation channels. With independent claims of GW190521 having significant
pre-merger eccentricity, we investigate what this implies for GW190521-like
binaries that form dynamically. The Laser Interferometer Space Antenna (LISA)
will also be sensitive to GW190521-like binaries if they are circular from an
isolated formation channel. We show, however, that GW190521-like binaries that
form dynamically may skip the LISA band entirely. To this end, we simulate
GW190521 analogues that dynamically form via post-Newtonian binary-single
scattering. From these scattering experiments, we find that GW190521-like
binaries may enter the LIGO-Virgo band with significant eccentricity as
suggested by recent studies, though well below an eccentricity of $e_{\rm 10Hz}
\lesssim 0.7$. Eccentric GW190521-like binaries further motivate the
astrophysical science case for a decihertz GW observatory, such as the
kilometer-scale version of the Midband Atomic Gravitational-wave
Interferometric Sensor (MAGIS). Pre-merger observations of GW190521-like
binaries with such a decihertz GW detector would be able to constrain the
eccentricity of GW190521-like binaries to greater precision than with just
LIGO-Virgo alone. These eccentricity constraints would also provide additional
insights into the possible environments that GW190521-like binaries form in. | astro-ph_HE |
Evidence for GeV Pair Halos around Low Redshift Blazars: We report on the results of a search for $\gamma$-ray pair halos with a
stacking analysis of low-redshift blazars using data from the Fermi Large Area
Telescope. For this analysis we used a number of a-priori selection criteria,
including the spatial and spectral properties of the Fermi sources. The angular
distribution of $\sim$ 1GeV photons around 24 stacked isolated
high-synchrotron-peaked BL Lacs with redshift $z<0.5$ shows an excess over that
of point-like sources. A statistical analysis yields a Bayes factor of
$\mathrm{log}_{10}B_{10}>2$, providing evidence in favor of extended emission
against the point-source hypothesis, consistent with expectations for pair
halos produced in the IGMF with strength $B_{\mathrm{IGMF}}\sim
10^{-17}-10^{-15}\mathrm{G}$. | astro-ph_HE |
The XMM-Newton survey of the Small Magellanic Cloud:
XMMUJ010633.1-731543 and XMMUJ010743.1-715953, two new Be/X-ray binary
systems: In the course of the XMM-Newton survey of the Small Magellanic Cloud (SMC),
two new bright X-ray sources were discovered exhibiting the spectral
characteris- tics of High Mass X-ray Binaries - but revealing only weak
evidence for pulsations in just one of the objects(at 153s in
XMMUJ010743.1-715953). The accurate X- ray source locations permit the
identification of these X-ray source with Be stars, thereby strongly suggesting
these systems are new Be/X-ray binaries. From blue spectra the proposed
classification for XMMUJ010633.1-731543 is B0.5-1Ve and for
XMMUJ010743.1-715953 it is B2IV-Ve. | astro-ph_HE |
Lorentz Violation: Motivation and new constraints: We review the main theoretical motivations and observational constraints on
Planck scale suppressed violations of Lorentz invariance. After introducing the
problems related to the phenomenological study of quantum gravitational
effects, we discuss the main theoretical frameworks within which possible
departures from Lorentz invariance can be described. In particular, we focus on
the framework of Effective Field Theory, describing several possible ways of
including Lorentz violation therein and discussing their theoretical viability.
We review the main low energy effects that are expected in this framework. We
discuss the current observational constraints on such a framework, focusing on
those achievable through high-energy astrophysics observations. In this context
we present a summary of the most recent and strongest constraints on QED with
Lorentz violating non-renormalizable operators. Finally, we discuss the present
status of the field and its future perspectives. | astro-ph_HE |
Investigating evidence for different black hole accretion modes since
redshift z~1: Chandra data in the COSMOS, AEGIS-XD and 4Ms CDFS are combined with
optical/near-IR photometry to determine the rest-frame U-V vs V-J colours of
X-ray AGN hosts at mean redshifts 0.40 and 0.85. This combination of colours
(UVJ) provides an efficient means of separating quiescent from star-forming,
including dust reddened, galaxies. Morphological information emphasises
differences between AGN split by their UVJ colours. AGN in quiescent galaxies
are dominated by spheroids, while star-forming hosts are split between bulges
and disks. The UVJ diagram of AGN hosts is then used to set limits on the
accretion density associated with evolved and star-forming systems. Most of the
black hole growth since z~1 is associated with star-forming hosts.
Nevertheless, ~15-20% of the X-ray luminosity density since z~1, is taking
place in the quiescent region of the UVJ diagram. For the z~0.40 subsample,
there is tentative evidence (2sigma significance), that AGN split by their UVJ
colours differ in Eddington ratio. AGN in star-forming hosts dominate at high
Eddington ratios, while AGN in quiescent hosts become increasingly important as
a fraction of the total population toward low Eddington ratios. At higher
redshift, z~0.8, such differences are significant at the 2sigma level only at
Eddington ratios >1e-3. These findings are consistent with scenarios in which
diverse accretion modes are responsible for the build-up of SMBHs at the
centres of galaxies. We compare our results with the GALFORM semi-analytic
model, which postulates two black hole fuelling modes, the first linked to
star-formation and the second occuring in passive galaxies. GALFORM predicts a
larger fraction of black hole growth in quiescent galaxies at z<1, compared to
the data. Relaxing the strong assumption of the model that passive AGN hosts
have zero star-formation rate could reconcile this disagreement. | astro-ph_HE |
Can very massive Population III stars produce a super-collapsar?: A fraction of the first generation of stars in the early Universe may be very
massive ($\gtrsim 300~\mathrm{M_\odot}$) as they form in metal-free
environments. Formation of black holes from these stars can be accompanied by
supermassive collapsars to produce long gamma-ray bursts of a unique type
having a very high total energy ($\sim 10^{54}~\mathrm{erg}$) as recently
suggested by several authors. We present new stellar evolution models of very
massive Population III stars including the effect of rotation to provide
theoretical constraints on super-collapsar progenitors. We find that the
angular momentum condition for super-collapsar can be fulfilled if magnetic
torques are ignored, in which case Eddington-Sweet circulations play the
dominant role for the transport of angular momentum. We further find that the
initial mass range for super-collapsar progenitors would be limited to
$300~\mathrm{M_\odot} \lesssim M \lesssim 700~\mathrm{M_\odot}$. However, all
of our very massive star models of this mass range end their lives as red
supergiants rather than blue supergiants, in good agreement with most of the
previous studies. The predicted final fate of these stars is either a
jet-powered type IIP supernova or an ultra-long, relatively faint gamma-ray
transient, depending on the initial amount of angular momentum. | astro-ph_HE |
Observation of Anisotropy in the Arrival Direction Distribution of TeV
Cosmic Rays with HAWC: The High-Altitude Water Cherenkov (HAWC) Observatory, located 4100 m above
sea level near Sierra Negra (19$^\circ$ N) in Mexico, is sensitive to gamma
rays and cosmic rays at TeV energies. The arrival direction distribution of
cosmic rays at these energies shows significant anisotropy on several angular
scales, with a relative intensity ranging between 10$^{-3}$ and 10$^{-4}$. We
present the results of a study of cosmic-ray anisotropy based on more than 86
billion cosmic-ray air showers recorded with HAWC since June 2013. The HAWC
cosmic-ray sky map, which has a median energy of 2 TeV, exhibits several
regions of significantly enhanced cosmic-ray flux. We present the energy
dependence of the anisotropy and the cosmic-ray spectrum in the regions of
significant excess. | astro-ph_HE |
A systematic study of photoionized emission and warm absorption
signatures of the NLS1 Mrk 335: We present an analysis of all the archival high resolution spectra of the
Narrow-line Seyfert 1 galaxy Mrk~335 obtained with Reflection Grating
Spectrometer (RGS) on board \textit{XMM-Newton}. The spectra show rich emission
and absorption features in low and intermediate flux intervals. We model the
emission lines with the \textsc{pion\_xs} grid and try to find any possible
correlation between the properties of the emitting gas and the source flux.
Current data does not allow detailed trace of the response of the line emitting
gas to the X-ray flux of Mrk~335, but the flux of the X-ray lines is
significantly less variable than the X-ray continuum. We also find that the
warm absorber's properties are not correlated with the flux variability. From
the latest \textit{XMM-Newton} observation in 2019 December, we find that the
photoionized emission and distant reflection components have not responded to
the flux drop of Mrk~335 from 2018 July. However, the possible existence of
partial covering absorber in the 2018--2019 low state of Mrk~335 makes it
difficult to constrain the scale of the emitting gas using this lack of
response. | astro-ph_HE |
Growing evidence that SNe Iax are not a one-parameter family: the case
of PS1-12bwh: In this study, we present observations of a type Iax supernova, PS1-12bwh,
discovered during the Pan-STARRS1 3$\pi$-survey. Our analysis was driven by
previously unseen pre-maximum, spectroscopic heterogeneity. While the light
curve and post-maximum spectra of PS1-12bwh are virtually identical to those of
the well-studied type Iax supernova, SN 2005hk, the $-$2 day spectrum of
PS1-12bwh does not resemble SN 2005hk at a comparable epoch; instead, we found
it to match a spectrum of SN 2005hk taken over a week earlier ($-$12 day). We
are able to rule out the cause as being incorrect phasing, and argue that it is
not consistent with orientation effects predicted by existing explosion
simulations. To investigate the potential source of this difference, we
performed radiative transfer modelling of both supernovae. We found that the
pre-maximum spectrum of PS1-12bwh is well matched by a synthetic spectrum
generated from a model with a lower density in the high velocity ($\gtrsim$6000
km~s$^{-1}$) ejecta than SN 2005hk. The observed differences between SN 2005hk
and PS1-12bwh may therefore be attributed primarily to differences in the high
velocity ejecta alone, while comparable densities for the lower velocity ejecta
would explain the nearly identical post-maximum spectra. These two supernovae
further highlight the diversity within the SNe Iax class, as well as the
challenges in spectroscopically identifying and phasing these objects,
especially at early epochs. | astro-ph_HE |
Superorbital modulation at GeV energies in the gamma-ray binary LS I +61
303: We report the results from our analysis of 8 years of the data for the
gamma-ray binary LS I +61 303, obtained with the Large Area Telescope onboard
the Fermi Gamma-Ray Space Telescope. We find a significant dip around the
binary's periastron in the superorbital light curves, and by fitting the light
curves with a sinusoidal function, clear phase shifts are obtained. The
superorbital modulation seen in the binary has been long known and different
scenarios have been proposed. Based on our results, we suggest that the
circumstellar disk around the Be companion of this binary has an elliptical
shape and the major axis of the disk rotates at the superorbital period of 1667
days. As a result, the density of the ambient material around the compact star
of the binary changes along the binary orbit over the superorbital period,
causing the phase shifts in the modulation, and around periastron, the compact
star probably enters the disk, causing the appearance of the dip. Numerical
simulations may be conducted in order to study the detailed physical processes
and verify our suggestion. | astro-ph_HE |
The Einstein@Home Gamma-ray Pulsar Survey. I. Search Methods,
Sensitivity and Discovery of New Young Gamma-ray Pulsars: We report on the results of a recent blind search survey for gamma-ray
pulsars in Fermi Large Area Telescope (LAT) data being carried out on the
distributed volunteer computing system, Einstein@Home. The survey has searched
for pulsations in 118 unidentified pulsar-like sources, requiring about 10,000
years of CPU core time. In total, this survey has resulted in the discovery of
17 new gamma-ray pulsars, of which 13 are newly reported in this work, and an
accompanying paper. These pulsars are all young, isolated pulsars with
characteristic ages between 12 kyr and 2 Myr, and spin-down powers between
$10^{34}$ and $4\times10^{36}$ erg/s. Two of these are the slowest spinning
gamma-ray pulsars yet known. One pulsar experienced a very large glitch $\Delta
f/f \approx 3.5\times10^{-6}$ during the Fermi mission. In this, the first of
two associated papers, we describe the search scheme used in this survey, and
estimate the sensitivity of our search to pulsations in unidentified Fermi-LAT
sources. One such estimate results in an upper limit of 57% for the fraction of
pulsed emission from the gamma-ray source associated with the Cas A supernova
remnant, constraining the pulsed gamma-ray photon flux that can be produced by
the neutron star at its center. We also present the results of precise timing
analyses for each of the newly detected pulsars. | astro-ph_HE |
A tale of two mergers: constraints on kilonova detection in two short
GRBs at z$\sim$0.5: We present a detailed multi-wavelength analysis of two short Gamma-Ray Bursts
(sGRBs) detected by the Neil Gehrels Swift Observatory: GRB 160624A at
$z=0.483$ and GRB 200522A at $z=0.554$. These sGRBs demonstrate very different
properties in their observed emission and environment. GRB 160624A is
associated to a late-type galaxy with an old stellar population ($\approx$3
Gyr) and moderate on-going star formation ($\approx$1 $M_{\odot}$ yr$^{-1}$).
Hubble and Gemini limits on optical/nIR emission from GRB 160624A are among the
most stringent for sGRBs, leading to tight constraints on the allowed kilonova
properties. In particular, we rule out any kilonova brighter than AT2017gfo,
disfavoring large masses of wind ejecta ($\lesssim$0.03 $M_\odot$). In
contrast, observations of GRB 200522A uncovered a luminous
($L_\textrm{F125W}\approx 10^{42}$ erg s$^{-1}$ at 2.3~d) and red ($r-H\approx
1.3$ mag) counterpart. The red color can be explained either by bright kilonova
emission powered by the radioactive decay of a large amount of wind ejecta
(0.03 $M_\odot$ $\lesssim$ $M$ $\lesssim$ 0.1 $M_\odot$) or moderate
extinction, $E(B-V)\approx0.1-0.2$ mag, along the line of sight. The location
of this sGRB in the inner regions of a young ($\approx$0.1 Gyr) star-forming
($\approx$2-6 $M_{\odot}$ yr$^{-1}$) galaxy and the limited sampling of its
counterpart do not allow us to rule out dust effects as contributing, at least
in part, to the red color. | astro-ph_HE |
Variety of disk wind-driven explosions in massive rotating stars: We perform a set of two-dimensional, non-relativistic, hydrodynamics
simulations for supernova-like explosion associated with stellar core collapse
of rotating massive stars to a system of a black hole and a disk connected by
the transfer of matter and angular momentum. Our model of the central engine
also includes the contribution of the disk wind. In this work, we specifically
investigate the wind-driven explosion of rotating, large-mass progenitor stars
with the zero-age main-sequence mass of $M_\mathrm{ZAMS}=20\,M_\odot$ from
arXiv:2008.09132 . This study is carried out using the open-source hydrodynamic
code Athena++, for which we implement a method to calculate self-gravity for
axially symmetric density distributions. We, then, investigate the explosion
properties and the $^{56}$Ni production as a function of (varying) some
features of the wind injection. We find a large variety of explosion energy
with $E_\mathrm{expl}$ ranging from $\sim 0.049\times10^{51}$~erg to $\sim
34\times10^{51}$~erg and ejecta mass $M_\mathrm{ej}$ from 0.58 to 6 $M_\odot$,
which shows a bimodal distribution in high- and low-energy branches. We
demonstrate that the resulting outcome of a highly- or sub-energetic explosion
for a certain stellar structure is mainly determined by the competition between
the ram pressure of the injected matter and that of the infalling envelope. In
the nucleosynthesis analysis the $^{56}$Ni mass produced in our models goes
from $< 0.2~M_\odot$ in the sub-energetic explosions to $2.1~M_\odot$ in the
highly-energetic ones. These results are consistent with the observational data
of stripped-envelope and high-energy SNe such as broad-lined type Ic SNe.
