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Directional association of TeV to PeV astrophysical neutrinos with radio
blazars: Recently we have shown that high-energy neutrinos above 200 TeV detected by
IceCube are produced within several parsecs in the central regions of
radio-bright blazars, that is active galactic nuclei with jets pointing towards
us. To independently test this result and extend the analysis to a wider energy
range, we use public data for all neutrino energies from seven years of IceCube
observations. The IceCube point-source likelihood map is analyzed against the
positions of blazars from a statistically complete sample selected by their
compact radio flux density. The latter analysis delivers a 3.0 sigma
significance with the combined post-trial significance of both studies being
4.1 sigma. The correlation is driven by a large number of blazars. Together
with fainter but physically similar sources not included in the sample, they
may explain the entire IceCube astrophysical neutrino flux as derived from
muon-track analyses. The neutrinos can be produced in interactions of
relativistic protons with X-ray self-Compton photons in parsec-scale blazar
jets. | astro-ph_HE |
Lunar gamma ray emission seen during the first year by Fermi: We report the detection of the lunar gamma-ray emission during the first year
of Fermi-LAT observations. Such emission is produced by cosmic ray nuclei
interacting with the lunar surface. Thanks to the solar minimum conditions and
the reduced effects of heliospheric modulation, the lunar flux was at its
maximum due to the increased flux of Galactic cosmic rays hitting the lunar
surface. Fermi-LAT instrument has a superior sensitivity, angular resolution,
and observes the whole sky every two orbits. It is the only gamma-ray mission
capable of detecting the lunar emission with high confidence and to monitor it
over the full 24th solar cycle. We also report the status of a search of the
gamma-ray emission from major planets and asteroid populations in the ecliptic
plane. | astro-ph_HE |
XMM-Newton Observes the Intrabinary Shock of PSR J1959+2048: In a multi-orbit (100 ks) $\mathrm{\it XMM-Newton}$ exposure of the original
black widow pulsar, PSR J1959+2048, we measure the strong orbital modulation
caused by intrabinary shock (IBS) emission. The IBS light curve peak appears
asymmetric, which we attribute to sweep-back effects in the companion wind. We
also see evidence for an X-ray eclipse by the companion and its wind. Together
with the IBS fit, this supports an edge-on $i\sim 90^\circ$ view of the system
and a modest $\sim 1.8M_\odot$ mass for the recycled pulsar. Our IBS fit
parameters imply a wind flux that, if persistent, would evaporated the
companion within a few Gyr. | astro-ph_HE |
Optical quasi-periodic oscillation and color behavior of blazar PKS
2155-304: PKS 2155-304 is a well studied BL Lac object in the southern sky. The
historical optical data during different period have been collected and
compiled. Light curves with a time span of 35 years have been constructed. The
R-band light curve has been analyzed by means of three methods: epoch folding
method, Jurkevich method and discrete correlation function (DCF) method. It is
derived that there is an evident periodic component of 317 days (i.e. 0.87 yr)
superposed on a long-term trend with large-amplitude variation in the light
curve. The variability of this source is accompanied by a slight color
variation, and the brightness and color index are correlated with each other.
On the long time-scale, PKS 2155-304 exhibits a tendency of
bluer-when-brighter, which means the spectrum becomes flatter when the source
brightens. | astro-ph_HE |
No X-Ray Excess from the HESS J1741-302 Region except a New Intermediate
Polar Candidate: With the Suzaku satellite, we observed an unidentified TeV gamma-ray source
HESS J1741$-$302 and its surroundings. No diffuse or point-like X-ray sources
are detected from the bright southern emission peak of HESS J1741$-$302. From
its neighborhood, we found a new intermediate polar candidate at the position
of $(\alpha, \delta)_{\rm J2000.0} = (\timeform{17h40m35.6s},
\timeform{-30D14m16s})$, which is designated as Suzaku J174035.6$-$301416. The
spectrum of Suzaku J174035.6$-$301416 exhibits emission lines at the energy of
6.4, 6.7 and 7.0 keV, which can be assigned as the K$\alpha$ lines from
neutral, He-like and H-like iron, respectively. A coherent pulsation is found
at a period of 432.1 $\pm$ 0.1 s. The pulse profile is quasi-sinusoidal in the
hard X-ray band (4$-$8 keV), but is more complicated in the soft X-ray band
(1$-$3 keV). The moderate period of pulsation, the energy flux, and the
presence of the iron K$\alpha$ lines indicate that Suzaku J174035.6$-$301416 is
likely an intermediate polar, a subclass of magnetized white dwarf binaries
(cataclysmic variables).
Based on these discoveries, we give some implications on the origin of GCDX
and brief comments on HESS J1741$-$302 and PSR B1737$-$30. | astro-ph_HE |
MC Simulations of the Broadband Spectra of Sagittarius A* through the
use of GRMHD: We present results of simulations of the spectrum of the accretion flow onto
the supermassive black hole in our Galactic Centre, Sagittarius A*, generated
with a coupling of Monte-Carlo (MC) radiation and general relativistic
magnetohydrodynamic (GRMHD) codes. In our modeling, we use the 2D HARM GRMHD
code to first model the physical parameters of the disk, then feed its results
into our 2D MC photon transport code. We will discuss results obtained which
fit radio, IR, and Chandra-obtained flaring or quiescent x-ray data points, as
well as the validity of the amount of scaling of input parameters (density,
temperature, and magnetic field) required to fit these points. HARM output will
be used to suggest whether the scaling is within reasonable limits. | astro-ph_HE |
Radio Constraints on $r$-process Nucleosynthesis by Collapsars: The heaviest elements in the Universe are synthesized through rapid neutron
capture ($r$-process) in extremely neutron rich outflows. Neutron star mergers
were established as an important $r$-process source through the multi-messenger
observation of GW170817. Collapsars were also proposed as a potentially major
source of heavy elements; however, this is difficult to probe through optical
observations due to contamination by other emission mechanisms. Here we present
observational constraints on $r$-process nucleosynthesis by collapsars based on
radio follow-up observations of nearby long gamma-ray bursts. We make the
hypothesis that late-time radio emission arises from the collapsar wind ejecta
responsible for forging $r$-process elements, and consider the constraints that
can be set on this scenario using radio observations of a sample of Swift/BAT
GRBs located within 2 Gpc. No radio counterpart was identified in excess of the
radio afterglow of the GRBs in our sample, limiting the collapsar $r$-process
contribution to $\lesssim0.2$ M$_\odot$ under the models we considered, with
constant circum-merger densities giving more stringent constraints. While our
results are in tension with collapsars being the majority $r$-process
production sites, the ejecta mass and velocity profile of collapsar winds is
not yet well modeled. As such, our results are currently subject to large
uncertainties, but further theoretical work could greatly improve them. | astro-ph_HE |
TeV gamma-ray survey of the northern sky using the ARGO-YBJ experiment: The ARGO-YBJ experiment is an extensive air shower array with full coverage
RPC detectors located at Yangbajing (4300 m asl, Tibet, China). It is operated
with high duty cycle (>86%) and a large field of view ($\sim$ 2sr). It
continuously monitors the entire overhead sky at $\gamma$-ray energies above
0.1 TeV. In the talk, we will present the result of the northern sky survey
(between declinations of -10$^{\circ}$ and 70$^{\circ}$) from an analysis of ~4
years of the ARGO-YBJ data (between July 2006 and February 2011). There are
four known TeV sources observed with significance greater than 5 S.D.. The
significance from Crab Nebula is more than 16 S.D.. 90% confidence level upper
limits to the flux from all directions in the sky are also presented, which
vary from 0.09 to 0.53 Crab unit for Crab-like point sources. | astro-ph_HE |
Constraining properties of neutron star merger outflows with radio
observations: The jet opening angle and inclination of GW170817 -- the first detected
binary neutron star merger -- were vital to understand its energetics, relation
to short gamma-ray bursts, and refinement of the standard siren-based
determination of the Hubble constant, $H_0$. These basic quantities were
determined through a combination of the radio lightcurve and Very Long Baseline
Interferometry (VLBI) measurements of proper motion. In this paper we discuss
and quantify the prospects for the use of radio VLBI observations and
observations of scintillation-induced variability to measure the source size
and proper motion of merger afterglows, and thereby infer properties of the
merger including inclination angle, opening angle and energetics. We show that
these techniques are complementary as they probe different parts of the
circum-merger density/inclination angle parameter space and different periods
of the temporal evolution of the afterglow. We also find that while VLBI
observations will be limited to the very closest events it will be possible to
detect scintillation for a large fraction of events beyond the range of current
gravitational wave detectors. Scintillation will also be detectable with next
generation telescopes such as the Square Kilometre Array, 2000 antenna Deep
Synoptic Array and the next generation Very Large Array, for a large fraction
of events detected with third generation gravitational wave detectors. Finally,
we discuss prospects for the measurement of the $H_0$ with VLBI observations of
neutron star mergers and compare this technique to other standard siren
methods. | astro-ph_HE |
An accurate and efficient numerical calculation of detonation waves in
multidimensional supernova simulations using a burning limiter and adaptive
quasi-statistical equilibrium: Resolving the small length-scale of thermonuclear detonation waves (TNDWs) in
supernovae is currently not possible in multidimensional full-star simulations.
Additionally, multidimensional simulations usually use small, oversimplistic
reaction networks and adopt an ad hoc transition criterion to nuclear
statistical equilibrium (NSE). The errors due to the applied approximations are
not well understood. We present here a new accurate and efficient numerical
scheme that accelerates the calculations by orders of magnitudes and allows the
structure of TNDWs to be resolved. The numerical scheme has two important
ingredients: (1) a burning limiter that broadens the width of the TNDW while
accurately preserving its internal structure, and (2) an adaptive separation of
isotopes into groups that are in nuclear statistical quasi-equilibrium, which
resolves the time-consuming burning calculation of reactions that are nearly
balanced out. Burning is calculated in situ employing the required large
networks without the use of post-processing or pre-describing the conditions
behind the TNDW. In particular, the approach to and deviation from NSE are
calculated self-consistently. The scheme can be easily implemented in
multidimensional codes. We test our scheme against accurate solutions of the
structure of TNDWs and against homogeneous expansion from NSE. We show that
with resolutions that are typical for multidimensional full-star simulations,
we reproduce the accurate thermodynamic trajectory (density, temperature, etc.)
to an accuracy that is better than a percent for the resolved scales (where the
burning limiter is not applied), while keeping the error for unresolved scales
(broadened by the burning limiter) within a few percent. | astro-ph_HE |
Hydrodynamics of Young Supernova Remnants and the Implications for their
Gamma-ray emission: Supernovae (SNe) are generally classified into Type I and Type II. Most SNe
(~ 80%), including all the subtypes of Type II, and Type Ib/c, arise from the
core-collapse of massive stars. During their lifetime, mass-loss from these
stars considerably modifies the medium around the stars. When the stars explode
as SNe, the resulting shock wave will expand in this wind-modified medium. In
contrast, Type Ia SNe will expand in a relatively uniform medium, but the
dynamics are different from those of core-collapse SNe. For young supernova
remnants, the properties of the ejecta as well as the surrounding medium are
important in determining the subsequent evolution of the SN shock wave, and the
dynamics and kinematics of the remnant. This will influence the acceleration of
particles at the SN shocks, and consequently affect the gamma-ray emission from
the remnant.
Herein we discuss the expected properties, especially the density structure,
of the medium around various types and sub-types of SNe, as suggested by
current stellar evolution models. Using analytic and semi-analytic models and
numerical simulations, we investigate how these affect the kinematics of the SN
shock waves, assess the impact this would have on the production of cosmic
rays, and show how it influences the time-evolution of the hadronic gamma-ray
emission from the remnant. In the case of SNRs evolving in a wind medium, the
emission should reach a maximum early on, and thereafter decrease with time.
For SNe in a constant density medium, the emission would be expected to
increase with time upto the advent of the Sedov stage. | astro-ph_HE |
Echo-Mapping of Swift J1753.5-0127: We present two epochs of coordinated X-ray-optical timing observations of the
black hole candidate Swift J1753.5-0127 during its 2005 outburst. The first
epoch in July occurred at outburst peak. Two consecutive nights of observations
using the McDonald Observatory Argos camera with the Rossi X-ray Timing
Explorer show a consistent correlation with an immediate response and an
extended tail lasting ~5s. The properties of the variability and the
correlation are consistent with thermal reprocessing in an accretion disk. The
shortness of the lag suggests a short orbital period consistent with that
recently claimed. The second epoch in August used the VLT FORS2 HIT mode again
in conjunction with RXTE. Again a repeatable correlation is seen between two
independent subsets of the data. In this case, though, the cross-correlation
function has an unusual structure comprising a dip followed by a double-peak.
We suggest that this may be equivalent to the dip plus single peak structure
seen by Kanbach et al. (2001) in XTE J1118+480 and attributed there to
synchrotron emission; a similar structure was seen during later activity of
Swift J1753.5-0127 by Durant et al. (2008). | astro-ph_HE |
Rates of Stellar Tidal Disruption as Probes of the Supermassive Black
Hole Mass Function: Rates of stellar tidal disruption events (TDEs) by supermassive black holes
(SMBHs) due to two-body relaxation are calculated using a large galaxy sample
(N=146) in order to explore the sensitivity of the TDE rates to observational
uncertainties, such as the parametrization of galaxy light profiles and the
stellar mass function. The largest uncertainty arises due to the poorly
constrained occupation fraction of SMBHs in low-mass galaxies, which otherwise
dominate the total TDE rate. The detection rate of TDE flares by optical
surveys is calculated as a function of SMBH mass and other observables for
several physically-motivated models of TDE emission. We also quantify the
fraction of galaxies that produce deeply penetrating disruption events. If the
majority of the detected events are characterized by super-Eddington
luminosities (such as disk winds, or synchrotron radiation from an off-axis
relativistic jet), then the measured SMBH mass distribution will tightly
constrain the low-end SMBH occupation fraction. If Eddington-limited emission
channels dominate, however, then the occupation fraction sensitivity is much
less pronounced in a flux-limited survey (although still present in a
volume-complete event sample). The SMBH mass distribution of the current sample
of TDEs, though highly inhomogeneous and encumbered by selection effects,
already suggests that Eddington-limited emission channels dominate. Even our
most conservative rate estimates appear to be in tension with much lower
observationally inferred TDE rates, and we discuss several possible resolutions
to this discrepancy. | astro-ph_HE |
Mergers of binary neutron star systems: a multi-messenger revolution: On 17 August 2017, less than two years after the direct detection of
gravitational radiation from the merger of two ~30 Msun black holes, a binary
neutron star merger was identified as the source of a gravitational wave signal
of ~100 s duration that occurred at less than 50 Mpc from Earth. A short GRB
was independently identified in the same sky area by the Fermi and INTEGRAL
satellites for high energy astrophysics, which turned out to be associated with
the gravitational event. Prompt follow-up observations at all wavelengths led
first to the detection of an optical and infrared source located in the
spheroidal galaxy NGC4993 and, with a delay of ~10 days, to the detection of
radio and X-ray signals. This paper revisits these observations and focusses on
the early optical/infrared source, which was thermal in nature and powered by
the radioactive decay of the unstable isotopes of elements synthesized via
rapid neutron capture during the merger and in the phases immediately following
it. The far-reaching consequences of this event for cosmic nucleosynthesis and
for the history of heavy elements formation in the Universe are also
illustrated. | astro-ph_HE |
Late-time radio observations of the short GRB200522A: constraints on the
magnetar model: GRB200522A is a short duration gamma-ray burst (GRB) at redshift $z$=0.554
characterized by a bright infrared counterpart. A possible, although not
unambiguous, interpretation of the observed emission is the onset of a luminous
kilonova powered by a rapidly rotating and highly-magnetized neutron star,
known as magnetar. A bright radio flare, arising from the interaction of the
kilonova ejecta with the surrounding medium, is a prediction of this model.
