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Bhjet: a public multi-zone, steady state jet + thermal corona spectral
model: Accreting black holes are sources of major interest in astronomy, particular
those launching jets because of their ability to accelerate particles, and
dramatically affect their surrounding environment up to very large distances.
The spatial, energy and time scales at which a central active black hole
radiates and impacts its environment depend on its mass. The implied
scale-invariance of accretion/ejection physics between black hole systems of
different central masses has been confirmed by several studies. Therefore,
designing a self-consistent theoretical model that can describe such systems,
regardless of their mass, is of crucial importance to tackle a variety of
astrophysical sources. We present here a new and significantly improved version
of a scale invariant, steady-state, multi-zone jet model, which we rename
bhjet, resulting from the efforts of our group to advance the modelling of
black hole systems. We summarise the model assumptions and basic equations, how
they have evolved over time, and the additional features that we have recently
introduced. These include additional input electron populations, the extension
to cyclotron emission in near-relativistic regime, an improved multiple inverse
Compton scattering method, external photon seeds typical of AGN and a
magnetically-dominated jet dynamical model as opposed to the pressure-driven
jet configuration present in older versions. In this paper, we publicly release
the code on Github and, in order to facilitate the user's approach to its many
possibilities, showcase a few applications as a tutorial. | astro-ph_HE |
The Long-term Monitoring Results of Insight-HXMT in the First 4 Yr
Galactic Plane Scanning Survey: The first X-ray source catalog of Insight-HXMT Galactic Plane (|b|<10deg)
Scanning Survey (GPSS) is presented based on the data accumulated from June
2017 to August 2021. The 4 yr limit sensitivities at main energy bands can
reach 8.2x10^(-12) erg/s/cm^2} (2-6 keV), 4.21x10^(-11) erg/s/cm^2 (7-40 keV)
and 2.78x10^(-11) erg/s/cm^2 (25-100 keV). More than 1300 sources have been
monitored at a wide band (1$-$100\,keV), of which 223 sources have a
signal-to-noise ratio greater than 5. We combined the GPSS data of Insight-HXMT
and MAXI and found it is feasible to obtain more complete long-term light
curves from their scanning results. The flux variabilities at different energy
bands of the 223 bright sources are analyzed based on the excess variances. It
is found that the fluxes of X-ray binaries are more active than those of
supernova remnants and isolated pulsars. Different types of binaries, e.g.,
low-mass X-ray binaries (LMXBs), high-mass X-ray binaries (HMXBs), neutron star
binaries, and black hole binaries, also distinctively show different
regularities. In addition, the relations between the hardness ratio (HR) and
excess variances, and HR and source types are analyzed. It is obvious that the
HRs of HMXBs tend to be harder than those of LMXBs and HMXBs tend to be more
active than those of LMXBs. | astro-ph_HE |
Enhancing gravitational wave astronomy with galaxy catalogues: Joint gravitational wave (GW) and electromagnetic (EM) observations, as a key
research direction in multi-messenger astronomy, will provide deep insight into
the astrophysics of a vast range of astronomical phenomena. Uncertainties in
the source sky location estimate from gravitational wave observations mean
follow-up observatories must scan large portions of the sky for a potential
companion signal. A general frame of joint GW-EM observations is presented by a
multi-messenger observational triangle. Using a Bayesian approach to
multi-messenger astronomy, we investigate the use of galaxy catalogue and host
galaxy information to reduce the sky region over which follow-up observatories
must scan, as well as study its use for improving the inclination angle
estimates for coalescing binary compact objects. We demonstrate our method
using a simulated neutron stars inspiral signal injected into simulated
Advanced detectors noise and estimate the injected signal sky location and
inclination angle using the Gravitational Wave Galaxy Catalogue. In this case
study, the top three candidates in rank have $72\%$, $15\%$ and $8\%$ posterior
probability of being the host galaxy, receptively. The standard deviation of
cosine inclination angle (0.001) of the neutron stars binary using
gravitational wave-galaxy information is much smaller than that (0.02) using
only gravitational wave posterior samples. | astro-ph_HE |
Massive White Dwarfs in the Galactic Center: A Chandra X-ray
Spectroscopy of Cataclysmic Variables: Previous X-ray observations toward the Nuclear Star Cluster (NSC) at the
Galactic center have discovered thousands of point sources, most of which were
believed to be cataclysmic variables (CVs), i.e., a white dwarf (WD) accreting
from a low-mass companion. However, the population properties of these CVs
remain unclear, which otherwise contain important information about the
evolutionary history of the NSC. In this work we utilize ultradeep archival
\textit{Chandra} observations to study the spectral properties of the NSC CVs,
in close comparison with those in the Solar vicinity. We find that the NSC CVs
have strong Fe XXV and Fe XXVI lines (both of which show equivalent widths
$\sim200-300$ eV), indicating metal-rich companions. Moreover, their Fe XXVI to
Fe XXV line flux ratio is used to diagnose the characteristic white dwarf mass
($M_{\rm WD}$) of NSC CVs. The results show that the CVs with $L_{\rm 2-10
keV}>6\times10^{31}$ erg s$^{-1}$ have a mean $M_{\rm WD}$ of
$\sim0.6/1.0\,M_{\odot}$ if they are magnetic/non-magnetic CVs; while those
with $L_{\rm 2-10 keV}$ between $1-6\times10^{31}$ erg s$^{-1}$ have a mean
$M_{\rm WD}$ of $\sim0.8/1.2\,M_{\odot}$ if they are magnetic/non-magnetic CVs.
All these \textit{Chandra}-detected CVs collectively contribute $\sim$30-50\%
of the unresolved 20-40 keV X-ray emission from the NSC. The CV population with
massive (i.e., $M_{\rm WD}\sim1.2M_{\odot}$) WDs have not been observed in the
Solar vicinity or the Galactic bulge, and they might have been formed via
dynamical encounters in the NSC. | astro-ph_HE |
Nitrogen K-shell photoabsorption: Reliable atomic data have been computed for the spectral modeling of the
nitrogen K lines, which may lead to useful astrophysical diagnostics. Data sets
comprise valence and K-vacancy level energies, wavelengths, Einstein
$A$-coefficients, radiative and Auger widths and K-edge photoionization cross
sections. An important issue is the lack of measurements which are usually
employed to fine-tune calculations so as to attain spectroscopic accuracy. In
order to estimate data quality, several atomic structure codes are used and
extensive comparisons with previous theoretical data have been carried out. In
the calculation of K photoabsorption with the Breit--Pauli $R$-matrix method,
both radiation and Auger damping, which cause the smearing of the K edge, are
taken into account. This work is part of a wider project to compute atomic data
in the X-ray regime to be included in the database of the popular {\sc xstar}
modeling code. | astro-ph_HE |
Exploring binary-neutron-star-merger scenario of short-gamma-ray bursts
by gravitational-wave observation: We elucidate the feature of gravitational waves (GWs) from binary neutron
star merger collapsing to a black hole by general relativistic simulation. We
show that GW spectrum imprints the coalescence dynamics, formation process of
disk, equation of state for neutron stars, total masses, and mass ratio. A
formation mechanism of the central engine of short $\gamma$-ray bursts, which
are likely to be composed of a black hole and surrounding disk, therefore could
be constrained by GW observation. | astro-ph_HE |
Outflow-Driven Transients from the Birth of Binary Black Holes I:
Tidally-Locked Secondary Supernovae: We propose a new type of electromagnetic transients associated with the birth
of binary black holes (BBHs), which may lead to merger events accompanied by
gravitational waves in $\sim0.1-1$ Gyr. We consider the newborn BBHs formed
through the evolution of isolated massive stellar binaries. For a close massive
binary, consisting of a primary black hole (BH) and a secondary Wolf-Rayet (WR)
star that are orbiting around each other, the spin period of the WR star can be
tidally synchronized to its orbital period. Then, the angular momentum of the
outer material of the WR star is large enough to form an accretion disk around
a newborn, secondary BH, following its core-collapse. This disk may produce an
energetic outflow with a kinetic energy of $\sim10^{50}-10^{52}$ erg and an
outflow velocity of $\sim10^{10}\rm~cm~s^{-1}$, resulting in an optical
transient with an absolute magnitude from $\sim -14$ to $\sim-17$ with a
duration of around a day. This type of transient also produces detectable radio
signals $\sim1-10$ years after the birth of BBHs, via synchrotron emission from
non-thermal electrons accelerated at external shocks. The predicted optical
transients have a shorter duration than ordinary core-collapse supernovae.
