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Comptonization by Reconnection Plasmoids in Black Hole Coronae III:
Dependence on the Guide Field in Pair Plasma: We perform two-dimensional particle-in-cell simulations of magnetic
reconnection for various strengths of the guide field (perpendicular to the
reversing field), in magnetically-dominated electron-positron plasmas. Magnetic
reconnection under such conditions could operate in accretion disk coronae
around black holes. There, it has been suggested that the trans-relativistic
bulk motions of reconnection plasmoids containing inverse-Compton-cooled
electrons could Compton-upscatter soft photons to produce the observed
non-thermal hard X-rays. Our simulations are performed for magnetizations $3
\leq \sigma \leq 40$ (defined as the ratio of enthalpy density of the reversing
field to plasma enthalpy density) and guide field strengths $0 \leq B_{\rm
g}/B_0 \leq 1$ (normalized to the reversing field strength $B_0$). We find that
the mean bulk energy of the reconnected plasma depends only weakly on the flow
magnetization but strongly on the guide field strength -- with $B_{\rm g}/B_0 =
1$ yielding a mean bulk energy twice smaller than $B_{\rm g}/B_0 = 0$.
Similarly, the dispersion of bulk motions around the mean -- a signature of
stochasticity in the plasmoid chain's motions -- is weakly dependent on
magnetization (for $\sigma \gtrsim 10$) but strongly dependent on the guide
field strength -- dropping by more than a factor of two from $B_{\rm g}/B_0 =
0$ to $B_{\rm g}/B_0 = 1$. In short, reconnection in strong guide fields
($B_{\rm g}/B_0 \sim 1$) leads to slower and more ordered plasmoid bulk motions
than its weak guide field ($B_{\rm g}/B_0 \sim 0$) counterpart. | astro-ph_HE |
NGC 4388: A Test Case for Relativistic Disk Reflection and Fe K
Fluorescence Features: We present a new analysis of the Suzaku X-ray spectrum of the Compton-thin
Seyfert 2 galaxy NGC 4388. The spectrum above $\sim$2 keV can be described by a
remarkably simple and rather mundane model, consisting of a uniform, neutral
spherical distribution of matter, with a radial column density of $2.58 \pm
0.02 \times 10^{23}$ cm$^{-2}$, and an Fe abundance of
$1.102^{+0.024}_{-0.021}$ relative to solar. The model does not require any
phenomenological adjustments to self-consistently account for the low-energy
extinction, the Fe K$\alpha$ and Fe K$\beta$ fluorescent emission lines, the Fe
K edge, and the Compton-scattered continuum from the obscuring material. The
spherical geometry is not a unique description, however, and the
self-consistent, solar abundance MYTORUS model, applied with toroidal and
non-toroidal geometries, gives equally good descriptions of the data. In all
cases, the key features of the spectrum are so tightly locked together that for
a wide range of parameters, a relativistic disk-reflection component
contributes no more than $\sim$2% to the net spectrum in the 2-20 keV band. We
show that the commonly invoked explanations for weak X-ray reflection features,
namely a truncated and/or very highly ionized disk, do not work for NGC 4388.
If relativistically-broadened Fe K$\alpha$ lines and reflection are ubiquitous
in Seyfert 1 galaxies, they should also be ubiquitous in Compton-thin Seyfert 2
galaxies. The case of NGC 4388 shows the need for similar studies of more
Compton-thin AGN to ascertain whether this is true. | astro-ph_HE |
Optical photometry and spectroscopy of the low-luminosity, broad-lined
Ic supernova iPTF15dld: Core-collapse stripped-envelope supernova (SN) explosions reflect the
diversity of physical parameters and evolutionary paths of their massive star
progenitors. We have observed the type Ic SN iPTF15dld (z = 0.047), reported by
the Palomar Transient Factory. Spectra were taken starting 20 rest-frame days
after maximum luminosity and are affected by a young stellar population
background. Broad spectral absorption lines associated with the SN are detected
over the continuum, similar to those measured for broad-lined, highly energetic
SNe Ic. The light curve and maximum luminosity are instead more similar to
those of low luminosity, narrow-lined Ic SNe. This suggests a behavior whereby
certain highly-stripped-envelope SNe do not produce a large amount of Ni56, but
the explosion is sufficiently energetic that a large fraction of the ejecta is
accelerated to higher-than-usual velocities. We estimate SN iPTF15dld had a
main sequence progenitor of 20-25 Msun, produced a Ni56 mass of ~0.1-0.2 Msun,
had an ejecta mass of [2-10] Msun, and a kinetic energy of [1-18] e51 erg. | astro-ph_HE |
Spin down during quiescence of the fastest known accretion-powered
pulsar: We present a timing solution for the 598.89 Hz accreting millisecond pulsar,
IGR J00291+5934, using Rossi X-ray Timing Explorer data taken during the two
outbursts exhibited by the source on 2008 August and September. We estimate the
neutron star spin frequency and we refine the system orbital solution. To
achieve the highest possible accuracy in the measurement of the spin frequency
variation experienced by the source in-between the 2008 August outburst and the
last outburst exhibited in 2004, we re-analysed the latter considering the
whole data set available. We find that the source spins down during quiescence
at an average rate of {\nu}dot_{sd}=(-4.1 +/- 1.2)E-15 Hz/s. We discuss
possible scenarios that can account for the long-term neutron star spin-down in
terms of either magneto-dipole emission, emission of gravitational waves, and a
propeller effect. If interpreted in terms of magneto-dipole emission, the
measured spin down translates into an upper limit to the neutron star magnetic
field, B<=3E+08 G, while an upper limit to the average neutron star mass
quadrupole moment of Q<=2E+36 g cm^2 is set if the spin down is interpreted in
terms of the emission of gravitational waves. | astro-ph_HE |
Multi-wavelength Photometry and Progenitor Analysis of the Nova V906 Car: We present optical and infrared photometry of the classical nova V906 Car,
also known as Nova Car 2018 and ASASSN-18fv, discovered by ASASS-SN survey on
16.32 March 2018 UT (MJD 58193.0). The nova reached its maximum on MJD 58222.56
at $V_{\rm{max}} = 5.84 \pm 0.09$ mag and had decline times of $t_{2,V} = 26.2
$ d and $t_{3,V} = 33.0 $ d. The data from Evryscope shows that the nova had
already brightened to $g'\simeq 13$\,mag five days before discovery, as
compared to its quiescent magnitude of $g=$20.13$\pm$0.03. The extinction
towards the nova, as derived from high resolution spectroscopy, shows an
estimate consistent with foreground extinction to the Carina Nebula of $A_V =
1.11_{-0.39}^{+0.54}$. The light curve resembles a rare C (cusp) class nova
with a steep decline slope of $\alpha=-3.94$ post cusp flare. From the
lightcurve decline rate, we estimate the mass of white dwarf to be $M_{WD}$ = $
< 0.8$M\textsubscript{\(\odot\)}, consistent with $M_{WD}=0.71^{+0.23}_{-0.19}$
derived from modelling the accretion disk of the system in quiescence. The
donor star is likely a K-M dwarf of 0.23-0.43\,\Msun, which is being heated by
its companion. | astro-ph_HE |
Swift monitoring of the central X-ray source in RCW 103: The X-ray source 1E 161348-5055 lies at the centre of the 2-kyr-old supernova
remnant RCW 103. Owing to its 24-ks modulation, orders-of-magnitude flux
variability over a few months/years, and lack of an obvious optical
counterpart, 1E 161348-5055 defies assignment to any known class of X-ray
sources. Starting from April 2006, Swift observed 1E 161348-5055 with its X-ray
telescope for 2 ks approximately once per month. During the five years covered,
the source has remained in a quiescent state, with an average observed flux of
1.7e-12 erg/cm^2/s (1-10 keV), about 20 times lower than the historical maximum
attained in its 1999-2000 outburst. The long time-span of the Swift data allows
us to obtain an accurate measure of the period of 1E 161348-5055 [P =
24030.42(2) s] and to derive the first upper limit on its period derivative
(|dP/dt| < 1.6e-9 s/s at 3 sigma). | astro-ph_HE |
Between the cosmic-ray `knee' and the `ankle': Contribution from star
clusters: We show that massive young star clusters may be possible candidates that can
accelerate Galactic cosmic rays (CRs) in the range of $10^7\hbox{--}10^9$ GeV
(between the `knee' and `ankle'). Various plausible scenarios such as
acceleration at the wind termination shock (WTS), supernova shocks inside these
young star clusters, etc. have been proposed,since it is difficult to
accelerate particles up to the $10^7\hbox{--}10^9$ GeV range in the standard
paradigm of CR acceleration in supernova remnants. We consider a model for the
production of different nuclei in CRs from massive stellar winds using the
observed distribution of young star clusters in the Galactic plane. We present
a detailed calculation of CR transport in the Galaxy, taking into account the
effect of diffusion, interaction losses during propagation, and particle
re-acceleration by old supernova remnants to determine the all-particle CR
spectrum. Using the maximum energy estimate from the Hillas criterion, we argue
that a young massive star cluster can accelerate protons up to a few tens of
PeV. Upon comparison with the observed data, our model requires a CR source
spectrum with an exponential cutoff of $5\times 10^7 Z$ GeV ($50\,Z$~PeV) from
these clusters together with a cosmic-ray injection fraction of $\sim 5\%$ of
the wind kinetic energy. We discuss the possibility of achieving these
requirements in star clusters, and the associated uncertainties, in the context
of considering star clusters as the natural accelerator of the `second
component' of Galactic cosmic rays. | astro-ph_HE |
Detection of X-ray flares from AX J1714.1-3912, the unidentified source
near RX J1713.7-3946: Molecular clouds are predicted to emit nonthermal X-rays when they are close
to particle-accelerating supernova remnants (SNRs), and the hard X-ray source
AX J1714.1-3912, near the SNR RX J1713.7-3946, has long been considered a
candidate for diffuse nonthermal emission associated with cosmic rays diffusing
from the remnant to a closeby molecular cloud. We aim at ascertaining the
nature of this source by analyzing two dedicated X-ray observations performed
with Suzaku and Chandra. We extracted images from the data in various energy
bands, spectra, and light curves and studied the long-term evolution of the
X-ray emission on the basis of the ~4.5 yr time separation between the two
observations. We found that there is no diffuse emission associated with AX
J1714.1-3912, which is instead the point-like source CXOU J171343.9-391205. We
discovered rapid time variability (timescale ~1 ks), together with a high
intrinsic absorption and a hard nonthermal spectrum (power law with photon
index Gamma~1.4). We also found that the X-ray flux of the source drops down by
1-2 orders of magnitude on a timescale of a few years. Our results suggest a
possible association between AX J1714.1-3912 and a previously unknown
supergiant fast X-ray transient, although further follow-up observations are
necessary to prove this association definitively. | astro-ph_HE |
Neutrino-Driven Outflows and the Elemental Abundance Patterns of Very
Metal-Poor Stars: The elemental abundances between strontium and silver ($Z = 38-47$) observed
in the atmospheres of very metal-poor stars (VMP) in the Galaxy may contain the
fingerprint of the weak $r$-process and $\nu p$-process occurring in early
core-collapse supernovae explosions. In this work, we combine various
astrophysical conditions based on a steady-state model to cover the richness of
the supernova ejecta in terms of entropy, expansion timescale, and electron
fraction. The calculated abundances based on different combinations of
conditions are compared with stellar observations with the aim of constraining
supernova ejecta conditions. We find that some conditions of the
neutrino-driven outflows consistently reproduce the observed abundances of our
sample. In addition, from the successful combinations, the neutron-rich
trajectories better reproduce the observed abundances of Sr-Zr ($Z= 38-40$),
while the proton-rich ones, Mo-Pd ($Z= 42-47$). | astro-ph_HE |
Progenitors and Explosion Properties of Supernova Remnants Hosting
Central Compact Objects: I. RCW 103 Associated with the Peculiar Source 1E
161348-5055: We present a Chandra and XMM-Newton imaging and spectroscopic study of the
supernova remnant (SNR) RCW 103 (G332.4-00.4) containing the Central Compact
Object 1E 161348-5055. The high resolution Chandra X-ray images reveal enhanced
emission in the south-eastern and north-western regions. Equivalent width line
images of Fe L, Mg, Si, and S using XMM-Newton data were used to map the
distribution of ejecta. The SNR was sectioned into 56 regions best
characterized by two-component thermal models. The harder component (kT~0.6
keV) is adequately fitted by the VPSHOCK non-equilibrium ionization model with
an ionization timescale ~ 1E11-1E12 cm^-3 s, and slightly enhanced abundances
over solar values. The soft component (kT~0.2 keV), fitted by the APEC model,
is well described by plasma in collisional ionization equilibrium with
abundances consistent with solar values. Assuming a distance of 3.1 kpc and a
Sedov phase of expansion into a uniform medium, we estimate an SNR age of 4.4
kyr, a swept-up mass M_sw=16 f_s^{-1/2} D_{3.1}^{5/2} solar masses, and a low
explosion energy E=3.7E49 f_s^{-1/2} D_{3.1}^{5/2} erg. This energy could be an
order of magnitude higher if we relax the Sedov assumption, the plasma has a
low filling factor, the plasma temperature is under-estimated, or if the SNR is
expanding into the progenitor's wind-blown bubble. Standard explosion models
did not match the ejecta yields. By comparing the fitted abundances to the most
recent core-collapse nucleosynthesis models, our best estimate yields a
low-mass progenitor around 12-13 solar masses, lower than previously reported.
We discuss degeneracies in the model fitting, particularly the effect of
altering the explosion energy on the progenitor mass estimate. | astro-ph_HE |
Inflows, Outflows, and a Giant Donor in the Remarkable Recurrent Nova
M31N 2008-12a? - Hubble Space Telescope Photometry of the 2015 Eruption: The recurrent nova M31N 2008-12a experiences annual eruptions, contains a
near-Chandrasekhar mass white dwarf, and has the largest mass accretion rate in
any nova system. In this paper, we present Hubble Space Telescope (HST)
WFC3/UVIS photometry of the late decline of the 2015 eruption. We couple these
new data with archival HST observations of the quiescent system and Keck
spectroscopy of the 2014 eruption. The late-time photometry reveals a rapid
decline to a minimum luminosity state, before a possible recovery /
re-brightening in the run-up to the next eruption. Comparison with accretion
disk models supports the survival of the accretion disk during the eruptions,
and uncovers a quiescent disk mass accretion rate of the order of
$10^{-6}\,M_\odot\,\mathrm{yr}^{-1}$, which may rise beyond
$10^{-5}\,M_\odot\,\mathrm{yr}^{-1}$ during the super-soft source phase - both
of which could be problematic for a number of well-established nova eruption
models. Such large accretion rates, close to the Eddington limit, might be
expected to be accompanied by additional mass loss from the disk through a wind
and even collimated outflows. The archival HST observations, combined with the
disk modeling, provide the first constraints on the mass donor;
$L_\mathrm{donor}=103^{+12}_{-11}\,L_\odot$,
$R_\mathrm{donor}=14.14^{+0.46}_{-0.47}\,R_\odot$, and $T_\mathrm{eff,
donor}=4890\pm110$ K, which may be consistent with an irradiated M31 red-clump
star. Such a donor would require a system orbital period $\gtrsim5$ days. Our
updated analysis predicts that the M31N 2008-12a WD could reach the
Chandrasekhar mass in < 20 kyr. | astro-ph_HE |
Simultaneous X-ray, gamma-ray, and Radio Observations of the repeating
Fast Radio Burst FRB 121102: We undertook coordinated campaigns with the Green Bank, Effelsberg, and
Arecibo radio telescopes during Chandra X-ray Observatory and XMM-Newton
observations of the repeating fast radio burst FRB 121102 to search for
simultaneous radio and X-ray bursts. We find 12 radio bursts from FRB 121102
during 70 ks total of X-ray observations. We detect no X-ray photons at the
times of radio bursts from FRB 121102 and further detect no X-ray bursts above
the measured background at any time. We place a 5$\sigma$ upper limit of
$3\times10^{-11}$ erg cm$^{-2}$ on the 0.5--10 keV fluence for X-ray bursts at
the time of radio bursts for durations $<700$ ms, which corresponds to a burst
energy of $4\times10^{45}$ erg at the measured distance of FRB 121102. We also
place limits on the 0.5--10 keV fluence of $5\times10^{-10}$ erg cm$^{-2}$ and
$1\times10^{-9}$ erg cm$^{-2}$ for bursts emitted at any time during the
XMM-Newton and Chandra observations, respectively, assuming a typical X-ray
burst duration of 5 ms. We analyze data from the Fermi Gamma-ray Space
Telescope Gamma-ray Burst Monitor and place a 5$\sigma$ upper limit on the
10--100 keV fluence of $4\times10^{-9}$ erg cm$^{-2}$ ($5\times10^{47}$ erg at
the distance of FRB 121102) for gamma-ray bursts at the time of radio bursts.