Whereas the available dataset remains open to multiple interpretations (e.g.
afterglow, r-process kilonova, magnetar-powered kilonova), long-term radio
monitoring of this burst may be key to discriminate between models. We present
our late-time upper limit on the radio emission of GRB200522A, carried out with
the Karl G. Jansky Very Large Array at 288 days after the burst. For kilonova
ejecta with energy $E_{\rm ej} \approx 10^{53} \rm erg$, as expected for a
long-lived magnetar remnant, we can already rule out ejecta masses $M_{\rm ej}
\lesssim0.03 \mathrm{M}_\odot$ for the most likely range of circumburst
densities $n\gtrsim 10^{-3}$ cm$^{-3}$. Observations on timescales of
$\approx$3-10 yr after the merger will probe larger ejecta masses up to $M_{\rm
ej} \sim 0.1 \mathrm{M}_\odot$, providing a robust test to the magnetar
scenario. | astro-ph_HE |
Associating Fast Radio Bursts with Extragalactic Radio Sources: General
Methodology and a Search for a Counterpart to FRB 170107: The discovery of a repeating fast radio burst has led to the first precise
localization, an association with a dwarf galaxy, and the identification of a
coincident persistent radio source. However, further localizations are required
to determine the nature of FRBs, the sources powering them, and the possibility
of multiple populations. Here we investigate the use of associated persistent
radio sources to establish FRB counterparts, taking into account the
localization area and the persistent source flux density. Due to the lower
areal number density of radio sources compared to faint optical sources, robust
associations can be achieved for less precise localizations as compared to
direct optical host galaxy associations. For generally larger localizations
which preclude robust associations, the number of candidate hosts can be
reduced based on the ratio of radio-to-optical brightness. We find that
confident associations with $\sim 0.01-$1 mJy sources, comparable to the
luminosity of the persistent source associated with FRB 121102 over the
redshift range $z \approx 0.1 - 1$, require FRB localizations of $\lesssim
20''$. In the absence of a robust association, constraints can be placed on the
luminosity of an associated radio source as a function of localization and DM.
For DM $\approx 1000 \rm \ pc \ cm^{-3}$, an upper limit comparable to the
luminosity of the FRB 121102 persistent source can be placed if the
localization is $\lesssim 10''$. We apply our analysis to the case of the ASKAP
FRB 170107, using optical and radio observations of the localization region. We
identify two candidate hosts based on a ratio of radio-to-optical brightness of
$\gtrsim 100$. We find that if one of these is associated with FRB 170107, the
resulting radio luminosity ($1 \times 10^{29} - 4 \times 10^{30} \ \rm erg \
s^{-1} \ Hz^{-1}$) is comparable to the luminosity of the FRB 121102 persistent
source. | astro-ph_HE |
The TANAMI Multiwavelength Program: Dynamic SEDs of Southern Blazars: Simultaneous broadband spectral and temporal studies of blazars are an
important tool for investigating active galactic nuclei (AGN) jet physics. We
study the spectral evolution between quiescent and flaring periods of 22
radio-loud AGN through multi-epoch, quasi-simultaneous broadband spectra. For
many of these sources these are the first broadband studies. We use a Bayesian
block analysis of \Fermi/LAT light curves in order to determine time ranges of
constant flux for constructing quasi-simultaneous SEDs. The shapes of the
resulting 81 SEDs are described by two logarithmic parabolas and a blackbody
spectrum where needed. For low states the peak frequencies and luminosities
agree well with the blazar sequence, higher luminosity implying lower peak
frequencies. This is not true for sources in a high state. The $\gamma$-ray
photon index in Fermi/LAT correlates with the synchrotron peak frequency in low
and intermediate states. No correlation is present in high states. The black
hole mass cannot be determined from the SEDs. Surprisingly, the thermal excess
often found in FSRQs at optical/UV wavelengths can be described by blackbody
emission and not an accretion disk spectrum. The "harder-when-brighter" trend,
typically seen in X-ray spectra of flaring blazars, is visible in the blazar
sequence. Our results for low and intermediate states, as well as the Compton
dominance, are in agreement with previous results. Black hole mass estimates
using the parameters from Bonchi (2013) are in agreement with some of the more
direct measurements. For two sources, estimates disagree by more than four
orders of magnitude, possibly due to boosting effects. The shapes of the
thermal excess seen predominantly in flat spectrum radio quasars are
inconsistent with a direct accretion disk origin. | astro-ph_HE |
From Supernova to Remnant: Tracking the Evolution of the Oldest Known
X-ray Supernovae: Core-collapse supernovae (SNe) expand into a medium created by winds from the
pre-SN progenitor. The SN explosion and resulting shock wave(s) heat up the
surrounding plasma, giving rise to thermal X-ray emission, which depends on the
density of the emitting material. Tracking the variation of the X-ray
luminosity over long periods of time thus allows for investigation of the
kinematics of the SN shock waves, the structure of the surrounding medium, and
the nature of the progenitor star. In this paper X-ray observations of five of
the oldest known X-ray supernovae - SN 1970G, SN 1968D, SN 1959D, SN 1957D and
SN 1941C - are analyzed, with the aim of reconstructing their light curves over
several decades. For those supernovae for which we can extract multi-epoch
data, the X-ray luminosity appears to decline with time, although with large
error bars. No increase in the X-ray emission from SN 1970G is found at later
epochs, contrary to previous reports. All five SNe show X-ray luminosities that
are of comparable magnitude. We compare the late-time X-ray luminosities of
these SNe to those of supernova remnants (SNRs) in the Galaxy which are a few
hundred years old, and find that when the tentative decline is taken into
account, the luminosity of the old SNe studied herein could fall below the
luminosity of some of the younger SNRs within a few hundred years. However, the
X-ray luminosity should begin to increase as the SNe expand in the Sedov phase,
thus reaching that of the observed SNRs. | astro-ph_HE |
An Exploration of X-ray Supernova Remnants in the Milky Way and Nearby
Galaxies: We probe the environmental properties of X-ray supernova remnants (SNRs) at
various points along their evolutionary journey, especially the S-T phase, and
their conformance with theoretically derived models of SNR evolution. The
remnant size is used as a proxy for the age of the remnant. Our data set
includes 34 Milky Way, 59 Large Magellanic Cloud (LMC), and 5 Small Magellanic
Cloud (SMC) SNRs. We select remnants that have been definitively typed as
either core-collapse (CC) or Type Ia supernovae, with well-defined size
estimates, and a thermal X-ray flux measured over the entire remnant. A catalog
of SNR size and X-ray luminosity is presented and plotted, with ambient density
and age estimates from the literature. Model remnants with a given density, in
the Sedov-Taylor (S-T) phase, are overplotted on the diameter-vs-luminosity
plot, allowing the evolutionary state and physical properties of SNRs to be
compared to each other, and to theoretical models. We find that small, young
remnants are predominantly Type Ia remnants or high luminosity CCs, suggesting
that many CC SNRs are not detected until after they have emerged from the
progenitor's wind-blown bubble. An examination of the distribution of SNR
diameters in the Milky Way and LMC reveals that LMC SNRs must be evolving in an
ambient medium which is 30% as dense as that in the Milky Way. This is
consistent with ambient density estimates for the Galaxy and LMC. | astro-ph_HE |
Unusual Emission Variations Near the Eclipse of A Black Widow PSR
J1720$-$0533: We report on an {unusually} bright observation of PSR J1720$-$0533 using the
Five-hundred-meter Aperture Spherical radio Telescope (FAST). The pulsar is in
a black widow system that {was discovered by the Commensal Radio Astronomy FAST
Survey (CRAFTS). By coincidence, a bright scintillation maximum was
simultaneous with the eclipse in our observation which allowed for precise
measurements of flux density variations, as well as dispersion measure (DM) and
polarization.} We found that there are quasi-periodic pulse emission variations
with a modulation period of $\sim$ {22\,s} during the ingress of the eclipse,
which could be caused by plasma lensing. {No such periodic modulation was found
during the egress of the eclipse. } {The linear polarization of the pulsar
disappears before the eclipse, even before there is a visually obvious change
in DM. We also found that the pulse scattering maybe play an important role in
the eclipse of PSR J1720$-$0533.} | astro-ph_HE |
A Survey of Fermi Catalog Sources using data from the Milagro Gamma-Ray
Observatory: The Fermi LAT has released a list of the most significant 205 sources with
three months of Fermi data (Bright Source List). The Milagro Gamma-Ray
Observatory is sensitive to gamma rays above 100 GeV with a peak sensitivity
between 10 and 30 TeV, overlapping and extending the energy range of Fermi. Of
the 34 Galactic LAT sources in the field of view of Milagro, 6 are observed
with significance greater than 5 sigma and 14 are observed at greater than 3
sigma. Of these 14 sources, 9 are pulsars. Since the VHE emission detected by
Milagro is often found to be extended and likely un-pulsed, the VHE component
presumably arises from the pulsar winds. Six of the 14 sources have not been
previously detected at TeV energies. The details of the Milagro survey will be
presented. We will also present the energy spectra of the high-significance
detections. Should the full 1-year source list be available prior to the
symposium, we will expand our analysis to include the larger Fermi catalog. | astro-ph_HE |
The first 48: Discovery and progenitor constraints on the Type Ia
supernova 2013gy: We present an early-phase $g$-band light curve and visual-wavelength spectra
of the normal Type Ia supernova (SN) 2013gy. The light curve is constructed by
determining the appropriate S-corrections to transform KAIT natural-system $B$-
and $V$-band photometry and Carnegie Supernova Project natural-system $g$-band
photometry to the Pan-STARRS1 $g$-band natural photometric system. A Markov
Chain Monte Carlo calculation provides a best-fit single power-law function to
the first ten epochs of photometry described by an exponent of
$2.16^{+0.06}_{-0.06}$ and a time of first light of MJD
56629.4$^{+0.1}_{-0.1}$, which is $1.93^{+0.12}_{-0.13}$ days (i.e., $<48$~hr)
before the discovery date (2013 December 4.84 UT) and $-19.10^{+0.12}_{-0.13}$
days before the time of $B$-band maximum (MJD 56648.5$\pm0.1$). The estimate of
the time of first light is consistent with the explosion time inferred from the
evolution of the Si II $\lambda$6355 Doppler velocity. Furthermore, discovery
photometry and previous nondetection limits enable us to constrain the
companion radius down to $R_c \leq 4\,R_{\odot}$. In addition to our early-time
constraints, we use a deep +235 day nebular-phase spectrum from Magellan/IMACS
to place a stripped H-mass limit of $< 0.018\,M_{\odot}$. Combined, these
limits effectively rule out H-rich nondegenerate companions. | astro-ph_HE |
X-ray Emission from SN 2012ca: A Type Ia-CSM Supernova Explosion in a
Dense Surrounding Medium: X-ray emission is one of the signposts of circumstellar interaction in
supernovae (SNe), but until now, it has been observed only in core-collapse
SNe. The level of thermal X-ray emission is a direct measure of the density of
the circumstellar medium (CSM), and the absence of X-ray emission from Type Ia
SNe has been interpreted as a sign of a very low density CSM. In this paper, we
report late-time (500--800 days after discovery) X-ray detections of SN 2012ca
in {\it Chandra} data. The presence of hydrogen in the initial spectrum led to
a classification of Type Ia-CSM, ostensibly making it the first SN~Ia detected
with X-rays. Our analysis of the X-ray data favors an asymmetric medium, with a
high-density component which supplies the X-ray emission. The data suggest a
number density $> 10^8$ cm$^{-3}$ in the higher-density medium, which is
consistent with the large observed Balmer decrement if it arises from
collisional excitation. This is high compared to most core-collapse SNe, but it
may be consistent with densities suggested for some Type IIn or superluminous
SNe. If SN 2012ca is a thermonuclear SN, the large CSM density could imply
clumps in the wind, or a dense torus or disk, consistent with the
single-degenerate channel. A remote possibility for a core-degenerate channel
involves a white dwarf merging with the degenerate core of an asymptotic giant
branch star shortly before the explosion, leading to a common envelope around
the SN. | astro-ph_HE |
Semi-implicit scheme for treating radiation under M1 closure in general
relativistic conservative fluid dynamics codes: A numerical scheme is described for including radiation in multi-dimensional
general-relativistic conservative fluid dynamics codes. In this method, a
covariant form of the M1 closure scheme is used to close the radiation moments,
and the radiative source terms are treated semi-implicitly in order to handle
both optically thin and optically thick regimes. The scheme has been
implemented in a conservative general relativistic radiation hydrodynamics code
KORAL. The robustness of the code is demonstrated on a number of test problems,
including radiative relativistic shock tubes, static radiation pressure
supported atmosphere, shadows, beams of light in curved spacetime, and
radiative Bondi accretion. The advantages of M1 closure relative to other
approaches such as Eddington closure and flux-limited diffusion are discussed,
and its limitations are also highlighted. | astro-ph_HE |
Gas perturbations in cool cores of galaxy clusters: effective equation
of state, velocity power spectra and turbulent heating: We present the statistical analysis of X-ray surface brightness and gas
density fluctuations in cool cores of ten, nearby and bright galaxy clusters
that have deep Chandra observations and show observational indications of
radio-mechanical AGN feedback. Within the central parts of cool cores the total
variance of fluctuations is dominated by isobaric and/or isothermal
fluctuations on spatial scales ~ 10-60 kpc, which are likely associated with
slow gas motions and bubbles of relativistic plasma. Adiabatic fluctuations
associated with weak shocks constitute less than 10 per cent of the total
variance in all clusters. The typical amplitude of density fluctuations is
small, ~ 10 per cent or less on scales of ~ 10-15 kpc. Subdominant contribution
of adiabatic fluctuations and small amplitude of density fluctuations support a
model of gentle AGN feedback as opposed to periodically explosive scenarios
which are implemented in some numerical simulations. Measured one-component
velocities of gas motions are typically below 100-150 km/s on scales < 50 kpc,
and can be up to ~ 300 km/s on ~ 100 kpc scales. The non-thermal energy is < 12
per cent of the thermal energy. Regardless of the source that drives these
motions the dissipation of the energy in such motions provides heat that is
sufficient to balance radiative cooling on average, albeit the uncertainties
are large. Presented results here support previous conclusions based on the
analysis of the Virgo and Perseus Clusters, and agree with the Hitomi
measurements. With next generation observatories like Athena and Lynx, these
techniques will be yet more powerful. | astro-ph_HE |
Variability of the X-ray Broad Iron Spectral Features in Active Galactic
Nuclei and Black-hole Binaries: The "broad iron spectral features" are often seen in X-ray spectra of Active
Galactic Nuclei (AGN) and black-hole binaries (BHB). These features may be
explained either by the "relativistic disc reflection" scenario or the "partial
covering" scenario: It is hardly possible to determine which model is valid
from time-averaged spectral analysis. Thus, X-ray spectral variability has been
investigated to constrain spectral models. To that end, it is crucial to study
iron structure of BHBs in detail at short time-scales, which is, for the first
time, made possible with the Parallel-sum clocking (P-sum) mode of XIS
detectors on board Suzaku. This observational mode has a time-resolution of
7.8~ms as well as a CCD energy-resolution. We have carried out systematic
calibration of the P-sum mode, and investigated spectral variability of the BHB
GRS 1915+105. Consequently, we found that the spectral variability of GRS
1915+105 does not show iron features at sub-seconds. This is totally different
from variability of AGN such as 1H0707--495, where the variation amplitude
significantly drops at the iron K-energy band. This difference can be naturally
explained in the framework of the "partial covering" scenario. | astro-ph_HE |
Upper Limit on Correlated Current Variations in the Crab Pulsar: The high energy emission of rotation powered pulsars is supposed to be
produced in "gaps" in the pulsar magnetosphere where charges are accelerated
and currents are produced. The rest of the magnetosphere is supposed to be
mostly a "force-free" plasma without any currents. Two important currents are
the main current that flows away from the pulsar, that produces the observed
radiation, and the current that returns to the pulsar to maintain charge
neutrality. This work attempts to study the return current in the Crab pulsar
using the soft X-ray data from the {\it{NICER}} observatory. It is assumed that
the two currents vary as a function of time. This would modulate the electric
fields in the "gaps", which would affect the observed X-ray flux. These flux
variations will show up only in the on-pulse phases, while those caused by the
Crab Nebula, instrumental effects, etc. will be present in the off-pulse phases
also. This work obtains the correlation coefficient of the flux variations in
the two peaks of the Crab pulsar, after removing the off-pulse flux variations.
