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An imperfect double: probing the physical origin of the low-frequency
QPO and its harmonic in black hole binaries: We extract the spectra of the strong low-frequency quasi-periodic oscillation
(QPO) and its harmonic during the rising phase of an outburst in the black-hole
binary XTE J1550-564. We compare these frequency resolved spectra to the
time-averaged spectrum and the spectrum of the rapid (<0.1s) variability. The
spectrum of the time averaged emission can be described by a disc, a Compton
upscattered tail, and its reflection. The QPO spectrum contains no detectable
disc, and the Compton spectrum is generally harder than in the time averaged
emission, and shows less reflection, making it very similar to the spectrum of
the rapid variability. The harmonic likewise contains no detectable disc
component, but has a Compton spectrum which is systematically softer than the
QPO, softer even than the Compton tail in the time averaged emission. We
interpret these results in the context of the Lense-Thirring model for the QPO,
where a precessing hot flow replaces the inner disc, and the harmonic is
produced by the angular dependence of Compton scattering within the hot flow.
We extend these models to include stratification of the hot flow, so that it is
softer (lower optical depth) at larger radii closer to the truncated disc, and
harder (higher optical depth) in the innermost parts of the flow where the
rapid variability is produced. The different optical depth with radius gives
rise to different angular dependence of the Comptonised emission, weighting the
fundamental to the inner parts of the hot flow, and the harmonic to the outer.
This is the first model which can explain both the spectrum of the QPO and its
harmonic in a self consistent geometry. | astro-ph_HE |
Multi-messenger Observations of Tidal Disruption Events: Using the Zwicky Transient Facility (ZTF) and other observatories, we have
identified three candidate Tidal Disruption Events (TDEs) in spatial and
temporal coincidence with high-energy neutrinos detected by IceCube: AT2019dsg,
AT2019fdr and AT2019aalc. All three of these events have been shown to be able
to produce high-energy neutrinos. In these proceedings, I will give an overview
of Tidal Disruption Events, outline our follow-up program with ZTF, describe
the observations carried out for each of those coincident events and highlight
their similarities and differences. | astro-ph_HE |
Binary Neutron Star Mergers and Short Gamma-Ray Bursts: Effects of
Magnetic Field Orientation, Equation of State, and Mass Ratio: We present fully GRMHD simulations of the merger of binary neutron star (BNS)
systems. We consider BNSs producing a hypermassive neutron star (HMNS) that
collapses to a spinning black hole (BH) surrounded by a magnetized accretion
disk in a few tens of ms. We investigate whether such systems may launch
relativistic jets and power short gamma-ray bursts. We study the effects of
different equations of state (EOSs), different mass ratios, and different
magnetic field orientations. For all cases, we present a detailed investigation
of the matter dynamics and of the magnetic field evolution, with particular
attention to its global structure and possible emission of relativistic jets.
The main result of this work is that we found the formation of an organized
magnetic field structure. This happens independently of EOS, mass ratio, and
initial magnetic field orientation. We also show that those models that produce
a longer-lived HMNS lead to a stronger magnetic field before collapse to BH.
Such larger fields make it possible, for at least one of our models, to resolve
the MRI and hence further amplify the magnetic field. However, by the end of
our simulations, we do not observe a magnetically dominated funnel and hence
neither a relativistic outflow. With respect to the recent simulations of Ruiz
et al 2016, we evolve models with lower and more realistic initial magnetic
field strengths and, because of computational reasons, we do not evolve the
accretion disk for the long timescales that seem to be required in order to see
a relativistic outflow. Since all our models produce a similar ordered magnetic
field structure, we expect that the results found in Ruiz et al 2016, where
they only considered an equal-mass system with an ideal fluid EOS, should be
general and, at least from a qualitative point of view, independent from
mass-ratio, magnetic field orientation, and EOS. | astro-ph_HE |
An independent determination of the distance to supernova SN 1987A by
means of the light echo AT 2019xis: Accurate distance determination to astrophysical objects is essential for the
understanding of their intrinsic brightness and size. The distance to SN 1987A
has been previously measured by the expanding photosphere method, and by using
the angular size of the circumstellar rings with absolute sizes derived from
light curves of narrow UV emission lines, with reported distances ranging from
46.77 kpc to 55 kpc. In this study, we independently determined the distance to
SN 1987A using photometry and imaging polarimetry observations of AT 2019xis, a
light echo of SN 1987A, by adopting a radiative transfer model of the light
echo developed in Ding et al. (2021). We obtained distances to SN 1987A in the
range from 49.09 $\pm$ 2.16 kpc to 59.39 $\pm$ 3.27 kpc, depending on the
interstellar polarization and extinction corrections, which are consistent with
the literature values. This study demonstrates the potential of using light
echoes as a tool for distance determination to astrophysical objects in the
Milky Way, up to kiloparsec level scales. | astro-ph_HE |
Anomalous Anisotropies of Cosmic Rays from Turbulent Magnetic Fields: The propagation of cosmic rays (CRs) in turbulent interstellar magnetic
fields is typically described as a spatial diffusion process. This formalism
predicts only a small deviation from an isotropic CR distribution in the form
of a dipole in the direction of the CR density gradient or relative background
flow. We show that the existence of a global CR dipole moment necessarily
generates a spectrum of higher multipole moments in the local CR distribution.
These "anomalous" anisotropies are a direct consequence of Liouville's theorem
in the presence of a local turbulent magnetic field. We show that the
predictions of this model are in excellent agreement with the observed power
spectrum of multi-TeV CRs. | astro-ph_HE |
Formation scenarios and mass-radius relation for neutron stars: Neutron star crust, formed via accretion of matter from a companion in a
low-mass X-ray binary (LMXB), has an equation of state (EOS) stiffer than that
of catalyzed matter. At a given neutron star mass, M, the radius of a star with
an accreted crust is therefore larger, by DR(M), than for usually considered
star built of catalyzed matter. Using a compressible liquid drop model of
nuclei, we calculate, within the one-component plasma approximation, the EOSs
corresponding to different nuclear compositions of ashes of X-ray bursts in
LMXB. These EOSs are then applied for studying the effect of different
formation scenarios on the neutron-star mass-radius relation. Assuming the SLy
EOS for neutron star's liquid core, derived by Douchin & Haensel (2001), we
find that at M=1.4 M_sun the star with accreted crust has a radius more than
100 m larger that for the crust of catalyzed matter. Using smallness of the
crust mass compared to M, we derive a formula that relates DR(M) to the
difference in the crust EOS. This very precise formula gives also analytic
dependence of DR on M and R of the reference star built of catalyzed matter.
The formula is valid for any EOS of the liquid core. Rotation of neutron star
makes DR(M) larger. We derive an approximate but very precise formula that
gives difference in equatorial radii, DR_eq(M), as a function of stellar
rotation frequency. | astro-ph_HE |
Telescope Array 10 Year Composition: Estimates of the composition of ultra high energy cosmic rays (UHECRs) can be
inferred by recording the depth of air shower maximum, $X_{\mathrm{max}}$, for
many showers and comparing the distributions to those predicted by Monte Carlo
simulations. Traditionally, UHECR composition has relied upon comparison of the
first and second moments of the $X_{\mathrm{max}}$ distributions to estimate
the compatibility between data and simulations, but with the large UHECR
datasets being built the current generation experiments better tests which
compare full distributions can be employed. Such tests can be used to
understand the accuracy with which UHECR composition can actually be understood
at the current level of statistics and quantitatively measure the significance
of agreement or disagreement with models in order to reject them. In this paper
we present the most recent results of 10 years of Telescope Array hybrid
$X_{\mathrm{max}}$ measurements which is found to agree with a predominantly
light composition. In previously published results we have demonstrated the
agreement of Telescope Array hybrid $X_{\mathrm{max}}$ data with single element
models using systematic shifting of the data in order to ensure the shapes of
the distributions are being compared. Here we present multi-component source
models fit to hybrid $X_{\mathrm{max}}$ data and report on the relative
fractions of those sources that best fit the data. Below $10^{19.1}$ eV TA
hybrid data is found to be compatible with mixtures composed of predominantly
light elements such as protons and helium. | astro-ph_HE |
Search for Coincident Gravitational Wave and Long Gamma-Ray Bursts from
4-OGC and the Fermi-GBM/Swift-BAT Catalog: The recent discovery of a kilonova associated with an apparent long-duration
gamma-ray burst has challenged the typical classification that long gamma-ray
bursts originate from the core collapse of massive stars and short gamma-ray
bursts are from compact binary coalescence. The kilonova indicates a neutron
star merger origin and suggests the viability of gravitational-wave and long
gamma-ray burst multimessenger astronomy. Gravitational waves play a crucial
role by providing independent information for the source properties. This work
revisits the archival 2015-2020 LIGO/Virgo gravitational-wave candidates from
the 4-OGC catalog which are consistent with a binary neutron star or neutron
star-black hole merger and the long-duration gamma-ray bursts from the
Fermi-GBM and Swift-BAT catalogs. We search for spatial and temporal
coincidence with up to 10 s time lag between gravitational-wave candidates and
the onset of long-duration gamma-ray bursts. The most significant candidate
association has only a false alarm rate of once every two years; given the
LIGO/Virgo observational period, this is consistent with a null result. We
report an exclusion distance for each search candidate for a fiducial
gravitational-wave signal and conservative viewing angle assumptions. | astro-ph_HE |
Resolving the X-ray obscuration in a low flux observation of the quasar
PDS 456: Simultaneous XMM-Newton, NuSTAR and HST observations, performed in March
2017, of the nearby ($z=0.184$) luminous quasar PDS 456 are presented. PDS 456
had a low X-ray flux compared to past observations, where the first of the two
new XMM-Newton observations occurred during a pronounced dip in the X-ray
lightcurve. The broad-band X-ray spectrum is highly absorbed, attenuated by a
soft X-ray absorber of column density $N_{\rm H}=6\times10^{22}$ cm$^{-2}$. An
increase in obscuration occurs during the dip, which may be due to an X-ray
eclipse. In addition, the persistent, fast Fe K outflow is present, with
velocity components of $-0.25c$ and $-0.4c$. The soft absorber is less ionized
($\log\xi=3$) compared to the iron K outflow ($\log\xi=5$) and is outflowing
with a velocity of approximately $-0.2c$. A soft X-ray excess is present below
1 keV against the highly absorbed continuum and can be attributed to the
re-emission from a wide angle wind. The complex X-ray absorption present in PDS
456 suggests that the wind is inhomogeneous, whereby the soft X-ray absorber
originates from denser clumps or filaments which may form further out along the
outflow. In contrast to the X-ray observations, the simultaneous UV spectrum of
PDS 456 is largely unabsorbed, where only a very weak broad absorption trough
is present bluewards of Ly$\alpha$, compared to a past observation in 2000 when
the trough was significantly stronger. The relative weakness of the UV
absorption may be due to the soft X-ray absorber being too highly ionized and
almost transparent in the UV band. | astro-ph_HE |
Proton synchrotron, plausible explanation for delayed VHE activity of 3C
279 in 2018: A nearly 11-day delayed very-high-energy(VHE) activity compared to the
Fermi-LAT flare from quasar 3C 279 was reported by H.E.S.S. on 28 January 2018.
3C 279 has long been considered a candidate site for particle acceleration;
hence such events may embed information about the high-energy phenomena. We
propose the production channel being leptonic for the multi-wavelength flare,
UV-Optical-Xrays-$\gamma$-rays, whereas the delayed VHE activity originated
from the proton synchrotron. Our model requires the magnetic field to be 2.3 G
and the proton luminosity (L$_{p}$) $1.56
\times 10^{46}$ erg/sec, whereas the lepton luminosity (L$_e$) $3.9
\times 10^{43}$ erg/sec. | astro-ph_HE |
Properties of unique hard X-ray dips observed from GRS 1915+105 and IGR
J17091-3624 and their implications: We report a comprehensive study on spectral and timing properties of hard
X-ray dips uniquely observed in some so-called variability classes of the
micro-quasars GRS 1915+105 and IGR J17091-3624. These dips are characterized by
a sudden decline in the 2.0-60.0 keV X-ray intensity by a factor of 4-12
simultaneous with the increase in hardness ratio by a factor of 2-4. Using 31
observations of GRS 1915+105 with RXTE/PCA, we show that different behavior are
observed in different types of variability classes, and we find that a
dichotomy is observed between classes with abrupt transitions vs those with
smoother evolution. For example, both energy-lag spectra and frequency-lag
spectra of hard X-ray dips in classes with abrupt transitions and shorter dip
intervals show hard-lag (hard photons lag soft photons), while both lag spectra
during hard dips in classes with smoother evolution and longer dip intervals
show soft-lag. Both lag time-scales are of the order of 100-600 msec. We also
show that timing and spectral properties of hard X-ray dips observed in light
curves of IGR J17091-3624 during its 2011 outburst are consistent with the
properties of the abrupt transitions in GRS 1915+105 rather than smooth
evolution. A global correlation between the X-ray intensity cycle time and hard
dip time is observed for both abrupt and smooth transition which may be due to
two distinct physical processes whose time-scales are eventually correlated. We
discuss implications of our results in the light of some generic models. | astro-ph_HE |
Gamma-Ray Burst observations with Fermi: After seven years of science operation, the Fermi mission has brought great
advances in the study of Gamma-ray Bursts (GRBs). Over 1600 GRBs have been
detected by the Gamma-ray Burst Monitor, and more than 100 of these are also
detected by the Large Area Telescope above 30 MeV. We will give an overview of
these observations, presenting the common properties in the GRB temporal and
spectral behavior at high energies. We will also highlight the unique
characteristics of some individual bursts. The main physical implications of
these results will be discussed, along with open questions regarding GRB
modeling in their prompt and temporally-extended emission phases. | astro-ph_HE |
Strongly Lensed Supermassive Black Hole Binaries as Nanohertz
Gravitational-Wave Sources: Supermassive black hole binary systems (SMBHBs) should be the most powerful
sources of gravitational waves (GWs) in the Universe. Once Pulsar Timing Arrays
(PTAs) detect the stochastic GW background from their cosmic merger history,
searching for individually resolvable binaries will take on new importance.
Since these individual SMBHBs are expected to be rare, here we explore how
strong gravitational lensing can act as a tool for increasing their detection
prospects by magnifying fainter sources and bringing them into view. Unlike for
electromagnetic waves, when the geometric optics limit is nearly always valid,
for GWs the wave-diffraction-interference effects can become important when the
wavelength of the GWs is larger than the Schwarzchild radius of the lens, i.e.