However, we find a tighter correlation between the explosion energy and the
ejecta mass than that observationally measured. | astro-ph_HE |
The Spectral Sharpness Angle of Gamma-ray Bursts: We explain the results of Yu et al. (2015b) of the novel sharpness angle
measurement to a large number of spectra obtained from the Fermi gamma-ray
burst monitor. The sharpness angle is compared to the values obtained from
various representative emission models: blackbody, single-electron synchrotron,
synchrotron emission from a Maxwellian or power-law electron distribution. It
is found that more than 91% of the high temporally and spectrally resolved
spectra are inconsistent with any kind of optically thin synchrotron emission
model alone. It is also found that the limiting case, a single temperature
Maxwellian synchrotron function, can only contribute up to 58+23 -18% of the
peak flux. These results show that even the sharpest but non-realistic case,
the single-electron synchrotron function, cannot explain a large fraction of
the observed spectra. Since any combination of physically possible synchrotron
spectra added together will always further broaden the spectrum, emission
mechanisms other than optically thin synchrotron radiation are likely required
in a full explanation of the spectral peaks or breaks of the GRB prompt
emission phase. | astro-ph_HE |
r-Process Lanthanide Production and Heating Rates in Kilonovae: r-Process nucleosynthesis in material ejected during neutron star mergers may
lead to radioactively powered transients called kilonovae. The timescale and
peak luminosity of these transients depend on the composition of the ejecta,
which determines the local heating rate from nuclear decays and the opacity.
Kasen et al. (2013, ApJ, 774, 25) and Tanaka & Hotokezaka (2013, ApJ, 775, 113)
pointed out that lanthanides can drastically increase the opacity in these
outflows. We use the new general-purpose nuclear reaction network SkyNet to
carry out a parameter study of r-process nucleosynthesis for a range of initial
electron fractions $Y_e$, initial specific entropies $s$, and expansion
timescales $\tau$. We find that the ejecta is lanthanide-free for $Y_e \gtrsim
0.22 - 0.30$, depending on $s$ and $\tau$. The heating rate is insensitive to
$s$ and $\tau$, but certain, larger values of $Y_e$ lead to reduced heating
rates, due to individual nuclides dominating the heating. We calculate
approximate light curves with a simplified gray radiative transport scheme. The
light curves peak at about a day (week) in the lanthanide-free (-rich) cases.
The heating rate does not change much as the ejecta becomes lanthanide-free
with increasing $Y_e$, but the light curve peak becomes about an order of
magnitude brighter because it peaks much earlier when the heating rate is
larger. We also provide parametric fits for the heating rates between 0.1 and
$100\,\text{days}$, and we provide a simple fit in $Y_e$, $s$, and $\tau$ to
estimate whether the ejecta is lanthanide-rich or not. | astro-ph_HE |
Sub-arcsecond radio and optical observations of the likely counterpart
to the gamma-ray source 2FGL J2056.7+4939: We have searched and reviewed all multi- wavelength data available for the
region towards the gamma-ray source 2FGL J2056.7+4939 in order to con- strain
its possible counterpart at lower energies. As a result, only a point-like
optical/infrared source with flat-spectrum radio emission is found to be
consistent with all X-ray and gamma-ray error circles. Its struc- ture is
marginally resolved at radio wavelengths at the sub-arcsecond level. An
extragalactic scenario appears to be the most likely interpretation for this
object. | astro-ph_HE |
Investigating the true nature of three hard X-ray sources: Many of the new high energy sources discovered both by INTEGRAL/IBIS and
Swift/BAT have been characterised thanks to extensive, multi-band follow-up
campaigns, but there are still objects whose nature remains to be asserted. In
this paper we investigate the true nature of three high energy sources, IGR
J12134-6015, IGR J16058-7253 and Swift J2037.2+4151, employing multiwavelength
data from the NIR to the X-rays. Through Gaia and ESO-VLT measurements and
through Swift/XRT X-ray spectral analysis, we re-evaluate the classification
for IGR J12134-6015, arguing that the source is a Galactic object and in
particular a Cataclysmic Variable. We were able to confirm, thanks to NuSTAR
observations, that the hard X-ray emission detected by INTEGRAL/IBIS and
Swift/BAT from IGR J16058-7253 is coming from two Seyfert 2 galaxies which are
both counterparts for this source. Through optical and X-ray spectral analysis
of Swift J2037.2+4151 we find that this source is likely part of the rare and
peculiar class of Symbiotic X-ray binaries and displays flux and spectral
variability as well as interesting spectral features, such as a blending of
several emission lines around the iron line complex. | astro-ph_HE |
Exploring properties of high-density matter through remnants of
neutron-star mergers: Remnants of neutron-star mergers are essentially massive, hot, differentially
rotating neutron stars, which are initially strongly oscillating. They
represent a unique probe for high-density matter because the oscillations are
detectable via gravitational-wave measurements and are strongly dependent on
the equation of state. The impact of the equation of state is apparent in the
frequency of the dominant oscillation mode of the remnant. For a fixed total
binary mass a tight relation between the dominant postmerger frequency and the
radii of nonrotating neutron stars exists. Inferring observationally the
dominant postmerger frequency thus determines neutron star radii with high
accuracy of the order of a few hundred meters. By considering symmetric and
asymmetric binaries of the same chirp mass, we show that the knowledge of the
binary mass ratio is not critical for this kind of radius measurements. We
summarize different possibilities to deduce the maximum mass of nonrotating
neutron stars. We clarify the nature of the three most prominent features of
the postmerger gravitational-wave spectrum and argue that the merger remnant
can be considered to be a single, isolated, self-gravitating object that can be
described by concepts of asteroseismology. The understanding of the different
mechanisms shaping the gravitational-wave signal yields a physically motivated
analytic model of the gravitational-wave emission, which may form the basis for
template-based gravitational-wave data analysis. We explore the observational
consequences of a scenario of two families of compact stars including hadronic
and quark stars. We find that this scenario leaves a distinctive imprint on the
postmerger gravitational-wave signal. In particular, a strong discontinuity in
the dominant postmerger frequency as function of the total mass will be a
strong indication for two families of compact stars. (abridged) | astro-ph_HE |
Intrinsic disc emission and the Soft X-ray Excess in AGN: (Abridged) Narrow Line Seyfert 1 (NLS1) galaxies have low mass black holes
and mass accretion rates close to (or exceeding) Eddington, so a standard
blackbody accretion disc should peak in the EUV. However, the lack of true
absorption opacity in the disc means that the emission is better approximated
by a colour temperature corrected blackbody, and this colour temperature
correction is large enough ($\sim 2.4$) that the bare disc emission from a zero
spin black hole can extend into the soft X-ray bandpass. Part of the soft X-ray
excess seen in these objects must be intrinsic emission from the disc unless
the vertical structure is very different to that predicted.
However, the soft excess is much broader than predicted by a bare disc
spectrum, indicating some Compton upscattering by cool, optically thick
material. We associate this with the disc itself, so it must ultimately be
powered by mass accretion. We build an energetically self consistent model
assuming that the emission thermalises at large radii, but that at smaller
radii the gravitational energy is split between powering optically thick
Comptonised disc emission (forming the soft X-ray excess) and an optically thin
corona above the disc (forming the tail to higher energies). We show examples
of this model fit to the extreme NLS1 REJ1034+396, and to the much lower
Eddington fraction Broad Line Seyfert 1 PG1048+231. We use these to guide our
fits and interpretations of three template spectra made from co-adding multiple
sources to track out a sequence of AGN spectra as a function of $L/L_{Edd}$.
The new model is publically available within the {\sc xspec} spectral fitting
package. | astro-ph_HE |
Magnetic-distortion-induced ellipticity and gravitational wave radiation
of neutron stars: millisecond magnetars in short GRBs, Galactic pulsars, and
magnetars: Neutron stars may sustain a non-axisymmetric deformation due to magnetic
distortion and are potential sources of continuous gravitational waves (GWs)
for ground-based interferometric detectors. With decades of searches using
available GW detectors, no evidence of a GW signal from any pulsar has been
observed. Progressively stringent upper limits of ellipticity have been placed
on Galactic pulsars. In this work, we use the ellipticity inferred from the
putative millisecond magnetars in short gamma-ray bursts (SGRBs) to estimate
their detectability by current and future GW detectors. For $\sim 1$ ms
magnetars inferred from the SGRB data, the detection horizon is $\sim 30$ Mpc
and $\sim 600$ Mpc for advanced LIGO (aLIGO) and Einstein Telescope (ET),
respectively. Using the ellipticity of SGRB millisecond magnetars as
calibration, we estimate the ellipticity and gravitational wave strain of
Galactic pulsars and magnetars assuming that the ellipticity is
magnetic-distortion-induced. We find that the results are consistent with the
null detection results of Galactic pulsars and magnetars with the aLIGO O1. We
further predict that the GW signals from these pulsars/magnetars may not be
detectable by the currently designed aLIGO detector. The ET detector may be
able to detect some relatively low frequency signals ($<50$ Hz) from some of
these pulsars. Limited by its design sensitivity, the eLISA detector seems not
suitable for detecting the signals from Galactic pulsars and magnetars. | astro-ph_HE |
Implications on the origin of cosmic rays in light of 10 TV spectral
softenings: Precise measurements of the energy spectra of cosmic rays (CRs) show various
kinds of features deviating from single power-laws, which give very interesting
and important implications on their origin and propagation. Previous
measurements from a few balloon and space experiments indicate the existence of
spectral softenings around 10 TV for protons (and probably also for Helium
nuclei). Very recently, the DArk Matter Particle Explorer (DAMPE) measurement
about the proton spectrum clearly reveals such a softening with a high
significance. Here we study the implications of these new measurements, as well
as the groundbased indirect measurements, on the origin of CRs. We find that a
single component of CRs fails to fit the spectral softening and the air shower
experiment data simultaneously. In the framework of multiple components, we
discuss two possible scenarios, the multiple source population scenario and the
background plus nearby source scenario. Both scenarios give reasonable fits to
the wide-band data from TeV to 100 PeV energies. Considering the anisotropy
observations, the nearby source model is favored. | astro-ph_HE |
Maximum mass of a hybrid star having a mixed phase region in the light
of pulsar PSR J1614-2230: Recent observation of pulsar PSR J1614-2230 with mass about 2 solar masses
poses a severe constraint on the equations of state (EOS) of matter describing
stars under extreme conditions. Neutron stars (NS) can reach the mass limits
set by PSR J1614-2230. But stars having hyperons or quark stars (QS) having
boson condensates, with softer EOS can barely reach such limits and are ruled
out. QS with pure strange matter also cannot have such high mass unless the
effect of strong coupling constant or colour superconductivity are taken into
account. In this work I try to calculate the upper mass limit for a hybrid
stars (HS) having a quark-hadron mixed phase. The hadronic matter (having
hyperons) EOS is described by relativistic mean field theory and for the quark
phase I use the simple MIT bag model. I construct the intermediate mixed phase
using Glendenning construction. HS with a mixed phase cannot reach the mass
limit set by PSR J1614-2230 unless I assume a density dependent bag constant.
For such case the mixed phase region is small. The maximum mass of a mixed
hybrid star obtained with such mixed phase region is $2.01 M_{\odot}$. | astro-ph_HE |
Mass and radius relations of quarkyonic stars using an excluded volume
model: Inspired by the excluded volume model for isospin symmetric quarkyonic
matter, we construct an `excluded volume' model for a charge neutral quarkyonic
phase whose hadronic sector contains only neutrons. We refer to this model as
quarkyonic neutron matter. We compute the equation of state for this model and
solve the Tolman-Oppenhermer-Volkoff equations to obtain mass and radius
relations relevant for neutron stars. The most straightforward extension of the
model for symmetric quarkyonic matter to quarkyonic neutron matter does not
satisfy the mass radius constraints from neutron star measurements. However, we
show that by incorporating appropriate nuclear interactions in the excluded
volume model one can produce mass-radius relations that lie within the
constraints obtained from gravitational waves of binary neutron star mergers
and maximum mass measurements of neutron stars. | astro-ph_HE |
Stellar and AGN feedback in isolated early-type galaxies: the role in
regulating star formation and ISM properties: Understanding how galaxies maintain the inefficiency of star formation with
physically self-consistent models is a central problem for galaxy evolution.
Although numerous theoretical models have been proposed in recent decades, the
debate still exists. By means of high-resolution two-dimensional hydrodynamical
simulations, we study the three feedback effects (the stellar wind heating, SNe
feedback, and AGN feedback) in suppressing star formation activities on the
evolution of early-type galaxies with different stellar masses. AGN feedback
models are updated based on \citet{Yuan2018}. The gas sources comes exclusively
from the mass losses of dying low-mass stars for most of our models. We find
that SNe feedback can keep star formation at a significantly low level for low
mass elliptical galaxies for a cosmological evolution time. For the high mass
galaxies, AGN feedback can efficiently offset the radiative cooling and thus
regulate the star formation activities. Such a suppression of star formation is
extremely efficient in the inner region of the galaxies. AGB heating cannot
account for this suppression for low and high mass galaxies. The X-ray
temperature $T_{\rm X}$ and luminosity $L_{\rm X}$ of hot plasma can be in
agreement with the observed data with the inclusion of effective feedback
processes. These results thus suggest that we can use $T_{\rm X}$ and $L_{\rm
X}$ to probe the role of different feedback processes. The inclusion of
additional gas sources can make the mass scale between SNe and AGN feedback
dominating in suppressing star formation decrease to an observationally
inferred value of a few $10^{10}~M_{\odot}$. | astro-ph_HE |
The polarized Gamma-Ray Burst GRB 061122: We report on the polarization measure, obtained with IBIS on board INTEGRAL,
of the prompt emission of GRB 061122. Over an 8 s interval containing the
brightest part of the Gamma-Ray Burst we put a lower limit on its polarization
fraction of 60% at 68% c.l. and of 33% at 90% c.l. on the 250-800 keV energy
range.
