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Indications of an unexpected signal associated with the GW170817 binary
neutron star inspiral: We report experimental evidence at the 2.5$\sigma$ level for an unexpected
signal associated with the GW170817 binary neutron star inspiral. This evidence
derives from a laboratory experiment simultaneously measuring the $\beta$-decay
rates of Si-32 and Cl-36 in a common detector. Whereas the Si-32 and Cl-36
decay rates show no statistical correlation before or after the inspiral, they
are highly correlated ($\sim 95\%$) in the 5 hour time interval immediately
following the inspiral. If we interpret this correlation as arising from the
influence of particles emitted during the inspiral, then we can estimate the
mass $m_{x}$ of these particles from the time delay between the gravity-wave
signal and a peak in the $\beta$-decay data. We find for particles of energy 10
MeV, $m_{x}$ $\lesssim$ 16 eV which includes the neutrino mass region $m_{\nu}$
$\lesssim$ 2 eV. The latter is based on existing limits for the masses $m_{i}$
of the three known neutrino flavors. Additionally, we find that the correlation
is even stronger if we include data in the 80 minute period before the arrival
of the gravity wave signal. Given the large number of radionuclides whose
decays are being monitored at any given time, we conjecture that other groups
may also be in a position to search for statistically suggestive fluctuations
of radionuclide decay rates associated with the GW170817 inspiral, and possibly
with other future inspirals. | astro-ph_HE |
X-ray Characteristics of NGC 3516: A View through the Complex Absorber: We consider new Suzaku data for NGC 3516 taken during 2009, along with other
recent X-ray observations of the source. The cumulative characteristics of NGC
3516 cannot be explained without invoking changes in the line-of-sight
absorption. Contrary to many other well-studied Seyfert galaxies, NGC 3516 does
not show a positive lag of hard X-ray photons relative to soft photons over the
timescales sampled. In the context of reverberation models for the X-ray lags,
the lack of such a signal in NGC 3516 is consistent with flux variations being
dominated by absorption changes. The lack of any reverberation signal in such a
highly variable source disfavors intrinsic continuum variability in this case.
Instead, the colorless flux variations observed at high flux states for NGC
3516 are suggested to be a consequence of Compton-thick clumps of gas crossing
the line-of-sight. | astro-ph_HE |
Analysis of Reaction-Diffusion Systems for Flame Capturing in Type Ia
Supernova Simulations: We present a study of numerical behavior of a thickened flame used in Flame
Capturing (FC, Khokhlov (1995)) for tracking thin unresolved physical flames in
deflagration simulations. We develop a steady-state procedure for calibrating
the flame model used, and test it against analytical results.
We observe numerical noises generated by original realization of the
technique. Alternative artificial burning rates are discussed, which produce
acceptably quiet flames. Two new quiet models are calibrated to yield required
"flame" speed and width, and further studied in 2D and 3D setting.
Landau-Darrieus type instabilities of the flames are observed. One model also
shows significantly anisotropic propagation speed on the grid, both effects
increasingly pronounced at larger matter expansion as a result of burning; this
makes the model unacceptable for use in type Ia supernova simulations. Another
model looks promising for use in flame capturing at fuel to ash density ratio
of order 3 and below. That "Model B" yields flames completely localized within
a region 6 cells wide at any expansion.
We study Markstein effect for flame models described, through direct
numerical simulations and through quasi-steady technique developed. Comparison
demonstrates that Markstein effect dominates instability effects on curved
flame speed for Model B in 2D simulations for fuel to ash density ratio of
about 2.5 and below. We find critical wavelength of LD instability by direct
simulations of perturbed nearly planar flames; these agree with analytical
estimates when Markstein number values found in this work are used. We conclude
that the behavior of model B is well understood, and optimal for FC
applications among all flame models proposed to date. | 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 |
A novel scenario for the possible X-ray line feature at ~3.5 keV: Charge
exchange with bare sulfur ions: Motivated by recent claims of a compelling ~3.5 keV emission line from nearby
galaxies and galaxy clusters, we investigate a novel plasma model incorporating
a charge exchange component obtained from theoretical scattering calculations.
Fitting this kind of component with a standard thermal model yields positive
residuals around 3.5 keV, produced mostly by S XVI transitions from principal
quantum numbers n > 8 to the ground. Such high-n states can only be populated
by the charge exchange process. In this scenario, the observed 3.5 keV line
flux in clusters can be naturally explained by an interaction in an effective
volume of ~1 kpc^3 between a ~3 keV temperature plasma and cold dense clouds
moving at a few hundred km/s. The S XVI lines at ~3.5 keV also provide a unique
diagnostic of the charge exchange phenomenon in hot cosmic plasmas. | astro-ph_HE |
The Peculiar Bursting Nature of CP Pup: The classical nova CP Puppis has been observed to have particularly puzzling
and peculiar properties. In particular, this classical nova displays occasional
bursts in its long-term ASAS-SN light curve. Here we report on 5 sectors of
TESS data displaying 2 of these rapid bursts, lasting ~1 day. Based on the
estimated lower energy limits of the bursts we discuss whether the bursts may
be examples of micronovae resulting from localised thermonuclear explosion.
Furthermore, its orbital period remains uncertain, with several inconsistent
periodic signals appearing in spectroscopic and photometric observations at
various wavelengths. Although we cannot unambiguously unravel the physical
origin of the signals, the previously suggested nature of CP Puppis as a long
orbital period system may be a viable explanation. The recurrence time of the
bursts in CP Puppis, together with the unexplained variable modulations make it
a prime candidate for intense monitoring. | astro-ph_HE |
Redback: A Bayesian inference software package for electromagnetic
transients: Fulfilling the rich promise of rapid advances in time-domain astronomy is
only possible through confronting our observations with physical models and
extracting the parameters that best describe what we see. Here, we introduce
{\sc Redback}; a Bayesian inference software package for electromagnetic
transients. {\sc Redback} provides an object-orientated {\sc python} interface
to over 12 different samplers and over 100 different models for kilonovae,
supernovae, gamma-ray burst afterglows, tidal disruption events, engine-driven
transients, X-ray afterglows of gamma-ray bursts driven by millisecond
magnetars among other explosive transients. The models range in complexity from
simple analytical and semi-analytical models to surrogates built upon numerical
simulations accelerated via machine learning. {\sc Redback} also provides a
simple interface for downloading and processing data from Swift, Fink, Lasair,
the open-access catalogues, and BATSE and fit this or private data. {\sc
Redback} can also be used as an engine to simulate transients for telescopes
such as the Zwicky Transient Facility and Vera Rubin with realistic cadences,
limiting magnitudes, and sky-coverage or a hypothetical user-constructed survey
with arbitrary settings. We also provide a more general simulation interface
suitable for target of opportunity observations with different telescopes. We
demonstrate through a series of examples how {\sc Redback} can be used as a
tool to simulate a large population of transients for realistic surveys, fit
models to real, simulated, or private data, multi-messenger inference and serve
as an end-to-end software toolkit for parameter estimation and interpreting the
nature of electromagnetic transients. | astro-ph_HE |
A theoretical color-velocity correlation for supernovae associated with
gamma-ray bursts: We carry out the first multi-dimensional radiative transfer calculations to
simultaneously compute synthetic spectra and light curves for models of
supernovae driven by fast bipolar outflows. These allow us to make
self-consistent predictions for the orientation dependence of both colour
evolution and spectral features. We compare models with different degrees of
asphericity and metallicity and find significant observable consequences of
both. In aspherical models, we find spectral and light curve features that vary
systematically with observer orientation. In particular, we find that the early
phase light curves are brighter and bluer when viewed close to the polar axis
but that the peak flux is highest for equatorial (off-axis) inclinations.
Spectral line features also depend systematically on observer orientation,
including the velocity of the SiII 6355A line. Consequently, our models predict
a correlation between line velocity and color that could assist the
identification of supernovae associated with off-axis jet-driven explosions.
The amplitude and range of this correlation depends on the degree of
asphericity, the metallicity and the epoch of observation but we find that it
is always present and acts in the same direction. | astro-ph_HE |
Evidence of jet induced optical microvariability in radio-loud Narrow
Line Seyfert 1 Galaxies: To quantify the role of radio jets for Intra-Night Optical Variability (INOV)
in Radio-Loud Narrow-Line Seyfert 1 (RLNLSy1) galaxies, we report the first
systematic comparative INOV study of 23 RLNLSy1 galaxies, with 15 RLNLSy1s
having confirmed detection of jets (jetted) and the remaining 8 RLNLSy1s having
no detection of jets (non-jetted) based on their Very Long Baseline Array
observations. We have monitored these two samples, respectively, in 37 and 16
sessions of a minimum 3-hour duration each. Based upon F$^{\eta}$-test at 99\%
confidence level with a typical INOV amplitude ($\psi$) detection threshold of
$>$ 3\%, we find the INOV duty cycles of 12\% for the sample of jetted
RLNLSy1s, however, none of the sources showed INOV in the sample of non-jetted
RLNLSy1s. Among the jetted RLNLSy1s, we find that the Duty Cycle (DC) for
jetted $\gamma$-ray detected ($\gamma$-ray) RLNLSy1s is found to be 34\% in
contrast to null INOV detection in the case of non-$\gamma$-ray RLNLSy1s. It
suggests that instead of the mere presence of a jet, relativistic beaming plays
a significant role for INOV in the case of low-luminous high accreting AGNs
such as NLSy1s in which dilution of the AGN's non-thermal optical emission by
the (much steadier) optical emission contributed by the nuclear accretion disc
is quite likely. Our study of jetted $\gamma$-ray RLNLSy1s shows more frequent
INOV detection for sources with higher apparent jet speed. Further, our results
also suggest that among the NLSy1s, only jetted $\gamma$-ray RNLSy1 galaxies DC
approaches blazar like DC. | astro-ph_HE |
On the paucity of Fast Radio Bursts at low Galactic latitudes: We examine the effect of Galactic diffractive interstellar scintillation as a
means of explaining the reported deficit of Fast Radio Burst (FRB) detections
at low Galactic latitude. We model the unknown underlying FRB flux density
distribution as a power law with a rate scaling as $S_\nu^{-5/2+\delta}$ and
account for the fact that the FRBs are detected at unknown positions within the
telescope beam. We find that the event rate of FRBs located off the Galactic
plane may be enhanced by a factor ~30-300% relative to objects near the
Galactic plane without necessarily affecting the slope of the distribution. For
FRBs whose flux densities are subject to relatively weak diffractive
scintillation, as is typical for events detected at high Galactic latitudes, we
demonstrate that an effect associated with Eddington bias is responsible for
the enhancement. The magnitude of the enhancement increases with the steepness
of the underlying flux density distribution, so that existing limits on the
disparity in event rates between high and low Galactic latitudes suggest that
the FRB population has a steep differential flux density distribution, scaling
as $S_\nu^{-3.5}$ or steeper. Existing estimates of the event rate in the flux
density range probed by the High Time Resolution Universe (HTRU) survey
overestimate the true rate by a factor of ~3. | astro-ph_HE |
The smooth cyclotron line in Her X-1 as seen with NuSTAR: Her X-1, one of the brightest and best studied X-ray binaries, shows a
cyclotron resonant scattering feature (CRSF) near 37 keV. This makes it an
ideal target for detailed study with the Nuclear Spectroscopic Telescope Array
(NuSTAR), taking advantage of its excellent hard X-ray spectral resolution. We
observed Her X-1 three times, coordinated with Suzaku, during one of the high
flux intervals of its 35d super-orbital period. This paper focuses on the shape
and evolution of the hard X-ray spectrum. The broad-band spectra can be fitted
with a powerlaw with a high-energy cutoff, an iron line, and a CRSF. We find
that the CRSF has a very smooth and symmetric shape, in all observations and at
all pulse-phases. We compare the residuals of a line with a Gaussian optical
depth profile to a Lorentzian optical depth profile and find no significant
differences, strongly constraining the very smooth shape of the line. Even
though the line energy changes dramatically with pulse phase, we find that its
smooth shape does not. Additionally, our data show that the continuum is only
changing marginally between the three observations. These changes can be
explained with varying amounts of Thomson scattering in the hot corona of the
accretion disk. The average, luminosity-corrected CRSF energy is lower than in
past observations and follows a secular decline. The excellent data quality of
NuSTAR provides the best constraint on the CRSF energy to date. | astro-ph_HE |
Constraints on Axion-like Particles from a Hard $X$-ray Observation of
Betelgeuse: We use the first observation of Betelgeuse in hard $X$-rays to perform a
novel search for axion-like particles (ALPs). Betelgeuse is not expected to be
a standard source of $X$-rays, but light ALPs produced in the stellar core
could be converted back into photons in the Galactic magnetic field, producing
a detectable flux that peaks in the hard $X$-ray band
($E_\gamma>10\mathrm{\,keV}$). Using a 50 ks observation of Betelgeuse by the
$NuSTAR$ satellite telescope, we find no significant excess of events above the
expected background. Using models of the regular Galactic magnetic field in the
direction of Betelgeuse, we set a 95% C.L. upper limit on the ALP-photon
coupling of ${g_{a\gamma}<(0.5-1.8)\times10^{-11}}$ GeV$^{-1}$ (depending on
magnetic field model) for ALP masses ${m_{a}<(5.5-3.5) \times10^{-11}}$ eV. | astro-ph_HE |
Using radial velocities to reveal black holes in binaries: a test case: Aims. Large radial velocity variations in the LAMOST spectra of giant stars
have been used to infer the presence of unseen companions. Some of them have
been proposed as possible black hole candidates. We test this selection by
investigating the classification of the one candidate having a known X-ray
counterpart (UCAC4 721-037069). Methods. We obtained time-resolved spectra from
the Liverpool Telescope and a 5ks observation from the Chandra observatory to
fully constrain the orbital parameters and the X-ray emission of this system.
Results. We find the source to be an eclipsing stellar binary that can be
classified as a RS CVn. The giant star fills its Roche Lobe and the binary mass
ratio is greater than one. The system may be an example of stable mass transfer
from an intermediate-mass star with a convective envelope. Conclusions. Using
only radial velocity to identify black hole candidates can lead to many false
positives. The presence of an optical orbital modulation, such as observed for
all LAMOST candidates, will in most cases indicate that this is a stellar
binary. | astro-ph_HE |
X-ray counterpart candidates for six new $γ$-ray pulsars: Using archival X-ray data we have found point-like X-ray counterpart
candidates positionally coincident with six $\gamma$-ray pulsars discovered
recently in the Fermi Gamma-ray Space Telescope data by the Einstein@Home
project. The candidates for PSRs J0002$+$6216, J0554$+$3107, J1844$-$0346 and
J1105$-$6037 are detected with Swift, and those for PSRs J0359$+$5414 and
J2017$+$3625 are detected with Chandra. Despite a low count statistics for some
candidates, assuming plausible constraints on the absorbing column density
towards the pulsars, we show that X-ray spectral properties for all of them are
consistent with those observed for other pulsars. J0359$+$5414 is the most
reliably identified object. We detect a nebula around it, whose spectrum and
extent suggest that this is a pulsar wind nebula powered by the pulsar.