$M_{\rm lens} \sim 10^8\,(\frac{f}{mHz})^{-1}\,M_\odot$. For the GW frequency
range explored in this work, the geometric optics limit holds. We investigate
GW signals from SMBHBs that might be detectable with current and future PTAs
under the assumption that quasars serve as bright beacons that signal a recent
merger. Using the black hole mass function derived from quasars and a
physically motivated magnification distribution, we expect to detect a few
strongly lensed binary systems out to $z \approx 2$. Additionally, for a range
of fixed magnifications $2 \leq \mu \leq 100$, strong lensing adds up to
$\sim$30 more detectable binaries for PTAs. Finally, we investigate the
possibility of observing both time-delayed electromagnetic signals and GW
signals from these strongly lensed binary systems -- that will provide us with
unprecedented multimessenger insights into their orbital evolution. | astro-ph_HE |
Properties of Low Luminosity Afterglow Gamma-ray Bursts: Aims: We characterize a sample of Gamma-Ray Bursts with low luminosity X-ray
afterglows (LLA GRBs), and study their properties. Method: We select a sample
consisting of the 12\% faintest X-ray afterglows from the total population of
long GRBs (lGRBs) with known redshift. We study their intrinsic properties
(spectral index, decay index, distance, luminosity, isotropic radiated energy
and peak energy) to assess whether they belong to the same population than the
brighter afterglow events. Results: We present strong evidences that these
events belong to a population of nearby events, different from that of the
general population of lGRBs. These events are faint during their prompt phase,
and include the few possible outliers of the Amati relation. Out of 14 GRB-SN
associations, 9 are in LLA GRB sample, prompting for caution when using SN
templates in observational and theoretical models for the general lGRBs
population. | astro-ph_HE |
Neutrino analysis of the September 2010 Crab Nebula flare and
time-integrated constraints on neutrino emission from the Crab using IceCube: We present the results for a search of high-energy muon neutrinos with the
IceCube detector in coincidence with the Crab nebula flare reported on
September 2010 by various experiments. Due to the unusual flaring state of the
otherwise steady source we performed a prompt analysis of the 79-string
configuration data to search for neutrinos that might be emitted along with the
observed gamma-rays. We performed two different and complementary data
selections of neutrino events in the time window of 10 days around the flare.
One event selection is optimized for discovery of E^-2 neutrino spectrum
typical of 1st order Fermi acceleration. A similar event selection has also
been applied to the 40-string data to derive the time-integrated limits to the
neutrino emission from the Crab. The other event selection was optimized for
discovery of neutrino spectra with softer spectral index and TeV energy
cut-offs as observed for various galactic sources in gamma-rays. The 90% CL
best upper limits on the Crab flux during the 10 day flare are 4.73 x 10^-11
cm-2 s-1 TeV-1 for an E^-2 neutrino spectrum and 2.50 x 10^-10 cm-2 s-1 TeV-1
for a softer neutrino spectra of E-2.7, as indicated by Fermi measurements
during the flare. IceCube has also set a time-integrated limit on the neutrino
emission of the Crab using 375.5 days of livetime of the 40-string
configuration data. This limit is compared to existing models of neutrino
production from the Crab and its impact on astrophysical parameters is
discussed. The most optimistic predictions of some models are already rejected
by the IceCube neutrino telescope with more than 90% CL. | astro-ph_HE |
Properties of intra-cluster low-mass X-ray binaries in Fornax globular
clusters: We present a study of the intra-cluster population of low-mass X-ray binaries
(LMXB) residing in globular clusters (GC) in the central 1 $deg^2$ of the
Fornax galaxy cluster. Differently from previous studies, which were restricted
to the innermost regions of individual galaxies, this work is aimed at
comparing the properties of the intra-cluster population of GC-LMXBs with those
of the host galaxy. We use VLT Survey Telescope (VST) and Chandra observations.
We identify 168 LMXBs residing in GCs and divide it into host-galaxy and
intra-cluster objects based on their distance from the nearest galaxy in terms
of effective radius. We found 82 intra-cluster GC-LMXBs and 86 objects that are
hosted in galaxies. We perform a Gaussian mixture model to divide the
population into red and blue GCs. As has been found for the innermost regions
of galaxies, LMXBs tend to form in red and bright GCs in intra-cluster space as
well. We find, however, that the likelihood of a red GC to host an LMXB
decreases with galactocentric distance, but it remains approximately constant
for the blue GC population. Investigating the X-ray properties of the LMXBs
residing in GCs, we find a difference in the X-ray luminosity function between
the intra-cluster and host-galaxy sample: both follow a power-law down to $\sim
8.5\times 10^{37}$ erg s$^{-1}$, which is consistent with field LMXBs for the
intra-cluster sample, while the latter agree with previous estimates for LMXBs
in GCs. We detect a tentative difference in the hardness ratio of two
populations, where the intra-cluster GC-LMXBs appear to have harder spectra
than the host-galaxy objects. We find the same trend when we compare red and
blue GC-LMXBs: the spectra of the blue sample are harder spectra than those of
the red sample. This result could suggest a relation between the spectral
properties of LMXBs and the host GC colour and therefore its metallicity. | astro-ph_HE |
Rapid Infrared Variability of Three Radio-loud Narrow-line Seyfert 1
Galaxies: A View from the Wide-field Infrared Survey Explorer: Using newly released data from the Wide-field Infrared Survey Explorer, we
report the discovery of rapid infrared variability in three radio-loud
narrow-line Seyfert 1 galaxies (NLS1s) selected from the 23 sources in the
sample of Yuan et al. (2008). J0849+5108 and J0948+0022 clearly show intraday
variability, while J1505+0326 has a longer measurable time scale within 180
days. Their variability amplitudes, corrected for measurement errors, are $\sim
0.1-0.2$ mag. The detection of intraday variability restricts the size of the
infrared-emitting region to $\sim 10^{-3}$ pc, significantly smaller than the
scale of the torus but consistent with the base of a jet. The three variable
sources are exceptionally radio-loud, have the highest radio brightness
temperature among the whole sample, and all show detected $\gamma$-ray emission
in Fermi/LAT observations. Their spectral energy distributions resemble those
of low-energy-peaked blazars, with a synchrotron peak around infrared
wavelengths. This result strongly confirms the view that at least some
radio-loud NLS1s are blazars with a relativistic jet close to our line of
sight. The beamed synchrotron emission from the jet contributes significantly
to and probably dominates the spectra in the infrared and even optical bands. | astro-ph_HE |
Assessing equation of state-independent relations for neutron stars with
nonparametric models: Relations between neutron star properties that do not depend on the nuclear
equation of state offer insights on neutron star physics and have practical
applications in data analysis. Such relations are obtained by fitting to a
range of phenomenological or nuclear physics equation of state models, each of
which may have varying degrees of accuracy. In this study we revisit
commonly-used relations and re-assess them with a very flexible set of
phenomenological nonparametric equation of state models that are based on
Gaussian Processes. Our models correspond to two sets: equations of state which
mimic hadronic models, and equations of state with rapidly changing behavior
that resemble phase transitions. We quantify the accuracy of relations under
both sets and discuss their applicability with respect to expected upcoming
statistical uncertainties of astrophysical observations. We further propose a
goodness-of-fit metric which provides an estimate for the systematic error
introduced by using the relation to model a certain equation-of-state set.
Overall, the nonparametric distribution is more poorly fit with existing
relations, with the I-Love-Q relations retaining the highest degree of
universality. Fits degrade for relations involving the tidal deformability,
such as the Binary-Love and compactness-Love relations, and when introducing
phase transition phenomenology. For most relations, systematic errors are
comparable to current statistical uncertainties under the nonparametric
equation of state distributions. | astro-ph_HE |
Very High-Energy ($>$50 GeV) Gamma-ray Flux Variability of Bright Fermi
Blazars: Understanding the high-energy emission processes and variability patterns are
two of the most challenging research problems associated with relativistic
jets. In particular, the long-term (months-to-years) flux variability at very
high energies (VHE, $>$50 GeV) has remained an unexplored domain so far. This
is possibly due to the decreased sensitivity of the Fermi Large Area Telescope
(LAT) above a few GeV, hence low photon statistics, and observing constraints
associated with the ground-based Cherenkov telescopes. This paper reports the
results obtained from the 0.05$-$2 TeV Fermi-LAT data analysis of a sample of
29 blazars with the primary objective to explore their months-to-year long VHE
flux variability behavior. This systematic search has led to, for the first
time, the detection of significant flux variations in 5 blazars at $>$99\%
confidence level, whereas, 8 of them exhibit variability albeit at a lower
confidence level ($\sim$95\%-99\%). A comparison of the 0.05$-$2 TeV flux
variations with that observed at 0.1$-$50 GeV band has revealed similar
variability behavior for most of the sources. However, complex variability
patterns that are not reflected contemporaneously in both energy bands were
also detected, thereby providing tantalizing clues about the underlying
radiative mechanisms. These results open up a new dimension to unravel the VHE
emission processes operating in relativistic jets, hence sowing the seeds for
their future observations with the upcoming Cherenkov Telescope Array. | astro-ph_HE |
Very Rapid High-Amplitude Gamma-ray Variability in Luminous Blazar PKS
1510-089 Studied with Fermi-LAT: Here we report on the detailed analysis of the gamma-ray light curve of a
luminous blazar PKS1510-089 observed in the GeV range with the Large Area
Telescope (LAT) onboard the Fermi satellite during the period 2011 September --
December. By investigating the properties of the detected three major flares
with the shortest possible time binning allowed by the photon statistics, we
find a variety of temporal characteristics and variability patterns. This
includes a clearly asymmetric profile (with a faster flux rise and a slower
decay) of the flare resolved on sub-daily timescales, a superposition of many
short uncorrelated flaring events forming the apparently coherent
longer-duration outburst, and a huge single isolated outburst unresolved down
to the timescale of three-hours. In the latter case we estimate the
corresponding gamma-ray flux doubling timescale to be below one hour, which is
extreme and never previously reported for any active galaxy in the GeV range.
The other unique finding is that the total power released during the studied
rapid and high-amplitude flares constitute the bulk of the power radiatively
dissipated in the source, and a significant fraction of the total kinetic
luminosity of the underlying relativistic outflow. Our analysis allows us to
access directly the characteristic timescales involved in shaping the energy
dissipation processes in the source, and to provide constraints on the location
and the structure of the blazar emission zone in PKS1510-089. | astro-ph_HE |
Application of the space-based optical interferometer towards measuring
cosmological distances of quasars: Measuring the quasar distance through joint analysis of spectroastrometry
(SA) and reverberation mapping (RM) observations is a new method for driving
the development of cosmology. In this paper, we carry out detailed simulation
and analysis to study the effect of four basic observational parameters
(baseline length, exposure time, equivalent diameter and spectral resolution)
on the data quality of differential phase curves (DPCs), furthermore on the
accuracy of distance measurement. In our simulation, we adopt an axis
symmetrical disc model of broad line region (BLR) to generate differential
phase signals. We find that the differential phases and their Poisson errors
could be amplified by extending the baseline, while the influence of OPD
(optical path difference) errors can be reduced during fitting the BLR model.
Longer exposure time or larger equivalent diameter helps reduce the absolute
Poisson error. Therefore, the relative error of DPCs could be reduce by
increasing any of the above three parameters, then the accuracy of distance
measurement could be improved. In contrast, the uncertainty of $D_{\rm{A}}$ (
absolute angular distances) could be improved with higher spectral resolution,
although the relative error of DPCs would be amplified. We show how the
uncertainty of distance measurement varies with the relative error of DPCs. For
our specific set of model parameters, without considering more complicated
structures and kinematics of BLRs in our simulation, it is found that the
relative error of DPCs $<$ 20$\%$ is a limit for accurate distance measurement.
The relative error of DPCs have a lower limit (roughly 5$\%$) and the
uncertainty of distance measurement can be better than 2$\%$. | astro-ph_HE |
Unexpected late-time temperature increase observed in two neutron star
crust cooling sources -- XTE~J1701-462 and EXO~0748-676: Transient LMXBs that host neutron stars (NSs) provide excellent laboratories
for probing the dense matter physics present in NS crusts. During accretion
outbursts in LMXBs, exothermic reactions may heat the NS crust, disrupting the
crust-core equilibrium. When the outburst ceases, the crust cools to restore
thermal equilibrium with the core. Monitoring this evolution allows us to probe
the dense matter physics in the crust. Properties of the deeper crustal layers
can be probed at later times after the end of the outburst. We report on the
unexpected late-time temperature evolution (>2000 days after the end of their
outbursts) of two NSs in LMXBs, XTE J1701-462 and EXO 0748-676. Although both
these sources exhibited very different outbursts (in terms of duration and the
average accretion rate), they exhibit an unusually steep decay of ~7 eV in the
observed effective temperature (occurring in a time span of ~700 days) around
~2000 days after the end of their outbursts. Furthermore, they both showed an
even more unexpected rise of ~3 eV in temperature (over a time period of
~500-2000 days) after this steep decay. This rise was significant at the
2.4{\sigma} and 8.5{\sigma} level for XTE J1701-462 and EXO 0748-676,
respectively. The physical explanation for such behaviour is unknown and cannot
be straightforwardly be explained within the cooling hypothesis. In addition,
this observed evolution cannot be well explained by low-level accretion either
without invoking many assumptions. We investigate the potential pathways in the
theoretical heating and cooling models that could reproduce this unusual
behaviour, which so far has been observed in two crust-cooling sources. Such a
temperature increase has not been observed in the other NS crust-cooling
sources at similarly late times, although it cannot be excluded that this might
be a result of the inadequate sampling obtained at such late times. | astro-ph_HE |
Towards a dynamical mass of the ultraluminous X-ray source NGC 5408 X-1: We obtained multi-epoch Very Large Telescope (VLT) optical spectroscopic data
in 2011 and 2012 on the ultraluminous X-ray source (ULX) NGC 5408 X-1. We
confirm that the HeII\lambda4686 line has a broad component with an average
FWHM of v=780\pm64 km/s with a variation of ~13% during observations spanning
over 4 years, and is consistent with the origin in the accretion disc. The
deepest optical spectrum does not reveal any absorption line from a donor star.