We performed late time optical and near infra-red imaging observations of the
GRB field using the Telescopio Nazionale Galileo (TNG), and the
Canada-France-Hawaii Telescope (CFHT). Our multi-band (ugrizYJHK) photometry
allowed us to identify the host galaxy of GRB 061122 and to build its SED.
Using a photometric redshift code we fitted these data, and derived the basic
properties of the galaxy, including its type and redshift, that we could
constrain to the interval [0.57, 2.10] at a 90% c.l., with a best fit value of
z=1.33.
The polarization measurement in different energy bands, together with the
distance determination, allowed us to put the most stringent limit (xi <
3.4x10-16) to date to a possible Lorentz Invariance Violation based on the
vacuum birefringence effect, predicted by some quantum-gravity theories. | astro-ph_HE |
Assessing the Observability of Hypernovae and Pair-Instability
Supernovae in the Early Universe: The era of the universe's first (Population III) stars is essentially
unconstrained by observation. Ultra-luminous and massive stars from this time
altered the chemistry of the cosmos, provided the radiative scaffolding to
support the formation of the first protogalaxies, and facilitated the creation
and growth of now-supermassive black holes. Unfortunately, because these stars
lie literally at the edge of the observable universe, they will remain beyond
the reach of even the next generation of telescopes such as the James Webb
Space Telescope and the Thirty-Meter Telescope. In this paper, we provide a
primer to supernovae modeling and the first stars to make our discussion
accessible to those new to or outside our field. We review recent work of the
Los Alamos Supernova Light Curve Project and Brigham Young University to
explore the possibility of probing this era through observations of the
spectacular deaths of the first stars. We find that many such brilliant
supernova explosions will be observable as far back as $\sim 99$% of the
universe's current age, tracing primordial star formation rates and the
locations of their protogalaxies on the sky. The observation of Population III
supernovae will be among the most spectacular discoveries in observational
astronomy in the coming decade. | astro-ph_HE |
Binary Black Hole Accretion From a Circumbinary Disk: Gas Dynamics
Inside the Central Cavity: We present the results of 2D hydrodynamical simulations of circumbinary disk
accretion using the finite-volume code DISCO. This code solves the 2D viscous
Navier-Stokes equations on a high-resolution moving mesh which shears with the
fluid flow, greatly reducing advection errors in comparison with a fixed grid.
We perform a series of simulations for binary mass ratios in the range 0.026 <
q < 1.0, each lasting longer than a viscous time so that we reach a
quasi-steady accretion state. In each case, we find that gas is efficiently
stripped from the inner edge of the circumbinary disk and enters the cavity
along accretion streams, which feed persistent "mini-disks" surrounding each
black hole. We find that for q > 0.1, the binary excites eccentricity in the
inner region of the circumbinary disk, creating an overdense lump which gives
rise to enhanced periodicity in the accretion rate. The dependence of the
periodicity on mass ratio may provide a method for observationally inferring
mass ratios from measurements of the accretion rate. We also find that for all
mass ratios studied, the magnitude of the accretion onto the secondary is
sufficient to drive the binary toward larger mass ratio. This suggests a
mechanism for biasing mass ratio distributions toward equal mass. | astro-ph_HE |
Nucleosynthesis in magnetorotational supernovae: impact of the magnetic
field configuration: The production of heavy elements is one of the main by-products of the
explosive end of massive stars. A long sought goal is finding differentiated
patterns in the nucleosynthesis yields, which could permit identifying a number
of properties of the explosive core. Among them, the traces of the magnetic
field topology are particularly important for \emph{extreme} supernova
explosions, most likely hosted by magnetorotational effects. We investigate the
nucleosynthesis of five state-of-the-art magnetohydrodynamic models with fast
rotation that have been previously calculated in full 3D and that involve an
accurate neutrino transport (M1). One of the models does not contain any
magnetic field and synthesizes elements around the iron group, in agreement
with other CC-SNe models in literature. All other models host a strong magnetic
field of the same intensity, but with different topology. For the first time,
we investigate the nucleosynthesis of MR-SNe models with a quadrupolar magnetic
field and a 90 degree tilted dipole. We obtain a large variety of ejecta
compositions reaching from iron nuclei to nuclei up to the third r-process
peak. We assess the robustness of our results by considering the impact of
different nuclear physics uncertainties such as different nuclear masses,
$\beta^{-}$-decays and $\beta^{-}$-delayed neutron emission probabilities,
neutrino reactions, fission, and a feedback of nuclear energy on the
temperature. We find that the qualitative results do not change with different
nuclear physics input. The properties of the explosion dynamics and the
magnetic field configuration are the dominant factors determining the ejecta
composition. | astro-ph_HE |
ThunderKAT: The MeerKAT Large Survey Project for Image-Plane Radio
Transients: ThunderKAT is the image-plane transients programme for MeerKAT. The goal as
outlined in 2010, and still today, is to find, identify and understand
high-energy astrophysical processes via their radio emission (often in concert
with observations at other wavelengths). Through a comprehensive and
complementary programme of surveying and monitoring Galactic synchrotron
transients (across a range of compact accretors and a range of other explosive
phenomena) and exploring distinct populations of extragalactic synchrotron
transients (microquasars, supernovae and possibly yet unknown transient
phenomena) - both from direct surveys and commensal observations - we will
revolutionise our understanding of the dynamic and explosive transient radio
sky. As well as performing targeted programmes of our own, we have made
agreements with the other MeerKAT large survey projects (LSPs) that we will
also search their data for transients. This commensal use of the other surveys,
which remains one of our key programme goals in 2016, means that the combined
MeerKAT LSPs will produce by far the largest GHz-frequency radio transient
programme to date. | astro-ph_HE |
The Luminosity Distribution of Short Gamma-Ray Bursts under a Structured
Jet Scenario: Luminosity of GRB 170817A is much lower than that of other sGRBs. The
measurement of the superluminal movement of the radio afterglow emission
confirms the presence of the relativistic jet, and the emission features can be
well explained by the structured jet model. In this paper, we calculate the
luminosity distribution of sGRBs and its evolution with redshift based on the
structured (Gaussian) jet model, and find that the typical luminosity increase
with redshift, for nearby sGRBs (such as for luminosity distance less than 200
Mpc) the typical gamma-ray luminosity is just around 10^47-10^48 erg s-1, which
naturally explains the very low radiation luminosity of GRB 170817A. We derived
the detection probability of sGRBs by Fermi-GBM and found that the expected
detection rate of sGRBs is only about 1 yr-1 within the distance of several
hundred Mpc. We explored the effect of the power-law index {\alpha} of the
merger time distribution on the observed characteristics and found that it had
little effect on the observed luminosity and viewing-angle distributions.
However, it is very interesting that, for different values of {\alpha}, the
distributions of the number of observed sGRBs are quite different, so it is
possible to determine the value of {\alpha} through observed distributions of
the number of sGRBs. We used the Bayesian method to make a quantitative
analysis and found that the value of {\alpha} may be identified when the number
of observed sGRBs with known redshifts is more than 200. Finally, we compare
our results of gamma-ray luminosity distribution with sGRBs with known
redshifts, and found that our results are consistent with the observation,
which implies that our simulation results can reproduce the observed luminosity
distribution well. | astro-ph_HE |
Multi-waveform cross-correlation search method for intermediate-duration
gravitational waves from gamma-ray bursts: Gamma-ray Bursts (GRBs) are flashes of gamma-rays thought to originate from
rare forms of massive star collapse (long GRBs), or from mergers of compact
binaries (short GRBs) containing at least one neutron star (NS). The nature of
the post-explosion / post-merger remnant (NS versus black hole, BH) remains
highly debated. In ~50% of both long and short GRBs, the temporal evolution of
the X-ray afterglow that follows the flash of gamma-rays is observed to
"plateau" on timescales of 100-10000 s since explosion, possibly signaling the
presence of energy injection from a long-lived, highly magnetized NS
(magnetar). The Cross-Correlation Algorithm (CoCoA) proposed by [R. Coyne et.
al., (2016)] aims to optimize searches for intermediate-duration (100-10000s)
gravitational waves (GWs) from GRB remnants. In this work, we test CoCoA on
real data collected with ground-based GW detectors. We further develop the
detection statistics on which CoCoA is based to allow for multi-waveform
searches spanning a physically-motivated parameter space, so as to account for
uncertainties in the physical properties of GRB remnants. | astro-ph_HE |
Enhanced X-ray emission from Lyman Break Analogues and a Possible
$L_{\rm X}$--SFR--Metallicity Plane: The source of energetic photons that heated and reionized the early Universe
remains uncertain. Early galaxies had low metallicity and recent population
synthesis calculations suggest that the number and luminosity of high-mass
X-ray binaries are enhanced in star-forming galaxies with low metallicity,
offering a potentially important and previously overlooked source of heating
and reionization. Lyman break analogue (LBA) galaxies are local galaxies that
strongly resemble the high-redshift, star-forming Lyman Break Galaxies and have
been suggested as local analogues to these metal-deficient galaxies found in
the early Universe. We studied a sample of ten LBAs in order to measure the
relation between star formation rate and X-ray luminosity. We found that for
LBAs with metallicities in the range $12+\log_{10}({\rm O/H}) = 8.15-8.80$, the
$L_X-$SFR relation was $\log_{10} (L_X/{\rm SFR}\, {[\rm erg\ s^{-1}\
M_{\odot}^{-1}\ yr]}) = 39.85(\pm 0.10)$ in the $0.5-8$~keV band with a
dispersion of $\sigma = 0.25$~dex. This is an enhancement of nearly a factor of
$2$ in the $L_{0.5-8\text{keV}}$--SFR relation relative to results for nearby,
near-solar metallicity galaxies. The enhancement is significant at the 98.2\%
level ($2.4\sigma$). Our enhanced $L_X/{\rm SFR}$ relation is consistent with
the metallicity-dependent predicted value from population synthesis models. We
discuss the possibility of a $L_X$--SFR--Metallicity plane for star-forming
galaxies. These results are important to our understanding of reionization and
the formation of early galaxies. | astro-ph_HE |
Detectability of Sub-Solar Mass Neutron Stars Through a Template Bank
Search: We study the detectability of gravitational-wave signals from sub-solar mass
binary neutron star systems by the current generation of ground-based
gravitational-wave detectors. We find that finite size effects from large tidal
deformabilities of the neutron stars and lower merger frequencies can
significantly impact the sensitivity of the detectors to these sources. By
simulating a matched-filter based search using injected binary neutron star
signals with tidal deformabilities derived from physically motivated equations
of state, we calculate the reduction in sensitivity of the detectors. We
conclude that the loss in sensitive volume can be as high as $78.4 \%$ for an
equal mass binary system of chirp mass $0.17 \, \textrm{M}_{\odot}$, in a
search conducted using binary black hole template banks. We use this loss in
sensitive volume, in combination with the results from the search for sub-solar
mass binaries conducted on data collected by the LIGO-Virgo observatories
during their first three observing runs, to obtain a conservative upper limit
on the merger rate of sub-solar mass binary neutron stars. Since the discovery
of a low-mass neutron star would provide new insight into formation mechanisms
of neutron stars and further constrain the equation of state of dense nuclear
matter, our result merits a dedicated search for sub-solar mass binary neutron
star signals. | astro-ph_HE |
The long-term optical evolution of the black hole candidate MAXI
J1659-152: We present 5 years of optical and infrared data of the black hole candidate
MAXI J1659-152 covering its 2010 outburst, decay and quiescence. Combining
optical data taken during the outburst decay, we obtain an orbital period of
2.414 $\pm$ 0.005 h, in perfect agreement with the value previously measured
from X-ray dips. In addition, we detect a clear H$\alpha$ excess in MAXI
J1659-152 with data taken during the outburst decay. We also detect a single
hump modulation most likely produced by irradiation. Assuming that the maximum
occurs at orbital phase 0.5, we constrain the phase of the X-ray dips to be ~
0.65. We also detect the quiescent optical counterpart at r' = 24.20 $\pm$
0.08, I = 23.32 $\pm$ 0.02 and H = 20.7 $\pm$ 0.1. These magnitudes provide
colour indices implying an M2-M5 donor star assuming 60% contribution from a
disc component in the r'-band. | astro-ph_HE |
Glitches: the exact quantum signatures of pulsars metamorphosis: The observed recurrence of glitches in pulsars and neutron stars carry rich
information about the evolution of their internal structures. In this article I
show that the glitch-events observed in pulsars are exact quantum signatures
for their metamorphosis into dark super-baryons (SBs), whose interiors are made
of purely incompressible superconducting gluon-quark superfluids. Here the
quantum nuclear shell model is adopted to describe the permitted energy levels
of the SB, which are assumed to be identical to the discrete spinning rates
$\Omega_{SB},$ that SBs are allowed to rotate with. Accordingly, a glitch-event
corresponds to a prompt spin-down of the superconducting SB from one energy
level to the next, thereby expelling a certain number of vortices, which in
turn spins-up the ambient medium. The process is provoked mainly by the
negative torque of the ambient dissipative nuclear fluid and by a universal
scalar field $\phi$ at the background of a supranuclear dense matter. As
dictated by the Onsager-Feynman equation, the prompt spin-down must be
associated with increase of the dimensions of the embryonic SB to finally
convert the entire pulsar into SB-Objects on the scale of Gyrs. Based on our
calculations, a Vela-like pulsar should display billions of glitches during its
lifetime, before it metamorphoses entirely into a maximally compact SB-object
and disappears from our observational windows. The present model predicts the
mass of SBs and $\Delta \Omega/\Omega$ in young pulsars to be relatively lower
than their older counterparts. | astro-ph_HE |
Quantifying How Density Gradients and Front Curvature Affect Carbon
Detonation Strength During Type Ia Supernovae: Accurately reproducing the physics behind the detonations of Type Ia
supernovae and the resultant nucleosynthetic yields is important for
interpreting observations of spectra and remnants. The scales of the processes
involved span orders of magnitudes, making the problem computationally
impossible to ever fully resolve in full star simulations in the present and
near future. In the lower density regions of the star, the curvature of the
detonation front will slow the detonation, affecting the production of
intermediate mass elements. We find that shock strengthening due to the density
gradient present in the outer layers of the progenitor is essential for
understanding the nucleosynthesis there, with burning extending well below the
density at which a steady-state detonation is extinct. We show that a complete
reaction network is not sufficient to obtain physical detonations at high
densities and modest resolution due to numerical mixing at the unresolved
reaction front. At low densities, below 6$\times$10$^{5}$ g cm$^{-3}$, it is
possible to achieve high enough resolution to separate the shock and the
reaction region,and the abundance structure predicted by fully resolved
quasi-steady-state calculations is obtained. For our best current benchmark
yields, we utilize a method in which the unresolved portion of Lagrangian
histories are reconstructed based on fully resolved quasi-steady-state
detonation calculations. These computations demonstrate that under-resolved
simulations agree approximately, $\sim$10\% in post-shock values of
temperature, pressure, density, and abundances, with expected detonation
structures sufficiently far from the under-resolved region, but that there is
still room for some improvement in the treatment of subgrid reactions in the
hydrodynamics to before better than 1$\%$ can be achieved at all densities. | astro-ph_HE |
