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Deep Synoptic Array science I: discovery of the host galaxy of FRB
20220912A: We report the detection and interferometric localization of the repeating
fast radio burst (FRB) source FRB 20220912A during commissioning observations
with the Deep Synoptic Array (DSA-110). Two bursts were detected from FRB
20220912A, one each on 2022 October 18 and 2022 October 25. The best-fit
position is (R.A. J2000, decl. J2000) = (23:09:04.9, +48:42:25.4), with a 90%
confidence error ellipse of $\pm2$ arcsec and $\pm1$ arcsec in right ascension
and declination respectively. The two bursts have disparate polarization
properties and temporal profiles. We find a Faraday rotation measure that is
consistent with the low value of $+0.6$ rad m$^{-2}$ reported by CHIME/FRB. The
DSA-110 localization overlaps with the galaxy PSO J347.2702+48.7066 at a
redshift $z=0.0771$, which we identify as the likely host. PSO
J347.2702$+$48.7066 has a stellar mass of approximately $10^{10}M_{\odot}$,
modest internal dust extinction, and a star-formation rate likely in excess of
$0.1\,M_{\odot}$ yr$^{-1}$. The host-galaxy contribution to the dispersion
measure is likely $\lesssim50$ pc cm$^{-3}$. The FRB 20220912A source is
therefore likely viewed along a tenuous plasma column through the host galaxy. | astro-ph_HE |
The XMM-Newton Slew view of IGRJ17361-4441: a transient in the globular
cluster NGC 6388: IGRJ17361-4441 is a hard transient recently observed by the INTEGRAL
satellite. The source, close to the center of gravity of the globular cluster
NGC 6388, quickly became the target of follow-up observations conducted by the
Chandra, Swift/XRT and RXTE observatories. Here, we concentrate in particular
on a set of observations conducted by the XMM-Newton satellite during two
slews, in order to get the spectral information of the source and search for
spectral variations. The spectral parameters determined by the recent
XMM-Newton slew observations were compared to the previously known results. The
maximum unabsorbed $X$-ray flux in the 0.5-10 keV band as detected by the
XMM-Newton slew observations is $\simeq 4.5\times 10^{-11}$ erg cm$^{-2}$
s$^{-1}$, i.e. consistent with that observed by the Swift/XRT satellite 15 days
earlier. The spectrum seems to be marginally consistent ($\Gamma\simeq
0.93-1.63$) with that derived from the previous high energy observation. | astro-ph_HE |
Radio Monitoring of the Tidal Disruption Event Swift J164449.3+573451.
IV. Continued Fading and Non-Relativistic Expansion: We present continued radio and X-ray observations of the previously
relativistic tidal disruption event (TDE) Swift J164449.3+573451 (\sw)
extending to about 9.4 years post disruption, as part of ongoing campaigns with
the Jansky Very Large Array (VLA) and the \textit{Chandra} X-ray observatory.
We find that the X-ray emission has faded below detectable levels, with an
upper limit of $\lesssim 3.5\times 10^{-15}$ erg cm$^{-2}$ s$^{-1}$ in a 100 ks
observation, while the radio emission continues to be detected and steadily
fade. Both are consistent with forward shock emission from a non-relativistic
outflow, although we find that the radio spectral energy distribution is better
fit at these late times with an electron power law index of $p\approx 3$ (as
opposed to $p\approx 2.5$ at earlier times). With the revised spectral index we
find $\epsilon_B\approx 0.01$ using the radio and X-ray data, and a density of
$\approx 0.04$ cm$^{3}$ at a radius of $R\approx 0.65$ pc ($R_{\rm sch}\approx
2\times 10^6$ R$_\odot$) from the black hole. The energy scale of the blastwave
is $\approx 10^{52}$ erg. We also report detections of \sw\ at 3 GHz from the
first two epochs of the VLA Sky Survey (VLASS), and find that $\sim 10^2$
off-axis \sw-like events to $z\sim 0.5$ may be present in the VLASS data.
Finally, we find that \sw\ itself will remain detectable for decades at radio
frequencies, although observations at sub-GHz frequencies will become
increasingly important to characterize its dynamical evolution. | astro-ph_HE |
How unique are pulsar wind nebulae models? Implementation of a
multi-parameter, automatic fitting for time-dependent spectra: Due to the computational cost of calculating a great number of variations of
the parameters, detailed radiative models of pulsar wind nebulae (PWNe) do not
usually contain fitting algorithms. As a consequence, most of the models in the
literature are, in fact, qualitative fits based on visual inspection. This is
particularly true when complex, time-dependent models are considered. Motivated
by improvements in the computational efficiency of the current PWN models that
were obtained in the last years, we here explore the inclusion of automatic
fitting algorithms into a fully time-dependent model. Incorporating an
efficient fitting tool based on the Nelder-Mead algorithm, we blindly find
fitting solutions for the Crab nebula and 3C 58 with a time-dependent radiation
model to compute the spectral and dynamical evolution of young and middle-aged
PWNe. This inclusion allows us, in addition of more faithfully determining the
quality of the fit, to tackle whether there exist degeneracy in the selected
PWNe models. We find both for Crab and 3C58, that the fits are well determined,
and that no other significantly different set of model parameters is able to
cope with experimental data equally well. The code is also able to consider the
system's age as a free parameter, recursively determining all other needed
magnitudes depending on age accordingly. We use this feature to consider
whether a detailed multi-frequency spectra can constrain the nebula age,
finding that in fact this is the case for the two PWNe studied. | astro-ph_HE |
Cosmic Neutron Star Merger Rate and Gravitational Waves constrained by
the R Process Nucleosynthesis: The cosmic evolution of the neutron star merger (NSM) rate can be deduced
from the observed cosmic star formation rate. This allows to estimate the rate
expected in the horizon of the gravitational wave detectors advanced Virgo and
ad LIGO and to compare those rates with independent predictions. In this
context, the rapid neutron capture process, or r process, can be used as a
constraint assuming NSM is the main astrophysical site for this nucleosynthetic
process. We compute the early cosmic evolution of a typical r process element,
Europium. Eu yields from NSM are taken from recent nucleosynthesis
calculations. The same approach allows to compute the cosmic rate of Core
Collapse SuperNovae (CCSN) and the associated evolution of Eu. We find that the
bulk of Eu observations at high iron abundance can be rather well fitted by
either CCSN or NSM scenarios. However, at lower metallicity, the early Eu
cosmic evolution favors NSM as the main astrophysical site for the r process. A
comparison between our calculations and spectroscopic observations at very low
metallicities allows to constrain the coalescence timescale in the NSM scenario
to about 0.1 to 0.2 Gyr. These values are in agreement with the coalescence
timescales of some observed binary pulsars. Finally, the cosmic evolution of Eu
is used to put constraints on the NSM rate, the merger rate in the horizon of
the gravitational wave detectors advanced Virgo/ad LIGO, as well as the
expected rate of electromagnetic counterparts to mergers (kilonovae) in large
near-infrared surveys. | astro-ph_HE |
NICER observations of the Crab pulsar glitch of 2017 November: Context: The Crab pulsar underwent its largest timing glitch on 2017 Nov 8.
The event was discovered at radio wavelengths, and was followed at soft X-ray
energies by observatories, such as XPNAV and NICER. aims: This work aims to
compare the glitch behavior at the two wavelengths mentioned above. Preliminary
work in this regard has been done by the X-ray satellite XPNAV. NICER with its
far superior sensitivity is expected to reveal much more detailed behavior.
methods: NICER has accumulated more than $301$ kilo seconds of data on the Crab
pulsar, equivalent to more than $3.3$ billion soft X-ray photons. These data
were first processed using the standard NICER analysis pipeline. Then the
arrival times of the X-ray photons were referred to the solar system's
barycenter. Then specific analysis was done to study the specific behavior
outlined in the following sections, while taking dead time into account.
results: The variation of the rotation frequency of the Crab pulsar and its
time derivative during the glitch is almost exactly similar at the radio and
X-ray energies. The following properties of the Crab pulsar remain essentially
constant before and after the glitch: the total X-ray flux; the flux, widths,
and peaks of the two components of its integrated profile; and the soft X-ray
spectrum. There is no evidence for giant pulses at X-ray energies. However, the
timing noise of the Crab pulsar shows quasi sinusoidal variation before the
glitch, with increasing amplitude, which is absent after the glitch.
conclusions: Even the strongest glitch in the Crab pulsar appears not to affect
all but one of the properties mentioned above, at either frequency. The fact
that the timing noise appears to change due to the glitch is an important clue
to unravel as this is still an unexplained phenomenon. | astro-ph_HE |
Study of measured pulsar masses and their possible conclusions: We study the statistics of 61 measured masses of neutron stars (NSs) in
binary pulsar systems, including 18 double NS (DNS) systems, 26 radio pulsars
(10 in our Galaxy) with white dwarf (WD) companions, 3 NSs with main-sequence
companions, 13 NSs in X-ray binaries, and one undetermined system. We derive a
mean value of M = 1.46 +/- 0.30 solar masses. When the 46 NSs with measured
spin periods are divided into two groups at 20 milliseconds, i.e., the
millisecond pulsar (MSP) group and others, we find that their mass averages
are, respectively, M=1.57 +/- 0.35 solar masses and M=1.37+/- 0.23 solar
masses. In the framework of the pulsar recycling hypothesis, this suggests that
an accretion of approximately 0.2 solar mass is sufficient to spin up a neutron
star and place it in the millisecond pulsar group. An empirical relation
between the accreting mass and MSP spin period is \Delta M=0.43 (solar
mass)(P/1 ms)^{-2/3}. UNlike the standard recycling process, if a MSP is formed
by the accretion induced collapse (AIC) of a white dwarf with a mass less than
Chandrasekha limit, e.g. 1.35 solar mass, then the binary MSPs involved in AICs
is not be higher than 20%, which imposes a constraint on the AIC origin of
MSPs. | astro-ph_HE |
Type IIP supernova SN 2016X in radio frequencies: Context: The study of radio emission from core-collapse supernovae (SNe)
probes the interaction of the ejecta with the circumstellar medium (CSM) and
reveals details of the mass-loss history of the progenitor. Aims: We report
observations of the type IIP supernova SN\,2016X during the plateau phase, at
ages between 21 and 75 days, obtained with the Karl G. Jansky Very Large Array
(VLA) radio observatory. Methods: We modelled the radio spectra as
self-absorbed synchrotron emission, and we characterised the shockwave and the
mass-loss rate of the progenitor. We also combined our results with previously
reported X-ray observations to verify the energy equipartition assumption.
Results: The properties of the shockwave are comparable to other type IIP
supernovae. The shockwave expands according to a self-similar law $R \propto
t^m$ with $m=0.76 \pm 0.08$, which is notably different from a constant
expansion. The corresponding shock velocities are approximately 10700 - 8000 km
s$^{-1}$ during the time of our observations. The constant mass-loss rate of
the progenitor is $\dot{M}=$ (7.8 $\pm$ 0.9)\,$\times 10^{-7} \alpha^{-8/19}
(\epsilon_B/0.1)^{-1} M_{\odot}$ yr$^{-1}$, for an assumed wind velocity of 10
km s$^{-1}$. We observe spectral steepening in the optically thin regime at the
earlier epochs, and we demonstrate that it is caused by electron cooling via
the inverse Compton effect. We show that the shockwave is characterised by a
moderate deviation from energy equipartition by a factor of $\epsilon_e /
\epsilon_B \approx 28$, being the second type IIP supernova to show such a
feature. | astro-ph_HE |
Nuclei in Strongly Magnetised Neutron Star Crusts: We discuss the ground state properties of matter in outer and inner crusts of
neutron stars under the influence of strong magnetic fields. In particular, we
demonstrate the effects of Landau quantization of electrons on compositions of
neutron star crusts. First we revisit the sequence of nuclei and the equation
of state of the outer crust adopting the Baym, Pethick and Sutherland (BPS)
model in the presence of strong magnetic fields and most recent versions of the
theoretical and experimental nuclear mass tables. Next we deal with nuclei in
the inner crust. Nuclei which are arranged in a lattice, are immersed in a
nucleonic gas as well as a uniform background of electrons in the inner crust.
The Wigner-Seitz approximation is adopted in this calculation and each lattice
volume is replaced by a spherical cell. The coexistence of two phases of
nuclear matter - liquid and gas, is considered in this case. We obtain the
equilibrium nucleus corresponding to each baryon density by minimizing the free
energy of the cell. We perform this calculation using Skyrme nucleon-nucleon
interaction with different parameter sets. We find nuclei with larger mass and
charge numbers in the inner crust in the presence of strong magnetic fields
than those of the zero field case for all nucleon-nucleon interactions
considered here. However, SLy4 interaction has dramatic effects on the proton
fraction as well as masses and charges of nuclei. This may be attributed to the
behaviour of symmetry energy with density in the sub-saturation density regime.
Further we discuss the implications of our results to shear mode oscillations
of magnetars. | astro-ph_HE |
New complex EAS installation of the Tien Shan Mountain Cosmic Ray
Station: We present a description of the new complex installation for the study of
extensive air showers which was created at the Tien Shan mountain cosmic ray
station, as well as the results of the test measurements made there in
2014-2016. At present, the system for registration of electromagnetic shower
component consists of $\sim$100 detector points built on the basis of plastic
scintillator plates with the sensitive area of 0.25m$^2$ and 1m$^2$, spread
equidistantly over $\sim$10$^4$m$^2$ space. The dynamic range of scintillation
amplitude measurements is currently about $(3-7)\cdot 10^4$, and there is a
prospect of it being extended up to $\sim$10$^6$. The direction of shower
arrival is defined by signal delays from a number of the scintillators placed
cross-wise at the periphery of the detector system. For the investigation of
nuclear active shower components there was created a multi-tier 55m$^2$
ionization-neutron calorimeter with a sum absorber thickness of
$\sim$1000g/cm$^2$, typical spatial resolution of the order of 10cm, and
dynamic range of ionization measurement channel about $\sim$10$^5$. Also, the
use of saturation-free neutron detectors is anticipated for registration of the
high- and low-energy hadron components in the region of shower core. A complex
of underground detectors is designed for the study of muonic and penetrative
nuclear-active components of the shower.