Dedicated optical transient surveys can detect them, and distinguish it from
ordinary SNe using the different features of its light curve and late-time
spectrum. In the paper I, we investigate disk-driven outflows from the
secondary BH, whereas possible signals from the primary BH will be discussed in
the paper II. | astro-ph_HE |
A NICER view of PSR J0030+0451: evidence for a global-scale multipolar
magnetic field: Recent modeling of NICER observations of thermal X-ray pulsations from the
surface of the isolated millisecond pulsar PSR J0030+0451 suggests that the hot
emitting regions on the pulsar's surface are far from antipodal, which is at
odds with the classical assumption that the magnetic field in the pulsar
magnetosphere is predominantly that of a centered dipole. Here, we review these
results and examine previous attempts to constrain the magnetospheric
configuration of PSR J0030+0451. To the best of our knowledge, there is in fact
no direct observational evidence that PSR J0030+0451's magnetic field is a
centered dipole. Developing models of physically motivated, non-canonical
magnetic field configurations and the currents that they can support poses a
challenging task. However, such models may have profound implications for many
aspects of pulsar research, including pulsar braking, estimates of birth
velocities, and interpretations of multi-wavelength magnetospheric emission. | astro-ph_HE |
Optical Spectroscopy of Bright Fermi LAT Blazars: We report on HET and Palomar 5 m spectroscopy of recently identified
$\gamma$-ray blazars in the {\it Fermi} LAT Bright Source List. These data
provide identifications for 10 newly discovered $\gamma$-ray flat spectrum
radio quasars (FSRQ) and six new BL Lacs plus improved spectroscopy for six
additional BL Lacs. We substantially improve the identification completeness of
the bright LAT blazars and give new redshifts and $z$ constraints, new
estimates of the black hole masses and new measurements of the optical SED. | 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 |
Diverse Properties of Interstellar Medium Embedding Gamma-Ray Bursts at
the Epoch of Reionization: Analysis is performed on ultra-high resolution large-scale cosmological
radiation-hydrodynamic simulations to, for the first time, quantify the
physical environment of long-duration gamma-ray bursts (GRBs) at the epoch of
reionization. We find that, on parsec scales, 13% of GRBs remain in high
density ($\ge 10^4$cm$^{-3}$) low-temperature star-forming regions, whereas 87%
of GRBs occur in low-density ($\sim 10^{-2.5}$cm$^{-3}$) high temperature
regions heated by supernovae. More importantly, the spectral properties of GRB
afterglows, such as the neutral hydrogen column density, total hydrogen column
density, dust column density, gas temperature and metallicity of intervening
absorbers, vary strongly from sightline to sightline. Although our model
explains extant limited observationally inferred values with respect to
circumburst density, metallicity, column density and dust properties, a
substantially larger sample of high-z GRB afterglows would be required to
facilitate a statistically solid test of the model. Our findings indicate that
any attempt to infer the physical properties (such as metallicity) of the
interstellar medium of the host galaxy based on a very small number of (usually
one) sightlines would be precarious. Utilizing high-z GRBs to probe
interstellar medium and intergalactic medium should be undertaken properly
taking into consideration the physical diversities of the interstellar medium. | astro-ph_HE |
Multi-wavelength observations of Isolated Neutron Stars: Almost 30 Isolated Neutron Stars (INSs) of different flavours have been
identified at optical, ultraviolet, or infrared (UVOIR) wavelengths. Here, I
present a short review of the historical background and describe the scientific
impact of INS observations in the UVOIR. Then, I focus on UVOIR observations of
rotation-powered pulsars, so far the most numerous class of INSs identified at
these wavelengths, and their observational properties. Finally, I present the
results of new UVOIR observations and an update of the follow-ups of gamma-ray
pulsars detected by Fermi. | astro-ph_HE |
Hot disk of the Swift J0243.6+6124 revealed by Insight-HXMT: We report on analysis of observations of the bright transient X-ray pulsar
\src obtained during its 2017-2018 giant outburst with Insight-HXMT,
\emph{NuSTAR}, and \textit{Swift} observatories. We focus on the discovery of a
sharp state transition of the timing and spectral properties of the source at
super-Eddington accretion rates, which we associate with the transition of the
accretion disk to a radiation pressure dominated (RPD) state, the first ever
directly observed for magnetized neutron star. This transition occurs at
slightly higher luminosity compared to already reported transition of the
source from sub- to super-critical accretion regime associate with onset of an
accretion column. We argue that this scenario can only be realized for
comparatively weakly magnetized neutron star, not dissimilar to other
ultra-luminous X-ray pulsars (ULPs), which accrete at similar rates. Further
evidence for this conclusion is provided by the non-detection of the transition
to the propeller state in quiescence which strongly implies compact
magnetosphere and thus rules out magnetar-like fields. | astro-ph_HE |
QCD, Gravitational Waves, and Pulsars: Investigations of the phase diagram of quantum chromodynamics (QCD) have
revealed that exotic new phases, the so called {\it color superconducting
phases}, may arise at very high baryon densities. It is speculated that these
exotic phases may arise in the cores of neutron stars. Focus on neutrons stars
has tremendously intensified in recent years with the direct detection of
gravitational waves (GW) by LIGO/Virgo from BNS merger events which has allowed
the possibility of directly probing the properties of the interior of a neutron
star. A remarkable phenomenon manifested by rapidly rotating neutron stars is
in their {\it avatar} as {\it Pulsars}. The accuracy of pulsar timing can reach
the level of one part in 10$^{15}$, comparable to that of atomic clocks. This
suggests that even a tiny deformation of the pulsar can leave its imprints on
the pulses by inducing tiny perturbations in the entire moment of inertia (MI)
tensor affecting the pulse timings, as well as the pulse profile (from wobbling
induced by off-diagonal MI components). This may allow a new probe of various
phase transitions occurring inside a pulsar core through induced density
fluctuations affecting the MI tensor. Such perturbations also naturally induce
a rapidly changing quadrupole moment of the star, thereby providing a new
source of gravitational wave emission. Another remarkable possibility arises
when we consider the effect of an external GW on neutron star. With the
possibility of detecting any minute changes in its configuration through pulse
observations, the neutron star has the potential of performing as a Weber
detector of gravitational wave. This brief review will focus on these specific
aspects of a pulsar. Specifically, the focus will be on the type of physics
which can be probed by utilizing the effect of changes in the MI tensor of the
pulsar on pulse properties. | astro-ph_HE |
Californium-254 and kilonova light curves: Neutron star mergers offer unique conditions for the creation of the heavy
elements and additionally provide a testbed for our understanding of this
synthesis known as the $r$-process. We have performed dynamical nucleosynthesis
calculations and identified a single isotope, $^{254}$Cf, which has a
particularly high impact on the brightness of electromagnetic transients
associated with mergers on the order of 15 to 250 days. This is due to the
anomalously long half-life of this isotope and the efficiency of fission
thermalization compared to other nuclear channels. We estimate the fission
fragment yield of this nucleus and outline the astrophysical conditions under
which $^{254}$Cf has the greatest impact to the light curve. Future
observations in the middle-IR which are bright during this regime could
indicate the production of actinide nucleosynthesis. | astro-ph_HE |
On the approximation of the black hole shadow with a simple polar curve: A black hole embedded within a bright, optically thin emitting region
imprints a nearly circular "shadow" on its image, corresponding to the
observer's line-of-sight into the black hole. The shadow boundary depends on
the black hole's mass and spin, providing an observable signature of both
properties via high resolution images. However, standard expressions for the
shadow boundary are most naturally parametrized by Boyer-Lindquist radii rather
than by image coordinates. We explore simple, approximate parameterizations for
the shadow boundary using ellipses and a family of curves known as limacons. We
demonstrate that these curves provide excellent and efficient approximations
for all black hole spins and inclinations. In particular, we show that the two
parameters of the limacon naturally account for the three primary shadow
deformations resulting from mass and spin: size, displacement, and asymmetry.