Using machine learning for transient classification in searches for
gravitational-wave counterparts: The large sky localization regions offered by the gravitational-wave
interferometers require efficient follow-up of the many counterpart candidates
identified by the wide field-of-view telescopes. Given the restricted telescope
time, the creation of prioritized lists of the many identified candidates
becomes mandatory. Towards this end, we use \text{\astrorapid}, a multi-band
photometric lightcurve classifier, to differentiate between kilonovae,
supernovae, and other possible transients. We demonstrate our method on the
photometric observations of real events. In addtion, the classification
performance is tested on simulated lightcurves, both ideally and realistically
sampled. We show that after only a few days of observations of an astronomical
object, it is possible to rule out candidates as supernovae and other known
transients. | astro-ph_HE |
Search for gravitational waves associated with gamma-ray bursts during
LIGO science run 6 and Virgo science runs 2 and 3: We present the results of a search for gravitational waves associated with
154 gamma-ray bursts (GRBs) that were detected by satellite-based gamma-ray
experiments in 2009-2010, during the sixth LIGO science run and the second and
third Virgo science runs. We perform two distinct searches: a modeled search
for coalescences of either two neutron stars or a neutron star and black hole;
and a search for generic, unmodeled gravitational-wave bursts. We find no
evidence for gravitational-wave counterparts, either with any individual GRB in
this sample or with the population as a whole. For all GRBs we place lower
bounds on the distance to the progenitor, under the optimistic assumption of a
gravitational-wave emission energy of 10^-2 M c^2 at 150 Hz, with a median
limit of 17 Mpc. For short hard GRBs we place exclusion distances on binary
neutron star and neutron star-black hole progenitors, using astrophysically
motivated priors on the source parameters, with median values of 16 Mpc and 28
Mpc respectively. These distance limits, while significantly larger than for a
search that is not aided by GRB satellite observations, are not large enough to
expect a coincidence with a GRB. However, projecting these exclusions to the
sensitivities of Advanced LIGO and Virgo, which should begin operation in 2015,
we find that the detection of gravitational waves associated with GRBs will
become quite possible. | astro-ph_HE |
A New Spin on an Old Black Hole: NuSTAR Spectroscopy of EXO 1846-031: The black hole candidate EXO 1846-031 underwent an outburst in 2019, after at
least 25 years in quiescence. We observed the system using \textit{NuSTAR} on
August 3rd, 2019. The 3--79 keV spectrum shows strong relativistic reflection
features. Our baseline model gives a nearly maximal black hole spin value of
$a=0.997_{-0.002}^{+0.001}$ ($1\sigma$ statistical errors). This high value
nominally excludes the possibility of the central engine harboring a neutron
star. Using several models, we test the robustness of our measurement to
assumptions about the density of the accretion disk, the nature of the corona,
the choice of disk continuum model, and addition of reflection from the outer
regions of the accretion disk. All tested models agree on a very high black
hole spin value and a high value for the inclination of the inner accretion
disk of $\theta\approx73^\circ$. We discuss the implications of this spin
measurement in the population of stellar mass black holes with known spins,
including LIGO events. | astro-ph_HE |
Jet Suppression by Accretion Disk Winds in the Microquasar GRS 1915+105: Stellar-mass black holes with relativistic jets, also known as microquasars,
mimic the behavior of quasars and active galactic nuclei. Because timescales
around stellar-mass black holes are orders of magnitude smaller than those
around more distant supermassive black holes, microquasars are ideal nearby
`laboratories' for studying the evolution of accretion disks and jet formation
in black-hole systems. Although studies of black holes have revealed a complex
array of accretion activity, the mechanisms that trigger and suppress jet
formation remain a mystery. Here we report the discovery of a broad emission
line during periods of intense hard X-ray flux in the microquasar GRS 1915+105,
and highly ionized narrow absorption lines during softer states. We argue that
the broad emission line arises when the inner accretion disk is illuminated by
hard X-rays, possibly from the jet. In contrast, during softer states, when the
jet is weak or absent, absorption lines appear as the powerful radiation field
around the black hole drives a hot wind off the accretion disk. Our analysis
strongly suggests that this wind carries enough mass away from the disk to halt
the flow of matter into the radio jet. | astro-ph_HE |
Supergiant Fast X-ray Transients with Swift: spectroscopic and temporal
properties: Supergiant fast X-ray transients (SFXTs) are a class of high-mass X-ray
binaries with possible counterparts in the high energy gamma rays. The Swift
SFXT Project has conducted a systematic investigation of the properties of
SFTXs on timescales ranging from minutes to years and in several intensity
states (from bright flares, to intermediate intensity states, and down to
almost quiescence). We also performed broad-band spectroscopy of outbursts, and
intensity-selected spectroscopy outside of outbursts. We demonstrated that
while the brightest phase of the outburst only lasts a few hours, further
activity is observed at lower fluxes for a remarkably longer time, up to weeks.
Furthermore, we assessed the fraction of the time these sources spend in each
phase, and their duty cycle of inactivity. We present the most recent results
from our investigation. The spectroscopic and, most importantly, timing
properties of SFXTs we have uncovered with Swift will serve as a guide in
search for the high energy emission from these enigmatic objects. | astro-ph_HE |
Improved early warning of compact binary mergers using higher modes of
gravitational radiation: A population study: A gravitational-wave (GW) early-warning of a compact-binary coalescence
event, with a sufficiently tight localisation skymap, would allow telescopes to
point in the direction of the potential electromagnetic counterpart before its
onset. This will enable astronomers to extract valuable information of the
complex astrophysical phenomena triggered around the time of the merger. Use of
higher-modes of gravitational radiation, in addition to the dominant mode
typically used in templated real-time searches, was recently shown to produce
significant improvements in early-warning times and skyarea localisations for a
range of asymmetric-mass binaries. In this work, we perform a large-scale study
to assess the benefits of this method for a population of compact binary merger
observations. In particular, we inject 100,000 such signals in Gaussian noise,
with component masses $m_1 \in \left[1, 60 \right] M_{\odot}$ and $m_2 \in
\left [1, 3 \right] M_{\odot}$. We consider three scenarios involving
ground-based detectors: the fifth (O5) observing run of the Advanced
LIGO-Virgo-KAGRA network, its projected Voyager upgrade, as well as a proposed
third generation (3G) network. We find that for fixed early warning times of
$20-60$ seconds, the inclusion of the higher modes can provide localisation
improvements of a factor of $\gtrsim 2$ for up to $\sim 60\%$ ($70 \%$) of the
neutron star-black hole systems in the O5 (Voyager) scenario. Considering only
those neutron star-black hole systems which can produce potential
electromagnetic counterparts, such improvements in the localisation can be
expected for $\sim 5-35\%$ $(20-50\%)$ binaries in O5 (Voyager), although the
localisation areas themselves depend on the distances. For the 3G scenario, a
significant fraction of the events have time gains of a minute to several
minutes, assuming fiducial target localisation areas of 100 to 1000 sq. deg. | astro-ph_HE |
DW Cancri in x-rays: We report on the $XMM$-Newton observation of DW Cnc, a candidate intermediate
polar candidate whose historical optical light curve shows the existence of
periods at $\simeq 38$, $\simeq 86$ and $\simeq 69$ minutes which were
interpreted as the white dwarf spin, the orbital and the spin-orbit beat
periodicities. By studying the $0.3-10$ keV light curves, we confirm the
existence of a period at $\simeq 38$ minutes and find in the OM light curve a
signature for a period at $75\pm 21$ minutes which is consistent with both the
orbital and spin-orbit beat. { These findings allow us to unveil without any
doubt, the nature of DW Cnc as an accreting intermediate polar. The EPIC and
RGS source spectra were analyzed and a best fit model, consisting of a
multi-temperature plasma, was found. The maximum temperature found when fitting
the data is $kT_{max}\simeq 31$ keV which can be interpreted as an upper limit
to the temperature of the shock. | astro-ph_HE |
Evidence for crust cooling in the transiently accreting 11-Hz X-ray
pulsar in the globular cluster Terzan 5: The temporal heating and subsequent cooling of the crusts of transiently
accreting neutron stars carries unique information about their structure and a
variety of nuclear reaction processes. We report on a new Chandra Director's
Discretionary Time observation of the globular cluster Terzan 5, aimed to
monitor the transiently accreting 11-Hz X-ray pulsar IGR J17480-2446 after the
cessation of its recent 10-week long accretion outburst. During the
observation, which was performed ~125 days into quiescence, the source displays
a thermal spectrum that fits to a neutron star atmosphere model with a
temperature for an observer at infinity of kT~92 eV. This is ~10% lower than
found ~75 days earlier, yet ~20% higher than the quiescent base level measured
prior to the recent outburst. This can be interpreted as cooling of the
accretion-heated neutron star crust, and implies that crust cooling is
observable after short accretion episodes. Comparison with neutron star thermal
evolution simulations indicates that substantial heat must be released at
shallow depth inside the neutron star, which is not accounted for in current
nuclear heating models. | astro-ph_HE |
Extreme Variability in a Long Duration Gamma-ray Burst Associated with a
Kilonova: The recent discovery of a kilonova from the long duration gamma-ray burst,
GRB 211211A, challenges classification schemes based on temporal information
alone. Gamma-ray properties of GRB 211211A reveal an extreme event, which
stands out among both short and long GRBs. We find very short variations (few
ms) in the lightcurve of GRB 211211A and estimate ~1000 for the Lorentz factor
of the outflow. We discuss the relevance of the short variations in identifying
similar long GRBs resulting from compact mergers. Our findings indicate that in
future gravitational wave follow-up campaigns, some long duration GRBs should
be treated as possible strong gravitational wave counterparts. | astro-ph_HE |
Correlations between radio and bolometric fluxes in GX 339-4 and
H1743-322: Compact radio jets are ubiquitous in stellar-mass black-hole binaries in
their hard spectral state. Empirical relations between the radio and
narrow-band X-ray fluxes have been used to understand the connection between
their accretion discs and jets. However, a narrow-band (e.g., 1--10 or 3--9
keV) X-ray flux can be a poor proxy for either the bolometric luminosity or the
mass accretion rate. Here, we study correlations between the radio and
unabsorbed broad-band X-ray fluxes, the latter providing good estimates of the
bolometric flux. We consider GX 339--4, the benchmark object for the main
branch of the correlation, and H1743--322, the first source found to be an
outlier of the correlation. The obtained power-law dependencies of the radio
flux on the bolometric flux have significantly different indices from those
found for the narrow X-ray bands. Also, the radio/bolometric flux correlations
for the rise of the outbursts are found to be significantly different from
those for the outburst decline. This points to a possible existence of a jet
hysteresis in the radio/X-ray source evolution, in addition to that seen in the
hardness/flux diagram of low-mass X-ray binaries. The correlation during the
rise of the outbursts is similar for both GX 339--4 and H1743--322. The
correlation for the decline of the outbursts for H1743--322 lies below that of
GX 339--4 at intermediate X-ray fluxes, whereas it approaches the standard
correlation at lower X-ray luminosities. We also compare these correlations to
those for the high-mass X-ray binaries Cyg X-1 and Cyg X-3. | astro-ph_HE |
Constraints on Blazar Jet Conditions During Gamma-Ray Flaring from
Radiative Transfer Modeling: As part of a program to investigate jet flow conditions during GeV gamma-ray
flares detected by Fermi, we are using UMRAO multi-frequency, centimeter-band
total flux density and linear polarization monitoring observations to constrain
radiative transfer models incorporating propagating shocks orientated at an
arbitrary angle to the flow direction. We describe the characteristics of the
model, illustrate how the data are used to constrain the models, and present
results for three program sources with diverse characteristics: PKS 0420-01, OJ
287, and 1156+295. The modeling of the observed spectral behavior yields
information on the sense, strength and orientation of the shocks producing the
radio-band flaring; on the energy distribution of the radiating particles; and
on the observer's viewing angle with respect to the jet independent of VLBI
data. We present evidence that, while a random component dominates the jet
magnetic field, a distinguishing feature of those radio events with an
associated gamma-ray flare is the presence of a weak but non-negligible ordered
magnetic field component along the jet axis. | astro-ph_HE |
X-Ray Luminous Binaries, Metallicity, and the Early Universe: High mass X-ray binaries (HMXBs) may have had a significant impact on the
heating of the intergalactic medium in the early universe. Study of HMXBs in
nearby, low metallicity galaxies that are local analogues to early galaxies can
help us understand early HMXBs. The total luminosity of HMXB populations is
dominated by sources at high luminosities. These sources exhibit X-ray spectra
that show curvature above 2 keV and the same is likely true of HMXB populations
at high redshifts. The spectral curvature changes the K-correction for X-rays
from HMXBs in a manner that weakens the constraints on X-ray emission of early
HMXBs obtained from the soft X-ray background. Applied to deep X-ray surveys of
star forming galaxies, the modified K-correction suggests a moderate increase
in the ratio of X-ray luminosity to star formation rate at intermediate
redshifts, z=3-5, and is consistent with a large enhancement at high redshifts,
z=6-7. | astro-ph_HE |
New unidentified H.E.S.S. Galactic sources: H.E.S.S. is one of the most sensitive instruments in the very high energy
(VHE; > 100 GeV) gamma-ray domain and has revealed many new sources along the
Galactic Plane. After the successful first VHE Galactic Plane Survey of 2004,
H.E.S.S. has continued and extended that survey in 2005-2008, discovering a
number of new sources, many of which are unidentified. Some of the unidentified
H.E.S.S. sources have several positional counterparts and hence several
different possible scenarios for the origin of the VHE gamma-ray emission;
their identification remains unclear. Others have so far no counterparts at any
other wavelength. Particularly, the lack of an X-ray counterpart puts serious
constraints on emission models. Several newly discovered and still unidentified
VHE sources are reported here. | astro-ph_HE |
Modeling pulsar time noise with long term power law decay modulated by
short term oscillations of the magnetic fields of neutron stars: We model the evolution of the magnetic fields of neutron stars as consisting
of a long term power-law decay modulated by short term small amplitude
oscillations. Our model predictions on the timing noise $\ddot\nu$ of neutron
stars agree well with the observed statistical properties and correlations of
normal radio pulsars. Fitting the model predictions to the observed data, we
found that their initial parameter implies their initial surface magnetic
dipole magnetic field strength ~ 5E14 G at ~0.4 year old and that the
oscillations have amplitude between E-8 to E-5 and period on the order of
years. For individual pulsars our model can effectively reduce their timing
residuals, thus offering the potential of more sensitive detections of
gravitational waves with pulsar timing arrays. Finally our model can also
re-produce their observed correlation and oscillations of the second derivative
of spin frequency, as well as the "slow glitch" phenomenon. | astro-ph_HE |
On the Tidal Capture of White Dwarfs by Intermediate-mass Black Holes in
Dense Stellar Environments: Intermediate-mass black holes (IMBHs) are the missing link between
stellar-mass and supermassive black holes, widely believed to reside in at
least some dense star clusters, but not yet observed directly. Tidal
disruptions of white dwarfs (WDs) are luminous only for black holes less
massive than $\sim 10^5\,M_{\odot}$, therefore providing a unique smoking gun
that could finally prove the existence of IMBHs beyond any reasonable doubt.