The full stack of data acquisition, detector calibration, and shower
parameters restoration procedures are now completed, and the newly obtained
shower size spectrum and lateral distribution of shower particles occur in
agreement with conventional data. Future studies in the field of
$10^{14}-10^{17}$eV cosmic ray physics to be held at the new shower
installation are discussed. | astro-ph_HE |
Observational characteristics of accretion onto black holes: These notes resulted from a series of lectures at the IAC winter school. They
are designed to help students, especially those just starting in subject, to
get hold of the fundamental tools used to study accretion powered sources. As
such, the references give a place to start reading, rather than representing a
complete survey of work done in the field. I outline Compton scattering and
blackbody radiation as the two predominant radiation mechanisms for accreting
black holes, producing the hard X-ray tail and disc spectral components,
respectively. The interaction of this radiation with matter can result in
photo-electric absorption and/or reflection. While the basic processes can be
found in any textbook, here I focus on how these can be used as a toolkit to
interpret the spectra and variability of black hole binaries (hereafter BHB)
and Active Galactic Nuclei (AGN). I also discuss how to use these to physically
interpret real data using the publicly available XSPEC spectral fitting package
(Arnaud et al 1996), and how this has led to current models (and controversies)
of the accretion flow in both BHB and AGN. | astro-ph_HE |
Fermi LAT Observations of LS 5039: The first results from observations of the high mass X-ray binary LS 5039
using the Fermi Gamma-ray Space Telescope data between 2008 August and 2009
June are presented. Our results indicate variability that is consistent with
the binary period, with the emission being modulated with a period of 3.903 +/-
0.005 days; the first detection of this modulation at GeV energies. The light
curve is characterized by a broad peak around superior conjunction in agreement
with inverse Compton scattering models. The spectrum is represented by a power
law with an exponential cutoff, yielding an overall flux (100 MeV - 300 GeV) of
4.9 +/- 0.5(stat) +/- 1.8(syst) x 10^-7 photon cm^-2 s^-1, with a cutoff at 2.1
+/- 0.3(stat) +/- 1.1(syst) GeV and photon index Gamma = 1.9 +/- 0.1(stat) +/-
0.3(syst). The spectrum is observed to vary with orbital phase, specifically
between inferior and superior conjunction. We suggest that the presence of a
cutoff in the spectrum may be indicative of magnetospheric emission similar to
the emission seen in many pulsars by Fermi. | astro-ph_HE |
Minutes-delayed jets from a neutron star companion in core collapse
supernovae: I study cases where a neutron star (NS; or a black hole) companion to a type
Ib or type Ic (stripped-envelope) core collapse supernova (CCSN) accretes mass
from the explosion ejecta and launches jets minutes to hours after explosion.
The NS orbits at a pre-explosion radius of a=1-5Ro. I find that when the ejecta
velocity drops to be <1000-1500km/s the ejecta gas that the NS accretes
possesses sufficient specific angular momentum to form an accretion disk around
the NS. The NS accretes a fraction of 3e-5 to 3e-4 of the ejecta mass through
an accretion disk over a time period of 10min to few hours. If the jets carry
about ten per cent of the accretion energy, then their total energy is a
fraction of about 0.003-0.03 of the kinetic energy of the ejecta. The
implications of these jets from a NS (or a black hole) companion to a CCSN are
the shaping the inner ejecta to have a bipolar morphology, energising the light
curve of the CCSN, and in some cases the possible enrichment of the inner
ejecta with r-process elements. | astro-ph_HE |
First discovery of new pulsars and RRATs with CHIME/FRB: We report the discovery of seven new Galactic pulsars with the Canadian
Hydrogen Intensity Mapping Experiment's Fast Radio Burst backend (CHIME/FRB).
These sources were first identified via single pulses in CHIME/FRB, then
followed up with CHIME/Pulsar. Four sources appear to be rotating radio
transients (RRATs), pulsar-like sources with occasional single pulse emission
with an underlying periodicity. Of those four sources, three have detected
periods ranging from 220 ms to 2.726 s. Three sources have more persistent but
still intermittent emission and are likely intermittent or nulling pulsars. We
have determined phase-coherent timing solutions for the latter three. These
seven sources are the first discovery of previously unknown Galactic sources
with CHIME/FRB and highlight the potential of fast radio burst detection
instruments to search for intermittent Galactic radio sources. | astro-ph_HE |
An improved infrastructure for the IceCube realtime system: The IceCube realtime alert system has been operating since 2016. It provides
prompt alerts on high-energy neutrino events to the astroparticle physics
community. The localization regions for the incoming direction of neutrinos are
published through NASA's Gamma-ray Coordinate Network (GCN). The IceCube
realtime system consists of infrastructure dedicated to the selection of alert
events, the reconstruction of their topology and arrival direction, the
calculation of directional uncertainty contours and the distribution of the
event information through public alert networks. Using a message-based workflow
management system, a dedicated software (SkyDriver) provides a representational
state transfer (REST) interface to parallelized reconstruction algorithms. In
this contribution, we outline the improvements of the internal infrastructure
of the IceCube realtime system that aims to streamline the internal handling of
neutrino events, their distribution to the SkyDriver interface, the collection
of the reconstruction results as well as their conversion into human- and
machine-readable alerts to be publicly distributed through different alert
networks. An approach for the long-term storage and cataloging of alert events
according to findability, accessibility, interoperability and reusability
(FAIR) principles is outlined. | astro-ph_HE |
Prospects for ultra-high-energy particle acceleration at relativistic
shocks: We study the acceleration of charged particles by ultra-relativistic shocks
using test-particle Monte-Carlo simulations. Two field configurations are
considered: (i) shocks with uniform upstream magnetic field in the plane of the
shock, and (ii) shocks in which the upstream magnetic field has a cylindrical
geometry. Particles are assumed to diffuse in angle due to frequent
non-resonant scattering on small-scale fields. The steady-state distribution of
particles' Lorentz factors is shown to approximately satisfy $dN/d\gamma
\propto \gamma^{-2.2}$ provided the particle motion is scattering dominated on
at least one side of the shock. For scattering dominated transport, the
acceleration rate scales as $t_{\rm acc}\propto t^{1/2}$, though recovers Bohm
scaling $t_{\rm acc}\propto t$ if particles become magnetised on one side of
the shock. For uniform field configurations, a limiting energy is reached when
particles are magnetised on both sides of the shock. For the cylindrical field
configuration, this limit does not apply, and particles of one sign of charge
will experience a curvature drift that redirects particles upstream. For the
non-resonant scattering model considered, these particles preferentially escape
only when they reach the confinement limit determined by the finite system
size, and the distribution approaches the escapeless limit $dN/d\gamma \propto
\gamma^{-1}$. The cylindrical field configuration resembles that expected for
jets launched by the Blandford $\&$ Znajek mechanism, the luminous jets of AGN
and GRBs thus provide favourable sites for the production of ultra-high energy
cosmic rays. | astro-ph_HE |
A search for gamma-ray prompt emission associated with the Lorimer Burst
FRB010724: No transient electromagnetic emission has yet been found in association to
fast radio bursts (FRBs), the only possible exception (3sigma confidence) being
the putative gamma-ray signal detected in Swift/BAT data in the energy band
15-150 keV at the time and position of FRB131104. Systematic searches for hard
X/gamma-ray counterparts to other FRBs ended up with just lower limits on the
radio/gamma-ray fluence ratios. In 2001, at the time of the earliest discovered
FRBs, the BeppoSAX Gamma-Ray Burst Monitor (GRBM) was one of the most sensitive
open sky gamma-ray monitors in the 40-700~keV energy band. During its lifetime,
one of the FRBs with the highest radio fluence ever recorded, FRB010724 (800 +-
400 Jy ms), also known as the Lorimer burst, was promptly visible to the GRBM.
Upon an accurate modeling of the GRBM background, eased by its equatorial
orbit, we searched for a possible gamma-ray signal in the first 400 s following
the FRB, similar to that claimed for FRB131104 and found no significant
emission down to a 5-sigma limit in the range (0.24-4.7)x10^-6 erg cm^-2
(corresponding to 1 and 400 s integration time, respectively), in the energy
band 40-700 keV. This corresponds to eta = F_radio/F_gamma>10^{8-9} Jy ms
erg^-1 cm^2, i.e. the deepest limit on the ratio between radio and gamma-ray
fluence, which rules out a gamma-ray counterpart similar to that of FRB131104.
We discuss the implications on the possible mechanisms and progenitors that
have been proposed in the literature, also taking into account its relatively
low dispersion measure (375 +- 3 pc cm^-3) and an inferred redshift limit of
z<0.4. | astro-ph_HE |
Type IIn supernovae as sources of high energy neutrinos: It is shown that high-energy astrophysical neutrinos observed in the IceCube
experiment can be produced by protons accelerated in extragalactic Type IIn
supernova remnants by shocks propagating in the dense circumstellar medium. The
nonlinear diffusive shock acceleration model is used for description of
particle acceleration. | astro-ph_HE |
Evidence for Unresolved Gamma-Ray Point Sources in the Inner Galaxy: We present a new method to characterize unresolved point sources (PSs),
generalizing traditional template fits to account for non-Poissonian photon
statistics. We apply this method to Fermi Large Area Telescope gamma-ray data
to characterize PS populations at high latitudes and in the Inner Galaxy. We
find that PSs (resolved and unresolved) account for ~50% of the total
extragalactic gamma-ray background in the energy range ~1.9 to 11.9 GeV. Within
10$^\circ$ of the Galactic Center with $|b| \geq 2^\circ$, we find that ~5-10%
of the flux can be accounted for by a population of unresolved PSs, distributed
consistently with the observed ~GeV gamma-ray excess in this region. The excess
is fully absorbed by such a population, in preference to dark-matter
annihilation. The inferred source population is dominated by near-threshold
sources, which may be detectable in future searches. | astro-ph_HE |
Can FSRQ 3C 345 be a Very High Energy Blazar Candidate?: The recent detection of very high energy (VHE) emissions from flat spectrum
radio quasars (FSRQs) at high redshifts has revealed that the universe is more
transparent to VHE $\gamma$-rays than it was expected. It has also questioned
the plausible VHE emission mechanism responsible for these objects.
Particularly for FSRQs, the $\gamma$-ray emission is attributed to the external
Compton process (EC). We perform a detailed spectral study of
\emph{Fermi}-detected FSRQ 3C 345 using synchrotron, synchrotron self Compton
(SSC) and EC emission mechanisms. The simultaneous data available in optical,
UV, X-ray, and $\gamma$-ray energy bands is statistically fitted under these
emission mechanisms using the $\chi^2$-minimization technique. Three high flux
states and one low flux state are chosen for spectral fitting. The broadband
spectral energy distribution (SED) during these flux states is fitted under
different target photon temperatures, and the model VHE flux is compared with
the 50\hspace{0.05cm}hr CTA sensitivity. Our results indicate a significant VHE
emission could be attained during the high flux state from MJD 59635-59715 when
the target photon temperature is within 900K to 1200K. Furthermore, our study
shows a clear trend of variation in the bulk Lorentz factor of the emission
region as the source transits through different flux states. We also note that
during high $\gamma$-ray flux states, an increase in external photon
temperature demands high bulk Lorentz factors, while this behaviour reverses in
case of low $\gamma$-ray flux state. | astro-ph_HE |
Jets at lowest mass accretion rates: We present results of recent observations and theoretical modeling of data
from black holes accreting at very low luminosities (L/L_Edd ~ 10^{-8}). We
discuss our newly developed time-dependent model for episodic ejection of
relativistic plasma within a jet framework, and a successful application of
this model to describe the origin of radio flares seen in Sgr A*, the Galactic
center black hole. Both the observed time lags and size-frequency relationships
are reproduced well by the model. We also discuss results from new Spitzer data
of the stellar black hole X-ray binary system A0620-00. Complemented by long
term SMARTS monitoring, these observations indicate that once the contribution
from the accretion disk and the donor star are properly included, the residual
mid-IR spectral energy distribution of A0620-00 is quite flat and consistent
with a non-thermal origin. The results above suggest that a significant
fraction of the observed spectral energy distribution originating near black
holes accreting at low luminosities could result from a mildly relativistic
outflow. The fact that these outflows are seen in both stellar-mass black holes
as well as in supermassive black holes at the heart of AGNs strengthens our
expectation that accretion and jet physics scales with mass. | astro-ph_HE |
The Long-term Variability of the X-ray Sources in M82: We investigate the long-term variability exhibited by the X-ray point sources
in the starburst galaxy M82. By combining 9 Chandra observations taken between
1999 and 2007, we detect 58 X-ray point sources within the D25 isophote of M82
down to a luminosity of ~ 10^37 ergs/s. Of these 58 sources, we identify 3
supernova remnant candidates and one supersoft source. Twenty-six sources in
M82 exhibit long-term (i.e., days to years) flux variability and 3 show
long-term spectral variability. Furthermore, we classify 26 sources as
variables and 10 as persistent sources. Among the total 26 variables, 17 varied
by a flux ratio of > 3 and 6 are transient candidates. By comparing with other
nearby galaxies, M82 shows extremely strong long-term X-ray variability that
47% of the X-ray sources are variables with a flux ratio of > 3. The strong
X-ray variability of M82 suggests that the population is dominated by X-ray
binaries. | astro-ph_HE |
SMBH Seeds: Model Discrimination with High Energy Emission Based on
Scaling Relation Evolution: We explore the expected X-ray (0.5-2keV) signatures from super massive black
holes (SMBHs) at high redshifts ($z\sim5-12$) assuming various models for their
seeding mechanism and evolution. The seeding models are approximated through
deviations from the M$_{BH}-\sigma$ relation observed in the local universe. We
use results from N-body simulations of the large-scale structure to estimate
the density of observable SMBHs. We focus on two families of seeding models:
(\textit{i}) light seed BHs from remnants of Pop-III stars; and (\textit{ii})
heavy seeds from the direct collapse of gas clouds. We investigate several
models for the accretion history, such as sub-Eddington accretion, slim disk
models allowing mild super-Eddington accretion and torque-limited growth
models. We consider observations with two instruments: (\textit{i}) the Chandra
X-ray observatory, and (\textit{ii}) the proposed Lynx. We find that all the
simulated models are in agreement with the current results from Chandra Deep
Field South (CDFS) - \textit{i.e.,} consistent with zero to a few observed
SMBHs in the field of view. In deep Lynx exposures, the number of observed
objects is expected to become statistically significant. We demonstrate the
capability to limit the phase space of plausible scenarios of the birth and
evolution of SMBHs by performing deep observations at a flux limit of
$1\times10^{-19}\mathrm{erg\,cm^{-2}\,s^{-1}}$. Finally, we estimate the
expected contribution from each model to the unresolved cosmic X-ray background
(CXRB), and show that our models are in agreement with current limits on the
CXRB and the expected contribution from unresolved quasars. We find that an
analysis of CXRB contributions down to the Lynx confusion limit yields valuable
information that can help identify the correct scenario for the birth and
evolution of SMBHs. | astro-ph_HE |
The Role of Nucleon Strangeness in Supernova Explosions: Recent hydrodynamical simulations of supernova (SN) evolution have
highlighted the importance of a thorough control over microscopic physics
responsible for such internal processes as neutrino heating. In particular, it
has been suggested that modifications to the neutrino-nucleon elastic cross
section can potentially play a crucial role in producing successful supernova
explosions. One possible source of such corrections can be found in a nonzero
value for the nucleon's strange helicity content $\Delta s$. In the present
analysis, however, we show that theoretical and experimental progress over the
past decade has suggested a comparatively small magnitude for $\Delta s$, such
that its sole effect is not sufficient to provide the physics leading to
supernova explosions. | astro-ph_HE |
Orbital Modulation of Gamma Rays from PSR~J2339$-$0533: We report on orbital modulation of the 100-600 MeV gamma-ray emission of the
$P_{\rm B}=4.6$ hr millisecond pulsar binary PSR J2339$-$0533 using 11 yr of
Fermi Large Area Telescope data. The modulation has high significance (chance
probability $p\approx 10^{-7}$), is approximately sinusoidal, peaks near pulsar
superior conjunction, and is detected only in the low-energy 100-600 MeV band.