These curves are convenient for parametric model fitting directly to
interferometric data, they reveal the degeneracies expected when estimating
black hole properties from images with practical measurement limitations, and
they provide a natural framework for parametric tests of the Kerr metric using
black hole images. | astro-ph_HE |
The Euclidean distribution of Fast Radio Bursts: We investigate whether current data on the distribution of observed flux
densities of Fast Radio Bursts (FRBs) are consistent with a constant source
density in Euclidean space. We use the number of FRBs detected in two surveys
with different characteristics along with the observed signal-to-noise ratios
of the detected FRBs in a formalism similar to a V/V_max-test to constrain the
distribution of flux densities. We find consistency between the data and a
Euclidean distribution. Any extension of this model is therefore not
data-driven and needs to be motivated separately. As a byproduct we also obtain
new improved limits for the FRB rate at 1.4 GHz, which had not been constrained
in this way before. | astro-ph_HE |
Suzaku investigation into the nature of the nearest ultraluminous X-ray
source, M33 X-8: The X-ray spectrum of the nearest ultraluminous X-ray source, M33 X-8,
obtained by Suzaku during 2010 January 11 -- 13, was closely analyzed to
examine its nature. It is, by far, the only data with the highest signal
statistic in 0.4 -- 10 keV range. Despite being able to reproduce the X-ray
spectrum, Comptonization of the disk photons failed to give a physically
meaningful solution. A modified version of the multi-color disk model, in which
the dependence of the disk temperature on the radius is described as r^(-p)
with p being a free parameter, can also approximate the spectrum. From this
model, the innermost disk temperature and bolometric luminosity were obtained
as T_in = 2.00-0.05+0.06 keV and L_disk = 1.36 x 10^39 (cos i)^(-1) ergs/s,
respectively, where i is the disk inclination. A small temperature gradient of
p = 0.535-0.005+0.004, together with the high disk temperature, is regarded as
the signatures of the slim accretion disk model, suggesting that M33 X-8 was
accreting at high mass accretion rate. With a correction factor for the slim
disk taken into account, the innermost disk radius, R_in =81.9-6.5+5.9 (cos
i)^(-0.5) km, corresponds to the black hole mass of M \sim 10 M_sun (cos
i)^(-0.5). Accordingly, the bolometric disk luminosity is estimated to be about
80 (cos i)^(-0.5)% of the Eddington limit. A numerically calculated slim disk
spectrum was found to reach a similar result. Thus, the extremely
super-Eddington luminosity is not required to explain the nature of M33 X-8.
This conclusion is utilized to argue for the existence of intermediate mass
black holes with M > 100 M_sun radiating at the sub/trans-Eddington luminosity,
among ultraluminous X-ray sources with L_disk > 10^(40) ergs/s. | astro-ph_HE |
Fast TeV variability from misaligned minijets in the jet of M87: The jet of the radio galaxy M87 is misaligned, resulting in a Doppler factor
delta~1 for emission of plasma moving parallel to the jet. This makes the
observed fast TeV flares on timescales of t_v~5R_g/c harder to understand as
emission from the jet. In previous work, we have proposed a jets-in-a-jet model
for the ultra-fast TeV flares with t_v<<R_g/c seen in Mrk 501 and PKS 2155-304.
Here, we show that about half of the minijets beam their emission outside the
jet cone. Minijets emitting off the jet axis result in rapidly evolving TeV
(and maybe lower energy) flares that can be observed in nearby radio galaxies.
The TeV flaring from M87 fits well into this picture, if M87 is a misaligned
blazar. | astro-ph_HE |
Favorable conditions for heavy element nucleosynthesis in rotating
proto-magnetar winds: The neutrino-driven wind cooling phase of proto-neutron stars (PNSs) follows
successful supernovae. Wind models without magnetic fields or rotation fail to
achieve the necessary conditions for production of the third $r$-process peak,
but robustly produce a weak $r$-process in neutron-rich winds. Using 2D
magnetohydrodynamic simulations with magnetar-strength magnetic fields and
rotation, we show that the PNS rotation rate significantly affects the
thermodynamic conditions of the wind. We show that high entropy material is
quasi-periodically ejected from the closed zone of the PNS magnetosphere with
the required thermodynamic conditions to produce heavy elements. We show that
maximum entropy $S$ of the material ejected depends systematically on the
magnetar spin period $P_{\star}$ and scales as $S \propto P_{\star}^{-5/6}$ for
sufficiently rapid rotation. We present results from simulations at a constant
neutrino luminosity representative of $\sim 1-2$ s after the onset of cooling
for $P_{\star}$ ranging from 5 ms to 200 ms and a few simulations with evolving
neutrino luminosity where we follow the evolution of the magnetar wind until
$10-14$ s after the onset of cooling. We estimate at magnetar polar magnetic
field strength $B_0=3\times 10^{15}$ G, $10^{15}$ G, and $5\times 10^{14}$ G
that neutron-rich magnetar winds can respectively produce at least $\sim
2-5\times 10^{-5}$ M$_{\odot}$, $\sim 3-4\times 10^{-6}$ M$_{\odot}$, and $\sim
2.5\times 10^{-8}$ M$_{\odot}$ of material with the required parameters for
synthesis of the third $r-$process peak, within $1-2$ s, 10 s, and 14 s in that
order after the onset of cooling. We show that proton-rich magnetar winds can
have favorable conditions for production of $p-$nuclei, even at a modest
$B_0=5\times 10^{14}$ G. | astro-ph_HE |
Magnetic Fields, Relativistic Particles, and Shock Waves in Cluster
Outskirts: It is only now, with low-frequency radio telescopes, long exposures with
high-resolution X-ray satellites and gamma-ray telescopes, that we are
beginning to learn about the physics in the periphery of galaxy clusters. In
the coming years, Sunyaev-Zeldovich telescopes are going to deliver further
great insights into the plasma physics of these special regions in the
Universe. The last years have already shown tremendous progress with detections
of shocks, estimates of magnetic field strengths and constraints on the
particle acceleration efficiency. X-ray observations have revealed shock fronts
in cluster outskirts which have allowed inferences about the microphysical
structure of shocks fronts in such extreme environments. The best indications
for magnetic fields and relativistic particles in cluster outskirts come from
observations of so-called radio relics, which are megaparsec-sized regions of
radio emission from the edges of galaxy clusters. As these are difficult to
detect due to their low surface brightness, only few of these objects are
known. But they have provided unprecedented evidence for the acceleration of
relativistic particles at shock fronts and the existence of muG strength fields
as far out as the virial radius of clusters. In this review we summarise the
observational and theoretical state of our knowledge of magnetic fields,
relativistic particles and shocks in cluster outskirts. | astro-ph_HE |
Intra-Day Variability Observations of Two Dozens of Blazars at 4.8 GHz: Two dozens of radio loud active galactic nuclei (AGNs) have been observed
with Urumqi 25 m radio telescope in order to search for intra-day variability
(IDV). The target sources are blazars (namely flat spectrum radio quasars and
BL Lac objects) which are mostly selected from the observing list of
RadioAstron AGN monitoring campaigns. The observations were carried out at 4.8
GHz in two sessions of 8-12 February 2014 and 7-9 March respectively. We report
the data reduction and the first results of observations. The results show that
the majority of the blazars exhibit IDV in 99.9% confidence level, some of them
show quite strong IDV. We find the strong IDV of blazar 1357 + 769 for the
first time. The IDV at centimeter-wavelength is believed to be predominately
caused by the scintillation of blazar emission through the local interstellar
medium in a few hundreds parsecs away from Sun. No significant correlation
between the IDV strength and either redshift or Galactic latitude is found in
our sample. The IDV timescale along with source structure and brightness
temperature analysis will be presented in a forthcoming paper. | astro-ph_HE |
Did a Kilonova Set Off in Our Galactic Backyard 3.5 Myr ago?: The recent detection of the live isotopes $^{60}{\rm Fe}$ and $^{244}{\rm
Pu}$ in deep ocean sediments dating back to the past 3-4 Myr poses a serious
challenge to the identification of their production site(s). While $^{60}{\rm
Fe}$ is usually attributed to standard core-collapse supernovae, actinides are
r-process nucleosynthesis yields, which are believed to be synthesized in rare
events, such as special classes of supernovae or binary mergers involving at
least one neutron star. Previous works concluded that a single binary neutron
star merger cannot explain the observed isotopic ratio. In this work, we
consider a set of numerical simulations of binary neutron star mergers
producing long-lived massive remnants expelling both dynamical and spiral-wave
wind ejecta. The latter, due to a stronger neutrino irradiation, also produce
iron-group elements. Assuming that large-scale mixing is inefficient before the
fading of the kilonova remnant and that the spiral-wave wind is sustained over
a 100-200 ms timescale, the ejecta emitted at mid-high latitudes provide a
$^{244}{\rm Pu}$ over $^{60}{\rm Fe}$ ratio compatible with observations. The
merger could have happened 80-150 pc away from the Earth and between 3.5 and
4.5 Myr ago. We also compute expected isotopic ratios for eight other live
radioactive nuclides showing that the proposed binary neutron star merger
scenario is distinguishable from other scenarios proposed in the literature. | astro-ph_HE |
The Discovery of the Most Accelerated Binary Pulsar: Pulsars in relativistic binary systems have emerged as fantastic natural
laboratories for testing theories of gravity, the most prominent example being
the double pulsar, PSR J0737$-$3039. The HTRU-South Low Latitude pulsar survey
represents one of the most sensitive blind pulsar surveys taken of the southern
Galactic plane to date, and its primary aim has been the discovery of new
relativistic binary pulsars. Here we present our binary pulsar searching
strategy and report on the survey's flagship discovery, PSR J1757$-$1854. A
21.5-ms pulsar in a relativistic binary with an orbital period of 4.4 hours and
an eccentricity of 0.61, this double neutron star (DNS) system is the most
accelerated pulsar binary known, and probes a relativistic parameter space not
yet explored by previous pulsar binaries. | astro-ph_HE |
NuSTAR Observations of the Compton-thick Active Galactic Nucleus and
Ultraluminous X-ray Source Candidate in NGC 5643: We present two NuSTAR observations of the local Seyfert 2 active galactic
nucleus (AGN) and an ultraluminous X-ray source (ULX) candidate in NGC 5643.