Associations of J0002$+$6216 and J1844$-$0346 with supernova remnants CTB 1 and
G28.6$-$0.1 are proposed. | astro-ph_HE |
AMS02 results support the secondary origin of cosmic ray positrons: We show that the recent AMS02 positron fraction measurement is consistent
with a secondary origin for positrons, and does not require additional primary
sources such as pulsars or dark matter. The measured positron fraction at high
energy saturates the previously predicted upper bound for secondary production
(Katz et al 2009), obtained by neglecting radiative losses. This coincidence,
which will be further tested by upcoming AMS02 data at higher energy, is a
compelling indication for a secondary source. Within the secondary model the
AMS02 data imply a cosmic ray propagation time in the Galaxy of < Myr and an
average traversed interstellar matter density of order 1/cc, comparable to the
density of the Milky Way gaseous disk, at a rigidity of 300 GV. | astro-ph_HE |
An in-depth investigation of 11 pulsars discovered by FAST: We present timing solutions and analyses of 11 pulsars discovered by the
Five-hundred-meter Aperture Spherical radio Telescope (FAST). These pulsars
were discovered using an ultra-wide bandwidth receiver in drift-scan
observations made during the commissioning phase of FAST, and were then
confirmed and timed using the 64-m Parkes Radio Telescope. Each pulsar has been
observed over a span of at least one year. Highlighted discoveries include PSR
J0344-0901, which displays mode-changing behaviour and may belong to the class
of so-called `swooshing' pulsars (alongside PSRs B0919+06 and B1859+07); PSR
J0803-0942, whose emission is almost completely linearly polarised; and PSRs
J1900-0134 and J1945+1211, whose well defined polarisation angle curves place
stringent constraints on their emission geometry. We further discuss the
detectability of these pulsars by earlier surveys, and highlight lessons
learned from our work in carrying out confirmation and monitoring observations
of pulsars discovered by a highly sensitive telescope, many of which may be
applicable to next-generation pulsar surveys. This paper marks one of the first
major releases of FAST-discovered pulsars, and paves the way for future
discoveries anticipated from the Commensal Radio Astronomy FAST Survey
(CRAFTS). | astro-ph_HE |
Disentangling Instrumental Features of the 130 GeV Fermi Line: We study the instrumental features of photons from the peak observed at
$E_\gamma=130$ GeV in the spectrum of Fermi-LAT data. We use the {\sc sPlots}
algorithm to reconstruct -- seperately for the photons in the peak and for
background photons -- the distributions of incident angles, the recorded time,
features of the spacecraft position, the zenith angles, the conversion type and
details of the energy and direction reconstruction. The presence of a striking
feature or cluster in such a variable would suggest an instrumental cause for
the peak. In the publically available data, we find several suggestive features
which may inform further studies by instrumental experts, though the size of
the signal sample is too small to draw statistically significant conclusions. | astro-ph_HE |
Self-consistent neutrino and UHE cosmic ray spectra from Mrk 421: We examine the neutrino and cosmic ray spectra resulting from two models of
fitting the spectral energy distribution (SED) of the blazar Mrk~421 using a
self-consistent leptohadronic code. The $\gamma$-ray emission is attributed to
either synchrotron radiation of ultra-high energy protons (LHs model) or to
synchrotron radiation from electrons that result from photopion interactions of
lower energy protons (LH$\pi$ model). Although both models succeed in fitting
satisfactorily the SED, the parameter values that they use result in
significantly different neutrino and cosmic-ray spectra. For the LH$\pi$ model,
which requires high proton energy density, we find that the neutrino spectrum
peaks at an energy $E_{\rm \nu,peak}= 3.3$ PeV which falls well within the
energy range of recent neutrino observations. While at the same time its peak
flux is just under the sensitivity limit of IC-40 observations, it cannot
produce ultra-high energy cosmic rays. In the LHs model, on the other hand,
neutrinos are far from being detectable because of their low flux and peak
energy at $E_{{\rm \nu,peak}} \simeq 100$ PeV. However, the propagation of
protons produced by the decay of escaping neutrons results in an ultra-high
energy cosmic ray flux close to that observed by Pierre Augere, HiRes and
Telescope Array at energies $E_p \simeq 30$ EeV. | astro-ph_HE |
Neutrino-driven massive stellar explosions in 3D fostered by magnetic
fields via turbulent $α$-effect: We investigate the influence of magnetic field amplification on the
core-collapse supernovae in highly magnetized progenitors through
three-dimensional simulations. By considering rotating models, we observe a
strong correlation between the exponential growth of the magnetic field in the
gain region and the initiation of shock revival, with a faster onset compared
to the non-rotating model. We highlight that the mean magnetic field
experiences exponential amplification as a result of $\alpha$-effect in the
dynamo process, which works efficiently with the increasing kinetic helicity of
the turbulence within the gain region. Our findings indicate that the
significant amplification of the mean magnetic fields leads to the development
of locally intense turbulent magnetic fields, particularly in the vicinity of
the poles, thereby promoting the revival of the shock by neutrino heating. | astro-ph_HE |
A systematic study of magnetar-powered hydrogen-rich supernovae: Context: It has been suggested that some supernovae (SNe) may be powered by a
magnetar formed at the moment of the explosion. While this scenario has mostly
been applied to hydrogen-free events, it may be possible also for hydrogen-rich
objects. Aims: We explore the effect of including a magnetar on the light
curves of supernovae with H-rich progenitors. Methods: We have applied a
version of our one-dimensional LTE radiation hydrodynamics code that takes into
account the relativistic motion of the ejecta caused by the extra energy
provided by the magnetar. For a fixed red supergiant (RSG) progenitor, we have
obtained a set of light curves that corresponds to different values of the
magnetar initial rotation energy and the spin-down timescale. The model is
applied to SN~2004em and OGLE-2014-SN-073, two peculiar Type II SNe with
long-rising SN1987A-like light curves, although with much larger luminosities.
Results: The presence of a plateau phase in either normal or superluminous
supernovae is one possible outcome, even if a magnetar is continuously
injecting energy into the ejecta.In other cases, the light curve shows a peak
but not a plateau. Also, there are intermediate events with a first peak
followed by a slow decline and a late break of the declining slope. Our models
show that bright and long rising morphologies are possible even assuming RSG
structures. Conclusions: A large number of supernova discoveries per year
reveal unexpected new types of explosions. According to our results, SLSNe II-P
are to be expected, as well as a variety of light curve morphologies that can
all be possible signs of a newly born magnetar. | astro-ph_HE |
Self-similar structure of a hot magnetized flow with thermal conduction: We have explored the structure of hot magnetized accretion flow with thermal
conduction. The importance of thermal conduction in hot accretion flows has
been confirmed by observations of the hot gas surrounding Sgr $A^*$ and a few
other nearby galactic nuclei. For a steady state structure of such accretion
flows a set of self similar solutions are presented. In this paper, we have
actually tried to re-check the solution presented by Abbassi et al. (2008)
using a physical constrain. In this study we find that Eq 29 places a new
constrain that limits answers presented by Abbassi et al. 2008. In that paper
the parameter space in which it is established in the new constrain was
plotted. However, the new requirement makes up only a small parameter space
with physically acceptable solutions. And now in this manuscript we have
followed the idea with more effort, and tried to find out how thermal
conduction influences the structur of the disks in a physical parameter space.
We have found out that the existence of thermal conduction will lead to
reduction of accretion and radial and azimuthal velocities as well as the
vertical thickness of the disk, which is slightly reduced. Moreover, the
surface density of the disk will increase when the thermal conduction becomes
important in the hot magnetized flow. | astro-ph_HE |
A combined timing/spectral study of IRAS 13224-3809 using XMM-Newton
data: We present the results from an X-ray variability study of IRAS 13224-3809.
This is probably the best source for X-ray reverberation studies since it is
X-ray bright, extremely variable, and it has been extensively observed with
XMM-Newton. We used all the archival XMM-Newton data from the three EPIC
cameras (to increase the signal-to-noise) and, given the many observations of
the source, we were able to compute the time-lags spectra in three different
flux levels/periods. We fitted the time-lags and energy spectra,
simultaneously, using a new X-ray reverberation code which computes the time
dependent reflection spectra of the disc as a response to an X-ray flash from a
point source located on the axis of the black-hole (BH) accretion disc
(lamp-post geometry). To the best of our knowledge, this is the first time for
an AGN that both time-lags and energy spectra are fitted by a model
simultaneously in different flux periods. The model fits in the case when the
BH is rapidly rotating are significantly better than the model fits in the case
of a Schwarzschild BH. This result strongly favours the hypothesis of a
rotating central BH in this source. We also detect significant variations in
the height of the X-ray corona. The X-ray height appears to increase from 3-5
gravitational radii when the X-ray luminosity is of the order of 1.5-3 percent
of the Eddington limit, up to 10 gravitational radii, when the luminosity
doubles. | astro-ph_HE |
Time-dependent Modeling of Pulsar Wind Nebulae: A spatially independent model that calculates the time evolution of the
electron spectrum in a spherically expanding pulsar wind nebula (PWN) is
presented, allowing one to make broadband predictions for the PWN's non-thermal
radiation. The source spectrum of electrons injected at the termination shock
of the PWN is chosen to be a broken power law. In contrast to previous PWN
models of a similar nature, the source spectrum has a discontinuity in
intensity at the transition between the low and high-energy components. To test
the model, it is applied to the young PWN G21.5--0.9, where it is found that a
discontinuous source spectrum can model the emission at all wavelengths better
than a continuous one. The model is also applied to the unidentified sources
HESS J1427--608 and HESS J1507--622. Parameters are derived for these two
candidate nebulae that are consistent with the values predicted for other PWNe.
For HESS J1427--608 a present-day magnetic field of $B\s{age}=0.4\,\mu\text{G}$
is derived. As a result of the small present-day magnetic field, this source
has a low synchrotron luminosity, while remaining bright at GeV/TeV energies.
It is therefore possible to interpret HESS J1427--608 within the ancient PWN
scenario. For the second candidate PWN HESS J1507--622, a present-day magnetic
field of $B\s{age}=1.7\,\mu\text{G}$ is derived. Furthermore, for this
candidate PWN a scenario is favoured in the present paper in which HESS
J1507--622 has been compressed by the reverse shock of the supernova remnant. | astro-ph_HE |
A search for ultra high energy neutrinos from TXS 0506+056 using the
Pierre Auger Observatory: Results of a search for ultra-high-energy neutrinos with the Pierre Auger
Observatory from the direction of the blazar TXS 0506+056 are presented. They
were obtained as part of the follow-up that stemmed from the detection of
high-energy neutrinos and gamma rays with IceCube, \textit{Fermi}-LAT, MAGIC,
and other detectors of electromagnetic radiation in several bands. The Pierre
Auger Observatory is sensitive to neutrinos in the energy range from 100 PeV to
100 EeV and in the zenith angle range from $\theta=60^\circ$ to
$\theta=95^\circ$, where the zenith angle is measured from the vertical
direction. No neutrinos from the direction of TXS 0506+056 have been found. The
results were analyzed in three periods: One of 6 months around the detection of
IceCube-170922A, coinciding with a flare period of TXS 0506+056, a second one
of 110 days during which the IceCube collaboration found an excess of 13
neutrinos from a direction compatible with TXS 0506+056, and a third one from
2004 January 1 up to 2018 August 31, over which the Pierre Auger Observatory
has been taking data. The sensitivity of the Observatory is addressed for
different spectral indices by considering the fluxes that would induce a single
expected event during the observation period. For indices compatible with those
measured by the IceCube collaboration the expected number of neutrinos at the
Observatory is well-below one. Spectral indices as hard as 1.5 would have to
apply in this energy range to expect a single event to have been detected. | astro-ph_HE |
Observable signatures of stellar-mass black holes in active galactic
nuclei: Stellar-mass black holes (BHs) are predicted to be embedded in the disks of
active galactic nuclei (AGN) due to gravitational drag and in-situ star
formation. However, clear evidence for AGN disk-embedded BHs is currently
lacking. Here, as possible electromagnetic signatures of these BHs, we
investigate breakout emission from shocks emerging around Blandford-Znajek jets
launched from accreting BHs in AGN disks. We assume that the majority of the
highly super-Eddington flow reaches the BH, produces a strong jet, and the jet
produces feedback that shuts off accretion and thus leads to episodic flaring.
While these assumptions are highly uncertain at present, they predict a
breakout emission characterized by luminous thermal emission in the X-ray
bands, and bright, broadband non-thermal emission from the infrared to the
gamma-ray bands. The flare duration depends on the BH's distance $r$ from the
central supermassive BH, varying between $10^3-10^6$ s for $r \sim 0.01-1$ pc.
This emission can be discovered by current and future infrared, optical, and
X-ray wide-field surveys and monitoring campaigns of nearby AGNs. | astro-ph_HE |
HAFFET: Hybrid Analytic Flux FittEr for Transients: The progenitors for many types of supernovae (SNe) are still unknown, and an
approach to diagnose their physical origins is to investigate the light curve
brightness and shape of a large set of SNe. However, it is often difficult to
compare and contrast the existing sample studies due to differences in their
approaches and assumptions, for example in how to eliminate host galaxy
extinction, and this might lead to systematic errors when comparing the
results. We therefore introduce the Hybrid Analytic Flux FittEr for Transients
(haffet), a Python-based software package that can be applied to download
photometric and spectroscopic data for transients from open online sources,
derive bolometric light curves, and fit them to semi-analytical models for
estimation of their physical parameters. In a companion study, we have
investigated a large collection of SNe Ib and Ic observed with the Zwicky
Transient Facility (ZTF) with haffet, and here we detail the methodology and
the software package to encourage more users. As large-scale surveys such as
ZTF and LSST continue to discover increasing numbers of transients, tools such
as haffet will be critical for enabling rapid comparison of models against data
in statistically consistent, comparable and reproducable ways. Additionally,
haffet is created with a Graphical User Interface mode, which we hope will
boost the efficiency and make the usage much easier. | astro-ph_HE |
A millisecond pulsar discovery in a survey of unidentified Fermi
$γ$-ray sources with LOFAR: Using LOFAR, we have performed a very-low-frequency (115-155 MHz) radio
survey for millisecond pulsars (MSPs). The survey targeted 52 unidentified
Fermi $\gamma$-ray sources. Employing a combination of coherent and incoherent
dedispersion, we have mitigated the dispersive effects of the interstellar
medium while maintaining sensitivity to fast-spinning pulsars. Toward 3FGL
J1553.1+5437 we have found PSR J1552+5437, the first MSP to be discovered
(through its pulsations) at a radio frequency $<$ 200 MHz. PSR J1552+5437 is an
isolated MSP with a 2.43 ms spin period and a dispersion measure of 22.9 pc
cm$^{-3}$. The pulsar has a very steep radio spectral index ($\alpha < -$2.8
$\pm$ 0.4). We obtain a phase-connected timing solution combining the 0.74
years of radio observations with $\gamma$-ray photon arrival times covering 7.5
years of Fermi observations. We find that the radio and $\gamma$-ray pulse
profiles of PSR J1552+5437 appear to be nearly aligned. The very steep spectrum
of PSR J1552+5437, along with other recent discoveries, hints at a population
of radio MSPs that have been missed in surveys using higher observing
frequencies. Detecting such steep spectrum sources is important for mapping the
population of MSPs down to the shortest spin periods, understanding their
emission in comparison to slow pulsars, and quantifying the prospects for
future surveys with low-frequency radio telescopes like SKA-Low and its
precursors. | astro-ph_HE |
An XMM-Newton Study of the Mixed-Morphology Supernova Remnant G346.6-0.2: We present an X-ray imaging and spectroscopic study of the molecular cloud
interacting mixed-morphology (MM) supernova remnant (SNR) G346.6-0.2 using
XMM-Newton. The X-ray spectrum of the remnant is well described by a
recombining plasma that most likely arises from adiabatic cooling, and has
sub-solar abundances of Mg, Si, and S. Our fits also suggest the presence of
either an additional power-law component with a photon index of $\sim$2, or an
additional thermal component with a temperature of $\sim$2.0 keV. We
investigate the possible origin of this component and suggest that it could
arise from either the Galactic ridge X-ray emission, an unidentified pulsar
wind nebula or X-ray synchrotron emission from high-energy particles
accelerated at the shock. However, deeper, high resolution observations of this
object are needed to shed light on the presence and origin of this feature.