No correlation was observed; its error of $0.000012$ sets an upper limit of
$0.036\%$ on the rms variation of correlated X-ray flux in the Crab pulsar.
Reasons exist for the return current variations to be correlated, while the
main current variations are probably uncorrelated. So the above number is
considered an upper limit on correlated return current variations, which may be
an important constraint for pulsar magnetospheric structure. | astro-ph_HE |
A multi-wavelength study of the hard and soft states of MAXI J1820+070
during its 2018 outburst: We present a comprehensive multi-wavelength spectral analysis of the black
hole X-ray binary MAXI J1820+070 during its 2018 outburst, utilizing AstroSat
far UV, soft and hard X-ray data, along with (quasi-)simultaneous optical and
X-ray data from Las Cumbres Observatory and NICER, respectively. In the soft
state, we detect soft X-ray and UV/optical excess components over and above the
intrinsic accretion disk emission ($kT_{\rm in}\sim 0.58$ keV) and a steep
X-ray power-law component. The soft X-ray excess is consistent with a
high-temperature blackbody ($kT\sim 0.79$ keV), while the UV/optical excess is
described by UV emission lines and two low-temperature blackbody components
($kT\sim 3.87$ eV and $\sim 0.75$ eV). Employing continuum spectral fitting, we
determine the black hole spin parameter ($a=0.77\pm0.21$), using the jet
inclination angle of $64^{\circ}\pm5^{\circ}$ and a mass spanning
$5-10M_{\odot}$. In the hard state, we observe a significantly enhanced
optical/UV excess component, indicating a stronger reprocessed emission in the
outer disk. Broad-band X-ray spectroscopy in the hard state reveals a
two-component corona, each associated with its reflection component, in
addition to the disk emission ($kT_{\rm in}\sim 0.19$ keV). The softer coronal
component dominates the bolometric X-ray luminosity and produces broader
relativistic reflection features, while the harder component gets reflected far
from the inner disk, yielding narrow reflection features. Furthermore, our
analysis in the hard state suggests a substantial truncation of the inner disk
($\gtrsim 51$ gravitational radii) and a high disk density ($\sim 10^{20}\ \rm
cm^{-3}$). | astro-ph_HE |
The Effect of a Cosmic Ray Precursor in SN 1006?: Like many young supernova remnants, SN 1006 exhibits what appear to be clumps
of ejecta close to or protruding beyond the main blast wave. In this paper we
examine 3 such protrusions along the east rim. They are semi-aligned with
ejecta fingers behind the shock-front, and exhibit emission lines from O VII
and O VIII. We first interpret them in the context of an upstream medium
modified by the saturated nonresonant Bell instability which enhances the
growth of Rayleigh-Taylor instabilities when advected postshock. We discuss
their apparent periodicity if the spacing is determined by properties of the
remnant or by a preferred size scale in the cosmic ray precursor. We also
briefly discuss the alternative that these structures have an origin in the
ejecta structure of the explosion itself. In this case the young evolutionary
age of SN 1006 would imply density structure within the outermost layers of the
explosion with potentially important implications for deflagration and
detonation in thermonuclear supernova explosion models. | astro-ph_HE |
Bose-Einstein condensate stars in combined Rastall-Rainbow gravity: We study zero and finite temperature static Bose-Einstein condensate (BEC)
stars in the combined Rastall-Rainbow (RR) theory of gravity by considering
different BEC equation of states (EoSs). We obtain the global properties of BEC
stars by solving the modified Tolman-Oppenheimer-Volkoff equations with values
of Rastall parameter $\kappa$ and Rainbow function $\Sigma$ chosen accordingly
to get the results in theories of Rastall, Rainbow and RR. We observe that the
parameter $\kappa$ has negligible effect on the maximum mass of the stars
considered, whereas $\Sigma$ alters it significantly, and increasing the value
of $\kappa$ beyond a certain limit results in unstable solutions for any value
of $\Sigma$. We report that the inclusion of temperature in our analysis
expands the parameter space by including more values of $\kappa$. However,
temperature has negligible effect on the maximum mass of the stellar profiles
in all the three theories. We find that the maximum masses and radii of the
stars within RR theory can have good agreement with the observational data on
pulsars for all the EoSs considered and in particular, the
Colpi-Wasserman-Shapiro EoS, which was ruled out in General Relativity (GR). We
also find that, in contrast to the results of GR, BEC stars consistent with
observations can be realised in the RR theory with smaller bosonic
self-interaction strength. | astro-ph_HE |
Spectral and polarimetric signatures of X-ray eclipses in AGN: X-ray observations of active galactic nuclei (AGN) show variability on
timescales ranging from a few hours up to a few days. Some of this variability
may be associated with occultation events by clouds in the broad line region.
In this work, we aim to model the spectral and polarization variability arising
from X-ray obscuration events, serving as probes of the relativistic effects
that dominate the emission from the innermost regions. We show that asymmetries
can be clearly detected in the AGN spectra as the cloud is shading different
parts of the accretion disc. We also show that these effects can be detected in
the temporal evolution of the polarization degree ($P$) and the polarization
position angle ($\Psi$). The variations in $P$ and $\Psi$ are highly dependent
on the inclination of the system, the position of the primary source and its
intrinsic polarization. Considering the disc-corona system only, for an
inclination $\theta = 30^\circ$ (60$^\circ$), $P$ increases up to $\sim 20$%
(30)%, in the 4-8 keV band, when the unpolarized primary source is obscured.
However, after accounting for the contribution of parsec-scale material
scattering the light in our line of sight (narrow-line region and molecular
torus), the variability is smoothed out and the polarization degree can be
reduced down to $\sim 1$% (2%). Our results suggest that the study of eclipses
in AGN with the next generation of X-ray spectral and polarimetric missions
could provide unique information on the physics and structure of the innermost
regions as well as of the parsec-scale material. | astro-ph_HE |
Too small to fail: characterizing sub-solar mass black hole mergers with
gravitational waves: The detection of a sub-solar mass black hole could yield dramatic new
insights into the nature of dark matter and early-Universe physics, as such
objects lack a traditional astrophysical formation mechanism. Gravitational
waves allow for the direct measurement of compact object masses during binary
mergers, and we expect the gravitational-wave signal from a low-mass
coalescence to remain within the LIGO frequency band for thousands of seconds.
However, it is unclear whether one can confidently measure the properties of a
sub-solar mass compact object and distinguish between a sub-solar mass black
hole or other exotic objects. To this end, we perform Bayesian parameter
estimation on simulated gravitational-wave signals from sub-solar mass black
hole mergers to explore the measurability of their source properties. We find
that the LIGO/Virgo detectors during the O4 observing run would be able to
confidently identify sub-solar component masses at the threshold of
detectability; these events would also be well-localized on the sky and may
reveal some information on their binary spin geometry. Further, next-generation
detectors such as Cosmic Explorer and the Einstein Telescope will allow for
precision measurement of the properties of sub-solar mass mergers and tighter
constraints on their compact-object nature. | astro-ph_HE |
A Model of White Dwarf Pulsar AR Scorpii: A 3.56-hour white dwarf (WD) - M dwarf (MD) close binary system, AR Scorpii,
was recently reported to show pulsating emission in radio, IR, optical, and UV,
with a 1.97-minute period, which suggests the existence of a WD with a rotation
period of 1.95 minutes. We propose a model to explain the temporal and spectral
characteristics of the system. The WD is a nearly perpendicular rotator, with
both open field line beams sweeping the MD stellar wind periodically. A bow
shock propagating into the stellar wind accelerates electrons in the wind.
Synchrotron radiation of these shocked electrons can naturally account for the
broad-band (from radio to X-rays) spectral energy distribution of the system. | astro-ph_HE |
Detailed Analysis of Filamentary Structure in the Weibel Instability: We present results of a 2D3V kinetic Vlasov simulation of the Weibel
instability. The kinetic Vlasov simulation allows us to investigate the
velocity distribution of dilute plasmas, in which the effect of collisions
between particles is negligible, and has the advantage that the accuracy of the
calculated velocity distribution does not depend on the density of plasmas at
each point in the physical space. We succeed in reproducing some features of
the Weibel instability shown by other simulations, for example, the
exponentially growing phase, the saturation of the magnetic field strength, the
formation of filamentary structure, and the coalescence of the filaments.
Especially, we concentrate on the behavior of the filaments after the
saturation of the magnetic field strength and find that there is a kind of
quasi-equilibrium states before the coalescence occurs. Furthermore, it is
found that an analytical solution for stationary states of the 2D3V
Vlasov-Maxwell system can reproduce some dominant features of the
quasi-equilibrium, e.g, the configuration of the magnetic field and the
velocity distribution at each point. The analytical expression could give a
plausible model for the transition layer of a collisionless shock where a
strong magnetic field generated by the Weibel instability provides an effective
dissipation process instead of collisions between particles. | astro-ph_HE |
On The Evolution of The Radio Pulsar PSR J1734-3333: Recent measurements showed that the period derivative of the 'high-B' radio
pulsar PSR J1734-3333 is increasing with time. For neutron stars evolving with
fallback disks, this rotational behavior is expected in certain phases of the
long-term evolution. Using the same model as employed earlier to explain the
evolution of anomalous X-ray pulsars and soft gamma-ray repeaters, we show that
the period, the first and second period derivatives and the X-ray luminosity of
this source can simultaneously acquire the observed values for a neutron star
evolving with a fallback disk. We find that the required strength of the dipole
field that can produce the source properties is in the range of 10^{12} -
10^{13} G on the pole of the neutron star. When the model source reaches the
current state properties of PSR J1734-3333, accretion onto the star has not
started yet, allowing the source to operate as a regular radio pulsar. Our
results imply that PSR J1734-3333 is at an age of ~ 3 x 10^4 - 2 x 10^5 years.
Such sources will have properties like the X-ray dim isolated neutron stars or
transient AXPs at a later epoch of weak accretion from the diminished fallback
disk. | astro-ph_HE |
Stellar oscillations in Eddington-inspired Born-Infeld gravity: We consider the stellar oscillations of relativistic stars in the
Eddington-inspired Born-Infeld gravity (EiBI). In order to examine the specific
frequencies, we derive the perturbation equations governing the stellar
oscillations in EiBI by linearizing the field equations, and numerically
determine the oscillation frequencies as changing the coupling parameter in
EiBI, $\kappa$, and stellar models. As a result, we find that the frequencies
depend strongly on the value of $\kappa$, where the frequencies in EiBI with
negative $\kappa$ become higher and those with positive $\kappa$ become lower
than the expectations in general relativity. We also find that, via the
observation of the fundamental frequency, one could distinguish EiBI with
$8\pi\epsilon_0|\kappa|\gtrsim 0.03$ from general relativity, independently of
the equation of state (EOS) for neutron star matter, where $\epsilon_0$ denotes
the nuclear saturation density and $\epsilon_0\kappa$ become dimensionless
parameter. With the further constraints on EOS, one might distinguish EiBI even
with $8\pi\epsilon_0|\kappa|\lesssim 0.03$ from general relativity. | astro-ph_HE |
Detections of simultaneous brightening of gamma-ray and optical
emissions of a distant blazar GB 1508+5714 at redshift 4.3: GB 1508+5714 is a high-redshift blazar ($z$ = 4.3) and a spectrally soft
$\gamma$-ray source has been detected in its direction. By analyzing 11.4-yr
{\it Fermi}-LAT data, significant long-term variability of the $\gamma$-ray
source is confirmed. More importantly, a $\gamma$-ray emission flare appeared
in an epoch of several tens of days in year 2018, when the flux is about four
times of the value from the global fit. Meanwhile, optical flares displayed in
both $r$- and $i$-bands from the {\it Zwicky} Transient Facility light curves.