Here, we investigate the tidal captures of WDs by IMBHs in dense star clusters,
and estimate a typical rate of $\sim 1\,{\rm Myr}^{-1}$ for galactic nuclei and
$\sim 0.01\,{\rm Myr}^{-1}$ for globular clusters. Following the capture, the
WD inspirals onto the IMBH producing gravitational waves detectable out to
$\sim100$ Mpc by LISA for $\sim 10^4\,M_{\odot}$ IMBHs. The subsequent tidal
stripping/disruption of the WD can also release bright X-ray and gamma-ray
emission with luminosities of at least $\gtrsim10^{40}\,\rm{erg\,s^{-1}}$,
detectable by \textit{Chandra}, \textit{Swift}, and upcoming telescopes, such
as the \textit{Einstein Probe}. | astro-ph_HE |
Radio Evolution of Supernova Remnants Including Non-linear Particle
Acceleration: Insights from Hydrodynamic Simulations: We present a model for the radio evolution of supernova remnants (SNRs)
obtained by using three-dimensional (3D) hydrodynamic simulations, coupled with
nonlinear kinetic theory of cosmic ray (CR) acceleration in SNRs. We model the
radio evolution of SNRs on a global level, by performing simulations for wide
range of the relevant physical parameters, such as the ambient density, the
supernova (SN) explosion energy, the acceleration efficiency and the magnetic
field amplification (MFA) efficiency. We attribute the observed spread of radio
surface brightnesses for corresponding SNR diameters to the spread of these
parameters. In addition to our simulations of type Ia SNRs, we also considered
SNR radio evolution in denser, nonuniform circumstellar environment, modified
by the progenitor star wind. These simulations start with the mass of the
ejecta substantially higher than in the case of a type Ia SN and presumably
lower shock speed. The magnetic field is understandably seen as very important
for the radio evolution of SNRs. In terms of MFA, we include both resonant and
non-resonant modes in our large scale simulations, by implementing models
obtained from first-principles, particle-in-cell (PIC) simulations and
non-linear magnetohydrodynamical (MHD) simulations. We test the quality and
reliability of our models on a sample consisting of Galactic and extragalactic
SNRs. Our simulations give $\Sigma-D$ slopes between -4 and -6 for the full
Sedov regime. Recent empirical slopes obtained for the Galactic samples are
around -5, while for the extragalactic samples are around -4. | astro-ph_HE |
Collective excitations in neutron-star crusts: We explore the spectrum of low-energy collective excitations in the crust of
a neutron star, especially in the inner region where neutron-proton clusters
are immersed in a sea of superfluid neutrons. The speeds of the different modes
are calculated systematically from the nuclear energy density functional theory
using a Skyrme functional fitted to essentially all experimental atomic mass
data. | astro-ph_HE |
Meeting the Challenge from Bright and Fast Gamma-Ray Flares of 3C 279: Bright and fast gamma-ray flares with hard spectra have been recently
detected from the blazar 3C 279, with apparent GeV luminosities up to $10^{49}$
erg/s. The source is observed to flicker on timescales of minutes with no
comparable optical-UV counterparts. Such observations challenge current models
of high-energy emissions from 3C 279 and similar blazar sources that are
dominated by relativistic jets along our line of sight with bulk Lorentz
factors up to $ \Gamma \sim 20$ launched by supermassive black holes. We
compute and discuss a model based on a clumpy jet comprising strings of compact
plasmoids as indicated by radio observations. We follow the path of the
synchrotron radiations emitted in the optical - UV bands by relativistic
electrons accelerated around the plasmoids to isotropic Lorentz factors $\gamma
\sim 1000$. These primary emissions are partly reflected back by a leading
member in the string that acts as a moving mirror for the approaching
companions. Around the plasmoids, shrinking \emph{gap} transient overdensities
of seed photons build up. These are upscattered into the GeV range by inverse
Compton interactions with the relativistic electrons accelerated in situ. We
show that such a combined process produces bright gamma-ray flares with minor
optical to X-ray enhancements. Main features of our model include: bright
gamma-ray flares with risetimes as short as a few minutes, occurring at
distances of order $10^{18} $ cm from the central black hole; Compton dominance
at GeV energies by factors up to some $10^2$; little reabsorption from local
photon-photon interactions. | astro-ph_HE |
A report on the type II X-ray burst from SMC X-1: We study RXTE PCA data for the high mass X-ray binary source SMC X-1 between
2003-10 and 2003-12 when the source was in high states. The source is found to
be frequently bursting which can be seen as flares in lightcurves on an average
of one in every 800 s, with an average of 4-5 X-ray burst per hour of type II.
We note that typically burst was short lasting for few tens of seconds in
addition few long bursts of more than hundred seconds were also observed. The
flares apparently occupied 2.5$\%$ of the total observing time of 225.5 ks. We
note a total of 272 flares with mean FWHM of the flare $\backsim$21 s. The rms
variability and the aperiodic variability are independent of flares. As
observed the pulse profiles of the lightcurves do not change its shape implying
that there is no change in the geometry of accretion disk due to burst. The
hardness ratio and the rms variability of lightcurves show no correlation with
the flares. The flare-fraction shows a positive correlation with the
peak-to-peak ratio of the primary and secondary peaks of the pulse profile. The
observed hardening or the softening of the spectrum cannot be correlated with
the flaring rate but may be due to the interstellar absorption of X-rays as
evident from the change in the hydrogen column density ($n_{H}$). It is found
that the luminosity of the source increases with the flaring rate. Considering
the viscous timescale equal to mean recurrence time of flares we fixed the
viscosity parameter $\alpha$ $\backsim$ 0.16. | astro-ph_HE |
Optical Spectral Variations of a Large Sample of Fermi Blazars: We have investigated the optical spectral behavior of a large sample of Fermi
blazars (40 FSRQs and 13 BL Lacs), and found two new universal optical spectral
behaviors. In the low state the optical spectrum gradually becomes softer
(steeper) or harder (flatter) but more and more slowly when the brightness
increases, and then tends to stable in the high state, which are briefly named
the redder-stable-when-brighter (RSWB) and bluer-stable-when-brighter (BSWB)
behaviors, respectively. 34 FSRQs and 7 BL Lacs exhibit clear RSWB behavior,
and 2 FSRQs and 5 BL Lacs show distinct BSWB behavior, which mean that FSRQs
favor more RSWB than BSWB behavior, while BL Lacs have no clear preference
among both behaviors. We have put forward a unified nonlinear formula to
quantitatively characterize the optical spectral behaviors of FSRQs and BL
Lacs, which can fit both kinds of behaviors very well. We argue that the RSWB
and BSWB behaviors originate from the same mechanism, and they are the
universal optical spectral behaviors for blazars. The frequently observed
redder-when-brighter (RWB) and bluer-when-brighter (BWB) trends can be
considered to be the approximations of the behaviors of RSWB and BSWB,
respectively. The rarely observed stable-when-brighter (SWB) trend can also be
viewed as an approximation or a special case of the RSWB or BSWB behavior. We
have developed a model with two constant-spectral-index components which can
not only explain well both two kinds of optical spectral behaviors, but also
successfully interpret the differential behaviors between FSRQs and BL Lacs. | astro-ph_HE |
Radio surface fluctuations in radio relics: Recent observations have revealed detailed structures of radio relics in a
wide range of frequencies. In this work, we perform three-dimensional
magnetohydrodynamical simulations of merger shocks propagating through a
turbulent magnetized intracluster medium, and employ on-the-fly Lagrangian
particles to explore the physical processes originating radio substructures and
their appearances in high and low-frequency observations. We employ two
cosmic-ray (CR) electron acceleration models: the fresh injection of electrons
from the thermal pool and the re-acceleration of mildly relativistic electrons.
We use the relative surface brightness fluctuations, $\delta S_{\nu}$, to
define a "degree of patchiness''. We find that: 1) Patchiness is produced if
the shock's surface has a distribution of Mach numbers, rather than a single
Mach number; 2) Radio relics appear patchier if the Mach number distribution
consists of a large percentage of low Mach numbers ($\mathcal{M}\lesssim2.5$);
3) As the frequency increases, the patchiness also becomes larger.
Nevertheless, if radio relics are patchy at high frequencies (e.g., 18.6 GHz),
they are necessarily also at low frequencies (e.g., 150 MHz); 4) To produce
noticeable differences in the patchiness at low and high frequencies, the shock
front should have a Mach number spread of $\sigma_{\mathcal{M}}\gtrsim0.3$-0.4;
5) The amount of the patchiness depends on the Mach number distribution as well
as the CR acceleration model. We propose $\delta S_{\nu}$ as a potential tool
for extracting merger shock properties and information about particle
acceleration processes at shocks in radio observations. | astro-ph_HE |
Cosmic-ray electron transport in the galaxy M 51: Context. Indirect observations of the cosmic-ray electron (CRE) distribution
via synchrotron emission is crucial for deepening the understanding of the CRE
transport in the interstellar medium, and in investigating the role of galactic
outflows.
Aims. In this paper, we quantify the contribution of diffusion- and
advection-dominated transport of CREs in the galaxy M51 considering relevant
energy loss processes.
Methods. We used recent measurement from M 51 that allow for the derivation
of the diffusion coefficient, the star formation rate, and the magnetic field
strength. With this input, we solved the 3D transport equation numerically
including the spatial dependence as provided by the measurements, using the
open-source transport framework CRPropa (v3.1). We included 3D transport
(diffusion and advection), and the relevant loss processes.
Results. We find that the data can be described well with the parameters from
recent measurements. For the best fit, it is required that the wind velocity,
following from the observed star formation rate, must be decreased by a factor
of 5. We find a model in which the inner galaxy is dominated by advective
escape and the outer galaxy is composed by both diffusion and advection.
Conclusions. Three-dimensional modelling of cosmic-ray transport in the
face-on galaxy M51 allows for conclusions about the strength of the outflow of
such galaxies by quantifying the need for a wind in the description of the
cosmic-ray signatures. This opens up the possibility of investigating galactic
winds in face-on galaxies in general. | astro-ph_HE |
When and where did GW150914 form?: The recent LIGO detection of gravitational waves (GW150914), likely
originating from the merger of two $\sim 30 M_\odot$ black holes suggests
progenitor stars of low metallicity ($[Z/Z_\odot] \lesssim 0.3$), constraining
when and where the progenitor of GW150914 may have formed. We combine estimates
of galaxy properties (metallicity, star formation rate and merger rate) across
cosmic time to predict the low redshift black hole - black hole merger rate as
a function of present day host galaxy mass, $M_\mathrm{gal}$, and the formation
redshift of the progenitor system $z_\mathrm{form}$ for different progenitor
metallicities $Z_\mathrm{c}$. At $Z_\mathrm{c}=0.1 Z_\odot$, the signal is
dominated by binaries in massive galaxies with $z_\mathrm{form}\simeq 2$, with
a small contribution from binaries formed around $z_\mathrm{form}\simeq 0.5$ in
dwarf galaxies. For $Z_\mathrm{c}=0.01Z_\odot$, fast mergers are possible and
very recent star formation in dwarfs likely dominates. Additional gravitational
wave detections from merging massive black holes will provide constraints on
the mass-metallicity relation and massive star formation at high redshifts. | astro-ph_HE |
Thermal synchrotron radiation from RRMHD simulations of the double
tearing mode reconnection - Application to the Crab flares: We study the magneto-hydrodynamic tearing instability occurring in a double
current sheet configuration when a guide field is present. This is investigated
by means of resistive relativistic magneto-hydrodynamic (RRMHD) simulations.
Following the dynamics of the double tearing mode (DTM), we are able to compute
synthetic synchrotron spectra in the explosive reconnection phase. The pulsar
striped wind model represents a site where such current sheets are formed,
including a guide field. The variability of the Crab nebula/pulsar system, seen
as flares, can be therefore naturally explained by the DTM explosive phase in
the striped wind. Our results indicate that the Crab GeV flare can be explained
by the double tearing mode in the striped wind region if the magnetization
parameter $\sigma$ is around $10^5$. | astro-ph_HE |
Gamma Rays From Blazars: Blazars are high-energy engines providing us natural laboratories to study
particle acceleration, relativistic plasma processes, magnetic field dynamics,
black hole physics. Key informations are provided by observations at
high-energy (in particular by Fermi/LAT) and very-high energy (by Cherenkov
telescopes). I give a short account of the current status of the field, with
particular emphasis on the theoretical challenges connected to the observed
ultra-fast variability events and to the emission of flat spectrum radio
quasars in the very high energy band. | astro-ph_HE |
Effects of nuclear matter and composition in core-collapse supernovae
and long-term proto-neutron star cooling: We study the influence of hot and dense matter in core-collapse supernovae by
adopting up-to-date nuclear equation of state (EOS) based on the microscopic
nuclear many-body frameworks. We explore effects of EOS based on the Dirac
Brueckner Hartree-Fock theory through comparisons with those based on the
variational method. We also examine effects of the differences in the
composition of nuclei and nucleons by using the same EOS by the variational
method but employing two different treatments in computations of nuclear
abundances. We perform numerical simulations of core-collapse supernovae
adopting the three EOSs. We also perform numerical simulations of the long-term
evolution over 70 s of the proto-neutron star cooling. We show that impacts by
different modeling of composition are remarkable as in those by different
treatments of uniform matter in the gravitational collapse, bounce, and shock
propagation. The cooling of proto-neutron star and the resulting neutrino
emission are also affected by the compositional difference even if the same
treatment in computing uniform matter of EOS. | astro-ph_HE |
The 999th Swift gamma-ray burst: Some like it thermal: We present a multiwavelength study of GRB 151027A. This is the 999th GRB
detected by the Swift satellite and it has a densely sampled emission in the
X-ray and optical band and has been observed and detected in the radio up to
140 days after the prompt. The multiwavelength light curve from 500 s to 140
days can be modelled through a standard forward shock afterglow but requires an
additional component to reproduce the early X-ray and optical emission. We
present TNG and LBT optical observations performed 19.6, 33.9 and 92.3 days
after the trigger which show a bump with respect to a standard afterglow flux
decay and are possibly interpreted as due to the underlying SN and host galaxy
(of 0.4 uJy in the R band). Radio observations, performed with SRT, Medicina,
EVN and VLBA between day 4 and 140, suggest that the burst exploded in an
environment characterised by a density profile scaling with the distance from
the source (wind profile). A remarkable feature of the prompt emission is the
presence of a bright flare 100 s after the trigger, lasting 70 seconds in the
soft X-ray band, which was simultaneously detected from the optical band up to
the MeV energy range. By combining Swift-BAT/XRT and Fermi-GBM data, the
broadband (0.3-1000 keV) time resolved spectral analysis of the flare reveals
the coexistence of a non-thermal (power law) and thermal blackbody components.