Based on its morphology, its Galactic latitude, the density of the surrounding
environment and its association with a dense molecular cloud, G346.6-0.2 most
likely arises from a massive progenitor that underwent core-collapse. | astro-ph_HE |
A search for VHE counterparts of Galactic Fermi bright sources and MeV
to TeV spectral characterization: Very high-energy (VHE; E>100 GeV) gamma-rays have been detected from a wide
range of astronomical objects, such as pulsar wind nebulae (PWNe), supernova
remnants (SNRs), giant molecular clouds, gamma-ray binaries, the Galactic
Center, active galactic nuclei (AGN), radio galaxies, starburst galaxies, and
possibly star-forming regions as well. At lower energies, observations using
the Large Area Telescope (LAT) onboard Fermi provide a rich set of data which
can be used to study the behavior of cosmic accelerators in the MeV to TeV
energy bands. In particular, the improved angular resolution of current
telescopes in both bands compared to previous instruments significantly reduces
source confusion and facilitates the identification of associated counterparts
at lower energies. In this paper, a comprehensive search for VHE gamma-ray
sources which are spatially coincident with Galactic Fermi/LAT bright sources
is performed, and the available MeV to TeV spectra of coincident sources are
compared. It is found that bright LAT GeV sources are correlated with TeV
sources, in contrast to previous studies using EGRET data. Moreover, a single
spectral component seems unable to describe the MeV to TeV spectra of many
coincident GeV/TeV sources. It has been suggested that gamma-ray pulsars may be
accompanied by VHE gamma-ray emitting nebulae, a hypothesis that can be tested
with VHE observations of these pulsars. | astro-ph_HE |
Hard X-ray selected giant radio galaxies -- III. The LOFAR view: Giant radio galaxies (GRGs), with extended structures reaching hundreds of
kpc, are among the most spectacular examples of ejection of relativistic plasma
from super-massive black holes. In this work, third of a series, we present LOw
Frequency ARray (LOFAR) images at 144 MHz, collected in the framework of the
LOFAR Two-metre Sky Survey Data Release 2 (LoTSS DR2), for nine sources
extracted from our sample of hard X-ray selected GRGs (HXGRG, i.e. from
INTEGRAL/IBIS and Swift/BAT catalogues at >20 keV). Thanks to the resolution
and sensitivity of LoTSS, we could probe the complex morphology of these GRGs,
unveiling cases with diffuse (Mpc-scale) remnant emission, presence of faint
off-axis wings, or a misaligned inner jet. In particular, for one source
(B21144+35B), we could clearly detect a $\sim$300 kpc wide off-axis emission,
in addition to an inner jet which orientation is not aligned with the lobes
axis. For another source (J1153.9+5848) a structure consistent with jet
precession was revealed, appearing as an X-shaped morphology with relic lobes
having an extension larger than the present ones, and with a different axis
orientation. From an environment analysis, we found 2 sources showing an
overdensity of cosmological neighbours, and a correspondent association with a
galaxy cluster from catalogues. Finally, a comparison with radio-selected GRGs
from LoTSS DR1 suggested that, on average, HXGRG can grow to larger extents.
These results highlight the importance of deep low-frequency observations to
probe the evolution of radio galaxies, and ultimately estimate the duty cycle
of their jets. | astro-ph_HE |
Structures in the cosmic ray energy spectra: All the components of cosmic rays have 'structure' in their energy spectra at
some level, ie deviations from a simple power law, and their examination is
relevant to the origin of the particles. Emphasis, here, is placed on the
large-scale structures in the spectra of nuclei (the 'knee' at about 3 PeV),
that of electrons and positrons (a shallow 'upturn' at about 100 GeV) and the
positron to electron plus positron ratio (an upturn starting at about 5 GeV).
Fine structure is defined as deviations from the smooth spectra which already
allow for the large-scale structure. Search for the fine structure has been
performed in the precise data on positron to electron plus positron ratio
measured by the AMS-02 experiment. Although no fine structure is indicated, it
could in fact be present at the few percent level. | astro-ph_HE |
Pair-Instability Supernovae in the Local Universe: The discovery of 150 - 300 M$_{\odot}$ stars in the Local Group and
pair-instability supernova candidates at low redshifts has excited interest in
this exotic explosion mechanism. Realistic light curves for pair-instability
supernovae at near-solar metallicities are key to identifying and properly
interpreting these events as more are found. We have modeled pair-instability
supernovae of 150 - 500 M$_{\odot}$ Z $\sim$ 0.1 - 0.4 Z$_{\odot}$ stars. These
stars lose up to 80% of their mass to strong line-driven winds and explode as
bare He cores. We find that their light curves and spectra are quite different
from those of Population III pair-instability explosions, which therefore
cannot be used as templates for low-redshift events. Although non-zero
metallicity pair-instability supernovae are generally dimmer than their
Population III counterparts, in some cases they will be bright enough to be
detected at the earliest epochs at which they can occur, the formation of the
first galaxies at $z \sim$ 10 - 15. Others can masquerade as dim, short
duration supernovae that are only visible in the local universe and that under
the right conditions could be hidden in a wide variety of supernova classes. We
also report for the first time that some pair-instability explosions can create
black holes with masses of $\sim$ 100 M$_{\odot}$. | astro-ph_HE |
Ultraviolet Diversity of Type Ia Supernovae: Ultraviolet (UV) observations of Type Ia supernovae (SNe Ia) probe the
outermost layers of the explosion, and UV spectra of SNe Ia are expected to be
extremely sensitive to differences in progenitor composition and the details of
the explosion. Here we present the first study of a sample of high
signal-to-noise ratio SN Ia spectra that extend blueward of 2900 A. We focus on
spectra taken within 5 days of maximum brightness. Our sample of ten SNe Ia
spans the majority of the parameter space of SN Ia optical diversity. We find
that SNe Ia have significantly more diversity in the UV than in the optical,
with the spectral variance continuing to increase with decreasing wavelengths
until at least 1800 A (the limit of our data). The majority of the UV variance
correlates with optical light-curve shape, while there are no obvious and
unique correlations between spectral shape and either ejecta velocity or
host-galaxy morphology. Using light-curve shape as the primary variable, we
create a UV spectral model for SNe Ia at peak brightness. With the model, we
can examine how individual SNe vary relative to expectations based on only
their light-curve shape. Doing this, we confirm an excess of flux for SN 2011fe
at short wavelengths, consistent with its progenitor having a subsolar
metallicity. While most other SNe Ia do not show large deviations from the
model, ASASSN-14lp has a deficit of flux at short wavelengths, suggesting that
its progenitor was relatively metal rich. | astro-ph_HE |
Combined Rastall and Rainbow theories of gravity with applications to
neutron stars: The possibility of modifications on general relativity is investigated. We
propose an alternative theory of gravity constructed with the combination of
Rastall and Rainbow theories. The hydrostatic equilibrium equations are
obtained in order to test the new theory in neutron stars, whose mass-radius
diagrams are obtained using modern equations of state of nuclear matter derived
from relativistic mean field models and compared with the ones computed by the
Tolman-Oppenheimer-Volkoff equations. We conclude that substantial
modifications are obtained even for very small alterations on the two free
parameters, making the reproduction of astrophysical observations an easy task. | astro-ph_HE |
The broad band spectral variability of MCG-6-30-15 observed by NuSTAR
and XMM-Newton: MCG-6-30-15, at a distance of 37 Mpc (z=0.008), is the archetypical Seyfert 1
galaxy showing very broad Fe K$\alpha$ emission. We present results from a
joint NuSTAR and XMM-Newton observational campaign that, for the first time,
allows a sensitive, time-resolved spectral analysis from 0.35 keV up to 80 keV.
The strong variability of the source is best explained in terms of intrinsic
X-ray flux variations and in the context of the light bending model: the
primary, variable emission is reprocessed by the accretion disk, which produces
secondary, less variable, reflected emission. The broad Fe K$\alpha$ profile
is, as usual for this source, well explained by relativistic effects occurring
in the innermost regions of the accretion disk around a rapidly rotating black
hole. We also discuss the alternative model in which the broadening of the Fe
K$\alpha$ is due to the complex nature of the circumnuclear absorbing
structure. Even if this model cannot be ruled out, it is disfavored on
statistical grounds. We also detected an occultation event likely caused by BLR
clouds crossing the line of sight. | astro-ph_HE |
Gamma-ray emission from PSR J0007+7303 using 7 years of Fermi Large Area
Telescope observations: Based on more than seven years of Fermi Large Area Telescope (LAT) Pass 8
data, we report on a detailed analysis of the bright gamma-ray pulsar (PSR)
J0007+7303. We confirm that PSR J0007+7303 is significantly detected as a point
source also during the off-peak phases with a TS value of 262 ($\sim$ 16
$\sigma$). In the description of PSR J0007+7303 off-peak spectrum, a power law
with an exponential cutoff at 2.7$\pm$1.2$\pm$1.3 GeV (the first/second
uncertainties correspond to statistical/systematic errors) is preferred over a
single power law at a level of 3.5 $\sigma$. The possible existence of a cutoff
hints at a magnetospheric origin of the emission. In addition, no extended
gamma-ray emission is detected compatible with either the supernova remnant
(CTA 1) or the very high energy (> 100 GeV) pulsar wind nebula. A flux upper
limit of 6.5$\times$10$^{-12}$ erg cm$^{-2}$ s$^{-1}$ in the 10-300 GeV energy
range is reported, for an extended source assuming the morphology of the
VERITAS detection. During on-peak phases, a sub-exponential cutoff is
significantly preferred ($\sim$11 $\sigma$) for representing the spectral
energy distribution, both in the phase-averaged and in the phase-resolved
spectra. Three glitches are detected during the observation period and we found
no flux variability at the time of the glitches or in the long-term behavior.
We also report the discovery of a previously unknown gamma-ray source in the
vicinity of PSR J0007+7303, Fermi J0020+7328, which we associate with the z =
1.781 quasar S5 0016+73. A concurrent analysis of this source is needed to
correctly characterize the behavior of CTA 1 and it is also presented in the
paper. | astro-ph_HE |
Formation of the First Two Black Hole-Neutron Star Mergers (GW200115 and
GW200105) from Isolated Binary Evolution: In this work we study the formation of the first two black hole-neutron star
(BHNS) mergers detected in gravitational waves (GW200115 and GW200105) from
massive stars in wide isolated binary systems - the isolated binary evolution
channel. We use 560 BHNS binary population synthesis model realizations from
Broekgaarden et al. (2021a) and show that the system properties (chirp mass,
component masses and mass ratios) of both GW200115 and GW200105 match
predictions from the isolated binary evolution channel. We also show that most
model realizations can account for the local BHNS merger rate densities
inferred by LIGO-Virgo. However, to simultaneously also match the inferred
local merger rate densities for BHBH and NSNS systems we find we need models
with moderate kick velocities ($\sigma\lesssim 10^2\,\rm{km}\,\rm{s}^{-1}$) or
high common-envelope efficiencies ($\alpha_{\rm{CE}}\gtrsim 2$) within our
model explorations. We conclude that the first two observed BHNS mergers can be
explained from the isolated binary evolution channel for reasonable model
realizations. | astro-ph_HE |
Evidence for Declination Dependence of the Ultrahigh Energy Cosmic Ray
Spectrum in the Northern Hemisphere: Telescope Array (TA) is the largest experiment in the Northern Hemisphere
studying ultrahigh energy cosmic rays. TA measurements of the cosmic ray
spectrum using the surface detector have the best statistical power in the
experiment, and observe the ankle of the spectrum and the high energy cutoff.
When the data are divided into two declination bands, above and below 24.8
degrees, the cutoff appears at $10^{19.64 \pm 0.04}$ ($10^{19.84 \pm 0.02}$) eV
in the lower (higher) band, an energy difference of 58\%. The global
significance of the difference is 4.3 standard deviations. The lack of an
instrumental cause of this difference implies it is astrophysical in nature. | astro-ph_HE |
High Metallicity LGRB Hosts: We present our imaging and spectroscopic observations of the host galaxies of
two dark long bursts with anomalously high metallicities, LGRB 051022 and LGRB
020819B, which in conjunction with another LGRB event with an optical afterglow
comprise the three LGRBs with high metallicity host galaxies in the Graham &
Fruchter (2013) sample. In Graham & Fruchter (2013), we showed that LGRBs
exhibit a strong and apparently intrinsic preference for low metallicity
environments (12+log(O/H) < 8.4 in the KK04 scale) in spite of these three
cases with abundances of about solar and above. These exceptions however are
consistent with the general star-forming galaxy population of comparable
brightness & redshift. This is surprising: even among a preselected sample of
high metallicity LGRBs, were the metal aversion to remain in effect for these
objects, we would expect their metallicity to still be lower than the typical
metallicity for the galaxies at that luminosity and redshift. Therefore we
deduce that it is possible to form an LGRB in a high metallicity environment
although with greater rarity.