Detections of the simultaneous $\gamma$-ray and optical brightening provide a
decisive evidence to pin down the association between the $\gamma$-ray source
and GB 1508+5714, which makes it the {\it first} identified $\gamma$-ray blazar
beyond redshift 4. A broadband spectral energy distribution in the high flux
state is constructed and the origin of the multiwavelength brightening is also
briefly discussed. Upcoming wide-deep-fast optical telescopes together with the
$\gamma$-ray sky surveyors will shed lights on the role that the AGN jets play
in the early cosmic time. | astro-ph_HE |
Tests of Lorentz Invariance Violation with Gamma Rays to probe Quantum
Gravity: We discuss the prospects of doing tests of Lorentz invariance with gamma-rays
observed with present and future ground based gamma-ray observatories. | astro-ph_HE |
Probing the origin of cosmic rays in Cygnus Cocoon using
ultrahigh-energy gamma-ray and neutrino observations: Recent ultrahigh energy gamma-ray observations by the HAWC up to 100 TeV and
LHAASO observatories up to 1.4 PeV energies from the direction of Fermi-LAT
4FGL source 4FGL J2028.6+4110e (Cygnus Cocoon), are indicative of a hadronic
origin over a leptonic process for their creation. The IceCube Neutrino
Observatory has reported IceCube-201120A, a neutrino event coming from the same
direction, suggesting that the Cygnus Cocoon may correspond to one of the most
plausible sources of high-energy cosmic rays. The apparent relationship of the
neutrino event with the observed ultra high energy gamma-rays from Cygnus
Cocoon is investigated in this work to study if it can be explained
consistently in hadronic interactions of accelerated cosmic rays with ambient
matter. Our findings reveal that leptonic mechanisms, together with pure
hadronic mechanisms, make a considerable contribution to the understanding of
the total electromagnetic spectrum as well as the observed neutrino event. The
estimate of expected muon neutrino events from the Cygnus cocoon agrees with
the one muon neutrino event detected so far in IceCube multi-year observations.
Thus, our results are indicative of the potential of the Cygnus Cocoon to be a
galactic cosmic ray source capable of accelerating at least up to PeV energies. | astro-ph_HE |
Understanding the cosmic ray positron flux: Recent precision measurements of the flux of cosmic ray positrons by the
Alpha Magnetic Spectrometer show that the spectrum has a marked softening
feature for energies close to one TeV. A possible interpretation of this result
is that the observed feature measures the maximum energy of a new hard source
of positrons perhaps associated to dark matter self--annihilation or decay, or
to positron accelerators. A gradual hardening of the positron flux centered at
$E \simeq 25$~GeV can also be understood as the signature of the transition
where the new source overtakes the conventional component due to secondary
production. This interpretation is simple and attractive, but it is not unique.
The alternative possibility, that the positron flux is entirely of secondary
origin, remains viable. In such a scenario the spectral softening observed by
AMS for positrons is generated by energy loss effects, and a feature of
similar, but not identical structure should be also visible in the $e^-$
spectrum. Spectral features similar to both the hardening and softening of the
positron flux are in fact observed for electrons and call for a consistent
explanation. Precision measurements of the $e^+$ and $e^-$ spectra in the TeV
and multi--TeV energy range are crucial to clarify the problem. | astro-ph_HE |
X-ray and Optical Study of Low Core Density Globular Clusters NGC6144
and E3: We report on the Chandra X-ray Observatory and Hubble Space Telescope
observation of two low core density globular clusters, NGC6144 and E3. By
comparing the number of X-ray sources inside the half-mass radius to those
outside, we found 6 X-ray sources within the half-mass radius of NGC6144, among
which 4 are expected to be background sources; 3 X-ray sources are also found
within the half-mass radius of E3, of which 3 is expected to be background
source. Therefore, we cannot exclude that all our sources are background
sources. However, combining the results from X-ray and optical observations, we
found that 1-2 sources in NGC6144 and 1 source in E3 are likely to be
cataclysmic variables and that 1 source in NGC6144 is an active binary, based
on the X-ray and optical properties. The number of faint X-ray sources in
NGC6144 and E3 found with Chandra and HST is higher than a prediction based on
collision frequency, but is closer to that based on mass. Our observations
strongly suggest that the compact binary systems in NGC6144 and E3 are
primordial in origin. | astro-ph_HE |
SN 2010jl: Optical to hard X-ray observations reveal an explosion
embedded in a ten solar mass cocoon: (Abridged) Some supernovae (SNe) may be powered by the interaction of the SN
ejecta with a large amount of circumstellar matter (CSM). Here we outline a
method to measure the mass of the optically thick CSM around such SNe. We
present observations of SN2010jl, including the first detection of a SN using
NuSTAR. The total radiated luminosity of SN2010jl is extreme, at least 9e50
erg. By modeling the visible-light data, we robustly show that the mass of the
circumstellar material within ~1e16 cm of the progenitor was in excess of 10
solar masses, likely ejected tens of years prior to the SN explosion. Our
modeling suggests that the shock velocity during shock breakout was ~6000 km/s,
decelerating to ~2600 km/s about two years after maximum light. Our late-time
NuSTAR+XMM spectra of the SN presumably provide the first direct measurement of
SN shock velocity two years after the SN maximum light -- measured to be in the
range of 2000 to 4500 km/s if the ions and electrons are in equilibrium, and
>~2000 km/s if they are not in equilibrium. This measurement is in agreement
with the shock velocity predicted by our modeling of the optical data. We also
show that the mean radial density distribution of the CSM roughly follows an
r^-2 law. A possible explanation for the massive CSM with a wind-like profile
is that they are the result of multiple pulsational pair instability events
prior to the SN explosion, separated from each other by years. | astro-ph_HE |
Hadronic vs leptonic models for $γ$-ray emission from VER J2227+608: Recent observations of VER J2227+608 reveal a broken power $\gamma$-ray
spectrum with the spectral index increasing from $\sim 1.8$ in the GeV energy
range to $\sim 2.3$ in the TeV range. Such a spectral break can be attributed
to radiative energy loss of energetic electrons in the leptonic scenario for
the $\gamma$-ray emission, which, in combination with characteristic age of the
nearby pulsar, can be used to constrain magnetic field in the emission region.
We show that the radio and X-ray observations can also be explained in such a
scenario. In the hadronic scenario, the spectral break can be attributed to
diffusion of energetic ions in a turbulent medium and detailed spectral
measurement can be used to constrain the diffusion coefficient. These two
models, however, predict drastically different spectra above 100 TeV, which
will be uncovered with future high-resolution observations, such as LHAASO
and/or CTA. | astro-ph_HE |
Glitches detected in southern radio pulsars: Parkes pulse arrival-time data for 165 radio pulsars spanning from 1990 to
2011 have been searched for period glitches. Forty-six events out of the
detected 107 glitches were found to be new contributions to the entire glitch
population which currently contains approximately 400 events. | astro-ph_HE |
Unwrapping the X-ray Spectra of Active Galactic Nuclei: Active galactic nuclei (AGN) are complex phenomena. At the heart of an AGN is
a relativistic accretion disk around a spinning supermassive black hole (SMBH)
with an X-ray emitting corona and, sometimes, a relativistic jet. On larger
scales, the outer accretion disk and molecular torus act as the reservoirs of
gas for the continuing AGN activity. And on all scales from the black hole
outwards, powerful winds are seen that probably affect the evolution of the
host galaxy as well as regulate the feeding of the AGN itself. In this review
article, we discuss how X-ray spectroscopy can be used to study each of these
components. We highlight how recent measurements of the high-energy cutoff in
the X-ray continuum by NuSTAR are pushing us to conclude that X-ray coronae are
radiatively-compact and have electron temperatures regulated by
electron-positron pair production. We show that the predominance of
rapidly-rotating objects in current surveys of SMBH spin is entirely
unsurprising once one accounts for the observational selection bias resulting
from the spin-dependence of the radiative efficiency. We review recent progress
in our understanding of fast (v~0.1-0.3c), highly-ionized (mainly visible in
FeXXV and FeXXVI lines), high-column density winds that may dominate
quasar-mode galactic feedback. Finally, we end with a brief look forward to the
promise of Astro-H and future X-ray spectropolarimeters. | astro-ph_HE |
Theoretically motivated search and detection of non-thermal pulsations
from PSRs J1747-2958, J2021+3651, and J1826-1256: Based on a theoretical selection of pulsars as candidates for detection at
X-ray energies, we present an analysis of archival X-ray observations done with
Chandra and XMM-Newton of PSR J1747-2958 (the pulsar in the "Mouse" nebula),
PSR J2021+3651 (the pulsar in the "Dragonfly" nebula), and PSR J1826-1256.
X-ray pulsations from PSR J1747-2958 and PSR J1826-1256 are detected for the
first time, and a previously reported hint of an X-ray pulsation from PSR
J2021+3651 is confirmed with a higher significance. We analyze these pulsars'
spectra in regards to the theoretically predicted energy distribution, finding
a remarkable agreement, and provide here a refined calculation of the model
parameters taking into account the newly derived X-ray spectral data. | astro-ph_HE |
Diffuse PeV neutrinos from EeV cosmic ray sources: semi-relativistic
hypernova remnants in star-forming galaxies: We argue that the excess of sub-PeV/PeV neutrinos recently reported by
IceCube could plausibly originate through pion-production processes in the same
sources responsible for cosmic rays (CRs) with energy above the second knee
around $10^{18}\,$eV. The pion production efficiency for escaping CRs that
produce PeV neutrinos is required to be $\gtrsim 0.1$ in such sources. On the
basis of current data, we identify semi-relativistic hypernova remants as
possible sources that satisfy the requirements. By virtue of their fast ejecta,
such objects can accelerate protons to EeV energies, which in turn can interact
with the dense surrounding medium during propagation in their host galaxies to
produce sufficient high-energy neutrinos via proton--proton ($pp$) collisions.
Their accompanying gamma ray flux can remain below the diffuse isotropic gamma
ray background observed by the {\it Fermi} Large Area Telescope (LAT). In order
to test this scenario and discriminate from alternatives, the density of target
protons/nuclei and the residence time of CRs in the interacting region are
crucial uncertainties that need to be clarified. As long as the neutrinos and
EeV CRs originate from the same source class, detection of $\gtrsim 10\,$PeV
neutrinos may be expected within 5-10 years' operation of IceCube. Together
with further observations in the PeV range, the neutrinos can help in revealing
the currently unknown sources of EeV CRs. | astro-ph_HE |
Signatures of Hierarchical Mergers in Black Hole Spin and Mass
distribution: Recent gravitational wave (GW) observations by LIGO/Virgo show evidence for
hierarchical mergers, where the merging BHs are the remnants of previous BH
merger events. These events may carry important clues about the astrophysical
host environments of the GW sources. In this paper, we present the
distributions of the effective spin parameter ($\chi_\mathrm{eff}$), the
precession spin parameter ($\chi_\mathrm{p}$), and the chirp mass
($m_\mathrm{chirp}$) expected in hierarchical mergers. Under a wide range of
assumptions, hierarchical mergers produce (i) a monotonic increase of the
average of the typical total spin for merging binaries, which we characterize
with ${\bar \chi}_\mathrm{typ}\equiv
\overline{(\chi_\mathrm{eff}^2+\chi_\mathrm{p}^2)^{1/2}}$, up to roughly the
maximum $m_\mathrm{chirp}$ among first-generation (1g) BHs, and (ii) a plateau
at ${\bar \chi}_\mathrm{typ}\sim 0.6$ at higher $m_\mathrm{chirp}$. We suggest
that the maximum mass and typical spin magnitudes for 1g BHs can be estimated
from ${\bar \chi}_\mathrm{typ}$ as a function of $m_\mathrm{chirp}$. The GW
data observed in LIGO/Virgo O1--O3a prefers an increase in ${\bar
\chi}_\mathrm{typ}$ at low $m_\mathrm{chirp}$, which is consistent with the
growth of the BH spin magnitude by hierarchical mergers, at $\sim 2 \sigma$
confidence. A Bayesian analysis suggests that 1g BHs have the maximum mass of
$\sim 15$--$30\,M_\odot$ if the majority of mergers are of high-generation BHs
(not among 1g-1g BHs), which is consistent with mergers in active galactic
nucleus disks and/or nuclear star clusters, while if mergers mainly originate
from globular clusters, 1g BHs are favored to have non-zero spin magnitudes of
$\sim 0.3$. We also forecast that signatures for hierarchical mergers in the
${\bar \chi}_\mathrm{typ}$ distribution can be confidently recovered once the
number of GW events increases to $\gtrsim O(100)$. | astro-ph_HE |
Accelerated binary black holes in globular clusters: forecasts and
detectability in the era of space-based gravitational-wave detectors: The motion of the center of mass of a coalescing binary black hole (BBH) in a
gravitational potential imprints a line-of-sight acceleration (LOSA) onto the
emitted gravitational wave (GW) signal. The acceleration could be sufficiently
large in dense stellar environments, such as globular clusters (GCs), to be
detectable with next-generation space-based detectors. In this work, we use
outputs of the \textsc{cluster monte carlo (cmc)} simulations of dense star
clusters to forecast the distribution of detectable LOSAs in DECIGO and LISA
eras. We study the effect of cluster properties -- metallicity, virial and
galactocentric radii -- on the distribution of detectable accelerations,
account for cosmologically-motivated distributions of cluster formation times,
masses, and metallicities, and also incorporate the delay time between the
formation of BBHs and their merger in our analysis. We find that larger
metallicities provide a larger fraction of detectable accelerations by virtue
of a greater abundance of relatively lighter BBHs, which allow a higher number
of GW cycles in the detectable frequency band. Conversely, smaller
metallicities result in fewer detections, most of which come from relatively
more massive BBHs with fewer cycles but larger LOSAs. We similarly find
correlations between the virial radii of the clusters and the fractions of
detectable accelerations. Our work, therefore, provides an important science
case for space-based GW detectors in the context of probing GC properties via
the detection of LOSAs of merging BBHs. | astro-ph_HE |
Interpreting the large amplitude X-ray variation of GRS 1915+105 and IGR
J17091-3624 as modulations of an accretion disc: Using the flux resolved spectroscopy for the first time, we analyse the
RXTE/PCA data of the black hole X-ray binaries GRS 1915+105 and IGR
J17091-3624, when both sources show large amplitude, quasi-regular oscillations
in 2.0-60.0 keV X-ray light curves. For different observations, we extract
spectra during the peak (spectrally soft) and dip (spectrally hard) intervals
of the oscillation, and find that their spectra are phenomenologically complex,
requiring at least two distinct spectral components. Besides a thermal
Comptonization component, we find that the disc emission is better modelled by
an index-free multicolour disc blackbody component (p-free disc model) rather
than that from a standard accretion disc. While the peak and dip spectra are
complex, remarkably, their difference spectra constructed by treating dip
spectra as the background spectra of the peak spectra, can be modelled as a
single p-free disc component. Moreover, the variability at different
time-scales and energy bands of the peak flux level is always greater than or
equal to the variability of the dip flux level, which strengthens the
possibility that the peak flux level may be due to an independent spectral
component added to the dip one. We also verify it using joint spectral analysis
of peak and dip spectra with a variable emission component. Our result
substantiates the standard hypothesis that the oscillations are due to the
limit cycle behaviour of an unstable radiation pressure dominated inner disc.