Our aim was to measure the radial velocity curve and estimate the parameters of
the binary system. We find an upper limit on the semi-amplitude of the radial
velocity of K=132\pm42 km/s. A search for a periodic signal in the data
resulted in no statistically significant period. The mass function and
constraints on the binary system imply a black hole mass of less than ~510
M_sun. Whilst, a disc irradiation model may imply a black hole mass smaller
than ~431-1985 M_sun, depending on inclination. Our data can also be consistent
with an unexplored orbital period range from a couple of hours to a few days,
thus with a stellar-mass black hole and a subgiant companion. | astro-ph_HE |
On the Diversity of Fallback Rates from Tidal Disruption Events with
Accurate Stellar Structure: The tidal disruption of stars by supermassive black holes (SMBHs) can be used
to probe the SMBH mass function, the properties of individual stars, and
stellar dynamics in galactic nuclei. Upcoming missions will detect thousands of
TDEs, and accurate theoretical modeling is required to interpret the data with
precision. Here we analyze the influence of more realistic stellar structure on
the outcome of TDEs; in particular, we compare the fallback rates -- being the
rate at which tidally-disrupted debris returns to the black hole -- from
progenitors generated with the stellar evolution code {\sc mesa} to $\gamma =
4/3$ and $\gamma = 5/3$ polytropes. We find that {\sc mesa}-generated density
profiles yield qualitatively-different fallback rates as compared to polytropic
approximations, and that only the fallback curves from low-mass ($1M_{\odot}$
or less), zero-age main-sequence stars are well fit by either a $\gamma = 4/3$
or $5/3$ polytrope. Stellar age has a strong affect on the shape of the
fallback curve, and can produce characteristic timescales (e.g., the time to
the peak of the fallback rate) that greatly differ from the polytropic values.
We use these differences to assess the degree to which the inferred black hole
mass from the observed lightcurve can deviate from the true value, and find
that the discrepancy can be at the order of magnitude level. Accurate stellar
structure also leads to a substantial variation in the critical impact
parameter at which the star is fully disrupted, and can increase the
susceptibility of the debris stream to fragmentation under its own
self-gravity. These results suggest that detailed modeling is required to
accurately interpret observed lightcurves of TDEs. | astro-ph_HE |
Spherical configuration of a super-dense hot compact object with
particular EoS: The equation of state (EoS) $P = P (\rho, ...)$ -- pressure as a function of
density and other thermodynamical quantities -- is what generates
particularities of mass--radius distribution $M (R)$ for super--dense compact
stellar bodies, the remnants of cosmic cataclysms. In view of recent nuclear
experiments, we propose one particular EoS, which admits the critical state
characterized by density $\rho_c$ and temperature $T_c$, and which under
certain conditions permits a radial distribution of the super--dense matter in
"liquid" phase. We establish such conditions and demonstrate that a stable
configuration is indeed possible (only) for temperatures smaller than the
critical one. Using Tolman--Oppenheimer--Volkoff equations for hydrostatic
equilibrium, we derive the mass--radius relation for the super--dense compact
objects with masses smaller than the Sun, $M \ll M_{\odot}$. The obtained
results are within the constraints established by both heavy--ion collision
experiments and theoretical studies of neutron--rich matter. | astro-ph_HE |
X-ray/UV/optical variability of NGC 4593 with Swift: Reprocessing of
X-rays by an extended reprocessor: We report the results of intensive X-ray, UV and optical monitoring of the
Seyfert 1 galaxy NGC 4593 with Swift. There is no intrinsic flux-related
spectral change in the the variable components in any band with small apparent
variations due only to contamination by a second constant component, possibly a
(hard) reflection component in the X-rays and the (red) host galaxy in the
UV/optical bands. Relative to the shortest wavelength band, UVW2, the lags of
the other UV and optical bands are mostly in agreement with the predictions of
reprocessing of high energy emission from an accretion disc. The U-band lag is,
however, far larger than expected, almost certainly because of reprocessed
Balmer continuum emission from the more distant broad line region gas. The UVW2
band is well correlated with the X-rays but lags by ~6x more than expected if
the UVW2 results from reprocessing of X-rays on the accretion disc. However, if
the lightcurves are filtered to remove variations on timescales >5d, the lag
approaches the expectation from disc reprocessing. MEMEcho analysis shows that
direct X-rays can be the driver of most of the variations in the UV/optical
bands as long as the response functions for those bands all have long tails (up
to 10d) in addition to a strong peak (from disc reprocessing) at short lag
(<1d). We interpret the tails as due to reprocessing from the surrounding gas.
Comparison of X-ray to UVW2 and UVW2 to V-band lags for 4 AGN, including NGC
4593, shows that all have UVW2 to V-band lags which exceed the expectations
from disc resprocessing by factor < 2. However the X-ray to UVW2 lags are,
mostly, in greater excess from the expectations from disc reprocessing and
differ between AGN. The largest excess is in NGC 4151. Absorption and
scattering may be affecting X-ray to UV lags. | astro-ph_HE |
Evidence for powerful winds and the associated reverse shock as the
origin of the Fermi bubbles: The Fermi bubbles are large gamma-ray-emitting structures. They are symmetric
about the Galactic Centre (GC), and their creation is therefore attributed to
intensive energy injection at the GC. In this study, we focus on the
non-equilibrium X-ray gas structures associated with the bubbles. We show that
a combination of the density, temperature, and shock age profiles of the X-ray
gas can be used to distinguish the energy injection mechanisms. By comparing
the results of numerical simulations with observations, we indicate that the
bubbles were created by a fast wind from the GC because it generates a strong
reverse shock and reproduces the observed temperature peak there. On the other
hand, instantaneous energy injection at the GC cannot reproduce the temperature
profile. The wind had a speed of ~1000 km/s, and blew for ~10^7 yr. Because the
mass flux of the wind is large, the entrainment of interstellar gas by
wide-angle outflows from the black hole is required. Thus, the wind may be the
same as active galactic nuclei outflows often observed in other galaxies and
thought to regulate the growth of galaxies and their central black holes. | astro-ph_HE |
Black hole lightning due to particle acceleration at subhorizon scales: Supermassive black holes with masses of millions to billions of solar masses
are commonly found in the centers of galaxies. Astronomers seek to image jet
formation using radio interferometry, but still suffer from insufficient
angular resolution. An alternative method to resolve small structures is to
measure the time variability of their emission. Here, we report on gamma-ray
observations of the radio galaxy IC 310 obtained with the MAGIC telescopes
revealing variability with doubling time scales faster than 4.8 min. Causality
constrains the size of the emission region to be smaller than 20\% of the
gravitational radius of its central black hole. We suggest that the emission is
associated with pulsar-like particle acceleration by the electric field across
a magnetospheric gap at the base of the radio jet. | astro-ph_HE |
Optical and Infrared Photometry of the Nearby SN 2017cbv: On 2017 March 11, the DLT40 Transient Discovery Survey discovered SN 2017cbv
in NGC5643, a Type 2 Seyfert Galaxy in the Lupus Constellation. SN 2017cbv went
on to become a bright Type Ia supernova, with a $V_{max}$ of 11.51 $\pm$ 0.05
mag. We present early time optical and infrared photometry of SN 2017cbv
covering the rise and fall of over 68 days. We find that SN 2017cbv has a broad
light curve $\Delta m_{15}(B)$ = 0.88 $\pm$ 0.07, a $B$-band maximum at
2457840.97 $\pm$ 0.43, a negligible host galaxy reddening where $E(B-V)_{host}$
$\approx$ 0, and a distance modulus of 30.49 $\pm$ 0.32 to the SN,
corresponding to a distance of $12.58_{-1.71}^{+1.98}$ Mpc. We also present the
results of two different numerical models we used for analysis in this paper:
SALT2, an empirical model for Type Ia supernova optical light curves that
accounts for variability components; and SNooPy, the CSP-II light-curve model
that covers both optical and near-infrared wavelengths and is used for distance
estimates. | astro-ph_HE |
Rapid spectral transition of the black hole binary V404 Cyg: During the June 2015 outburst of the black hole binary V404 Cyg, rapid
changes in the X-ray brightness and spectra were common. The INTEGRAL
monitoring campaign detected spectacular Eddington-limited X-ray flares, but
also rapid variations at much lower flux levels. On 2015 June 21 at 20 h 50
min, the 3-10 keV JEM-X data as well as simultaneous optical data started to
display a gradual brightening from one of these low-flux states. This was
followed 15 min later by an order-of-magnitude increase of flux in the 20-40
keV IBIS/ISGRI light curve in just 15 s. The best-fitting model for both the
pre- and post-transition spectra required a Compton-thick partially covering
absorber. The absorber parameters remained constant, but the spectral slope
varied significantly during the event, with the photon index decreasing from
$\Gamma \approx 3.7$ to $\Gamma \approx 2.3$. We propose that the rapid 20-40
keV flux increase was either caused by a spectral state transition that was
hidden from our direct view, or that there was a sudden reduction in the amount
of Compton down-scattering of the primary X-ray emission in the disk outflow. | astro-ph_HE |
Astro 2020 Science White Paper: Cosmic-ray Antinuclei as Messengers for
Dark Matter: The origin of dark matter is a driving question of modern physics. Low-energy
antideuterons provide a "smoking gun" signature of dark matter annihilation or
decay, essentially free of astrophysical background. Low-energy antiprotons are
a vital partner for this analysis, and low-energy antihelium could provide
further discovery space for new physics. In the coming decade, AMS-02 will
continue accumulating the large statistics and systematic understanding
necessary for it to probe rare antinuclei signatures, and GAPS, which is the
first experiment optimized specifically for low-energy cosmic antinuclei, will
begin several Antarctic balloon campaigns. The connection of cosmic-ray
antinuclei and dark matter is reviewed and the outlook in light of experimental
progress is presented. | astro-ph_HE |
Binary evolution with LOFT: This is a White Paper in support of the mission concept of the Large
Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We
discuss the potential of LOFT for the study of very faint X-ray binaries,
orbital period distribution of black hole X-ray binaries and neutron star spin
up. For a summary, we refer to the paper. | astro-ph_HE |
Old Data, New Forensics: The First Second of SN 1987A Neutrino Emission: The next Milky Way supernova will be an epochal event in multi-messenger
astronomy, critical to tests of supernovae, neutrinos, and new physics.
Realizing this potential depends on having realistic simulations of core
collapse. We investigate the neutrino predictions of nearly all modern models
(1-, 2-, and 3-d) over the first $\simeq$1 s, making the first detailed
comparisons of these models to each other and to the SN 1987A neutrino data.
Even with different methods and inputs, the models generally agree with each
other. However, even considering the low neutrino counts, the models generally
disagree with data. What can cause this? We show that neither neutrino
oscillations nor different progenitor masses appear to be a sufficient
solution. We outline urgently needed work. | astro-ph_HE |
Strong Calcium Emission Indicates that the Ultraviolet-Flashing Type Ia
SN 2019yvq was the Result of a Sub-Chandrasekhar Mass Double-Detonation
Explosion: We present nebular spectra of the Type Ia supernova (SN Ia) SN 2019yvq, which
had a bright flash of blue and ultraviolet light after exploding, followed by a
rise similar to other SNe Ia. Although SN 2019yvq displayed several other rare
characteristics such as persistent high ejecta velocity near peak brightness,
it was not especially peculiar and if the early "excess" emission were not
observed, it would likely be included in cosmological samples. The excess flux
can be explained by several different physical models linked to the details of
the progenitor system and explosion mechanism. Each has unique predictions for
the optically thin emission at late times. In our nebular spectra, we detect
strong [Ca II] $\lambda\lambda$7291, 7324 and Ca NIR triplet emission,
consistent with a double-detonation explosion. We do not detect H, He, or [O I]
emission, predictions for some single-degenerate progenitor systems and violent
white dwarf mergers. The amount of swept-up H or He is <2.8 x 10^-4 and 2.4 x
10^-4 Msun, respectively. Aside from strong Ca emission, the SN 2019yvq nebular
spectrum is similar to those of typical SNe Ia with the same light-curve shape.