From this we conclude that there are three possible explanations for the
presence of the LGRBs observed in high metallicity hosts as seen to date: (1)
LGRBs do not occur in high metallicity environments and those seen in high
metallicity hosts are in fact occurring in low metallicity environments that
have become associated with otherwise high metallicity hosts but remain
unenriched. (2) The LGRB formation mechanism while preferring low metallicity
environments does not strictly require it resulting in a gradual decline in
burst formation with increasing metallicity. (3) The typical low metallicity
LGRBs and the few high metallicity cases are the result of physically different
burst formation pathways with only the former affected by the metallicity and
the later occurring much more infrequently. | astro-ph_HE |
Unexpected emission pattern adds to the enigma of fast radio bursts: Observations of millisecond-long radio bursts from beyond the Milky Way have
revealed a repeat pattern with a roughly 16-day period -- a finding that adds
to the enigma of the origin of these bursts. | astro-ph_HE |
A variable ionized disk wind in MAXI J1803-298 revealed by NICER: We present the results from the NICER observation data of MAXI J1803-298
across the entire 2021 outburst. In the intermediate and soft state, we detect
significant absorption lines at $\sim 7.0$ keV and $\sim 6.7$ keV, arising from
the X-ray disk wind outflowing with a velocity of hundreds of km per second
along our line of sight. The fitting results from photoionized model suggest
that the wind is driven by thermal pressure and the mass-loss rate is low. We
find a clear transition for iron from predominantly H-like to predominantly
He-like during the intermediate-to-soft state transition. Our results indicate
this transition for iron is caused by the evolution of the illuminating
spectrum and the slow change of the geometric properties of the disk wind
together. The coexistence of disk wind and QPOs features in intermediate state
is also reported. Our study makes MAXI J1803-298 the first source in which a
transition from optical wind to X-ray wind is detected, offering new insights
into the evolution of disk winds across an entire outburst and long-term
coupling of accretion disks and mass outflows around accreting black holes. | astro-ph_HE |
Afterglow Observations of Fermi-LAT Gamma-Ray Bursts and the Emerging
Class of Hyper-Energetic Events: We present broadband (radio, optical, and X-ray) light curves and spectra of
the afterglows of four long-duration gamma-ray bursts (GRBs 090323, 090328,
090902B, and 090926A) detected by the Gamma-Ray Burst Monitor (GBM) and Large
Area Telescope (LAT) instruments on the Fermi satellite. With its wide spectral
bandpass, extending to GeV energies, Fermi is sensitive to GRBs with very large
isotropic energy releases (10e54 erg). Although rare, these events are
particularly important for testing GRB central-engine models. When combined
with spectroscopic redshifts, our afterglow data for these four events are able
to constrain jet collimation angles, the density structure of the circumburst
medium, and both the true radiated energy release and the kinetic energy of the
outflows. In agreement with our earlier work, we find that the relativistic
energy budget of at least one of these events (GRB 090926A) exceeds the
canonical value of 10e51 erg by an order of magnitude. Such energies pose a
severe challenge for models in which the GRB is powered by a magnetar or
neutrino-driven collapsar, but remain compatible with theoretical expectations
for magneto-hydrodynamical (MHD) collapsar models. Our jet opening angles
(theta) are similar to those found for pre-Fermi GRBs, but the large initial
Lorentz factors (Gamma_0) inferred from the detection of GeV photons imply
theta Gamma_0 ~ 70-90, values which are above those predicted in MHD models of
jet acceleration. Finally, we find that these Fermi-LAT events preferentially
occur in a low-density circumburst environment, and we speculate that this
might result from the lower mass-loss rates of their lower-metallicity
progenitor stars. Future studies of Fermi-LAT afterglows in the radio with the
order-of-magnitude improvement in sensitivity offered by the EVLA should
definitively establish the relativistic energy budgets of these events. | astro-ph_HE |
Discovery of the source HESS J1356-645 associated with the young and
energetic PSR J1357-6429: Several newly discovered very-high-energy (VHE; E > 100 GeV) gamma-ray
sources in the Galaxy are thought to be associated with energetic pulsars.
Among them, middle-aged (> 1E+4 yr) systems exhibit large centre-filled VHE
nebulae, offset from the pulsar position, which result from the complex
relationship between the pulsar wind and the surrounding medium, and reflect
the past evolution of the pulsar. Imaging Atmospheric Cherenkov Telescopes
(IACTs) have been successful in revealing extended emission from these sources
in the VHE regime. Together with radio and X-ray observations, this
observational window allows one to probe the energetics and magnetic field
inside these large-scale nebulae. H.E.S.S., with its large field of view,
angular resolution of < 0.1deg and unprecedented sensitivity, has been used to
discover a large population of such VHE sources. In this paper, the H.E.S.S.
data from the continuation of the Galactic Plane Survey (-80deg < l < 60deg,
|b| < 3deg), together with the existing multi-wavelength observations, are
used. A new VHE gamma-ray source was discovered at R.A. (J2000) = 13h56m00s,
Dec. (J2000) = -64d30m00s with a 2' statistical error in each coordinate,
namely HESS J1356-645. The source is extended, with an intrinsic Gaussian width
of (0.20 +/- 0.02)deg. Its integrated energy flux between 1 and 10 TeV of 8E-12
erg cm-2 s-1 represents ~ 11% of the Crab Nebula flux in the same energy band.
The energy spectrum between 1 and 20 TeV is well described by a power law dN/dE
~ E-Gamma with photon index Gamma = 2.2 +/- 0.2stat +/- 0.2sys. The inspection
of archival radio images at three frequencies and the analysis of X-ray data
from ROSAT/PSPC and XMM-Newton/MOS reveal the presence of faint non-thermal
diffuse emission coincident with HESS J1356-645. HESS J1356-645 is most likely
associated with the young and energetic pulsar PSR J1357-6429 (Abridged) | astro-ph_HE |
General relativistic treatment of $f$-mode oscillations of hyperonic
stars: We present a systematic study of $f$-mode oscillations in neutron stars
containing hyperons, extending recent results obtained within the Cowling
approximation to linearized General Relativity. Employing a relativistic mean
field model, we find that the Cowling approximation can overestimate the
quadrupolar $f$-mode frequency of neutron stars by up to 30\% compared to the
frequency obtained in the linearized general relativistic formalism. Imposing
current astrophysical constraints, we derive updated empirical relations for
gravitational wave asteroseismology. The frequency and damping time of
quadrupole $f$-mode oscillations of hyperonic stars are found to be in the
range of 1.47 - 2.45kHz and 0.13 - 0.51 sec respectively. Our correlation
studies demonstrate that among the various parameters of the nucleonic and
hyperonic sectors of the model, the nucleon effective mass shows the strongest
correlation with mode characteristics and neutron star observables. Estimates
for the detectability of $f$-modes in a transient burst of gravitational waves
from isolated hyperonic stars is also provided. | astro-ph_HE |
Timing and spectral variability of high mass X-ray pulsar GX 301--2 over
orbital phases observed by Insight-HXMT: We report the orbital X-ray variability of high mass X-ray binary (HMXB)
GX301--2. GX301--2 undergone a spin up process in 2018--2020 with the period
evolving from $\sim$ 685 s to 670 s. The energy resolved pulse-profiles of the
pulsar in 1--60 keV varied from single peaked and sinusoidal shapes to
multi-peaked across different orbital phases. Pulse fractions evolving over
orbit had negative correlations with the X-ray flux. The broad-band X-ray
energy spectrum of the pulsar can be described with a partial covering negative
positive cutoff power-law continuum model. Near the periastron passage of the
pulsar we found a strong variation in the additional column density ($NH_{2}$),
which correlated with variation of the flux. Curves of growth for both Fe
K$\alpha$ and Fe K$\beta$ lines were plotted to investigate the distribution of
matter around neutron star. We have also found the evidence of two cyclotron
absorption lines in the phase-averaged spectra in GX301--2, with one line of
30--42 keV and the other line varying in 48--56 keV. Both two line's centroid
energies show the similar relationship with X-ray luminosity: positive
correlation in lower luminosity range, and negative relation above a critical
luminosity of $10^{37}$ erg s$^{-1}$. We estimated the surface magnetic field
of the neutron star in GX301--2 at ~$(0.5-2)\times 10^{13}$ G. Two cyclotron
line energies have a nearly fixed ratio of ~1.63 while having a low strength
ratio (~0.05), suggesting that these two features may actually be one line. | astro-ph_HE |
Exploring the role of X-ray reprocessing and irradiation in the
anomalous bright optical outbursts of A0538-66: In 1981, the Be/X-ray binary (Be/XRB) A0538-66 showed outbursts characterized
by high peak luminosities in the X-ray and optical bands. The optical outbursts
were qualitatively explained as X-ray reprocessing in a gas cloud surrounding
the binary system. Since then, further important information about A0538-66
have been obtained, and sophisticated photoionization codes have been developed
to calculate the radiation emerging from a gas nebula illuminated by a central
X-ray source. In the light of the new information and tools available, we
studied again the enhanced optical emission displayed by A0538-66 to understand
the mechanisms responsible for these unique events among the class of Be/XRBs.
We performed about 10^5 simulations of a gas envelope photoionized by an X-ray
source. We assumed for the shape of the gas cloud either a sphere or a
circumstellar disc observed edge-on. We studied the effects of varying the main
properties of the envelope and the influence of different input X-ray spectra
on the optical/UV emission emerging from the photoionized cloud. We compared
the computed spectra with the IUE spectrum and photometric UBV measurements
obtained during the outburst of 29 April 1981. We also explored the role played
by the X-ray heating of the surface of the donor star irradiated by the X-ray
emission of the neutron star (NS). We found that reprocessing in a spherical
cloud with a shallow radial density distribution can reproduce the optical/UV
emission. To our knowledge, this configuration has never been observed either
in A0538-66 during other epochs or in other Be/XRBs. We found, contrary to the
case of most other Be/XRBs, that the optical/UV radiation produced by the X-ray
heating of the surface of the donor star irradiated by the NS is
non-negligible, due to the particular orbital parameters of this system that
bring the NS very close to its companion. | astro-ph_HE |
Type Ia supernovae from violent mergers of carbon-oxygen white dwarfs:
polarisation signatures: The violent merger of two carbon-oxygen white dwarfs has been proposed as a
viable progenitor for some Type Ia supernovae. However, it has been argued that
the strong ejecta asymmetries produced by this model might be inconsistent with
the low degree of polarisation typically observed in Type Ia supernova
explosions. Here, we test this claim by carrying out a spectropolarimetric
analysis for the model proposed by Pakmor et al. (2012) for an explosion
triggered during the merger of a 1.1 M$_{\odot}$ and 0.9 M$_{\odot}$
carbon-oxygen white dwarf binary system. Owing to the asymmetries of the
ejecta, the polarisation signal varies significantly with viewing angle. We
find that polarisation levels for observers in the equatorial plane are modest
($\lesssim$ 1 per cent) and show clear evidence for a dominant axis, as a
consequence of the ejecta symmetry about the orbital plane. In contrast,
orientations out of the plane are associated with higher degrees of
polarisation and departures from a dominant axis. While the particular model
studied here gives a good match to highly-polarised events such as SN 2004dt,
it has difficulties in reproducing the low polarisation levels commonly
observed in normal Type Ia supernovae. Specifically, we find that significant
asymmetries in the element distribution result in a wealth of strong
polarisation features that are not observed in the majority of currently
available spectropolarimetric data of Type Ia supernovae. Future studies will
map out the parameter space of the merger scenario to investigate if
alternative models can provide better agreement with observations. | astro-ph_HE |
Optical polarization variations in the blazar PKS 1749+096: We report on the variation in the optical polarization of the blazar PKS
1749+096 observed in 2008--2015. The degree of polarization (PD) tends to
increase in short flares having a time-scale of a few days. The object favors a
polarization angle (PA) of $40^\circ$--$50^\circ$ at the flare maxima, which is
close to the position angle of the jet ($20^\circ$--$40^\circ$). Three clear
polarization rotations were detected in the negative PA direction associated
with flares. In addition, a rapid and large decrease in the PA was observed in
the other two flares, while another two flares showed no large PA variation.
The light curve maxima of the flares possibly tend to lag behind the PD maxima
and color-index minima. The PA became $-50^\circ$ to $-20^\circ$ in the decay
phase of active states, which is almost perpendicular to the jet position
angle. We propose a scenario to explain these observational features, where
transverse shocks propagate along curved trajectories. The favored PA at the
flare maxima suggests that the observed variations were governed by the
variations in the Doppler factor, $\delta$. Based on this scenario, the minimum
viewing angle of the source, $\theta_\mathrm{min}=4.8^\circ$--$6.6^\circ$, and
the location of the source, $\Delta r\gtrsim 0.1$pc, from the central black
hole were estimated. In addition, the acceleration of electrons by the shock
and synchrotron cooling would have a time-scale similar to that of the change
in $\delta$. The combined effect of the variation in $\delta$ and
acceleration/cooling of electrons is probably responsible for the observed
diversity of the polarization variations in the flares. | astro-ph_HE |
A new universal cosmic-ray knee near the magnetic rigidity 10 TV with
the NUCLEON space observatory: Data from the NUCLEON space observatory give a strong indication of the
existence of a new universal cosmic ray "knee", which is observed in all groups
of nuclei, including heavy nuclei, near a magnetic rigidity of about 10\,TV.