The BB component contributes up to 35% of the luminosity in the 0.3-1000 keV
band. The gamma-ray emission observed in Swift-BAT and Fermi-GBM anticipates
and lasts less than the soft X-ray emission as observed by Swift-XRT, arguing
against a Comptonization origin. The BB component could either be produced by
an outflow becoming transparent or by the collision of a fast shell with a
slow, heavy and optically thick fireball ejected during the quiescent time
interval between the initial and later flares of the burst. | astro-ph_HE |
Time-resolved Spectral Study Of Fermi GRBs Having Single Pulses: We analyze gamma-ray bursts (GRBs) detected by \textit{Fermi}/Gamma-Ray Burst
Monitor (GBM) and having single pulse. We fit the light curves with a model
having exponential rise and decay parts. We perform a detailed time-resolved
spectroscopy using four models: Band, blackbody with a power-law (BBPL),
multicolour blackbody with a power-law (mBBPL) and two blackbodies with a
power-law (2BBPL). We find that models other than the BBPL give better
$\chi_{red}^2$ for the "hard-to-soft" (HTS) pulses, while for the "intensity
tracking" (IT) pulses, the BBPL model is statistically as good as the other
models. Interestingly, the energy at the peak of the spectrum resulting from
the BBPL model ($\sim3kT$), is always lower than that of the $\nu F_{\nu}$
spectrum of the Band function. The values of the low energy photon index
($\alpha$) of the Band function are often higher than the fundamental single
particle synchrotron limit, especially for the HTS pulses. Specifically we find
two extreme cases --- for GRB~110817A (HTS GRB) $\alpha$ is always higher,
while for GRB~100528A (IT GRB) $\alpha$ is always within the synchrotron
regime. The PL component of the BBPL model always starts with a delay compared
to the BB component, and it lingers at the later part of the prompt emission
phase. For three HTS GRBs, namely, GRB~081224, GRB~100707A and GRB~110721A this
behaviour is particularly significant and interestingly there are reported LAT
detections for them. Finally, we argue that various evidences hint that neither
BBPL nor Band model is acceptable, while 2BBPL and mBBPL are the most
acceptable models for the set of GRBs we have analyzed. | astro-ph_HE |
Deep Chandra observations of Pictor A: We report on deep Chandra observations of the nearby broad-line radio galaxy
Pictor A, which we combine with new Australia Telescope Compact Array (ATCA)
observations. The new X-ray data have a factor 4 more exposure than
observations previously presented and span a 15-year time baseline, allowing a
detailed study of the spatial, temporal and spectral properties of the AGN,
jet, hotspot and lobes. We present evidence for further time variation of the
jet, though the flare that we reported in previous work remains the most
significantly detected time-varying feature. We also confirm previous tentative
evidence for a faint counterjet. Based on the radio through X-ray spectrum of
the jet and its detailed spatial structure, and on the properties of the
counterjet, we argue that inverse-Compton models can be conclusively rejected,
and propose that the X-ray emission from the jet is synchrotron emission from
particles accelerated in the boundary layer of a relativistic jet. For the
first time, we find evidence that the bright western hotspot is also
time-varying in X-rays, and we connect this to the small-scale structure in the
hotspot seen in high-resolution radio observations. The new data allow us to
confirm that the spectrum of the lobes is in good agreement with the
predictions of an inverse-Compton model and we show that the data favour models
in which the filaments seen in the radio images are predominantly the result of
spatial variation of magnetic fields in the presence of a relatively uniform
electron distribution. | astro-ph_HE |
Observations of V404 Cygni during the 2015 outburst by the Nasu
telescope array at 1.4 GHz: Waseda University Nasu telescope array is a spatial fast Fourier transform
(FFT) interferometer consisting of eight linearly aligned antennas with 20 m
spherical dishes. This type of interferometer was developed to survey transient
radio sources with an angular resolution as high as that of a 160 m dish with a
field of view as wide as that of a 20 m dish. We have been performing
drift-scan-mode observations, in which the telescope scans the sky around a
selected declination as the earth rotates. The black hole X-ray binary V404
Cygni underwent a new outburst in 2015 June after a quiescent period of 26
years. Because of the interest in black hole binaries, a considerable amount of
data on this outburst at all wavelengths was accumulated. Using the above
telescope, we had been monitoring V404 Cygni daily from one month before the
X-ray outburst, and two radio flares at 1.4 GHz were detected on June 21.73 and
June 26.71. The flux density and time-scale of each flare were 313+/-30 mJy and
1.50+/-0.49 days, 364+/-30 mJy and 1.70+/-0.16 days, respectively. We have also
confirmed the extreme variation of radio spectra within a short period by
collecting other radio data observed with several radio telescopes. Such
spectral behaviors are considered to reflect the change in the opacity of the
ejected blobs associated with these extreme activities in radio and X-ray. Our
1.4 GHz radio data are expected to be helpful for studying the physics of the
accretion and ejection phenomena around black holes. | astro-ph_HE |
Merging White Dwarfs and Thermonuclear Supernovae: Thermonuclear supernovae result when interaction with a companion reignites
nuclear fusion in a carbon-oxygen white dwarf, causing a thermonuclear runaway,
a catastrophic gain in pressure, and the disintegration of the whole white
dwarf. It is usually thought that fusion is reignited in near-pycnonuclear
conditions when the white dwarf approaches the Chandrasekhar mass. I briefly
describe two long-standing problems faced by this scenario, and our suggestion
that these supernovae instead result from mergers of carbon-oxygen white
dwarfs, including those that produce sub-Chandrasekhar mass remnants. I then
turn to possible observational tests, in particular those that test the absence
or presence of electron captures during the burning. | astro-ph_HE |
Vortex Pinning in Neutron Stars, Slip-stick Dynamics, and the Origin of
Spin Glitches: We study pinning and unpinning of superfluid vortices in the inner crust of a
neutron star using 3-dimensional dynamical simulations. Strong pinning occurs
for certain lattice orientations of an idealized, body-centered cubic lattice,
and occurs generally in an amorphous or impure nuclear lattice. The pinning
force per unit length is $\sim 10^{16}$ dyn cm$^{-1}$ for a vortex-nucleus
interaction that is repulsive, and $\sim 10^{17}$ dyn cm$^{-1}$ for an
attractive interaction. The pinning force is strong enough to account for
observed spin jumps (glitches). Vortices forced through the lattice move with a
slip-stick character; for a range of superfluid velocities, the vortex can be
in either a cold, pinned state or a hot unpinned state, with strong excitation
of Kelvin waves on the vortex. This two-state nature of vortex motion sets the
stage for large-scale vortex movement that creates an observable spin glitch.
We argue that the vortex array is likely to become tangled as a result of
repeated unpinnings and repinnings. We conjecture that during a glitch, the
Kelvin-wave excitation spreads rapidly along the direction of the mean
superfluid vorticity and slower in the direction perpendicular to it, akin to
an anisotropic deflagration. | astro-ph_HE |
Searching for energy-resolved quasi-periodic oscillations in AGN: X-ray quasi-periodic oscillations (QPOs) in AGN allow us to probe and
understand the nature of accretion in highly curved space-time, yet the most
robust form of detection (i.e. repeat detections over multiple observations)
has been limited to a single source to-date, with only tentative claims of
single observation detections in several others. The association of those
established AGN QPOs with a specific spectral component has motivated us to
search the XMM-Newton archive and analyse the energy-resolved lightcurves of 38
bright AGN. We apply a conservative false alarm testing routine folding in the
uncertainty and covariance of the underlying broad-band noise. We also explore
the impact of red-noise leak and the assumption of various different forms
(power-law, broken power-law and lorentzians) for the underlying broad-band
noise. In this initial study, we report QPO candidates in 6 AGN (7 including
one tentative detection in MRK~766) from our sample of 38, which tend to be
found at characteristic energies and, in four cases, at the same frequency
across at least two observations, indicating they are highly unlikely to be
spurious in nature. | astro-ph_HE |
Searching for TeV emission from GRBs: the status of the H.E.S.S. GRB
programme: H.E.S.S. is an array of five Imaging Atmospheric Cherenkov Telescopes (IACTs)
located 1800 m above sea level in the Khomas Highland of Namibia and is
sensitive to very-high-energy (VHE) gamma rays between tens of GeV to tens of
TeV. The very-high background rejection capabilities of IACTs provide excellent
sensitivity of H.E.S.S. to GRBs. In this contribution the status of the
H.E.S.S. GRB programme, already started in 2003, is reviewed. A highlight is
the recent addition of the fifth telescope, which is the world's largest IACT.
Its 600 square metre mirror lowers the energy threshold to tens of GeV and
provides an effective area that is ten thousands of times larger than Fermi-LAT
at these energies. The higher performance drive system will reduce the response
time to a GRB alert, which will significantly enhance the chances of a H.E.S.S.
GRB detection. Recent results on selected GRBs will be shown. | astro-ph_HE |
EAS optical Cherenkov signatures of tau neutrinos for space and
suborbital detectors: Multi-messenger observations of transient astrophysical sources have the
potential to characterize the highest energy accelerators and the most extreme
environments in the Universe. Detection of neutrinos, in particular tau
neutrinos generated by neutrino oscillations in transit from their sources to
Earth, is possible for neutrino energies above 10 PeV using optical Cherenkov
detectors imaging upward-moving extensive air showers (EAS). These EAS are
produced from Earth interacting tau neutrinos leading to tau leptons that
subsequently decay in the atmosphere. We compare neutrino detection
sensitivities for generic short- and long-burst transient neutrino sources and
sensitivities to the diffuse neutrino flux for the second generation Extreme
Universe Space Observatory on a Super Pressure Balloon (EUSO-SPB2)
balloon-borne mission and the proposed space-based Probe of Extreme
Multi-Messenger Astrophysics (POEMMA) mission. | astro-ph_HE |
Kink-driven magnetic reconnection in relativistic jets: consequences for
X-ray polarimetry of BL Lacs: We investigate with relativistic MHD simulations the dissipation physics of
BL Lac jets, by studying the synchrotron polarization signatures of particles
accelerated by the kink instability in a magnetically-dominated plasma column.
The nonlinear stage of the kink instability generates current sheets, where
particles can be efficiently accelerated via magnetic reconnection. We identify
current sheets as regions where s = J d/B is above some predefined threshold
(where B is the field strength, J the current density and d the grid scale),
and assume that the particle injection efficiency scales as proportional to the
square of the current. X-ray emitting particles have short cooling times, so
they only probe the field geometry of their injection sites. In contrast,
particles emitting in the optical band, which we follow self-consistently as
they propagate away from their injection sites while cooling, sample a larger
volume, and so they may be expected to produce different polarimetric
signatures. We find that the degree of polarization is roughly the same between
X-ray and optical bands, because even the optical-emitting particles do not
travel far from the current sheet where they were injected, due to lack of
sufficient kink-generated turbulence. The polarization angle shows a different
temporal evolution between the two bands, due to the different regions probed
by X-ray and optical emitting particles. In view of the upcoming IXPE
satellite, our results can help constrain whether kink-induced reconnection (as
opposed to shocks) can be the source of multi-wavelength emission from BL Lacs. | astro-ph_HE |
The inner knot of the Crab nebula: We model the inner knot of the Crab Nebula as a synchrotron emission coming
from the non-spherical MHD termination shock of relativistic pulsar wind. The
post-shock flow is mildly relativistic; as a result the Doppler-beaming has a
strong impact on the shock appearance. The model can reproduce the knot
location, size, elongation, brightness distribution, luminosity and
polarization provided the effective magnetization of the section of the pulsar
wind producing the knot is low, $\sigma \leq 1$. In the striped wind model,
this implies that the striped zone is rather wide, with the magnetic
inclination angle of the Crab pulsar $\ge 45^\circ$; this agrees with the
previous model-dependent estimate based on the gamma-ray emission of the
pulsar. We conclude that the tiny knot is indeed a bright spot on the surface
of a quasi-stationary magnetic relativistic shock and that this shock is a site
of efficient particle acceleration. On the other hand, the deduced low
magnetization of the knot plasma implies that this is an unlikely site for the
Crab's gamma-ray flares, if they are related to the fast relativistic magnetic
reconnection events. | astro-ph_HE |
Revealing x-ray and gamma ray temporal and spectral similarities in the
GRB 190829A afterglow: Gamma-ray bursts (GRBs), which are bright flashes of gamma rays from
extragalactic sources followed by fading afterglow emission, are associated
with stellar core collapse events. We report the detection of very-high-energy
(VHE) gamma rays from the afterglow of GRB 190829A, between 4 and 56 hours
after the trigger, using the High Energy Stereoscopic System (H.E.S.S.). The
low luminosity and redshift of GRB 190829A reduce both internal and external
absorption, allowing determination of its intrinsic energy spectrum. Between
energies of 0.18 and 3.3 tera-electron volts, this spectrum is described by a
power law with photon index of 2.07 $\pm$ 0.09, similar to the x-ray spectrum.
The x-ray and VHE gamma-ray light curves also show similar decay profiles.