The modulation is confined to the on-pulse interval, suggesting that the
variation is in the 2.9-ms pulsed signal itself. This contrasts with the few
other known systems exhibiting GeV orbital modulations, as these are unpulsed
and generally associated with beamed emission from an intrabinary shock. The
origin of the modulated pulsed signal is not yet clear, although we describe
several scenarios, including Compton upscattering of photons from the heated
companion. This would require high coherence in the striped pulsar wind. | astro-ph_HE |
Multi-Messenger Searches in Astrophysics: Multi-messenger astronomy has experienced an explosive development in the
past few years. While not being a particularly young field, it has recently
attracted a lot of attention by several major discoveries and unprecedented
observation campaigns covering the entity of the electromagnetic spectrum as
well as observations of cosmic rays, neutrinos, and gravitational waves. The
exploration of synergies is in full steam and requires close cooperation
between different instruments. Here I give an overview over the subject of
multi-messenger astronomy and its virtues compared to classical "single
messenger" observations, present the recent break throughs of the field, and
discuss some of its organisational and technical challenges. | astro-ph_HE |
A Comparison of Gamma-ray Burst Subgroups Measured by RHESSI and BATSE: A sample of almost 400 Gamma-ray bursts (GRBs) detected by the RHESSI
satellite is studied statistically. We focus on GRB duration and hardness ratio
and use the statistical chi^2 test and the F-test to compare the number of GRB
subgroups in the RHESSI database with that of the BATSE database. Although some
previous articles based on the BATSE catalog claim the existence of an
intermediate GRB subgroup, besides long and short, we have not found a
statistically significant intermediate subgroup in the RHESSI data. | astro-ph_HE |
Constraints from LIGO O3 data on gravitational-wave emission due to
r-modes in the glitching pulsar PSR J0537-6910: We present a search for continuous gravitational-wave emission due to r-modes
in the pulsar PSR J0537-6910 using data from the LIGO-Virgo Collaboration
observing run O3. PSR J0537-6910 is a young energetic X-ray pulsar and is the
most frequent glitcher known. The inter-glitch braking index of the pulsar
suggests that gravitational-wave emission due to r-mode oscillations may play
an important role in the spin evolution of this pulsar. Theoretical models
confirm this possibility and predict emission at a level that can be probed by
ground-based detectors. In order to explore this scenario, we search for r-mode
emission in the epochs between glitches by using a contemporaneous timing
ephemeris obtained from NICER data. We do not detect any signals in the
theoretically expected band of 86-97 Hz, and report upper limits on the
amplitude of the gravitational waves. Our results improve on previous amplitude
upper limits from r-modes in J0537-6910 by a factor of up to 3 and place
stringent constraints on theoretical models for r-mode driven spin-down in PSR
J0537-6910, especially for higher frequencies at which our results reach below
the spin-down limit defined by energy conservation. | astro-ph_HE |
Radius-expansion burst spectra from 4U 1728-34: an ultracompact binary?: Recent theoretical and observational studies have shown that ashes from
thermonuclear burning may be ejected during radius-expansion bursts, giving
rise to photoionisation edges in the X-ray spectra. We report a search for such
features in Chandra spectra observed from the low-mass X-ray binary 4U 1728-34.
We analysed the spectra from four radius-expansion bursts detected in 2006
July, and two in 2002 March, but found no evidence for discrete features. We
estimate upper limits for the equivalent widths of edges of a few hundred eV,
which for the moderate temperatures observed during the bursts, are comparable
with the predictions. During the 2006 July observation 4U 1728-34 exhibited
weak, unusually frequent bursts (separated by <2 hr in some cases), with
profiles and alpha-values characteristic of hydrogen-poor fuel. Recurrence
times as short as those measured are insufficient to exhaust the accreted
hydrogen at solar composition, suggesting that the source accretes hydrogen
deficient fuel, for example from an evolved donor. The detection for the first
time of a 10.77 min periodic signal in the persistent intensity, perhaps
arising from orbital modulation, supports this explanation, and suggests that
this system is an ultracompact binary similar to 4U 1820-30. | astro-ph_HE |
X-ray bolometric corrections for Compton-thick active galactic nuclei: We present X-ray bolometric correction factors, $\kappa_{Bol}$ ($\equiv
L_{Bol}/L_X$), for Compton-thick (CT) active galactic nuclei (AGN) with the aim
of testing AGN torus models, probing orientation effects, and estimating the
bolometric output of the most obscured AGN. We adopt bolometric luminosities,
$L_{Bol}$, from literature infrared (IR) torus modeling and compile published
intrinsic 2--10 keV X-ray luminosities, $L_{X}$, from X-ray torus modeling of
NuSTAR data. Our sample consists of 10 local CT AGN where both of these
estimates are available. We test for systematic differences in $\kappa_{Bol}$
values produced when using two widely used IR torus models and two widely used
X-ray torus models, finding consistency within the uncertainties. We find that
the mean $\kappa_{Bol}$ of our sample in the range
$L_{Bol}\approx10^{42}-10^{45}$ erg/s is log$_{10}\kappa_{Bol}=1.44\pm0.12$
with an intrinsic scatter of $\sim0.2$ dex, and that our derived $\kappa_{Bol}$
values are consistent with previously established relationships between
$\kappa_{Bol}$ and $L_{Bol}$ and $\kappa_{Bol}$ and Eddington ratio. We
investigate if $\kappa_{Bol}$ is dependent on $N_H$ by comparing our results on
CT AGN to published results on less-obscured AGN, finding no significant
dependence. Since many of our sample are megamaser AGN, known to be viewed
edge-on, and furthermore under the assumptions of AGN unification whereby
unobscured AGN are viewed face-on, our result implies that the X-ray emitting
corona is not strongly anisotropic. Finally, we present $\kappa_{Bol}$ values
for CT AGN identified in X-ray surveys as a function of their observed $L_X$,
where an estimate of their intrinsic $L_{X}$ is not available, and redshift,
useful for estimating the bolometric output of the most obscured AGN across
cosmic time. | astro-ph_HE |
A Fast-Evolving, Luminous Transient Discovered by K2/Kepler: For decades optical time-domain searches have been tuned to find ordinary
supernovae, which rise and fall in brightness over a period of weeks. Recently,
supernova searches have improved their cadences and a handful of fast-evolving
luminous transients (FELTs) have been identified. FELTs have peak luminosities
comparable to Type Ia supernovae, but rise to maximum in $<10$ days and fade
from view in $<$month. Here we present the most extreme example of this class
thus far, KSN2015K, with a rise time of only 2.2 days and a time above
half-maximum ($t_{1/2}$) of only 6.8 days. Here we show that, unlike Type Ia
supernovae, the light curve of KSN2015K was not powered by the decay of
radioactive elements. We further argue that it is unlikely that it was powered
by continuing energy deposition from a central remnant (a magnetar or black
hole). Using numerical radiation hydrodynamical models, we show that the light
curve of KSN2015K is well fit by a model where the supernova runs into external
material presumably expelled in a pre-supernova mass loss episode. The rapid
rise of KSN2015K therefore probes the venting of photons when a hypersonic
shock wave breaks out of a dense extended medium. | astro-ph_HE |
Pulse energy distribution for RRAT J0139+33 according to observations at
the frequency 111 MHz: Using five year monitoring observations, we did a blind search for pulses for
rotating radio transient (RRAT) J0139+33 and PSR B0320+39. At the interval \pm
1.5m of the time corresponding to the source passing through the meridian, we
detected 39377 individual pulses for the pulsar B0320+39 and 1013 pulses for
RRAT J0139+33. The share of registered pulses from the total number of observed
periods for the pulsar B0320+39 is 74%, and for the transient J0139+33 it is
0.42%. Signal-to-noise ratio (S/N) for the strongest registered pulses is
approximately equal to: S/N = 262 (for B0320+39) and S/N = 154 (for J0139+33).
Distributions of the number of detected pulses in S/N units for the pulsar
and for the rotating transient are obtained. The distributions could be
approximated with a lognormal and power dependencies. For B0320+39 pulsar, the
dependence is lognormal, it turns into a power dependence at high values of
S/N, and for RRAT J0139+33, the distribution of pulses by energy is described
by a broken (bimodal) power dependence with an exponent of about 0.4 and 1.8
(S/N < 19 and S/N > 19).
We have not detected regular (pulsar) emission of J0139+33. Analysis of the
obtained data suggests that RRAT J0139+33 is a pulsar with giant pulses. | astro-ph_HE |
Forbidden Line Emission from Type Ia Supernova Remnants Containing
Balmer-Dominated Shells: Balmer-dominated shells in supernova remnants (SNRs) are produced by
collisionless shocks advancing into a partially neutral medium, and are most
frequently associated with Type Ia supernovae. We have analyzed Hubble Space
Telescope (HST) images and VLT/MUSE or AAT/WiFeS observations of five Type Ia
SNRs containing Balmer-dominated shells in the LMC: 0509-67.5, 0519-69.0,
N103B, DEM L71, and 0548-70.4. Contrary to expectations, we find bright
forbidden line emission from small dense knots embedded in four of these SNRs.
The electron densities in some knots are higher than 10$^4$ cm$^{-3}$. The size
and density of these knots are not characteristic for interstellar medium (ISM)
-- they most likely originate from a circumstellar medium (CSM) ejected by the
SN progenitor. Physical property variations of dense knots in the SNRs appear
to reflect an evolutionary effect. The recombination timescales for high
densities are short, and HST images of N103B taken 3.5 yr apart already show
brightness changes in some knots. VLT/MUSE observations detect [Fe XIV] line
emission from reverse shocks into SN ejecta as well as forward shocks into the
dense knots. Faint [O III] line emission is also detected from the Balmer shell
in 0519-69.0, N103B, and DEM L71. We exclude the postshock origin because the
[O III] line is narrow. For the preshock origin, we considered three
possibilities: photoionization precursor, cosmic ray precursor, and neutral
precursor. We conclude that the [O III] emission arises from oxygen that has
been photoionized by [He II] $\lambda$304 photons and is then collisionally
excited in a shock precursor heated mainly by cosmic rays. | astro-ph_HE |
Ultra-wide Bandwidth Observations of 19 pulsars with Parkes telescope: Flux densities are basic observation parameters to describe pulsars. In the
most updated pulsar catalog, 24% of the listed radio pulsars have no flux
density measurement at any frequency. Here, we report the first flux density
measurements, spectral indices, pulse profiles, and correlations of the
spectral index with pulsar parameters for 19 pulsars employing the
Ultra-Wideband Low (UWL) receiver system installed on the Parkes radio
telescope. The results for spectral indices of 17 pulsars are in the range
between -0.6 and -3.10. The polarization profiles of thirteen pulsars are
shown. There is a moderate correlation between the spectral index and spin
frequency. For most pulsars detected, the S/N ratio of pulse profile is not
high, so DM, Faraday rotation measure (RM), and polarization can not be
determined precisely. Twenty-nine pulsars were not detected in our
observations. We discuss the possible explanations for why these pulsars were
not detected. | astro-ph_HE |
Absorption effects in the blazar's gamma-ray spectra due to luminous
stars crossing the jet: Gamma-ray emission in active galaxies is likely produced within the inner
jet, or in the close vicinity of the supermassive black hole (SMBH) at
sub-parsec distances. Gamma rays have to pass through the surrounding massive
stellar cluster which luminous stars can accidentally appear close to the
observer's line of sight. In such a case, soft radiation of massive stars can
create enough target for transient absorption of the gamma rays in multi-GeV to
TeV energy range. We consider the effect of such stellar encounters on the
gamma-ray spectrum produced within the massive stellar cluster surrounding a
central SMBH. We predict characteristic, time-dependent effects on the
gamma-ray spectra due to the encounter with the single luminous star and also
stellar binary system. We conclude that during the encounter, the gamma-ray
spectrum of an active galaxy should steepen at tens of GeV and harden in the
range of hundreds of GeV. As an example, we consider such effects on the
spectra observed from a typical blazar, 1ES\ 1959+650 (in an active state) and
also in the case of a radio galaxy M87 (in a low state). It is shown that
observation of such transient characteristic features in the gamma-ray spectra,
observed from blazars and radio galaxies, lays within the sensitivity of the
future Cherenkov Telescope Array. | astro-ph_HE |
Opacities of Singly and Doubly Ionised Neodymium and Uranium for
Kilonova Emission Modeling: Even though the electromagnetic counterpart AT2017gfo to the binary neutron
star merger GW170817 is powered by the radioactive decay of r-process nuclei,
only few tentative identifications of light r-process elements have been made
so far. One of the major limitations for the identification of heavy nuclei is
incomplete or missing atomic data. While substantial progress has been made on
lanthanide atomic data over the last few years, for actinides there has been
less emphasis, with the first complete set of opacity data only recently
published. We perform atomic structure calculations of neodymium $(Z=60)$ as
well as the corresponding actinide uranium $(Z=92)$. Using two different codes
(FAC and HFR) for the calculation of the atomic data, we investigate the
accuracy of the calculated data (energy levels and electric dipole transitions)
and their effect on kilonova opacities. For the FAC calculations, we optimise
the local central potential and the number of included configurations and use a
dedicated calibration technique to improve the agreement between theoretical
and available experimental atomic energy levels (AELs). For ions with vast
amounts of experimental data available, the presented opacities agree quite
well with previous estimations. On the other hand, the optimisation and
calibration method cannot be used for ions with only few available AELs. For
these cases, where no experimental nor benchmarked calculations are available,
a large spread in the opacities estimated from the atomic data obtained with
the various atomic structure codes is observed.We find that the opacity of
uranium is almost double the neodymium opacity. | astro-ph_HE |
The First Fermi LAT Gamma-Ray Burst Catalog: In three years of observations since the beginning of nominal science
operations in August 2008, the Large Area Telescope (LAT) on board the Fermi
Gamma Ray Space Telescope has observed high-energy (>20 MeV) \gamma-ray
emission from 35 gamma-ray bursts (GRBs). Among these, 28 GRBs have been
detected above 100 MeV and 7 GRBs above ~ 20 MeV. The first Fermi-LAT catalog
of GRBs is a compilation of these detections and provides a systematic study of
high-energy emission from GRBs for the first time. To generate the catalog, we
examined 733 GRBs detected by the Gamma-Ray Burst Monitor (GBM) on Fermi and
processed each of them using the same analysis sequence. Details of the
methodology followed by the LAT collaboration for GRB analysis are provided. We
summarize the temporal and spectral properties of the LAT-detected GRBs. We
also discuss characteristics of LAT-detected emission such as its delayed onset
and longer duration compared to emission detected by the GBM, its power-law
temporal decay at late times, and the fact that it is dominated by a power-law
spectral component that appears in addition to the usual Band model. | astro-ph_HE |
The variability timescales and brightness temperatures of radio flares
from stars to supermassive black holes: In this paper we compile the analysis of ~ 200 synchrotron flare events from
~ 90 distinct objects/events for which the distance is well established, and
hence the peak luminosity can be accurately estimated. For each event we
measure this peak and compare it to the rise and decay timescales, as fit by
exponential functions, which allows us in turn to estimate a minimum brightness
temperature for all the events. The astrophysical objects from which the flares
originate vary from flare stars to supermassive black holes in active galactic
nuclei, and include both repeating phenomena and single cataclysmic events
(such as supernovae and gamma ray burst afterglows). The measured timescales
vary from minutes to longer than years, and the peak radio luminosities range
over 22 orders of magnitude. Despite very different underlying phenomena,
including relativistic and non-relativistic regimes, and highly collimated
versus isotropic phenomena, we find a broad correlation between peak radio
luminosity and rise/decay timescales, approximately of the form L ~ t^5. This
rather unexpectedly demonstrates that the estimated minimum brightness
temperature, when based upon variability timescales, and with no attempt to
correct for relativistic boosting, is a strongly rising function of source
luminosity. It furthermore demonstrates that variability timescales could be
used as an early diagnostic of source class in future radio transient surveys.