Universality means the same position of the knee in the magnetic rigidity scale
for all groups of nuclei. The knee is observed by both methods of measurement
of particles energy implemented in the NUCLEON observatory---the calorimetric
method and the kinematic method KLEM. This new cosmic ray "knee" is probably
connected with the limit of acceleration of cosmic rays by some generic or
nearby source of cosmic rays. | astro-ph_HE |
How, where and when do cosmic rays reach ultrahigh energies?: Understanding the origins of ultrahigh energy cosmic rays (UHECRs) - which
reach energies in excess of $10^{20}~{\rm eV}$ - stretches particle
acceleration physics to its very limits. In this review, we discuss how such
energies can be reached, using general arguments that can often be derived on
the back of an envelope. We explore possible particle acceleration mechanisms,
with special attention paid to shock acceleration. Informed by the arguments
derived, we discuss where UHECRs might come from and which classes of powerful
astrophysical objects could be UHECR sources; generally, we favour radio
galaxies, GRB afterglows and other sources which are not too compact and
dissipate prodigious amounts of energy on large scales, allowing them to
generate large products $\beta B R$ without the CRs undergoing restrictive
losses. Finally, we discuss when UHECRs are accelerated by highlighting the
importance of source variability, and explore the intriguing possibility that
the UHECR arrival directions are partly a result of "echoes" from magnetic
structures in the local Universe. | astro-ph_HE |
eROSITA Science Book: Mapping the Structure of the Energetic Universe: eROSITA is the primary instrument on the Russian SRG mission. In the first
four years of scientific operation after its launch, foreseen for 2014, it will
perform a deep survey of the entire X-ray sky. In the soft X-ray band (0.5-2
keV), this will be about 20 times more sensitive than the ROSAT all sky survey,
while in the hard band (2-10 keV) it will provide the first ever true imaging
survey of the sky at those energies. Such a sensitive all-sky survey will
revolutionize our view of the high-energy sky, and calls for major efforts in
synergic, multi-wavelength wide area surveys in order to fully exploit the
scientific potential of the X-ray data. The design-driving science of eROSITA
is the detection of very large samples (~10^5 objects) of galaxy clusters out
to redshifts z>1, in order to study the large scale structure in the Universe,
test and characterize cosmological models including Dark Energy. eROSITA is
also expected to yield a sample of around 3 millions Active Galactic Nuclei,
including both obscured and un-obscured objects, providing a unique view of the
evolution of supermassive black holes within the emerging cosmic structure. The
survey will also provide new insights into a wide range of astrophysical
phenomena, including accreting binaries, active stars and diffuse emission
within the Galaxy, as well as solar system bodies that emit X-rays via the
charge exchange process. Finally, such a deep imaging survey at high spectral
resolution, with its scanning strategy sensitive to a range of variability
timescales from tens of seconds to years, will undoubtedly open up a vast
discovery space for the study of rare, unpredicted, or unpredictable
high-energy astrophysical phenomena. In this living document we present a
comprehensive description of the main scientific goals of the mission, with
strong emphasis on the early survey phases. | astro-ph_HE |
Investigation of Dense Gas Towards Relativistic Outflow Sources: We probe the interstellar medium towards the objects Circinus X-1, a low-mass
X-ray binary with relativistic jets; and the highly energetic Westerlund 2
stellar cluster, which is located towards TeV gamma-ray emission and
interesting arc- and jet-like features seen in Nanten 12CO data. We have mapped
both regions with the Mopra radio telescope, in 7 mm and 12 mm wavebands,
looking for evidence of disrupted/dense gas caused by the interaction between
high energy outflows and the ISM. Towards Westerlund 2, peaks in CS(J=1-0)
emission indicate high density gas towards the middle of the arc and the
endpoint of the jet; and radio recombination line emission is seen overlapping
the coincident HII region RCW49. Towards Circinus X-1, 12CO(J=1-0) Nanten data
reveals three molecular clouds that lie in the region of Cir X-1. Gas
parameters for each cloud are presented here. | astro-ph_HE |
Dark matter in the solar system I: The distribution function of WIMPs at
the Earth from solar capture: The next generation of dark matter (DM) direct detection experiments and
neutrino telescopes will probe large swaths of dark matter parameter space. In
order to interpret the signals in these experiments, it is necessary to have
good models of both the halo DM streaming through the solar system and the
population of DM bound to the solar system. In this paper, the first in a
series of three on DM in the solar system, we present simulations of orbits of
DM bound to the solar system by solar capture in a toy solar system consisting
of only the Sun and Jupiter, assuming that DM consists of a single species of
weakly interacting massive particle (WIMP). We describe how the size of the
bound WIMP population depends on the WIMP mass, spin-independent cross section,
and spin-dependent cross section. Using a standard description of the Galactic
DM halo, we find that the maximum enhancement to the direct detection event
rate, consistent with current experimental constraints on the WIMP-nucleon
cross section, is < 1% relative to the event rate from halo WIMPs, while the
event rate from neutrinos from WIMP annihilation in the center of the Earth is
unlikely to meet the threshold of next-generation, km^3-sized (IceCube, KM3NeT)
neutrino telescopes. | astro-ph_HE |
Predictions for the Rates of Compact Binary Coalescences Observable by
Ground-based Gravitational-wave Detectors: We present an up-to-date, comprehensive summary of the rates for all types of
compact binary coalescence sources detectable by the Initial and Advanced
versions of the ground-based gravitational-wave detectors LIGO and Virgo.
Astrophysical estimates for compact-binary coalescence rates depend on a number
of assumptions and unknown model parameters, and are still uncertain. The most
confident among these estimates are the rate predictions for coalescing binary
neutron stars which are based on extrapolations from observed binary pulsars in
our Galaxy. These yield a likely coalescence rate of 100 per Myr per Milky Way
Equivalent Galaxy (MWEG), although the rate could plausibly range from 1 per
Myr per MWEG to 1000 per Myr per MWEG. We convert coalescence rates into
detection rates based on data from the LIGO S5 and Virgo VSR2 science runs and
projected sensitivities for our Advanced detectors. Using the detector
sensitivities derived from these data, we find a likely detection rate of 0.02
per year for Initial LIGO-Virgo interferometers, with a plausible range between
0.0002 and 0.2 per year. The likely binary neutron-star detection rate for the
Advanced LIGO-Virgo network increases to 40 events per year, with a range
between 0.4 and 400 per year. | astro-ph_HE |
Tilted black hole accretion disc models of Sagittarius A*: time-variable
millimetre to near-infrared emission: High-resolution, multi-wavelength, and time-domain observations of the
Galactic centre black hole candidate, Sgr A*, allow for a direct test of
contemporary accretion theory. To date, all models have assumed alignment
between the accretion disc and black hole angular momentum axes, but this is
unjustified for geometrically thick accretion flows like that onto Sgr A*.
Instead, we calculate images and spectra from a set of simulations of accretion
flows misaligned ('tilted') by 15 degrees from the black hole spin axis and
compare them with millimetre (mm) to near-infrared (NIR) observations.
Non-axisymmetric standing shocks from eccentric fluid orbits dominate the
emission, leading to a wide range of possible image morphologies. These effects
invalidate previous parameter estimates from model fitting, including estimates
of the dimensionless black hole spin, except possibly at low values of spin or
tilt. At 1.3mm, the images have crescent morphologies, and the black hole
shadow may still be accessible to future mm-VLBI observations. Shock heating
leads to high energy electrons (T > 10^12 K), which can naturally produce the
observed NIR flux, spectral index, and rapid variability ('flaring'). This NIR
emission is uncorrelated with that in the mm, which also agrees with
observations.
These are the first models to self-consistently explain the time-variable mm
to NIR emission of Sgr A*. Predictions of the model include significant
structural changes observable with mm-VLBI on both the dynamical (hour) and
Lense-Thirring precession (day-year) timescales; and ~30-50 microarcsecond
changes in centroid position from extreme gravitational lensing events during
NIR flares, detectable with the future VLT instrument GRAVITY. If the observed
NIR emission is caused by shock heating in a tilted accretion disc, then the
Galactic centre black hole has a positive, non-zero spin parameter (a > 0). | astro-ph_HE |
Relations Between Neutron-Star Parameters in the Hartle-Thorne
Approximation: Using stellar structure calculations in the Hartle-Thorne approximation, we
derive analytic expressions connecting the ellipticity of the stellar surface
to the compactness, the spin angular momentum, and the quadrupole moment of the
spacetime. We also obtain empirical relations between the compactness, the spin
angular momentum, and the spacetime quadrupole. Our formulae reproduce the
results of numerical calculations to within a few percent and help reduce the
number of parameters necessary to model the observational appearance of
moderately spinning neutron stars. This is sufficient for comparing theoretical
spectroscopic and timing models to observations that aim to measure the masses
and radii of neutron stars and to determine the equation of state prevailing in
their interiors. | astro-ph_HE |
Free motion around black holes with discs or rings: between
integrability and chaos -- V: Complete integrability of geodesic motion, the well known feature of the
fields of isolated stationary black holes, can easily be "spoilt" by the
presence of some additional source (even if highly symmetric). In previous
papers, we used various methods to show how free time-like motion becomes
chaotic if the gravitational field of the Schwarzschild black hole is perturbed
by that of a circular disc or ring, considering specifically the inverted first
disc of the Morgan-Morgan counter-rotating family and the Bach-Weyl ring as the
additional sources. The present paper focuses on two new points. First, since
the Bach-Weyl thin ring is physically quite unsatisfactory, we now repeat some
of the analysis for a different, Majumdar-Papapetrou--type (extremally charged)
ring around an extreme Reissner-Nordstr\"om black hole, and compare the results
with those obtained before. We also argue that such a system is in fact more
relevant astrophysically than it may seem. Second, we check numerically, for
the latter system as well as for the Schwarzschild black hole encircled by the
inverted Morgan-Morgan disc, how indicative is the geometric (curvature)
criterion for chaos suggested by Sota, Suzuki & Maeda (1996). We also add a
review of the literature where the relevance of geometric criteria in general
relativity (as well as elsewhere) has been discussed for decades. | astro-ph_HE |
Kinematics of the jet in M87 on scales of 100 -- 1000 Schwarzschild
radii: Very long baseline interferometry (VLBI) imaging of radio emission from
extragalactic jets provides a unique probe of physical mechanisms governing the
launching, acceleration, and collimation of relativistic outflows. The
two-dimensional structure and kinematics of the jet in M\,87 (NGC\,4486) have
been studied by applying the Wavelet-based Image Segmentation and Evaluation
(WISE) method to 11 images obtained from multi-epoch Very Long Baseline Array
(VLBA) observations made in January-August 2007 at 43 GHz ($\lambda = 7$ mm).
The WISE analysis recovers a detailed two-dimensional velocity field in the jet
in M\,87 at sub-parsec scales. The observed evolution of the flow velocity with
distance from the jet base can be explained in the framework of MHD jet
acceleration and Poynting flux conversion. A linear acceleration regime is
observed up to $z_{obs} \sim 2$\,mas. The acceleration is reduced at larger
scales, which is consistent with saturation of Poynting flux conversion.
Stacked cross correlation analysis of the images reveals a pronounced
stratification of the flow. The flow consists of a slow, mildly relativistic
layer (moving at $\beta \sim 0.5\,c$), associated either with instability
pattern speed or an outer wind, and a fast, accelerating stream line (with
$\beta \sim 0.92$, corresponding to a bulk Lorentz factor $\gamma \sim 2.5$). A
systematic difference of the apparent speeds in the northern and southern limbs
of the jet is detected, providing evidence for jet rotation. The angular
velocity of the magnetic field line associated with this rotation suggests that
the jet in M87 is launched in the inner part of the disk, at a distance $r_0
\sim 5\, R_\mathrm{s}$ from the central engine. The combined results of the
analysis imply that MHD acceleration and conversion of Poynting flux to kinetic
energy play the dominant roles in collimation and acceleration of the flow in
M\,87. | astro-ph_HE |
Radial diffusion in corotating magnetosphere of Pulsar PSR J0737-3039B: Rich observational phenomenology associated with Pulsar B in PSR
J0737$-$3039A/B system resembles in many respects phenomena observed in the
Earth and Jupiter magnetospheres, originating due to the wind-magnetosphere
interaction. We consider particle dynamics in the fast corotating magnetosphere
of Pulsar B, when the spin period is shorter than the third adiabatic period.
We demonstrate that trapped particles occasionally experience large radial
variations of the L-parameter (effective radial distance) due to the parametric
interaction of the gyration motion with the large scale electric fields induced
by the deformations of the magnetosphere, in what could be called a
betatron-induced diffusion. The dynamics of particles from the wind of Pulsar A
trapped inside Pulsar B magnetosphere is governed by Mathieu's equation, so
that the parametrically unstable orbits are occasionally activated; particle
dynamics is not diffusive per se. The model explains the high plasma density on
the closed field lines of Pulsar B, and the fact that the observed eclipsing
region is several times smaller than predicted by the hydrodynamic models. | astro-ph_HE |
News from the very-high-energy sky seen with H.E.S.S: The H.E.S.S. experiment, the largest Cherenkov telescope array to date, has
been observing the sky at TeV energies for the past 16 years. Its location in
the Southern hemisphere provides H.E.S.S. with equally good access to Galactic
and extra-galactic sources. The focus of observations is now gradually shifting
from discoveries of new TeV emitters to studies of transients and the detailed
exploration of known sources. A few recent highlights from these observations
are presented here, with a focus on extragalactic observations. | astro-ph_HE |
Detecting Gravitational-waves from Extreme Mass Ratio Inspirals using
Convolutional Neural Networks: Extreme mass ratio inspirals (EMRIs) are among the most interesting
gravitational wave (GW) sources for space-borne GW detectors. However,
successful GW data analysis remains challenging due to many issues, ranging
from the difficulty of modeling accurate waveforms, to the impractically large
template bank required by the traditional matched filtering search method. In
this work, we introduce a proof-of-principle approach for EMRI detection based
on convolutional neural networks (CNNs). We demonstrate the performance with
simulated EMRI signals buried in Gaussian noise. We show that over a wide range
of physical parameters, the network is effective for EMRI systems with a
signal-to-noise ratio larger than 50, and the performance is most strongly
related to the signal-to-noise ratio. The method also shows good generalization
ability towards different waveform models. Our study reveals the potential
applicability of machine learning technology like CNNs towards more realistic
EMRI data analysis. | astro-ph_HE |
A numerical approach to the non-uniqueness problem of cosmic ray
two-fluid equations at shocks: Cosmic rays (CRs) are frequently modeled as an additional fluid in
hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations of astrophysical
flows. The standard CR two-fluid model is described in terms of three
conservation laws (expressing conservation of mass, momentum and total energy)
and one additional equation (for the CR pressure) that cannot be cast in a
satisfactory conservative form. The presence of non-conservative terms with
spatial derivatives in the model equations prevents a unique weak solution
behind a shock. We investigate a number of methods for the numerical solution
of the two-fluid equations and find that, in the presence of shock waves, the
results generally depend on the numerical details (spatial reconstruction, time
stepping, the CFL number, and the adopted discretization). All methods converge
to a unique result if the energy partition between the thermal and non-thermal
fluids at the shock is prescribed using a subgrid prescription. This highlights
the non-uniqueness problem of the two-fluid equations at shocks. From our
numerical investigations, we report a robust method for which the solutions are
insensitive to the numerical details even in absence of a subgrid prescription,
although we recommend a subgrid closure at shocks using results from kinetic
theory. The subgrid closure is crucial for a reliable post-shock solution and
also its impact on large scale flows because the shock microphysics that
determines CR acceleration is not accurately captured in a fluid approximation.