We also present a deep search for a persistent X-ray source using all of the
X-ray observations taken to date and place a 5$\sigma$ upper limit on the
0.5--10 keV flux of $4\times10^{-15}$ erg s$^{-1}$ cm$^{-2}$ ($3\times10^{41}$
erg~s$^{-1}$ at the distance of FRB 121102). We discuss these non-detections in
the context of the host environment of FRB 121102 and of possible sources of
fast radio bursts in general. | astro-ph_HE |
MAGIC observations provide compelling evidence of the hadronic multi-TeV
emission from the putative PeVatron SNR G106.3+2.7: The SNR G106.3+2.7, detected at 1--100 TeV energies by different $\gamma$-ray
facilities, is one of the most promising PeVatron candidates. This SNR has a
cometary shape which can be divided into a head and a tail region with
different physical conditions. However, it is not identified in which region
the 100 TeV emission is produced due to the limited position accuracy and/or
angular resolution of existing observational data. Additionally, it remains
unclear whether the origin of the $\gamma$-ray emission is leptonic or
hadronic. With the better angular resolution provided by these new MAGIC data
compared to earlier $\gamma$-ray datasets, we aim to reveal the acceleration
site of PeV particles and the emission mechanism by resolving the SNR
G106.3+2.7 with 0.1$^\circ$ resolution at TeV energies. We detected extended
$\gamma$-ray emission spatially coincident with the radio continuum emission at
the head and tail of SNR G106.3+2.7. The fact that we detected a significant
$\gamma$-ray emission with energies above 6.0 TeV from the tail region only
suggests that the emissions above 10 TeV, detected with air shower experiments
(Milagro, HAWC, Tibet AS$\gamma$ and LHAASO), are emitted only from the SNR
tail. Under this assumption, the multi-wavelength spectrum of the head region
can be explained with either hadronic or leptonic models, while the leptonic
model for the tail region is in contradiction with the emission above 10 TeV
and X-rays. In contrast, the hadronic model could reproduce the observed
spectrum at the tail by assuming a proton spectrum with a cutoff energy of
$\sim 1$ PeV for the tail region. Such a high energy emission in this
middle-aged SNR (4--10 kyr) can be explained by considering the scenario that
protons escaping from the SNR in the past interact with surrounding dense gases
at present. | astro-ph_HE |
A JWST Near- and Mid-Infrared Nebular Spectrum of the Type Ia Supernova
2021aefx: We present JWST near- and mid-infrared spectroscopic observations of the
nearby normal Type Ia supernova SN 2021aefx in the nebular phase at $+255$ days
past maximum light. Our Near Infrared Spectrograph (NIRSpec) and Mid Infrared
Instrument (MIRI) observations, combined with ground-based optical data from
the South African Large Telescope (SALT), constitute the first complete optical
$+$ NIR $+$ MIR nebular SN Ia spectrum covering 0.3$-$14 $\mu$m. This spectrum
unveils the previously unobserved 2.5$-$5 $\mu$m region, revealing strong
nebular iron and stable nickel emission, indicative of high-density burning
that can constrain the progenitor mass. The data show a significant improvement
in sensitivity and resolution compared to previous Spitzer MIR data. We
identify numerous NIR and MIR nebular emission lines from iron-group elements
and as well as lines from the intermediate-mass element argon. The argon lines
extend to higher velocities than the iron-group elements, suggesting stratified
ejecta that are a hallmark of delayed-detonation or double-detonation SN Ia
models. We present fits to simple geometric line profiles to features beyond
1.2 $\mu$m and find that most lines are consistent with Gaussian or spherical
emission distributions, while the [Ar III] 8.99 $\mu$m line has a distinctively
flat-topped profile indicating a thick spherical shell of emission. Using our
line profile fits, we investigate the emissivity structure of SN 2021aefx and
measure kinematic properties. Continued observations of SN 2021aefx and other
SNe Ia with JWST will be transformative to the study of SN Ia composition,
ionization structure, density, and temperature, and will provide important
constraints on SN Ia progenitor and explosion models. | astro-ph_HE |
The Varying Kinematics of Multiple Ejecta from the Black Hole X-ray
Binary MAXI J1820+070: During a 2018 outburst, the black hole X-ray binary MAXI J1820+070 was
comprehensively monitored at multiple wavelengths as it underwent a hard to
soft state transition. During this transition a rapid evolution in X-ray timing
properties and a short-lived radio flare were observed, both of which were
linked to the launching of bi-polar, long-lived relativistic ejecta. We provide
detailed analysis of two Very Long Baseline Array observations, using both time
binning and a new dynamic phase centre tracking technique to mitigate the
effects of smearing when observing fast-moving ejecta at high angular
resolution. We identify a second, earlier ejection, with a lower proper motion
of $18.0\pm1.1$ mas day$^{-1}$. This new jet knot was ejected $4\pm1$ hours
before the beginning of the rise of the radio flare, and $2\pm1$ hours before a
switch from type-C to type-B X-ray quasi-periodic oscillations (QPOs). We show
that this jet was ejected over a period of $\sim6$ hours and thus its ejection
was contemporaneous with the QPO transition. Our new technique locates the
original, faster ejection in an observation in which it was previously
undetected. With this detection we revised the fits to the proper motions of
the ejecta and calculated a jet inclination angle of $(64\pm5)^\circ$, and jet
velocities of $0.97_{-0.09}^{+0.03}c$ for the fast-moving ejecta ($\Gamma>2.1$)
and $(0.30\pm0.05)c$ for the newly-identified slow-moving ejection
($\Gamma=1.05\pm0.02$). We show that the approaching slow-moving component is
predominantly responsible for the radio flare, and is likely linked to the
switch from type-C to type-B QPOs, while no definitive signature of ejection
was identified for the fast-moving ejecta. | astro-ph_HE |
Predicting Short-duration GRB Rates in the Advanced LIGO Volume: Starting with models for the compact object merger event rate, the
short-duration Gamma-ray Burst (sGRB) luminosity function, and the Swift/BAT
detector, we calculate the observed Swift sGRB rate and its uncertainty. Our
probabilistic sGRB world model reproduces the observed number distributions in
redshift and flux for 123 Swift/BAT detected sGRBs and can be used to predict
joint sGRB/LIGO detection rates. We discuss the dependence of the rate
predictions on the model parameters and explore how they vary with increasing
experimental sensitivity. In particular, the number of bursts in the LIGO
volume depends strongly on the parameters that govern sGRB beaming. Our results
suggest that nearby sGRBs should be observed to have broader jets on average
($\theta_{\rm jet}\gtrsim 30$ degrees), as compared to the narrowly-beamed
appearance of cosmological sGRBs due to detection selection effect driving
observed jet angle. Assuming all sGRBs are due to compact object mergers,
within a $D < 200$ Mpc aLIGO volume, we predict $0.18^{+0.19}_{-0.08}$ sGRB/GW
associations all-sky per year for on-axis events at Swift sensitivities,
increasing to $1.2^{+1.9}_{-0.6}$ with the inclusion of off-axis events. We
explore the consistency of our model with GW170817/GRB~170817A in the context
of structured jets. Predictions for future experiments are made. | astro-ph_HE |
The unreasonable weakness of r-process cosmic rays in the
neutron-star-merger nucleosynthesis scenario: We reach the robust conclusion that, by combining the observed cosmic rays of
r-process elements with the fact that the velocity of the neutron-star-merger
ejecta is much higher than that of the supernova ejecta, either (1) the reverse
shock in the neutron-star-merger ejecta is a very inefficient accelerator that
converts less than 0.003% of the ejecta kinetic energy to the cosmic-ray energy
or (2) the neutron star merger is not the origin of the Galactic r-process
elements. We also find that the acceleration efficiency should be less than
0.1% for the reverse shock of the supernova ejecta with the observed cosmic
rays lighter than the iron. | astro-ph_HE |
Formation of Lower Mass-gap Black Hole--Neutron Star Binary Mergers
through Super-Eddington Stable Mass Transfer: Super-Eddington accretion of neutron stars (NSs) has been suggested both
observationally and theoretically. In this paper, we propose that NSs in
close-orbit binary systems with companions of helium (He) stars, most of which
systems form after the common-envelope phase, could experience super-Eddington
stable Case BB/BC mass transfer (MT), and can sometimes occur accretion-induced
collapses (AICs) to form lower mass-gap black holes (mgBHs). Our detailed
binary evolution simulations reveal that AIC events tend to happen if the
primaries NS have an initial mass $\gtrsim1.7\,M_\odot$ with an accretion rate
of $\gtrsim300$ times the Eddington limit. These mgBHs would have a mass nearly
equal to or slightly higher than the NS maximum mass. The remnant mgBH--NS
binaries after the core collapses of He stars are potential progenitors of
gravitational-wave (GW) source. Multimessenger observation between GW and
kilonova signals from a population of high-mass binary NS and mgBH--NS mergers
formed through super-Eddington stable MT are helpful in constraining the
maximum mass and equation of state of NSs. S230529ay, a mgBH--NS merger
candidate recently detected in the fourth observing run of the LIGO-Virgo-KAGRA
Collaboration, could possibly originate from this formation scenario. | astro-ph_HE |
Long-term multi-wavelength study of 1ES 0647+250: The BL Lac object 1ES 0647+250 is one of the few distant $\gamma$-ray
emitting blazars detected at very high energies (VHE, $\gtrsim$100 GeV) during
a non-flaring state. It was detected with the MAGIC telescopes during its low
activity in the years 2009-2011, as well as during three flaring activities in
the years 2014, 2019 and 2020, with the highest VHE flux in the latter epoch.
An extensive multi-instrument data set was collected within several coordinated
observing campaigns throughout these years. We aim to characterise the
long-term multi-band flux variability of 1ES 0647+250, as well as its broadband
spectral energy distribution (SED) during four distinct activity states
selected in four different epochs, in order to constrain the physical
parameters of the blazar emission region under certain assumptions. We evaluate
the variability and correlation of the emission in the different energy bands
with the fractional variability and the Z-transformed Discrete Correlation
Function, as well as its spectral evolution in X-rays and $\gamma$ rays. Owing
to the controversy in the redshift measurements of 1ES 0647+250 reported in the
literature, we also estimate its distance in an indirect manner through the
comparison of the GeV and TeV spectra from simultaneous observations with
Fermi-LAT and MAGIC during the strongest flaring activity detected to date.
Moreover, we interpret the SEDs from the four distinct activity states within
the framework of one-component and two-component leptonic models, proposing
specific scenarios that are able to reproduce the available multi-instrument
data. | astro-ph_HE |
Energetic Gamma Radiation from Rapidly Rotating Black Holes: Supermassive black holes are believed to be the central power house of active
galactic nuclei. Applying the pulsar outer-magnetospheric particle accelerator
theory to black-hole magnetospheres, we demonstrate that an electric field is
exerted along the magnetic field lines near the event horizon of a rotating
black hole. In this particle accelerator (or a gap), electrons and positrons
are created by photon-photon collisions and accelerated in the opposite
directions by this electric field, efficiently emitting gamma-rays via
curvature and inverse-Compton processes. It is shown that a gap arises around
the null charge surface formed by the frame-dragging effect, provided that
there is no current injection across the gap boundaries. The gap is dissipating
a part of the hole's rotational energy, and the resultant gamma-ray luminosity
increases with decreasing plasma accretion from the surroundings. Considering
an extremely rotating supermassive black hole, we show that such a gap
reproduces the significant very-high-energy (VHE) gamma-ray flux observed from
the radio galaxy IC 310, provided that the accretion rate becomes much less
than the Eddington rate particularly during its flare phase. It is found that
the curvature process dominates the inverse-Compton process in the
magnetosphere of IC~310, and that the observed power-law-like spectrum in VHE
gamma-rays can be explained to some extent by a superposition of the curvature
emissions with varying curvature radius. It is predicted that the VHE spectrum
extends into higher energies with increasing VHE photon flux. | astro-ph_HE |
Magnetar as the Central Engine of AT2018cow: Optical, Soft X-Ray, and
Hard X-Ray Emission: AT2018cow is the most extensively observed and widely studied fast blue
optical transient to date; its unique observational properties challenge all
existing standard models. In this paper, we model the luminosity evolution of
the optical, soft X-ray, and hard X-ray emission, as well as the X-ray spectrum
of AT2018cow with a magnetar-centered engine model. We consider a two-zone
model with a striped magnetar wind in the interior and an expanding ejecta
outside. The soft and hard X-ray emission of AT2018cow can be explained by the
leakage of high-energy photons produced by internal gradual magnetic
dissipation in the striped magnetar wind, while the luminous thermal UV/optical
emission results from the thermalization of the ejecta by the captured photons.
The two-component energy spectrum yielded by our model with a quasi-thermal
component from the optically thick region of the wind superimposed on an
optically thin synchrotron component well reproduces the X-ray spectral shape
of AT2018cow. The Markov Chain Monte Carlo fitting results suggest that in
order to explain the very short rise time to peak of the thermal optical
emission, a low ejecta mass $M_{\rm ej}\approx0.1~M_\odot$ and high ejecta
velocity $v_{\rm SN}\approx0.17c$ are required. A millisecond magnetar with
$P_0\approx3.7~\rm ms$ and $B_p\approx2.4\times10^{14}~\rm G$ is needed to
serve as the central engine of AT2018cow. | astro-ph_HE |
K-shell photoionization of Nickel ions using R-matrix: We present R-matrix calculations of photoabsorption and photoionization cross
sections across the K edge of the Li-like to Ca-like ions stages of Ni.
Level-resolved, Breit-Pauli calculations were performed for the Li-like to
Na-like stages. Term-resolved calculations, which include the mass-velocity and
Darwin relativistic corrections, were performed for the Mg-like to Ca-like ion
stages. This data set is extended up to Fe-like Ni using the distorted wave
approximation as implemented by AUTOSTRUCTURE. The R-matrix calculations
include the effects of radiative and Auger dampings by means of an optical
potential. The damping processes affect the absorption resonances converging to
the K thresholds causing them to display symmetric profiles of constant width
that smear the otherwise sharp edge at the K-shell photoionization threshold.