Comparing to double-detonation models, we find that the Ca emission is
consistent with a model with a total progenitor mass of 1.15 Msun. However, we
note that a lower progenitor mass better explains the early light-curve and
peak luminosity. The unique properties of SN 2019yvq suggest that thick
He-shell double-detonations only account for $1.1^{+2.1}_{-1.1}\%$ of the total
"normal" SN Ia rate. SN 2019yvq is one of the best examples yet that multiple
progenitor channels appear necessary to reproduce the full diversity of
"normal" SNe Ia. | astro-ph_HE |
Extreme Gravity and Fundamental Physics: Future gravitational-wave observations will enable unprecedented and unique
science in extreme gravity and fundamental physics answering questions about
the nature of dynamical spacetimes, the nature of dark matter and the nature of
compact objects. | astro-ph_HE |
X-ray Polarization of Gamma-Ray Bursts: The degree and the temporal evolution of linear polarization in the prompt
and afterglow emission of gamma-ray bursts is a very robust diagnostic of some
key features of gamma-ray bursts jets and their micro and macro physics. In
this contribution, I review the current status of the theory of polarized
emission from GRB jets during the prompt, optical flash, and afterglow
emission. I compare the theoretical predictions to the available observations
and discuss the future prospect from both the theoretical and observational
standpoints. | astro-ph_HE |
Impact of the Cherenkov Telescope Array (CTA) altitude on Dark Matter
searches in the Milky Way Halo: Observations of dwarf galaxies and of the Milky Way halo with current
ground-based Cherenkov telescopes have resulted in interesting limits on the
cross-section for dark matter (DM) self- annihilation for WIMP masses above
some 100 GeV. The future Cherenkov Telescope Array (CTA) is expected to further
explore the parameter space of dark matter candidates that are predicted in
extensions of the standard model of particle physics. Due to its low energy
threshold (of order of few tens of GeV) and high sensitivity, CTA will also
probe lower WIMP masses than current experiments, but the actual performance in
this regime will be influenced by the altitude of the observatory above sea
level. Using the response of possible CTA candidate arrays to simulated photons
and hadrons, we estimate how searches for a WIMP annihilation signal from the
Milky Way halo will be influenced by altitude of different possible CTA sites. | astro-ph_HE |
Spectroscopic r-Process Abundance Retrieval for Kilonovae II:
Lanthanides in the Inferred Abundance Patterns of Multi-Component Ejecta from
the GW170817 Kilonova: In kilonovae, freshly-synthesized $r$-process elements imprint features on
optical spectra, as observed in AT2017gfo, the counterpart to the GW170817
binary neutron star merger. However, measuring the $r$-process compositions of
the merger ejecta is computationally challenging. Vieira et al. (2023)
introduced Spectroscopic $r$-Process Abundance Retrieval for Kilonovae (SPARK),
a software tool to infer elemental abundance patterns of the ejecta, and
associate spectral features with particular species. Previously, we applied
SPARK to the 1.4 day spectrum of AT2017gfo and inferred its abundance pattern
for the first time, characterized by electron fraction $Y_e=0.31$, a
substantial abundance of strontium, and a dearth of lanthanides and heavier
elements. This ejecta is consistent with wind from a remnant hypermassive
neutron star and/or accretion disk. We now extend our inference to spectra at
2.4 and 3.4 days, and test the need for multicomponent ejecta, where we
stratify the ejecta in composition. The ejecta at 1.4 and 2.4 days is described
by the same single blue component. At 3.4 days, a new redder component with
lower $Y_e=0.16$ and a significant abundance of lanthanides emerges. This new
redder component is consistent with dynamical ejecta and/or neutron-rich ejecta
from a magnetized accretion disk. As expected from photometric modelling, this
component emerges as the ejecta expands, the photosphere recedes, and the
earlier bluer component dims. At 3.4 days, we find an ensemble of lanthanides,
with the presence of cerium most concrete. This presence of lanthanides has
important implications for the contribution of kilonovae to the $r$-process
abundances observed in the Universe. | astro-ph_HE |
Multimessenger signal from phase transition of neutron star to quark
star: Aim: To study the multimessenger nature of the signal that can result from
the phase transition of a neutron star to a quark star and their corresponding
astrophysical observations. Methods: The phase transition process is initiated
by the abrupt pressure and density changes at the star center, giving rise to a
shock which deconfines matter followed by a weak front converting excess down
to strange quarks to attain absolute stability. This process's effects are
investigated by understanding how the energy escapes from the star in the form
of neutrino-antineutrino annihilation. For such annihilation process, the
corresponding energy deposition rate is calculated. Structural changes due to
the energy loss have been investigated in the likes of misalignment angle
evolution of the star and its astrophysical observation through gravitational
waves. Results: The energy and time signature for the neutrino-antineutrino
annihilation is compared with the observed isotropic energy for a short
gamma-ray burst. The misalignment angle evolves to align the star's tilt axis,
which can lead to the sudden increase or decrease of radio intensity from the
pulsar. The corresponding gravitational wave emission, both continuous and
burst, all lead towards multimessenger signals coming from the phase
transition. | astro-ph_HE |
Constraining Progenitors of Observed LMXBs Using CARB Magnetic Braking: We present a new method for constraining the mass transfer evolution of low
mass X-ray binaries (LMXBs) - a reverse population synthesis technique. This is
done using the detailed 1D stellar evolution code MESA (Modules for Experiments
in Stellar Astrophysics) to evolve a high-resolution grid of binary systems
spanning a comprehensive range of initial donor masses and orbital periods. We
use the recently developed Convection And Rotation Boosted (CARB) magnetic
braking scheme. The CARB magnetic braking scheme is the only magnetic braking
prescription capable of reproducing an entire sample of well studied persistent
LMXBs -- those with their mass ratios, periods and mass transfer rates that
have been observationally determined. Using the reverse population synthesis
technique, where we follow any simulated system that successfully reproduces an
observed LMXB backwards, we have constrained possible progenitors for each
observed well-studied persistent LMXB. We also determined that the minimum
number of LMXB formations in the Milky Way is 1500 per Gyr if we exclude Cyg
X-2. For Cyg X-2, the most likely formation rate is 9000 LMXB per Gyr. The
technique we describe can be applied to any observed LMXB with well-constrained
mass ratios, period and mass transfer rate. With the upcoming GAIA DR3
containing information on binary systems, this technique can be applied to the
data release to search for progenitors of observed persistent LMXBs. | astro-ph_HE |
Novel dark matter constraints from antiprotons in the light of AMS-02: We evaluate dark matter (DM) limits from cosmic-ray antiproton observations
using the recent precise AMS-02 measurements. We properly take into account
cosmic-ray propagation uncertainties fitting at the same time DM and
propagation parameters, and marginalizing over the latter. We find a
significant (~4.5 sigma) indication of a DM signal for DM masses near 80 GeV,
with a hadronic annihilation cross-section close to the thermal value, sigma v
~3e-26 cm3s-1. Intriguingly, this signal is compatible with the DM
interpretation of the Galactic center gamma-ray excess. Confirmation of the
signal will require a more accurate study of the systematic uncertainties,
i.e., the antiproton production cross-section, and modelling of the solar
modulation effect. Interpreting the AMS-02 data in terms of upper limits on
hadronic DM annihilation, we obtain strong constraints excluding a thermal
annihilation cross-section for DM masses below about 50 GeV and in the range
between approximately 150 and 500 GeV, even for conservative propagation
scenarios. Except for the range around 80 GeV, our limits are a factor 4
stronger than the limits from gamma-ray observations of dwarf galaxies. | astro-ph_HE |
Cosmic-ray Antinuclei as Messengers of New Physics: Status and Outlook
for the New Decade: The precise measurement of cosmic-ray antinuclei serves as an important means
for identifying the nature of dark matter and other new astrophysical
phenomena, and could be used with other cosmic-ray species to understand
cosmic-ray production and propagation in the Galaxy. For instance, low-energy
antideuterons would provide a "smoking gun" signature of dark matter
annihilation or decay, essentially free of astrophysical background. Studies in
recent years have emphasized that models for cosmic-ray antideuterons must be
considered together with the abundant cosmic antiprotons and any potential
observation of antihelium. Therefore, a second dedicated Antideuteron Workshop
was organized at UCLA in March 2019, bringing together a community of theorists
and experimentalists to review the status of current observations of cosmic-ray
antinuclei, the theoretical work towards understanding these signatures, and
the potential of upcoming measurements to illuminate ongoing controversies.
This review aims to synthesize this recent work and present implications for
the upcoming decade of antinuclei observations and searches. This includes
discussion of a possible dark matter signature in the AMS-02 antiproton
spectrum, the most recent limits from BESS Polar-II on the cosmic antideuteron
flux, and reports of candidate antihelium events by AMS-02; recent collider and
cosmic-ray measurements relevant for antinuclei production models; the state of
cosmic-ray transport models in light of AMS-02 and Voyager data; and the
prospects for upcoming experiments, such as GAPS. This provides a roadmap for
progress on cosmic antinuclei signatures of dark matter in the coming years. | astro-ph_HE |
A Numerical Model for the Multi-wavelength Lightcurves of PSR J0030+0451: Recent modeling of Neutron Star Interior Composition Explorer(NICER)
observations of the millisecond pulsar PSR J0030+0451 suggests that the
magnetic field of the pulsar is non-dipolar. We construct a magnetic field
configuration where foot points of the open field lines closely resemble the
hotspot configuration from NICER observations. Using this magnetic field as
input, we perform force-free simulations of the magnetosphere of PSR
J0030+0451, showing the three-dimensional structure of its plasma-filled
magnetosphere. Making simple and physically motivated assumptions about the
emitting regions, we are able to construct the multi-wavelength lightcurves
that qualitatively agree with the corresponding observations. The agreement
suggests that multipole magnetic structures are the key to modeling this type
of pulsars, and can be used to constrain the magnetic inclination angle and the
location of radio emission. | astro-ph_HE |
Spheroidal force-free neutron star magnetospheres: Abridged. Fast rotating and self-gravitating astrophysical objects suffer
strong deformations from centrifugal forces. If moreover they are magnetized,
they generate an electromagnetic wave that is perturbed accordingly. When
stellar objects are also surrounded by an ideal plasma, a magnetosphere is
formed. We study the electromagnetic configuration of a force-free
magnetosphere encompassing an ideal spheroidal rotating conductor as an inner
boundary. We put special emphasize to millisecond period neutron star
magnetospheres, those showing a significant oblate shape. Force-free solutions
are computed by numerical integration of the time-dependent Maxwell equations
in spheroidal coordinates. Relevant quantities such as the magnetic field
structure, the spin down luminosity, the polar cap rims and the current density
are shown. We find that the force-free magnetic field produced by spheroidal
stars remains very similar to their spherical counterpart. However, the spin
down luminosity slightly decreases with increasing oblateness or prolateness.
Moreover the polar cap area increases and always mostly encompasses the
equivalent spherical star polar cap rims. The polar cap current density is also
drastically affected. | astro-ph_HE |
A study of the accretion mechanisms of the High Mass X-ray Binary IGR
J00370+6122: IGR J00370+6122 is a high-mass X-ray binary, of which the primary is a B1 Ib
star, whereas the companion is suggested to be a neutron star by the detection
of 346-s pulsation in one-off 4-ks observation. To better understand the nature
of the compact companion, the present work performs timing and spectral studies
of the X-ray data of this object, taken with XMM-Newton, Swift, Suzaku, RXTE,
and INTEGRAL. In the XMM-Newton data, a sign of coherent 674 s pulsation was
detected, for which the previous 346-s period may be the 2nd harmonic. The
spectra exhibited the "harder when brighter" trend in the 1$-$10 keV range, and
a flat continuum without clear cutoff in the 10$-$80 keV range. These
properties are both similar to those observed from several low-luminosity
accreting pulsars, including X Persei in particular. Thus, the compact object
in IGR J00370+6122 is considered to be a magnetized neutron star with a rather
low luminosity. The orbital period was refined to $15.6649 \pm 0.0014$ d. Along
the orbit, the luminosity changes by 3 orders of magnitude, involving a sudden
drop from $\sim 4 \times 10^{33}$ to $\sim 1\times10^{32}$ erg s$^{-1}$ at an
orbital phase of 0.3 (and probably vice verse at 0.95). Although these
phenomena cannot be explained by a simple Hoyle-Lyttleton accretion from the
primary's stellar winds, they can be explained when incorporating the propeller
effect with a strong dipole magnetic field of $\sim 5 \times10^{13}$ G.
Therefore, the neutron star in IGR J00370+6122 may have a stronger magnetic
field compared to ordinary X-ray pulsars. | astro-ph_HE |
A Ray-Tracing Algorithm for Spinning Compact Object Spacetimes with
Arbitrary Quadrupole Moments. II. Neutron Stars: A moderately spinning neutron star acquires an oblate shape and a spacetime
with a significant quadrupole moment. These two properties affect its apparent
surface area for an observer at infinity, as well as the lightcurve arising
from a hot spot on its surface. In this paper, we develop a ray-tracing
algorithm to calculate the apparent surface areas of moderately spinning
neutron stars making use of the Hartle-Thorne metric. This analytic metric
allows us to calculate various observables of the neutron star in a way that
depends only on its macroscopic properties and not on the details of its
equation of state. We use this algorithm to calculate the changes in the
apparent surface area, which could play a role in measurements of neutron star
radii and, therefore, in constraining their equation of state. We show that
whether the spinning neutron star appears larger or smaller than its
non-rotating counterpart depends primarily on its equatorial radius. For
neutron stars with radii ~10 km, the corrections to the Schwarzschild spacetime
cause the apparent surface area to increase with spin frequency. In contrast,
for neutron stars with radii ~15 km, the oblateness of the star dominates the
spacetime corrections and causes the apparent surface area to decrease with
increasing spin frequency. In all cases, the change in the apparent geometric
surface area for the range of observed spin frequencies is < 5% and hence only
a small source of error in the measurement of neutron star radii. | astro-ph_HE |
Possible changes of state and relevant timescales for a neutron star in
LS I +61°303: The properties of the short, energetic bursts recently observed from the
gamma-ray binary LS I +61{\deg}303, are typical of those showed by high
magnetic field neutron stars, and thus provide a strong indication in favor of
a neutron star being the compact object in the system. Here, we discuss the
transitions among the states accessible to a neutron star in a system like LS I
+61{\deg}303, such as the ejector, propeller and accretor phases, depending on
the NS spin period, magnetic field and rate of mass captured. We show how the
observed bolometric luminosity (>= few x 1E35 erg/s), and its broad-band
spectral distribution, indicate that the compact object is most probably close
to the transition between working as an ejector all along its orbit, and being
powered by the propeller effect when it is close to the orbit periastron, in a
so-called flip-flop state. By assessing the torques acting onto the compact
object in the various states, we follow the spin evolution of the system,
evaluating the time spent by the system in each of them. Even taking into
account the constraint set by the observed gamma-ray luminosity, we found that
the total age of the system is compatible with being ~5-10 kyr, comparable to
the typical spin-down ages of high-field neutron stars. The results obtained
are discussed in the context of the various evolutionary stages expected for a
neutron star with a high mass companion. | astro-ph_HE |
Measurement of Anisotropy and Search for UHECR Sources: Ultra-high energy cosmic rays (UHECRs) are particles, likely protons and/or
nuclei, with energies up to $10^{20}$ eV that are observed through the giant
air showers they produce in the atmosphere. These particles carry the
information on the most extreme phenomena in the Universe. At these energies,
even charged particles could be magnetically rigid enough to keep track of, or
even point directly to, the original positions of their sources on the sky. The
discovery of anisotropy of UHECRs would thus signify opening of an entirely new
window onto the Universe. With the construction and operation of the new
generation of cosmic ray experiments -- the Pierre Auger Observatory in the
Southern hemisphere and the Telescope Array in the Northern one -- the study of
these particles, the most energetic ever detected, has experienced a jump in
statistics as well as in the data quality, allowing for a much better
sensitivity in searching for their sources. In this review, we summarize the
searches for anisotropies and the efforts to identify the sources of UHECRs
which have been carried out using these new data. | astro-ph_HE |
Antiproton Flux in Cosmic Ray Propagation Models with Anisotropic
Diffusion: Recently a cosmic ray propagation model has been introduced, where
anisotropic diffusion is used as a mechanism to allow for $\mathcal{O}(100)$
km/s galactic winds. This model predicts a reduced antiproton background flux,
suggesting an excess is being observed. We implement this model in GALPROP
v50.1 and perform a $\chi^2$ analysis for B/C, $^{10}$Be/$^{9}$Be, and the
recent PAMELA $\bar{p}/p$ datasets. By introducing a power-index parameter
$\alpha$ that dictates the dependence of the diffusion coefficient $D_{xx}$ on
height $|z|$ away from the galactic plane, we confirm that isotropic diffusion
models with $\alpha=0$ cannot accommodate high velocity convective winds
suggested by ROSAT, while models with $\alpha=1$ ($D_{xx}\propto |z|$) can give
a very good fit. A fit to B/C and $^{10}$Be/$^{9}$Be data predicts a lower
$\bar{p}/p$ flux ratio than the PAMELA measurement at energies between
approximately 2 GeV to 20 GeV. A combined fit including in addition the
$\bar{p}/p$ data is marginal, suggesting only a partial contribution to the
measured antiproton flux. | astro-ph_HE |
On the GeV excess in the diffuse γ-ray emission towards the
Galactic Center: The Fermi-LAT \gamma-ray data have been used to study the morphological and
spectral features of the so-called GeV excess - a diffuse radiation component
recently discovered towards the Galactic centre. We used the likelihood method
to analyze Fermi-LAT data. Our study does confirm the existence of such an
extra component in the diffuse \gamma-ray emission at GeV energies. Based on a
detailed morphological analysis, a spatial template that fits the data best was
generated and adopted. Using this template, the energy distribution of
\gamma-rays was derived in the 0.3-30 GeV energy interval. The spectrum
appeared to have less distinct ('bump'-like) structure than previous reported.