These similar characteristics in the x-ray and gamma-ray bands challenge GRB
afterglow emission scenarios. | astro-ph_HE |
Associated molecular and atomic clouds with X-ray shell of superbubble
30 Doradus C in the LMC: 30 Doradus C is a superbubble which emits the brightest nonthermal X- and TeV
gamma-rays in the Local Group. In order to explore detailed connection between
the high energy radiation and the interstellar medium, we have carried out new
CO and HI observations using the Atacama Large Millimeter$/$Submillimeter Array
(ALMA), Atacama Submillimeter Telescope Experiment, and the Australia Telescope
Compact Array with resolutions of up to 3 pc. The ALMA data of $^{12}$CO($J$ =
1-0) emission revealed 23 molecular clouds with the typical diameters of
$\sim$6-12 pc and masses of $\sim$600-10000 $M_{\odot}$. The comparison with
the X-rays of $XMM$-$Newton$ at $\sim$3 pc resolution shows that X-rays are
enhanced toward these clouds. The CO data were combined with the HI to estimate
the total interstellar protons. Comparison of the interstellar proton column
density and the X-rays revealed that the X-rays are enhanced with the total
proton. These are most likely due to the shock-cloud interaction modeled by the
magnetohydrodynamical simulations (Inoue et al. 2012, ApJ, 744, 71). Further,
we note a trend that the X-ray photon index varies with distance from the
center of the high-mass star cluster, suggesting that the cosmic-ray electrons
are accelerated by one or multiple supernovae in the cluster. Based on these
results we discuss the role of the interstellar medium in cosmic-ray particle
acceleration. | astro-ph_HE |
Effect of dark matter on the shadow of a distorted and deformed compact
object: This work investigates observational properties, namely the shadow and photon
ring structure, of emission profiles originated near compact objects. In
particular, we consider a distorted and deformed compact object characterized
by quadrupoles and surrounded by an optically thin and geometrically thin
accretion disk with different emission profiles modeled by the Johnson's
Standard-Unbound (SU) distribution in the reference frame of the emitter. Under
these assumptions, we produce the observed intensity profiles and shadow images
for a face-on observer. Our results indicate that, due to the fact that
modifications of the quadrupole parameters affect the radius of the innermost
stable circular orbit (ISCO) and the photon sphere (PS), the observed shadow
images and their properties are significantly influenced by the quadrupole
parameters and emission profiles. Furthermore, we analyze the impact of the
presence of a dark matter halo in the observational imprints considered and
verify that both the increase in the matter contained in the halo or decrease
in the length-scale of the halo lead to an increase in the size of the observed
shadow. Our results indicate potential degeneracies between the observational
features of distorded and deformed compact objects with those of spherically
symmetric blackholes, which could be assessed by a comparison with the current
and future generation of optical experiments in gravitational physics. | astro-ph_HE |
Progenitor, environment, and modelling of the interacting transient, AT
2016jbu (Gaia16cfr): We present the bolometric lightcurve, identification and analysis of the
progenitor candidate, and preliminary modelling of AT2016jbu (Gaia16cfr). We
find a progenitor consistent with a $\sim$22--25~$M_{\odot}$ yellow hypergiant
surrounded by a dusty circumstellar shell, in agreement with what has been
previously reported. We see evidence for significant photometric variability in
the progenitor, as well as strong H$\alpha$ emission consistent with
pre-existing circumstellar material. The age of the environment as well as the
resolved stellar population surrounding AT2016jbu, support a progenitor age of
$>$10 Myr, consistent with a progenitor mass of $\sim$22~$M_{\odot}$. A joint
analysis of the velocity evolution of AT2016jbu, and the photospheric radius
inferred from the bolometric lightcurve shows the transient is consistent with
two successive outbursts/explosions. The first outburst ejected material with
velocity $\sim$650$kms^{-1}$, while the second, more energetic event, ejected
material at $\sim$4500$kms^{-1}$. Whether the latter is the core-collapse of
the progenitor remains uncertain. We place a limit on the ejected $^{56}$Ni
mass of $<$0.016$M_{\odot}$. Using the BPASS code, we explore a wide range of
possible progenitor systems, and find that the majority of these are in
binaries, some of which are undergoing mass transfer or common envelope
evolution immediately prior to explosion. Finally, we use the SNEC code to
demonstrate that the low-energy explosion within some of these binary systems,
together with sufficient CSM, can reproduce the overall morphology of the
lightcurve of AT2016jbu. | astro-ph_HE |
Supernova Search with the AMANDA / IceCube Detectors: Since 1997 the neutrino telescope AMANDA at the geographic South Pole has
been monitoring our Galaxy for neutrino bursts from supernovae. Triggers were
introduced in 2004 to submit burst candidates to the Supernova Early Warning
System SNEWS. From 2007 the burst search was extended to the much larger
IceCube telescope, which now supersedes AMANDA. By exploiting the low
photomultiplier noise in the antarctic ice (on average 280Hz for IceCube),
neutrino bursts from nearby supernovae can be identified by the induced
collective rise in the pulse rates. Although only a counting experiment,
IceCube will provide the world's most precise measurement of the time profile
of a neutrino burst near the galactic center. The sensitivity to neutrino
properties such as the theta13 mixing angle and the neutrino hierarchy are
discussed as well as the possibility to detect the deleptonization burst. | astro-ph_HE |
The glitches and rotational history of the highly energetic young pulsar
PSR J0537$-$6910: We present a timing and glitch analysis of the young X-ray pulsar PSR
J0537$-$6910, located within the Large Magellanic Cloud, using 13 years of data
from the now decommissioned Rossi X-ray Timing Explorer. Rotating with a spin
period of 16 ms, PSR J0537$-$6910 is the fastest spinning and most energetic
young pulsar known. It also displays the highest glitch activity of any known
pulsar. We have found 42 glitches over the data span, corresponding to a glitch
rate of 3.2 yr$^{-1}$, with an overall glitch activity rate of $8.8\times
10^{-7}\,$yr$^{-1}$. The high glitch frequency has allowed us to study the
glitch behavior in ways that are inaccessible in other pulsars. We observe a
strong linear correlation between spin frequency glitch magnitude and wait time
to the following glitch. We also find that the post-glitch spin-down recovery
is well described by a single two-component model fit to all glitches for which
we have adequate input data. This consists of an exponential amplitude $A =
(7.6 \pm 1.0)\times 10^{-14}\,$s$^{-2}$ and decay timescale $\tau =
27_{-6}^{+7}\,$d, and linear slope $m = (4.1\pm 0.4)\times
10^{-16}\,$s$^{-2}\,$d$^{-1}$. The latter slope corresponds to a second
frequency derivative $\ddot{\nu} = (4.7\pm 0.5) \times 10^{-22}\,$s$^{-3}$,
from which we find an implied braking index $n=7.4 \pm 0.8$. We also present a
maximum-likelihood technique for searching for periods in event-time data,
which we used to both confirm previously published values and determine
rotation frequencies in later observations. We discuss the implied constraints
on glitch models from the observed behavior of this system, which we argue
cannot be fully explained in the context of existing theories. | astro-ph_HE |
X-ray emission of contact binary variables within 1 kpc: By assembling the largest sample to date of X-ray emitting EW-type binaries
(EWXs), we carried out correlation analyses for the X-ray luminosity
log$L_{\textrm{X}}$, and X-ray activity level
log($L_{\textrm{X}}$/$L_{\textrm{bol}}$) versus the orbital period $P$ and
effective temperature $T_{\rm eff}$. We find strong $P$-log$L_{\textrm{X}}$ and
$P$-log($L_{\textrm{X}}$/$L_{\textrm{bol}}$) correlations for EWXs with $P$ <
0.44 days and we provide the linear parametrizations for these relations, on
the basis of which the orbital period can be treated as a good predictor for
log$L_{\textrm{X}}$ and log($L_{\textrm{X}}$/$L_{\textrm{bol}}$). The
aforementioned binary stellar parameters are all correlated with
log$L_{\textrm{X}}$, while only $T_{\rm eff}$ exhibits a strong correlation
with log($L_{\textrm{X}}$/$L_{\textrm{bol}}$). Then, EWXs with higher
temperature show lower X-ray activity level, which could indicate the thinning
of the convective area related to the magnetic dynamo mechanism. The total
X-ray luminosity of an EWX is essentially consistent with that of an X-ray
saturated main sequence star with the same mass as its primary, which may imply
that the primary star dominates the X-ray emission. The monotonically
decreasing $P$-log($L_{\textrm{X}}$/$L_{\textrm{bol}}$) relation and the short
orbital periods indicate that EWXs could all be in the X-ray saturated state,
and they may inherit the changing trend of the saturated X-ray luminosities
along with the mass shown by single stars. For EWXs, the orbital period, mass,
and effective temperature increase in concordance. We demonstrate that the
period $P=0.44$ days corresponds to the primary mass of $\sim1.1 \rm M_\odot$,
beyond which the saturated X-ray luminosity of single stars will not continue
to increase with mass. This explains the break in the positive
$P$-log$L_{\textrm{X}}$ relation for EWXs with $P>0.44$ days. | astro-ph_HE |
High Resolution X-ray Spectroscopy of the Seyfert 1, Mrk 1040. Revealing
the Failed Nuclear Wind with Chandra: High resolution X-ray spectroscopy of the warm absorber in the nearby X-ray
bright Seyfert 1 galaxy, Mrk 1040 is presented. The observations were carried
out in the 2013-2014 timeframe using the Chandra High Energy Transmission
Grating with a total exposure of 200 ks. A multitude of absorption lines from
Ne, Mg and Si are detected from a wide variety of ionization states. In
particular, the detection of inner K-shell absorption lines from Ne, Mg and Si,
from charge states ranging from F-like to Li-like ions, suggests the presence
of a substantial amount of low ionization absorbing gas, illuminated by a steep
soft X-ray continuum. The observations reveal at least 3 warm absorbing
components ranging in ionization parameter from $\log\xi = 0-2$ and with column
densities of $N_{\rm H} =1.5-4.0 \times 10^{21}$cm$^{-2}$. The velocity
profiles imply that the outflow velocities of the absorbing gas are low and
within $\pm100$ km s$^{-1}$ of the systemic velocity of Mrk 1040, which
suggests any outflowing gas may have stalled in this AGN on large enough
scales. The warm absorber is likely located far from the black hole, within 300
pc of the nucleus and is spatially coincident with emission from an extended
Narrow Line Region as seen in the HST images. The iron K band spectrum reveals
only narrow emission lines, with Fe K$\alpha$ at 6.4 keV consistent with
originating from reflection off Compton thick pc-scale reprocessing gas. | astro-ph_HE |
Extremal energy shifts of radiation from a ring near a rotating black
hole: Radiation from a narrow circular ring shows a characteristic double-horn
profile dominated by photons having energy around the maximum or minimum of the
allowed range, i.e. near the extremal values of the energy shift. The energy
span of a spectral line is a function of the ring radius, black hole spin, and
observer's view angle. We describe a useful approach to calculate the extremal
energy shifts in the regime of strong gravity. Then we consider an accretion
disk consisting of a number of separate nested annuli in the equatorial plane
of Kerr black hole, above the innermost stable circular orbit (ISCO). We
suggest that the radial structure of the disk emission could be reconstructed
using the extremal energy shifts of the individual rings deduced from the broad
wings of a relativistic spectral line. | astro-ph_HE |
Gamma-Ray and X-Ray Observations of the Periodic-Repeater FRB 180916
During Active Phases: FRB 180916 is a most intriguing source at 150 Mpc distance capable of
producing repeating fast radio bursts with a periodic 16.35 day temporal
pattern. We report on the X-ray and $\gamma$-ray observations of FRB 180916
obtained by AGILE and Swift. We focused on the recurrent 5-day time intervals
of active radio bursting and present results obtained on Feb. 3 - 8; Feb. 25;
Mar. 5 - 10; Mar. 22 - 28, 2020 during a multiwavelength campaign involving
high-energy and radio observations. We also searched for temporal coincidences
at millisecond timescales between all known radio bursts of FRB 180916 and
X-ray and MeV events detectable by AGILE. We do not detect any simultaneous
event or any extended X-ray and $\gamma$-ray emission on timescales of
hours/days/weeks. Our cumulative X-ray (0.3-10 keV) flux upper limit of $5
\times\,10^{-14} \rm \, erg \, cm^{-2} s^{-1}$ (obtained during 5-day active
intervals) translates into an isotropic luminosity upper limit of $L_{X,UL}
\sim 1.5 \times\, 10^{41} \rm erg \, s^{-1}$. Observations above 100 MeV over a
many-year timescale provide an average luminosity upper limit one order of
magnitude larger. These results provide the so-far most stringent limits on
high-energy emission from FRB 180916 and constrain the dissipation of magnetic
energy from a magnetar-like source of radius $R_m$, internal magnetic field
$B_m$ and dissipation timescale $\tau_d$ to satisfy the relation $R_{m,6}^3
B_{m,16}^2 \tau_{d,8}^{-1} \lesssim 1$, where $R_{m,6}$ is $R_m$ in units of
$10^6$ cm, $B_{m,16}$ is $B_m$ in units of $10^{16}$ G, and $\tau_{d,8}$ in
units of $10^8$ s. | astro-ph_HE |
Equations of state for supernovae and compact stars: We review various theoretical approaches for the equation of state (EoS) of
dense matter, relevant for the description of core-collapse supernovae, compact
stars and compact star mergers. The emphasis is put on models that are
applicable to all of these scenarios. Such EoS models have to cover large
ranges in baryon number density, temperature and isospin asymmetry. The
characteristics of matter change dramatically within these ranges, from a
mixture of nucleons, nuclei, and electrons to uniform, strongly interacting
matter containing nucleons, and possibly other particles such as hyperons or
quarks. As the development of an EoS requires joint efforts from many
directions we consider different theoretical approaches and discuss relevant
experimental and observational constraints which provide insights for future
research. Finally, results from applications of the discussed EoS models are
summarized. | astro-ph_HE |
Neural Simulation-Based Inference of the Neutron Star Equation of State
directly from Telescope Spectra: Neutron stars provide a unique opportunity to study strongly interacting
matter under extreme density conditions. The intricacies of matter inside
neutron stars and their equation of state are not directly visible, but
determine bulk properties, such as mass and radius, which affect the star's
thermal X-ray emissions. However, the telescope spectra of these emissions are
also affected by the stellar distance, hydrogen column, and effective surface
temperature, which are not always well-constrained. Uncertainties on these
nuisance parameters must be accounted for when making a robust estimation of
the equation of state. In this study, we develop a novel methodology that, for
the first time, can infer the full posterior distribution of both the equation
of state and nuisance parameters directly from telescope observations. This
method relies on the use of neural likelihood estimation, in which normalizing
flows use samples of simulated telescope data to learn the likelihood of the
neutron star spectra as a function of these parameters, coupled with
Hamiltonian Monte Carlo methods to efficiently sample from the corresponding
posterior distribution. Our approach surpasses the accuracy of previous
methods, improves the interpretability of the results by providing access to
the full posterior distribution, and naturally scales to a growing number of
neutron star observations expected in the coming years. | astro-ph_HE |
Cosmic-ray spectral anomaly at GeV-TeV energies as due to
re-acceleration by weak shocks in the Galaxy: Recent cosmic-ray measurements by the ATIC, CREAM and PAMELA experiments have
found an apparent hardening of the energy spectrum at TeV energies. Although
the origin of the hardening is not clearly understood, possible explanations
include hardening in the cosmic-ray source spectrum, changes in the cosmic-ray
propagation properties in the Galaxy and the effect of nearby sources. In this
contribution, we propose that the spectral anomaly might be an effect of
re-acceleration of cosmic rays by weak shocks in the Galaxy. After acceleration
by strong supernova remnant shock waves, cosmic rays undergo diffusive
propagation through the Galaxy. During the propagation, cosmic rays may again
encounter expanding supernova remnant shock waves, and get re-accelerated. As
the probability of encountering old supernova remnants is expected to be larger
than the young ones due to their bigger size, re-acceleration is expected to be
produced mainly by weaker shocks. Since weaker shocks generate a softer
particle spectrum, the resulting re-accelerated component will have a spectrum
steeper than the initial cosmic-ray source spectrum produced by strong shocks.