These data are important for the modeling of features found in photoionized
plasmas. | astro-ph_HE |
Chandra observations of NGC4342, an optically faint, X-ray gas-rich
early-type galaxy: Chandra X-ray observations of NGC4342, a low stellar mass (M_K=-22.79 mag)
early-type galaxy, show luminous, diffuse X-ray emission originating from hot
gas with temperature of kT~0.6 keV. The observed 0.5-2 keV band luminosity of
the diffuse X-ray emission within the D_25 ellipse is L_0.5-2keV = 2.7 x 10^39
erg/s. The hot gas has a significantly broader distribution than the stellar
light, and shows strong hydrodynamic disturbances with a sharp surface
brightness edge to the northeast and a trailing tail. We identify the edge as a
cold front and conclude that the distorted morphology of the hot gas is
produced by ram pressure as NGC4342 moves through external gas. From the
thermal pressure ratios inside and outside the cold front, we estimate the
velocity of NGC4342 and find that it moves supersonically (M~2.6) towards the
northeast. Outside the optical extent of the galaxy we detect ~17 bright
(L_0.5-8keV > 3 x 10^37 erg/s) excess X-ray point sources. The excess sources
are presumably low-mass X-ray binaries (LMXBs) located in metal-poor globular
clusters (GCs) in the extended dark matter halo of NGC4342. Based on the number
of excess sources and the average frequency of bright LMXBs in GCs, we estimate
that NGC4342 may host roughly 850-1700 GCs. In good agreement with this,
optical observations hint that NGC4342 may harbor 1200 +/- 500 GCs. This number
corresponds to a GC specific frequency of S_N = 19.9 +/- 8.3, which is among
the largest values observed in full-size galaxies. | astro-ph_HE |
Gamma-Ray Burst long lasting X-ray flaring activity: In this paper we shed light on late time (i.e. with peak time t_{pk} \gtrsim
1000 s) flaring activity. We address the morphology and energetic of flares in
the window \sim 10^3-10^6 s to put constraints on the temporal evolution of the
flare properties and to identify possible differences in the mechanism
producing the early and late time flaring emission, if any. This requires the
complete understanding of the observational biases affecting the detection of
X-ray flares superimposed on a fading continuum at t > 1000 s. We consider all
the Swift GRBs that exhibit late time flares. Our sample consists of 36 flares,
14 with redshift measurements. We inherit the strategy of data analysis from
Chincarini et al. (2010) in order to make a direct comparison with the early
time flare properties. The morphology of the flare light curve is the same for
both early time and late time flares, while they differ energetically. The
width of late time flares increases with time similarly to the early time
flares. Simulations confirmed that the increase of the width with time is not
due to the decaying statistics, at least up to 10^4 s. The energy output of
late time flares is one order of magnitude lower than the early time flare one,
being \sim 1% E_{prompt}. The evolution of the peak luminosity as well as the
distribution of the peak flux-to-continuum ratio for late time flares indicate
that the flaring emission is decoupled from the underlying continuum,
differently from early time flares/steep decay. A sizable fraction of late time
flares are compatible with afterglow variability. The internal shock origin
seems the most promising explanation for flares. However, some differences that
emerge between late and early time flares suggest that there could be no unique
explanation about the nature of late time flares. | astro-ph_HE |
On the Geometry of Curvature Radiation and Implications for Subpulse
Drifting: The phenomenon of subpulse drifting offers unique insights into the emission
geometry of pulsars, and is commonly interpreted in terms of a rotating
carousel of "spark" events near the stellar surface. We develop a detailed
geometric model for the emission columns above a carousel of sparks that is
entirely calculated in the observer's inertial frame, and which is consistent
with the well-understood rotational effects of aberration and retardation. We
explore the observational consequences of the model, including (1) the
appearance of the reconstructed beam pattern via the cartographic transform and
(2) the morphology of drift bands and how they might evolve as a function of
frequency. The model, which is implemented in the software package PSRGEOM, is
applicable to a wide range of viewing geometries, and we illustrate its
implications using PSRs B0809+74 and B2034+19 as examples. Some specific
predictions are made with respect to the difference between subpulse evolution
and microstructure evolution, which provides a way to further test our model. | astro-ph_HE |
Low Mass X-ray Binary Simulation Data Release: We present DABS (Database of Accreting Binary Simulations), an open-access
database of modelled Low Mass X-ray Binaries (LMXBs). DABS has been created
using evolutionary tracks of neutron star and black hole LMXBs, spanning a
large set of initial conditions for the accretor mass, donor mass, and orbital
period. The LMXBs are evolved with the Convection and Rotation Boosted Magnetic
Braking prescription. The most important asset of this online database is the
tool PEAS (Progenitor Extractor for Accreting Systems). This tool can be used
to predict the progenitors of any user-entered LMXB system and view their
properties before the start of mass transfer. This prediction can facilitate
preliminary searches for the progenitors of observed LMXBs, which can help in
streamlining further detailed analyses. The PEAS tool can also be used to
constrain population synthesis techniques that specialize in supernova kicks in
binaries and common envelope outcomes. | astro-ph_HE |
Electromagnetic Helicity in Classical Physics: This pedagogical note revisits the concept of electromagnetic helicity in
classical systems. In particular, magnetic helicity and its role in mean field
dynamo theories is briefly discussed highlighting the major mathematical
inconsistency in most of these theories---violation of magnetic helicity
conservation. A short review of kinematic dynamo theory and its classic
theorems is also presented in the Appendix. | astro-ph_HE |
On the relation between hard X-ray photon index versus accretion rate
for super-Eddington accreting quasars: We investigate whether the hard X-ray photon index (${\Gamma}$) versus
accretion rate correlation for super-Eddington accreting quasars is different
from that for sub-Eddington accreting quasars. We construct a sample of 113
bright quasars from the Sloan Digital Sky Survey Data Release 14 quasar
catalog, including 38 quasars as the super-Eddington subsample and 75 quasars
as the sub-Eddington subsample. We derive black-hole masses using a
simple-epoch virial mass formula based on the ${\rm H\beta}$ lines, and we use
the standard thin disk model to derive the dimensionless accretion rates
($\dot{\mathscr{M}}$) for our sample. The X-ray data for these quasars are
collected from the Chandra and XMM-Newton archives. We fit the hard X-ray
spectra using a single power-law model to obtain ${\Gamma}$ values. We find a
statistically significant ($R_{\rm S}=0.43$, $p=7.75\times{10}^{-3}$)
correlation between ${\Gamma}$ and $\dot{\mathscr{M}}$ for the super-Eddington
subsample. The ${\Gamma}$-$\dot{\mathscr{M}}$ correlation for the sub-Eddington
subsample is also significant, but weaker ($R_{\rm S}=0.30$,
$p=9.98\times{10}^{-3}$). Linear regression analysis shows that ${\rm
\Gamma}=(0.34\pm0.11){\rm log}{\dot{\mathscr{M}}}+(1.71\pm0.17)$ and ${\rm
\Gamma}=(0.09\pm0.04){\rm log}{\dot{\mathscr{M}}}+(1.93\pm0.04)$ for the super-
and sub-Eddington subsamples, respectively. The ${\Gamma}$-$\dot{\mathscr{M}}$
correlations of the two subsamples are different, suggesting different
disk-corona connections in these two types of systems. We propose one
qualitative explanation of the steeper ${\Gamma}$-$\dot{\mathscr{M}}$
correlation in the super-Eddington regime that involves larger seed photon
fluxes received by the compact coronae from the thick disks in super-Eddington
accreting quasars. | astro-ph_HE |
XMM-Newton and Swift Observations of the Seyfert 1 AGN NGC 5940: Probing the physics of the accretion flow around active galactic nuclei (AGN)
is crucial to understanding their emission mechanisms as well as being able to
constrain the geometrical and variability properties of the different regions
around them. The soft X-ray excess -- usually observed below
$\sim2\,\mathrm{keV}$ in excess of the dominant X-ray powerlaw continuum -- is
one prominent feature that is commonly seen in type 1 Seyfert AGN and therefore
readily provides a useful diagnostic of the accretion flow mechanism around
these systems. NGC 5940 is a Seyfert 1 AGN which reveals strong, prominent soft
X-ray excess below $\sim2\,\mathrm{keV}$ as seen in both its XMM-Newton and
Swift observations. Model fit to the data revealed that this feature could be
equally well explained by the ionised partial covering, the thermal
Comptonisation and the blurred reflection models. Although the other models
cannot be decisively ruled out with the data at hand, the lack of significant
broad iron $K_{\alpha}$ as well as any significant emission/absorption line
features in the reflection grating spectrometer (RGS) data tend to favour the
thermal Comptonisation origin for the soft X-ray excess in NGC 5940. | astro-ph_HE |
First observation of MeV gamma-ray universe with bijective imaging
spectroscopy using the Electron-Tracking Compton Telescope aboard SMILE-2+: MeV gamma-rays provide a unique window for the direct measurement of line
emissions from radioisotopes, but observations have made little significant
progress after COMPTEL/{\it CGRO}. To observe celestial objects in this band,
we are developing an electron-tracking Compton camera (ETCC), which realizes
both bijective imaging spectroscopy and efficient background reduction gleaned
from the recoil electron track information. The energy spectrum of the
observation target can then be obtained by a simple ON-OFF method using a
correctly defined point spread function on the celestial sphere. The
performance of celestial object observations was validated on the second
balloon SMILE-2+ installed with an ETCC having a gaseous electron tracker with
a volume of 30$\times$30$\times$30 cm$^3$. Gamma-rays from the Crab nebula were
detected with a significance of 4.0$\sigma$ in the energy range 0.15--2.1 MeV
with a live time of 5.1 h, as expected before launching. Additionally, the
light curve clarified an enhancement of gamma-ray events generated in the
Galactic center region, indicating that a significant proportion of the final
remaining events are cosmic gamma rays. Independently, the observed intensity
and time variation were consistent with the pre-launch estimates except in the
Galactic center region. The estimates were based on the total background of
extragalactic diffuse, atmospheric, and instrumental gamma-rays after
accounting for the variations in the atmospheric depth and rigidity during the
level flight. The Crab results and light curve strongly support our
understanding of both the detection sensitivity and the background in real
observations. This work promises significant advances in MeV gamma-ray
astronomy. | astro-ph_HE |
Feature Extraction on Synthetic Black Hole Images: The Event Horizon Telescope (EHT) recently released the first horizon-scale
images of the black hole in M87. Combined with other astronomical data, these
images constrain the mass and spin of the hole as well as the accretion rate
and magnetic flux trapped on the hole. An important question for EHT is how
well key parameters such as spin and trapped magnetic flux can be extracted
from present and future EHT data alone. Here we explore parameter extraction
using a neural network trained on high resolution synthetic images drawn from
state-of-the-art simulations. We find that the neural network is able to
recover spin and flux with high accuracy. We are particularly interested in
interpreting the neural network output and understanding which features are
used to identify, e.g., black hole spin. Using feature maps, we find that the
network keys on low surface brightness features in particular. | astro-ph_HE |
The High Energy X-ray Probe (HEX-P): A New Window into Neutron Star
Accretion: Accreting neutron stars (NSs) represent a unique laboratory for probing the
physics of accretion in the presence of strong magnetic fields ($B\gtrsim 10^8$
G). Additionally, the matter inside the NS itself exists in an ultra-dense,
cold state that cannot be reproduced in Earth-based laboratories. Hence,
observational studies of these objects are a way to probe the most extreme
physical regimes. Here we present an overview of the field and discuss the most
important outstanding problems related to NS accretion. We show how these open
questions regarding accreting NSs in both low-mass and high-mass X-ray binary
systems can be addressed with the High-Energy X-ray Probe (HEX-P) via simulated
data. In particular, with the broad X-ray passband and improved sensitivity
afforded by a low X-ray background, HEX-P will be able to 1) distinguish
between competing continuum emission models; 2) provide tighter upper limits on
NS radii via reflection modeling techniques that are independent and
complementary to other existing methods; 3) constrain magnetic field geometry,
plasma parameters, and accretion column emission patterns by characterizing
fundamental and harmonic cyclotron lines and exploring their behavior with
pulse phase; 4) directly measure the surface magnetic field strength of highly
magnetized NSs at the lowest accretion luminosities; as well as 5) detect
cyclotron line features in extragalactic sources and probe their dependence on
luminosity in the super-Eddington regime in order to distinguish between
geometrical evolution and accretion-induced decay of the magnetic field. In
these ways HEX-P will provide an essential new tool for exploring the physics
of NSs, their magnetic fields, and the physics of extreme accretion. | astro-ph_HE |
Cooling of neutron stars and hybrid stars with a stiff hadronic EoS: Within the "nuclear medium cooling" scenario of neutron stars all reliably
known temperature - age data, including those of the central compact objects in
the supernova remnants of Cassiopeia A and XMMU-J1732, can be comfortably
explained by a set of cooling curves obtained by variation of the star mass
within the range of typical observed masses. The recent measurements of the
high masses of the pulsars PSR J1614-2230 and PSR J0348-0432 on the one hand,
and of the low masses for PSR J0737-3039B and the companion of PSR J1756-2251
on the other, provide independent proof for the existence of neutron stars with
masses in a broad range from $\sim 1.2$ to 2 $M_\odot$. The values $M>2
M_{\odot}$ call for sufficiently stiff equations of state for neutron star
matter. We investigate the response of the set of neutron star cooling curves
to a stiffening of the nuclear equation of state so that maximum masses of
about $2.4 M_\odot$ would be accessible and to a deconfinement phase transition
from such stiff nuclear matter in the outer core to color superconducting quark
matter in the inner core. Without readjustment of cooling inputs the mass range
required to cover all cooling data for the stiff DD2 equation of state should
include masses of $2.426 M_\odot$ for describing the fast cooling of CasA while
the existence of a quark matter core accelerates the cooling so that CasA
cooling data are described with a hybrid star of mass $1.674 M_\odot$. | astro-ph_HE |
Delving Into X-ray Obscuration of Type 2 AGN, Near and Far: Using self-consistent, physically motivated models, we investigate the X-ray
obscuration in 19 Type 2 [OIII] 5007 \AA\ selected AGN, 9 of which are local
Seyfert 2 galaxies and 10 of which are Type 2 quasar candidates. We derive
reliable line-of-sight and global column densities for these objects, which is
the first time this has been reported for an AGN sample; 4 AGN have
significantly different global and line-of-sight column densities. Five sources
are heavily obscured to Compton-thick. We comment on interesting sources
revealed by our spectral modeling, including a candidate ``naked'' Sy2. After
correcting for absorption, we find that the ratio of the rest-frame, 2-10 keV
luminosity (L$_{\rm 2-10keV,in}$) to L$_{\rm [OIII]}$ is 1.54 $\pm$ 0.49 dex
which is essentially identical to the mean Type 1 AGN value. The Fe K$\alpha$
luminosity is significantly correlated with L$_{\rm [OIII]}$, but with
substantial scatter. Finally, we do not find a trend between L$_{\rm
2-10keV,in}$ and global or line-of-sight column density, between column density
and redshift, between column density and scattering fraction or between
scattering fraction and redshift. | astro-ph_HE |
Giant Primeval Magnetic Dipoles: Macroscopic magnetic dipoles are considered as cosmic dark matter. Permanent
magnetism in relativistic field structures can involve some form of
superconductivity, one example being current-carrying string loops (`springs')
with vanishing net tension. We derive the cross section for free classical
dipoles to collide, finding it depends weakly on orientation when mutual
precession is rapid. The collision rate of `spring' loops with tension ${\cal
T} \sim 10^{-8}c^4/G$ in galactic halos approaches the measured rate of fast
radio bursts (FRBs) if the loops comprise most of the dark matter. A large
superconducting dipole (LSD) with mass $\sim 10^{20}$ g and size $\sim 1$ mm
will form a $\sim 100$ km magnetosphere moving through interstellar plasma.