As an illustration of radio transients parameter space, we compare the
synchrotron events with coherent bursts at higher brightness temperatures to
illustrate which regions of radio transient parameter space have been explored. | astro-ph_HE |
Suzaku and BeppoSAX X-ray Spectra of the Persistently Accreting
Neutron-Star Binary 4U 1705-44: We present an analysis of the broad-band spectra of 4U~1705--44 obtained with
{\it Suzaku} in 2006--2008 and by {\it BeppoSAX} in 2000. The source exhibits
two distinct states: the hard state shows emission from 1 to 150 keV, while the
soft state is mostly confined to be $<40$ keV. We model soft-state continuum
spectra with two thermal components, one of which is a multicolor accretion
disk and the other is a single-temperature blackbody to describe the boundary
layer, with additional weak Comptonization represented by either a simple power
law or the SIMPL model by Steiner et al. The hard-state continuum spectra are
modeled by a single-temperature blackbody for the boundary layer plus strong
Comptonization, modeled by a cutoff power law. While we are unable to draw firm
conclusions about the physical properties of the disk in the hard state, the
accretion disk in the soft state appears to approximately follow $L\propto
T^{3.2}$. The deviation from $L\propto T^4$, as expected from a constant inner
disk radius, might be caused by a luminosity-dependent spectral hardening
factor and/or real changes of the inner disk radius in some part of the soft
state. The boundary layer apparent emission area is roughly constant from the
hard to the soft states, with a value of about 1/11 of the neutron star
surface. The magnetic field on the surface of the NS in 4U~1705--44 is
estimated to be less than about $1.9\times 10^8$ G, assuming that the disk is
truncated by the ISCO or by the neutron star surface. Broad relativistic Fe
lines are detected in most spectra and are modeled with the diskline model. The
strength of the Fe lines is found to correlate well with the boundary layer
emission in the soft state. In the hard state, the Fe lines are probably due to
illumination of the accretion disk by the strong Comptonization emission. | astro-ph_HE |
Bumpy Declining Light Curves Are Common in Hydrogen-poor Superluminous
Supernovae: Recent work has revealed that the light curves of hydrogen-poor (Type I)
superluminous supernovae (SLSNe), thought to be powered by magnetar central
engines, do not always follow the smooth decline predicted by a simple magnetar
spin-down model. Here we present the first systematic study of the prevalence
and properties of "bumps" in the post-peak light curves of 34 SLSNe. We find
that the majority (44-76%) of events cannot be explained by a smooth magnetar
model alone. We do not find any difference in supernova properties between
events with and without bumps. By fitting a simple Gaussian model to the
light-curve residuals, we characterize each bump with an amplitude,
temperature, phase, and duration. We find that most bumps correspond with an
increase in the photospheric temperature of the ejecta, although we do not see
drastic changes in spectroscopic features during the bump. We also find a
moderate correlation ($\rho\approx0.5$; $p\approx0.01$) between the phase of
the bumps and the rise time, implying that such bumps tend to happen at a
certain "evolutionary phase," $(3.7\pm1.4)t_\mathrm{rise}$. Most bumps are
consistent with having diffused from a central source of variable luminosity,
although sources further out in the ejecta are not excluded. With this
evidence, we explore whether the cause of these bumps is intrinsic to the
supernova (e.g., a variable central engine) or extrinsic (e.g., circumstellar
interaction). Both cases are plausible, requiring low-level variability in the
magnetar input luminosity, small decreases in the ejecta opacity, or a thin
circumstellar shell or disk. | astro-ph_HE |
Nonlinear variations in axisymmetric accretion: We subject the stationary solutions of inviscid and axially symmetric
rotational accretion to a time-dependent radial perturbation, which includes
nonlinearity to any arbitrary order. Regardless of the order of nonlinearity,
the equation of the perturbation bears a form that is similar to the metric
equation of an analogue acoustic black hole. We bring out the time dependence
of the perturbation in the form of a Li\'enard system, by requiring the
perturbation to be a standing wave under the second order of nonlinearity. We
perform a dynamical systems analysis of the Li\'enard system to reveal a saddle
point in real time, whose implication is that instabilities will develop in the
accreting system when the perturbation is extended into the nonlinear regime.
We also model the perturbation as a high-frequency travelling wave, and carry
out a Wentzel-Kramers-Brillouin analysis, treating nonlinearity iteratively as
a very feeble effect. Under this approach both the amplitude and the energy
flux of the perturbation exhibit growth, with the acoustic horizon segregating
the regions of stability and instability. | astro-ph_HE |
Classification of pulsars with Dirichlet process Gaussian mixture model: Young isolated neutron stars (INS) most commonly manifest themselves as
rotationally powered pulsars (RPPs) which involve conventional radio pulsars as
well as gamma-ray pulsars (GRPs) and rotating radio transients (RRATs). Some
other young INS families manifest themselves as anomalous X-ray pulsars (AXPs)
and soft gamma-ray repeaters (SGRs) which are commonly accepted as magnetars,
i.e. magnetically powered neutron stars with decaying superstrong fields. Yet
some other young INS are identified as central compact objects (CCOs) and X-ray
dim isolated neutron stars (XDINSs) which are cooling objects powered by their
thermal energy. Older pulsars, as a result of a previous long episode of
accretion from a companion, manifest themselves as millisecond pulsars and more
commonly appear in binary systems. We use Dirichlet process Gaussian mixture
model (DPGMM), an unsupervised machine learning algorithm, for analyzing the
distribution of these pulsar families in the parameter space of period and
period derivative. We compare the average values of the characteristic age,
magnetic dipole field strength, surface temperature and transverse velocity of
all discovered clusters. We verify that DPGMM is robust and provides hints for
inferring relations between different classes of pulsars. We discuss the
implications of our findings for the magneto-thermal spin evolution models and
fallback discs. | astro-ph_HE |
Annihilation of positrons from AGN jets as a possible source of cosmic
gamma-ray background at energies below 511 keV: The origin of the diffuse gamma-ray background in the range from hundreds keV
to several MeV is not known conclusively. From current models and observations
it is believed that, at least partially, this background is formed by blazars
and remnants of supernovae (SN) of type Ia in distant galaxies. However, these
contributions are not sufficient to reproduce the observed level of the signal.
In this work we propose another source which could contribute to this
background, namely the jets of active galactic nuclei (AGN). The composition of
jets is not known, but there are observational hints that the fraction of
positrons there is substantial. Positrons are partially evacuated to the
intergalactic medium and partially mix with the circumgalactic medium and
annihilate there comparatively quickly. Using the AGN luminosity function, we
estimated the positron production rate and the contribution of the positron
annihilation to the cosmic background below 511 keV. We also estimated the
analogous contribution from positron annihilation within SN Ia remnants in
distant galaxies. The contribution of AGNs is estimated to be a factor of 5 -
10 smaller than the observed background intensity, and the contribution from
SNe is yet smaller by one order of magnitude. Nevertheless, the contribution of
AGNs appeared to be larger than the contribution of blazars estimated from
Swift-BAT and Fermi-LAT observations. The main uncertainty in our model is the
fraction of positrons remaining in the circumgalactic medium which makes our
estimation an upper limit. | astro-ph_HE |
Jet production in black-hole X-ray binaries and active galactic nuclei:
mass feeding and advection of magnetic fields: Relativistic jets are observed only in the low/hard and intermediate states
of X-ray binaries (XRBs), and are switched off in the thermal state, but they
appear to be present in both low-luminosity and luminous active galactic nuclei
(AGNs). It is widely believed that strong large-scale magnetic fields is a
crucial ingredient in jet production; such fields can be attained only through
efficient advection from the outer disc. We suggest that geometrically thin
accretion discs with magnetic outflows are present in luminous radio-loud AGNs;
this is likely because the interstellar medium provides both mass and
sufficient magnetic flux to the outer disc. Most angular momentum of such disc
is removed by the outflows, and the radial velocity of the disc is
significantly increased compared to viscous drift velocity. This facilitates
efficient magnetic field advection through the disc to produce a strong field
near the black hole in luminous AGNs, which helps launch relativistic jets. In
XRBs, the magnetic fields of the gas from companion stars are too weak to drive
outflows from outer discs. Jets are therefore switched off in the thermal state
due to inefficient magnetic field advection in the disc. | astro-ph_HE |
Interaction between Molecular Clouds and MeV-TeV Cosmic-ray Protons
Escaped from Supernova Remnants: Recent discovery of the X-ray neutral iron line (Fe I K\alpha at 6.40 keV)
around several supernova remnants (SNRs) show that MeV cosmic-ray (CR) protons
are distributed around the SNRs and are interacting with neutral gas there. We
propose that these MeV CRs are the ones that have been accelerated at the SNRs
together with GeV-TeV CRs. In our analytical model, the MeV CRs are still
confined in the SNR when the SNR collides with molecular clouds. After the
collision, the MeV CRs leak into the clouds and produce the neutral iron line
emissions. On the other hand, GeV-TeV CRs had already escaped from the SNRs and
emit gamma-rays through interaction with molecular clouds surrounding the SNRs.
We apply this model to the SNRs W28 and W44 and show that it can reproduce the
observations of the iron line intensities and the gamma-ray spectra. This can
be another support of a hadronic scenario for the gamma-ray emissions from
these SNRs. | astro-ph_HE |
Spin-Down of the Long-Period Accreting Pulsar 4U 2206+54: 4U 2206+54 is a high mass X-ray binary which has been suspected to contain a
neutron star accreting from the wind of its companion BD +53 2790. Reig et al.
have recently detected 5560 s period pulsations in both RXTE and INTEGRAL
observations which they conclude are due to the spin of the neutron star. We
present observations made with Suzaku which are contemporaneous with their RXTE
observation of this source. We find strong pulsations at a period of 5554 +/- 9
s in agreement with their results. We also present a reanalysis of BeppoSAX
observations of 4U 2206+54 made in 1998, in which we find strong pulsations at
a period of 5420 +/- 28 seconds, revealing a spin-down trend in this
long-period accreting pulsar. Analysis of these data suggests that the neutron
star in this system is an accretion-powered magnetar. | astro-ph_HE |
Low-Latency Algorithm for Multi-messenger Astrophysics (LLAMA) with
Gravitational-Wave and High-Energy Neutrino Candidates: We describe in detail the online data analysis pipeline that was used in the
multi-messenger search for common sources of gravitational waves (GWs) and
high-energy neutrinos (HENs) during the second observing period (O2) of
Advanced LIGO and Advanced Virgo. Beyond providing added scientific insight
into source events, low-latency coincident HENs can offer better localization
than GWs alone, allowing for faster electromagnetic follow-up. Transitioning
GW+HEN analyses to low-latency, automated pipelines is therefore
mission-critical for future multi-messenger efforts. The O2 Low-Latency
Algorithm for Multi-messenger Astrophysics (\pipeline) also served as a
proof-of-concept for future online GW+HEN searches and led to a codebase that
can handle other messengers as well. During O2, the pipeline was used to take
LIGO/Virgo GW candidates as triggers and search in realtime for temporally
coincident HEN candidates provided by the IceCube Collaboration that fell
within the \ninetyCR of the reconstructed GW skymaps. The algorithm used NASA's
Gamma-ray Coordinates Network to report coincident alerts to LIGO/Virgo's
electromagnetic follow-up partners. | astro-ph_HE |
3D Relativistic MHD simulations of the gamma-ray binaries: In gamma-ray binaries neutron star is orbiting a companion that produces a
strong stellar wind. We demonstrate that observed properties of "stellar
wind"-"pulsar wind" interaction depend both on the overall wind thrust ratio,
as well as more subtle geometrical factors: the relative direction of the
pulsar's spin, the plane of the orbit, the direction of motion, and the
instantaneous line of sight. Using fully 3D relativistic magnetohydrodynamical
simulations we find that the resulting intrinsic morphologies can be
significantly orbital phase-dependent: a given system may change from
tailward-open to tailward-closed shapes. As a result, the region of unshocked
pulsar wind can change by an order of magnitude over a quarter of the orbit. We
calculate radiation maps and synthetic light curves for synchrotron (X-ray) and
Inverse-Compton emission (GeV-TeV), taking into account $\gamma-\gamma$
absorption. Our modeled light curves are in agreement with the phase-dependent
observed light curves of LS5039. | astro-ph_HE |
An unusual transient following the short GRB 071227: We present X-ray and optical observations of the short duration gamma-ray
burst GRB 071227 and its host at $z=0.381$, obtained using \textit{Swift},
Gemini South and the Very Large Telescope. We identify a short-lived and
moderately bright optical transient, with flux significantly in excess of that
expected from a simple extrapolation of the X-ray spectrum at 0.2-0.3 days
after burst. We fit the SED with afterglow models allowing for high extinction
and thermal emission models that approximate a kilonova to assess the excess'
origins. While some kilonova contribution is plausible, it is not favoured due
to the low temperature and high luminosity required, implying superluminal
expansion and a large ejecta mass of $\sim 0.1$ M$_{\odot}$. We find, instead,
that the transient is broadly consistent with power-law spectra with additional
dust extinction of $E(B-V)\sim0.4$ mag, although a possibly thermal excess
remains in the \textit{z}-band. We investigate the host, a spiral galaxy with
an edge-on orientation, resolving its spectrum along its major axis to
construct the galaxy rotation curve and analyse the star formation and chemical
properties. The integrated host emission shows evidence for high extinction,
consistent with the afterglow findings. The metallicity and extinction are
consistent with previous studies of this host and indicate the galaxy is a
typical, but dusty, late-type SGRB host. | astro-ph_HE |
Modelling interaction of relativistic and non-relativistic winds in
binary system PSR B1259-63/SS2883 - II. Impact of magnetization and
anisotropy of the pulsar wind: In this paper, we present a numerical study of the properties of the flow
produced by the collision of a magnetized anisotropic pulsar wind with its
environment in binary system. We compare the impact of both the magnetic field
and the wind anisotropy to the benchmark case of a purely hydrodynamical (HD)
interaction of isotropic winds, which has been studied in detail by Bogovalov
et al. (2008). We consider the interaction in axisymmetric approximation, i.e.