Together with archival data from Chandra, XMM-Newton and Swift-BAT, we perform
a high-quality broadband spectral analysis of the AGN over two decades in
energy ($\sim$0.5-100 keV). Previous X-ray observations suggested that the AGN
is obscured by a Compton-thick (CT) column of obscuring gas along our
line-of-sight. However, the lack of high-quality $\gtrsim$ 10 keV observations,
together with the presence of a nearby X-ray luminous source, NGC 5643 X-1, had
left significant uncertainties in the characterization of the nuclear spectrum.
NuSTAR now enables the AGN and NGC 5643 X-1 to be separately resolved above 10
keV for the first time and allows a direct measurement of the absorbing column
density toward the nucleus. The new data show that the nucleus is indeed
obscured by a CT column of $N_{\rm{H}}$ $\gtrsim$ 5 $\times$ 10$^{24}$
cm$^{-2}$. The range of 2-10 keV absorption-corrected luminosity inferred from
the best fitting models is $L_{2-10,\rm{int}} =$ (0.8-1.7) $\times$ 10$^{42}$
erg s$^{-1}$, consistent with that predicted from multiwavelength intrinsic
luminosity indicators. We also study the NuSTAR data for NGC 5643 X-1, and show
that it exhibits evidence for a spectral cut-off at energy, $E$ $\sim$ 10 keV,
similar to that seen in other ULXs observed by NuSTAR. Along with the evidence
for significant X-ray luminosity variations in the 3-8 keV band from 2003-2014,
our results further strengthen the ULX classification of NGC 5643 X-1. | astro-ph_HE |
Absorption of GRB X-ray Afterglows by The Missing Baryons: Confronting
Observations with Cosmological Simulations: A large fraction of the baryons at low redshift are undetected, and likely
reside in the tenuous, hot intergalactic medium (IGM). One way to probe the
missing baryons is through their absorption of bright sources. The anomalous
absorption excess in the X-ray afterglows of $\gamma$-ray bursts (GRBs) has
been suggested to result from the missing baryons. In order to test this
hypothesis, the present paper employs IllustrisTNG simulations to compute the
X-ray absorption effect on cosmological distances. The simulation shows that
ionization of H and He in the IGM leaves the metals responsible for $>60\%$ of
the X-ray opacity of high-$z$ sources (the ionization of He isn't available in
the simulation, we used here external knowledge). The high-$z$ asymptotic
optical depth at 0.5\,keV in the simulation reaches $0.15\pm0.07$, while the
GRB afterglow values tend to $\approx 0.4$, implying the missing baryons can
account for a significant fraction of the observed opacity. The remaining
discrepancy is ascribed mainly to the low average metallicity in the
simulation, which drops from 0.06 solar at $z=0$ to 0.01 at $z=3$, and which is
below previously measured values. | astro-ph_HE |
Instability of twisted magnetar magnetospheres: We present three-dimensional force-free electrodynamics simulations of
magnetar magnetospheres that demonstrate the instability of certain degenerate,
high energy equilibrium solutions of the Grad-Shafranov equation. This result
indicates the existence of an unstable branch of twisted magnetospheric
solutions and allows to formulate an instability criterion. The rearrangement
of magnetic field lines as a consequence of this instability triggers the
dissipation of up to 30% of the magnetospheric energy on a thin layer above the
magnetar surface. During this process, we predict an increase of the mechanical
stresses onto the stellar crust, which can potentially result in a global
mechanical failure of a significant fraction of it. We find that the estimated
energy release and the emission properties are compatible with the observed
giant flare events. The newly identified instability is a candidate for
recurrent energy dissipation, which could explain part of the phenomenology
observed in magnetars. | astro-ph_HE |
The link between short Gamma-ray bursts and Gravitational Waves:
perspectives for the THESEUS mission: The knowledge of the class of short Gamma-Ray Bursts (GRBs), characterised by
a duration of the gamma-ray emission $\leq 2$ s, experienced an impressive
boost in the last decade. In particular, the discovery of short GRB afterglows
in 2005 with Swift and HETE-II provided the first insight into their energy
scale, environments and host galaxies. The lack of detection of associated
supernovae proved that they are not related to the death of massive stars. The
increasing evidence for compact object binary progenitors makes short GRBs one
of the most promising sources of gravitational waves for the forthcoming
Advanced LIGO/Virgo science runs. To this end, the spectacular detection of the
first electromagnetic counterpart of the gravitational wave event GW\,170817
originated by the coalescence of a double neutron star (NS) system, represents
a first hystorical milestone. The (weak) short GRB\,170817A associated to this
GW event provided the long-sought evidence that at least a fraction of short
GRBs are originated by NS-NS merging and suggested the intriguing possibility
that relativistic jets can be launched in the process of a NS-NS merger. The
THESEUS mission, thanks to the diversity of intstrumentation, fast pointing and
flexible schedule will represent a key facility in the multi-messenger
astronomy era. | astro-ph_HE |
Time of Flight and Supernova Progenitor Effects on the Neutrino Halo: We argue that the neutrino halo, a population of neutrinos that have
undergone direction-changing scattering in the stellar envelope of a
core-collapse supernova (CCSNe), is sensitive to neutrino emission history
through time of flight. We show that the constant time approximation, commonly
used in calculating the neutrino halo, does not capture the spatiotemporal
evolution of the halo neutrino population and that correcting for time of
flight can produce conditions which may trigger fast neutrino flavor
conversion. We also find that there exists a window of time early in all CCSNe
where the neutrino halo population is sufficiently small that it may be
negligible. This suggests that collective neutrino oscillation calculations
which neglect the Halo may be well founded at sufficiently early times. | astro-ph_HE |
X-ray counterpart of gravitational waves due to binary neutron star
mergers: light curves, luminosity functions, and event rate densities: Zhang (2013) proposed a type of GRB-less X-ray transient associated with
double neutron star (NS-NS) mergers under the conjecture of a rapidly-spinning
magnetar merger product with the line of sight off the short GRB jet. We
investigate possible light curves of these transients by considering different
observer's viewing angles. We perform Monte Carlo simulations to calculate the
peak luminosity function (LF) and event rate density of these X-ray transients.