Although hydromagnetic drag is generally weak, it is strong enough to capture
some LSDs into long-lived rings orbiting supermassive black holes (SMBHs) that
form by the direct collapse of massive gas clouds. Repeated collisions near
young SMBHs could dominate the global collision rate, thereby broadening the
dipole mass spectrum. Colliding LSDs produce tiny, hot electromagnetic
explosions. The accompanying paper shows that these explosions couple
effectively to propagating low-frequency electromagnetic modes, with output
peaking at 0.01-1 THz. We describe several constraints on, and predictions of,
LSDs as cosmic dark matter. The shock formed by an infalling LSD triggers
self-sustained thermonuclear burning in a C/O (ONeMg) white dwarf (WD) of mass
$\gtrsim 1\,M_\odot$ ($1.3\,M_\odot$). The spark is generally located well off
the center of the WD. The rate of LSD-induced explosions matches the observed
rate of Type Ia supernovae. | astro-ph_HE |
Penetration of cosmic rays into dense molecular clouds: role of diffuse
envelope: A flux of cosmic rays (CRs) propagating through a diffuse ionized gas can
excite MHD waves, thus generating magnetic disturbances. We propose a generic
model of CR penetration into molecular clouds through their diffuse envelopes,
and identify the leading physical processes controlling their transport on the
way from a highly ionized interstellar medium to a dense interior of the cloud.
The model allows us to describe a transition between a free streaming of CRs
and their diffusive propagation, determined by the scattering on the
self-generated disturbances. A self consistent set of equations, governing the
diffusive transport regime in an envelope and the MHD turbulence generated by
the modulated CR flux, is essentially characterized by two dimensionless
numbers. We demonstrate a remarkable mutual complementarity of different
mechanisms leading to the onset of the diffusive regime, which results in a
universal energy spectrum of the modulated CRs. In conclusion, we briefly
discuss implications of our results for several fundamental astrophysical
problems, such as the spatial distribution of CRs in the Galaxy as well as the
ionization, heating, and chemistry in dense molecular clouds. | astro-ph_HE |
Testing the millisecond pulsar scenario of the Galactic center gamma-ray
excess with very high energy gamma-rays: The recent analyses of the Fermi Large Area Telescope data show an extended
GeV $\gamma$-ray excess on top of the expected diffuse background in the
Galactic center region, which can be explained with annihilating dark matter or
a population of millisecond pulsars (MSPs). We propose to observe the very high
energy $\gamma$-rays for distinguishing the MSP scenario from the dark matter
scenario. The GeV $\gamma$-ray MSPs should release most energy to the
relativistic $e^{\pm}$ wind, which will diffuse in the Galaxy and radiate TeV
$\gamma$-rays through inverse Compton scattering and bremsstrahlung processes.
By calculating the spectrum and spatial distribution, we show that such
emission is detectable with the next generation very high energy $\gamma$-ray
observatory, the Cherenkov Telescope Array (CTA), under reasonable model
parameters. It is essential to search for the multi-wavelength counterparts to
the GeV $\gamma$-ray excess for solving this mystery in the high energy
universe. | astro-ph_HE |
Study of general relativistic magnetohydrodynamic accretion flow around
black holes: We present a novel approach to study the global structure of steady,
axisymmetric, advective, geometrically thin, magnetohydrodynamic (MHD)
accretion flow around black holes in full general relativity (GR). Considering
ideal MHD conditions and relativistic equation of state (REoS), we solve the
governing equations to obtain all possible smooth global accretion solutions.
We examine the dynamical and thermodynamical properties of accreting matter in
terms of the flow parameters, namely energy (${\cal E}$), angular momentum
(${\cal L}$), and local magnetic fields. For a thin GRMHD flow, we observe that
toroidal component ($b^\phi$) of the magnetic fields generally dominates over
radial component ($b^r$) at the disk equatorial plane. This evidently suggests
that toroidal magnetic field indeed plays important role in regulating the disk
dynamics. We further notice that the disk remains mostly gas pressure ($p_{\rm
gas}$) dominated ($\beta = p_{\rm gas}/p_{\rm mag} > 1$, $p_{\rm mag}$ refers
magnetic pressure) except at the near horizon region, where magnetic fields
become dynamically important ($\beta \sim 1$). We observe that Maxwell stress
is developed that eventually yields angular momentum transport inside the disk.
Towards this, we calculate the viscosity parameter ($\alpha$) that appears to
be radially varying. In addition, we examine the underlying scaling relation
between $\alpha$ and $\beta$, which clearly distinguishes two domains coexisted
along the radial extent of the disk. Finally, we discuss the utility of the
present formalism in the realm of GRMHD simulation studies. | astro-ph_HE |
Supernovae Powered by Collapsar Accretion in Gamma-Ray Burst Sources: The association of long-duration gamma-ray bursts (LGRBs) with Type Ic
supernovae presents a challenge to supernova explosion models. In the collapsar
model for LGRBs, gamma rays are produced in an ultrarelativistic jet launching
from the magnetosphere of the black hole that forms in the aftermath of the
collapse of a rotating progenitor star. The jet is collimated along the star's
rotation axis, but the concomitant luminous supernova should be
relatively--though certainly not entirely--spherical, and should synthesize a
substantial mass of 56Ni. Our goal is to provide a qualitative assessment of
the possibility that accretion of the progenitor envelope onto the black hole,
which powers the LGRB, could also deposit sufficient energy and nickel mass in
the envelope to produce a luminous supernova. For this, the energy dissipated
near the black hole during accretion must be transported outward, where it can
drive a supernova-like shockwave. Here we suggest that the energy is
transported by convection and develop an analytical toy model, relying on
global mass and energy conservation, for the dynamics of stellar collapse. The
model suggests that a ~10,000 km/s shock can be driven into the envelope and
that ~10^51 erg explosions are possible. The efficiency with which the
accretion energy is being transferred to the envelope is governed by the
competition of advection and convection at distances ~100-1,000 km from the
black hole and is sensitive to the values of the convective mixing length, the
magnitude of the effective viscous stress, and the specific angular momentum of
the infalling envelope. Substantial masses of 56Ni may be synthesized in the
convective accretion flow over the course of tens of seconds from the initial
circularization of the infalling envelope around the black hole. The
synthesized nickel is convectively mixed with a much larger mass of unburned
ejecta. | astro-ph_HE |
Superbroad Component in Emission Lines of SS 433: We have detected new components in stationary emission lines of SS 433; these
are the superbroad components that are low-contrast substrates with a width of
2000--2500 km s-1 in He I $\lambda4922$ and H$\beta$ and 4000--5000 km s-1 in
He II $\lambda4686$. Based on 44 spectra taken during four years of
observations from 2003 to 2007, we have found that these components in the He
II and He I lines are eclipsed by the donor star; their behavior with
precessional and orbital phases is regular and similar to the behavior of the
optical brightness of SS 433. The same component in H$\beta$ shows neither
eclipses nor precessional variability. We conclude that the superbroad
components in the helium and hydrogen lines are different in origin. Electron
scattering is shown to reproduce well the superbroad component of H$\beta$ at a
gas temperature of 20--35 kK and an optical depth for Thomson scattering $\tau
\approx$ 0.25--0.35. The superbroad components of the helium lines are probably
formed in the wind from the supercritical accretion disk. We have computed a
wind model based on the concept of Shakura-Sunyaev supercritical disk
accretion. The main patterns of the He II line profiles are well reproduced in
this model: not only the appearance of the superbroad component but also the
evolution of the central two-component part of the profile of this line during
its eclipse by the donor star can be explained. | astro-ph_HE |
Statistics of X-ray flares of Sagittarius A*: evidence for solar-like
self-organized criticality phenomenon: X-ray flares have routinely been observed from the supermassive black hole,
Sagittarius A$^\star$ (Sgr A$^\star$), at our Galactic center. The nature of
these flares remains largely unclear, despite of many theoretical models. In
this paper, we study the statistical properties of the Sgr A$^\star$ X-ray
flares, by fitting the count rate (CR) distribution and the structure function
(SF) of the light curve with a Markov Chain Monte Carlo (MCMC) method. With the
3 million second \textit{Chandra} observations accumulated in the Sgr A$^\star$
X-ray Visionary Project, we construct the theoretical light curves through
Monte Carlo simulations. We find that the $2-8$ keV X-ray light curve can be
decomposed into a quiescent component with a constant count rate of
$\sim6\times10^{-3}~$count s$^{-1}$ and a flare component with a power-law
fluence distribution $dN/dE\propto E^{-\alpha_{\rm E}}$ with $\alpha_{\rm
E}=1.65\pm0.17$. The duration-fluence correlation can also be modelled as a
power-law $T\propto E^{\alpha_{\rm ET}}$ with $\alpha_{\rm ET} < 0.55$ ($95\%$
confidence). These statistical properties are consistent with the theoretical
prediction of the self-organized criticality (SOC) system with the spatial
dimension $S = 3$. We suggest that the X-ray flares represent plasmoid
ejections driven by magnetic reconnection (similar to solar flares) in the
accretion flow onto the black hole. | astro-ph_HE |
Measuring the delay time distribution of binary neutron stars. II. Using
the redshift distribution from third-generation gravitational wave detectors
network: We investigate the ability of current and third-generation gravitational wave
(GW) detectors to determine the delay time distribution (DTD) of binary neutron
stars (BNS) through a direct measurement of the BNS merger rate as a function
of redshift. We assume that the DTD follows a power law distribution with a
slope $\Gamma$ and a minimum merger time $t_{\rm min}$, and also allow the
overall BNS formation efficiency per unit stellar mass to vary. By convolving
the DTD and mass efficiency with the cosmic star formation history, and then
with the GW detector capabilities, we explore two relevant regimes. First, for
the current generation of GW detectors, which are only sensitive to the local
universe, but can lead to precise redshift determinations via the
identification of electromagnetic counterparts and host galaxies, we show that
the DTD parameters are strongly degenerate with the unknown mass efficiency and
therefore cannot be determined uniquely. Second, for third-generation detectors
such as Einstein Telescope (ET) and Cosmic Explorer (CE), which will detect BNS
mergers at cosmological distances, but with a redshift uncertainty inherent to
GW-only detections ($\delta(z)/z\approx 0.1z$), we show that the DTD and mass
efficiency can be well-constrained to better than 10\% with a year of
observations. This long-term approach to determining the DTD through a direct
mapping of the BNS merger redshift distribution will be supplemented by more
near term studies of the DTD through the properties of BNS merger host galaxies
at $z\approx 0$ (Safarzadeh & Berger 2019). | astro-ph_HE |
On the origin of supergiant fast X-ray transients: A fraction of high-mass X-ray binaries are supergiant fast X-ray transients.
These systems have on average low X-ray luminosities, but display short flares
during which their X-ray luminosity rises by a few orders of magnitude. The
leading model for the physics governing this X-ray behaviour suggests that the
winds of the donor OB supergiants are magnetized. In agreement with this model,
the first spectropolarimetric observations of the SFXT IGR J11215-5952 using
the FORS2 instrument at the Very Large Telescope indicate the presence of a kG
longitudinal magnetic field. Based on these results, it seems possible that the
key difference between supergiant fast X-ray transients and other high-mass
X-ray binaries are the properties of the supergiant's stellar wind and the
physics of the wind's interaction with the neutron star magnetosphere. | astro-ph_HE |
Cosmological effects on the observed flux and fluence distributions of
gamma-ray bursts: Several claims have been put forward that an essential fraction of
long-duration BATSE gamma-ray bursts should lie at redshifts larger than 5.
This point-of-view follows from the natural assumption that fainter objects
should, on average, lie at larger redshifts. However, redshifts larger than 5
are rare for bursts observed by Swift. The purpose of this article is to show
that the most distant bursts in general need not be the faintest ones. We
derive the cosmological relationships between the observed and emitted
quantities, and arrive at a prediction that is tested on the ensembles of
BATSE, Swift and Fermi bursts. This analysis is independent on the assumed
cosmology, on the observational biases, as well as on any gamma-ray burst
model. We arrive to the conclusion that apparently fainter bursts need not, in
general, lie at large redshifts. Such a behaviour is possible, when the
luminosities (or emitted energies) in a sample of bursts increase more than the
dimming of the observed values with redshift. In such a case dP(z)/dz > 0 can
hold, where P(z) is either the peak-flux or the fluence. This also means that
the hundreds of faint, long-duration BATSE bursts need not lie at high
redshifts, and that the observed redshift distribution of long Swift bursts
might actually represent the actual distribution. | astro-ph_HE |
Deep Neural Networks for Estimation of Gamma-Ray Burst Redshifts: While the available set of Gamma-ray Burst (GRB) data with known redshift is
currently limited, a much larger set of GRB data without redshift is available
from different instruments. This data includes well-measured prompt gamma-ray
flux and spectral information. We estimate the redshift of a selection of these
GRBs detected by Fermi-GBM and Konus-Wind using Machine Learning techniques
that are based on spectral parameters. We find that Deep Neural Networks with
Random Forest models employing non-linear relations among input parameters can
reasonably reproduce the pseudo-redshift distribution of GRBs, mimicking the
distribution of GRBs with spectroscopic redshift. Furthermore, we find that the
pseudo-redshift samples of GRBs satisfy (i) Amati relation between the peak
photon energy of the time-averaged energy spectrum in the cosmological rest
frame of the GRB ${E}_{\rm i, p}$ and the isotropic-equivalent radiated energy
${E}_{\rm iso}$ during the prompt phase; and (ii) Yonetoku relation between
${E}_{\rm i, p}$ and isotropic-equivalent luminosity ${L}_{\rm iso}$, both
measured during the peak flux interval. | astro-ph_HE |
Search for photon line-like signatures from Dark Matter annihilations
with H.E.S.S: Gamma-ray line signatures can be expected in the very-high-energy (VHE;
E_\gamma > 100 GeV) domain due to self-annihilation or decay of dark matter
(DM) particles in space. Such a signal would be readily distinguishable from
astrophysical \gamma-ray sources that in most cases produce continuous spectra
which span over several orders of magnitude in energy. Using data collected
with the H.E.S.S. \gamma-ray instrument, upper limits on line-like emission are
obtained in the energy range between ~500 GeV and ~25 TeV for the central part
of the Milky Way halo and for extragalactic observations, complementing recent
limits obtained with the Fermi-LAT instrument at lower energies. No
statistically significant signal could be found. For monochromatic \gamma-ray
line emission, flux limits of (2x10^-7 - 2x10^-5) m^-2 s^-1 sr^-1 and (1x10^-8
- 2x10^-6) m^-2 s^-1 sr^-1 are obtained for the central part of the Milky Way
halo and extragalactic observations, respectively. For a DM particle mass of 1
TeV, limits on the velocity-averaged DM annihilation cross section < \sigma v
>(\chi\chi -> \gamma\gamma) reach ~10^-27 cm^3 s^-1, based on the Einasto
parametrization of the Galactic DM halo density profile. | astro-ph_HE |
Evidence for two neutrinos bursts from SN1987A: The SN1987A in the Giant Magellanic Cloud was an amazing and extraordinary
event because it was detected in real time for different neutrinos experiments
($\nu$s) around the world. Approximate $\sim25$ events were observed in three
different experiments: Kamiokande II (KII) $\sim 12$,
Irvine-Michigan-Brookhaven (IMB) $\sim 8$ e Baksan $\sim 5$, plus a contrived
burst at Mont Blanc (Liquid Scintillator Detector - LSD) later dismissed
because of energetic requirements (Aglietta et al. 1988). The neutrinos have an
important play role into the neutron star newborn: at the moment when the
supernova explodes the compact object remnant is freezing by neutrinos
($\sim99\%$ energy is lost in the few seconds of the explosion). The work is
motivated by neutrinos' event in relation arrival times where there is a
temporal gap between set of events ($\sim6\mbox{s}$). The first part of dataset
came from the ordinary mechanism of freezing and the second part suggests
different mechanism of neutrinos production. We tested two models of cooling
for neutrinos from SN1987A: 1st an exponential cooling is an ordinary model of
cooling and 2nd a two-step temperature model that it considers two bursts
separated with temporal gap. Our analysis was done with Bayesian tools ({\it
Bayesian Information Criterion} - BIC) The result showed strong evidence in
favor of a two-step model against one single exponential cooling
($\ln\mbox{B}_{ij} > 5.0$), and suggests the existence of two neutrino bursts
at the moment the neutron star was born. | astro-ph_HE |
Combined analyses of the antiproton production from cosmic-ray
interactions and its possible dark matter origin: Recent cosmic-ray (CR) studies have claimed the possibility of an excess on
the antiproton flux over the predicted models at around $10$ GeV, which can be
the signature of dark matter annihilating into hadronic final states that
subsequently form antiprotons. However, this excess is subject to many
uncertainties related to the evaluation of the antiproton spectrum produced
from spallation interactions of CRs. In this work, we implement a combined
Markov-Chain Monte Carlo analysis of the secondary ratios of B, Be and Li and
the antiproton-to-proton ratio ($\bar{p}/p$), while also including nuisance
parameters to consider the uncertainties related to the spallation cross
sections. This study allows us to constrain the Galactic halo height and the
rest of propagation parameters, evaluate the impact of cross sections
uncertainties in the determination of the antiproton spectrum and test the
origin of the excess of antiprotons. In this way, we provide a set of
propagation parameters and scale factors for renormalizing the cross sections
parametrizations that allow us to reproduce all the ratios of B, Be, Li and
$\bar{p}$ simultaneously. We show that the energy dependence of the $\bar{p}/p$
ratio is compatible with a pure secondary origin. We find that the energy
dependence of the evaluated $\bar{p}/p$ spectrum matches the AMS-02 data at
energies above $\sim3$GeV, although there is still a nearly constant $\sim10\%$
excess of $\bar{p}$ over our prediction. We discuss that this discrepancy is
more likely explained from a $\sim10\%$ scaling in the cross sections of
antiproton production, rather than a component of dark matter leading to
antiprotons. In particular, we find that the best-fit WIMP mass ($\sim 300$
GeV) needed to explain the discrepancy lies above the constraints from most
indirect searches of dark matter and the resultant fit is poorer than with a
cross sections scaling. | astro-ph_HE |
Testing astrophysical models for the PAMELA positron excess with cosmic
ray nuclei: The excess in the positron fraction reported by the PAMELA collaboration has
been interpreted as due to annihilation or decay of dark matter in the Galaxy.