We argue that the morphology of this radiation component has a bipolar rather
than a spherically symmetric structure as has been assumed a priori in previous
studies. This finding excludes the associations of the GeV excess with Dark
Matter. We briefly discuss the radiation mechanisms and possible source
populations that could be responsible for this new component of diffuse gamma
radiation. | astro-ph_HE |
Discovery of a jet-like structure with overionized plasma in the SNR
IC443: IC443 is a supernova remnant located in a quite complex environment since it
interacts with nearby clouds. Indications for the presence of overionized
plasma have been found though the possible physical causes of overionization
are still debated. Moreover, because of its peculiar position and proper
motion, it is not clear if the pulsar wind nebula (PWN) within the remnant is
the relic of the IC443 progenitor star or just a rambling one seen in
projection on the remnant. Here we address the study of IC443 plasma in order
to clarify the relationship PWN-remnant, the presence of overionization and the
origin of the latter. We analyzed two \emph{XMM-Newton} observations producing
background-subtracted, vignetting-corrected and mosaicked images in two
different energy bands and we performed a spatially resolved spectral analysis
of the X-ray emission. We identified an elongated (jet-like) structure with
Mg-rich plasma in overionization. The head of the jet is interacting with a
molecular cloud and the jet is aligned with the position of the PWN at the
instant of the supernova explosion. Interestingly, the direction of the jet of
ejecta is somehow consistent with the direction of the PWN jet. Our discovery
of a jet of ejecta in IC443 enlarge the sample of core-collapse SNRs with
collimated ejecta structures. IC443's jet is the first one which shows
overionized plasma, possibly associated with the adiabatic expansion of ejecta.
The match between the jet's direction and the original position of the PWN
strongly supports the association between the neutron star and IC443. | astro-ph_HE |
Growth of magnetic fields in accreting millisecond pulsars: R-modes can generate strong magnetic fields in the core of accreting
millisecond neutron stars (NSs). The diffusion of these fields outside the core
causes the growth of the external magnetic field and thus it affects the
evolution of the spin down rates $\dot{P}$ of the millisecond pulsars (MSPs).
The diffusion of the internal magnetic field provides a new evolutionary path
for the MSPs. This scenario could explain the large $\dot{P}$ of the pulsars
J1823-3021A and J1824-2452A. | astro-ph_HE |
Fermi LAT Measurements of the Diffuse Gamma-Ray Emission at Intermediate
Galactic Latitudes: The diffuse Galactic gamma-ray emission is produced by cosmic rays (CRs)
interacting with the interstellar gas and radiation field. Measurements by the
Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton
Gamma-Ray Observatory indicated excess gamma-ray emission > 1 GeV relative to
diffuse Galactic gamma-ray emission models consistent with directly measured CR
spectra (the so-called ``EGRET GeV excess''). The excess emission was observed
in all directions on the sky, and a variety of explanations have been proposed,
including beyond-the-Standard-Model scenarios like annihilating or decaying
dark matter. The Large Area Telescope (LAT) instrument on the Fermi Gamma-ray
Space Telescope has measured the diffuse gamma-ray emission with unprecedented
sensitivity and resolution. We report on LAT measurements of the diffuse
gamma-ray emission for energies 100 MeV to 10 GeV and Galactic latitudes 10
deg. <= |b| <= 20 deg. The LAT spectrum for this region of the sky is well
reproduced by the diffuse Galactic gamma-ray emission models mentioned above
and inconsistent with the EGRET GeV excess. | astro-ph_HE |
Joint constraints on the field-cluster mixing fraction, common envelope
efficiency, and globular cluster radii from a population of binary hole
mergers via deep learning: The recent release of the second Gravitational-Wave Transient Catalog
(GWTC-2) has increased significantly the number of known GW events, enabling
unprecedented constraints on formation models of compact binaries. One pressing
question is to understand the fraction of binaries originating from different
formation channels, such as isolated field formation versus dynamical formation
in dense stellar clusters. In this paper, we combine the $\texttt{COSMIC}$
binary population synthesis suite and the $\texttt{CMC}$ code for globular
cluster evolution to create a mixture model for black hole binary formation
under both formation scenarios. For the first time, these code bodies are
combined self-consistently, with $\texttt{CMC}$ itself employing
$\texttt{COSMIC}$ to track stellar evolution. We then use a deep-learning
enhanced hierarchical Bayesian analysis to constrain the mixture fraction $f$
between formation models, while simultaneously constraining the common envelope
efficiency $\alpha$ assumed in $\texttt{COSMIC}$ and the initial cluster virial
radius $r_v$ assumed in $\texttt{CMC}$. Under specific assumptions about other
uncertain aspects of isolated binary and globular cluster evolution, we report
the median and $90\%$ confidence interval of three physical parameters
$(f,\alpha,r_v)=(0.20^{+0.32}_{-0.18},2.26^{+2.65}_{-1.84},2.71^{+0.83}_{-1.17})$.
This simultaneous constraint agrees with observed properties of globular
clusters in the Milky Way and is an important first step in the pathway toward
learning astrophysics of compact binary formation from GW observations. | astro-ph_HE |
Multi-wavelength temporal and spectral study of PKS 0402-362: We study the long-term behavior of the bright gamma-ray blazar PKS 0402-362.
Over a span of approximately 12.5 years, from August 2008 to January 2021, we
gathered Fermi-LAT temporal data and identified three distinct periods of
intense $\gamma$-ray activity. Notably, the second period exhibited the highest
brightness ever observed in this particular source. We observed most of the
$\gamma$-ray flare peaks to be asymmetric in profile suggesting a slow cooling
time of particles or the varying Doppler factor as the main cause of these
flares. The $\gamma$-ray spectrum is fitted with power-law and log-parabola
models, and in both cases, the spectral index is very steep. The lack of time
lags between optical-IR and $\gamma$-ray emissions indicates the presence of a
single-zone emission model. Using this information, we modeled the broadband
SEDs with a simple one-zone leptonic model using the publicly available code
`GAMERA'. The particle distribution index is found to be the same as expected
in diffusive shock acceleration suggesting it as the main mechanism of particle
acceleration to very high energy up to 4 - 6 GeV. During the different flux
phases, we observed that the thermal disk dominates the optical emission,
indicating that this source presents a valuable opportunity to investigate the
connection between the disk and the jet. | astro-ph_HE |
New limits on the local Lorentz invariance violation of gravity in the
Standard-Model Extension with pulsars: Lorentz Violation (LV) is posited as a possible relic effect of quantum
gravity at low energy scales. The Standard-Model Extension provides an
effective field-theoretic framework for examining possible deviations
attributed to LV. With their high observational accuracy, pulsars serve as
ideal laboratories for probing LV. In the presence of LV, both the spin
precession of solitary pulsars and orbital dynamics of binary pulsars would
undergo modifications. Observations of pulse profiles and times of arrival
(TOAs) of pulses allow for an in-depth investigation of these effects, leading
to the establishment of strict limits on LV coefficients. We revisit the
project of limiting local LV with updated pulsar observations. We employ a new
parameter estimation method and utilize state-of-the-art pulsar timing
observation data and get new limits on 8 linear combinations of LV coefficients
based on 25 tests from 12 different systems. Compared to previous limits from
pulsars, precision has improved by a factor of two to three. Additionally, we
explore prospects for further improvements from pulsars. Simulation results
indicate that more observations of spin precession in solitary millisecond
pulsars could significantly enhance the accuracy of spatial LV coefficients,
potentially by three to four orders of magnitude. As observational data
accumulate, pulsars are anticipated to increasingly contribute to the tests of
LV. | astro-ph_HE |
First 100 ms of a long-lived magnetized neutron star formed in a binary
neutron star merger: The recent multimessenger observation of the short gamma-ray burst (SGRB) GRB
170817A together with the gravitational wave (GW) event GW170817 provides
evidence for the long-standing hypothesis associating SGRBs with binary neutron
star (BNS) mergers. The nature of the remnant object powering the SGRB, which
could have been either an accreting black hole (BH) or a long-lived magnetized
neutron star (NS), is, however, still uncertain. General relativistic
magnetohydrodynamic (GRMHD) simulations of the merger process represent a
powerful tool to unravel the jet launching mechanism, but so far most
simulations focused the attention on a BH as the central engine, while the
long-lived NS scenario remains poorly investigated. Here, we explore the latter
by performing a GRMHD BNS merger simulation extending up to ~100 ms after
merger, much longer than any previous simulation of this kind. This allows us
to (i) study the emerging structure and amplification of the magnetic field and
observe a clear saturation at magnetic energy $E_\mathrm{mag} \sim 10^{51}$
erg, (ii) follow the magnetically supported expansion of the outer layers of
the remnant NS and its evolution into an ellipsoidal shape without any
surrounding torus, and (iii) monitor density, magnetization, and velocity along
the axis, observing no signs of jet formation. We also argue that the
conditions at the end of the simulation disfavor later jet formation on
subsecond timescales if no BH is formed. Furthermore, we examine the rotation
profile of the remnant, the conversion of rotational energy associated with
differential rotation, the overall energy budget of the system, and the
evolution of the GW frequency spectrum. Finally, we perform an additional
simulation where we induce the collapse to a BH ~70 ms after merger, in order
to gain insights on the prospects for massive accretion tori in case of a late
collapse. We find that... | astro-ph_HE |
Electron capture of strongly screening nuclides $^{56}$Fe, $^{56}$Co,
$^{56}$Ni , $^{56}$Mn ,$^{56}$Cr and $^{56}$V in presupernova: According to the Shell-Model Monte Carlo method, basing on the Random Phase
Approximation and the linear response theory, we carried out an estimation on
electron capture of strongly screening nuclides $^{56}$Fe, $^{56}$Co, $^{56}$Ni
, $^{56}$Mn ,$^{56}$Cr and $^{56}$V in strong electron screening (SES)in
presupernova. The
EC rates are decreased greatly and even exceed $21.5\%$ in SES. We also
compare our results with those of Aufderheide (AFUD), which calculated by the
method of Aufderheide in SES. Our results are agreed reasonably well with AUFD
at higher density-temperature surroundings (e.g. $\rho_7>60, T_9=15.40$) and
the maximum error is $\sim $0.5$\%$. However, the maximum error is $\sim
$13.0$\%$ at lower density surroundings (e.g. $^{56}$Cr at $\rho_7=10,
T_9=15.40, Y_e=0.41$ ). On the other hand, we also compared our results in SES
with those of FFN's and Nabi's, which is in the case without SES. The
comparisons show that our results are lower more than one order magnitude than
FFN's, but about $7.23\%$ than Nabi's. | astro-ph_HE |
Disentangling the Air Shower Components Using Scintillation and Water
Cherenkov Detectors: We consider a ground array of scintillation and water Cherenkov detectors
with the purpose of determining the muon content of air showers. The different
response characteristics of these two types of detectors to the components of
the air shower provide a way to infer their relative contributions. We use a
detailed simulation to estimate the impact of parameters, such as scintillation
detector size, in the determination of the size of the muon component. | astro-ph_HE |
Mass Distribution and Maximum Mass of Neutron Stars: Effects of Orbital
Inclination Angle: Matter at ultra-high densities finds a physical realization inside neutron
stars. One key property is their maximum mass, which has far-reaching
implications for astrophysics and the equation of state of ultra dense matter.
In this work, we employ Bayesian analysis to scrutinize the mass distribution
and maximum mass threshold of galactic neutron stars. We compare two distinct
models to assess the impact of assuming a uniform distribution for the most
important quantity, the cosine of orbital inclination angles ($i$), which has
been a common practice in previous analyses. This prevailing assumption yields
a maximum mass of $2.25$~$M_\odot$ (2.15--3.32~$M_\odot$ within $90\%$
confidence), with a strong peak around the maximum value. However, in the
second model, which indirectly includes observational constraints of $i$, the
analysis supports a mass limit of $2.56^{+0.87}_{-0.58}~M_\odot$ ($2\sigma$
uncertainty), a result that points in the same direction as some recent results
gathered from gravitational wave observations, although their statistics are
still limited. This work stresses the importance of an accurate treatment of
orbital inclination angles, and contributes to the ongoing debate about the
maximum neutron star mass, further emphasizing the critical role of
uncertainties in the individual neutron star mass determinations. | astro-ph_HE |
Interacting Kilonovae: Long-lasting Electromagnetic Counterparts to
Binary Mergers in the Accretion Disks of Active Galactic Nuclei: We investigate the dynamics and electromagnetic (EM) signatures of neutron
star-neutron star (NS-NS) or neutron star-black hole (NS-BH) merger ejecta that
occurs in the accretion disk of an active galactic nucleus (AGN). We find that
the interaction between ejecta and disk gas leads to important effects on the
dynamics and radiation. We show five stages of the ejecta dynamics:
gravitational slowing down, coasting, Sedov-Taylor deceleration in the disk,
re-acceleration after the breakout from the disk surface, and
momentum-conserved snowplow phase. Meanwhile, the radiation from the ejecta is
so bright that its typical peak luminosity reaches a few times
$10^{43}-10^{44}~\rm erg~s^{-1}$. Since most of the radiation energy has
converted from the kinetic energy of merger ejecta, we call such an explosive
phenomenon an interacting kilonova (IKN). It should be emphasized that IKNe are
very promising, bright EM counterparts to NS-NS/BH-NS merger events in AGN
disks. The bright peak luminosity and long rising time (i.e., ten to twenty
days in UV bands, thirty to fifty days in optical bands, and one hundred days
to hundreds of days in IR bands) allow most survey telescopes to have ample
time to detect an IKN. However, the peak brightness, peak time, and evolution
pattern of the light curve of an IKN are similar to a superluminous supernova
in a galactic nucleus and a tidal disruption event making it difficult to
distinguish between them. But it also suggests that IKNe might have been
present in recorded AGN transients. | astro-ph_HE |
The temperature of hot gas halos of early-type galaxies: Recently, the temperature T and luminosity L_X of the hot gas halos of early
type galaxies have been derived with unprecedented accuracy from Chandra data,
for 30 galaxies covering a wider range of galactic luminosity (and central
velocity dispersion sigma_c) than before. This work investigates the origin of
the observed temperatures, by examining the relationship between them and the
galaxy structure, the gas heating due to Type Ia supernovae (SNIa's) and the
gravitational potential, and the dynamical status of the gas flow. In galaxies
with sigma_c<200 km/s, the T's are close to a fiducial average temperature for
the gas when in outflow; at 200<sigma_c (km/s)<250, the T's are generally lower
than this, and unrelated with sigma_c, which requires a more complex gas flow
status; at larger sigma_c, the T's may increase as sigma_c^2, as expected for
infall heating, though heating from SNIa's, independent of sigma_c, should be
dominant. All observed T's are larger than the virial temperature, by up to
~0.5 keV. This additional heating can be provided in the X-ray brightest
galaxies by SNIa's and infall heating, with a SNIa's energy input even lower
than in standard assumptions; in the X-ray fainter ones it can be provided by
SNIa's, whose energy input would be required close to the full standard value
at the largest sigma_c. This same energy input, though, would produce
temperatures larger than observed at low sigma_c, if entirely thermalized. The
values of the observed T's increase from outflows to inflows; the gas is
relatively hotter in outflows, though, if the T's are rescaled by the virial
temperature. For 200<sigma_c(km/s)<250, lower L_X values tend to correspond to
lower T's, which deserves further investigation. | astro-ph_HE |
Probing the dipole of the diffuse gamma-ray background: We measured the dipole of the diffuse $\gamma$-ray background (DGB)
identifying a highly significant time-independent signal coincidental with that
of the Pierre Auger UHECR. The DGB dipole is determined from flux maps in
narrow energy bands constructed from 13 years of observations by the Large Area
Telescope (LAT) of the {\it Fermi} satellite. The $\gamma$-ray maps were
clipped iteratively of sources and foregrounds similar to that done for the
cosmic infrared background. The clipped narrow energy band maps were then
assembled into one broad energy map out to the given energy starting at
$E=2.74$ Gev, where the LAT beam falls below the sky's pixel resolution. Next
we consider cuts in Galactic latitude and longitude to probe residual
foreground contaminations from the Galactic Plane and Center. In the broad
energy range $2.74 < E\leq115.5$ GeV the measured dipoles are stable with
respect to the various Galactic cuts, consistent with an extragalactic origin.