Critical test problems, limitations of fluid modeling, and future directions
are discussed. | astro-ph_HE |
A Luminous Be+White Dwarf Supersoft Source in the Wing of the SMC: MAXI
J0158-744: We present a multi-wavelength analysis of the very fast X-ray transient MAXI
J0158-744, which was detected by MAXI/GSC on 2011 November 11. The subsequent
exponential decline of the X-ray flux was followed with Swift observations, all
of which revealed spectra with low temperatures (~100eV) indicating that MAXI
J0158-744 is a new Supersoft Source (SSS). The Swift X-ray spectra near maximum
show features around 0.8 keV that we interpret as possible absorption from
OVIII, and emission from O, Fe, and Ne lines. We obtained SAAO and ESO optical
spectra of the counterpart early in the outburst and several weeks later. The
early spectrum is dominated by strong Balmer and HeI emission, together with
weaker HeII emission. The later spectrum reveals absorption features that
indicate a B1/2IIIe spectral type, and all spectral features are at velocities
consistent with the Small Magellanic Cloud. At this distance, it is a luminous
SSS (>10^37 erg/s) but whose brief peak luminosity of >10^39 erg/s in the 2-4
keV band makes it the brightest SSS yet seen at "hard" X-rays. We propose that
MAXI J0158-744 is a Be-WD binary, and the first example to possibly enter ULX
territory. The brief hard X-ray flash could possibly be a result of the
interaction of the ejected nova shell with the B star wind in which the white
dwarf (WD) is embedded. This makes MAXI J0158-744 only the third Be/WD system
in the Magellanic Clouds, but it is by far the most luminous. The properties of
MAXI J0158-744 give weight to previous suggestions that SSS in nearby galaxies
are associated with early-type stellar systems. | astro-ph_HE |
Collective baryon decay and gravitational collapse: While it is widely believed that the gravitational collapse of a sufficiently
large mass will lead to a density singularity and an event horizon, we propose
that this never happens when quantum effects are taken into account. In
particular, we propose that when the conditions become ripe for a trapped
surface to form, a quantum critical surface sweeps over the collapsing body,
transforming the nucleons in the collapsing matter into a lepton/photon gas
together with a positive vacuum energy. This will happen regardless of the
matter density at the time a trapped surface starts to form, and as a result we
predict that at least in all cases of gravitational collapse involving ordinary
matter, a large fraction of the rest mass of the collapsing matter will be
converted into a burst of neutrinos, and {\gamma}-rays. We predict that the
peak luminosity of these bursts is only weakly dependent on the mass of the
collapsing object, and is on the order of ({\epsilon}_q/m_Pc^2)^1/4c^5/G, where
{\epsilon}_q is the mean energy of a nucleon parton and m_P is the Planck mass.
The duration of the bursts will depend the mass of the collapsing objects; in
the case of stellar core collapse we predict that the duration of both the
neutrino and {\gamma}-ray bursts will be on the order of 10 seconds. | astro-ph_HE |
Updating the orbital ephemeris of the dipping source XB 1254-690 and the
distance to the source: XB 1254-690 is a dipping low mass X-ray binary system hosting a neutron star
and showing type I X-ray bursts. We aim at obtaining more accurate orbital
ephemeris and at constraining the orbital period derivative of the system for
the first time. In addition, we want to better constrain the distance to the
source in order to locate the system in a well defined evolutive scenario. We
apply for the first time an orbital timing technique to XB 1254-690, using the
arrival times of the dips present in the light curves that have been collected
during 26 years of X-ray pointed observations performed from different space
missions. We estimate the dip arrival times using a statistical method that
weights the count-rate inside the dip with respect to the level of the
persistent emission outside the dip. We fit the obtained delays as a function
of the orbital cycles both with a linear and a quadratic function. We infer the
orbital ephemeris of XB 1254-690 improving the accuracy of the orbital period
with respect to previous estimates. We infer a mass of M$_{2}=0.42\pm 0.04$
M$_{\odot}$ for the donor star, in agreement with the estimations already
present in literature, assuming that the star is in thermal equilibrium while
it transfers part of its mass via the inner Lagrangian point, and assuming a
neutron star mass of 1.4 M$_{\odot}$. Using these assumptions, we also
constrain the distance to the source, finding a value of 7.6$\pm 0.8$ kpc.
Finally, we discuss the evolution of the system suggesting that it is
compatible with a conservative mass transfer driven by magnetic braking. | astro-ph_HE |
Hidden Hearts of Neutrino Active Galaxies: Recent multimessenger studies have provided evidence for high-energy neutrino
sources that are opaque to GeV-TeV gamma rays. We present model-independent
studies on the connection between neutrinos and gamma rays in the active galaxy
NGC 1068, and find that the neutrinos most likely come from regions within
about 30-100 Schwarzschild radii. This is especially the case if neutrinos are
produced via the photomeson production process, although the constraints could
be alleviated if hadronuclear interactions are dominant. We consider the most
favorable neutrino production regions, and discuss coronae, jets, winds, and
their interactions with dense material. The results strengthen the importance
of understanding dissipation mechanisms near the coronal region and the outflow
base. There could be a connection between active galactic nuclei with
near-Eddington accretion and tidal disruptions events, in that neutrinos are
produced in the obscured vicinity of supermassive black holes. | astro-ph_HE |
Accreting Millisecond X-Ray Pulsars: Accreting Millisecond X-Ray Pulsars (AMXPs) are astrophysical laboratories
without parallel in the study of extreme physics. In this chapter we review the
past fifteen years of discoveries in the field. We summarize the observations
of the fifteen known AMXPs, with a particular emphasis on the multi-wavelength
observations that have been carried out since the discovery of the first AMXP
in 1998. We review accretion torque theory, the pulse formation process, and
how AMXP observations have changed our view on the interaction of plasma and
magnetic fields in strong gravity. We also explain how the AMXPs have deepened
our understanding of the thermonuclear burst process, in particular the
phenomenon of burst oscillations. We conclude with a discussion of the open
problems that remain to be addressed in the future. | astro-ph_HE |
The progenitor of binary millisecond radio pulsar PSR J1713+0747
(Research Note): PSR J1713+0747 is a binary system comprising millisecond radio pulsar with a
spin period of 4.57 ms, and a low-mass white dwarf (WD) companion orbiting the
pulsar with a period of 67.8 days. Using the general relativistic Shapiro
delay, the masses of the WD and pulsar components were previously found to be
$0.28\pm 0.03 M_{\odot}$ and $1.3\pm 0.2 M_{\odot}$ (68% confidence),
respectively. Standard binary evolution theory suggests that PSR J1713+0747
evolved from a low-mass X-ray binary (LMXB). Here, we test this hypothesis. We
used a binary evolution code and a WD evolution code to calculate evolutionary
sequences of LMXBs that could result in binary millisecond radio pulsars such
as PSR J1713+0747. During the mass exchange, the mass transfer is
nonconservative. Because of the thermal and viscous instabilities developing in
the accretion disk, the neutron star accretes only a small part of the incoming
material. We find that the progenitor of PSR J1713+0747 can be modelled as an
LMXB including a donor star with mass $1.3-1.6 M_{\odot}$ and an initial
orbital period ranging from 2.40 to 4.15 days. If the cooling timescale of the
WD is 8 Gyr, its present effective temperature is between 3870 and 4120 K,
slightly higher than the observed value. We estimate a surface gravity of ${\rm
Log} (g) \approx 7.38 - 7.40$. | astro-ph_HE |
Deriving fundamental parameters of millisecond pulsars via AIC in white
dwarfs: We present a study of the observational properties of Millisecond Pulsars
(MSPs) by way of their magnetic fields, spin periods and masses. These
measurements are derived through the scenario of Accretion Induced Collapse
(AIC) of white dwarfs (WDs) in stellar binary systems, in order to provide a
greater understanding of the characteristics of MSP populations. In addition,
we demonstrate a strong evolutionary connection between neutron stars and WDs
with binary companions from a stellar binary evolution perspective via the AIC
process. | astro-ph_HE |
PS1-14bj: A Hydrogen-Poor Superluminous Supernova With a Long Rise and
Slow Decay: We present photometry and spectroscopy of PS1-14bj, a hydrogen-poor
superluminous supernova (SLSN) at redshift $z=0.5215$ discovered in the last
months of the Pan-STARRS1 Medium Deep Survey. PS1-14bj stands out by its
extremely slow evolution, with an observed rise of $\gtrsim 125$ rest-frame
days, and exponential decline out to $\sim 250$ days past peak at a measured
rate of $0.01~{\rm mag~day}^{-1}$, consistent with fully-trapped $^{56}$Co
decay. This is the longest rise time measured in a SLSN to date, and the first
SLSN to show a rise time consistent with pair-instability supernova (PISN)
models. Compared to other slowly-evolving SLSNe, it is spectroscopically
similar to the prototype SN2007bi at maximum light, though lower in luminosity
($L_{\rm peak} \simeq 4.6 \times 10^{43} {\rm erg s}^{-1}$) and with a flatter
peak than previous events. PS1-14bj shows a number of peculiar properties,
including a near-constant color temperature for $>200$ days past peak, and
strong emission lines from [O III] $\lambda$5007 and [O III] $\lambda$4363 with
a velocity width of $\sim$3400 km/s, in its late-time spectra. These both
suggest there is a sustained source of heating over very long timescales, and
are incompatible with a simple $^{56}$Ni-powered/PISN interpretation. A
modified magnetar model including emission leakage at late times can reproduce
the light curve, in which case the blue continuum and [O III] features are
interpreted as material heated and ionized by the inner pulsar wind nebula
becoming visible at late times. Alternatively, the late-time heating could be
due to interaction with a shell of H-poor circumstellar material. | astro-ph_HE |
Particle acceleration in relativistic turbulence: A theoretical
appraisal: We discuss the physics of stochastic particle acceleration in relativistic
magnetohydrodynamic (MHD) turbulence, combining numerical simulations of
test-particle acceleration in synthetic wave turbulence spectra with detailed
analytical estimates. In particular, we study particle acceleration in wavelike
isotropic fast mode turbulence, in Alfv\'en and slow Goldreich-Sridhar type
wave turbulence (properly accounting for anisotropy effects), including
resonance broadening due to wave decay and pitch-angle randomization. At high
particle rigidities, the contributions of those three modes to acceleration are
comparable to within an order of magnitude, as a combination of several effects
(partial disappearance of transit-time damping for fast modes, increased
scattering rate for Alfv\'en and slow modes due to resonance broadening).
Additionally, we provide analytical arguments regarding acceleration beyond the
regime of MHD wave turbulence, addressing the issue of nonresonant acceleration
in a turbulence comprised of structures rather than waves, as well as the issue
of acceleration in small-scale parallel electric fields. Finally, we compare
our results to the existing literature and provide ready-to-use formulas for
applications to high-energy astrophysical phenomenology. | astro-ph_HE |
About the physical nature of some peculiarities of the primary cosmic
radiation nuclei and gamma quanta spectra: About 20 years ago we published the data concerning some peculiarities of the
behavior of cosmic radiation EAS hadron component spectra. The results pointed
to the possible existence in the interstellar space of the background of weakly
interacting objects of the mass (the energy of the resonance oscillations) of
the order of 37 eV. The absence of such background particle creation in
accelerator experiments may mean that it is not the elementary particle but the
object of some different nature.
On the other hand, the experimental data of the last years are pointing to
the existence of cosmic gamma radiation with the specific spectrum having the
steep right front again in the region of the order of 37 eV and the left front
falling down to the energies of the order less than 10-6 eV. Obviously, no
elementary object may possess such spectrum of frequencies or a decay spectrum.
Such spectrum may have for example the topological defect of the space.
Further the possibility will be presented of exactly such explanation of the
cosmic rays primary radiation spectra peculiarities experimentally observed. It
is based on the hypothesis about the discreteness of the space and existence in
it of the topological defects distributed with sufficient density. In the
frames of the proposed model some essential experimental peculiarities of the
primary cosmic radiation nuclei and gamma quanta spectra find the unified
explanation. | astro-ph_HE |
Intra-Night Optical Variability Monitoring of Fermi Blazars: First
Results from 1.3 m J C Bhattacharya Telescope: We report the first results obtained from our campaign to characterize the
intranight-optical variability (INOV) properties of {\it Fermi}~detected
blazars, using the observations from the recently commissioned 1.3 m J C
Bhattacharya telescope (JCBT). During the first run, we were able to observe 17
blazars in the Bessel $R$ filter for $\sim$137 hrs. Using $C$ and scaled
$F$-statistics, we quantify the extent of INOV and derive the duty cycle (DC)
which is the fraction of time during which a source exhibits a substantial flux
variability. We find a high DC of 40\% for BL Lac objects and the flat spectrum
radio quasars are relatively less variable (DC $\sim15$\%). However, when
estimated for blazars sub-classes, a high DC of $\sim$59\% is found in low
synchrotron peaked (LSP) blazars, whereas, intermediate and high synchrotron
peaked objects have a low DC of $\sim$11\% and 13\%, respectively. We find
evidences about the association of the high amplitude INOV with the
$\gamma$-ray flaring state. We also notice a high polarization during the
elevated INOV states (for the sources that have polarimetric data available),
thus supporting the jet based origin of the observed variability. We plan to
enlarge the sample and utilize the time availability from the small telescopes,
such as 1.3 m JCBT, to strengthen/verify the results obtained in this work and
those existed in the literature. | astro-ph_HE |
First Multi-wavelength Campaign on the Gamma-ray-loud Active Galaxy IC
310: The extragalactic VHE gamma-ray sky is rich in blazars. These are jetted
active galactic nuclei viewed at a small angle to the line-of-sight. Only a
handful of objects viewed at a larger angle are known so far to emit above 100
GeV. Multi-wavelength studies of such objects up to the highest energies
provide new insights into the particle and radiation processes of active
galactic nuclei. We report the results from the first multi-wavelength campaign
observing the TeV detected nucleus of the active galaxy IC 310, whose jet is
observed at a moderate viewing angle of 10 deg - 20 deg. The multi-instrument
campaign was conducted between 2012 Nov. and 2013 Jan., and involved
observations with MAGIC, Fermi, INTEGRAL, Swift, OVRO, MOJAVE and EVN. These
observations were complemented with archival data from the AllWISE and 2MASS
catalogs. A one-zone synchrotron self-Compton model was applied to describe the
broad-band spectral energy distribution. IC 310 showed an extraordinary TeV
flare at the beginning of the campaign, followed by a low, but still detectable
TeV flux. Compared to previous measurements, the spectral shape was found to be
steeper during the low emission state. Simultaneous observations in the soft
X-ray band showed an enhanced energy flux state and a harder-when-brighter
spectral shape behaviour. No strong correlated flux variability was found in
other frequency regimes. The broad-band spectral energy distribution obtained
from these observations supports the hypothesis of a double-hump structure. The
harder-when-brighter trend in the X-ray and VHE emission is consistent with the
behaviour expected from a synchrotron self-Compton scenario. The
contemporaneous broad-band spectral energy distribution is well described with
a one-zone synchrotron self-Compton model using parameters that are comparable
to those found for other gamma-ray-emitting misaligned blazars. | astro-ph_HE |
Pevatron at the Galactic Center: Multi-Wavelength Signatures from
Millisecond Pulsars: Diffuse TeV emission has been observed by H.E.S.S. in the Galactic Center
region, in addition to the GeV gamma rays observed by Fermi. We propose that a
population of unresolved millisecond pulsars located around the Galactic
Center, suggested as possible candidates for the diffuse Galactic Center excess
observed by Fermi, accelerate cosmic rays up to very high energies, and are
thus also responsible for the TeV excess. We model analytically the diffusion
of these accelerated protons and their interaction with the molecular clouds,
producing gamma rays. The spatial and spectral dependences of the gamma rays
produced can reproduce the H.E.S.S. observations, for a population of $\sim
10^4-10^5$ millisecond pulsars above the cosmic-ray luminosity $10^{34}\,{\rm
erg\,s}^{-1}$, with moderate acceleration efficiency. More precise measurements
at the highest energies would allow us to constrain the properties of the
pulsar population, such as the magnetic field or initial spin distributions. | astro-ph_HE |
Ne X X-ray Emission due to Charge Exchange in M82: Recent X-ray observations of star-forming galaxies such as M82 have shown the
Ly beta/Ly alpha line ratio of Ne X to be in excess of predictions for thermal
electron impact excitation. Here we demonstrate that the observed line ratio
may be due to charge exchange and can be used to constrain the ion kinetic
energy to be <500 eV/u. This is accomplished by computing spectra and line
ratios via a range of theoretical methods and comparing these to experiments
with He over astrophysically relevant collision energies. The charge exchange
emission spectra calculations were performed for Ne[10+] +H and Ne[10+] +He
using widely applied approaches including the atomic orbital close coupling,
classical trajectory Monte Carlo, and multichannel Landau- Zener (MCLZ)
methods. A comparison of the results from these methods indicates that for the
considered energy range and neutrals (H, He) the so-called "low-energy
l-distribution" MCLZ method provides the most likely reliable predictions. | astro-ph_HE |
Neutrinos from Choked Jets Accompanied by Type-II Supernovae: The origin of the IceCube neutrinos is still an open question. Upper limits
from diffuse gamma-ray observations suggest that the neutrino sources are
either distant or hidden from gamma-ray observations. It is possible that the
neutrinos are produced in jets that are formed in the core-collapsing massive
stars and fail to break out, the so-called choked jets. We study neutrinos from
the jets choked in the hydrogen envelopes of red supergiant stars. Fast
photo-meson cooling softens the neutrino spectrum, making it difficult to
explain the PeV neutrinos observed by IceCube in a one-component scenario, but
a two-component model can explain the spectrum. Furthermore, we predict that a
newly born jet-driven type-II supernova may be observed to be associated with a
neutrino burst detected by IceCube. | astro-ph_HE |
Recent multi-wavelength campaigns in the Fermi-GST era: Since 2008 the Fermi/LAT instrument has delivered highly time-resolved
gamma-ray spectra and detailed variability curves for a steadily increasing
number of AGN. For detailed AGN studies the Fermi/LAT data have to be combined
with, and accompanied by, dedicated ground- and space-based multi-frequency
observations. In this framework, the Fermi AGN team has realized a detailed
plan for multi-wavelength campaigns including a large suite of cm/mm/sub-mm
band instruments. Many of those campaigns have been triggered, often for
sources detected in flaring states. We review here a few interesting results
recently obtained during three such campaigns, namely for the flat-spectrum
radio quasar 3C 279, the Narrow Line Seyfert 1 PMN J0948+0022 and quasar 3C
454.3. | astro-ph_HE |
Pair Production in Low Luminosity Galactic Nuclei: Electron-positron pairs may be produced near accreting black holes by a
variety of physical processes, and the resulting pair plasma may be accelerated
and collimated into a relativistic jet. Here we use a self-consistent dynamical
and radiative model to investigate pair production by \gamma\gamma collisions
in weakly radiative accretion flows around a black hole of mass M and accretion
rate \dot{M}. Our flow model is drawn from general relativistic
magnetohydrodynamic simulations, and our radiation field is computed by a Monte
Carlo transport scheme assuming the electron distribution function is thermal.