the pulsar rotation axis is assumed to be oriented along the line between the
pulsar and the optical star and the effects related to the pulsar orbiting are
neglected. The impact of the magnetic field is studied for the case of weak
magnetization (with magnetization parameter $\sigma<0.1$), which is consistent
with conventional models of pulsar winds. The effects related to anisotropy in
pulsar winds are modeled assuming that the kinetic energy flux in a
non-magnetized pulsar wind is strongly anisotropic, with the minimum at the
pulsar rotation axis and the maximum in the perpendicular direction. We show
that, although both considered effects change the shape of the region occupied
by the terminated pulsar wind, their impact appears to be small. In particular,
for the magnetization of the pulsar wind below 0.1, the magnetic field pressure
remains well below the plasma pressure in the post-shock region. Thus, in the
case of interaction of a pulsar with the stellar wind environment (opposite to
the case of plerions, i.e. the pulsar interaction with the interstellar medium,
when the magnetic field becomes dynamically important independently on the wind
magnetization) the HD approach represents a feasible approximation for
numerical modelling. | astro-ph_HE |
Changes in the pulse phase dependence of X-ray emission lines in 4U
1626-67 with a torque reversal: We report results from an observation with the XMM-Newton observatory of a
unique X-ray pulsar 4U 1626-67. EPIC-pn data during the current spin-up phase
of 4U 1626-67 have been used to study pulse phase dependence of low energy
emission lines. We found strong variability of low energy emission line at
0.915 keV with the pulse phase, varying by a factor of 2, much stronger than
the continuum variability. Another interesting observation is that behavior of
one of the low energy emission lines across the pulse phase is quite different
from that observed during the spin-down phase. This indicates that the
structures in the accretion disk that produce pulse phase dependence of
emission features have changed from spin-down to spin-up phase. This is well
supported by the differences in the timing characteristics (like pulse
profiles, QPOs etc) between spin-down and spin-up phases. We have also found
that during the current spin-up phase of 4U 1626-67, the X-ray pulse profile
below 2 keV is different compared to the spin-down phase. The X-ray light curve
also shows flares which produce a feature around 3 mHz in power density
spectrum of 4U 1626-67. Since flares are dominant at lower energies, the
feature around 3 mHz is prominent at low energies. | astro-ph_HE |
Long-term periodicity in LSI+61303 as beat frequency between orbital and
precessional rate: Context: In the binary system LSI+61303 the peak flux density of the radio
outburst, which is related to the orbital period of 26.4960 +/- 0.0028d,
exibits a modulation of 1667 +/- 8 d. The radio emission at high spatial
resolution appears structured in a precessing jet with a precessional period of
27-28 d. Aims: How close is the precessional period of the radio jet to the
orbital period? Any periodicity in the radio emission should be revealed by
timing analysis. The aim of this work is to establish the accurate value of the
precessional period. Methods: We analyzed 6.7 years of the Green Bank
Interferometer database at 2.2 GHz and 8.3 GHz with the Lomb-Scargle and phase
dispersion minimization (PDM) methods and performed simulations. Results: The
periodograms show two periodicities, P1 = 26.49 +/- 0.07 d (\nu1=0.03775
d^{-1}) and P2 = 26.92 +/- 0.07 d (\nu2 = 0.03715 d^{-1}). Whereas radio
outbursts have been known to have nearly orbital occurrence P1 with timing
residuals exhibiting a puzzling sawtooth pattern, we probe in this paper that
they are actually periodical outbursts and that their period is Paverage=
(2/(\nu1 + \nu2)= 26.70 +/- 0.05 d. The period Paverage as well as the
long-term modulation Pbeat=1/(\nu1 - \nu2)=1667 +/- 393 d result from the beat
of the two close periods, the orbital P1 and the precessional P2 periods.
Conclusions: The precessional period, indicated by the astrometry to be of
27--28 d, is P2=26.92 d. The system \lsi seems to be one more case in astronomy
of beat, i.e., a phenomenon occurring when two physical processes create stable
variations of nearly equal frequencies. The very small difference in frequency
creates a long-term variation of period 1/(\nu1-\nu2). The long-term modulation
of 1667 d results from the beat of the two close orbital and precessional
rates. | astro-ph_HE |
A propeller scenario for the gamma-ray emission of low-mass X-ray
binaries: The case of XSS J12270-4859: XSS J12270-4859 is the only low mass X-ray binary (LMXB) with a proposed
persistent gamma-ray counterpart in the Fermi-LAT domain, 2FGL 1227.7-4853.
Here, we present the results of the analysis of recent INTEGRAL observations,
aimed at assessing the long-term variability of the hard X-ray emission, and
thus the stability of the accretion state. We confirm that the source behaves
as a persistent hard X-ray emitter between 2003 and 2012. We propose that XSS
J12270-4859 hosts a neutron star in a propeller state, a state we investigate
in detail, developing a theoretical model to reproduce the associated X-ray and
gamma-ray properties. This model can be understood as being of a more general
nature, representing a viable alternative by which LMXBs can appear as
gamma-ray sources. In particular, this may apply to the case of millisecond
pulsars performing a transition from a state powered by the rotation of their
magnetic field, to a state powered by matter in-fall, such as that recently
observed from the transitional pulsar PSR J1023+0038. While the surface
magnetic field of a typical NS in a LMXB is lower by more than four orders of
magnitude than the much more intense fields of neutron stars accompanying
high-mass binaries, the radius at which the matter in-flow is truncated in a
NS-LMXB system is much lower. The magnetic field at the magnetospheric
interface is then orders of magnitude larger at this interface, and as
consequence, so is the power to accelerate electrons. We demonstrate that the
cooling of the accelerated electron population takes place mainly through
synchrotron interaction with the magnetic field permeating the interface, and
through inverse Compton losses due to the interaction between the electrons and
the synchrotron photons they emit. We found that self-synchrotron Compton
processes can explain the high energy phenomenology of XSS J12270-4859. | astro-ph_HE |
The impact of resistive electric fields on particle acceleration in
reconnection layers: In the context of particle acceleration in high-energy astrophysical
environments featuring magnetic reconnection, the importance of the resistive
term of the electric field compared to the convective one is still under
debate. In this work, we present a quantitative analysis through 2D
magnetohydrodynamic numerical simulations of tearing-unstable current sheets
coupled to a test-particles approach, performed with the PLUTO code. We find
that the resistive field plays a significant role in the early-stage
energization of high-energy particles. Indeed, these particles are firstly
accelerated due to the resistive electric field when they cross an X-point,
created during the fragmentation of the current sheet. If this preliminary
particle acceleration mechanism dominated by the resistive field is neglected,
particles cannot reach the same high energies. Our results support therefore
the conclusion that the resistive field is not only non-negligible but it does
actually play an important role in the particle acceleration mechanism. | astro-ph_HE |
Resonant energization of particles by radio AGN: A new mechanism of particle acceleration, based on the resonant interaction
of a classical electromagnetic wave (EM) with a quantum wave (associated with a
relativistic particle), is explored.
In a model illustrative calculation, we study the fate of a Klein Gordon wave
subjected to the intense radio frequency waves generated in the vicinity of an
active galactic nuclei (AGN). In the framework of the paper we examine a
quantum wave associated with a relativistic particle, and it is shown that the
group velocity of the wave approaches the speed of light, implying that the
particles resonantly exchange energy with EM waves, eventually leading to
acceleration of particles to very high energies.
For typical parameters of under accreting Eddington radio AGN, it is shown
that the resonant energization could catapult particles to extreme energies
$\sim 10^{16-20}$eV. | astro-ph_HE |
Non-relativistic perpendicular shocks modeling young supernova remnants:
nonstationary dynamics and particle acceleration at forward and reverse
shocks: For parameters that are applicable to the conditions at young supernova
remnants, we present results of 2D3V particle-in-cell simulations of a
non-relativistic plasma shock with a large-scale perpendicular magnetic field
inclined at 45-deg angle to the simulation plane to approximate 3D physics. We
developed an improved clean setup that uses the collision of two plasma slabs
with different density and velocity, leading to the development of two
distinctive shocks and a contact discontinuity. The shock formation is mediated
by Weibel-type filamentation instabilities that generate magnetic turbulence.
Cyclic reformation is observed in both shocks with similar period, for which we
note global variations on account of shock rippling and local variations
arising from turbulent current filaments. The shock rippling occurs on spatial
and temporal scales given by gyro-motions of shock-reflected ions. The drift
motion of electrons and ions is not a gradient drift, but commensurates with E
x B drift. We observe a stable suprathermal tail in the ion spectra, but no
electron acceleration because the amplitude of Buneman modes in the shock foot
is insufficient for trapping relativistic electrons. We see no evidence of
turbulent reconnection. A comparison with other 2D simulation results suggests
that the plasma beta and the ion-to-electron mass ratio are not decisive for
efficient electron acceleration, but pre-acceleration efficacy might be reduced
with respect to the 2D results once three-dimensional effects are fully
accounted for. Other microphysical factors may also be at play to limit the
amplitude of Buneman waves or prevent return of electrons to the foot region. | astro-ph_HE |
Subgrid modeling of neutrino oscillations in astrophysics: Approximating neutrino oscillations as subgrid physics is an appealing
prospect for simulators of core-collapse supernovae and neutron-star mergers.
Because flavor instabilities quickly lead to quasisteady states in oscillation
calculations, it is widely believed that flavor mixing can be approximated in
astrophysical simulations by mapping unstable states onto the appropriate
asymptotic ones. Subgrid models of this kind, however, are not self-consistent.
The miscidynamic theory of quantum-coherent gases furnishes a subgrid model
that is. | astro-ph_HE |
High resolution X-ray spectroscopy of Supergiant HMXB 4U1700$-$37 during
the compact object eclipse: We present an analysis of the first observation of the iconic High Mass X-ray
Binary \so with the \chandra High Energy Transmission Gratings during an X-ray
eclipse. The goal of the observation was to study the structure/physical
conditions in the clumpy stellar wind through high resolution spectroscopy. We
find that: a) emission line brightness from K shell transitions, corresponding
to near neutral species, directly correlates with continuum illumination.
However, these lines do not greatly diminish during eclipse. This is readily
explained if fluorescence K$\alpha$ emission comes from the bulk of the wind.
b) The highly ionised Fexxv and Fexxvi Ly$\alpha$ diminish during eclipse.
Thus, they must be produced in the vicinity of the compact object where $\log
\xi >3$. c) to describe the emission line spectrum, the sum of two self
consistent photo ionisation models with low ionisation ($\log \xi\sim -1$) and
high ionisation ($\log \xi\sim 2.4$) is required. From their emission measures,
the clump-to-interclump density ratio can be estimated to be $n_c/n_i\sim 300$.
To fit the complex He-like \ion{Si}{xiii}{} profile, the plasma requires a
broadening with $v_{\rm bulk}\sim 840$ km s$^{-1}$. Reproducing the observed
$r\approx f$ line fluxes requires the addition of a third collisionally ionised
plasma. d) Emission lines widths appear unresolved at the \textsc{hetg}
gratings resolution with exception of Silicon. There is no clear radial
segregation between (quasi)neutral and ionised species, consistent with cold
wind clumps interspersed in a hot rarefied interclump medium. | astro-ph_HE |
Probing the evolution of the EAS muon content in the atmosphere with
KASCADE-Grande: The evolution of the muon content of very high energy air showers (EAS) in
the atmosphere is investigated with data of the KASCADE-Grande observatory. For
this purpose, the muon attenuation length in the atmosphere is obtained to
$\Lambda_\mu = 1256 \, \pm 85 \, ^{+229}_{-232}(\mbox{syst})\, \mbox{g/cm}^2$
from the experimental data for shower energies between $10^{16.3}$ and
$10^{17.0} \, \mbox{eV}$. Comparison of this quantity with predictions of the
high-energy hadronic interaction models QGSJET-II-02, SIBYLL 2.1, QGSJET-II-04
and EPOS-LHC reveals that the attenuation of the muon content of measured EAS
in the atmosphere is lower than predicted. Deviations are, however, less
significant with the post-LHC models. The presence of such deviations seems to
be related to a difference between the simulated and the measured zenith angle
evolutions of the lateral muon density distributions of EAS, which also causes
a discrepancy between the measured absorption lengths of the density of shower
muons and the predicted ones at large distances from the EAS core. The studied
deficiencies show that all four considered hadronic interaction models fail to
describe consistently the zenith angle evolution of the muon content of EAS in
the aforesaid energy regime. | astro-ph_HE |
Proton acceleration in pulsar magnetospheres: Pulsars have been identified as good candidates for the acceleration of
cosmic rays, up to ultra-high energies. However, a precise description of the
acceleration processes at play is still to be established. Using 2D
particle-in-cell simulations, we study proton acceleration in axisymmetric
pulsar magnetospheres. Protons and electrons are extracted from the neutron
star surface by the strong electric field induced by the rotation of the star,
and electrons and positrons are produced in the magnetosphere through pair
production process. As pair production has a crucial impact on electromagnetic
fields, on gaps and thus on particle acceleration, we study its influence on
the maximum energy and luminosity of protons escaping the magnetosphere.
Protons are accelerated and escape in all our simulations. However, the
acceleration sites are different for the protons and the pairs. As shown in
previous studies, pairs are accelerated to their highest energies at the
Y-point and in the equatorial current sheet, where magnetic reconnection plays
and important role. In contrast, protons gain most of their kinetic energy
below the light-cylinder radius within the separatrix current layers, but they
are not confined within the equatorial current sheet. Their maximum Lorentz
factors can reach $15\%$ to $75\%$ of the maximum Lorentz factor obtained by
acceleration through the full vacuum potential drop from pole to equator, and
increase with decreasing pair production. Their luminosity can reach $0.2\%$ to
$4\%$ of the theoretical spin down luminosity of an aligned pulsar, and the
minimum luminosity is obtained at the transition between the force-free and
electrosphere regimes. These estimates support that millisecond pulsars could
accelerate cosmic rays up to PeV energies and that new born millisecond pulsars
could accelerate cosmic rays up to ultra-high energies. | astro-ph_HE |
The population of X-ray supernova remnants in the Large Magellanic Cloud: We present a comprehensive X-ray study of the population of supernova
remnants (SNRs) in the LMC. Using primarily XMM-Newton, we conduct a systematic
spectral analysis of LMC SNRs to gain new insights on their evolution and the
interplay with their host galaxy. We combined all the archival XMM observations
of the LMC with those of our Very Large Programme survey. We produced X-ray
images and spectra of 51 SNRs, out of a list of 59. Using a careful modelling
of the background, we consistently analysed all the X-ray spectra and measure
temperatures, luminosities, and chemical compositions. We investigated the
spatial distribution of SNRs in the LMC and the connection with their
environment, characterised by various SFHs. We tentatively typed all LMC SNRs
to constrain the ratio of core-collapse to type Ia SN rates in the LMC. We
compared the X-ray-derived column densities to HI maps to probe the
three-dimensional structure of the LMC. This work provides the first
homogeneous catalogue of X-ray spectral properties of LMC SNRs. It offers a
complete census of LMC SNRs exhibiting Fe K lines (13% of the sample), or
revealing contribution from hot SN ejecta (39%). Abundances in the LMC ISM are
found to be 0.2-0.5 solar, with a lower [$\alpha$/Fe] than in the Milky Way.
The ratio of CC/type Ia SN in the LMC is $N_{\mathrm{CC}}/N_{\mathrm{Ia}} =
1.35(_{-0.24}^{+0.11})$, lower than in local SN surveys and galaxy clusters.