More prosaically, it has been ascribed to direct production of positrons by
nearby pulsars, or due to pion production during stochastic acceleration of
hadronic cosmic rays in nearby sources. We point out that measurements of
secondary nuclei produced by cosmic ray spallation can discriminate between
these possibilities. New data on the titanium-to-iron ratio from the ATIC-2
experiment support the hadronic source model above and enable a prediction to
be made for the boron-to-carbon ratio at energies above 100 GeV. Presently, all
cosmic ray data are consistent with the positron excess being astrophysical in
origin. | astro-ph_HE |
Construction of a Multidimensional Parallel Adaptive Mesh Refinement
Special Relativistic Hydrodynamics Code for Astrophysical Applications: We have developed a new computer code, RELDAFNA, to solve the conservative
equations of special relativistic hydrodynamics (SRHD) using adaptive mesh
refinement (AMR) on parallel computers. We have implemented a
characteristic-wise, finite volume Godunov scheme using the full characteristic
decomposition of the SRHD equations, to achieve second and third order accuracy
in space (both PLM and PPM reconstruction). For time integration, we use the
method of directional splitting with symmetrization, which is second order
accurate in time. We have also implemented second and third order Runge-Kutta
time integration scheme for comparison. In addition to the hydrodynamics
solvers we have implemented approximate Riemann solvers along with an exact
Riemann solver. We examine the ability of RELDAFNA to accurately simulate
special relativistic flows efficiently in number of processors, computer memory
and over all integration time. We show that a wide variety of test problems can
be solved as accurately as solved by higher order programs, such as RAM,
GENESIS, or PLUTO, but with a less number of variables kept in memory and
computer calculations than most schemes, an advantage which is crucial for 3D
high resolution simulations to be of practical use for scientific research in
computational astrophysics. RELDAFNA has been tested in one, two and three
dimensions and in Cartesian, cylindrical and spherical geometries. We present
the ability of RELDAFNA to assist with the understanding of open questions in
high energy astrophysics which involve relativistic flows. | astro-ph_HE |
Dwarf (Twin) Neutron Stars I: Did GW170817 Involve One?: Dwarf neutron stars are stable twins of neutron stars but with a maximum mass
less than that of neutron stars. Their existence brings into concordance the
seemingly conflicting information on the size of neutron stars inferred from
gravitational waves from GW170817, from the NICER mission, and from the PREX-II
experiment. Their distinctive characteristics lead to rich and falsifiable
predictions that are expected to be tested in the near future. If corroborated,
the existence of dwarf neutron stars would substantially improve our
understanding of the QCD phase diagram and offer valuable insights into the
dark sector. | astro-ph_HE |
Test of the superdiffusion model in the interstellar medium around the
Geminga pulsar: The TeV $\gamma$-ray halo around the Geminga pulsar is an important indicator
of cosmic-ray (CR) propagation in the local zone of the Galaxy as it reveals
the spatial distribution of the electrons and positrons escaping from the
pulsar. Considering the intricate magnetic field in the interstellar medium
(ISM), it is proposed that superdiffusion model could be more realistic to
describe the CR propagation than the commonly used normal diffusion model. In
this work, we test the superdiffusion model in the ISM around the Geminga
pulsar by fitting to the surface brightness profile of the Geminga halo
measured by HAWC. Our results show that the chi-square statistic monotonously
increases as $\alpha$ decreases from 2 to 1, where $\alpha$ is the
characteristic index of superdiffusion describing the degree of fractality of
the ISM and $\alpha=2$ corresponds to the normal diffusion model. We find that
model with $\alpha<1.32$ (or $<1.4$, depending on the data used in fit) is
disfavored at 95\% confidence level. Superdiffusion model with $\alpha$ close
to 2 can well explain the morphology of the Geminga halo, while it predicts
much higher positron flux on the Earth than the normal diffusion model. This
has important implication for the interpretation of the CR positron excess. | astro-ph_HE |
On the triple pulsar profiles generated by ordinary mode: A detailed study of the refraction of an ordinary wave in the magnetosphere
of radio pulsars was carried out. For this, a consistent theory of the
generation of secondary particles was constructed, which essentially takes into
account the dependence of the number density and the energy spectrum of
secondary particles on the distance from the magnetic axis. This made it
possible to determine with high accuracy the refraction of the ordinary O-mode
in the central region of the outflowing plasma, which makes it possible to
explain the central peak of three-humped mean radio profiles. As shown by
detailed numerical calculations, in most cases it is possible to reproduce
quite well the observed mean profiles of radio pulsars. | astro-ph_HE |
Are Superluminous Supernovae Powered By Collision Or By Millisecond
Magnetars?: Using our previously derived simple analytic expression for the bolometric
light curves of supernovae, we demonstrate that the collision of the fast
debris of ordinary supernova explosions with relatively slow-moving shells from
pre-supernova eruptions can produce the observed bolometric light curves of
superluminous supernovae (SLSNe) of all types. These include both, those which
can be explained as powered by spinning-down millisecond magnetars and those
which cannot. That and the observed close similarity between the bolometric
light-curves of SLSNe and ordinary interacting SNe suggest that SLSNe are
powered mainly by collisions with relatively slow moving circumstellar shells
from pre-supernova eruptions rather than by the spin-down of millisecond
magnetars born in core collapse supernova explosions. | astro-ph_HE |
Dark Matter Annihilation in the Universe: The astronomical dark matter is an essential component of the Universe and
yet its nature is still unresolved. It could be made of neutral and massive
elementary particles which are their own antimatter partners. These dark matter
species undergo mutual annihilations whose effects are briefly reviewed in this
article. Dark matter annihilation plays a key role at early times as it sets
the relic abundance of the particles once they have decoupled from the
primordial plasma. A weak annihilation cross section naturally leads to a
cosmological abundance in agreement with observations. Dark matter species
subsequently annihilate - or decay - during Big Bang nucleosynthesis and could
play havoc with the light element abundances unless they offer a possible
solution to the $^{7}$Li problem. They could also reionize the intergalactic
medium after recombination and leave visible imprints in the cosmic microwave
background. But one of the most exciting aspects of the question lies in the
possibility to indirectly detect the dark matter species through the rare
antimatter particles - antiprotons, positrons and anti-deuterons - which they
produce as they currently annihilate inside the galactic halo. Finally, the
effects of dark matter annihilation on stars is discussed. | astro-ph_HE |
Determining the Origin of Very-high-energy Gamma Rays from Galactic
Sources by Future Neutrino Observations: Recently, the Large High Altitude Air Shower Observatory (LHAASO) identified
12 $\gamma$-ray sources emitting gamma rays with energies above 100 TeV, making
them potential PeV cosmic-ray accelerators (PeVatrons). Neutrino observations
are crucial in determining whether the gamma-ray radiation process is of
hadronic or leptonic origin. In this paper, we study three detected sources,
LHAASO J1908+0621, LHAASO J2018+3651, and LHAASO J2032+4102, which are also the
most promising galactic high-energy neutrino candidate sources with the lowest
pre-trial p-value based on the stacking searches testing for excess neutrino
emission by IceCube Neutrino Observatory. We study the lepto-hadronic scenario
for the observed multiband spectra of these LHAASO sources considering the
possible counterpart source of the LHAASO sources. The very-high-energy gamma
rays are entirely attributed to the hadronic contribution, therefore the most
optimistic neutrino flux can be derived. Then, we evaluate the statistical
significance (p-value) as a function of the observation time of IceCube and the
next-generation IceCube-Gen2 neutrino observatory respectively. Our results
tend to disfavor that all gamma rays above $100\,\rm GeV$ from LHAASO
J1908+0621 are of purely hadronic origin based on current IceCube observations,
but the purely hadronic origin of gamma rays above $100\,\rm TeV$ is still
possible. By IceCube-Gen2, the origin of gamma rays above $100\,\rm TeV$ from
LHAASO J1908+0621 can be further determined at a $5\sigma$ significance level
within a running time of $\sim 3$ years. For LHAASO J2018+3651 and LHAASO
J2032+4102, the required running time of IceCube-Gen2 is $\sim 10$ years
($3\sigma$) and $\sim 10$ years ($5\sigma$), respectively. Future observations
by the next-generation neutrino telescope will be crucial to understanding the
particle acceleration and radiation processes inside the sources. | astro-ph_HE |
A multi-wavelength study of the gamma-ray binary candidate HESS
J1832-093: We investigate the nature of the unidentified very-high-energy (VHE)
gamma-ray object, HESS J1832-093, in a multi-wavelength context. Based on X-ray
variability and spectral index ($\Gamma_X\sim\,1.5$), and its broad-band
spectrum (which was remarkably similar to HESS J0632+057, a confirmed gamma-ray
binary), HESS J1832-093 has been considered to be a strong gamma-ray binary
candidate in previous works. In this work, we provide further evidence for this
scenario. We obtained a spectrum of its IR counterpart using Gemini/Flamingo,
finding absorption lines that are usually seen in massive stars, in particular
O stars. We also obtained a rather steep ATCA spectrum
($\alpha=-1.18^{+1.04}_{-0.88}$) which prefers a gamma-ray binary over an AGN
scenario. Based on spatial-spectral analysis and variability search, we found
that 4FGL J1832.9-0913 is possible to be associated with SNR G22.7-0.2 rather
than with HESS J1832-093 only. | astro-ph_HE |
Jets, Disks and Winds from Spinning Black Holes: Nature or Nurture?: A brief summary is given of an alternative interpretation of the Event
Horizon Telescope observations of the massive black hole in the nucleus of the
nearby galaxy M87. It is proposed that the flow is primarily powered by the
black hole rotation, not the release of gravitational energy by the infalling
gas. Consequently, the observed millimetre emission is produced by an
"ergomagnetosphere" that connects the black hole horizon to an "ejection disk"
from which most of the gas supplied at a remote "magnetopause" is lost through
a magnetocentrifugal wind. It is argued that the boundary conditions at high
latitude on the magnetopause play a crucial role in the collimation of the
relativistic jets. The application of these ideas to other types of source is
briefly discussed. | astro-ph_HE |
Detecting Kozai-Lidov imprints on the gravitational waves of
intermediate-mass black holes in galactic nuclei: A third object in the vicinity of a binary system causes variations in the
eccentricity and the inclination of the binary through the Kozai-Lidov effect.
We examine if such variations leave a detectable imprint on the gravitational
waves of a binary consisting of intermediate mass black holes and stellar mass
objects. As a proof of concept, we present an example where LISA may detect the
Kozai-Lidov modulated gravitational wave signals of such sources from at least
a distance of 1Mpc if the perturbation is caused by a supermassive black hole
tertiary. Although the quick pericenter precession induced by general
relativity significantly reduces the appropriate parameter space for this
effect by quenching the Kozai-Lidov oscillations, we still find reasonable
parameters where the Kozai-Lidov effect may be detected with high
signal-to-noise ratios. | astro-ph_HE |
Model on pulsed GeV radiation from magnetars: We discuss a possible scenario for radiation mechanism of pulsed GeV
gamma-rays from magnetars. The magnetars have shown frequent X-ray bursts,
which would be triggered by crust fractures and could release the energy of
order of ~10^{41-42}erg. If the location of the crust cracking of the magnetic
field is close to the magnetic pole, part of the released energy may excite the
Alfevn wave that can propagate into outer magnetosphere. The oscillation of the
magnetic field induces the available potential drop ~10^{15}Volts, which can
accelerate the electrons and/or positrons to the Lorentz factor ~10^{7} in the
outer magnetosphere. The curvature radiation process at outer magnetosphere can
produce GeV gamma-rays. If the radiation process is occurred above r~5x 10^7cm
from the stellar surface, the emitted GeV gamma-rays can escape from the
pair-creation process with the X-rays and/or the magnetic field. The expected
luminosity of the GeV emissions is order of L_{\gamma}< 10^{35} erg/s, and the
radiation process will last for a temporal scale of years. The expected pulse
profiles have a broad shape with sometimes sharp peaks. We apply the model to
AXP 1E~2259+586. | astro-ph_HE |
Cosmic Frontier Indirect Dark Matter Detection Working Group Summary: As part of the Snowmass process, the Cosmic Frontier Indirect-Detection
subgroup (CF2) has drawn on input from the Cosmic Frontier and the broader
Particle Physics community to produce this document. The purposes of this
report are to identify opportunities for dark matter science through indirect
detection, to give an overview of the primary scientific drivers for indirect
searches for dark matter, and to survey current and planned experiments that
have, as a large part of their scientific program, the goal of searching for
indirect (or astrophysical) signatures of dark matter. We primarily address
existing experiments with a large U.S. role, or future experiments where a U.S.
contribution is sought. We also address the limitations of this technique, and
answer the tough questions relevant to this subgroup posed by the HEP community
through the Snowmass process. | astro-ph_HE |
Stripped-envelope core-collapse supernova $^{56}$Ni masses: Persistently
larger values than supernovae type II: The mass of synthesised radioactive material is an important power source for
all supernova (SN) types. Anderson 2019 recently compiled literature values and
obtained $^{56}$Ni distributions for different core-collapse supernovae
(CC-SNe), showing that the $^{56}$Ni distribution of stripped envelope CC-SNe
(SE-SNe: types IIb, Ib, and Ic) is highly incompatible with that of hydrogen
rich type II SNe (SNe-II). This motivates questions on differences in
progenitors, explosion mechanisms, and $^{56}$Ni estimation methods. Here, we
re-estimate the nucleosynthetic yields of $^{56}$Ni for a well-observed and
well-defined sample of SE-SNe in a uniform manner. This allows us to
investigate whether the observed SN-II--SE-SN $^{56}$Ni separation is due to
real differences between these SN types, or because of systematic errors in the
estimation methods. We compiled a sample of well observed SE-SNe and measured
$^{56}$Ni masses through three different methods proposed in the literature.