For a reasonable set of model parameters, it is shown that such re-accelerated
component can dominate the GeV energy region while the non-reaccelerated
component dominates at higher energies, explaining the observed GeV-TeV
spectral anomaly. | astro-ph_HE |
A radio, optical, UV and X-ray view of the enigmatic changing look
Active Galactic Nucleus 1ES~1927+654 from its pre- to post-flare states: The nearby type-II AGN 1ES1927+654 went through a violent changing-look (CL)
event beginning December 2017 during which the optical and UV fluxes increased
by four magnitudes over a few months, and broad emission lines newly appeared
in the optical/UV. By July 2018 the X-ray coronal emission had completely
vanished, only to reappear a few months later. In this work we report the
evolution of the radio, optical, UV and X-rays from the pre-flare state through
mid-2021 with new and archival data from the Very Long Baseline Array (VLBA),
the European VLBI Network, the Very Large Array (VLA), the Telescopio Nazionale
Galileo (TNG), Gran Telescopio Canarias (GTC), The Neil Gehrels Swift
observatory and XMM-Newton. The main results from our work are: (1) The source
has returned to its pre-CL state in optical, UV, and X-ray; the disk-corona
relation has been re-established as has been in the pre-CL state, with an
$\alpha_{\rm OX}\sim 1.02$. The optical spectra are dominated by narrow
emission lines. (2) The UV light curve follows a shallower slope of $\propto
t^{-0.91\pm 0.04}$ compared to that predicted by a tidal disruption event. We
conjecture that a magnetic flux inversion event is the possible cause for this
enigmatic event. (3) The compact radio emission which we tracked in the pre-CL
(2014), during CL (2018) and post-CL(2021) at spatial scales $<1$ pc was at its
lowest level during the changing look event in 2018, nearly contemporaneous
with a low $2-10$ keV emission. The radio to X-ray ratio of the compact source
$L_{\rm Radio}/L_{\rm X-ray}\sim 10^{-5.5}$, follows the Gudel-Benz relation,
typically found in coronally active stars, and several AGN. (4) We do not
detect any presence of nascent jets at the spatial scales of $\sim 5-10$ pc. | astro-ph_HE |
Fermi Large Area Telescope observations of Local Group galaxies:
Detection of M31 and search for M33: Cosmic rays (CRs) can be studied through the galaxy-wide gamma-ray emission
that they generate when propagating in the interstellar medium. The comparison
of the diffuse signals from different systems may inform us about the key
parameters in CR acceleration and transport. We aim to determine and compare
the properties of the CR-induced gamma-ray emission of several Local Group
galaxies. We use 2 years of nearly continuous sky-survey observations obtained
with the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope to
search for gamma-ray emission from M31 and M33. We compare the results with
those for the Large Magellanic Cloud, the Small Magellanic Cloud, the Milky
Way, and the starburst galaxies M82 and NGC253. We detect a gamma-ray signal at
5sigma significance in the energy range 200 MeV-20 GeV that is consistent with
originating from M31. The integral photon flux above 100MeV amounts to 9.1 +/-
1.9 (stat) +/- 1.0 (sys) x10e-9 ph/cm2/s. We find no evidence for emission from
M33 and derive an upper limit on the photon flux >100MeV of 5.1 x10e-9 ph/cm2/s
(2sigma). Comparing these results to the properties of other Local Group
galaxies, we find indications for a correlation between star formation rate and
gamma-ray luminosity that also holds for the starburst galaxies. The gamma-ray
luminosity of M31 is about half that of the Milky Way, which implies that the
ratio between the average CR densities in M31 and the Milky Way amounts to 0.35
+/- 0.25. The observed correlation between gamma-ray luminosity and star
formation rate suggests that the flux of M33 is not far below the current upper
limit from the LAT observations. | astro-ph_HE |
Variations of the physical parameters of the blazar Mrk 421 based on the
analysis of the spectral energy distributions: We report on the variations of the physical parameters of the jet observed in
the blazar Mrk 421, and discuss the origin of X-ray flares in the jet, based on
the analysis of the several spectral energy distributions (SEDs). The SEDs are
modeled using the one-zone synchrotron self-Compton (SSC) model and its
parameters determined using a Markov chain Monte Carlo method. The lack of data
at TeV energies means many of the parameters cannot be uniquely determined and
are correlated. These are studied in detail. We found that the optimal solution
can be uniquely determined only when we apply a constraint to one of four
parameters: the magnetic field (B), Doppler factor, size of the emitting
region, and normalization factor of the electron energy distribution. We used
31 sets of SED from 2009 to 2014 with optical-UV data observed with UVOT/Swift
and the Kanata telescope, X-ray data with XRT/Swift, and gamma-ray data with
the Fermi Large Area Telescope (LAT). The result of our SED analysis suggests
that, in the X-ray faint state, the emission occurs in a relatively small area
(~ 10^16 cm) with relatively strong magnetic field (B~10^-1 G). The X-ray
bright state shows a tendency opposite to that of the faint state, that is, a
large emitting area (~10^18 cm), probably in the downstream of the jet and weak
magnetic field (B~10^-3 G). The high X-ray flux was due to an increase in the
maximum energy of electrons. On the other hand, the presence of two kinds of
emitting areas implies that the one-zone model is unsuitable to reproduce, at
least a part of the observed SEDs. | astro-ph_HE |
Accretion-powered pulsations in an apparently quiescent neutron star
binary: Accreting millisecond X-ray pulsars are an important subset of low-mass X-ray
binaries in which coherent X-ray pulsations can be observed during occasional,
bright outbursts (X-ray luminosity $L_X\sim 10^{36}$ erg s$^{-1}$). These
pulsations show that matter is being channeled onto the neutron star's magnetic
poles. However, such sources spend most of their time in a low-luminosity,
quiescent state ($L_X\lesssim 10^{34}$ erg s$^{-1}$), where the nature of the
accretion flow onto the neutron star (if any) is not well understood. Here we
report that the millisecond pulsar/low-mass X-ray binary transition object PSR
J1023+0038 intermittently shows coherent X-ray pulsations at luminosities
nearly 100 times fainter than observed in any other accreting millisecond X-ray
pulsar. We conclude that in spite of its low luminosity PSR J1023+0038
experiences episodes of channeled accretion, a discovery that challenges
existing models for accretion onto magnetized neutron stars. | astro-ph_HE |
Indications of Negative Evolution for the Sources of the Highest Energy
Cosmic Rays: Using recent measurements of the spectrum and chemical composition of the
highest energy cosmic rays, we consider the sources of these particles. We find
that the data strongly prefers models in which the sources of the ultra-high
energy cosmic rays inject predominantly intermediate mass nuclei, with
comparatively few protons or heavy nuclei, such as iron or silicon. If the
number density of sources per comoving volume does not evolve with redshift,
the injected spectrum must be very hard ($\alpha\simeq 1$) in order to fit the
spectrum observed at Earth. Such a hard spectral index would be surprising and
difficult to accommodate theoretically. In contrast, much softer spectral
indices, consistent with the predictions of Fermi acceleration ($\alpha\simeq
2$), are favored in models with negative source evolution. With this
theoretical bias, these observations thus favor models in which the sources of
the highest energy cosmic rays are preferentially located within the
low-redshift universe. | astro-ph_HE |
Stellar wind accretion and accretion disk formation: applications to
neutron star high mass X-ray binaries: Recent X-ray observations have revealed the complexity and diversity of
high-mass X-ray binaries (HMXBs). This diversity challenges a classical
understanding of the accretion process onto the compact objects. In this study,
we reinforce the conventional concept of the nature of wind-fed accretion onto
a neutron star considering the geometrical effect of radiatively accelerated
wind, and re-evaluate the transported angular momentum by using a simple wind
model. Our results suggest that even in an OB-type HMXB fed by stellar wind, a
large amount of angular momentum could be transported to form an accretion disk
due to the wind-inhomogeneity, if the binary separation is tight enough and/or
stellar wind is slow. We apply our model into actual systems such as LMC X-4
and OAO 1657-415, and discuss the possibility of disk formations in these
systems. | astro-ph_HE |
Stability of three-dimensional relativistic jets: implications for jet
collimation: The stable propagation of jets in FRII sources is remarkable if one takes
into account that large-scale jets are subjected to potentially highly
disruptive three-dimensional (3D) Kelvin-Helmholtz instabilities. Numerical
simulations can address this problem and help clarify the causes of this
remarkable stability. Following previous studies of the stability of
relativistic flows in two dimensions (2D), it is our aim to test and extend the
conclusions of such works to three dimensions. We present numerical simulations
for the study of the stability properties of 3D, sheared, relativistic flows.
This work uses a fully parallelized code Ratpenat that solves equations of
relativistic hydrodynamics in 3D. The results of the present simulations
confirm those in 2D. We conclude that the growth of resonant modes in sheared
relativistic flows could be important in explaining the long-term collimation
of extragalactic jets. | astro-ph_HE |
Non-parametric Bayesian reconstruction of Galactic magnetic fields using
Information Field Theory: The inclusion of line-of-sight information in
ultra-high energy cosmic ray backtracking: (abridged) Ultra-high energy cosmic rays (UHECRs) are extremely energetic
charged particles with energies surpassing $10^{18}$ eV. Their sources remain
elusive, obscured by deflections caused by the Galactic magnetic field (GMF).
This challenge is further complicated by our limited understanding of the
three-dimensional structure of the GMF, as current GMF observations consist
primarily of quantities integrated along the line-of-sight (LOS). Nevertheless,
data from upcoming stellar polarisation surveys along with Gaia's stellar
parallax data are expected to yield local GMF measurements.. In this work, we
employ methods of Bayesian statistical inference in order to sample the
posterior distribution of the GMF within part of the Galaxy. By assuming a
known rigidity and arrival direction of an UHECR, we backtrack its trajectory
through various GMF configurations drawn from the posterior distribution. Our
objective is to rigorously evaluate our algorithm's performance in scenarios
that closely mirror the setting of expected future applications. In pursuit of
this, we condition the posterior to synthetic integrated LOS measurements of
the GMF, in addition to synthetic local POS-component measurements. In this
proof of concept work, we assume the ground truth to be a magnetic field
produced by a dynamo simulation of the Galactic ISM. Our results demonstrate
that for all locations of the observed arrival direction on the POS, our
algorithm is able to substantially update our knowledge on the original arrival
direction of UHECRs with rigidity $E/Z = 5 \times 10^{19}$ eV, even in the case
of complete absence of LOS information. If integrated data is included in the
inference, then the regions of the celestial sphere where the maximum error
occurs diminishes greatly. Even in those regions the maximum error is
diminished by a factor of about $3$ in the specific setting studied. | astro-ph_HE |
Mass of intermediate black hole in the source M82 X-1 restricted by
models of twin high-frequency quasi-periodic oscillations: We apply the relativistic precession (RP) model with its variants and the
resonance epicyclic model with its variants, based on the frequencies of the
geodesic epicyclic motion in the field of a Kerr black hole, to put limits on
the mass of the black hole in the ultraluminous X-ray source M82 X-1
demonstrating twin high-frequency quasi-periodic oscillations (HF QPOs) with
the frequency ratio near 3:2. The mass limits implied by the geodesic HF QPO
models are compared to those obtained due to the model of string loop
oscillations around a stable equilibrium position. Assuming the whole range of
the black hole dimensionless spin, 0 < a < 1, the restrictions on the black
hole mass related to the twin HF QPOs are widely extended and strongly model
dependent; nevertheless, they give the lower limit M_{M82 X-1} > 130 M_{sun}
confirming existence of an intermediate black hole in the M82 X-1 source. The
upper limit given by one of the variants of the geodesic twin HF QPO models
goes up to M_{M82 X-1}<1500 M_{sun}. The range 37-210 mHz of the low-frequency
QPOs observed in the M82 X-1 source introduces additional restrictive limits on
the black hole mass, if we model the low-frequency QPOs by nodal precession of
the epicyclic motion. The nodal precession model restrictions combined with
those implied by the geodesic models of the twin HF QPOs give allowed ranges of
the M82 X-1 black hole parameters, namely 140 M_{sun}<M_{M82 X-1}<660 M_{sun}
for the mass parameter and 0.05<a_{M82 X-1}<0.6 for the spin parameter. | astro-ph_HE |
Determining the optimal locations for shock acceleration in
magnetohydrodynamical jets: Observations of relativistic jets from black holes systems suggest that
particle acceleration often occurs at fixed locations within the flow. These
sites could be associated with critical points that allow the formation of
standing shock regions, such as the magnetosonic modified fast point. Using the
self-similar formulation of special relativistic magnetohydrodynamics by
Vlahakis & K\"onigl, we derive a new class of flow solutions that are both
relativistic and cross the modified fast point at a finite height. Our
solutions span a range of Lorentz factors up to at least 10, appropriate for
most jets in X-ray binaries and active galactic nuclei, and a range in injected
particle internal energy. A broad range of solutions exists, which will allow
the eventual matching of these scale-free models to physical boundary
conditions in the analysis of observed sources. | astro-ph_HE |
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