Comparison of X-ray luminosity functions of SNRs in Local Group galaxies
reveals an intriguing excess of bright objects in the LMC. We confirm that 30
Doradus and the LMC Bar are offset from the main disc of the LMC, to the far
and near sides, respectively. (abridged) | astro-ph_HE |
Detection of Millihertz Quasi-Periodic Oscillations in the X-Ray Binary
1RXS J180408.9$-$342058: Millihertz quasi-periodic oscillations (mHz QPOs) observed in neutron-star
low-mass X-ray binaries (NS LMXBs) are generally explained as marginally stable
thermonuclear burning on the neutron star surface. We report the discovery of
mHz QPOs in an XMM-Newton observation of the transient 1RXS J180408.9$-$342058,
during a regular bursting phase of its 2015 outburst. We found significant
periodic signals in the March observation, with frequencies in the range
$5-8\,\mathrm{mHz}$, superimposed on a strong $\sim1/f$ power-law noise
continuum. Neither the QPO signals nor the power-law noise were present during
the April observation, which exhibited a $2.5\times$ higher luminosity and had
correspondingly more frequent bursts. When present, the QPO signal power
decreases during bursts and disappears afterwards, similar to the behaviour in
other sources. 1RXS J180408.9$-$342058 is the eighth source known to date that
exhibits such QPOs driven by thermonuclear burning. We examine the range of
properties of the QPO signals in different sources. Whereas the observed
oscillation profile is similar to that predicted by numerical models, the
amplitudes are significantly higher, challenging their explanation as
originating from marginally stable burning. | astro-ph_HE |
The magnetic field structure in CTA 102 from high resolution mm-VLBI
observations during the flaring state in 2016-2017: Investigating the magnetic field structure in the innermost regions of
relativistic jets is fundamental to shed light on the crucial physical
processes giving rise to the jet formation, as well as to its extraordinary
radiation output up to gamma-ray energies. We study the magnetic field
structure of the quasar CTA 102 with 3 and 7 mm-VLBI polarimetric observations,
reaching an unprecedented resolution (~50 microarcsec). We also investigate the
variability and physical process occurring in the source during the observing
period which coincides with a very active state of the source till
high-energies. The Faraday rotation analysis between 3 and 7mm shows a gradient
in rotation measure with a maximum value of ~6X10^4 rad/m^2 and intrinsic
electric vector position angles (EVPAs) oriented around the centroid of the
core, suggesting the presence of large-scale helical magnetic fields. Such a
magnetic field structure is also visible in 7 mm images when a new superluminal
component is crossing the core region. The 7mm EVPAs orientation is different
when the component is exiting the core or crossing a stationary feature at ~0.1
mas. The interaction between the superluminal component and a recollimation
shock at ~0.1 mas could have triggered the multi-wavelengths flares. The
variability Doppler factor associated with such interaction is large enough to
explain the high energy emission, as we infer from the analysis of gamma-ray
and X-ray data, and it is in agreement with the Doppler factor obtained to
explain the extraordinary optical flare by Raiteri et al.(2017). | astro-ph_HE |
TMRT observations of 26 pulsars at 8.6 GHz: Integrated pulse profiles at 8.6~GHz obtained with the Shanghai Tian Ma Radio
Telescope (TMRT) are presented for a sample of 26 pulsars. Mean flux densities
and pulse width parameters of these pulsars are estimated. For eleven pulsars
these are the first high-frequency observations and for a further four, our
observations have a better signal-to-noise ratio than previous observations.
For one (PSR J0742-2822) the 8.6~GHz profiles differs from previously observed
profiles. A comparison of 19 profiles with those at other frequencies shows
that in nine cases the separation between the outmost leading and trailing
components decreases with frequency, roughly in agreement with
radius-to-frequency mapping, whereas in the other ten the separation is nearly
constant. Different spectral indices of profile components lead to the
variation of integrated pulse profile shapes with frequency. In seven pulsars
with multi-component profiles, the spectral indices of the central components
are steeper than those of the outer components. For the 12 pulsars with
multi-component profiles in the high-frequency sample, we estimate the core
width using gaussian fitting and discuss the width-period relationship. | astro-ph_HE |
X-ray emission from optical novae in M 31: The first supersoft source (SSS) identification with an optical nova in M 31
was based on ROSAT observations. Twenty additional X-ray counterparts (mostly
identified as SSS by their hardness ratios) were detected using archival ROSAT,
XMM-Newton and Chandra observations obtained before July 2002. Based on these
results optical novae seem to constitute the major class of SSS in M 31. An
analysis of archival Chandra HRC-I and ACIS-I observations obtained from July
2004 to February 2005 demonstrated that M 31 nova SSS states lasted from months
to about 10 years. Several novae showed short X-ray outbursts starting within
50 d after the optical outburst and lasting only two to three months. The
fraction of novae detected in soft X-rays within a year after the optical
outburst was more than 30%. Ongoing optical nova monitoring programs, optical
spectral follow-up and an up-to-date nova catalogue are essential for the X-ray
work. Re-analysis of archival nova data to improve positions and find
additional nova candidates are urgently needed for secure recurrent nova
identifications. Dedicated XMM-Newton/Chandra monitoring programs for X-ray
emission from optical novae covering the center area of M 31 continue to
provide interesting new results (e.g. coherent 1105s pulsations in the SSS
counterpart of nova M31N 2007-12b). The SSS light curves of novae allow us -
together with optical information - to estimate the mass of the white dwarf, of
the ejecta and the burned mass in the outburst. Observations of the central
area of M 31 allow us - in contrast to observations in the Galaxy - to monitor
many novae simultaneously and proved to be prone to find many interesting SSS
and nova types. | astro-ph_HE |
Deep Hard X-ray Survey of the Large Magellanic Cloud: Results of the deep survey of the Large Magellanic Cloud (LMC), performed
with the INTEGRAL observatory, are presented. The long exposure (~7 Ms) allowed
us to detect twenty one sources in this sky region: ten belonging to the LMC
itself (7 HMXBs, 2 PSRs, 1 LMXB), six of extragalactic origin and three
belonging to other galaxies from the Local Group - the Milky Way (2 sources)
and Small Magellanic Cloud (1 source). Four new hard X-ray sources of these 21
ones were discovered during the survey in addition to IGR J05414-6858 reported
earlier; two of them were identified with extragalactic objects. We report also
for the first time the detection of a hard X-ray emission from the Crab-like
pulsar PSR J0537-6910 and identification of the hard X-ray source IGR
J05305-6559 with the high-mass X-ray binary EXO 053109-6609. | astro-ph_HE |
Properties of relativistic hot accretion flow around rotating black hole
with radially varying viscosity: We examine the effect of variable viscosity parameter ($\alpha$) in
relativistic, low angular momentum advective accretion flow around rotating
black holes. Following the recent simulation studies of magnetohydrodynamic
disk that reveal the radial variation of $\alpha(r)$, we theoretically
investigate the properties of the global transonic accretion flow considering a
one-dimensional power law prescription of viscosity parameter as $\alpha(r)
\propto r^{\theta}$, where the viscosity exponent $\theta$ is a constant. In
doing so, we adopt the relativistic equation of state and solve the fluid
equations that govern the flow motion inside the disk. We find that depending
on the flow parameters, accretion flow experiences centrifugally supported
shock transition and such shocked accretion solutions continue to exist for
wide ranges of the flow energy, angular momentum, accretion rate and viscosity
exponent, respectively. Due to shock compression, the hot and dense post-shock
flow (hereafter PSC) can produce the high energy radiations after reprocessing
the soft photons from the pre-shock flow via inverse Comptonization. Since PSC
is usually described using shock radius ($r_s$), compression ratio ($R$) and
shock strength ($S$), we study the role of $\theta$ in deciding $r_s$, $R$ and
$S$, respectively. Moreover, we obtain the parameter space for shock and find
that possibility of shock formation diminishes as $\theta$ is increased.
Finally, we compute the limiting value of $\theta$ ($i.e., \theta^{\rm max}$)
that admits shock and find that flow can sustain more viscosity when it
accretes onto rapidly rotating ($a_{\rm k} \rightarrow 1$) black hole in
comparison to weakly rotating ($a_{\rm k} \rightarrow 0$) black hole. | astro-ph_HE |
Fermi-LAT observations of the LIGO/Virgo event GW170817: We present the Fermi Large Area Telescope (LAT) observations of the binary
neutron star merger event GW170817 and the associated short gamma-ray burst
(SGRB) GRB\,170817A detected by the Fermi Gamma-ray Burst Monitor. The LAT was
entering the South Atlantic Anomaly at the time of the LIGO/Virgo trigger
($t_{\rm GW}$) and therefore cannot place constraints on the existence of
high-energy (E $>$ 100 MeV) emission associated with the moment of binary
coalescence. We focus instead on constraining high-energy emission on longer
timescales. No candidate electromagnetic counterpart was detected by the LAT on
timescales of minutes, hours, or days after the LIGO/Virgo detection. The
resulting flux upper bound (at 95\% C.L.\/) from the LAT is
$4.5\times$10$^{-10}$ erg cm$^{-2}$ s$^{-1}$ in the 0.1--1 GeV range covering a
period from T0 + 1153 s to T0 + 2027 s. At the distance of GRB\,170817A, this
flux upper bound corresponds to a luminosity upper bound of 9.7$\times10^{43}$
erg s$^{-1}$, which is 5 orders of magnitude less luminous than the only other
LAT SGRB with known redshift, GRB\,090510. We also discuss the prospects for
LAT detection of electromagnetic counterparts to future gravitational wave
events from Advanced LIGO/Virgo in the context of GW170817/GRB\,170817A. | astro-ph_HE |
Gamma-ray Emitting Narrow-Line Seyfert 1 Galaxies: Past, Present, and
Future: This article reviews our current understanding about $\gamma$-ray detected
narrow-line Seyfert 1 ($\gamma$-NLSy1) galaxies. The detection with the Large
Area Telescope onboard {\it Fermi}~Gamma-ray Space Telescope has provided the
strongest evidence for the presence of closely aligned relativistic jet in
these intriguing active galactic nuclei (AGN) and opened up a realm to explore
the physical conditions needed to launch the jet in a different central engine
and host galaxy environment than that is known for blazars. Promising results
acquired from various multi-wavelength campaigns are converging to a scenario
in which the $\gamma$-NLSy1 galaxies can be considered as `young' blazars.
These enigmatic sources hold the key to unravel the jet triggering mechanism
and evolution of the AGN phase of a galaxy, in general. As such,
$\gamma$-NLSy1s should be considered as one of the top priority targets for
next generation observational facilities. | astro-ph_HE |
Gaia's Detectability of Black Hole-Main Sequence Star Binaries Formed in
Open Clusters: Black hole-main sequence star (BH-MS) binaries are one of the targets of the
future data releases of the astrometric satellite {\it Gaia}. They are supposed
to be formed in two main sites: a galactic field and star clusters. However,
previous work has never predicted the number of BH-MS binaries originating in
the latter site. In this paper, we estimate the number of BH-MS binaries formed
in open clusters and detectable with {\it Gaia} based on the results of {\it
N}-body simulations. By considering interstellar extinction in the Milky Way
(MW) and observational constraints, we predict $\sim 10$ BH-MS binaries are
observable. We also find that chemical abundance patterns of companion MSs will
help us to identify the origin of the binaries as star clusters. Such MSs are
not polluted by outflows of the BH progenitors, such as stellar winds and
supernova ejecta. Chemical anomalies might be a good test to confirm the origin
of binaries with relatively less massive MSs ($\lesssim 5M_{\odot}$), orbital
periods ($\sim 1.5\;$year) and higher eccentricities ($e \gtrsim 0.1$). | astro-ph_HE |
Discovery of a ~5 day characteristic timescale in the Kepler power
spectrum of Zw 229-15: We present time series analyses of the full Kepler dataset of Zw 229-15. This
Kepler light curve --- with a baseline greater than three years, composed of
virtually continuous, evenly sampled 30-minute measurements --- is
unprecedented in its quality and precision. We utilize two methods of power
spectral analysis to investigate the optical variability and search for
evidence of a bend frequency associated with a characteristic optical
variability timescale. Each method yields similar results. The first
interpolates across data gaps to use the standard Fourier periodogram. The
second, using the CARMA-based time-domain modeling technique of Kelly et al.
(2014), does not need evenly-sampled data. Both methods find excess power at
high frequencies that may be due to Kepler instrumental effects. More
importantly both also show strong bends ({\Delta}{\alpha} ~ 2) at timescales of
~5 days, a feature similar to those seen in the X-ray PSDs of AGN but never
before in the optical. This observed ~5 day timescale may be associated with
one of several physical processes potentially responsible for the variability.