Arnett's rule -as previously shown - gives $^{56}$Ni masses for SE-SNe that are
considerably higher than SNe-II. While for the distributions calculated using
both the Khatami&Kasen prescription and Tail $^{56}$Ni masses are offset to
lower values than `Arnett values', their $^{56}$Ni distributions are still
statistically higher than that of SNe II. Our results are strongly driven by a
lack of SE-SN with low $^{56}$Ni masses (that are in addition strictly lower
limits). The lowest SE-SN $^{56}$Ni mass in our sample is of 0.015M$_\odot$,
below which are more than 25$\%$ of SNe II. We conclude that there exists real,
intrinsic differences in the mass of synthesised radioactive material between
SNe II and SE-SNe . Any proposed current or future CCSN progenitor scenario and
explosion mechanism must be able to explain why and how such differences arise,
or outline a yet to be fully explored bias in current SN samples. | astro-ph_HE |
GRB jet structure and the jet break: We investigate the shape of the jet break in within-beam gamma-ray burst
(GRB) optical afterglows for various lateral jet structure profiles. We
consider cases with and without lateral spreading and a range of inclinations
within the jet core half-opening angle, $\theta_c$. We fit model and observed
afterglow lightcurves with a smoothly-broken power-law function with a
free-parameter $\kappa$ that describes the sharpness of the break. We find that
the jet break is sharper ($\kappa$ is greater) when lateral spreading is
included than in the absence of lateral spreading. For profiles with a
sharp-edged core, the sharpness parameter has a broad range of
$0.1\lesssim\kappa\lesssim4.6$, whereas profiles with a smooth-edged core have
a narrower range of $0.1\lesssim\kappa\lesssim2.2$ when models both with and
without lateral spreading are included. For sharp-edged jets, the jet break
sharpness depends strongly on the inclination of the system within $\theta_c$,
whereas for smooth-edged jets, $\kappa$ is more strongly dependent on the size
of $\theta_c$. Using a sample of 20 GRBs we find nine candidate smooth-edged
jet structures and eight candidate sharp-edged jet structures, while the
remaining three are consistent with either. The shape of the jet break, as
measured by the sharpness parameter $\kappa$, can be used as an initial check
for the presence of lateral structure in within-beam GRBs where the afterglow
is well-sampled at and around the jet-break time. | astro-ph_HE |
A 350-MHz Green Bank Telescope Survey of Unassociated Fermi LAT Sources:
Discovery and Timing of Ten Millisecond Pulsars: We have searched for radio pulsations towards 49 Fermi Large Area Telescope
(LAT) 1FGL Catalog $\gamma$-ray sources using the Green Bank Telescope at 350
MHz. We detected 18 millisecond pulsars (MSPs) in blind searches of the data;
10 of these were discoveries unique to our survey. Sixteen are binaries, with
eight having short orbital periods $P_B < 1$ day. No radio pulsations from
young pulsars were detected, although three targets are coincident with
apparently radio-quiet $\gamma$-ray pulsars discovered in LAT data. Here, we
give an overview of the survey and present radio and $\gamma$-ray timing
results for the 10 MSPs discovered. These include the only isolated MSP
discovered in our survey and six short-$P_B$ binary MSPs. Of these, three have
very low-mass companions ($M_c$ $\ll$ 0.1M$_{\odot}$) and hence belong to the
class of black widow pulsars. Two have more massive, non-degenerate companions
with extensive radio eclipses and orbitally modulated X-ray emission consistent
with the redback class. Significant $\gamma$-ray pulsations have been detected
from nine of the discoveries. This survey and similar efforts suggest that the
majority of Galactic $\gamma$-ray sources at high Galactic latitudes are either
MSPs or relatively nearby non-recycled pulsars, with the latter having on
average a much smaller radio/$\gamma$-ray beaming ratio as compared to MSPs. It
also confirms that past surveys suffered from an observational bias against
finding short-$P_B$ MSP systems. | astro-ph_HE |
High-energy emission of variable objects in the OMC--VAR catalogue: OMC-VAR is the first catalogue of variable sources observed by the Optical
Monitoring Camera (OMC) on board INTEGRAL. It includes photometry and
variability data for more than 5000 sources of very different nature. In order
to study the multi-wavelength similarities and differences of the engines
powering AGN and X-ray binaries, especially those powered by a black hole, we
have searched for counterparts in several high-energy and infrared catalogues,
including the 4th IBIS/ISGRI soft gamma-ray survey catalogue, the XMM-Newton
Serendipitous Source catalogue, the 2MASS All-Sky catalogue of Point Sources,
the 2MASS Extended Source Catalog and the WISE All-Sky Data Release.
Preliminary multiwavelength results for the IBIS counterparts are presented and
discussed. | astro-ph_HE |
Evidence for Astrophysical Muon Neutrinos from the Northern Sky with
IceCube: Results from the IceCube Neutrino Observatory have recently provided
compelling evidence for the existence of a high energy astrophysical neutrino
flux utilizing a dominantly Southern Hemisphere dataset consisting primarily of
nu_e and nu_tau charged current and neutral current (cascade) neutrino
interactions. In the analysis presented here, a data sample of approximately
35,000 muon neutrinos from the Northern sky was extracted from data taken
during 659.5 days of livetime recorded between May 2010 and May 2012. While
this sample is composed primarily of neutrinos produced by cosmic ray
interactions in the Earth's atmosphere, the highest energy events are
inconsistent with a hypothesis of solely terrestrial origin at 3.7 sigma
significance. These neutrinos can, however, be explained by an astrophysical
flux per neutrino flavor at a level of Phi(E_nu) = 9.9^{+3.9}_{-3.4} times
10^{-19} GeV^{-1} cm^{-2} sr^{-1} s^{-1} ({E_nu / 100 TeV})^{-2}, consistent
with IceCube's Southern Hemisphere dominated result. Additionally, a fit for an
astrophysical flux with an arbitrary spectral index was performed. We find a
spectral index of 2.2^{+0.2}_{-0.2}, which is also in good agreement with the
Southern Hemisphere result. | astro-ph_HE |
Absolute timing of the Crab pulsar with the INTEGRAL/SPI telescope: We have investigated the pulse shape evolution of the Crab pulsar emission in
the hard X-ray domain of the electromagnetic spectrum. In particular, we have
studied the alignment of the Crab pulsar phase profiles measured in the hard
X-rays and in other wavebands. To obtain the hard X-ray pulse profiles, we have
used six year (2003-2009, with a total exposure of about 4 Ms) of publicly
available data of the SPI telescope on-board of the INTEGRAL observatory,
folded with the pulsar time solution derived from the Jodrell Bank Crab Pulsar
Monthly Ephemeris. We found that the main pulse in the hard X-ray 20-100 keV
energy band is leading the radio one by $8.18\pm0.46$ milliperiods in phase, or
$275\pm15 \mu s$ in time. Quoted errors represent only statistical
uncertainties.Our systematic error is estimated to be $\sim 40 \mu s$ and is
mainly caused by the radio measurement uncertainties. In hard X-rays, the
average distance between the main pulse and interpulse on the phase plane is
$0.3989\pm0.0009$. To compare our findings in hard X-rays with the soft 2-20
keV X-ray band, we have used data of quasi-simultaneous Crab observations with
the PCA monitor on-board the Rossi X-Ray Timing Explorer (RXTE) mission. The
time lag and the pulses separation values measured in the 3-20 keV band are
$0.00933\pm0.00016$ (corresponding to $310\pm6 \mu s$) and $0.40016\pm0.00028$
parts of the cycle, respectively. While the pulse separation values measured in
soft X-rays and hard X-rays agree, the time lags are statistically different.
Additional analysis show that the delay between the radio and X-ray signals
varies with energy in the 2 - 300 keV energy range. We explain such a behaviour
as due to the superposition of two independent components responsible for the
Crab pulsed emission in this energy band. | astro-ph_HE |
The spin of the black hole GRS 1716-249 determined from the hard
intermediate state: We present three simultaneous/quasi-simultaneous NuSTAR and Swift datasets of
the black hole GRS 1716-249 in its hard intermediate state. The accretion disk
in this state may have reached the innermost stable circular orbit, and the
NuSTAR spectra show a broad relativistic iron line and a strong Compton hump.
To measure the black hole spin, we construct a joint model consisting of a
relativistic disk model kerrbb and a reflection model relxill, to fit the
continuum and the reflection components, respectively. By applying this model
to each dataset independently, a consistent result is obtained on the black
hole spin and the disk inclination. The black hole spin is a* >~ 0.92, and the
inclination angle (i) is around 40-50 degree, based on the measurements of all
datasets. In the third dataset, a high black hole mass (M_BH) is strongly
disfavored by the spectral fits. By unfreezing the black hole mass, we find
a*>0.92, i=49.9^{+1.0}_{-1.3} degree and M_BH<8.0 Msun, at a 90% confidence
level. Considering the lower limit derived from a previous optical constraint,
M_BH is in a range of 4.9-8.0 Msun. | astro-ph_HE |
Uncovering Red and Dusty Ultraluminous X-ray Sources with Spitzer: We present a mid-infrared (IR) sample study of nearby ultraluminous X-ray
sources (ULXs) using multi-epoch observations with the Infrared Array Camera
(IRAC) on the Spitzer Space Telescope. Spitzer/IRAC observations taken after
2014 were obtained as part of the Spitzer Infrared Intensive Transients Survey
(SPIRITS). Our sample includes 96 ULXs located within 10 Mpc. Of the 96~ULXs,
12 have candidate counterparts consistent with absolute mid-IR magnitudes of
supergiants, and 16 counterparts exceeded the mid-IR brightness of single
supergiants and are thus more consistent with star clusters or non-ULX
background active galactic nuclei (AGN). The supergiant candidate counterparts
exhibit a bi-modal color distribution in a Spitzer/IRAC color-magnitude
diagram, where "red" and "blue" ULXs fall in IRAC colors $[3.6] - [4.5]\sim0.7$
and $[3.6] - [4.5]\sim0.0$, respectively. The mid-IR colors and absolute
magnitudes of 4 "red" and 5 "blue" ULXs are consistent with that of supergiant
B[e] (sgB[e]) and red supergiant (RSG) stars, respectively. While "blue",
RSG-like mid-IR ULX counterparts likely host RSG mass donors, we propose the
"red" counterparts are ULXs exhibiting the "B[e] phenomenon'' rather than hosts
of sgB[e] mass donors. We show that the mid-IR excess from the "red" ULXs is
likely due to thermal emission from circumstellar or circumbinary dust. Using
dust as a probe for total mass, we estimate mass-loss rates of
$\dot{M}\sim1\times10^{-4}$ M$_\odot$ yr$^{-1}$ in dust-forming outflows of red
ULXs. Based on the transient mid-IR behavior and its relatively flat spectral
index, $\alpha=-0.19\pm0.1$, we suggest that the mid-IR emission from Holmberg
IX X-1 originates from a variable jet. | astro-ph_HE |
Gamma-Ray Burst Central Engines: Black Hole Vs. Magnetar: Discovered over forty years ago, Gamma-Ray Bursts (GRBs) remain a forefront
topic in modern astrophysics. Perhaps the most fundamental question associated
with GRBs is the nature of the astrophysical agent (or agents) that ultimately
powers them: the central engine. In this review, I focus on the possible
central engines of long-duration GRBs, and the constraints that present
observations place on these models. Long GRBs are definitively associated with
the deaths of massive stars, but whether the central engine is an accreting
black hole or a rapidly-spinning, highly-magnetized neutron star (a
"proto-magnetar") remains unsettled. This distinction has been brought into
particular focus by recent MHD simulations of the core-collapse of massive,
rotating "collapsar progenitors," which suggest that powerful
magneto-centrifugal outflows from the proto-neutron star may stave off black
hole formation entirely. Although both black hole and magnetar GRB models
remain viable, I argue that the magnetar model is more mature in the sense that
it provides quantitative explanations for the durations, energies, Lorentz
factors, and collimation of long GRB outflows. Given these virtues, one
promising strategy to break the present stalemate is to further develop the
magnetar model until inescapable (and falsifiable) predictions emerge. This
course of action signals a renewed challenge to translate time-dependent jet
properties (power, magnetization, and Lorentz factor) into observables
(gamma-ray light curves and spectra). | astro-ph_HE |
The fast radio burst population evolves, consistent with the
star-formation rate: Fast radio bursts (FRBs) are extremely powerful sources of radio waves
observed at cosmological distances. We use a sophisticated model of FRB
observations -- presented in detail in a companion paper -- to fit FRB
population parameters using large samples of FRBs detected by ASKAP and Parkes,
including seven sources with confirmed host galaxies. Our fitted parameters
demonstrate that the FRB population evolves with redshift in a manner
consistent with, or faster than, the star-formation rate (SFR), ruling out a
non-evolving population at 99.9\% C.L. Our estimated maximum FRB energy is
$\log_{10} E_{\rm max} [{\rm erg}] = 41.84_{-0.18}^{+0.49}$ (68\% C.L.)
assuming a 1\,GHz emission bandwidth, with slope of the cumulative luminosity
distribution $\gamma=-1.16_{-0.12}^{+0.11}$. We find a log-mean host DM
contribution of $145_{-60}^{+64}$\,pc\,cm$^{-3}$ on top of a typical local (ISM
and halo) contribution of $\sim80$\,pc\,cm$^{-3}$, which is higher than most
literature values. These results are consistent with the model of FRBs arising
as the high-energy limit of magnetar bursts, but allow for FRB progenitors that
evolve faster than the SFR. | astro-ph_HE |
Anomalous bremsstrahlung and the structure of cosmic ray
electron-positron fluxes at the GeV-TeV energy range: We reveal that the energy spectra of electrons-positrons in primary cosmic
rays measured at atmosphere top have double structures: an excess component
$\Phi^s_{e^+}(E)=\Phi^s_{e^-}(E)$ around $400 GeV$, which origins from a strong
$e^+e^-$-source and the distorted background $\Phi^0_{e^-}(E)$. We supposed
that the difference between AMS-CALET and Fermi-LAT-DAMPE data origins from the
energy loss of the fluxes due to the anomalous bremsstrahlung effect at a
special window. The evolution of spectra under anomalous bremsstrahlung effect
satisfies an improved electromagnetic cascade equation. The above spectra are
parameterized and they can be regarded as the subjects exploring new physics.
We suggest to check the previous applications of the Bethe-Heitler formula in
the study of the propagation of high energy electrons and photons. | astro-ph_HE |
Magnetic field reversal in the turbulent environment around a repeating
fast radio burst: Fast radio bursts (FRBs) are brief, intense flashes of radio waves from
unidentified extragalactic sources. Polarized FRBs originate in highly
magnetized environments. We report observations of the repeating FRB 20190520B
spanning seventeen months , which show its amount of Faraday rotation is highly
variable and twice changes its sign. The FRB also depolarizes below radio
frequencies around 1 to 3 GHz. We interpret these properties as due to change
in the parallel component of the integrated magnetic field along the
line-of-sight, including reversals. This could result from propagation through
a turbulent, magnetized screen of plasma located between $10^{-5}$ to 100
parsecs of the FRB source. This is consistent with the bursts passing through
the stellar wind of a binary companion of the FRB source. | astro-ph_HE |
Non-linear diffusive shock acceleration: A recipe for injection of
electrons: Prescriptions for electron injection into the diffusive shock acceleration
process are required in many practical considerations of cosmic-ray
astrophysics, particularly in modeling of the synchrotron emission of
astrophysical sources. In particle-in-cell simulations of quasi-parallel
magnetized collisionless shocks, we analyse the evolution of particle spectra.