However, in this interpretation, the flux variation of the unstable disc can be
several order of magnitudes as expected from some theoretical simulations and
need not be fine tuned to match the factor ~10 variation seen between the peak
and dip levels. | astro-ph_HE |
Neutron star dynamics under time dependent external torques: The two component model describes neutron star dynamics incorporating the
response of the superfluid interior. Conventional solutions and applications
involve constant external torques, as appropriate for radio pulsars on
dynamical timescales. We present the general solution of two component dynamics
under arbitrary time dependent external torques, with internal torques that are
linear in the rotation rates, or with the extremely non-linear internal torques
due to vortex creep. The two-component model incorporating the response of
linear or nonlinear internal torques can now be applied not only to radio
pulsars but also to magnetars and to neutron stars in binary systems, with
strong observed variability and noise in the spin-down or spin-up rates. Our
results allow the extraction of the time dependent external torques from the
observed spin-down (or spin-up) time series, $\dot{\Omega}(t)$. Applications
are discussed. | astro-ph_HE |
Supermassive Black Hole Winds in X-rays -- SUBWAYS. I. Ultra-fast
outflows in QSOs beyond the local Universe: We present a new X-ray spectroscopic study of $22$ luminous
($2\times10^{45}\lesssim L_{\rm bol}\rm /erg\,s^{-1} \lesssim 2\times10^{46}$)
active galactic nuclei (AGNs) at intermediate-redshift ($0.1 \lesssim z
\lesssim 0.4$), as part of the SUpermassive Black hole Winds in the x-rAYS
(SUBWAYS) sample, mostly composed of quasars (QSOs) and type\,1 AGN. Here, 17
targets were observed with \textit{XMM-Newton} between 2019--2020 and the
remaining 5 are from previous observations. The aim of this large campaign
($1.45\,\rm Ms$ duration) is to characterise the various manifestations of
winds in the X-rays driven from supermassive black holes in AGN. In this paper
we focus on the search and characterization of ultra-fast outflows (UFOs),
which are typically detected through blueshifted absorption troughs in the
Fe\,K band ($E>7\,\rm keV$). By following Monte Carlo procedures, we confirm
the detection of absorption lines corresponding to highly ionised iron (e.g.,
Fe\,\textsc{xxv}\,H$\alpha$, Fe\,\textsc{xxvi}\,Ly$\alpha$) in 7/22 sources at
the $\gtrsim95\%$ confidence level (for each individual line). The global
combined probability of such absorption features in the sample is $>99.9\%$.
The SUBWAYS campaign extends at higher luminosity and redshifts than previous
local studies on Seyferts, obtained using \xmm and \suzaku observations. We
find a UFO detection fraction of $\sim30\%$ on the total sample that is in
agreement with the previous findings. This work independently provides further
support for the existence of highly-ionised matter propagating at mildly
relativistic speed ($\gtrsim0.1c$) in a considerable fraction of AGN over a
broad range of luminosities, which is expected to play a key role in the
self-regulated AGN feeding-feedback cycle, as also supported by hydrodynamical
multiphase simulations. | astro-ph_HE |
Effects of a non-universal IMF and binary parameter correlations on
compact binary mergers: Binary population synthesis provides a direct way of studying the effects of
different choices of binary evolution models and initial parameter
distributions on present-day binary compact merger populations, which can then
be compared to empirical properties such as observed merger rates. Samples of
zero-age main sequence binaries to be evolved by such codes are typically
generated from an universal IMF and simple, uniform, distributions for orbital
period $P$, mass ratio $q$ and eccentricity $e$. More recently, however,
mounting observational evidence has suggested the non-universality of the IMF
and the existence of correlations between binary parameters. In this study, we
implement a metallicity- and redshift-dependent IMF alongside correlated
distributions for $P$, $q$ and $e$ in order to generate representative
populations of binaries at varying redshifts, which are then evolved with the
COMPAS code in order to study the variations in merger rates and overall
population properties. | astro-ph_HE |
The link between broad emission line fluctuations and non-thermal
emission from the inner AGN jet: AGN reverberate when the broad emission lines respond to changes of the
ionizing thermal continuum emission. Reverberation measurements have been
commonly used to estimate the size of the broad-line region (BLR) and the mass
of the central black hole. However, reverberation mapping studies have been
mostly performed on radio-quiet sources where the contribution of the jet can
be neglected. In radio-loud AGN, jets and outflows may affect substantially the
relation observed between the ionizing continuum and the line emission. To
investigate this relation, we have conducted a series of multiwavelength
studies of radio-loud AGN, combining optical spectral line monitoring with
regular VLBI observations. Our results suggest that at least a fraction of the
broad-line emitting material can be located in a sub-relativistic outflow
ionized by non-thermal continuum emission generated in the jet at large
distances (> 1pc) from the central engine of AGN. This finding may have a
strong impact on black hole mass estimates based on measured widths of the
broad emission lines and on the gamma-ray emission mechanisms. | astro-ph_HE |
Dark matter annihilation and jet quenching phenomena in the early
universe: Dark-matter particles like neutralinos should decouple from the hot cosmic
plasma at temperatures of about 40 GeV. Later they can annihilate each other
into standard-model particles, which are injected into the dense primordial
plasma and quickly loose energy. This process is similar to jet quenching in
ultrarelativistic heavy-ion collisions, actively studied in RHIC and LHC
experiments. Using empirical information from heavy-ion experiments I show that
the cosmological (anti)quark and gluon jets are damped very quickly until the
plasma remains in the deconfined phase. The charged hadron and lepton jets are
strongly damped until the recombination of electrons and protons. The
consequences of energy transfer by the annihilation products to the cosmic
matter are discussed. | astro-ph_HE |
Radiation Pressure and Mass Ejection in Rho-like States of GRS 1915+105: We present a unifying scenario to address the physical origin of the
diversity of X-ray lightcurves within the rho variability class of the
microquasar GRS 1915+105. This 'heartbeat' state is characterized by a bright
flare that recurs every ~50-100 seconds, but the profile and duration of the
flares varies significantly from observation to observation. Based on a
comprehensive, phase-resolved study of heartbeats in the RXTE archive, we
demonstrate that very different X-ray lightcurves do not require origins in
different accretion processes. Indeed, our detailed comparison of the
phase-resolved spectra of a double-peaked oscillation and a single-peaked
oscillation shows that different cycles can have basically similar X-ray
spectral evolution. We argue that all heartbeat oscillations can be understood
as the result of a combination of a thermal-viscous radiation pressure
instability, a local Eddington limit in the disk, and a sudden,
radiation-pressure-driven evaporation or ejection event in the inner accretion
disk. This ejection appears to be a universal, fundamental part of the rho
state, and is largely responsible for a hard X-ray pulse seen in the lightcurve
of all cycles. We suggest that the detailed shape of oscillations in the mass
accretion rate through the disk is responsible for the phenomenological
differences between different rho-type lightcurves, and we discuss how future
time-dependent simulations of disk instabilities may provide new insights into
the role of radiation pressure in the accretion flow. | astro-ph_HE |
Radioactive heating rate of r-process elements and macronova light curve: We study the heating rate of r-process nuclei and thermalization of decay
products in neutron star merger ejecta and macronova (kilonova) light curves.
Thermalization of charged decay products, i.e., electrons, $\alpha$-particles,
and fission fragments is calculated according to their injection energy. The
$\gamma$-ray thermalization processes are also properly calculated by taking
the $\gamma$-ray spectrum of each decay into account. We show that the
$\beta$-decay heating rate at later times approaches a power-law decline as
$\propto t^{-2.8}$, which agrees with the result of Waxman et al. (2019). We
present a new analytic model to calculate macronova light curves, in which the
density structure of the ejecta is accounted for. We demonstrate that the
observed bolometric light curve and temperature evolution of the macronova
associated with GW170817 are reproduced well by the $\beta$-decay heating rate
with the solar r-process abundance pattern. We interpret the break in the
observed bolometric light curve around a week as a result of the diffusion wave
crossing a significant part of the ejecta rather than a thermalization break.
We also show that the time-weighted integral of the bolometric light curve
(Katz integral) is useful to provide an estimate of the total r-process mass
from the observed data, which is independent of the highly uncertain radiative
transfer. For the macronova in GW170817, the ejecta mass is robustly estimated
as $\approx 0.05M_{\odot}$ for $A_{\rm min}\leq 72$ and $85\leq A_{\rm min}\leq
130$ with the solar r-process abundance pattern. The code for computation of
the heating rate and light curve for given initial nuclear abundances is
publicly available. | astro-ph_HE |
Search for Individual Ultrahigh Energy Cosmic Ray Sources in the Future
Data: We propose a new way to detect individual bright Ultra-High Energy Cosmic Ray
(UHECR) sources above background if the Galactic Magnetic Field (GMF) gives the
main contribution to UHECR deflections. This method can be directly applied to
maps given by experiments. It consists in starting from at least two high
energy events above 6x10^19 eV, and looking at lower energy tails. We test the
efficiency of the method and investigate its dependence on different
parameters. In case of detection, the source position and the local GMF
deflection power are reconstructed. Both reconstructions are strongly affected
by the turbulent GMF. With the parameters adopted in this study, for 68 % of
reconstructed sources, the angular position is less than one degree from the
real one. For typical turbulent field strengths of 4 \mu G at the Earth
position and 1.5 kpc extension in the halo, one can reconstruct the deflection
power with 25 % precision in 68 % of cases. | astro-ph_HE |
On the equivalent width of the Fe K$α$ line produced by a dusty
absorber in active galactic nuclei: Obscured AGNs provide an opportunity to study the material surrounding the
central engine. Geometric and physical constraints on the absorber can be
deduced from the reprocessed AGN emission. In particular, the obscuring gas may
reprocess the nuclear X-ray emission producing a narrow Fe K$\alpha$ line and a
Compton reflection hump. In recent years, models of the X-ray reflection from
an obscuring torus have been computed; however, although the reflecting gas may
be dusty, the models do not yet take into account the effects of dust on the
predicted spectrum. We study this problem by analyzing two sets of models, with
and without the presence of dust, using the one dimensional photo-ionization
code Cloudy. The calculations are performed for a range of column densities
($22 <{\rm log}[N_H(\rm cm^{-2})]< 24.5$ ) and hydrogen densities ( $6 <{\rm
log}[n_H(\rm cm^{-3})]< 8$). The calculations show the presence of dust can
enhance the Fe K$\alpha$ equivalent width (EW) in the reflected spectrum by
factors up to $\approx$ 8 for Compton thick (CT) gas and a typical ISM grain
size distribution. The enhancement in EW with respect to the reflection
continuum is due to the reduction in the reflected continuum intensity caused
by the anisotropic scattering behaviour of dust grains. This effect will be
most relevant for reflection from distant, predominately neutral gas, and is a
possible explanation for AGNs which show a strong Fe K$\alpha$ EW and a
relatively weak reflection continuum. Our results show it is an important to
take into account dust while modeling the X-ray reflection spectrum, and that
inferring a CT column density from an observed Fe K$\alpha$ EW may not always
be valid. Multi-dimensional models are needed to fully explore the magnitude of
the effect. | astro-ph_HE |
ALMA CO Observations of the Gamma-Ray Supernova Remnant RX J1713.7-3946:
Discovery of Shocked Molecular Cloudlets and Filaments at 0.01 pc scales: RX J1713.7-3946 is a unique core-collapse SNR that emits bright TeV
gamma-rays and synchrotron X-rays caused by cosmic rays, in addition to
interactions with interstellar gas clouds. We report here on results of ALMA
$^{12}$CO($J$ = 1-0) observations toward the northwestern shell of the SNR. We
newly found three molecular complexes consisting of dozens of shocked molecular
cloudlets and filaments with typical radii of $\sim$0.03-0.05 pc and densities
of $\sim$$10^4$ cm$^{-3}$. These cloudlets and filaments are located not only
along synchrotron X-ray filaments, but also in the vicinity of X-ray hotspots
with month or year-scale time variations. We argue that X-ray hotspots were
generated by shock-cloudlet interactions through magnetic-field amplification
up to mG. The ISM density contrast of $\sim$$10^5$, coexistence of molecular
cloudlets and low-density diffuse medium of $\sim$0.1 cm$^{-3}$, is consistent
with such a magnetic field amplification as well as a wind-bubble scenario. The
small-scale cloud structures also affect hadronic gamma-ray spectra considering
the magnetic field amplification on surface and inside clouds. | astro-ph_HE |
Correlated variability of the reflection fraction with the X-ray flux
and spectral index for Mkn 478: The X-ray spectrum of Mkn 478 is known to be dominated by a strong soft
excess which can be described using relativistic blurred reflection. Using
observations from {\it XMM-Newton}, {\it AstroSat} and {\it Swift}, we show
that for the long-term ($\sim$ years) and intermediate-term (days to months)
variability, the reflection fraction is anti-correlated with the flux and
spectral index, which implies that the variability is due to the hard X-ray
producing corona moving closer to and further from the black hole. Using
flux-resolved spectroscopy of the {\it XMM-Newton} data, we show that the
reflection fraction has the same behaviour with flux and index on short
time-scales of hours. The results indicate that both the long-term and
short-term variability of the source is determined by the same physical
mechanism of strong gravitational light bending causing enhanced reflection and
low flux as the corona moves closer to the black hole. | astro-ph_HE |
Fermi LAT Measurements of the Diffuse Gamma-Ray Emission at Intermediate
Galactic Latitudes: The diffuse Galactic gamma-ray emission is produced by cosmic rays (CRs)
interacting with the interstellar gas and radiation field. Measurements by the
Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton
Gamma-Ray Observatory indicated excess gamma-ray emission > 1 GeV relative to
diffuse Galactic gamma-ray emission models consistent with directly measured CR
spectra (the so-called ``EGRET GeV excess''). The excess emission was observed
in all directions on the sky, and a variety of explanations have been proposed,
including beyond-the-Standard-Model scenarios like annihilating or decaying
dark matter. The Large Area Telescope (LAT) instrument on the Fermi Gamma-ray
Space Telescope has measured the diffuse gamma-ray emission with unprecedented
sensitivity and resolution. We report on LAT measurements of the diffuse
gamma-ray emission for energies 100 MeV to 10 GeV and Galactic latitudes 10
deg. <= |b| <= 20 deg. The LAT spectrum for this region of the sky is well
reproduced by the diffuse Galactic gamma-ray emission models mentioned above
and inconsistent with the EGRET GeV excess. | astro-ph_HE |
Effects of shock and turbulence properties on electron acceleration: Using test particle simulations we study electron acceleration at
collisionless shocks with a two-component model turbulent magnetic field with
slab component including dissipation range. We investigate the importance of
shock normal angle $\theta_{Bn}$, magnetic turbulence level
$\left(b/B_0\right)^2$, and shock thickness on the acceleration efficiency of
electrons. It is shown that at perpendicular shocks the electron acceleration
efficiency is enhanced with the decreasing of $\left(b/B_0\right)^2$, and at
$\left(b/B_0\right)^2=0.01$ the acceleration becomes significant due to strong
drift electric field with long time particles staying near the shock front for
shock drift acceleration (SDA). In addition, at parallel shocks the electron
acceleration efficiency is increasing with the increasing of
$\left(b/B_0\right)^2$, and at $\left(b/B_0\right)^2=10.0$ the acceleration is
very strong due to sufficient pitch-angle scattering for first-order Fermi
acceleration, as well as due to large local component of magnetic field
perpendicular to shock normal angle for SDA. On the other hand, the high
perpendicular shock acceleration with $\left(b/B_0\right)^2=0.01$ is stronger
than the high parallel shock acceleration with ($\left(b/B_0\right)^2=10.0$),
the reason might be the assumption that SDA is more efficient than first-order
Fermi acceleration. Furthermore, for oblique shocks, the acceleration
efficiency is small no matter the turbulence level is low or high. Moreover,
for the effect of shock thickness on electron acceleration at perpendicular
shocks, we show that there exists the bend-over thickness, $L_{\text{diff,b}}$.