We argue that the pair production rate scales as r^{-6} M^{-1} \dot{M}^{6}. We
confirm this numerically and calibrate the scaling relation. This relation is
self-consistent in a wedge in M, \dot{M} parameter space. If \dot{M} is too low
the implied pair density over the poles of the black hole is below the
Goldreich-Julian density and \gamma\gamma pair production is relatively
unimportant; if \dot{M} is too high the models are radiatively efficient. We
also argue that for a power-law spectrum the pair production rate should scale
with the observables L_X \equiv X-ray luminosity and M as L_X^2 M^{-4}. We
confirm this numerically and argue that this relation likely holds even for
radiatively efficient flows. The pair production rates are sensitive to black
hole spin and to the ion-electron temperature ratio which are fixed in this
exploratory calculation. We finish with a brief discussion of the implications
for Sgr A* and M87. | astro-ph_HE |
The blazar sequence 2.0: I discuss the spectral energy distribution (SED) of all blazars with redshift
detected by the {\it Fermi} satellite and listed in the 3LAC catalog. I will
update the so called "blazar sequence" from the phenomenological point of view,
with no theory or modelling. I will show that: i) pure data show that jet and
accretion power are related; ii) the updated blazar sequence maintains the
properties of the old version, albeit with a less pronounced dominance of the
$\gamma$--ray emission; iii) at low bolometric luminosities, two different type
of objects have the same high energy power: low black hole mass flat spectrum
radio quasars and high mass BL Lacs. Therefore, at low luminosities, there is a
very large dispersion of SED shapes; iv) in low power BL Lacs, the contribution
of the host galaxy is important. Remarkably, the luminosity distribution of the
host galaxies of BL Lacs are spread in a very narrow range; v) a simple sum of
two smoothly joining power laws can describe the blazar SEDs very well. | astro-ph_HE |
Measuring the Spins of Accreting Black Holes: A typical galaxy is thought to contain tens of millions of stellar-mass black
holes, the collapsed remnants of once massive stars, and a single nuclear
supermassive black hole. Both classes of black holes accrete gas from their
environments. The accreting gas forms a flattened orbiting structure known as
an accretion disk. During the past several years, it has become possible to
obtain measurements of the spins of the two classes of black holes by modeling
the X-ray emission from their accretion disks. Two methods are employed, both
of which depend upon identifying the inner radius of the accretion disk with
the innermost stable circular orbit (ISCO), whose radius depends only on the
mass and spin of the black hole. In the Fe K method, which applies to both
classes of black holes, one models the profile of the
relativistically-broadened iron line with a special focus on the
gravitationally redshifted red wing of the line. In the continuum-fitting
method, which has so far only been applied to stellar-mass black holes, one
models the thermal X-ray continuum spectrum of the accretion disk. We discuss
both methods, with a strong emphasis on the continuum-fitting method and its
application to stellar-mass black holes. Spin results for eight stellar-mass
black holes are summarized. These data are used to argue that the high spins of
at least some of these black holes are natal, and that the presence or absence
of relativistic jets in accreting black holes is not entirely determined by the
spin of the black hole. | astro-ph_HE |
Inferring the core-collapse supernova explosion mechanism with
three-dimensional gravitational-wave simulations: A detection of a core-collapse supernova signal with an Advanced LIGO and
Virgo gravitational-wave detector network will allow us to measure
astrophysical parameters of the source. In real advanced gravitational-wave
detector data there are transient noise artifacts that may mimic a true
gravitational-wave signal. In this paper, we outline a procedure implemented in
the Supernova Model Evidence Extractor (SMEE) that determines if a
core-collapse supernova signal candidate is a noise artefact, a
rapidly-rotating core-collapse supernova signal, or a neutrino explosion
mechanism core-collapse supernova signal. Further to this, we use the latest
available three-dimensional gravitational-wave core-collapse supernova
simulations, and we outline a new procedure for the rejection of background
noise transients when only one detector is operational. We find the minimum SNR
needed to detect all waveforms is reduced when using three-dimensional
waveforms as signal models. | astro-ph_HE |
A New Transient Ultraluminous X-ray Source in NGC 7090: We report on the discovery of a new, transient ultraluminous X-ray source
(ULX) in the galaxy NGC 7090. This new ULX, which we refer to as NGC 7090 ULX3,
was discovered via monitoring with $Swift$ during 2019-20, and to date has
exhibited a peak luminosity of $L_{\rm{X}} \sim 6 \times 10^{39}$ erg s$^{-1}$.
Archival searches show that, prior to its recent transition into the ULX
regime, ULX3 appeared to exhibit a fairly stable luminosity of $L_{\rm{X}} \sim
10^{38}$ erg s$^{-1}$. Such strong long-timescale variability may be
reminiscent of the small population of known ULX pulsars, although deep
follow-up observations with $XMM$-$Newton$ and $NuSTAR$ do not reveal any
robust X-ray pulsation signals. Pulsations similar to those seen from known ULX
pulsars cannot be completely excluded, however, as the limit on the pulsed
fraction of any signal that remains undetected in these data is $\lesssim$20\%.
The broadband spectrum from these observations is well modelled with a simple
thin disc model, consistent with sub-Eddington accretion, which may instead
imply a moderately large black hole accretor ($M_{\rm{BH}} \sim 40 ~
M_{\odot}$). Similarly, though, more complex models consistent with the
super-Eddington spectra seen in other ULXs (and the known ULX pulsars) cannot
be excluded given the limited signal-to-noise of the available broadband data.
The nature of the accretor powering this new ULX therefore remains uncertain. | astro-ph_HE |
Jet Launching from Merging Magnetized Binary Neutron Stars with
Realistic Equations of State: We perform general relativistic, magnetohydrodynamic (GRMHD) simulations of
binary neutron stars in quasi-circular orbit that merge and undergo delayed or
prompt collapse to a black hole (BH). The stars are irrotational and modeled
using an SLy or an H4 nuclear equation of state. To assess the impact of the
initial magnetic field configuration on jet launching, we endow the stars with
a purely poloidal magnetic field that is initially unimportant dynamically and
is either confined to the stellar interior or extends from the interior into
the exterior as in typical pulsars. Consistent with our previous results, we
find that only the BH + disk remnants originating from binaries that form
hypermassive neutron stars (HMNSs) and undergo delayed collapse can drive
magnetically-powered jets. We find that the closer the total mass of the binary
is to the threshold value for prompt collapse, the shorter is the time delay
between the gravitational wave peak amplitude and jet launching. This time
delay also strongly depends on the initial magnetic field configuration. We
also find that seed magnetic fields confined to the stellar interior can launch
a jet over $\sim 25\,\rm ms$ later than those with pulsar-like magnetic fields.
The lifetime of the jet [$\Delta t\lesssim 150\,\rm ms$] and its outgoing
Poynting luminosity [$L_{\rm EM}\sim 10^{52\pm 1}\rm erg/s$] are consistent
with typical short gamma-ray burst central engine lifetimes, as well as with
the Blandford--Znajek mechanism for launching jets and their associated
Poynting luminosities. Our numerical results also suggest that the dynamical
ejection of matter can be enhanced by the magnetic field. Therefore, GRMHD
studies are required to fully understand kilonova signals from GW170818-like
events. | astro-ph_HE |
Black hole spin of Cyg X-1 determined from the softest state ever
observed: We show the softest ever spectrum from Cyg~X-1, detected in 2013 with Suzaku.
This has the weakest high energy Compton tail ever seen from this object, so
should give the cleanest view of the underlying disk spectrum, and hence the
best determination of black hole spin from disk continuum fitting. Using the
standard model of a disk with simple non-thermal Comptonisation to produce the
weak high energy tail gives a high spin black hole. However, we get a
significantly better fit by including an additional, low temperature thermal
Comptonisation component, which allows a much lower black hole spin.
Corroboration of the existence of an additional Compton component comes from
the frequency dependent hard lags seen in the rapid variability in archival
high/soft state data. These can not be explained if the continuum is a single
non-thermal Comptonisation component, but are instead consistent with a
radially stratified, multi zone Comptonisation spectrum, where the spectrum is
softer further from the black hole. A complex multi-zone Comptonisation
continuum is required to explain both spectra and timing together, and this has
an impact on the derived black hole spin. | astro-ph_HE |
A general relativistic model of accretion disks with coronae surrounding
Kerr black holes: We calculate the structure of a standard accretion disk with corona
surrounding a massive Kerr black hole in general relativistic frame, in which
the corona is assumed to be heated by the reconnection of the strongly buoyant
magnetic fields generated in the cold accretion disk. The emergent spectra of
the accretion disk-corona systems are calculated by using the relativistic
ray-tracing method. We propose a new method to calculate the emergent
Comptonized spectra from the coronae. The spectra of the disk-corona systems
with a modified $\alpha$-magnetic stress show that both the hard X-ray spectral
index and the hard X-ray bolometric correction factor $L_{\rm bol}/L_{\rm
X,2-10keV}$ increase with the dimensionless mass accretion rate, which are
qualitatively consistent with the observations of active galactic nuclei
(AGNs). The fraction of the power dissipated in the corona decreases with
increasing black hole spin parameter $a$, which leads to lower electron
temperatures of the coronas for rapidly spinning black holes. The X-ray
emission from the coronas surrounding rapidly spinning black holes becomes weak
and soft. The ratio of the X-ray luminosity to the optical/UV luminosity
increases with the viewing angle, while the spectral shape in the X-ray band is
insensitive with the viewing angle. We find that the spectral index in the
infrared waveband depends on the mass accretion rate and the black hole spin
$a$, which deviates from $f_\nu\propto\nu^{1/3}$ expected by the standard thin
disk model. | astro-ph_HE |
Multifrequency Variability and Correlations from Extensive Observing
Campaigns of Mkn 421 and Mkn 501 in 2009: We are performing an unprecedentedly long and dense monitoring of the
multi-frequency (radio to TeV) emission from the classical TeV blazars Mkn 421
and Mkn 501. These objects are among the brightest X-ray/TeV blazars in the sky
and among the few sources whose spectral energy distributions (SED) can be
completely characterised by the current instruments. This is a multi-year and
multi-instrument programme which includes the participation of VLBA, Swift,
RXTE, MAGIC, VERITAS, Whipple, the Fermi/LAT Gamma-ray Observatory, GASP-WEBT,
among other collaborations and instruments which combined provide the most
detailed temporal and energy coverage of these sources to date. In this
proceedings we will focus mostly on the results obtained with the
multi-frequency data from 2009, for which the SEDs of Mkn 421 and Mkn 501 are
very similar and can be described by a one-zone synchrotron self-Compton
scenario. We will report on the multi-frequency variability derived from these
data. | astro-ph_HE |
Magnetically Advected Winds: Observations of X-ray absorption lines in magnetically driven disk winds
around black hole binaries and active galactic nuclei yield a universal radial
density profile rho proportional to r^{-1.2} in the wind. This is in
disagreement with the standard Blandford & Payne profile rho_BP proportional to
r^{-1.5} expected when the magnetic field is neither advected nor diffusing
through the accretion disk. In order to account for this discrepancy, we
establish a new paradigm for magnetically driven astrophysical winds according
to which the large scale ordered magnetic field that threads the disk is
continuously generated by the Cosmic Battery around the inner edge of the disk
and continuously diffuses outward. We obtain self-similar solutions of such
magnetically advected winds (MAW) and discuss their observational
ramifications. | astro-ph_HE |
5.5 years multi-wavelength variability of Mrk 421: evidences of leptonic
emission from the radio to TeV: Mrk 421 is a high-synchrotron-peaked blazar featuring bright and persistent
GeV and TeV emission. We use the longest and densest ongoing unbiased observing
campaign obtained at TeV and GeV energies during 5.5 years with the FACT
telescope and the Fermi-LAT detector. The contemporaneous multi-wavelength
observations were used to characterize the variability of the source and to
constrain the underlying physical mechanisms. We study and correlate light
curves obtained by nine different instruments from radio to gamma rays and
found two significant results. The TeV and X-ray light curves are very well
correlated with lag, if any, shorter than a day. The GeV light curve varies
independently and accurately leads the variations observed at long wavelengths,
in particular in the radio band. We find that the observations match the
predictions of leptonic models and suggest that the physical conditions vary
along the jet, when the emitting region moves outwards. | astro-ph_HE |
Investigating High Mass X-ray Binaries at hard X-rays with INTEGRAL: The INTEGRAL archive developed at INAF-IASF Milano with the available public
observations from late 2002 to 2016 is investigated to extract the X-ray
properties of 58 High Mass X-ray Binaries (HMXBs). This sample consists of
sources hosting either a Be star (Be/XRBs) or an early-type supergiant
companion (SgHMXBs), including the Supergiant Fast X-ray Transients (SFXTs).