The $\gamma$-ray sky's dipole/monopole ratio is much greater than that expected
from the DGB clustering component and the Compton-Getting effect origin with
reasonable velocities. At $\simeq (6.5-7)\%$ it is similar to the Pierre Auger
UHECRs with $E_{\rm UHECR}\ge 8$ EeV pointing to a common origin of the two
dipoles. However, the DGB flux associated with the found DGB dipole reaches
parity with that of the UHECR around $E_{\rm UHECR}\le 1$ EeV, perhaps arguing
for a non-cascading mechanism if the DGB dipole were to come from the higher
energy UHECRs. The signal/noise of the DGB dipole is largest in the $5-30$ GeV
range, possibly suggesting the $\gamma$-photons at these energies are the ones
related to cosmic rays. | astro-ph_HE |
About the measurements of the hard X-ray background: We analyze uncertainties in the cosmic X-ray background measurements
performed by the INTEGRAL observatory. We find that the most important effect
limiting the accuracy of the measurements is related to the intrinsic
background variation in detectors. Taking into account all of the uncertainties
arising during the measurements we conclude that the X-ray background intensity
obtained in the INTEGRAL observations is compatible with the historic X-ray
background observations performed by the HEAO-1 satellite. | astro-ph_HE |
Compact Symmetric Objects -- III Evolution of the High-Luminosity Branch
and a Possible Connection with Tidal Disruption Events: We use a sample of 54 Compact Symmetric Objects (CSOs) to confirm that there
are two unrelated CSO classes: an edge-dimmed, low-luminosity class (CSO~1),
and an edge-brightened, high-luminosity class (CSO~2). Using blind tests, we
show that CSO~2s consist of three sub-classes: CSO 2.0, having prominent
hot-spots at the leading edges of narrow jets and/or narrow lobes; CSO~2.2,
without prominent hot-spots, and with broad jets and/or lobes; and CSO~2.1,
which exhibit mixed properties. Most CSO 2s do not evolve into larger
jetted-AGN, but spend their whole life-cycle as CSOs of size $\lesssim$500 pc
and age $\lesssim$5000 yr. The minimum energies needed to produce the radio
luminosity and structure in CSO~2s range from $\sim~10^{-4}\,M_\odot{c}^2$ to
$\sim7\,M_\odot{c}^2$. We show that the transient nature of most CSO~2s, and
their birthrate, can be explained through ignition in the tidal disruption
events of giant stars. We also consider possibilities of tapping the spin
energy of the supermassive black hole, and tapping the energy of the accretion
disk. Our results demonstrate that CSOs constitute a large family of AGN in
which we have thus far studied only the brightest. More comprehensive CSO
studies, with higher sensitivity, resolution, and dynamic range, will
revolutionize our understanding of AGN and the central engines that power them. | astro-ph_HE |
Resonance suppression of the r-mode instability in superfluid neutron
stars: Accounting for muons and entrainment: We calculate the finite-temperature r-mode spectrum of a superfluid neutron
star accounting for both muons in the core and the entrainment between neutrons
and protons. We show that the standard perturbation scheme, considering the
rotation rate as an expansion parameter, breaks down in this case. We develop
an original perturbation scheme which circumvents this problem by treating both
the perturbations due to rotation and (weak) entrainment simultaneously.
Applying this scheme, we propose a simple method for calculating the superfluid
r-mode eigenfrequency in the limit of vanishing rotation rate. We also
calculate the r-mode spectrum at finite rotation rate for realistic
microphysics input (adopting, however, the Newtonian framework and Cowling
approximation when considering perturbed oscillation equations) and show that
the normal r-mode exhibits resonances with superfluid r-modes at certain values
of temperatures and rotation frequencies in the parameter range relevant to
neutron stars in low-mass X-ray binaries (LMXBs). This turns the recently
suggested phenomenological model of resonance r-mode stabilization into a
quantitative theory, capable of explaining observations. A strong dependence of
resonance rotation rates and temperatures on the neutron superfluidity model
allows us to constrain the latter by confronting our calculations with the
observations of neutron stars in LMXBs. | astro-ph_HE |
Statistical studies of supernova environments: Investigations of the environments of SNe allow statistical constraints to be
made on progenitor properties. We review progress that has been made in this
field. Pixel statistics using tracers of e.g. star formation within galaxies
show differences in the explosion sites of, in particular SNe types II and Ibc
(SNe II and SNe Ibc), suggesting differences in population ages. Of particular
interest is that SNe Ic are significantly more associated with H-alpha emission
than SNe Ib, implying shorter lifetimes for the former. In addition, such
studies have shown that the interacting SNe IIn do not explode in regions
containing the most massive stars, which suggests that at least a significant
fraction of their progenitors arise from the lower end of the core-collapse SN
mass range. Host HII region spectroscopy has been obtained for a significant
number of core-collapse events, however definitive conclusions have to-date
been elusive. Single stellar evolution models predict that the fraction of SNe
Ibc to SNe II should increase with increasing metallicity, due to the
dependence of mass-loss rates on progenitor metallicity. We present a
meta-analysis of host HII region oxygen abundances for CC SNe. It is concluded
that the SN II to SN Ibc ratio shows little variation with oxygen abundance,
with only a suggestion that the ratio increases in the lowest bin. Radial
distributions of different SNe are discussed, where a central excess of SNe Ibc
has been observed within disturbed galaxy systems, which is difficult to
ascribe to metallicity or selection effects. Environment studies are evolving
to enable studies at higher spatial resolutions than previously possible, while
in addition the advent of wide-field integral field unit instruments allows
galaxy-wide spectral analyses which will provide fruitful results to this
field. Some example contemporary results are shown in that direction. | astro-ph_HE |
The optical and near-infrared spectrum of the Crab pulsar with X-shooter: Pulsars are well studied all over the electromagnetic spectrum, and the Crab
pulsar may be the most studied object in the sky. Nevertheless, a high-quality
optical to near-infrared spectrum of the Crab or any other pulsar has not been
published to date. Obtaining a properly flux-calibrated spectrum enables us to
measure the spectral index of the pulsar emission, without many of the caveats
from previous studies. This was the main aim of this project, but we could also
detect absorption and emission features from the pulsar and nebula over an
unprecedentedly wide wavelength range. A spectrum was obtained with the
X-shooter spectrograph on the Very Large Telescope. Particular care was given
to the flux-calibration of these data. A high signal-to-noise spectrum of the
Crab pulsar was obtained from 300 to 2400nm. The spectral index fitted to this
spectrum is flat with alpha_nu=0.16 +- 0.07. For the emission lines we measure
a maximum velocity of 1600 km/s, whereas the absorption lines from the material
between us and the pulsar is unresolved at the 50 km/s resolution. A number of
Diffuse Interstellar Bands and a few near-IR emission lines that have
previously not been reported from the Crab are highlighted. | astro-ph_HE |
Multiwavelength temporal and spectral study of TeV blazar 1ES 1727+502
during 2014 to 2021: One of the most important questions in blazar physics is the origin of
broadband emission and fast-flux variation. In this work, we studied the
broadband temporal and spectral properties of a TeV blazar 1ES 1727+502 and
explore the one-zone synchrotron-self Compton (SSC) model to fit the broadband
spectral energy distribution (SED). We collected the long-term (2014-2021)
multiband data which includes both the low and high flux states of the source.
The entire light curve is divided into three segments of different flux states
and the best-fit parameters obtained by broadband SED modeling corresponding to
three flux states were then compared. The TeV blazar 1ES 1727+502 has been
observed to show the brightest flaring episode in X-ray followed by optical-UV
and gamma-ray. The fractional variability estimated during various segments
behaves differently in multiple wavebands, suggesting a complex nature of
emission in this source. This source has shown a range of variability time from
days scale to month scale during this long period of observations between
2014-2021. A "harder-when-brighter" trend is not prominent in X-ray but seen in
optical-UV and an opposite trend is observed in gamma-ray. The complex nature
of correlation among various bands is observed. The SED modeling suggests that
the one-zone SSC emission model can reproduce the broadband spectrum in the
energy range from optical-UV to very high energy gamma-ray. | astro-ph_HE |
Search for extended gamma ray emission in Markarian 421 using VERITAS
observations: Very high energy (VHE: >100 GeV) gamma rays coming from AGN can pair-produce
on the intergalactic background light generating an electromagnetic cascade. If
the Intergalactic Magnetic Field (IGMF) is sufficiently strong, this cascade
may result in an extended isotropic emission of photons around the source, or
halo. Using VERITAS observations of the blazar Markarian 421, we search for
extended emission by comparing the source angular distribution (${\theta}^2$)
from a quiescent period with one coming from a flare period, which can be
considered as halo-free. ${\chi}^2$ test showed no significant statistical
differences between the samples, suggesting that the effect is either
non-existent or too weak to be detected. We calculated upper limits for the
extended flux considering different angle ranges, the most stringent being <8%
of the Crab Nebulae flux (C.U), in the range $0\deg \leq {\theta} \leq 0.1\deg$ . | astro-ph_HE |
Enforcing causality in nonrelativistic equations of state at finite
temperature: We present a thermodynamically consistent method by which equations of state
based on nonrelativistic potential models can be modified so that they respect
causality at high densities, both at zero and finite temperature (entropy). We
illustrate the application of the method using the high density phase
parametrization of the well known APR model in its pure neutron matter
configuration as an example. We also show that, for models with only contact
interactions, the adiabatic speed of sound is independent of the temperature in
the limit of very large temperature. This feature is approximately valid for
models with finite-range interactions as well, insofar as the temperature
dependence they introduce to the Landau effective mass is weak. In addition,
our study reveals that in first principle nonrelativistic models of hot and
dense matter, contributions from higher than two-body interactions must be
screened at high density to preserve causality. | astro-ph_HE |
Low-Mass X-ray Binaries in Globular Clusters: Puzzles and Solutions: In dense stellar systems, dynamical interactions between objects lead to
frequent formation of exotic stellar objects, unusual binaries, and systems of
higher multiplicity. They are especially important for the formation of low
mass X-ray binaries (LMXBs), which are not only formed 100 times more
efficiently than in the field, but also have a puzzling dependence on
metallicity. In this contribution we review how compact objects are formed and
retained, the mechanisms of dynamical formation and the specifics of the
evolution of mass-transferring binaries with neutron stars and black holes in
globular clusters - those two kinds of compact objects have different favored
paths to become luminous in X-ray. We describe how stellar evolution affects
ostensibly purely dynamical formation, producing the observed metallicity
dependence for LMXBs. We also discuss the next puzzle to be solved on our
journey to understand the link between LMXBs and millisecond pulsars formation. | astro-ph_HE |
Mode switching and oscillations in PSR B1828-11: The young pulsar PSR B1828-11 has long been known to show correlated shape
and spin-down changes with timescales of roughly 500 and 250 days, perhaps
associated with large-scale magnetospheric switching. Here we present
multi-hour observations with the Parkes and Green Bank Telescopes at multiple
phases across the roughly 500-day cycle and show that the pulsar undergoes
mode-changing between two stable, extreme profile states. The fraction of time
spent in each profile state naturally accounts for the observed overall "shape
parameter" (defined to be 0 for wide profiles and 1 for narrow ones); this and
the variable rate of the mode transitions are directly related to the spin-down
changes. We observe that the mode transition rate could plausibly function as
an additional parameter governing the chaotic behaviour in this object which
was proposed earlier by Seymour and Lorimer. Free precession is not needed to
account for the variations. | astro-ph_HE |
Multi-frequency VLBI Observations of the M 84 Inner Jet/Counterjet: Observational studies of inner-most regions of the edge-on jets in nearby
active galactic nuclei (AGN) are crucial to understand their kinematics and
morphology. For the inner jet of the nearby low luminosity AGN in M 84, we
present new high-sensitivity observations with very long baseline
interferometry since 2019, as well as archival Very Long Baseline Array
observations in 2014. We find that the compact core in M 84 has an
inverted-to-flat spectrum from 1.5 to 88 GHz. Based on the turnover frequency
of $4.2\pm 0.2$ GHz in the spectrum, we estimated a magnetic field strength of
1-10mG and an electron number density of $\sim 10^5 cm^{-3}$ in the core
region. Three inner jet components within $\sim 3$ mas from the core are
identified and traced in the images at 22 GHz, whose apparent speeds are 0.11
c, 0.27 c, and 0.32 c, respectively. We calculate the viewing angle of $\sim58$
degree for the inner jet based on the proper motion and the flux ratio of
jet-to-counterjet. A propagating sinusoidal model with a wavelength of $\sim
3.4$ mas is used to fit the helical morphology of the jet extended to 20 mas
($\sim 2.2\times 10^4$ Schwarzschild Radii). | astro-ph_HE |
Fermi-LAT detection of G118.4+37.0: a supernova remnant in the Galactic
halo seen around the Calvera pulsar: The discovery of a non-thermal radio ring of low surface brightness about one
degree in diameter has been recently reported around the location in the sky of
the Calvera pulsar, at a high Galactic latitude. The radio properties point to
it likely being a new supernova remnant (SNR), G118.4+37.0. We report an
analysis of almost 14 years of observations of this region by the gamma-ray
Large Area Telescope onboard the Fermi satellite. We detect extended GeV
emission consistent with the size and location of the radio source, which
confirms the presence of relativistic particles. The spectrum of the
high-energy emission is fully compatible with an origin in the same
relativistic particles producing the radio emission. These features and its
similarities to other isolated SNRs establish this source as the remnant of a
supernova. A simple model of the non-thermal emission from radio to GeV
energies resulting from leptonic emission from electrons produced by the SNR is
presented. G118.4+37.0 and other similar isolated remnants could be part of a
radio-dim SNR population evolving in low density environments showing hard GeV
emission of leptonic origin. Future deeper surveys in radio and gamma-rays
could discover new members of the group. | astro-ph_HE |
Hydrodynamics of Core-Collapse Supernovae at the Transition to
Explosion. I. Spherical Symmetry: We study the transition to runaway expansion of an initially stalled
core-collapse supernova shock. The neutrino luminosity, mass accretion rate,
and neutrinospheric radius are all treated as free parameters. In spherical
symmetry, this transition is mediated by a global non-adiabatic instability
that develops on the advection time and reaches non-linear amplitude. Here we
perform high-resolution, time-dependent hydrodynamic simulations of stalled
supernova shocks with realistic microphysics to analyze this transition. We
find that radial instability is a sufficient condition for runaway expansion if
the neutrinospheric parameters do not vary with time and if heating by the
accretion luminosity is neglected. For a given unstable mode, transition to
runaway occurs when fluid in the gain region reaches positive specific energy.