By considering that a fraction of massive neutron stars may be supra-massive
and later collapse into black holes after spinning down, we investigate how the
predicted LF depends on the equation of state (EoS) of the central object and
the geometry of the system. In general, the LF can be fit by two log-normal
distributions peaking around $10^{46.4}$ and $10^{49.6}$ $\rm erg\,s^{-1}$,
corresponding to the trapped and free zones, respectively. For the majority of
the EoS models, the current non-detection is consistent with having a free zone
solid angle at most a few times of the solid angle of the short GRB jet. The
event rate density of these X-ray transients is around a few tens of $\rm
Gpc^{-3}yr^{-1}$ for luminosity above $10^{45}$ $\rm erg\,s^{-1}$. We predict
that future X-ray telescopes (such as Einstein Probe) with sensitivity $\sim
10^{-11}$ $\rm erg\,s^{-1}\,cm^{-2}$ would detect as many as several tens of
such transients per year per steradian. Within 200 Mpc, the aLIGO average range
for NS-NS mergers, the estimated event rate of these transients is about 1
transient per year all sky. | astro-ph_HE |
Long-term monitoring of LS I+61303 with INTEGRAL: LS I+61303 is one of the few high-mass X-ray binaries that have been recently
observed at TeV and GeV energies. Here we investigate the hard X-ray spectral
and timing properties of this source using the IBIS/ISGRI instrument on-board
the INTEGRAL satellite. We carry out a systematic analysis based on all
available INTEGRAL data since December 28, 2002 up to April 30, 2008. The total
exposure time analyzed amounts to 2.1 Ms, hence more than doubling the previous
reported sample. The source is best detected in the 18-60 keV band, with a
significance level of 12.0 sigma. The hard X-ray data are best fit with a
simple power law with a photon index of ~ 1.7+-0.2. We detect a periodical
signal at 27 +- 4 days, matching the orbital period of 26.496 days previously
reported at other wavelengths. The hard X-rays orbital lightcurve is obtained
and compared with those derived at other frequencies. | astro-ph_HE |
The role of metallicity in high mass X-ray binaries in galaxy formation
models: Recent theoretical works claim that high-mass X-ray binaries (HMXBs) could
have been important sources of energy feedback into the interstellar and
intergalactic media, playing a major role in the reionization epoch. A
metallicity dependence of the production rate or luminosity of the sources is a
key ingredient generally assumed but not yet probed. Aims: Our goal is to
explore the relation between the X-ray luminosity (Lx) and star formation rate
of galaxies as a possible tracer of a metallicity dependence of the production
rates and/or X-ray luminosities of HMXBs. Methods: We developed a model to
estimate the Lx of star forming galaxies based on stellar evolution models
which include metallicity dependences. We applied our X-ray binary models to
galaxies selected from hydrodynamical cosmological simulations which include
chemical evolution of the stellar populations in a self-consistent way.
Results: Our models successfully reproduce the dispersion in the observed
relations as an outcome of the combined effects of the mixture of stellar
populations with heterogeneous chemical abundances and the metallicity
dependence of the X-ray sources. We find that the evolution of the Lx as a
function of SFR of galaxies could store information on possible metallicity
dependences of the HMXB sources. A non-metallicity dependent model predicts a
non-evolving relation while any metallicity dependence should affect the slope
and the dispersion as a function of redshift. Our results suggest the
characteristics of the Lx evolution can be linked to the nature of the
metallicity dependence of the production rate or the Lx of the stellar sources.
By confronting our models with current available observations of strong
star-forming galaxies, we find that only chemistry-dependent models reproduce
the observed trend for z < 4. | astro-ph_HE |
Multiwavelength behaviour of the blazar 3C279: decade-long study from
$γ$-ray to radio: We report the results of decade-long (2008-2018) $\gamma$-ray to 1 GHz radio
monitoring of the blazar 3C 279, including GASP/WEBT, $\it{Fermi}$ and
$\it{Swift}$ data, as well as polarimetric and spectroscopic data. The X-ray
and $\gamma$-ray light curves correlate well, with no delay > 3 hours, implying
general co-spatiality of the emission regions. The $\gamma$-ray-optical
flux-flux relation changes with activity state, ranging from a linear to a more
complex dependence. The behaviour of the Stokes parameters at optical and radio
wavelengths, including 43 GHz VLBA images, supports either a predominantly
helical magnetic field or motion of the radiating plasma along a spiral path.
Apparent speeds of emission knots range from 10 to 37c, with the highest values
requiring bulk Lorentz factors close to those needed to explain $\gamma$-ray
variability on very short time scales. The Mg II emission line flux in the
`blue' and `red' wings correlates with the optical synchrotron continuum flux
density, possibly providing a variable source of seed photons for inverse
Compton scattering. In the radio bands we find progressive delays of the most
prominent light curve maxima with decreasing frequency, as expected from the
frequency dependence of the $\tau=1$ surface of synchrotron self-absorption.
The global maximum in the 86 GHz light curve becomes less prominent at lower
frequencies, while a local maximum, appearing in 2014, strengthens toward
decreasing frequencies, becoming pronounced at $\sim5$ GHz. These tendencies
suggest different Doppler boosting of stratified radio-emitting zones in the
jet. | astro-ph_HE |
Type Ic Supernova of a 22 $\text{M}_{\odot}$ Progenitor: Type Ic supernovae (SNe Ic) are a sub-class of core-collapse supernovae that
exhibit no helium or hydrogen lines in their spectra. Their progenitors are
thought to be bare carbon-oxygen cores formed during the evolution of massive
stars that are stripped of their hydrogen and helium envelopes sometime before
collapse. SNe Ic present a range of luminosities and spectral properties, from
luminous GRB-SNe with broad-lined spectra to less luminous events with
narrow-line spectra. Modelling SNe\,Ic reveals a wide range of both kinetic
energies, ejecta masses, and $^{56}\text{Ni}$ masses. To explore this diversity
and how it comes about, light curves and spectra are computed from the ejecta
following the explosion of an initially 22 $\text{M}_{\odot}$ progenitor that
was artificially stripped of its hydrogen and helium shells, producing a bare
CO core of $\sim$ 5 $\text{M}_{\odot}$, resulting in an ejected mass of $\sim$
4 $\text{M}_{\odot}$, which is an average value for SNe Ic. Four different
explosion energies are used that cover a range of observed SNe. Finally,
$^{56}\text{Ni}$ and other elements are artificially mixed in the ejecta using
two approximations to determine how element distribution affects light curves
and spectra. The combination of different explosion energy and degree of mixing
produces spectra that roughly replicate the distribution of near-peak
spectroscopic features of SNe Ic. High explosion energies combined with
extensive mixing can produce red, broad-lined spectra, while minimal mixing and
a lower explosion energy produce bluer, narrow-lined spectra. | astro-ph_HE |
ASAS-SN search for optical counterparts of gravitational-wave events
from the third observing run of Advanced LIGO/Virgo: We report on the search for electromagnetic counterparts to the nine
gravitational-wave events with a $>$60\% probability of containing a neutron
star during the third (O3) LIGO-Virgo Collaboration (LVC) observing run with
the All-Sky Automated Survey for SuperNovae (ASAS-SN). No optical counterparts
associated with a gravitational wave event was found. However, thanks to its
network of telescopes, the average area visible to at least one ASAS-SN site
during the first 10 hours after the trigger contained $\sim$30\% of the
integrated source location probability. Through a combination of normal
operations and target-of-opportunity observations, ASAS-SN observations of the
highest probability fields began within one hour of the trigger for four of the
events. After 24 hours, ASAS-SN observed $>$60\% of total probability for three
events and $>$40\% for all but one of the events. This is the largest area
coverage to a depth of $g = 18.5$ mag from any survey with published coverage
statistics for seven of the nine events. With its observing strategy, five
sites around the world, and a large field of view, ASAS-SN will be one of the
leading surveys to optically search for nearby neutron star mergers during LVC
O4. | astro-ph_HE |
Star formation, supernovae, iron, and alpha: consistent cosmic and
Galactic histories: Recent versions of the observed cosmic star-formation history (SFH) have
resolved an inconsistency with the stellar mass density history. We show that
the revised SFH also scales up the delay-time distribution (DTD) of Type Ia
supernovae (SNe Ia), as determined from the observed volumetric SN Ia rate
history, aligning it with other field-galaxy SN Ia DTD measurements. The
revised-SFH-based DTD has a $t^{-1.1 \pm 0.1}$ form and a
Hubble-time-integrated production efficiency of $N/M_\star=1.3\pm0.1$ SNe Ia
per $1000~{\rm M_\odot}$ of formed stellar mass. Using these revised histories
and updated empirical iron yields of the various SN types, we re-derive the
cosmic iron accumulation history. Core-collapse SNe and SNe Ia have contributed
about equally to the total mass of iron in the Universe today. We find the
track of the average cosmic gas element in the [$\alpha$/Fe] vs. [Fe/H]
abundance-ratio plane. The track is broadly similar to the observed main locus
of Galactic stars in this plane, indicating a Milky Way (MW) SFH similar in
form to the cosmic one. We easily find a simple MW SFH that makes the track
closely match this stellar locus. Galaxy clusters appear to have a
higher-normalization DTD. This cluster DTD, combined with a short-burst MW SFH
peaked at $z=3$, produces a track that matches remarkably well the observed
"high-$\alpha$" locus of MW stars, suggesting the halo/thick-disk population
has had a galaxy-cluster-like formation mode. Thus, a simple two-component SFH,
combined with empirical DTDs and SN iron yields, suffices to closely reproduce
the MW's stellar abundance patterns. | astro-ph_HE |
Dissipation of the striped pulsar wind: Rapidly rotating neutron stars blow a relativistic, magnetized wind mainly
composed of electron-positron pairs. The free expansion of the wind terminates
far from the neutron star where a weakly magnetized pulsar wind nebula forms,
implying efficient magnetic dissipation somewhere upstream. The wind current
sheet that separates the two magnetic polarities is usually considered as the
most natural place for magnetic dissipation via relativistic reconnection, but
its efficiency remains an open question. Here, the goal of this work is to
revisit this issue in light of the most recent progress in the understanding of
reconnection and pulsar electrodynamics. We perform large two-dimensional
particle-in-cell simulations of the oblique rotator to capture the multi-scale
evolution of the wind. We find that the current sheet breaks up into a
dynamical chain of magnetic islands separated by secondary thin current sheets.