We find that in the later stages of shock evolution, the initially strong
suprathermal part in the ion spectra fades, thus leaving the spectra composed
of a Maxwellian and a power law. Once the electron and ion spectra flatten and
become parallel, we find that the amounts of cosmic ray ions and electrons
become similar. We make the step towards relating the micro and macro-scale
physics by applying this injection rule to Blasi's semi-analytical model of
non-linear diffusive shock acceleration, in order to obtain the particle
spectra and electron-to-proton ratio $K_{\mathrm{ep}}$ at high energies. By
using shock jump conditions that include the electron heating, we find
$K_{\mathrm{ep}}$ as a function of Mach number. For Mach number $\sim$ 100, our
model finely reproduces the typically observed ratio for Galactic cosmic-rays
$K_{\mathrm{ep}} \sim$ 1:100 in the test particle regime. | astro-ph_HE |
3D simulations of microquasar jets in clumpy stellar winds: High-mass microquasars consist of a massive star and a compact object, the
latter producing jets that will interact with the stellar wind. The evolution
of the jets, and ultimately their radiative outcome, could depend strongly on
the inhomogeneity of the wind, which calls for a detailed study. The
hydrodynamics of the interaction between a jet and a clumpy wind is studied,
focusing on the global wind and single clump-jet interplay. We have performed,
using the code \textit{Ratpenat}, three-dimensional numerical simulations of a
clumpy wind interacting with a mildly relativistic jet, and of individual
clumps penetrating into a jet. For typical wind and jet velocities, filling
factors of about > 0.1 are already enough for the wind to be considered as
clumpy. An inhomogeneous wind makes the jet more unstable when crossing the
system. Kinetic luminosities of the order 1.e37 erg/s allow the jet to reach
the borders of a compact binary with an O star, as in the smooth wind case,
although with a substantially higher degree of disruption. When able to enter
into the jet, clumps are compressed and heated during a time of about their
size divided by the sound speed in the shocked clump. Then, clumps quickly
disrupt, mass-loading and slowing down the jet. We conclude that moderate wind
clumpiness makes already a strong difference with the homogeneous wind case,
enhancing jet disruption, mass-loading, bending, and likely energy dissipation
in the form of emission. All this can have observational consequences at
high-energies and also in the large scale radio jets. | astro-ph_HE |
XMM-Newton and Chandra observations of the unidentified Fermi-LAT source
3FGL J1016.5-6034: A Young Pulsar with a Nebula?: We report the discovery of a bright X-ray source in the XMM-Newton and
Chandra X-ray Observatory (CXO) images of the unidentified Fermi-LAT source
3FGL J1016.5-6034. The XMM-Newton spectrum of the source is well fit by an
absorbed blackbody+power-law model with a temperature, $kT=0.20\pm0.02$ keV,
and photon index $\Gamma=1.8\pm0.1$. The CXO resolves the same source into a
point source (CXOU J101546.0-602939) and a surrounding compact nebula seen up
to about $30''$ from the point source. The CXO spectrum of the nebula can be
described by an absorbed power-law with $\Gamma=1.7\pm0.3$ and is partly
responsible for the non-thermal emission observed in the XMM-Newton spectrum.
The XMM-Newton images also reveal faint extended emission on arcminute scales.
These properties strongly suggest that the X-ray source and the accompanying
extended emission are a newly discovered young pulsar with a pulsar wind
nebula. We also analyze $\sim10$ years of Fermi-LAT data and find that the
improved LAT source localization is consistent with the position of CXOU
J101546.0-602939. | astro-ph_HE |
Have Cherenkov telescopes detected a new light boson?: Recent observations by H.E.S.S. and MAGIC strongly suggest that the Universe
is more transparent to very-high-energy gamma rays than previously thought. We
show that this fact can be reconciled with standard blazar emission models
provided that photon oscillations into a very light Axion-Like Particle occur
in extragalactic magnetic fields. A quantitative estimate of this effect indeed
explains the observed data and in particular the spectrum of blazar 3C279. | astro-ph_HE |
Nonparametric Representation of Neutron Star Equation of State Using
Variational Autoencoder: We introduce a new nonparametric representation of the neutron star (NS)
equation of state (EoS) by using the variational autoencoder (VAE). As a deep
neural network, the VAE is frequently used for dimensionality reduction since
it can compress input data to a low-dimensional latent space using the encoder
component and then reconstruct the data using the decoder component. Once a VAE
is trained, one can take the decoder of the VAE as a generator. We employ
100,000 EoSs that are generated using the nonparametric representation method
based on \citet{2021ApJ...919...11H} as the training set and try different
settings of the neural network, then we get an EoS generator (trained VAE's
decoder) with four parameters. We use the mass\textendash{}tidal-deformability
data of binary neutron star (BNS) merger event GW170817, the
mass\textendash{}radius data of PSR J0030+0451, PSR J0740+6620, PSR J0437-4715,
and 4U 1702-429, and the nuclear constraints to perform the joint Bayesian
inference. The overall results of the analysis that includes all the
observations are $R_{1.4}=12.59^{+0.36}_{-0.42}\,\rm km$,
$\Lambda_{1.4}=489^{+114}_{-110}$, and $M_{\rm max}=2.20^{+0.37}_{-0.19}\,\rm
M_\odot$ ($90\%$ credible levels), where $R_{1.4}$/$\Lambda_{1.4}$ are the
radius/tidal-deformability of a canonical $1.4\,\rm M_\odot$ NS, and $M_{\rm
max}$ is the maximum mass of a non-rotating NS. The results indicate that the
implementation of the VAE techniques can obtain the reasonable results, while
accelerate calculation by a factor of $\sim$ 3\textendash10 or more, compared
with the original method. | astro-ph_HE |
Brightest Fermi-LAT Flares of PKS 1222+216: Implications on Emission and
Acceleration Processes: We present a high time resolution study of the two brightest $\gamma$-ray
outbursts from a blazar PKS 1222+216 observed by the \textit{Fermi} Large Area
Telescope (LAT) in 2010. The $\gamma$-ray light-curves obtained in four
different energy bands: 0.1--3, 0.1--0.3, 0.3--1 and 1--3 GeV, with time bin of
6 hr, show asymmetric profiles with a similar rise time in all the bands but a
rapid decline during the April flare and a gradual one during the June. The
light-curves during the April flare show $\sim 2$ days long plateau in 0.1--0.3
GeV emission, erratic variations in 0.3--1 GeV emission, and a daily recurring
feature in 1--3 GeV emission until the rapid rise and decline within a day. The
June flare shows a monotonic rise until the peak, followed by a gradual decline
powered mainly by the multi-peak 0.1--0.3 GeV emission. The peak fluxes during
both the flares are similar except in the 1--3 GeV band in April which is twice
the corresponding flux during the June flare. Hardness ratios during the April
flare indicate spectral hardening in the rising phase followed by softening
during the decay. We attribute this behavior to the development of a shock
associated with an increase in acceleration efficiency followed by its decay
leading to spectral softening. The June flare suggests hardening during the
rise followed by a complicated energy dependent behavior during the decay.
Observed features during the June flare favor multiple emission regions while
the overall flaring episode can be related to jet dynamics. | astro-ph_HE |
Mechanism of Outflows in Accretion System: Advective Cooling Cannot
Balance Viscous Heating?: Based on no-outflow assumption, we investigate steady state, axisymmetric,
optically thin accretion flows in spherical coordinates. By comparing the
vertically integrated advective cooling rate with the viscous heating rate, we
find that the former is generally less than 30% of the latter, which indicates
that the advective cooling itself cannot balance the viscous heating. As a
consequence, for radiatively inefficient flows with low accretion rates such as
$\dot M \la 10^{-3} \dot M_{Edd}$, where $\dot M_{Edd}$ is the Eddington
accretion rate, the viscous heating rate will be larger than the sum of the
advective cooling rate and the radiative cooling one. Thus, no thermal
equilibrium can be established under the no-outflow assumption. We therefore
argue that in such case outflows ought to occur and take away more than 70% of
the thermal energy generated by viscous dissipation. Similarly, for optically
thick flows with extremely large accretion rates such as $\dot M \ga 10 \dot
M_{Edd}$, outflows should also occur due to the limited advection and the low
efficiency of radiative cooling. Our results may help to understand the
mechanism of outflows found in observations and numerical simulations. | 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 |
Search for correlations between the arrival directions of IceCube
neutrino events and ultrahigh-energy cosmic rays detected by the Pierre Auger
Observatory and the Telescope Array: This paper presents the results of different searches for correlations
between very high-energy neutrino candidates detected by IceCube and the
highest-energy cosmic rays measured by the Pierre Auger Observatory and the
Telescope Array. We first consider samples of cascade neutrino events and of
high-energy neutrino-induced muon tracks, which provided evidence for a
neutrino flux of astrophysical origin, and study their cross-correlation with
the ultrahigh-energy cosmic ray (UHECR) samples as a function of angular
separation. We also study their possible directional correlations using a
likelihood method stacking the neutrino arrival directions and adopting
different assumptions on the size of the UHECR magnetic deflections. Finally,
we perform another likelihood analysis stacking the UHECR directions and using
a sample of through-going muon tracks optimized for neutrino point-source
searches with sub-degree angular resolution. No indications of correlations at
discovery level are obtained for any of the searches performed. The smallest of
the p-values comes from the search for correlation between UHECRs with IceCube
high-energy cascades, a result that should continue to be monitored. | astro-ph_HE |
A Deep Observation of Gamma-ray Emission from Cassiopeia A using VERITAS: Supernova remnants (SNRs) have long been considered the leading candidates
for the accelerators of cosmic rays within the Galaxy through the process of
diffusive shock acceleration. The connection between SNRs and cosmic rays is
supported by the detection of high energy (HE; 100 MeV to 100 GeV) and very
high energy (VHE; 100 GeV to 100 TeV) gamma rays from young and middle-aged
SNRs. However, the interpretation of the gamma-ray observations is not unique.
This is because gamma rays can be produced both by electrons through
non-thermal Bremsstrahlung and inverse Compton scattering, and by protons
through proton-proton collisions and subsequent neutral-pion decay. To
disentangle and quantify the contributions of electrons and protons to the
gamma-ray flux, it is necessary to measure precisely the spectra and morphology
of SNRs over a broad range of gamma-ray energies. Cassiopeia A (Cas A) is one
such young SNR (~ 350 years) which is bright in radio and X-rays. It has been
detected as a bright point source in HE gamma rays by Fermi-LAT and in VHE
gamma rays by HEGRA, MAGIC and VERITAS. Cas A has been observed with VERITAS
for more than 60 hours, tripling the published exposure. The observations span
2007-2013, and half of the data were taken at large zenith angles to boost the
effective area above few TeV. We will present the detailed spectral and
morphological results from the complete dataset. | astro-ph_HE |
Gamma-ray Observation of the Cygnus Region in the 100 TeV Energy Region: We report observations of gamma-ray emissions with energies in the 100 TeV
energy region from the Cygnus region in our Galaxy. Two sources are
significantly detected in the directions of the Cygnus OB1 and OB2
associations. Based on their positional coincidences, we associate one with a
pulsar PSR J2032+4127 and the other mainly with a pulsar wind nebula PWN
G75.2+0.1 with the pulsar moving away from its original birthplace situated
around the centroid of the observed gamma-ray emission. This work would
stimulate further studies of particle acceleration mechanisms at these
gamma-ray sources. | astro-ph_HE |
Dead zone in the polar-cap accelerator of pulsars: We study plasma flows above pulsar polar caps using time-dependent
simulations of plasma particles in the self-consistent electric field. The flow
behavior is controlled by the dimensionless parameter alpha=(j/c rho_GJ) where
j is the electric current density and rho_GJ is the Goldreich-Julian charge
density. The region of the polar cap where 0<alpha<1 is a "dead zone" --- in
this zone particle acceleration is inefficient and pair creation is not
expected even for young, rapidly rotating pulsars. Pulsars with polar caps near
the rotation axis are predicted to have a hollow-cone structure of radio
emission, as the dead zone occupies the central part of the polar cap. Our
results apply to charge-separated flows of electrons (j<0) or ions (j>0). In
the latter case, we consider the possibility of a mixed flow consisting of
different ion species, and observe the development of two-stream instability.
The dead zone at the polar cap is essential for the development of an outer gap
near the null surface rho_GJ=0. | astro-ph_HE |
Towards an Understanding of Changing-Look Quasars: An Archival
Spectroscopic Search in SDSS: The uncertain origin of the recently-discovered `changing-looking' quasar
phenomenon -- in which a luminous quasar dims significantly to a quiescent
state in repeat spectroscopy over ~10 year timescales -- may present unexpected
challenges to our understanding of quasar accretion. To better understand this
phenomenon, we take a first step to building a sample of changing-look quasars
with a systematic but simple archival search for these objects in the Sloan
Digital Sky Survey Data Release 12. By leveraging the >10 year baselines for
objects with repeat spectroscopy, we uncover two new changing-look quasars, and
a third discovered previously. Decomposition of the multi-epoch spectra and
analysis of the broad emission lines suggest that the quasar accretion disk
emission dims due to rapidly decreasing accretion rates (by factors of >2.5),
while disfavoring changes in intrinsic dust extinction for the two objects
where these analyses are possible. Broad emission line energetics also support
intrinsic dimming of quasar emission as the origin for this phenomenon rather
than transient tidal disruption events or supernovae. Although our search
criteria included quasars at all redshifts and transitions from either
quasar-like to galaxy-like states or the reverse, all of the clear cases of
changing-look quasars discovered were at relatively low-redshift (z ~ 0.2 -
0.3) and only exhibit quasar-like to galaxy-like transitions. | astro-ph_HE |
Linear Analysis of Fast-Pairwise Collective Neutrino Oscillations in
Core-Collapse Supernovae based on the Results of Boltzmann Simulations: Neutrinos are densely populated deep inside the core of massive stars after
their gravitational collapse to produce supernova explosions and form compact
stars such as neutron stars (NS) and black holes (BH). It has been considered
that they may change their flavor identities through so-called fast-pairwise
conversions induced by mutual forward scatterings. If that is really the case,
the dynamics of supernova explosion will be influenced, since the conversion
may occur near the neutrino sphere, from which neutrinos are effectively
emitted. In this paper, we conduct a pilot study of such possibilities based on
the results of fully self-consistent, realistic simulations of a core-collapse
supernova explosion in two spatial dimensions under axisymmetry. As we solved
the Boltzmann equations for neutrino transfer in the simulation not as a
post-process but in real time, the angular distributions of neutrinos in
momentum space for all points in the core at all times are available, a
distinct feature of our simulations. We employ some of these distributions
extracted at a few selected points and times from the numerical data and apply
linear analysis to assess the possibility of the conversion. We focus on the
vicinity of the neutrino sphere, where different species of neutrinos move in
different directions and have different angular distributions as a result. This
is a pilot study for a more thorough survey that will follow soon. We find no
positive sign of conversion unfortunately at least for the spatial points and
times we studied in this particular model. We hence investigate rather in
detail the condition for the conversion by modifying the neutrino distributions
rather arbitrarily by hand. | astro-ph_HE |
Constraining the size of the Comptonizing medium by modeling the energy
dependent time-lags of kHz QPOs of Neutron star system: In earlier works, we had shown that the observed soft lags and r.m.s versus
energy of the lower kHz QPO of neutron star binaries can be explained in the
framework of a thermal Comptonization model. It was also shown that such an
interpretation can provide estimates of the size and geometry of the
Comptonizing medium. Here we study the dependence of these estimates on the
time-averaged spectral model assumed and on the frequency of the QPO. We use
the high quality time lag and r.m.s obtained during March 3rd 1996 observation
of 4U 1608-52 by RXTE as well as other observations of the source at different
QPO frequencies where a single time-lag between two broad energy bands have
been reported. We compare the results obtained when assuming that the
time-averaged spectra are represented by the spectrally degenerate "hot" and
"cold" seed photon spectral models. We find that for the "hot" seed photon
model the medium size is in the range of 0.3-2.0 kms and the size decreases
with increasing QPO frequency. On the other hand for the "cold" seed photon
model the range for the sizes are much larger 0.5-20 kms and hence perhaps show
no variation with QPO frequency. Our results emphasis the need for broad band
spectral information combined with high frequency timing to lift this
degeneracy. We further show that the r.m.s as a function of energy for the
upper kHz QPO is similar to the lower one and indeed we find that the driver
for this QPO should be temperature variations of the corona identical to the
lower kHz QPO. However, the time lag reported for the upper kHz QPO is hard,
which if confirmed, would challenge the simple Comptonization model presented
here. It would perhaps imply that reverberation lags are also important and/or
the dominate spectral component is not a single temperature medium but a
multi-temperature complex one. | astro-ph_HE |
Chasing candidate Supergiant Fast X-ray Transients in the 1,000 orbits
INTEGRAL/IBIS catalog: We report results from an investigation at hard X-rays (above 18 keV) and
soft X-rays (below 10 keV) of a sample of X-ray transients located on the
Galactic plane and detected with the bursticity method, as reported in the
latest 1,000 orbits INTEGRAL/IBIS catalog. Our main aim has been to individuate
those with X-rays characteristics strongly resembling Supergiant Fast X-ray
Transients (SFXTs). As a result, we found four unidentified fast X-ray
transients which now can be considered good SFXT candidates. In particular,
three transients (IGR J16374-5043, IGR J17375-3022 and IGR J12341-6143) were
very poorly studied in the literature before the current work, and our findings
largely improved the knowledge of their X-ray characteristics. The other
transient (XTE J1829-098) was previously studied in detail only below 10 keV,
conversely the current work provides the first detailed study in outburst above
18 keV. In addition we used archival infrared observations of the transients to
pinpoint, among the field objects, their best candidate counterpart. We found
that their photometric properties are compatible with an early type spectral
classification, further supporting our proposed nature of SFXTs. Infrared
spectroscopy is advised to confirm or disprove our interpretation. The reported
findings allowed a significant increase of the sample of candidate SFXTs known
to date, effectively doubling their number. | astro-ph_HE |
Infrared study of H1743-322 in outburst: a radio-quiet and NIR-dim
microquasar: The X-ray binary, black hole candidate, and microquasar H1743-322 exhibited a
series of X-ray outbursts between 2003 and 2008. We took optical/infrared (OIR)
observations with the ESO/NTT telescope during 3 of these outbursts (2003,
2004, and 2008), to study its spectral energy distribution (SED).