The acceleration efficiency does not change evidently if the shock thickness is
much smaller than $L_{\text{diff,b}}$. However, if the shock thickness is much
larger than $L_{\text{diff,b}}$, the acceleration efficiency starts to drop
abruptly. | astro-ph_HE |
TESS Shines Light on the Origin of the Ambiguous Nuclear Transient
ASASSN-18el: We analyze high-cadence data from the Transiting Exoplanet Survey Satellite
(TESS) of the ambiguous nuclear transient (ANT) ASASSN-18el. The optical
changing-look phenomenon in ASASSN-18el has been argued to be due to either a
drastic change in the accretion rate of the existing active galactic nucleus
(AGN) or the result of a tidal disruption event (TDE). Throughout the TESS
observations, short-timescale stochastic variability is seen, consistent with
an AGN. We are able to fit the TESS light curve with a damped-random-walk (DRW)
model and recover a rest-frame variability amplitude of $\hat{\sigma} = 0.93
\pm 0.02$ mJy and a rest-frame timescale of $\tau_{DRW} = 20^{+15}_{-6}$ days.
We find that the estimated $\tau_{DRW}$ for ASASSN-18el is broadly consistent
with an apparent relationship between the DRW timescale and central
supermassive black hole mass. The large-amplitude stochastic variability of
ASASSN-18el, particularly during late stages of the flare, suggests that the
origin of this ANT is likely due to extreme AGN activity rather than a TDE. | astro-ph_HE |
Log-normal flux distribution of bright Fermi blazars: We present the results of the $\gamma$-ray flux distribution study on the
brightest blazars which are observed by the \emph{Fermi}-LAT. We selected 50
brightest blazars based on the maximum number of detection reported in the LAT
third AGN catalog. We performed standard unbinned maximum likelihood analysis
on the LAT data during the period between August 2008 and December 2016, in
order to obtain the average monthly flux. After quality cuts, blazars for which
at least 90\% of the total flux was survived were selected for the further
study, and this includes 19 FSRQs and 19 BL Lacs. The Anderson-Darling and
$\chi^2$ tests suggest that the integrated monthly flux follow a log-normal
distribution for all sources, except for three FSRQs for which neither a normal
nor a log-normal distribution was preferred. A double log-normal flux
distribution tendency were observed in these sources, though it has to be
confirmed with improved statistics. We also found that, the standard deviation
of the log-normal flux distribution increases with the mean spectral index of
the blazar, and can be fitted with a line of slope 0.24$\pm$0.04. We repeat our
study on three additional brightest unclassified blazars to identify their flux
distribution properties. Based on the features of their log-normal flux
distribution, we infer these unclassified blazars may be closely associated
with FSRQs. We also highlight that considering the log-normal behavior of the
flux distribution of blazars, averaging their long term flux in linear scale
can largely under estimate the nominal flux and this discrepancy can propagate
down to the estimation of source parameters through spectral modeling. | astro-ph_HE |
The Role of Superluminal Electromagnetic Waves in Pulsar Wind
Termination Shocks: The dynamics of a standing shock front in a Poynting-flux dominated
relativistic flow is investigated by using a one-dimensional, relativistic,
two-fluid simulation. An upstream flow containing a circularly polarized,
sinusoidal magnetic shear wave is considered, mimicking a wave driven by an
obliquely rotating pulsar. It is demonstrated that this wave is converted into
large amplitude electromagnetic waves with superluminal phase speeds by
interacting with the shock when the shock-frame frequency of the wave exceeds
the proper plasma frequency. The superluminal waves propagate in the upstream,
modify the shock structure substantially, and form a well-developed precursor
region ahead of a subshock. Dissipation of Poynting flux occurs in the
precursor as well as in the downstream region through a parametric instability
driven by the superluminal waves. The Poynting flux remaining in the downstream
region is carried entirely by the superluminal waves. The downstream plasma is
therefore an essentially unmagnetized, relativistically hot plasma with a
non-relativistic flow speed, as suggested by observations of pulsar wind
nebulae. | astro-ph_HE |
The NANOGrav 11-year Data Set: High-precision timing of 45 Millisecond
Pulsars: We present high-precision timing data over time spans of up to 11 years for
45 millisecond pulsars observed as part of the North American Nanohertz
Observatory for Gravitational Waves (NANOGrav) project, aimed at detecting and
characterizing low-frequency gravitational waves. The pulsars were observed
with the Arecibo Observatory and/or the Green Bank Telescope at frequencies
ranging from 327 MHz to 2.3 GHz. Most pulsars were observed with approximately
monthly cadence, with six high--timing-precision pulsars observed weekly, and
all were observed at widely separated frequencies at each observing epoch in
order to fit for time-variable dispersion delays. We describe our methods for
data processing, time-of-arrival (TOA) calculation, and the implementation of a
new, automated method for removing outlier TOAs. We fit a timing model for each
pulsar that includes spin, astrometric, and, if necessary, binary parameters,
in addition to time-variable dispersion delays and parameters that quantify
pulse-profile evolution with frequency. The new timing solutions provide three
new parallax measurements, two new Shapiro delay measurements, and two new
measurements of large orbital-period variations. We fit models that
characterize sources of noise for each pulsar. We find that 11 pulsars show
significant red noise, with generally smaller spectral indices than typically
measured for non-recycled pulsars, possibly suggesting a different origin.
Future papers will use these data to constrain or detect the signatures of
gravitational-wave signals. | astro-ph_HE |
Exploding neutron stars in close binaries: The discovery of GW signal from merging neutron stars by LIGO on 17th August
2017 was followed by a short GRB170817A discovered by FERMI and INTEGRAL 1.7
seconds after the loss of the GW signal when it just reached its maximum. Here
we present a reproduction of the first paper (published by us in 1984)
predicting a short GRB after GW signal of merging neutron stars. Our paper
followed the scenario by Clark and Eardley (1977) who predicted a catastrophic
disruption of a neutron star in a binary 1.7 seconds after the peak of GW
signal. Our next paper in 1990 predicted all the main properties of the short
GRB with quite a reasonable accuracy. Typos in English translation are
corrected and a few comments are added in the current publication as numbered
footnotes (the only footnote from the original paper is marked by an asterisk). | astro-ph_HE |
Characterising the rotational irregularities of the Vela pulsar from 21
yr of phase-coherent timing: Pulsars show two classes of rotational irregularities that can be used to
understand neutron-star interiors and magnetospheres: glitches and timing
noise. Here we present an analysis of the Vela pulsar spanning nearly 21 yr of
observation and including 8 glitches. We identify the relative pulse number of
all of the observations between glitches, with the only pulse-number
ambiguities existing over glitch events. We use the phase coherence of the
timing solution to simultaneously model the timing noise and glitches in a
Bayesian framework, allowing us to select preferred models for both. We find
the glitches can be described using only permanent and transient changes in
spin frequency, i.e., no step changes in frequency derivative. For all of the
glitches, we only need two exponentially decaying changes in spin frequency to
model the transient components. In contrast to previous studies, we find that
the dominant transient components decay on a common $\approx$ 1300 d time
scale, and that a larger fraction ( $\gtrsim 25\%$) of glitch amplitudes are
associated with these transient components. We also detect shorter-duration
transient components of $\approx$ 25 d, as previously observed, but are limited
in sensitivity to events with shorter durations by the cadence of our
observations. The timing noise is well described by a steep power-law process
that is independent of the glitches and subdominant to the glitch recovery. The
braking index is constrained to be $<$ 8 with 95% confidence. This methodology
can be used to robustly measure the properties of glitches and timing noise in
other pulsars. | astro-ph_HE |
Stability of interlinked neutron vortex and proton flux-tube arrays in a
neutron star -- III. Proton feedback: The coupled, time-dependent Gross-Pitaevskii and Ginzburg-Landau equations
are solved simultaneously in three dimensions to investigate the equilibrium
state and far-from-equilibrium, spin-down dynamics of an interpenetrating
neutron superfluid and proton type-II superconductor, as an idealized
description of the outer core of a neutron star. The simulations generalize
previous calculations without the time-dependent Ginzburg-Landau equation,
where proton feedback is absent. If the angle $\theta$ between the rotation and
magnetic axes does not equal zero, the equilibrium state consists of
geometrically complicated neutron vortex and proton flux-tube tangles, as the
topological defects pin to one another locally but align with different axes
globally. During spin-down, new types of motion are observed. For $\theta = 0$,
entire vortices pair rectilinearly with flux tubes and move together while
pinned. For $\theta \neq 0$, vortex segments pair with segments from one or
more flux tubes, and the paired segments move together while pinned. The degree
to which proton feedback impedes the deceleration of the crust is evaluated as
a function of $\theta$ and the pinning strength, $\eta$. Key geometric
properties of vortex-flux-tube tangles, such as filament length, mean
curvature, and polarity are analysed. It is found that proton feedback smooths
the deceleration of the crust, reduces the rotational glitch sizes, and
stabilizes the vortex tangle dynamics. The dimensionless control parameters in
the simulations are mutually ordered to match what is expected in a real
neutron star, but their central values and dynamics ranges differ from reality
by many orders of magnitude due to computational limitations. | astro-ph_HE |
The drifting subpulses of PSR B0031-07 and its synchronously modulated
radio polarization: We establish that for PSR B0031-07 the orthogonal polarization modes switch
at a single pulse level synchronously with the periodic drifting subpulses seen
in total intensity. There are only four other pulsars known for which this
phenomenon is observed. PSR B0031-07 is unique as it is the only source in this
group which has multiple stable drift modes. For both drift modes visible at
our observing frequency centered at 1369 MHz, the modulation of polarization
modes is synchronous with the drifting subpulses. In one of the drift modes, a
discontinuity in the modulation pattern of polarization properties occurs
halfway through the pulse, coinciding with a slight change in the slope of the
intensity drift band. In contrast to what has been suggested for this pulsar in
the past, this, plus other differences in the polarization of the modulated
emission observed for the two drift modes, suggests that a drift mode change is
more than a change in the underlying carousel radius and that magnetospheric
propagation effects play an important role. The ellipticity evolves
asymmetrically in time during the modulation cycle, which in the framework of a
carousel model implies that the polarized sub-beams are asymmetric with respect
to the sense of circulation, something which is not observed for other pulsars.
Birefringence in the magnetosphere, resulting in the orthogonal polarization
modes to spatially separate, is not enough to explain these results. It is
argued that more complex magnetospheric processes, which possibly allow
conversion between orthogonal polarization modes, play a role. | astro-ph_HE |
Indirect evidence of GeV Dark Matter: Recently, an excess of GeV gamma ray near the Galactic Centre has
beenvreported. The spectrum obtained can be best fitted with the
annihilationvof $30-40$ GeV dark matter particles through $b \bar{b}$ channel.
In this letter, I show that this annihilation model can also solve the
mysteries of heating source in x-ray plasma and the unexpected high gamma-ray
luminosity. The cross section constrained by these observations give excellent
agreements with both the predicted range by using Fermi-LAT data and the
canonical thermal relic abundance cross section. | astro-ph_HE |
Relativistic Viscous Radiation Hydrodynamic Simulations of Geometrically
Thin Disks: I. Thermal and Other Instabilities: We present results from two-dimensional, general relativistic, viscous,
radiation hydrodynamic numerical simulations of Shakura-Sunyaev thin disks
accreting onto stellar mass Schwarzschild black holes. We consider cases on
both the gas- and radiation-pressure-dominated branches of the thermal
equilibrium curve, with mass accretion rates spanning the range from $\dot{M} =
0.01 L_\mathrm{Edd}/c^2$ to $10 L_\mathrm{Edd}/c^2$. The simulations directly
test the stability of this standard disk model on the different branches. We
find clear evidence of thermal instability for all radiation-pressure-dominated
disks, resulting universally in the vertical collapse of the disks, which in
some cases then settle onto the stable, gas-pressure-dominated branch. Although
these results are consistent with decades-old theoretical predictions, they
appear to be in conflict with available observational data from black hole
X-ray binaries. We also find evidence for a radiation-pressure-driven
instability that breaks the unstable disks up into alternating rings of high
and low surface density on a timescale comparable to the thermal collapse.
Since radiation is included self-consistently in the simulations, we are able
to calculate lightcurves and power density spectra (PDS). For the most part, we
measure radiative efficiencies (ratio of luminosity to mass accretion rate)
close to 6%, as expected for a non-rotating black hole. The PDS appear as
broken power laws, with a break typically around 100 Hz. There is no evidence
of significant excess power at any frequencies, i.e. no quasi-periodic
oscillations are observed. | astro-ph_HE |
Properties of strongly magnetized ultradense matter and their imprints
on magnetar pulsations: We investigate the effect of strong magnetic fields on the adiabatic radial
oscillations of hadronic stars. We describe magnetized hadronic matter within
the framework of the relativistic nonlinear Walecka model and integrate the
equations of relativistic radial oscillations to determine the fundamental
pulsation mode. We consider that the magnetic field increases, in a density
dependent way, from the surface, where it has a typical magnetar value of
$10^{15}$ G, to the interior of the star where it can be as large as $3 \times
10^{18}$ G. We show that magnetic fields of the order of $10^{18}$ G at the
stellar core produce a significant change in the frequency of neutron star
pulsations with respect to unmagnetized objects. If radial pulsations are
excited in magnetar flares, they can leave an imprint in the flare lightcurves
and open a new window for the study of highly magnetized ultradense matter. | astro-ph_HE |
A simplified view of blazars: why BL Lacertae is actually a quasar in
disguise: We put forward a scenario where blazars are classified as flat-spectrum radio
quasars, BL Lacs, low synchrotron, or high synchrotron peaked objects according
to a varying combination of Doppler boosted radiation from the jet, emission
from the accretion disk, the broad line region, and light from the host galaxy.