The sheet thickness increases linearly with radius while the Poynting flux
decreases monotonically as reconnection proceeds. The radius of complete
annihilation of the stripes is given by the plasma multiplicity parameter at
the light cylinder. Current starvation within the sheets does not occur before
complete dissipation as long as there is enough charges where the sheets form.
Particles are efficiently heated up to a characteristic energy set by the
magnetization parameter at the light cylinder. Energetic pulsed synchrotron
emission peaks close to the light cylinder, and presents sub-pulse variability
associated with the formation of plasmoids in the sheet. This study suggests
that the striped component of the wind dissipates far before reaching the
termination shock in isolated pulsars, even in very-high-multiplicity systems
such as the Crab pulsar. Pulsars in binary systems may provide the best
environments to study magnetic dissipation in the wind. | astro-ph_HE |
Acceleration and propagation of ultra high energy cosmic rays: We review the physics of the highest energy cosmic rays. The discovery of
their sources, still unknown, will reveal the most energetic astrophysical
objects in the universe and could unveil new physics beyond the standard model
of particle physics. We discuss the details of propagation of these high energy
particles, their interaction with astrophysical photon backgrounds and
intergalactic magnetic fields, and the production of secondary cosmogenic
particles associated to their transport. We examine different models of
acceleration, reviewing the principal astrophysical objects that could energise
cosmic rays until the highest energies. Given the uncertainties in the observed
mass composition, we review the two alternative scenarios of a composition made
only by protons or by protons and heavier nuclei; discussing the consequences
of the two scenarios in terms of sources, acceleration mechanisms and
production of secondary cosmogenic gamma rays and neutrinos. | astro-ph_HE |
CLUES on Fermi-LAT prospects for the extragalactic detection of munuSSM
gravitino Dark Matter: The munuSSM is a supersymmetric model that has been proposed to solve the
problems generated by other supersymmetric extensions of the standard model of
particle physics. Given that R-parity is broken in the munuSSM, the gravitino
is a natural candidate for decaying dark matter since its lifetime becomes much
longer than the age of the Universe. In this model, gravitino dark matter could
be detectable through the emission of a monochromatic gamma ray in a two-body
decay.
We study the prospects of the Fermi-LAT telescope to detect such
monochromatic lines in 5 years of observations of the most massive nearby
extragalactic objects.
The dark matter halo around the Virgo galaxy cluster is selected as a
reference case, since it is associated to a particularly high signal-to-noise
ratio and is located in a region scarcely affected by the astrophysical diffuse
emission from the galactic plane.
The simulation of both signal and background gamma-ray events is carried out
with the Fermi Science Tools, and the dark matter distribution around Virgo is
taken from a N-body simulation of the nearby extragalactic Universe, with
constrained initial conditions provided by the CLUES project.
We find that a gravitino with a mass range of 0.6 to 2 GeV, and with a
lifetime range of about 3x10^27 to 2x10^28 s would be detectable by the
Fermi-LAT with a signal-to-noise ratio larger than 3. We also obtain that
gravitino masses larger than about 4 GeV are already excluded in the munuSSM by
Fermi-LAT data of the galactic halo | astro-ph_HE |
LeHaMoC: a versatile time-dependent lepto-hadronic modeling code for
high-energy astrophysical sources: Recent associations of high-energy neutrinos with active galactic nuclei
(AGN) have revived the interest in leptohadronic models of radiation from
astrophysical sources. The rapid increase in the amount of acquired
multi-messenger data will require soon fast numerical models that may be
applied to large source samples. We develop a time-dependent leptohadronic
code, LeHaMoC, that offers several notable benefits compared to other existing
codes, such as versatility and speed. LeHaMoC solves the Fokker-Planck
equations of photons and relativistic particles (i.e. electrons, positrons,
protons, and neutrinos) produced in a homogeneous magnetized source that may
also be expanding. The code utilizes a fully implicit difference scheme that
allows fast computation of steady-state and dynamically evolving physical
problems. We first present test cases where we compare the numerical results
obtained with LeHaMoC against exact analytical solutions and numerical results
computed with ATHE$\nu$A, a well-tested code of similar philosophy but
different numerical implementation. We find a good agreement (within 10-30%)
with the numerical results obtained with ATHE$\nu$A without evidence of
systematic differences. We then demonstrate the capabilities of the code
through illustrative examples. First, we fit the spectral energy distribution
from a jetted AGN in the context of a synchrotron-self Compton model and a
proton-synchrotron model using Bayesian inference. Second, we compute the
high-energy neutrino signal and the electromagnetic cascade induced by hadronic
interactions in the corona of NGC 1068. LeHaMoC is easily customized to model a
variety of high-energy astrophysical sources and has the potential to become a
widely utilized tool in multi-messenger astrophysics. | astro-ph_HE |
AT2017gfo: an anisotropic and three-component kilonova counterpart of
GW170817: The detection of a kilo/macronova electromagnetic counterpart (AT2017gfo) of
the first gravitational wave signal compatible with the merger of two neutron
stars (GW170817) has confirmed the occurrence of r-process nucleosynthesis in
this kind of events. The blue and red components of AT2017gfo have been
interpreted as the signature of multi-component ejecta in the merger dynamics.
However, the explanation of AT2017gfo in terms of the properties of the ejecta
and of the ejection mechanisms is still incomplete. In this work, we analyse
AT2017gfo with a new semi-analytic model of kilo/macronova inferred from
general relativistic simulations of the merger and long-term numerical models
of the merger aftermath. The model accounts for the anisotropic emission from
the three known mass ejecta components: dynamic, winds and secular outflows
from the disk. The early multi-band light-curves of AT2017gfo can only be
explained by the presence of a relatively low opacity component of the ejecta
at high latitudes. This points to the key role of weak interactions in setting
the ejecta properties and determining the nucleosynthetic yields. Our model
constrains also the total ejected mass associated to AT2017gfo to be between
$0.042$ and $0.077 M_{\odot}$; the observation angle of the source to be
between $\pi/12$ and $7\pi/36 $; and the mass of the disk to be $ \gtrsim 0.08
M_{\odot}$. | astro-ph_HE |
Bondi on spherically symmetric accretion: Hermann Bondi's 1952 paper "On spherically symmetrical accretion" is
recognized as one of the foundations of accretion theory. Although Bondi later
remarked that it was "not much more than an examination exercise", his
mathematical analysis of spherical accretion on to a point mass has found broad
use across fields of astrophysics that were embryonic or non-existent at the
time of the paper's publication. In this non-technical review, I describe the
motivations for Bondi's work, and briefly discuss some of the applications of
Bondi accretion in high energy astrophysics, galaxy formation, and star
formation. | astro-ph_HE |
Indications of non-conservative mass-transfer in AMXPs: Context. Since the discovery of the first Accreting Millisecond X-ray Pulsar
SAX J1808.4-3658 in 1998, the family of these sources kept growing on.
Currently, it counts 22 members. All AMXPs are transients with usually very
long quiescence periods, implying that mass accretion rate in these systems is
quite low and not constant. Moreover, for at least three sources, a
non-conservative evolution was also proposed.