INTEGRAL light curves (sampled at 2 ks) are used to build their hard X-ray
luminosity distributions, returning the source duty cycles, the range of
variability of the X-ray luminosity and the time spent in each luminosity
state. The phenomenology observed with INTEGRAL, together with the source
variability at soft X-rays taken from the literature, allows us to obtain a
quantitative overview of the main sub-classes of massive binaries in accretion
(Be/XRBs, SgHMXBs and SFXTs). Although some criteria can be derived to
distinguish them, some SgHMXBs exist with intermediate properties, bridging
together persistent SgHMXBs and SFXTs. | astro-ph_HE |
PHEMTO : Polarimetric High Energy Modular Telescope Observatory: With the opening of the X and gamma--ray windows in the sixties, thanks to to
sounding rockets and satellite-borne instruments, extremely energetic and
violent phenomena were discovered and subsequently found to be ubiquitous in
the Universe. Observations in the high energy domain are fundamental for
understanding how matter is organized and behaves around black holes;
unravelling how these extreme objects influence their environments on a very
large scale; and finding the still elusive obscured massive objects in the
centre of galaxies. Other major problems in contemporary astrophysics, such as
the understanding of acceleration processes at shocks of all sizes (those of
pulsar wind nebulae, supernova remnants, but also at larger scales those of
Active Galactic Nuclei radio lobes) in relation to the origin of cosmic-rays,
or the definitive characterization of the debated non-thermal X-ray energy
content of clusters of galaxies, also requires observations at very high
energies. An observatory type medium mission operating from around 1 keV to
about 600 keV can provide direct insights into these major questions. The
essential characteristics will be coverage of the full energy range by
telescopes featuring a large throughput and arc-second resolution optics,
coupled to a compact focal plane assembly, with excellent imaging resolution
and spectroscopy. In addition, the mission will provide unique polarimetry
measurements in the hard X-ray domain, an important new diagnostic tool at
energies for which the non-thermal processes dominate. The Polarimetric
High-Energy Modular Telescope Observatory (PHEMTO) is designed to have
performance several orders of magnitude better than the present hard X-ray
instruments. This gives to PHEMTO the improvements in scientific performance
needed for a mission in the 2050 era. | astro-ph_HE |
Variable jet properties in GRB110721A: Time resolved observations of the
jet photosphere: {\it Fermi Gamma-ray Space Telescope} observations of GRB110721A have
revealed two emission components from the relativistic jet: emission from the
photosphere, peaking at $\sim 100$ keV and a non-thermal component, which peaks
at $\sim 1000$ keV. We use the photospheric component to calculate the
properties of the relativistic outflow. We find a strong evolution in the flow
properties: the Lorentz factor decreases with time during the bursts from
$\Gamma \sim 1000$ to $\sim 150$ (assuming a redshift $z=2$; the values are
only weakly dependent on unknown efficiency parameters). Such a decrease is
contrary to the expectations from the internal shocks and the isolated magnetar
birth models. Moreover, the position of the flow nozzle measured from the
central engine, $r_0$, increases by more than two orders of magnitude. Assuming
a moderately magnetised outflow we estimate that $r_0$ varies from $10^6$ cm to
$\sim 10^9$ cm during the burst. We suggest that the maximal value reflects the
size of the progenitor core. Finally, we show that these jet properties
naturally explain the observed broken power-law decay of the temperature which
has been reported as a characteristic for GRB pulses. | astro-ph_HE |
LOFAR Detection of 110-188 MHz Emission and Frequency-Dependent Activity
from FRB 20180916B: FRB 20180916B is a well-studied repeating fast radio burst source. Its
proximity (~150 Mpc), along with detailed studies of the bursts, have revealed
many clues about its nature -- including a 16.3-day periodicity in its
activity. Here we report on the detection of 18 bursts using LOFAR at 110-188
MHz, by far the lowest-frequency detections of any FRB to date. Some bursts are
seen down to the lowest-observed frequency of 110 MHz, suggesting that their
spectra extend even lower. These observations provide an order-of-magnitude
stronger constraint on the optical depth due to free-free absorption in the
source's local environment. The absence of circular polarization and nearly
flat polarization angle curves are consistent with burst properties seen at
300-1700 MHz. Compared with higher frequencies, the larger burst widths
(~40-160 ms at 150 MHz) and lower linear polarization fractions are likely due
to scattering. We find ~2-3 rad/m^2 variations in the Faraday rotation measure
that may be correlated with the activity cycle of the source. We compare the
LOFAR burst arrival times to those of 38 previously published and 22 newly
detected bursts from the uGMRT (200-450 MHz) and CHIME/FRB (400-800 MHz).
Simultaneous observations show 5 CHIME/FRB bursts when no emission is detected
by LOFAR. We find that the burst activity is systematically delayed towards
lower frequencies by ~3 days from 600 MHz to 150 MHz. We discuss these results
in the context of a model in which FRB 20180916B is an interacting binary
system featuring a neutron star and high-mass stellar companion. | astro-ph_HE |
Dense matter equation of state for neutron star mergers: In simulations of binary neutron star mergers, the dense matter equation of
state (EOS) is required over wide ranges of density and temperature as well as
under conditions in which neutrinos are trapped, and the effects of magnetic
fields and rotation prevail. Here we assess the status of dense matter theory
and point out the successes and limitations of approaches currently in use. A
comparative study of the excluded volume (EV) and virial approaches for the
$np\alpha$ system using the equation of state of Akmal, Pandharipande and
Ravenhall for interacting nucleons is presented in the sub-nuclear density
regime. Owing to the excluded volume of the $\alpha$-particles, their mass
fraction vanishes in the EV approach below the baryon density 0.1 fm$^{-3}$,
whereas it continues to rise due to the predominantly attractive interactions
in the virial approach. The EV approach of Lattimer et al. is extended here to
include clusters of light nuclei such as d, $^3$H and $^3$He in addition to
$\alpha$-particles. Results of the relevant state variables from this
development are presented and enable comparisons with related but slightly
different approaches in the literature. We also comment on some of the sweet
and sour aspects of the supra-nuclear EOS. The extent to which the neutron star
gravitational and baryon masses vary due to thermal effects, neutrino trapping,
magnetic fields and rotation are summarized from earlier studies in which the
effects from each of these sources were considered separately. Increases of
about $20\% (\gtrsim 50\%)$ occur for rigid (differential) rotation with
comparable increases occurring in the presence of magnetic fields only for
fields in excess of $10^{18}$ Gauss. Comparatively smaller changes occur due to
thermal effects and neutrino trapping. Some future studies to gain further
insight into the outcome of dynamical simulations are suggested. | astro-ph_HE |
Hall cascades versus instabilities in neutron star magnetic fields: The Hall effect is an important nonlinear mechanism affecting the evolution
of magnetic fields in neutron stars. Studies of the governing equation, both
theoretical and numerical, have shown that the Hall effect proceeds in a
turbulent cascade of energy from large to small scales. We investigate the
small-scale Hall instability conjectured to exist from the linear stability
analysis of Rheinhardt and Geppert. Identical linear stability analyses are
performed to find a suitable background field to model Rheinhardt and Geppert's
ideas. The nonlinear evolution of this field is then modelled using a
three-dimensional pseudospectral numerical MHD code. Combined with the
background field, energy was injected at the ten specific eigenmodes with the
greatest positive eigenvalues as inferred by the linear stability analysis.
Energy is transferred to different scales in the system, but not into small
scales to any extent that could be interpreted as a Hall instability. Any
instabilities are overwhelmed by a late-onset turbulent Hall cascade, initially
avoided by the choice of background field, but soon generated by nonlinear
interactions between the growing eigenmodes. The Hall cascade is shown here,
and by several authors elsewhere, to be the dominant mechanism in this system. | astro-ph_HE |
Suzaku Observation of X-ray Variability in Soft State LMC X-1: This paper reports the results of Suzaku observation of the spectral
variation of the black hole binary LMCX-1 in the soft state. The observationwas
carried out in 2009 from July 21 to 24. the obtained net count rate was
$\sim$30 counts s$^{-1}$ in the 0.5--50 keV band with $\sim$10% peak-to-peak
flux variation. The time-averaged X-ray spectrum cannot be described by a
multi-color disk and single Compton component with its reflection, but requires
additional Comptonized emissions. This double Compton component model allows a
slightly larger inner radius of the multi-color disk, implying a lower spin
parameter. Significant spectral evolution was observed above 8 keV along with a
flux decrease on a timescale of $\sim$10$^4$--10$^5$ s. By spectral fitting, we
show that this behavior is well explained by changes in the hard Comptonized
emission component in contrast to the maintained disk and soft Comptonized
emission. | astro-ph_HE |
Observation of TeV Gamma Rays from the Fermi Bright Galactic Sources
with the Tibet Air Shower Array: Using the Tibet-III air shower array, we search for TeV gamma-rays from 27
potential Galactic sources in the early list of bright sources obtained by the
Fermi Large Area Telescope at energies above 100 MeV. Among them, we observe 7
sources instead of the expected 0.61 sources at a significance of 2 sigma or
more excess. The chance probability from Poisson statistics would be estimated
to be 3.8 x 10^-6. If the excess distribution observed by the Tibet-III array
has a density gradient toward the Galactic plane, the expected number of
sources may be enhanced in chance association. Then, the chance probability
rises slightly, to 1.2 x 10^-5, based on a simple Monte Carlo simulation. These
low chance probabilities clearly show that the Fermi bright Galactic sources
have statistically significant correlations with TeV gamma-ray excesses. We
also find that all 7 sources are associated with pulsars, and 6 of them are
coincident with sources detected by the Milagro experiment at a significance of
3 sigma or more at the representative energy of 35 TeV. The significance maps
observed by the Tibet-III air shower array around the Fermi sources, which are
coincident with the Milagro >=3sigma sources, are consistent with the Milagro
observations. This is the first result of the northern sky survey of the Fermi
bright Galactic sources in the TeV region. | astro-ph_HE |
Is the 130 GeV Line Real? A Search for Systematics in the Fermi-LAT Data: Our recent claims of a Galactic center feature in Fermi-LAT data at
approximately 130 GeV have prompted an avalanche of papers proposing
explanations ranging from dark matter annihilation to exotic pulsar winds.
Because of the importance of such interpretations for physics and astrophysics,
a discovery will require not only additional data, but a thorough investigation
of possible LAT systematics. While we do not have access to the details of each
event reconstruction, we do have information about each event from the public
event lists and spacecraft parameter files. These data allow us to search for
suspicious trends that could indicate a spurious signal. We consider several
hypotheses that might make an instrumental artifact more apparent at the
Galactic center, and find them implausible. We also search for an instrumental
signature in the Earth limb photons, which provide a smooth reference spectrum
for null tests. We find no significant 130 GeV feature in the Earth limb
sample. However, we do find a marginally significant 130 GeV feature in Earth
limb photons with a limited range of detector incidence angles. This raises
concerns about the 130 GeV Galactic center feature, even though we can think of
no plausible model of instrumental behavior that connects the two. A modest
amount of additional limb data would tell us if the limb feature is a
statistical fluke. If the limb feature persists, it would raise doubts about
the Pass 7 processing of E > 100 GeV events. At present we find no instrumental
systematics that could plausibly explain the excess Galactic center emission at
130 GeV. | astro-ph_HE |
Cooling and Instabilities in Colliding Flows: Collisional self-interactions occurring in protostellar jets give rise to
strong shocks, the structure of which can be affected by radiative cooling
within the flow. To study such colliding flows, we use the AstroBEAR AMR code
to conduct hydrodynamic simulations in both one and three dimensions with a
power law cooling function. The characteristic length and time scales for
cooling are temperature dependent and thus may vary as shocked gas cools. When
the cooling length decreases sufficiently rapidly the system becomes unstable
to the radiative shock instability, which produces oscillations in the position
of the shock front; these oscillations can be seen in both the one and three
dimensional cases. Our simulations show no evidence of the density clumping
characteristic of a thermal instability, even when the cooling function meets
the expected criteria. In the three-dimensional case, the nonlinear thin shell
instability (NTSI) is found to dominate when the cooling length is sufficiently
small. When the flows are subjected to the radiative shock instability,
oscillations in the size of the cooling region allow NTSI to occur at larger
cooling lengths, though larger cooling lengths delay the onset of NTSI by
increasing the oscillation period. | astro-ph_HE |
Multiwavelength Study of an X-ray Tidal Disruption Event Candidate in
NGC 5092: We present multiwavelength studies of a transient X-ray source, XMMSL1
J131952.3+225958, associated with the galaxy NGC 5092 at $z=0.023$ detected in
the XMM-Newton SLew survey (XMMSL). The source brightened in the 0.2--2 keV
band by a factor of $>20$ in 2005 as compared with previous flux limits and
then faded by a factor of $>200$ as observed with it XMM-Newton in 2013 and
with it Swift in 2018. At the flaring state, the X-ray spectrum can be modeled
with a blackbody at a temperature of $\sim$ 60 eV and an overall luminosity of
$\sim$ $1.5 \times 10^{43}$ erg s$^{-1}$. A UV flare and optical flare were
also detected with the Galaxy Evolution Explorer and the Sloan Digital Sky
Survey, respectively, within several months of the X-ray flare, whose
nonstellar UV--optical spectrum can be described with a blackbody at a
temperature of $\sim$ $(1-2) \times 10^4$ K and a luminosity of $\sim$ $(2-6)
\times 10^{43}$ erg s$^{-1}$. Interestingly, mid-infrared monitoring
observations of NGC 5092 with the Wide-field Infrared Survey Explorer 5--13 yr
later show a continuous flux decline. These dramatic variability properties,
from the X-ray through UV and optical to infrared, appear to be orderly,
suggestive of a stellar tidal disruption event (TDE) by a massive black hole,
confirming the postulation by Kanner et al.(2013). This TDE candidate belongs
to a rare sample with contemporaneous bright emission detected in the X-ray,
UV, and optical, which are later echoed by dust-reprocessed light in the
mid-infrared. The black hole has a mass of $\sim$ $5 \times 10^{7} \rm
M_{\odot}$, residing in a galaxy that is dominated by a middle-aged stellar
population of 2.5 Gyr. | astro-ph_HE |
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