A plausible association could be made with light-crossing, dynamical or thermal
timescales, depending on the assumed value of the accretion disk size and on
unobserved disk parameters such as {\alpha} and H/R. This timescale is not
consistent with the viscous timescale, which would be years in a ~10^7 Solar
mass AGN such as Zw 229-15. However there must be a second bend on long (>~1
year) timescales, and that feature could be associated with the viscous
timescale. | astro-ph_HE |
The population properties of spinning black holes using
Gravitational-wave Transient Catalog 3: Binary black holes formed via different pathways are predicted to have
distinct spin properties. Measuring these properties with gravitational waves
provides an opportunity to unveil the origins of binary black holes. Recent
work draws conflicting conclusions regarding the spin distribution observed by
LIGO--Virgo--KAGRA (LVK). Some analyses suggest that a fraction of the observed
black-hole spin vectors are significantly misaligned (by $>90^\circ$) relative
to the orbital angular momentum. This has been interpreted to mean that some
binaries in the LVK dataset are assembled dynamically in dense stellar
environments. Other analyses find support for a sub-population of binaries with
negligible spin and no evidence for significantly misaligned spin -- a result
consistent with the field formation scenario. In this work, we study the spin
properties of binary black holes in the third LVK gravitational-wave transient
catalog. We find that there is insufficient data to resolve the existence of a
sub-population of binaries with negligible black-hole spin (the presence of
this sub-population is supported by a modest Bayes factor of 1.7). We find
modest support for the existence of mergers with extreme spin tilt angles $>
90^\circ$ (the presence of extreme-tilt binaries is favored by a Bayes factor
of 10.1). Only one thing is clear: at least some of the LVK binaries formed in
the field. At most $89\%$ of binaries are assembled dynamically (99\%
credibility), though, the true branching fraction could be much lower, even
negligible. | astro-ph_HE |
Taking a break: paused accretion in the symbiotic binary RT Cru: Symbiotic binaries sometimes hide their symbiotic nature for significant
periods of time. There is mounting observational evidence that in those
symbiotics that are powered solely by accretion of red-giant's wind material
onto a white dwarf, without any quasi-steady shell burning on the surface of
the white dwarf, the characteristic emission lines in the optical spectrum can
vanish, leaving the semblance of an isolated red giant spectrum. Here we
present compelling evidence that this disappearance of optical emission lines
from the spectrum of RT Cru during 2019 was due to a decrease in the accretion
rate, which we derive by modeling the X-ray spectrum. This drop in accretion
rate leads to a lower flux of ionizing photons and thus to faint/absent
photoionization emission lines in the optical spectrum. We observed the white
dwarf symbiotic RT Cru with XMM-Newton and Swift in X-rays and UV and collected
ground-based optical spectra and photometry over the last 33 years. This
long-term coverage shows that during most of the year 2019, the accretion rate
onto the white dwarf was so low, $\dot{M}= (3.2\pm 0.06)\, \times$10$^{-11}$
$M_{\odot}$ yr$^{-1}$ (d/2.52 kpc)$^2$, that the historically detected hard
X-ray emission almost vanished, the UV flux faded by roughly 5 magnitudes, the
$U$, $B$ and $V$ flickering amplitude decreased, and the Balmer lines virtually
disappeared from January through March 2019. Long-lasting low-accretion
episodes as the one reported here may hamper the chances of RT Cru experiencing
nova-type outburst despite the high-mass of the accreting white dwarf. | astro-ph_HE |
A QPO in NGC 4945 from Archival RXTE Data: We report the discovery of a ~6-week quasi-periodic oscillation (QPO) in
archival NGC 4945 data observed by the Rossi X-ray Timing Explorer (RXTE)
satellite. QPOs are an important observable in accretion disks and have been
studied extensively in both neutron star (NS) and black hole (BH) X-ray
binaries (XRB). QPOs should be present in Active Galactic Nuclei (AGN) if
galactic black holes and supermassive black holes (SMBH) are governed by a
common set of physical processes. The search for QPOs in AGN has proven
difficult because the timescales would be much longer, due to their higher
mass. RXTE AGN light curves spanning 1996 to 2011 provide an excellent and
perhaps unique opportunity to search for low-frequency QPOs. We investigated
the 533 RXTE observations made of the Seyfert-2 AGN, NGC 4945. During a large
cluster of observations in 2006-2007 (194 observations, spanning 396 days), the
Lomb-Scargle periodogram shows a candidate QPO at 0.274 $\mu$Hz (period
$\approx$ 42.2 days). We estimate the uncertainties using the False Alarm
Probability (FAP). We discuss the possible identification of this feature with
the Lense-Thirring precession period. | astro-ph_HE |
International X-ray Observatory (IXO) Assessment Study Report for the
ESA Cosmic Vision 2015-2025: The International X-Ray Observatory (IXO) will address fundamental questions
in astrophysics, including "When did the first SMBH form? How does large scale
structure evolve? What happens close to a black hole? What is the connection
between these processes? What is the equation of state of matter at
supra-nuclear density?" This report presents an overview of the assessment
study phase of the IXO candidate ESA L-class Cosmic Vision mission. We provide
a description of the IXO science objectives, the mission implementation and the
payload. The performance will offer more than an order of magnitude improvement
in capability compared with Chandra and XMM-Newton. This observatory-class
facility comprises a telescope with highly nested grazing incidence optics with
a performance requirement of 2.5 sq.m. of effective area at 1.25 keV with a 5"
PSF. There is an instrument complement that provides capabilities in imaging,
spatially resolved spectroscopy, timing, polarimetry and high resolution
dispersive spectroscopy. Since earlier submissions to the Astro2010 Decadal
Survey, substantial technological progress has been made, particularly in the
mirror development. Risk reduction measures and important programmatic choices
have also been identified. An independent internal Technical and Programmatic
Review has also been carried out by ESA, concluding with positive
recommendations. Subject to successful conclusion of agreements between the
partner space agencies, IXO is fully ready to proceed to further definition,
moving towards an eventual launch in 2021-2022. | astro-ph_HE |
High-energy cosmic particles: A review of the status of the knowledge in the field of High-energy cosmic
particles is presented. The spectrum, arrival direction distribution and
composition measurements are summarized, together with some implications for
the understanding of the cosmic ray origin and their propagation. Special
emphasis is put in the ultra-high energy range, corresponding to particles of
extragalactic origin. | astro-ph_HE |
Impact of initial mass functions on the dynamical channel of
gravitational wave sources: Dynamically formed black hole (BH) binaries (BBHs) are important sources of
gravitational waves (GWs). Globular clusters (GCs) provide a major environment
to produce such BBHs, but the total mass of the known GCs is small compared to
that in the Galaxy; thus, the fraction of BBHs formed in GCs is also small.
However, this assumes that GCs contain a canonical initial mass function (IMF)
similar to that of field stars. This might not be true because several studies
suggest that extreme dense and metal-poor environment can result in top-heavy
IMFs, where GCs may originate. Although GCs with top-heavy IMFs were easily
disrupted or have become dark clusters, the contribution to the GW sources can
be significant. Using a high-performance and accurate $N$-body code,
\textsc{petar}, we investigate the effect of varying IMFs by carrying out four
star-by-star simulations of dense GCs with the initial mass of $5\times10^5
M_\odot$ and the half-mass radius of $2$~pc. We find that the BBH merger rate
does not monotonically correlate with the slope of IMFs. Due to a rapid
expansion, top-heavy IMFs lead to less efficient formation of merging BBHs. The
formation rate continuously decreases as the cluster expands because of the
dynamical heating caused by BHs. However, in star clusters with a top-heavier
IMF, the total number of BHs is larger, and therefore, the final contribution
to merging BBHs can still be more than from clusters with the standard IMF, if
the initial cluster mass and density is higher than those used in our model. | astro-ph_HE |
The impact of Faraday effects on polarized black hole images of
Sagittarius A*: We study model images and polarization maps of Sagittarius A* at 230 GHz. We
post-process GRMHD simulations and perform a fully relativistic radiative
transfer calculation of the emitted synchrotron radiation to obtain polarized
images for a range of mass accretion rates and electron temperatures. At low
accretion rates, the polarization map traces the underlying toroidal magnetic
field geometry. At high accretion rates, we find that Faraday rotation internal
to the emission region can depolarize and scramble the map. We measure the net
linear polarization fraction and find that high accretion rate "jet-disc"
models are heavily depolarized and are therefore disfavoured. We show how Event
Horizon Telescope measurements of the polarized "correlation length" over the
image provide a model-independent upper limit on the strength of these Faraday
effects, and constrain plasma properties like the electron temperature and
magnetic field strength. | astro-ph_HE |
On radiative acceleration in spine-sheath structured blazar jets: It has been proposed that blazar jets are structured, with a fast spine
surrounded by a slower sheath or layer. This structured jet model explains some
properties of their emission and morphology. Because of their relative motion,
the radiation produced by one component is seen amplified by the other, thus
enhancing the inverse Compton emission of both. Radiation is emitted
anisotropically in the comoving frames, and causes the emitting plasma to
recoil. As seen in the observer frame, this corresponds to a deceleration of
the fastest component (the spine) and an acceleration of the slower one (the
layer). While the deceleration of the spine has already been investigated, here
we study for the first time the acceleration of the sheath and find
self-consistent velocity profile solutions for both the spine and the sheath
while accounting for radiative cooling. We find that the sheath can be
accelerated to the velocities required by the observations if its leptons
remain energetic in the acceleration region, assumed to be of the order of 100
Schwarzschild radii, demanding continuous injection of energetic particles in
that region. | astro-ph_HE |
Population Synthesis of Isolated Neutron Stars with magneto-rotational
evolution II: from radio-pulsars to magnetars: Population synthesis studies constitute a powerful method to reconstruct the
birth distribution of periods and magnetic fields of the pulsar population.
When this method is applied to populations in different wavelengths, it can
break the degeneracy in the inferred properties of initial distributions that
arises from single-band studies. In this context, we extend previous works to
include $X$-ray thermal emitting pulsars within the same evolutionary model as
radio-pulsars. We find that the cumulative distribution of the number of X-ray
pulsars can be well reproduced by several models that, simultaneously,
reproduce the characteristics of the radio-pulsar distribution. However, even
considering the most favourable magneto-thermal evolution models with fast
field decay, log-normal distributions of the initial magnetic field
over-predict the number of visible sources with periods longer than 12 s. We
then show that the problem can be solved with different distributions of
magnetic field, such as a truncated log-normal distribution, or a binormal
distribution with two distinct populations. We use the observational lack of
isolated NSs with spin periods P>12 s to establish an upper limit to the
fraction of magnetars born with B > 10^{15} G (less than 1\%). As future
detections keep increasing the magnetar and high-B pulsar statistics, our
approach can be used to establish a severe constraint on the maximum magnetic
field at birth of NSs. | astro-ph_HE |
Modelling the disk atmosphere of the low mass X-ray binary EXO 0748-676: Low mass X-ray binaries exhibit ionized emission from an extended disk
atmosphere that surrounds the accretion disk. However, its nature and geometry
is still unclear. In this work we present a spectral analysis of the extended
atmosphere of EXO 0748-676 using high-resolution spectra from archival
XMM-Newton observations. We model the RGS spectrum that is obtained during the
eclipses. This enables us to model the emission lines that come only from the
extended atmosphere of the source, and study its physical structure and
properties. The RGS spectrum reveals a series of emission lines consistent with
transitions of O VIII, O VII, Ne IX and N VII. We perform both Gaussian line
fitting and photoionization modelling. Our results suggest that there are two
photoionization gas components, out of pressure equilibrium with respect to
each other. One with ionization parameter of 2.5 and a large opening angle, and
one with 1.3. The second component is possibly covering a smaller fraction of
the source. From the density diagnostics of the O vii triplet using
photoionization modelling, we detect a rather high density plasma of > 10^13
cm^-3 for the lower ionization component. This latter component also displays
an inflow velocity. We propose a scenario where the high ionization component
constitutes an extended upper atmosphere of the accretion disk. The lower
ionization component may instead be a clumpy gas created from the impact of the
accretion stream with the disk. | astro-ph_HE |
A new possibility of the fast neutrino-flavor conversion in the
pre-shock region of core-collapse supernova: We make a strong case that the fast neutrino-flavor conversion, one of the
collective flavor oscillation modes, commonly occurs in core-collapse
supernovae (CCSNe). It is confirmed in the numerical data obtained in realistic
simulations of CCSNe but the argument is much more generic and applicable
universally: the coherent neutrino-nucleus scattering makes the electron lepton
number (ELN) change signs at some inward direction and trigger the flavor
conversion in the outward direction in the pre-shock region. Although the ELN
crossing is tiny and that is why it has eluded recognition so far, it is still
large enough to induce the flavor conversion. Our findings will have an
important observational consequences for CCSNe neutrinos. | astro-ph_HE |
Pre-burst neutrinos of gamma-ray bursters accompanied by high-energy
photons: Previous researches on high-energy neutrino events from gamma-ray bursters
(GRBs) suggest a neutrino speed variation $v(E)=c(1\pm
E/E^{\nu}_{\mathrm{LV}})$ with ${E}^{\nu}_{\rm LV}=(6.4\pm 1.5)\times10^{17}~{
\rm GeV}$, together with an intrinsic time difference ${\Delta {t}_{\rm
in}=(-2.8\pm 0.7)\times10^2~{\rm s}}$, which means that high-energy neutrinos
come out about 300~s earlier than low-energy photons in the source reference
system. Considering the possibility that pre-bursts of neutrinos may be
accompanied by high-energy photons, in this work we search for high-energy
photon events with earlier emission time from 100 to 1000~s before low-energy
photons at source by analyzing Fermi Gamma-ray Space Telescope (FGST) data. We
perform the searching of photon events with energies larger than 100~MeV, and
find 14 events from 48 GRBs with known redshifts. Combining these events with a
$1.07~\rm{TeV}$ photon event observed by the Major Atmospheric Gamma Imaging
Cherenkov telescopes (MAGIC), we suggest a pre-burst stage with a long duration
period of several minutes of high energy neutrino emissions accompanied by high
energy photons at the GRB source. | astro-ph_HE |
Long term study of the light curve of PKS 1510-089 in GeV energies: We have analyzed data from the Flat Spectrum Radio Quasar PKS 1510-089
collected over a period of 8 years from August 2008 to December 2016 with the
Fermi-LAT. We have identified several flares of this highly variable source,
studied their temporal and spectral properties in detail and compared with
previous works on flares of PKS 1510-089. Five major flares and few
sub-flares/sub-structures have been identified in our study. The fastest
variability time is found to be 1.30$\pm$0.18 hr between MJD 55852.063 and
55852.188 where we estimate the minimum size of the emission region to be $4.85
\times 10^{15}$ cm. In most of the flares the spectral energy distributions are
better fitted with Logparabolic distribution compared to simple Power law or
Power law with exponential cut-offs. This has strong physics implications
regarding the nature of the high energy gamma-ray emission region. | astro-ph_HE |
Radio impulsive events in quiet solar corona and Axion Quark Nugget Dark
Matter: The Murchison Widefield Array (MWA) recorded \cite{Mondal-2020} impulsive
radio events in the quiet solar corona at frequencies 98, 120, 132, and 160
MHz. We propose that these radio events are the direct manifestation of dark
matter annihilation events within the axion quark nugget (AQN) framework. It
has been argued \cite{Zhitnitsky:2017rop,Raza:2018gpb} that the AQN
annihilation events in the quiet solar corona can be identified with the
nanoflares conjectured by Parker \cite{Parker-1983}. We further support this
claim by demonstrating that observed impulsive radio events \cite{Mondal-2020},
including their rate of appearance, their temporal and spatial distributions
and their energetics, are matching the generic consequences of AQN
annihilations in the quiet corona. We propose to test this idea by analyzing
the correlated clustering of impulsive radio events in different frequency
bands. These correlations are expressed in terms of the time delays between
radio events in different frequency bands, measured in seconds. We also make
generic predictions for low (80 and 89 MHz) and high (179, 196, 217 and 240
MHz) frequency bands, that have been recorded, but not published, by
\cite{Mondal-2020}. We finally suggest to test our proposal by studying
possible cross-correlation between MWA radio signals and Solar Orbiter
recording of extreme UV photons (a.k.a. "campfires"). | astro-ph_HE |
A high-resolution view of the jets in 3C 465: We present new high-resolution and high-sensitivity studies of the jets in
the WAT source 3C 465, using deep transverse-resolved radio observations from
e-MERLIN, and with complementary observations from the VLA. We derive a lower
limit $\beta_{\rm j}$ = ($\nu_{\rm j}$/$c$) $\gtrsim$ 0.5 for the jet speed,
and an upper limit $\theta_{\rm j}$ $\lesssim$ 61$^{\circ}$ for the jet angle
to the line of sight. The jet spectral index ($\alpha$, defined in the sense $S
\propto \nu^{\alpha}$) is fairly constant (<$\alpha_{\rm jet}$> = $-$0.7), and
spectral flattening within 4.4 kpc of the core coincides with bright knots and
is consistent with the site of X-ray particle acceleration at the base of the
radio jet found in previous studies. There is little difference between the
spectra of the two hotspot components, plausibly indicating that electron
populations of the same properties are injected there. The NW and SE plumes are
approximately homologous structures, with variations in mass injection and
propagation in external pressure and density gradients in the two regions
plausibly accounting for the slightly steeper spectrum in the NW plume,
<$\alpha_{\rm NWp}$> = $-$1.43 compared with the SE plume, <$\alpha_{\rm SEp}$>
= $-$1.38. Our synchrotron lifetime model supports plausible reacceleration of
particles within the plume materials. Overall, our results show that the
first-order Fermi process at mildly relativistic and non-relativistic shocks is
the most likely acceleration mechanism at play in 3C 465 and distinguish
differences between the acceleration at $\beta_{\rm j}$ $>$ 0.5 and $\beta_{\rm
j}$ $<$ 0.5. The former case can accelerate electrons to higher Lorentz
factors. | astro-ph_HE |
GRB 090510: a genuine short-GRB from a binary neutron star coalescing
into a Kerr-Newman black hole: In a new classification of merging binary neutron stars (NSs) we separate
short gamma-ray bursts (GRBs) in two sub-classes. The ones with
$E_{iso}\lesssim10^{52}$ erg coalesce to form a massive NS and are indicated as
short gamma-ray flashes (S-GRFs). The hardest, with $E_{iso}\gtrsim10^{52}$
erg, coalesce to form a black hole (BH) and are indicated as genuine short-GRBs
(S-GRBs). Within the fireshell model, S-GRBs exhibit three different
components: the P-GRB emission, observed at the transparency of a
self-accelerating baryon-$e^+e^-$ plasma; the prompt emission, originating from
the interaction of the accelerated baryons with the circumburst medium; the
high-energy (GeV) emission, observed after the P-GRB and indicating the
formation of a BH. GRB 090510 gives the first evidence for the formation of a
Kerr BH or, possibly, a Kerr-Newman BH. Its P-GRB spectrum can be fitted by a
convolution of thermal spectra whose origin can be traced back to an axially
symmetric dyadotorus. A large value of the angular momentum of the newborn BH
is consistent with the large energetics of this S-GRB, which reach in the
1--10000 keV range $E_{iso}=(3.95\pm0.21)\times10^{52}$ erg and in the 0.1--100
GeV range $E_{LAT}=(5.78\pm0.60)\times10^{52}$ erg, the most energetic GeV
emission ever observed in S-GRBs. The theoretical redshift $z_{th}=0.75\pm0.17$
that we derive from the fireshell theory is consistent with the spectroscopic
measurement $z=0.903\pm0.003$, showing the self-consistency of the theoretical
approach. All S-GRBs exhibit GeV emission, when inside the Fermi-LAT field of
view, unlike S-GRFs, which never evidence it. The GeV emission appears to be
the discriminant for the formation of a BH in GRBs, confirmed by their observed
overall energetics. | astro-ph_HE |
Dependence of the LMXB population on stellar age: We investigate the dependence of the low-mass X-ray binary (LMXB) population
in early-type galaxies on stellar age, by selecting 20 massive nearby
early-type galaxies from the Chandra archive occupying a relatively narrow
range of masses and spanning a broad range of ages, from 1.6 Gyr to more than
10 Gyrs, with the median value of 6 Gyrs. With the ~ 2000 X-ray point sources
detected in total, we correlated the specific number of LMXBs in each galaxy
with its stellar age and globular cluster (GC) content. We found a correlation
between the LMXB population and stellar age: older galaxies tend to possess
about ~50% more LMXBs (per unit stellar mass) than the younger ones. The
interpretation of this dependence is complicated by large scatter and a rather
strong correlation between stellar age and GC content of galaxies in our
sample. We present evidence suggesting that the more important factor may be
the evolution of the LMXB population with time. Its effect is further amplified
by the larger GC content of older galaxies and correspondingly, the larger
numbers of dynamically formed binaries in them. We also found clear evolution
of the X-ray luminosity function (XLF) with age, that younger galaxies have
more bright sources and fewer faint sources per unit stellar mass. The XLF of
LMXBs in younger galaxies appears to extend significantly beyond E39 erg/s.