We detect rapid flares of duration ~5 mn in the high time-resolution IR
lightcurve. We identify H and He emission lines in the IR spectra, coming from
the accretion disk. The IR SED exhibits the spectral index typically associated
with the X-ray high, soft state in our observations taken during the 2003 and
2004 outbursts, while the index changes to one that is typical of the X-ray
low, hard state during the 2008 outburst. During this last outburst, we
detected a change of slope in the NIR spectrum between the J and Ks bands,
where the JH part is characteristic of an optically thick disk emission, while
the HKs part is typical of optically thin synchrotron emission. Furthermore,
the comparison of our IR data with radio and X-ray data shows that H1743-322
exhibits a faint jet both in radio and NIR domains. Finally, we suggest that
the companion star is a late-type main sequence star located in the Galactic
bulge.
These OIR photometric and spectroscopic observations of the microquasar
H1743-322, the first of this source to be published in a broad multiwavelength
context, allow us to unambiguously identify two spectra of different origins in
the OIR domain, evolving from optically thick thermal emission to optically
thin synchrotron emission toward longer wavelengths. Comparing these OIR
observations with other black hole candidates suggests that H1743-322 behaves
like a radio-quiet and NIR-dim black hole in the low, hard state. This study
will be useful when quantitatively comparing the overall contribution of the
compact jet and accretion flow in the energy budget of microquasars. | astro-ph_HE |
Simulations of coalescing black holes: We describe the methods and results of numerical simulations of coalescing
black holes. The simulation in dynamical spacetime covers the inspiral, merger,
and ringdown phases. We analyze the emission of gravitational waves and
properties of a black hole being the merger product. We discuss the results in
the context of astrophysical environment of black holes that exist in the
Universe. | astro-ph_HE |
Massive BH Binaries as Periodically-Variable AGN: Massive black-hole (MBH) binaries, which are expected to form following the
merger of their parent galaxies, produce gravitational waves which will be
detectable by Pulsar Timing Arrays at nanohertz frequencies (year periods).
While no confirmed, compact MBH binary systems have been seen in
electromagnetic observations, a large number of candidates have recently been
identified in optical surveys of AGN variability. Using a combination of
cosmological, hydrodynamic simulations; comprehensive, semi-analytic binary
merger models; and analytic AGN spectra and variability prescriptions; we
calculate the expected electromagnetic detection rates of MBH binaries as
periodically variable AGN. In particular, we consider two independent
variability models: (i) Doppler boosting due to large orbital velocities, and
(ii) hydrodynamic variability in which the fueling of MBH accretion disks is
periodically modulated by the companion. Our models predict that numerous MBH
binaries should be present and distinguishable in the existing data. In
particular, our fiducial models produce an expectation value of $0.2$ (Doppler)
and $5$ (hydrodynamic) binaries to be identifiable in CRTS, while $20$ and
$100$ are expected after five years of LSST observations. The brightness
variations in most systems are too small to be distinguishable, but almost
$1\%$ of AGN at redshifts $z \lesssim 0.6$ could be in massive binaries. We
analyze the predicted binary parameters of observable systems and their
selection biases, and include an extensive discussion of our model parameters
and uncertainties. | astro-ph_HE |
Evolution of Accretion Disc Geometry of GRS~1915+105 during its $χ$
state as revealed by TCAF solution: The evolution of the C-type low frequency quasi-periodic oscillations
(LFQPOs) and associated time lag in transient black hole sources as a function
of time can be explained by variation of the Compton cloud size in a Two
Component Advective Flow solution (TCAF). A similar study of a persistent
source, GRS~1915+105, has not been attempted. We fit the evolution of QPOs with
propagatory oscillating shock (POS) solution for two sets of so-called
$\chi$-state observations and find that the shock steadily recedes with almost
constant velocity when QPO frequency is decreasing and the spectrum is
hardening. The shock moves inward with a constant velocity $v_0=473.0$ cm
s$^{-1}$ and $v_0=400.0$ cm s$^{-1}$ respectively in these two cases, when the
QPO frequency is increasing and the spectrum softens. This behavior is similar
to what was observed in XTE~J1550-564 during the 1998 outburst. The time lag
measured at the QPO frequency varies in a similar way as the size of the
Compton cloud. Most interestingly, in both the cases, the lag switches sign
(hard lag to soft lag) at a QPO frequency of $\sim 2.3 - 2.5$ Hz irrespective
of the energy of photons. We find, at very low frequencies $< 1$ Hz, the
Comptonizing Efficiency (CE) increases with QPO frequency and at higher QPO
frequencies the trend is opposite. The time lags become mostly positive at all
energies when CE is larger than $\sim 0.85\%$ for both the sources. | astro-ph_HE |
Potential for Precision Measurement of Low-Energy Antiprotons with GAPS
for Dark Matter and Primordial Black Hole Physics: The general antiparticle spectrometer (GAPS) experiment is a proposed
indirect dark matter search focusing on antiparticles produced by WIMP (weakly
interacting massive particle) annihilation and decay in the Galactic halo. In
addition to the very powerful search channel provided by antideuterons, GAPS
has a strong capability to measure low-energy antiprotons (0.07 $\le$ E $\le$
0.25 GeV) as dark matter signatures. This is an especially effective means for
probing light dark matter, whose existence has been hinted at in the direct
dark matter searches, including the recent result from the CDMS-II experiment.
While severely constrained by LUX and other direct dark matter searches, light
dark matter candidates are still viable in an isospin-violating dark matter
scenario and halo-independent analysis. Along with the excellent antideuteron
sensitivity, GAPS will be able to detect an order of magnitude more low-energy
antiprotons, compared to BESS, PAMELA and AMS-02, providing a precision
measurement of low-energy antiproton flux and a unique channel for probing
light dark matter models. Additionally, dark matter signatures from gravitinos
and Kaluza-Klein right-handed neutrinos as well as evidence of primordial black
hole evaporation can be observed through low-energy antiproton search. | astro-ph_HE |
Effects of nuclear matter and composition in core-collapse supernovae
and long-term proto-neutron star cooling: We study the influence of hot and dense matter in core-collapse supernovae by
adopting up-to-date nuclear equation of state (EOS) based on the microscopic
nuclear many-body frameworks. We explore effects of EOS based on the Dirac
Brueckner Hartree-Fock theory through comparisons with those based on the
variational method. We also examine effects of the differences in the
composition of nuclei and nucleons by using the same EOS by the variational
method but employing two different treatments in computations of nuclear
abundances. We perform numerical simulations of core-collapse supernovae
adopting the three EOSs. We also perform numerical simulations of the long-term
evolution over 70 s of the proto-neutron star cooling. We show that impacts by
different modeling of composition are remarkable as in those by different
treatments of uniform matter in the gravitational collapse, bounce, and shock
propagation. The cooling of proto-neutron star and the resulting neutrino
emission are also affected by the compositional difference even if the same
treatment in computing uniform matter of EOS. | astro-ph_HE |
Phase-Resolved Spectroscopy of the Low-Mass X-ray Binary V801 Ara: We present phase-resolved optical spectra of the low mass X-ray binary system
V801 Ara. The spectra, obtained in 2014 with IMACS on the Magellan/Baade
telescope at Las Campanas Observatory, cover the full binary orbit of 3.8
hours. They contain strong emission features allowing us to map the emission of
HAlpha, HBeta, He II 4686, and the Bowen blend at 4640. The radial velocity
curves of the Bowen blend shows significantly stronger modulation at the
orbital period than HAlpha as expected for the former originating on the
secondary with the latter consistent with emission dominated by the disk. Our
tomograms of HAlpha and HBeta are the most detailed studies of these lines for
V801 to date and they clearly detect the accretion disk. The HBeta emission
extends to higher velocities than HAlpha, suggesting emission from closer to
the neutron star and differentiating temperature variance in the accretion disk
for the first time. The center of the accretion disk appears offset from the
center-of-mass of the neutron star as has been seen in several other X-ray
binaries. This is often interpreted to imply disk eccentricity. Our tomograms
do not show strong evidence for a hot spot at the point where the accretion
stream hits the disk. This could imply a reduced accretion rate or could be due
to the spot being drowned out by bright accretion flow around it. There is
enhanced emission further along the disk, however, which implies gas stream
interaction downstream of the hot spot. | astro-ph_HE |
Pulse Profile Modeling of Thermonuclear Burst Oscillations I: The Effect
of Neglecting Variability: We study the effects of the time-variable properties of thermonuclear X-ray
bursts on modeling their millisecond-period burst oscillations. We apply the
pulse profile modeling technique that is being used in the analysis of
rotation-powered millisecond pulsars by the Neutron Star Interior Composition
Explorer (NICER) to infer masses, radii, and geometric parameters of neutron
stars. By simulating and analyzing a large set of models, we show that
overlooking burst time-scale variability in temperatures and sizes of the hot
emitting regions can result in substantial bias in the inferred mass and
radius. To adequately infer neutron star properties, it is essential to develop
a model for the time variable properties or invest a substantial amount of
computational time in segmenting the data into non-varying pieces. We discuss
prospects for constraints from proposed future X-ray telescopes. | astro-ph_HE |
A low-mass binary neutron star: long-term ejecta evolution and kilonovae
with weak blue emission: We study the long-term evolution of ejecta formed in a binary neutron star
(BNS) merger that results in a long-lived remnant NS by performing a
hydrodynamics simulation with the outflow data of a numerical relativity
simulation as the initial condition. At the homologously expanding phase, the
total ejecta mass reaches $\approx0.1\,M_\odot$ with an average velocity of
$\approx0.1\,c$ and lanthanide fraction of $\approx 0.005$. We further perform
the radiative transfer simulation employing the obtained ejecta profile. We
find that, contrary to a naive expectation from the large ejecta mass and low
lanthanide fraction, the optical emission is not as bright as that in
GW170817/AT2017gfo, while the infrared emission can be brighter. This light
curve property is attributed to preferential diffusion of photons toward the
equatorial direction due to the prolate ejecta morphology, large opacity
contribution of Zr, Y, and lanthanides, and low specific heating rate of the
ejecta. Our results suggest that these light curve features could be used as an
indicator for the presence of a long-lived remnant NS. We also found that the
bright optical emission broadly consistent with GW170817/AT2017gfo is realized
for the case that the high-velocity ejecta components in the polar region are
suppressed. These results suggest that the remnant in GW170817/AT2017gfo is
unlikely to be a long-lived NS, but might have collapsed to a black hole within
${\cal O}(0.1)$ s. | astro-ph_HE |
A VLA search for 5 GHz radio transients and variables at low Galactic
latitudes: We present the results of a 5 GHz survey with the Very Large Array, designed
to search for short-lived (<1 day) transients and to characterize the
variability of radio sources at milli-Jansky levels. A total sky area of 2.66
deg^2, spread over 141 fields at low Galactic latitudes was observed 16 times
with a cadence sampling timescales of days, months and years. Most of the data
were searched for transients in near real time. Candidates were followed up
using visible light telescopes (1-2 hr delays) and the X-Ray Telescope on board
the Swift satellite. The final processing of the data revealed a single
possible transient with a flux density of 2.4 mJy. This implies a transients,
>1.8 mJy, sky surface density of 0.039 (-0.032/+0.13) deg^-2. This areal
density is consistent with the sky surface density of transients from the Bower
et al. survey extrapolated to 1.8 mJy. Our observed transient areal density is
consistent with a Neutron Stars (NSs) origin for these events. Furthermore, we
use the data to measure the sources variability on days to years time scales,
and we present the variability structure function of 5 GHz sources. The mean
structure function shows a fast increase on ~1 day time scale, followed by a
slower increase on time scales of up to 10 days. On time scales between 10-60
days the structure function is roughly constant. We find that >30% of the
unresolved sources brighter than 1.8 mJy are variable at the >4 sigma
confidence level, presumably due mainly to refractive scintillation. | astro-ph_HE |
Dependence of inner accretion disk stress on parameters: the
Schwarzschild case: We explore the parameter dependence of inner disk stress in black hole
accretion by contrasting the results of a number of simulations, all employing
3-d general relativistic MHD in a Schwarzschild spacetime. Five of these
simulations were performed with the intrinsically conservative code HARM3D,
which allows careful regulation of the disk aspect ratio, H/R; our simulations
span a range in H/R from 0.06 to 0.17. We contrast these simulations with two
previously reported simulations in a Schwarzschild spacetime in order to
investigate possible dependence of the inner disk stress on magnetic topology.
In all cases, much care was devoted to technical issues: ensuring adequate
resolution and azimuthal extent, and averaging only over those time-periods
when the accretion flow is in approximate inflow equilibrium. We find that the
time-averaged radial-dependence of fluid-frame electromagnetic stress is almost
completely independent of both disk thickness and poloidal magnetic topology.
It rises smoothly inward at all radii (exhibiting no feature associated with
the ISCO) until just outside the event horizon, where the stress plummets to
zero. Reynolds stress can also be significant near the ISCO and in the plunging
region; the magnitude of this stress, however, depends on both disk thickness
and magnetic topology. The two stresses combine to make the net angular
momentum accreted per unit rest-mass 7-15% less than the angular momentum of
the ISCO. | astro-ph_HE |
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