We find approximate instability criteria that accurately describe the behavior
of the system over a wide region of parameter space. The threshold neutrino
luminosities are in general different than the limiting value for a
steady-state solution. We hypothesize that multidimensional explosions arise
from the excitation of unstable large-scale modes of the turbulent background
flow, at threshold luminosities that are lower than in the laminar case. | astro-ph_HE |
A NICER look at the state transitions of the black hole candidate MAXI
J1535-571 during its reflares: The black hole candidate and X-ray binary MAXI J1535-571 was discovered in
September 2017. During the decay of its discovery outburst, and before
returning to quiescence, the source underwent at least four reflaring events,
with peak luminosities of $\sim$10$^{35-36}$ erg s$^{-1}$ (d/4.1 kpc)$^2$. To
investigate the nature of these flares, we analysed a sample of NICER
observations taken with almost daily cadence. In this work we present the
detailed spectral and timing analysis of the evolution of the four reflares.
The higher sensitivity of NICER at lower energies, in comparison with other
X-ray detectors, allowed us to constrain the disc component of the spectrum at
$\sim$0.5 keV. We found that during each reflare the source appears to trace
out a q-shaped track in the hardness-intensity diagram similar to those
observed in black hole binaries during full outbursts. MAXI J1535-571 transits
between the hard state (valleys) and softer states (peaks) during these flares.
Moreover, the Comptonised component is undetected at the peak of the first
reflare, while the disc component is undetected during the valleys. Assuming
the most likely distance of 4.1 kpc, we find that the hard-to-soft transitions
take place at the lowest luminosities ever observed in a black hole transient,
while the soft-to-hard transitions occur at some of the lowest luminosities
ever reported for such systems. | astro-ph_HE |
Inclination Dependence of The Time-Lag -- Photon-Index Correlation in
BHXRBs and its Explanation with a Simple Jet Model: Recently, we reported an observational correlation between a) the time-lag of
the hard (9 - 15 keV) with respect to the soft (2 - 5 keV) X-ray photons in
black-hole X-ray binaries (BHXRBs) and b) the power-law photon index $\Gamma$
of the X-ray spectrum. This was physically explained with a simple jet model,
i.e., a model where the Comptonization (the Compton upscattering of soft
photons) happens in the jet. Here, we report the inclination dependence of this
correlation, which we also explain with our jet model. Photons that emerge at
different polar angles from the jet axis have different spectra and different
time-lags. Because of this, we can explain quantitatively the type-B QPOs of GX
339-4 as resulting from a precessing jet. | astro-ph_HE |
Superluminal Waves and the Structure of Pulsar Wind Termination Shocks: The termination shock of a pulsar wind is located roughly where the ram
pressure matches that of the surrounding medium. Downstream of the shock, MHD
models of the diffuse nebular emission suggest the plasma is weakly magnetized.
However, the transition from a Poynting-dominated MHD wind to a
particle-dominated flow is not well understood. We discuss a solution of this
"sigma problem" in which a striped wind converts into a strong, superluminal
electromagnetic wave. This mode slows down as it propagates radially, and its
ram pressure tends to a constant value at large radius, a property we use to
match the solution to the surrounding nebula. The wave thus forms a pre-cursor
to the termination shock, which occurs at the point where the wave dissipates.
Possible damping and dissipation mechanisms are discussed qualitatively. | astro-ph_HE |
Impact of Rastall gravity on mass, radius and sound speed of the pulsar
PSR J0740+6620: Millisecond pulsars are perfect laboratories to test possible matter-geometry
coupling and its physical implications in light of recent Neutron Star Interior
Composition Explorer (NICER) observations. We apply Rastall field equations of
gravity, where matter and geometry are nonminimally coupled, to Krori-Barua
interior spacetime whereas the matter source is assumed to be anisotropic
fluid. We show that all physical quantities inside the star can be expressed in
terms of Rastall, $\epsilon$, and compactness, $C=2GM/Rc^2$, parameters. Using
NICER and X-ray Multi-Mirror X-ray observational constraints on the mass and
radius of the pulsar PSR J0740+6620 we determine Rastall parameter to be at
most $\epsilon=0.041$ in the positive range. The obtained solution provides a
stable compact object; in addition the squared sound speed does not violate the
conjectured sound speed $c_s^2\leq c^2/3$ unlike the general relativistic
treatment. We note that no equations of state are assumed; the model however
fits well with linear patterns with bag constants. In general, for
$\epsilon>0$, the theory predicts a slightly larger size star in comparison to
general relativity for the same mass. This has been explained as an additional
force, due to matter-geometry coupling, in the hydrodynamic equilibrium
equation, which contributes to partially diminish the gravitational force
effect. Consequently, we calculate the maximal compactness as allowed by the
strong energy condition to be $C = 0.735$ which is $\sim 2\%$ higher than
general relativity prediction. Moreover, for the surface density at saturation
nuclear density $\rho_{\text{nuc}} = 2.7\times 10^{14}$ g/cm$^3$ we estimate
the maximum mass $M=4 M_\odot$ at radius $R=16$ km. | astro-ph_HE |
Identification of an X-ray Pulsar in the BeXRB system IGR J18219$-$1347: We report on observations of the candidate Be/X-ray binary IGR J18219$-$1347
with \textit{Swift}/XRT, \textit{NuSTAR}, and \textit{NICER} during Type-I
outbursts in March and June 2020. Our timing analysis revealed the spin period
of a neutron star with $P_\textrm{spin}=52.46$ s. This periodicity, combined
with the known orbital period of $72.4$ d, indicates that the system is a
BeXRB. Furthermore, by comparing the infrared counterpart's spectral energy
distribution to known BeXRBs, we confirm this classification and set a distance
of approximately $10-15$ kpc for the source. The source's broadband X-ray
spectrum ($1.5-50$ keV) is described by an absorbed power-law with photon index
$\Gamma$\,$\sim$\,$0.5$ and cutoff energy at $\sim$\,$13$ keV. | astro-ph_HE |
A real-time transient detector and the Living Swift-XRT Point Source
catalogue: We present the Living Swift-XRT Point Source catalogue (LSXPS) and real-time
transient detector. This system allows us for the first time to carry out
low-latency searches for new transient X-ray events fainter than those
available to the current generation of wide-field imagers, and report their
detection in near real-time. Previously, such events could only be found in
delayed searches, e.g. of archival data; our low-latency analysis now enables
rapid and ongoing follow up of these events, enabling the probing of timescales
previously inaccessible. The LSXPS is, uniquely among X-ray catalogues, updated
in near real-time, making this the first up-to-date record of the point sources
detected by a sensitive X-ray telescope: the Swift-X-ray Telescope (XRT). The
associated upper limit calculator likewise makes use of all available data
allowing contemporary upper limits to be rapidly produced on-demand. These
facilities, which enable the low-latency transient system are also fully
available to the community, providing a powerful resource for time-domain and
multi-messenger astrophysics. | astro-ph_HE |
SN 2020udy: a SN Iax with strict limits on interaction consistent with a
helium-star companion: Early observations of transient explosions can provide vital clues to their
progenitor origins. In this paper we present the nearby Type Iax (02cx-like)
supernova (SN), SN 2020udy that was discovered within hours ($\sim$7 hr) of
estimated first light. An extensive dataset of ultra-violet, optical, and
near-infrared observations was obtained, covering out to $\sim$150 d after
explosion. SN 2020udy peaked at -17.86$\pm$0.43 mag in the r band and evolved
similarly to other 'luminous' SNe Iax, such as SNe 2005hk and 2012Z. Its
well-sampled early light curve allows strict limits on companion interaction to
be placed. Main-sequence companion stars with masses of 2 and 6 M$_\odot$ are
ruled out at all viewing angles, while a helium-star companion is allowed from
a narrow range of angles (140-180$^\circ$ away from the companion). The spectra
and light curves of SN2020udy are in good agreement with those of the 'N5def'
deflagration model of a near Chandrasekhar-mass carbon-oxygen white dwarf.
However, as has been seen in previous studies of similar luminosity events, SN
2020udy evolves slower than the model. Broad-band linear polarisation
measurements taken at and after peak are consistent with no polarisation, in
agreement with the predictions of the companion-star configuration from the
early light curve measurements. The host galaxy environment is low metallicity
and is consistent with a young stellar population. Overall, we find the most
plausible explosion scenario to be the incomplete disruption of a CO white
dwarf near the Chandrasekhar-mass limit, with a helium-star companion. | astro-ph_HE |
On the morphology of outbursts of accreting millisecond X-ray pulsar
Aquila X-1: We present the X-ray light curves of the last two outbursts --2014 & 2016--
of the well known accreting millisecond X-ray pulsar (AMXP) Aquila X-1 using
the monitor of all sky X-ray image (MAXI) observations in the $2-20$ keV band.
After calibrating the (MAXI) count rates to the all-sky monitor (ASM) level, we
report that the 2016 outburst is the most energetic event of Aql X-1, ever
observed from this source. We show that 2016 outburst is a member of the
long-high class according to the classification presented by G\"ung\"or et al.
with $\sim68$ cnt/s maximum flux and $\sim60$ days duration time and the
previous outburst, 2014, belongs to the short-low class with $\sim 25$ cnt/s
maximum flux and $\sim 30$ days duration time. In order to understand
differences between outbursts, we investigate the possible dependence of the
peak intensity to the quiescent duration leading to the outburst and find that
the outbursts following longer quiescent episodes tend to reach higher peak
energetic. | astro-ph_HE |
The 450 days X-ray monitoring of the changing-look AGN 1ES 1927+654: 1ES 1927+654 is a nearby active galactic nucleus (AGN) which underwent a
changing-look event in early 2018, developing prominent broad Balmer lines
which were absent in previous observations. We have followed up this object in
the X-rays with an ongoing campaign that started in May 2018, and that includes
265 NICER (for a total of 678ks) and 14 Swift/XRT (26ks) observations, as well
as three simultaneous XMM-Newton/NuSTAR (158/169 ks) exposures. In the X-rays,
1ES 1927+654 shows a behaviour unlike any previously known AGN. The source is
extremely variable both in spectral shape and flux, and does not show any
correlation between X-ray and UV flux on timescales of hours or weeks/months.
After the outburst the power-law component almost completely disappeared, and
the source showed an extremely soft continuum dominated by a blackbody
component. The temperature of the blackbody increases with the luminosity,
going from $kT\sim 80$eV (for a 0.3--2keV luminosity of $L_{0.3-2}\sim
10^{41.5}\rm\,erg\,s^{-1}$) to $\sim 200$eV (for $L_{0.3-2}\sim
10^{44}\rm\,erg\,s^{-1}$). The spectra show evidence of ionized outflows, and
of a prominent feature at $\sim 1$keV, which can be reproduced by a broad
emission line. The unique characteristics of 1ES 1927+654 in the X-ray band
suggest that it belongs to a new type of changing-look AGN. Future X-ray
surveys might detect several more objects with similar properties. | astro-ph_HE |
Modelling the Type Ic SN 2004aw: a Moderately Energetic Explosion of a
Massive C+O Star without a GRB: An analysis of the Type Ic supernova (SN) 2004aw is performed by means of
models of the photospheric and nebular spectra and of the bolometric light
curve. SN2004aw is shown not to be ``broad-lined'', contrary to previous
claims, but rather a ``fast-lined'' SN Ic. The spectral resemblance to the
narrow-lined Type Ic SN1994I, combined with the strong nebular [O I] emission
and the broad light curve, point to a moderately energetic explosion of a
massive C+O star. The ejected 56Ni mass is ~0.2 Msun. The ejecta mass as
constrained by the models is ~3-5 Msun, while the kinetic energy is estimated
as KE ~3-6 e51 ergs. The ratio KE/Mej, the specific energy which influences the
shape of the spectrum, is therefore ~1. The corresponding zero-age
main-sequence mass of the progenitor star may have been ~23-28 Msun. Tests show
that a flatter outer density structure may have caused a broad-lined spectrum
at epochs before those observed without affecting the later epochs when data
are available, implying that our estimate of KE is a lower limit. SN2004aw may
have been powered by either a collapsar or a magnetar, both of which have been
proposed for gamma-ray burst-supernovae. Evidence for this is seen in the
innermost layers, which appear to be highly aspherical as suggested by the
nebular line profiles. However, any engine was not extremely powerful, as the
outer ejecta are more consistent with a spherical explosion and no gamma-ray
burst was detected in coincidence with SN2004aw. | astro-ph_HE |
A ring-like accretion structure in M87 connecting its black hole and jet: The nearby radio galaxy M87 is a prime target for studying black hole
accretion and jet formation^{1,2}. Event Horizon Telescope observations of M87
in 2017, at a wavelength of 1.3 mm, revealed a ring-like structure, which was
interpreted as gravitationally lensed emission around a central black hole^3.