Such bright sources seem to be less frequent in older galaxies. We found that 6
out of ~ 12 (ultra-) luminous sources are located in GCs. | astro-ph_HE |
Excitation of Trapped Waves in Simulations of Tilted Black Hole
Accretion Disks with Magnetorotational Turbulence: We analyze the time dependence of fluid variables in general relativistic,
magnetohydrodynamic simulations of accretion flows onto a black hole with
dimensionless spin parameter a/M=0.9. We consider both the case where the
angular momentum of the accretion material is aligned with the black hole spin
axis (an untilted flow) and where it is misaligned by 15 degrees (a tilted
flow). In comparison to the untilted simulation, the tilted simulation exhibits
a clear excess of inertial variability, that is, variability at frequencies
below the local radial epicyclic frequency. We further study the radial
structure of this inertial-like power by focusing on a radially extended band
at 118 (M/10Msol)^-1 Hz found in each of the three analyzed fluid variables.
The three dimensional density structure at this frequency suggests that the
power is a composite oscillation whose dominant components are an over dense
clump corotating with the background flow, a low order inertial wave, and a low
order inertial-acoustic wave. Our results provide preliminary confirmation of
earlier suggestions that disk tilt can be an important excitation mechanism for
inertial waves. | astro-ph_HE |
Energy function, formation rate and low-metallicity environment of fast
radio bursts: In this paper, we investigate the energy function, formation rate and
environment of fast radio bursts (FRBs) using Parkes sample and Australian
Square Kilometer Array Pathfinder (ASKAP) sample. For the first time, the
metallicity effect on the formation rate is considered. If FRBs are produced by
the mergers of compact binaries, the formation rate of FRBs should have a time
delay relative to cosmic star formation rate (CSFR). We get the time delay is
about 3-5 Gyr and the index of differential energy function $\gamma$
($dN/dE\propto E^{-\gamma}$) is between 1.6 and 2.0 from redshift cumulative
distribution. The value of $\gamma$ is similar to that of FRB 121102, which
indicates single bursts may share the same physical mechanism with the
repeaters. In another case, if the formation rate of FRB is proportional to the
SFR without time delay, the index $\gamma$ is about 2.3. In both cases, we find
that FRBs may prefer to occur in low-metallicity environment with $ 12
+\log(\rm{O/H}) \simeq 8.40$, which is similar to those of long gamma-ray
bursts (GRBs) and hydrogen-poor superluminous supernovae (SLSNe-I). | astro-ph_HE |
The discovery of a rotating radio transient J1918$-$0449 with intriguing
emission properties with the five hundred meter aperture spherical radio
telescope: In this study, we report on a detailed single pulse analysis of the radio
emission from a rotating radio transient (RRAT) J1918$-$0449 which is the first
RRAT discovered with the five hundred meter aperture spherical radio telescope
(FAST). The sensitive observations were carried out on 30 April 2021 using the
FAST with a central frequency of 1250 MHz and a short time resolution of 49.152
$\mu$s, which forms a reliable basis to probe single pulse emission properties
in detail. The source was successively observed for around 2 hours. A total of
83 dispersed bursts with significance above 6$\sigma$ are detected over 1.8
hours. The source's DM and rotational period are determined to be 116.1$\pm$0.4
\pcm \ and 2479.21$\pm$0.03 ms, respectively. The share of registered pulses
from the total number of observed period is 3.12\%. No underlying emission is
detected in the averaged off pulse profile. For bursts with fluence larger than
10 Jy ms, the pulse energy follows a power-law distribution with an index of
$-3.1\pm0.4$, suggesting the existence of bright pulse emission. We find that
the distribution of time between subsequent pulses is consistent with a
stationary Poisson process and find no evidence of clustering over the 1.8 h
observations, giving a mean burst rate of one burst every 66 s. Close
inspection of the detected bright pulses reveals that 21 pulses exhibit
well-defined quasi-periodicities. The subpulse drifting is present in
non-successive rotations with periodicity of $2.51\pm0.06$ periods. Finally,
possible physical mechanisms are discussed. | astro-ph_HE |
UHECR: Signatures and Models: The signatures of Ultra High Energy (E >1 EeV) proton propagation through CMB
radiation are pair-production dip and GZK cutoff. The visible characteristics
of these two spectral features are ankle, which is intrinsic part of the dip,
beginning of GZK cutoff in the differential spectrum and E_{1/2} in integral
spectrum. Measured by HiRes and Telescope Array (TA) these characteristics
agree with theoretical predictions. However, directly measured mass composition
remains a puzzle. While HiRes and TA detectors observe the proton dominated
mass composition, the data of Auger detector strongly evidence for nuclei mass
composition becoming progressively heavier at energy higher than 4 EeV and
reaching Iron at energy about 35 EeV. The models based on the Auger and
HiRes/TA data are considered independently and classified using the transition
from galactic to extragalactic cosmic rays. The ankle cannot provide this
transition. since data of all three detector at energy (1 - 3) EeV agree with
pure proton composition (or at least not heavier than Helium). If produced in
Galaxy these particles result in too high anisotropy. This argument excludes or
strongly disfavours all ankle models with ankle energy E_a > 3 EeV. The
calculation of elongation curves, X_{\max}(E), for different ankle models
strengthens further this conclusion. Status of other models, the dip, mixed
composition and Auger based models are discussed. | astro-ph_HE |
Effective potential energy in St\ormer's problem for an inclined
rotating magnetic dipole: We discuss the dynamics of a charged nonrelativistic particle in
electromagnetic field of a rotating magnetized celestial body. The equations of
motion of the particle are obtained and some particular solutions are found.
Effective potential energy is defined on the base of the first constant of
motion. Regions accessible and inaccessible for a charged particle motion are
studied and depicted for different values of a constant of motion. | astro-ph_HE |
XMM-Newton and Swift observations of XTE J1743-363: XTEJ1743-363 is a poorly known hard X-ray transient, that displays short and
intense flares similar to those observed from Supergiant Fast X-ray Transients.
The probable optical counterpart shows spectral properties similar to those of
an M8 III giant, thus suggesting that XTEJ1743-363 belongs to the class of the
Symbiotic X-ray Binaries. In this paper we report on the first dedicated
monitoring campaign of the source in the soft X-ray range with XMM-Newton and
Swift/XRT. T hese observations confirmed the association of XTEJ1743-363 with
the previously suggested M8 III giant and the classification of the source as a
member of the Symbiotic X-ray binaries. In the soft X-ray domain, XTEJ1743-363
displays a high absorption (~6x10^22 cm^-2 ) and variability on time scales of
hundreds to few thousand seconds, typical of wind accreting systems. A
relatively faint flare (peak X-ray flux 3x10^-11 erg/cm^2/s) lasting ~4 ks is
recorded during the XMM-Newton observation and interpreted in terms of the wind
accretion scenario. | astro-ph_HE |
Evidence for simultaneous jets and disk winds in luminous low-mass X-ray
binaries: Recent work on jets and disk winds in low-mass X-ray binaries (LMXBs)
suggests that they are to a large extent mutually exclusive, with jets observed
in spectrally hard states and disk winds observed in spectrally soft states. In
this paper we use existing literature on jets and disk winds in the luminous
neutron star (NS) LMXB GX 13+1, in combination with archival Rossi X-ray Timing
Explorer data, to show that this source is likely able to produce jets and disk
winds simultaneously. We find that jets and disk winds occur in the same
location on the source's track in its X-ray color-color diagram. A further
study of literature on other luminous LMXBs reveals that this behavior is more
common, with indications for simultaneous jets and disk winds in the black hole
LMXBs V404 Cyg and GRS 1915+105 and the NS LMXBs Sco X-1 and Cir X-1. For the
three sources for which we have the necessary spectral information, we find
that the simultaneous jets/winds all occur in their spectrally hardest states.
Our findings indicate that in LMXBs with luminosities above a few tens of
percent of the Eddington luminosity, jets and disk winds are not mutually
exclusive, and that the presence of disk winds does not necessarily result in
jet suppression. | astro-ph_HE |
Gamma-Ray Bursts at high and very high energies: Gamma-Ray Bursts (GRBs) are extra-galactic and extremely energetic transient
emissions of gamma rays, which are thought to be associated with the death of
massive stars or the merger of compact objects in binary systems. Their huge
luminosities involve the presence a newborn stellar-mass black hole emitting a
relativistic collimated outflow, which accelerates particles and produces
non-thermal emissions from the radio domain to the highest energies. In this
article, I review recent progresses in the understanding of GRB jet physics
above 100 MeV, based on Fermi observations of bright GRBs. I discuss the
physical implications of these observations and their impact on GRB modeling,
and I present some prospects for GRB observation at very high energies in the
near future. | astro-ph_HE |
Constraints and prospects on gravitational wave and neutrino emission
using GW150914: The recent LIGO observation of gravitational waves from a binary black hole
merger triggered several follow-up searches from both electromagnetic wave as
well as neutrino observatories. Since in general, it is expected that all
matter has been removed from the binary black hole environment long before the
merger, no neutrino emission is expected from such mergers. Still, it remains
interesting to test this hypothesis. The ratio of the energy emitted in
neutrinos with respect to gravitational waves represents a useful parameter to
constrain the environment of such astrophysical events. In addition to putting
constraints by use of the non-detection of counterpart neutrinos, it is also
possible to consider the diffuse neutrino flux measured by the IceCube
observatory as the maximum contribution from an extrapolated full class of
BBHs. Both methods currently lead to similar bounds on the fraction of energy
that can be emitted in neutrinos. Nevertheless, combining both methods should
allow to strongly constrain the source population in case of a future neutrino
counterpart detection. The proposed approach can and will be applied to
potential upcoming LIGO events, including binary neutron stars and black
hole-neutron star mergers, for which a neutrino counterpart is expected. | astro-ph_HE |
Self Organized Criticality in an one dimensional magnetized grid.
Application to GRB X-ray afterglows: A simplified one dimensional grid is used to model the evolution of
magnetized plasma flow. We implement diffusion laws similar to those so-far
used to model magnetic reconnection with Cellular Automata. As a novelty, we
also explicitly superimpose a background flow. The aim is to numerically
investigate the possibility that Self-Organized Criticality appears in a one
dimensional magnetized flow. The cellular automaton's cells store information
about the parameter relevant to the evolution of the system being modelled.
Under the assumption that this parameter stands for the magnetic field, the
magnetic energy released by one volume during one individual relaxation event
is also computed. Our results show that indeed in this system Self-Organized
Criticality is established. The possible applications of this model to the
study of the X-ray afterglows of GRBs is also briefly considered. | astro-ph_HE |
Fluid Instabilities of Magnetar-Powered Supernovae: Magnetar-powered supernova explosions are competitive models for explaining
very luminous optical transits. However, these explosion models were mainly
calculated in 1D. Radiation emitted from the magnetar snowplows into the
previous supernovae ejecta and causes a nonphysical dense shell (spike) found
in previous 1D studies. This suggests that strong fluid instabilities may have
developed within the magnetar-powered supernovae. Such fluid instabilities
emerge at the region where luminous transits later occur, so they can affect
the consequent observational signatures. We examine the magnetar-powered
supernovae with 2D hydrodynamics simulations and find that the 1D dense shell
transforms into the development of Rayleigh-Taylor and thin shell instabilities
in 2D. The resulting mixing is able to fragment the entire shell and break the
spherical symmetry of supernovae ejecta. | astro-ph_HE |
High-Energy gamma-ray Astronomy and String Theory: There have been observations, first from the MAGIC Telescope (July 2005) and
quite recently (September 2008) from the FERMI Satellite Telescope, on
non-simultaneous arrival of high-energy photons from distant celestial sources.
In each case, the highest energy photons were delayed, as compared to their
lower-energy counterparts. Although the astrophysics at the source of these
energetic photons is still not understood, and such non simultaneous arrival
might be due to non simultaneous emission as a result of conventional physics
effects, nevertheless, rather surprisingly, the observed time delays can also
fit excellently some scenarios in quantum gravity, predicting Lorentz violating
space-time "foam" backgrounds with a non-trivial subluminal vacuum refractive
index suppressed linearly by a quantum gravity scale of the order of the
reduced Planck mass. In this pedagogical talk, I discuss the MAGIC and FERMI
findings in this context and I argue on a theoretical model of space-time foam
in string/brane theory that can accommodate the findings of those experiments
in agreement with all other stringent tests of Lorentz invariance. However, I
stress the current ambiguities/uncertainties on the source mechanisms, which
need to be resolved first before definite conclusions are reached regarding
quantum gravity foam scenarios. | astro-ph_HE |
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