Aims. Our purpose is to study the long term averaged mass-accretion rates in
all the Accreting Millisecond X-ray Pulsars discovered so far, to investigate a
non-conservative mass-transfer scenario.
Methods. We calculated the expected mass-transfer rate under the hypothesis
of a conservative evolution based on their orbital periods and on the (minimum)
mass of the secondary (as derived from the mass function), driven by
gravitational radiation and/or magnetic braking. Using this theoretical
mass-transfer, we determined the expected accretion luminosity of the systems.
Thus, we achieved the lower limit to the distance of the sources by comparing
the computed theoretical luminosity and the observed flux averaged over a time
period of 20 years. Then, the lower limit to the distance of the sources has
been compared to the value of the distance reported in literature to evaluate
how reasonable is the hypothesis of a conservative mass-transfer.
Results. Based on a sample of 18 sources, we found strong evidences of a
non-conservative mass-transfer for five sources, for which the estimated
distance lower limits are higher than their known distances. We also report
hints for mass outflows in other six sources. The discrepancy can be fixed
under the hypothesis of a non-conservative mass-transfer in which a fraction of
the mass transferred onto the compact object is swept away from the system,
likely due to the (rotating magnetic dipole) radiation pressure of the pulsar. | astro-ph_HE |
The First Two Years of Electromagnetic Follow-Up with Advanced LIGO and
Virgo: We anticipate the first direct detections of gravitational waves (GWs) with
Advanced LIGO and Virgo later this decade. Though this groundbreaking technical
achievement will be its own reward, a still greater prize could be observations
of compact binary mergers in both gravitational and electromagnetic channels
simultaneously. During Advanced LIGO and Virgo's first two years of operation,
2015 through 2016, we expect the global GW detector array to improve in
sensitivity and livetime and expand from two to three detectors. We model the
detection rate and the sky localization accuracy for binary neutron star (BNS)
mergers across this transition. We have analyzed a large, astrophysically
motivated source population using real-time detection and sky localization
codes and higher-latency parameter estimation codes that have been expressly
built for operation in the Advanced LIGO/Virgo era. We show that for most BNS
events the rapid sky localization, available about a minute after a detection,
is as accurate as the full parameter estimation. We demonstrate that Advanced
Virgo will play an important role in sky localization, even though it is
anticipated to come online with only one-third as much sensitivity as the
Advanced LIGO detectors. We find that the median 90% confidence region shrinks
from ~500 square degrees in 2015 to ~200 square degrees in 2016. A few distinct
scenarios for the first LIGO/Virgo detections emerge from our simulations. | astro-ph_HE |
Possible hard X-ray shortages in bursts from KS 1731-260 and 4U 1705-44: Aims: A hard X-ray shortage, implying the cooling of the corona, was observed
during bursts of IGR J17473-272, 4U 1636-536, Aql X-1, and GS 1826-238. Apart
from these four sources, we investigate here an atoll sample, in which the
number of bursts for each source is larger than 5, to explore the possible
additional hard X-ray shortage during {\it Rossi X-ray timing explorer (RXTE)}
era. Methods: According to the source catalog that shows type-I bursts, we
analyzed all the available pointing observations of these sources carried out
by the {\it RXTE} proportional counter array (PCA). We grouped and combined the
bursts according to their outburst states and searched for the possible hard
X-ray shortage while bursting. Results: We found that the island states of KS
1731-260 and 4U 1705-44 show a hard X-ray shortage at significant levels of 4.5
and 4.7 $\sigma$ and a systematic time lag of $0.9 \pm 2.1$ s and $2.5 \pm 2.0$
s with respect to the soft X-rays, respectively. While in their banana branches
and other sources, we did not find any consistent shortage. | astro-ph_HE |
The Role of Fission in Neutron Star Mergers and its Impact on the
r-Process Peaks: Comparing observational abundance features with nucleosynthesis predictions
of stellar evolution or explosion simulations can scrutinize two aspects: (a)
the conditions in the astrophysical production site and (b) the quality of the
nuclear physics input utilized. We test the abundance features of r-process
nucleosynthesis calculations for the dynamical ejecta of neutron star merger
simulations based on three different nuclear mass models: The Finite Range
Droplet Model (FRDM), the (quenched version of the) Extended Thomas Fermi Model
with Strutinsky Integral (ETFSI-Q), and the Hartree-Fock-Bogoliubov (HFB) mass
model. We make use of corresponding fission barrier heights and compare the
impact of four different fission fragment distribution models on the final
r-process abundance distribution. In particular, we explore the abundance
distribution in the second r-process peak and the rare-earth sub-peak as a
function of mass models and fission fragment distributions, as well as the
origin of a shift in the third r-process peak position. The latter has been
noticed in a number of merger nucleosynthesis predictions. We show that the
shift occurs during the r-process freeze-out when neutron captures and
{\beta}-decays compete and an (n,{\gamma})-({\gamma},n) equilibrium is not
maintained anymore. During this phase neutrons originate mainly from fission of
material above A = 240. We also investigate the role of {\beta}-decay
half-lives from recent theoretical advances, which lead either to a smaller
amount of fissioning nuclei during freeze-out or a faster (and thus earlier)
release of fission neutrons, which can (partially) prevent this shift and has
an impact on the second and rare-earth peak as well. | astro-ph_HE |
Supergiant, fast, but not so transient 4U 1907+09: We have investigated the dipping activity observed in the high-mass X-ray
binary 4U 1907+09 and shown that the source continues to pulsate in the "off"
state, noting that the transition between the "on" and "off" states may be
either dip-like or flare-like. This behavior may be explained in the framework
of the "gated accretion" scenario proposed to explain the flares in supergiant
fast X-ray transients (SFXTs). We conclude that 4U 1907+09 might prove to be a
missing link between the SFXTs and ordinary accreting pulsars. | astro-ph_HE |
The Fermi Haze: A Gamma-Ray Counterpart to the Microwave Haze: The Fermi Gamma-Ray Space Telescope reveals a diffuse inverse Compton signal
in the inner Galaxy with a similar spatial morphology to the microwave haze
observed by WMAP, supporting the synchrotron interpretation of the microwave
signal. Using spatial templates, we regress out pi0 gammas, as well as IC and
bremsstrahlung components associated with known soft-synchrotron counterparts.
We find a significant gamma-ray excess towards the Galactic center with a
spectrum that is significantly harder than other sky components and is most
consistent with IC from a hard population of electrons. The morphology and
spectrum are consistent with it being the IC counterpart to the electrons which
generate the microwave haze seen at WMAP frequencies. In addition, the implied
electron spectrum is hard; electrons accelerated in supernova shocks in the
disk which then diffuse a few kpc to the haze region would have a softer
spectrum. We describe the full sky Fermi maps used in this analysis and make
them available for download. | astro-ph_HE |
A Systematic Search for Periodically Varying Quasars in Pan-STARRS1: An
Extended Baseline Test in Medium Deep Survey Field MD09: We present a systematic search for periodically varying quasars and
supermassive black hole binary (SMBHB) candidates in the Pan-STARRS1 (PS1)
Medium Deep Survey's MD09 field. From a color-selected sample of 670 quasars
extracted from a multi-band deep-stack catalog of point sources, we locally
select variable quasars and look for coherent periods with the Lomb-Scargle
periodogram. Three candidates from our sample demonstrate strong variability
for more than ~3 cycles, and their PS1 light curves are well fitted to
sinusoidal functions. We test the persistence of the candidates' apparent
periodic variations detected during the 4.2 years of the PS1 survey with
archival photometric data from the SDSS Stripe 82 survey or new monitoring with
the Large Monolithic Imager at the Discovery Channel Telescope. None of the
three periodic candidates (including PSO J334.2028+1.4075) remain persistent
over the extended baseline of 7 - 14 years, corresponding to a detection rate
of < 1 in 670 quasars in a search area of 5 deg^2. Even though SMBHBs should be
a common product of the hierarchal growth of galaxies, and periodic variability
in SMBHBs has been theoretically predicted, a systematic search for such
signatures in a large optical survey is strongly limited by its temporal
baseline and the "red noise" associated with normal quasar variability. We show
that follow-up long-term monitoring (>5 cycles) is crucial to our search for
these systems. | astro-ph_HE |
Identification of AGN in the XMM-Newton X-ray survey of the SMC: Context: Finding Active Galactic Nuclei (AGN) behind the Magellanic Clouds
(MCs) is difficult because of the high stellar density in these fields.