Here we report images of M87 obtained in 2018, at a wavelength of 3.5 mm,
showing that the compact radio core is spatially resolved. High-resolution
imaging shows a ring-like structure of 8.4_{-1.1}^{+0.5} Schwarzschild radii in
diameter, approximately 50% larger than that seen at 1.3 mm. The outer edge at
3.5 mm is also larger than that at 1.3 mm. This larger and thicker ring
indicates a substantial contribution from the accretion flow with absorption
effects in addition to the gravitationally lensed ring-like emission. The
images show that the edge-brightened jet connects to the accretion flow of the
black hole. Close to the black hole, the emission profile of the jet-launching
region is wider than the expected profile of a black-hole-driven jet,
suggesting the possible presence of a wind associated with the accretion flow. | astro-ph_HE |
Predicting the X-ray polarization of type-2 Seyfert galaxies: Infrared, optical and ultraviolet spectropolarimetric observations have
proven to be ideal tools for the study of the hidden nuclei of type-2 active
galactic nuclei (AGN) and for constraining the composition and morphology of
the sub-parsec scale emission components. In this paper, we extend the analysis
to the polarization of the X-rays from type-2 AGN. Combining two radiative
transfer codes, we performed the first simulations of photons originating in
the gravity dominated vicinity of the black hole and scattering in structures
all the way out to the parsec-scale torus and polar winds. We demonstrate that,
when strong gravity effects are accounted for, the X-ray polarimetric signal of
Seyfert-2s carries as much information about the central AGN components as
spectropolarimetric observations of Seyfert-1s. The spectropolarimetric
measurements can constrain the spin of the central supermassive black hole even
in edge-on AGN, the hydrogen column density along the observer's line-of-sight,
and the composition of the polar outflows. However, the polarization state of
the continuum source is washed out by multiple scattering, and should not be
measurable unless the initial polarization is exceptionally strong. Finally, we
estimate that modern X-ray polarimeters, either based on the photo-electric
effect or on Compton scattering, will require long observational times on the
order of a couple of mega-seconds to be able to properly measure the
polarization of type-2 AGN. | astro-ph_HE |
Oscillation of high-energy neutrinos from choked jets in stellar and
merger ejecta: We present a comprehensive study on oscillation of high-energy neutrinos from
two different environments: blue supergiant progenitors that may harbor
low-power gamma-ray burst (GRB) jets and neutron star merger ejecta that would
be associated with short gamma-ray bursts. We incorporate the radiation
constraint that gives a necessary condition for nonthermal neutrino production,
and account for the time evolution of the jet, which allows us to treat
neutrino oscillation in matter more accurately. For massive star progenitors,
neutrino injection inside the star can lead to nonadiabatic oscillation
patterns in the 1 TeV - 10 TeV and is also visible in the flavor ratio. For
neutron star merger ejecta, we find a similar behavior in the 100 GeV - 10 TeV
region and the oscillation may result in a $\nu_e$ excess around 1 TeV. These
features, which enable us to probe the progenitors of long and short GRBs,
could be seen by future neutrino detectors with precise flavor ratio
measurements. We also discuss potential contributions to the diffuse neutrino
flux measured by IceCube, and find parameter sets allowing choked low-power GRB
jets to account for the neutrino flux in the 10 TeV - 100 TeV range without
violating the existing constraints. | astro-ph_HE |
X-ray line formation in the spectrum of SS 433: The mechanisms for the formation of X-ray lines in the spectrum of SS 433 are
investigated by taking into account the radiative transfer inside the jets. The
results of Monte Carlo numerical simulations are presented. The effect of a
decrease in line intensity due to scattering inside the jet turns out to be
pronounced, but it does not exceed 60% in magnitude on the entire grid of
parameters. The line broadening due to scattering, nutational motion, and the
contribution of satellites can lead to overestimates of the jet opening angle
$\Theta$ from the line widths in Chandra X-ray observations. The fine structure
of the lines turns out to be very sensitive to the scattering effects. This
makes its investigation by planned X-ray observatories equipped with
high-resolution spectrometers (primarily Astro-H) a powerful tool for
diagnosing the parameters of the jets in SS 433. | astro-ph_HE |
A radio structure resolved at the deca-parsec scale in radio-quiet
quasar PDS 456 with an extremely powerful X-ray outflow: Active galactic nuclei (AGN) accreting at rates close to the Eddington limit
can host radiatively driven mildly relativistic outflows. Some of these X-ray
absorbing but powerful outflows may produce strong shocks resulting in a
significant non-thermal emission. This outflow-driven radio emission may be
detectable in the radio-quiet quasar PDS 456 since it has a bolometric
luminosity reaching the Eddington limit and a relativistic wide-aperture X-ray
outflow with a kinetic power high enough to quench the star formation in its
host galaxy. To investigate this possibility, we performed very-long-baseline
interferometric (VLBI) observations of the quasar with the European VLBI
Network (EVN) at 5 GHz. The EVN image with the full resolution reveals two
faint and diffuse radio components with a projected separation of about 20 pc
and an average brightness temperature of around two million Kelvin. In relation
to the optical sub-mas-accuracy position measured by the Gaia mission, the two
components are very likely on opposite sides of an undetected radio core. The
VLBI structure at the deca-pc scale can thus be either a young jet or a
bidirectional radio-emitting outflow, launched in the vicinity of a strongly
accreting central engine. Two diffuse components at the hecto-pc scale, likely
the relic radio emission from the past AGN activity, are tentatively detected
on each side in the low-resolution EVN image. | astro-ph_HE |
Observations of diffuse fluxes of cosmic neutrinos: In this contribution the current observational results for the diffuse flux
of high-energy astrophysical neutrinos are reviewed. In order to understand the
science implications, the measurements in different detection channels are
discussed and results are compared. The discussion focuses is the energy
spectrum, the flavor ratio and large scale anisotropy. | astro-ph_HE |
The Compton Spectrometer and Imager: The Compton Spectrometer and Imager (COSI) is a NASA Small Explorer (SMEX)
satellite mission in development with a planned launch in 2027. COSI is a
wide-field gamma-ray telescope designed to survey the entire sky at 0.2-5 MeV.
It provides imaging, spectroscopy, and polarimetry of astrophysical sources,
and its germanium detectors provide excellent energy resolution for emission
line measurements. Science goals for COSI include studies of 0.511 MeV emission
from antimatter annihilation in the Galaxy, mapping radioactive elements from
nucleosynthesis, determining emission mechanisms and source geometries with
polarization measurements, and detecting and localizing multimessenger sources.
The instantaneous field of view for the germanium detectors is >25% of the sky,
and they are surrounded on the sides and bottom by active shields, providing
background rejection as well as allowing for detection of gamma-ray bursts and
other gamma-ray flares over most of the sky. In the following, we provide an
overview of the COSI mission, including the science, the technical design, and
the project status. | astro-ph_HE |
Comments on "Imaging Reanalyses of EHT Data": On June 14, 2022, the EHT collaboration (hereafter EHTC) made the web page
(https://eventhorizontelescope.org/blog/imaging-reanalyses-eht-data) with the
title "Imaging Reanalyses of EHT Data," in which they made comments on our
recent Miyoshi et al .2022 published in the Astrophysical Journal.
We investigated the EHTC comments and found that all of the five points
raised by the EHTC are subjective and unsubstantiated claims. Thus they do not
prove the correctness of the result of EHTC. Sincerely we hope that the EHTC
will publish, not a collection of unsubstantiated claims, but a discussion
based on scientific arguments. | astro-ph_HE |
Gamma Ray Lines from a Universal Extra Dimension: Indirect Dark Matter searches are based on the observation of secondary
particles produced by the annihilation or decay of Dark Matter. Among them,
gamma-rays are perhaps the most promising messengers, as they do not suffer
deflection or absorption on Galactic scales, so their observation would
directly reveal the position and the energy spectrum of the emitting source.
Here, we study the detailed gamma-ray energy spectrum of Kaluza--Klein Dark
Matter in a theory with 5 Universal Extra Dimensions. We focus in particular on
the two body annihilation of Dark Matter particles into a photon and another
particle, which produces monochromatic photons, resulting in a line in the
energy spectrum of gamma rays. Previous calculations in the context of the five
dimensional UED model have computed the line signal from annihilations into
\gamma \gamma, but we extend these results to include \gamma Z and \gamma H
final states. We find that these spectral lines are subdominant compared to the
predicted \gamma \gamma signal, but they would be important as follow-up
signals in the event of the observation of the \gamma \gamma line, in order to
distinguish the 5d UED model from other theoretical scenarios. | astro-ph_HE |
Estimation of Plasma Parameters in an Accretion Column near the Surface
of Accreting White Dwarfs from Their Flux Variability: We consider the behavior of matter in the accretion column that emerges under
accretion in binary systems near the surface of a white dwarf. The plasma
heated in a standing shock wave near the white dwarf surface efficiently
radiates in the X-ray energy band. We suggest a method for estimating
post-shock plasma parameters, such as the density, temperature, and height of
the hot zone, from the power spectrum of its X-ray luminosity variability. The
method is based on the fact that the flux variability amplitude for the hot
region at various Fourier frequencies depends significantly on its cooling
time, which is determined by the parameters of the hot zone in the accretion
column. This allows the density and temperature of the hot matter to be
estimated. We show that the characteristic cooling time can be efficiently
determined from the break frequency in the power spectrum of the X-ray flux
variability for accreting white dwarfs. The currently available X-ray
instruments do not allow such measurements to be made because of an
insufficient collecting area, but this will most likely become possible with
new-generation large-area X-ray spectrometers. | astro-ph_HE |
Application of the MST clustering to the high energy gamma-ray sky. IV -
Blazar candidates found as possible counterparts of photon clusters: We present the results of a cluster search in the Fermi-LAT Pass 8 gamma-ray
sky by means of the Minimum Spanning Tree algorithm, at energies higher than 10
GeV and at Galactic latitudes higher than 25 degrees. The selected clusters
have a minimum number of photons higher than or equal to 5, a high degree of
concentration, and are without a clear corresponding counterpart in blazar
catalogues. A sample of 30 possible gamma-ray sources was obtained. These
objects were verified by applying the standard Maximum Likelihood analysis on
the Fermi-LAT data. A search for possible radio counterparts in a circle having
a radius of 6 arcmin was performed, finding several interesting objects, the
majority of them without optical spectroscopical data. These can be considered
as new blazar candidates. Some of them were already noticed as possible blazars
or Active Galactic Nuclei in previous surveys, but never associated with high
energy emission. These possible counterparts are reported and their properties
are discussed. | astro-ph_HE |
IceCube as a Multi-messenger Follow-up Observatory for Astrophysical
Transients: The recent association between IC-170922A and the blazar TXS0506+056
highlights the importance of real-time observations for identifying possible
astrophysical neutrino sources. Thanks to its near-100\% duty cycle, 4$\pi$
steradian field of view, and excellent sensitivity over many decades of energy,
IceCube is well suited both to generate alerts for follow-up by other
instruments and to rapidly follow up alerts generated by other instruments.
Detection of neutrinos in coincidence with transient astrophysical phenomena
serves as a smoking gun for hadronic processes and supplies essential
information about the identities and mechanisms of cosmic-ray accelerators. In
2016, the IceCube Neutrino Observatory established a pipeline to rapidly search
for neutrinos from astrophysical transients on timescales ranging from a
fraction of a second to multiple weeks. Since then, 67 dedicated analyses have
been performed searching for associations between IceCube neutrinos and
astrophysical transients reported by radio, optical, X-ray, and gamma-ray
instruments in addition to searching for lower energy neutrino signals in
association with IceCube's own high-energy alerts. We present the event
selection, maximum likelihood analysis method, and sensitivity of the IceCube
real-time pipeline. We also summarize the results of all follow-up analyses to
date. | astro-ph_HE |
Cosmic Gamma Ray Bursts: Gamma ray bursts (GRBs) are astronomical phenomena detected at highest
energies. The gamma ray photons carry energies on the order of
mega-electronovolts and arrive to us from the point-like sources that are
uniformly distributed on the sky. A typical burst has a form of a pulse that
lasts for about a minute. As the Earth atmosphere is not transparent to the
very high energy radiation, the bursts are detected by means of the telescopes
onboard satellites that are placed on the orbit. The total energetics of GRB
events, which is given by the integrated energy flux by the detector unit area,
implies that we are witnessing very powerful explosions, where an enormously
great power is released within a short time. There is only one way to obtain
such huge energies in cosmos: the disruption of a star. | astro-ph_HE |
Multimessenger observations of counterparts to IceCube-190331A: High-energy neutrinos are a promising tool for identifying astrophysical
sources of high and ultra-high energy cosmic rays (UHECR). Prospects of
detecting neutrinos at high energies ($\gtrsim$TeV) from blazars have been
boosted after the recent association of IceCube-170922A and TXS 0506+056. We
investigate the high-energy neutrino, IceCube-190331A, a high-energy starting
event (HESE) with a high likelihood of being astrophysical in origin. We
initiated a Swift/XRT and UVOT tiling mosaic of the neutrino localisation, and
followed up with ATCA radio observations, compiling a multiwavelength SED for
the most likely source of origin. NuSTAR observations of the neutrino location
and a nearby X-ray source were also performed. We find two promising
counterpart in the 90% confidence localisation region and identify the
brightest as the most likely counterpart. However, no Fermi/LAT $\gamma$-ray
source and no prompt Swift/BAT source is consistent with the neutrino event. At
this point it is unclear whether any of the counterparts produced
IceCube-190331A. We note that the Helix Nebula is also consistent with the
position of the neutrino event, and we calculate that associated particle
acceleration processes cannot produce the required energies to generate a
high-energy HESE neutrino. | astro-ph_HE |
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