We thoroughly test this new approach, which builds upon unified schemes, using
Monte Carlo simulations and show that it can provide simple answers to a number
of long-standing open issues. We also demonstrate that selection effects play a
very important role in the diversity observed in radio and X-ray samples and in
the correlation between luminosity and peak frequency of the synchrotron power
(the so-called "blazar sequence"). It turns out that sources so far classified
as BL Lacs on the basis of their observed weak, or undetectable, emission lines
are of two physically different classes: intrinsically weak-lined objects, more
common in X-ray selected samples, and heavily diluted broad-lined sources, more
frequent in radio selected samples, which explains some of the confusion in the
literature. | astro-ph_HE |
From Supernova to Remnant: Tracking the Evolution of the Oldest Known
X-ray Supernovae: Core-collapse supernovae (SNe) expand into a medium created by winds from the
pre-SN progenitor. The SN explosion and resulting shock wave(s) heat up the
surrounding plasma, giving rise to thermal X-ray emission, which depends on the
density of the emitting material. Tracking the variation of the X-ray
luminosity over long periods of time thus allows for investigation of the
kinematics of the SN shock waves, the structure of the surrounding medium, and
the nature of the progenitor star. In this paper X-ray observations of five of
the oldest known X-ray supernovae - SN 1970G, SN 1968D, SN 1959D, SN 1957D and
SN 1941C - are analyzed, with the aim of reconstructing their light curves over
several decades. For those supernovae for which we can extract multi-epoch
data, the X-ray luminosity appears to decline with time, although with large
error bars. No increase in the X-ray emission from SN 1970G is found at later
epochs, contrary to previous reports. All five SNe show X-ray luminosities that
are of comparable magnitude. We compare the late-time X-ray luminosities of
these SNe to those of supernova remnants (SNRs) in the Galaxy which are a few
hundred years old, and find that when the tentative decline is taken into
account, the luminosity of the old SNe studied herein could fall below the
luminosity of some of the younger SNRs within a few hundred years. However, the
X-ray luminosity should begin to increase as the SNe expand in the Sedov phase,
thus reaching that of the observed SNRs. | astro-ph_HE |
3 to 12 millimetre studies of dense gas towards the western rim of
supernova remnant RX J1713.7-3946: The young X-ray and gamma-ray-bright supernova remnant RXJ1713.7-3946 (SNR
G347.3-0.5) is believed to be associated with molecular cores that lie within
regions of the most intense TeV emission. Using the Mopra telescope, four of
the densest cores were observed using high-critical density tracers such as
CS(J=1-0,J=2-1) and its isotopologue counterparts, NH3(1,1) and (2,2) inversion
transitions and N2H+(J=1-0) emission, confirming the presence of dense gas
>10^4cm^-3 in the region. The mass estimates for Core C range from 40M_{\odot}
(from CS(J=1-0)) to 80M_{\odot} (from NH3 and N2H+), an order of magnitude
smaller than published mass estimates from CO(J=1-0) observations. We also
modelled the energy-dependent diffusion of cosmic-ray protons accelerated by
RXJ1713.7-3946 into Core C, approximating the core with average density and
magnetic field values. We find that for considerably suppressed diffusion
coefficients (factors \chi=10^{-3} down to 10^{-5} the galactic average), low
energy cosmic-rays can be prevented from entering the inner core region. Such
an effect could lead to characteristic spectral behaviour in the GeV to TeV
gamma-ray and multi-keV X-ray fluxes across the core. These features may be
measurable with future gamma-ray and multi-keV telescopes offering arcminute or
better angular resolution, and can be a novel way to understand the level of
cosmic-ray acceleration in RXJ1713.7-3946 and the transport properties of
cosmic-rays in the dense molecular cores. | astro-ph_HE |
On the Origin of Diffuse Radio Emission in Abell 85 -- Insights from new
GMRT Observations: Extended, steep, and ultra-steep spectrum radio emission in a galaxy cluster
is usually associated with recent mergers. Simulations show that radio
phoenixes are aged radio galaxy lobes whose emission reactivates when a low
Mach shock compresses it. A85 hosts a textbook example of a radio phoenix at
about 320 kpc southwest of the cluster center. We present a new high resolution
325 MHz GMRT radio map illustrating this radio phoenix's complex and
filamentary structure. The full extent of the radio structure is revealed for
the first time from these radio images of A85. Using archival \textit{Chandra}
X-ray observations, we applied an automated 2-D shock finder to the X-ray
surface brightness and Adaptive Circular Binning (ACB) temperature maps which
confirmed a bow shock at the location of the radio phoenix. We also compared
the Mach number from the X-ray data with the radio-derived Mach number in the
same region using multi-frequency radio observations and find that they are
consistent within the 1$\sigma$ error level. | astro-ph_HE |
Supernova Explosions inside Carbon-Oxygen Circumstellar Shells: Motivated by a recent discovery of Supernova 2010gx and numerical results of
Fryer et al.(2010), we simulate light curves for several type I supernova
models, enshrouded by dense circumstellar shells, or "super-wind", rich in
carbon and oxygen and having no hydrogen. We demonstrate that the most luminous
events like SN2010gx can be explained by those models at moderate explosion
energies (2-3) foe if the total mass of SN ejecta and a shell is (3-5) Msun and
the radius of the shell is ~10^{16} cm. | astro-ph_HE |
The time-dependent one-zone hadronic model - First principles: We present a time-dependent approach to the one-zone hadronic model in the
case where the photon spectrum is produced by ultrarelativistic protons
interacting with soft photons that are produced from protons and low magnetic
fields. Assuming that protons are injected at a certain rate in a homogeneous
spherical volume containing a magnetic field, the evolution of the system can
be described by five coupled kinetic equations, for protons, electrons,
photons, neutrons, and neutrinos. Photopair and photopion interactions are
modelled using the results of Monte-Carlo simulations and, in particular from
the SOPHIA code for the latter. The coupling of energy losses and injection
introduces a self-consistency in our approach and allows the study of the
comparative relevancy of processes at various conditions, the efficiency of the
conversion of proton luminosity to radiation, the resulting neutrino spectra,
and the effects of time variability on proton injection, among other topics. We
present some characteristic examples of the temporal behaviour of the system
and show that this can be very different from the one exhibited by leptonic
models. Furthermore, we argue that, contrary to the wide-held belief, there are
parameter regimes where the hadronic models can become quite efficient.
However, to keep the free parameters at a minimum and facilitate an in-depth
study of the system, we have only concentrated on the case where protons are
injected; i.e., we did not consider the effects of a co-accelerated leptonic
component. | astro-ph_HE |
A generic method to constrain the dark matter model parameters from
Fermi observations of dwarf spheroids: Observation of gamma-rays from dwarf galaxies is an effective way to search
for particle dark matter. Using 4-year data of Fermi-LAT observations on a
series of Milky Way satellites, we develop a general way to search for the
signals from dark matter annihilation in such objects. Instead of giving prior
information about the energy spectrum of dark matter annihilation, we bin the
Fermi-LAT data into several energy bins and build a likelihood map in the
"energy bin - flux" plane. The final likelihood of any spectrum can be easily
derived through combining the likelihood of all the energy bins. It gives
consistent result with that directly calculated using the Fermi Scientific
Tool. This method is very efficient for the study of any specific dark matter
models with gamma-rays. We use the new likelihood map with Fermi-LAT 4 year
data to fit the parameter space in three representative dark matter models: i)
toy dark matter model, ii) effective dark matter operators, and iii)
supersymmetric neutralino dark matter. | astro-ph_HE |
The spin and orbit of the newly discovered pulsar IGR J17480-2446: We present an analysis of the spin and orbital properties of the newly
discovered accreting pulsar IGR J17480-2446, located in the globular cluster
Terzan 5. Considering the pulses detected by the Rossi X-ray Timing Explorer at
a period of 90.539645(2) ms, we derive a solution for the 21.27454(8) hr binary
system. The binary mass function is estimated to be 0.021275(5) Msun,
indicating a companion star with a mass larger than 0.4 Msun. The X-ray pulsar
spins up while accreting at a rate of between 1.2 and 1.7E-12 Hz/s, in
agreement with the accretion of disc matter angular momentum given the observed
luminosity.
We also report the detection of pulsations at the spin period of the source
during a Swift observation performed ~2 d before the beginning of the RXTE
coverage. Assuming that the inner disc radius lies in between the neutron star
radius and the corotation radius while the source shows pulsations, we estimate
the magnetic field of the neutron star to be within ~2E8 G and ~2.4E10 G. From
this estimate, the value of the spin period and of the observed spin-up rate,
we associate this source with the still poorly sampled population of slow,
mildly recycled, accreting pulsars. | astro-ph_HE |
TeV gamma-ray survey of the Northern sky using the ARGO-YBJ detector: The ARGO-YBJ detector is an extensive air shower array that has been used to
monitor the northern $\gamma$-ray sky at energies above 0.3 TeV from 2007
November to 2013 January. In this paper, we present the results of a sky survey
in the declination band from $-10^{\circ}$ to $70^{\circ}$, using data recorded
over the past five years. With an integrated sensitivity ranging from 0.24 to
$\sim$1 Crab units depending on the declination, six sources have been detected
with a statistical significance greater than 5 standard deviations. Several
excesses are also reported as potential $\gamma$-ray emitters. The features of
each source are presented and discussed. Additionally, $95\%$ confidence level
upper limits of the flux from the investigated sky region are shown. Specific
upper limits for 663 GeV $\gamma$-ray AGNs inside the ARGO-YBJ field of view
are reported. The effect of the absorption of $\gamma$-rays due to the
interaction with extragalactic background light is estimated. | astro-ph_HE |
Suzaku Discovery of a Slowly Varying Hard X-ray Continuum from the Type
I Seyfert Galaxy NGC 3516: The bright type I Seyfert galaxy NGC 3516 was observed by {\it Suzaku} twice,
in 2005 October 12--15 and 2009 October 28--November 2, for a gross time
coverage of 242 and 544 ksec and a net exposure of 134 and 255 ksec,
respectively. The 2--10 keV luminosity was $2.8 \times 10^{41}$ erg s$^{-1}$ in
2005, and $1.6 \times 10^{41}$ erg s$^{-1}$ in 2009. The 1.4--1.7 keV and 2--10
keV count rates both exhibited peak-to-peak variations by a factor of $\sim2$
in 2005, while $\sim 4$ in 2009. In either observation, the 15--45 keV count
rate was less variable. The 2--10 keV spectrum in 2005 was significantly more
convex than that in 2009. Through a count-count-plot technique, the 2--45 keV
signals in both data were successfully decomposed in a model-independent way
into two distinct broadband components. One is a variable emission with a
featureless spectral shape, and the other is a non-varying hard component
accompanied by a prominent Fe-K emission line at 6.33 keV (6.40 keV in the rest
frame). The former was fitted successfully by an absorbed power-law model,
while the latter requires a new hard continuum in addition to a reflection
component from distant materials. The spectral and variability differences
between the two observations are mainly attributed to long-term changes of this
new hard continuum, which was stable on time scales of several hundreds ksec. | astro-ph_HE |
Full Monte-Carlo description of the Moscow State University Extensive
Air Shower experiment: The Moscow State University Extensive Air Shower (EAS-MSU) array studied
high-energy cosmic rays with primary energies ~(1-500) PeV in the Northern
hemisphere. The EAS-MSU data are being revisited following recently found
indications to an excess of muonless showers, which may be interpreted as the
first observation of cosmic gamma rays at ~100 PeV. In this paper, we present a
complete Monte-Carlo model of the surface detector which results in a good
agreement between data and simulations. The model allows us to study the
performance of the detector and will be used to obtain physical results in
further studies. | astro-ph_HE |
Cosmic-ray acceleration and escape from post-adiabatic Supernova
remnants: Supernova remnants are known to accelerate cosmic rays on account of their
non-thermal emission of radio waves, X-rays, and gamma rays. Although there are
many models for the acceleration of cosmic rays in Supernova remnants, the
escape of cosmic rays from these sources is yet understudied.
We use our time-dependent acceleration code RATPaC to study the acceleration
of cosmic rays and their escape in post-adiabatic Supernova remnants and
calculate the subsequent gamma-ray emission from inverse-Compton scattering and
Pion decay.
We performed spherically symmetric 1-D simulations in which we simultaneously
solve the transport equations for CRs, magnetic turbulence, and the
hydrodynamical flow of the thermal plasma in a volume large enough to keep all
CRs in the simulation. The transport equations for cosmic-rays and magnetic
turbulence are coupled via the cosmic-ray gradient and the spatial diffusion
coefficient of the cosmic rays, while the cosmic-ray feedback onto the shock
structure can be ignored. Our simulations span 100kyrs, thus covering the
remnants evolution until the beginning of the post-adiabatic phase.
At later stages of the evolution cosmic rays over a wide range of energy can
reside outside of the remnant, creating spectra that are softer than predicted
by standard DSA and feature breaks in the 10-100 GeV-range. The total spectrum
of cosmic rays released into the interstellar medium has a spectral index of
s~2.4 above roughly 10 GeV which is close to that required by Galactic
propagation models. We further find the gamma-ray luminosity to peak around an
age of 4,000 years for inverse-Compton-dominated high-energy emission. Remnants
expanding in low-density media emit generally more inverse-Compton radiation
matching the fact that the brightest known supernova remnants - RCW86, Vela Jr,
HESSJ1731-347 and RXJ1713.7-3946 - are all expanding in low density
environments. | astro-ph_HE |
Cosmic ray driven Galactic winds: The escape of cosmic rays from the Galaxy leads to a gradient in the cosmic
ray pressure that acts as a force on the background plasma, in the direction
opposite to the gravitational pull. If this force is large enough to win
against gravity, a wind can be launched that removes gas from the Galaxy,
thereby regulating several physical processes, including star formation. The
dynamics of these cosmic ray driven winds is intrinsically non-linear in that
the spectrum of cosmic rays determines the characteristics of the wind
(velocity, pressure, magnetic field) and in turn the wind dynamics affects the
cosmic ray spectrum. Moreover, the gradient of the cosmic ray distribution
function causes excitation of Alfven waves, that in turn determine the
scattering properties of cosmic rays, namely their diffusive transport. These
effects all feed into each other so that what we see at the Earth is the result
of these non-linear effects. Here we investigate the launch and evolution of
such winds, and we determine the implications for the spectrum of cosmic rays
by solving together the hydrodynamical equations for the wind and the transport
equation for cosmic rays under the action of self-generated diffusion and
advection with the wind and the self-excited Alfven waves. | astro-ph_HE |
Environmental dependence of Type IIn supernova properties: Type IIn supernovae occur when stellar explosions are surrounded by dense
hydrogen-rich circumstellar matter. The dense circumstellar matter is likely
formed by extreme mass loss from their progenitors shortly before they explode.
The nature of Type IIn supernova progenitors and the mass-loss mechanism
forming the dense circumstellar matter are still unknown. In this work, we
investigate if there are any correlations between Type IIn supernova properties
and their local environments. We use Type IIn supernovae with well-observed
light-curves and host-galaxy integral field spectroscopic data so that we can
estimate both supernova and environmental properties. We find that Type IIn
supernovae with a higher peak luminosity tend to occur in environments with
lower metallicity and/or younger stellar populations. The circumstellar matter
density around Type IIn supernovae is not significantly correlated with
metallicity, so the mass-loss mechanism forming the dense circumstellar matter
around Type IIn supernovae might be insensitive to metallicity. | astro-ph_HE |
X-Ray Emission from the Galactic Supernova Remnant G12.0-0.1: We present results of the Suzaku/XIS observation around the radio supernova
remnant (SNR) G12.0-0.1. No significant diffuse emission extending in or along
the radio shell was observed. Instead two compact X-ray sources, Suzaku
J181205-1835 and Suzaku J181210-1842, were found in or near G12.0-0.1. Suzaku
J181205-1835 is located at the northwest of the radio shell of G12.0-0.1. The
X-ray profile is slightly extended over the point spread function of the Suzaku
telescope. The X-ray spectrum has no line-like structure and is well
represented by a power-law model with a photon index of 2.2 and an absorption
column of N_{H}=4.9x10^{22} cm^{-2}. The distances of Suzaku J181205-1835 and
G12.0-0.1 are estimated from the absorption column and the Sigma-D relation,
respectively, and are nearly the same with each other. These results suggest
that Suzaku J181205-1835 is a candidate of a pulsar wind nebula associated with
G12.0-0.1. From its location, Suzaku J181210-1842 would be unrelated object to
G12.0-0.1. The X-ray profile is point-like and the spectrum is thin thermal
emission with Fe K-lines at 6.4, 6.7, and 6.97 keV, similar to those of
cataclysmic variables. | astro-ph_HE |
GRBs on probation: testing the UHE CR paradigm with IceCube: Gamma ray burst (GRB) fireballs provide one of very few astrophysical
environments where one can contemplate the acceleration of cosmic rays to
energies that exceed 10^20 eV. The assumption that GRBs are the sources of the
observed cosmic rays generates a calculable flux of neutrinos produced when the
protons interact with fireball photons. With data taken during construction
IceCube has already reached a sensitivity to observe neutrinos produced in
temporal coincidence with individual GRBs provided that they are the sources of
the observed extragalactic cosmic rays. We here point out that the GRB origin
of cosmic rays is also challenged by the IceCube upper limit on a possible
diffuse flux of cosmic neutrinos which should not be exceeded by the flux
produced by all GRB over Hubble time. Our alternative approach has the
advantage of directly relating the diffuse flux produced by all GRBs to
measurements of the cosmic ray flux. It also generates both the neutrino flux
produced by the sources and the associated cosmogenic neutrino flux in a
synergetic way. | astro-ph_HE |
An up-scattered cocoon emission model of Gamma-Ray Burst high-energy
lags: The Fermi Gamma-ray Space Telescope recently detected the most energetic
gamma-ray burst so far, GRB 080916C, and reported its detailed temporal
properties in an extremely broad spectral range: (i) the time-resolved spectra
are well described by broken power-law forms over the energy range of $10 {\rm
keV}-10$ GeV, (ii) the high-energy emission (at $\epsilon > 100$ MeV) is
delayed by $\approx 5$s with respect to the $\epsilon \lesssim 1$ MeV emission,
and (iii) the emission onset times shift towards later times in the higher
energy bands. We show that this behavior of the high-energy emission can be
explained by a model in which the prompt emission consists of two components:
one is the emission component peaking at $\epsilon \sim 1$ MeV due to the
synchrotron-self-Compton radiation of electrons accelerated in the internal
shock of the jet and the other is the component peaking at $\epsilon \sim 100$
MeV due to up-scattering of the photospheric X-ray emission of the expanding
cocoon (i.e., the hot bubble produced by dissipation of the jet energy inside
the progenitor star) off the same electrons in the jet. Based on this model, we
derive some constraints on the radius of the progenitor star and the total
energy and mass of the cocoon of this GRB, which may provide information on the
structure of the progenitor star and the physical conditions of the jet
propagating in the star. The up-scattered cocoon emission could be important
for other Fermi-GRBs as well. We discuss some predictions of this model,
including a prompt bright optical emission and a soft X-ray excess. | astro-ph_HE |
Extragalactic cosmic ray sources with very small contribution in the
particle flux on the Earth and their study: Possible existence of extragalactic ultra-high energy cosmic ray sources
giving a very small particle flux on the Earth is considered. Accretion discs
around supermassive black holes where particles are accelerated in electric
fields are discussed as an illustration of such sources. Due to acceleration
mechanism particle injection spectra are assumed to be hard. In this case
particle flux on the Earth is too low for detection. But particles produce in
space a noticeable flux of diffuse gamma-ray emission via electromagnetic
cascades. It should be accounted for when analyzing other source models and
dark matter models. Also neutrinos are produced in cascades, and at energies E
>10^19 eV cascade neutrino spectra depends on injection spectra. Therefore it
is proposed to study cosmic ray sources under consideration using data on
gamma-ray and neutrino emission. | astro-ph_HE |
Strong late-time circumstellar interaction in the peculiar supernova
iPTF14hls: We present a moderate-resolution spectrum of the peculiar Type II supernova
iPTF14hls taken on day 1153 after discovery. This spectrum reveals the clear
signature of shock interaction with dense circumstellar material (CSM). We
suggest that this CSM interaction may be an important clue for understanding
the extremely unusual photometric and spectroscopic evolution seen over the
first 600 days of iPTF14hls. The late-time spectrum shows a double-peaked
intermediate-width H-alpha line indicative of expansion speeds around 1000
km/s, with the double-peaked shape hinting at a disc-like geometry in the CSM.
If the CSM was highly asymmetric, perhaps in a disc or torus that was ejected
from the star 3-6 years prior to explosion, then the CSM interaction could have
been overrun and hidden below the SN ejecta photosphere from a wide range of
viewing angles. In that case, CSM interaction luminosity would have been
thermalized well below the photosphere, possibly sustaining the high luminosity
without exhibiting the traditional observational signatures of strong CSM
interaction (narrow H-alpha emission and X-rays). Variations in density
structure of the CSM could account for the multiple rebrightenings of the
lightcurve. We propose that enveloped CSM interaction as seen in some recent
SNe, rather than an entirely new explosion mechanism, may be adequate to
explain the peculiar evolution of iPTF14hls. | astro-ph_HE |
The origin of the first neutron star -- neutron star merger: The first neutron star-neutron star (NS-NS) merger was discovered on August
17, 2017 through gravitational waves (GW170817) and followed with
electromagnetic observations. This merger was detected in an old elliptical
galaxy with no recent star formation. We perform a suite of numerical
calculations to understand the formation mechanism of this merger. We probe
three leading formation mechanisms of double compact objects: classical
isolated binary star evolution, dynamical evolution in globular clusters and
nuclear cluster formation to test whether they are likely to produce NS-NS
mergers in old host galaxies. Our simulations with optimistic assumptions show
current NS-NS merger rates at the level of 10^-2 yr^-1 from binary stars, 5 x
10^-5 yr^-1 from globular clusters and 10^-5 yr^-1 from nuclear clusters for
all local elliptical galaxies (within 100 Mpc^3). These models are thus in
tension with the detection of GW170817 with an observed rate 1.5 yr^-1 (per 100
Mpc^3; LIGO/Virgo estimate). Our results imply that either (i) the detection of
GW170817 by LIGO/Virgo at their current sensitivity in an elliptical galaxy is
a statistical coincidence; or that (ii) physics in at least one of our three
models is incomplete in the context of the evolution of stars that can form
NS-NS mergers; or that (iii) another very efficient (unknown) formation channel
with a long delay time between star formation and merger is at play. | astro-ph_HE |
Characteristics of the diffuse astrophysical electron and tau neutrino
flux with six years of IceCube high energy cascade data: We report on the first measurement of the astrophysical neutrino flux using
particle showers (cascades) in IceCube data from 2010 -- 2015. Assuming
standard oscillations, the astrophysical neutrinos in this dedicated cascade
sample are dominated ($\sim 90 \%$) by electron and tau flavors. The flux,
observed in the sensitive energy range from $16\,\mathrm{TeV}$ to
$2.6\,\mathrm{PeV}$, is consistent with a single power-law model as expected
from Fermi-type acceleration of high energy particles at astrophysical sources.
We find the flux spectral index to be $\gamma=2.53\pm0.07$ and a flux
normalization for each neutrino flavor of $\phi_{astro} = 1.66^{+0.25}_{-0.27}$
at $E_{0} = 100\, \mathrm{TeV}$, in agreement with IceCube's complementary muon
neutrino results and with all-neutrino flavor fit results. In the measured
energy range we reject spectral indices $\gamma\leq2.28$ at $\ge3\sigma$
significance level. Due to high neutrino energy resolution and low atmospheric
neutrino backgrounds, this analysis provides the most detailed characterization
of the neutrino flux at energies below $\sim100\,{\rm{TeV}}$ compared to
previous IceCube results. Results from fits assuming more complex neutrino flux
models suggest a flux softening at high energies and a flux hardening at low
energies (p-value $\ge 0.06$). The sizable and smooth flux measured below $\sim
100\,{\rm{TeV}}$ remains a puzzle. In order to not violate the isotropic
diffuse gamma-ray background as measured by the Fermi-LAT, it suggests the
existence of astrophysical neutrino sources characterized by dense environments
which are opaque to gamma-rays. | astro-ph_HE |
Modeling Magnetic Disk-Wind State Transitions in Black Hole X-ray
Binaries: We analyze three prototypical black hole (BH) X-ray binaries (XRBs), \4u1630,
\gro1655\ and \h1743, in an effort to systematically understand the intrinsic
state transition of the observed accretion-disk winds between \windon\ and
\windoff\ states by utilizing state-of-the-art {\it Chandra}/HETGS archival
data from multi-epoch observations. We apply our magnetically-driven wind
models in the context of magnetohydrodynamic (MHD) calculations to constrain
their (1) global density slope ($p$), (2) their density ($n_{17}$) at the foot
point of the innermost launching radius and (3) the abundances of heavier
elements ($A_{\rm Fe,S,Si}$). Incorporating the MHD winds into {\tt xstar}
photoionization calculations in a self-consistent manner, we create a library
of synthetic absorption spectra given the observed X-ray continua. Our analysis
clearly indicates a characteristic bi-modal transition of multi-ion X-ray
winds; i.e. the wind density gradient is found to steepen (from $p \sim
1.2-1.4$ to $\sim 1.4-1.5$) while its density normalization declines as the
source transitions from \windon\ to \windoff\ state. The model implies that the
ionized wind {\it remains physically present} even in \windoff\ state, despite
its absent appearance in the observed spectra. Super-solar abundances for
heavier elements are also favored. Our global multi-ion wind models, taking
into account soft X-ray ions as well as Fe K absorbers, show that the internal
wind condition plays an important role in wind transitions besides
photoionization changes. % Simulated {\it XRISM}/Resolve and {\it Athena}/X-IFU
spectra are presented to demonstrate a high fidelity of the multi-ion wind
model for better understanding of these powerful ionized winds in the coming
decades. | astro-ph_HE |
Calculating transient rates from surveys: We have developed a method to determine the transient surface density and
transient rate for any given survey, using Monte-Carlo simulations. This method
allows us to determine the transient rate as a function of both the flux and
the duration of the transients in the whole flux-duration plane rather than one
or a few points as currently available methods do. It is applicable to every
survey strategy that is monitoring the same part of the sky, regardless the
instrument or wavelength of the survey, or the target sources. We have
simulated both top-hat and Fast Rise Exponential Decay light curves,
highlighting how the shape of the light curve might affect the detectability of
transients. Another application for this method is to estimate the number of
transients of a given kind that are expected to be detected by a survey,
provided that their rate is known. | astro-ph_HE |
A search for lines in the bright X-ray afterglow of GRB120711A: GRB120711A, discovered and rapidly localized by the INTEGRAL satellite,
attracted particular interest due to its high gamma-ray fluence, very bright
X-ray afterglow, and the detection of a prompt optical transient and of
long-lasting emission at GeV energies. A follow-up observation carried out with
the XMM-Newton satellite has provided an X-ray spectrum in the 0.3-10 keV with
unprecedented statistics for a GRB afterglow 20 hours after the burst. The
spectrum is well fit by a power-law with photon index 1.87+-0.01, modified by
absorption in our Galaxy and in the GRB host at z=1.4. A Galactic absorption
consistent with that estimated from neutral hydrogen observations is obtained
only with host metallicity lower than 0.05 of the Solar value. We report the
results of a sensitive search for emission and absorption lines using the
matched filter smoothing method (Rutledge and Sako 2003). No statistically
significant lines were found. The upper limits on the equivalent width of
emission lines, derived through Monte Carlo simulations, are few tens of eV, a
factor about 10 lower than that of the possible lines reported in the
literature for other bursts. | astro-ph_HE |
Long duration radio transients lacking optical counterparts are possibly
Galactic Neutron Stars: (abridged) Recently, a new class of radio transients in the 5-GHz band was
detected by Bower et al. We present new deep near-Infrared (IR) observations of
the field containing these transients, and find no counterparts down to a
limiting magnitude of K=20.4 mag. We argue that the bright (>1 Jy) radio
transients recently reported by Kida et al. are consistent with being
additional examples of the Bower et al. transients. We refer to these groups of
events as "long-duration radio transients". The main characteristics of this
population are: time scales longer than 30 minute but shorter than several
days; rate, ~10^3 deg^-2 yr^-1; progenitors sky surface density of >60 deg^-2
(95% C.L.) at Galactic latitude ~40 deg; 1.4-5 GHz spectral slopes, f_\nu ~
\nu^alpha, with alpha>0; and most notably the lack of any counterparts in
quiescence in any wavelength. We rule out an association with many types of
objects. Galactic brown-dwarfs or some sort of exotic explosions remain
plausible options. We argue that an attractive progenitor candidate for these
radio transients is the class of Galactic isolated old neutron stars (NS). We
confront this hypothesis with Monte-Carlo simulations of the space distribution
of old NSs, and find satisfactory agreement for the large areal density.
Furthermore, the lack of quiescent counterparts is explained quite naturally.
In this framework we find: the mean distance to events in the Bower et al.
sample is of order kpc; the typical distance to the Kida et al. transients are
constrained to be between 30 pc and 900 pc (95% C.L.); these events should
repeat with a time scale of order several months; and sub-mJy level bursts
should exhibit Galactic latitude dependence. We discuss possible mechanisms
giving rise to the observed radio emission. | astro-ph_HE |
A pulsar in a binary with a compact object in the mass gap between
neutron stars and black holes: Among the compact objects observed in gravitational wave merger events a few
have masses in the gap between the most massive neutron stars (NSs) and least
massive black holes (BHs) known. Their nature and the formation of their
merging binaries are not well understood. We report on pulsar timing
observations using the Karoo Array Telescope (MeerKAT) of PSR J0514-4002E, an
eccentric binary millisecond pulsar in the globular cluster NGC 1851 with a
total binary mass of $3.887 \pm 0.004$ solar masses. The companion to the
pulsar is a compact object and its mass (between $2.09$ and $2.71$ solar
masses, 95% confidence interval) is in the mass gap, so it either is a very
massive NS or a low-mass BH. We propose the companion was formed by a merger
between two earlier NSs. | astro-ph_HE |
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