Although the first AGN behind the Small Magellanic Cloud (SMC) were reported in
the 1980s, it is only recently that the number of AGN known behind the SMC has
increased by several orders of magnitude. Aims: The mid-infrared colour
selection technique has been proven to be an efficient means of identifying
AGN, especially obscured sources. The X-ray regime is complementary in this
regard and we use XMM-Newton observations to support the identification of AGN
behind the SMC. Methods: We present a catalogue of AGN behind the SMC by
correlating an updated X-ray point source catalogue from our XMM-Newton survey
of the SMC with already known AGN from the literature as well as a list of
candidates obtained from the ALLWISE mid-infrared colour selection criterion.
We studied the properties of the sample with respect to their redshifts,
luminosities and X-ray spectral characteristics. We also identified the
near-infrared counterpart of the sources from the VISTA observations. Results:
The redshift and luminosity distributions of the sample (where known) indicate
that we detect sources from nearby Seyfert galaxies to distant and obscured
quasars. The X-ray hardness ratios are compatible with those typically expected
for AGN. The VISTA colours and variability are also consistent in this regard.
A positive correlation was observed between the integrated X-ray flux (0.2--12
keV) and the ALLWISE and VISTA magnitudes. We further present a sample of new
candidate AGN and candidates for obscured AGN. All of these make an interesting
subset for further follow-up studies. An initial spectroscopic follow-up of 6
out of the 81 new candidates showed all six sources are active galaxies, albeit
two with narrow emission lines. | astro-ph_HE |
A dispersion excess from pulsar wind nebulae and supernova remnants:
Implications for pulsars and FRBs: Young pulsars and the pulsar wind nebulae (PWNe) or supernova remnants (SNRs)
that surround them are some of the most dynamic and high-powered environments
in our Universe. With the rise of more sensitive observations, the number of
pulsar-SNR and PWN associations (hereafter, SNR/PWN) has increased, yet we do
not understand to which extent this environment influences the pulsars'
impulsive radio signals. We studied the dispersive contribution of SNRs and
PWNe on Galactic pulsars, and considered their relevance to fast radio bursts
(FRBs) such as FRB 121102. We investigated the dispersion measure (DM)
contribution of SNRs and PWNe by comparing the measured DMs of Galactic pulsars
in a SNR/PWN to the DM expected only from the intervening interstellar
electrons, using the NE2001 model. We find that a two-$\sigma$ DM contribution
of SNRs and PWNe to the pulsar signal exists, amounting to $21.1 \pm 10.6$ pc
cm$^{-3}$. The control sample of pulsars unassociated with a SNR/PWN shows no
excess. We model the SNR and PWN electron densities for each young pulsar in
our sample and show that these indeed predict an excess of this magnitude. By
extrapolating to the kind of fast-spinning, high magnetic field, young pulsars
that may power FRBs, we show their SNR and PWN are capable of significantly
contributing to the observed DM. | astro-ph_HE |
Returning radiation in strong gravity around black holes: Reverberation
from the accretion disc: We study reflected X-ray emission that returns to the accretion disc in the
strong gravitational fields around black holes using General Relativistic ray
tracing and radiative transfer calculations. Reflected X-rays that are produced
when the inner regions of the disc are illuminated by the corona are subject to
strong gravitational light bending, causing up to 47 per cent of the reflected
emission to be returned to the disc around a rapidly spinning black hole,
depending upon the scale height of the corona. The iron K line is enhanced
relative to the continuum by 25 per cent, and the Compton hump by up to a
factor of three. Additional light travel time between primary and secondary
reflections increases the reverberation time lag measured in the iron K band by
49 per cent, while the soft X-ray lag is increased by 25 per cent and the
Compton hump response time is increased by 60 per cent. Measured samples of
X-ray reverberation lags are shown to be consistent with X-rays returning to
the accretion disc in strong gravity. Understanding the effects of returning
radiation is important in interpreting reverberation observations to probe
black holes. Reflected X-rays returning to the disc can be uniquely identified
by blueshifted returning iron K line photons that are Compton scattered from
the inner disc, producing excess, delayed emission in the 3.5-4.5keV energy
range that will be detectable with forthcoming X-ray observatories,
representing a unique test of General Relativity in the strong field limit. | astro-ph_HE |
Multiwavelength flaring activity of PKS1510-089: Aims: In this work, we analyse the multiwavelength brightness variations and
flaring activity of FSRQ PKS1510-089, aiming to constrain the position of the
emission sources.
Methods: We report 7 mm (43 GHz) radio and R-band polarimetric observations
of PKS1510-089. The radio observations were performed at the Itapetinga Radio
Observatory, while the polarimetric data were obtained at the Pico dos Dias
Observatory. The 7 mm observations cover the period between 2011 and 2013,
while the optical polarimetric observations were made between 2009 and 2012.
Results: At 7 mm, we detected a correlation between four radio and
$\gamma$-ray flares with a delay of about 54 days between them; the higher
frequency counterpart occurred first. Using optical polarimetry, we detected a
large variation in polarization angle (PA) within two days associated with the
beginning of a $\gamma$-ray flare. Complementing our data with other data
obtained in the literature, we show that PA presented rotations associated with
the occurrence of flares.
Conclusions: Our results can be explained by a shock-in-jet model, in which a
new component is formed in the compact core producing an optical and/or
$\gamma$-ray flare, propagates along the jet, and after some time becomes
optically thin and is detected as a flare at radio frequencies. The variability
in the polarimetric parameters can also be reproduced; we can explain large
variation in both PA and polarization degree (PD), in only one of them, or in
neither, depending on the differences in PA and PD between the jet and the new
component. | astro-ph_HE |
Equation-of-State Constraints on the Neutron-Star Binding Energy and
Tests of Binary Formation Scenarios: The second supernova that forms double-neutron-star systems is expected to
occur in a progenitor that is ultra-stripped due to binary interactions. Thus,
the secondary neutron star's mass as well as the post-supernova binary's
orbital parameters will depend on the nature of the collapsing progenitor core.
Since neutron stars are in the strong-gravity regime, their binding energy
makes up a significant fraction of their total mass-energy budget. The second
neutron star's binding energy may thus provide a unique insight as to whether
its progenitor was a low-mass iron core or an oxygen-neon-magnesium core. I
obtain relations for the baryonic mass and binding energy incorporating both a
hadronic equation-of-state catalog as well as recent multi-messenger
neutron-star observations. With these relations, I obtain the first tight
constraints on the baryonic mass and binding energy of three neutron stars that
are thought to have formed from an ultra-stripped progenitor. With these
constraints, I test if each neutron star is consistent with forming from an
ONeMg core that undergoes an electron-capture supernova. From these tests, I
find that this scenario can be ruled out for one of three neutron stars.
Neutron-star properties and the dense-matter equation of state can thus help
distinguish binary formation scenarios. | astro-ph_HE |
Evidence of a non-conservative mass transfer in the ultra-compact X-ray
source XB 1916-053: The dipping source XB 1916-053 is a compact binary system with an orbital
period of 50 min harboring a neutron star. Using ten new {\it Chandra}
observations and one {\it Swift/XRT} observation, we are able to extend the
baseline of the orbital ephemeris; this allows us to exclude some models that
explain the dip arrival times. The Chandra observations provide a good plasma
diagnostic of the ionized absorber and allow us to determine whether it is
placed at the outer rim of the accretion disk or closer to the compact object.
From the available observations we are able to obtain three new dip arrival
times extending the baseline of the orbital ephemeris from 37 to 40 years. From
the analysis of the dip arrival times we confirm an orbital period derivative
of $\dot{P}=1.46(3) \times 10^{-11}$ s s$^{-1}$. We show that the $\dot{P}$
value and the luminosity values are compatible with a mass accretion rate lower
than 10\% of the mass transfer rate. We show that the mass ratio $q=m_2/m_1$ of
0.048 explains the apsidal precession period and the nodal precession period.
The observed absorption lines are associated with the presence of \ion{Ne}{x},
\ion{Mg}{xii}, \ion{Si}{xiv}, \ion{S}{xvi,} and \ion{Fe}{xxvi} ions. We observe
a redshift in the absorption lines between $1.1 \times 10^{-3}$ and $1.3 \times
10^{-3}$. By interpreting it as gravitational redshift, as recently discussed
in the literature, we find that the ionized absorber is placed at a distance of
$10^8$ cm from the neutron star with a mass of 1.4 M$_{\odot}$ and has a
hydrogen atom density greater than $10^{15}$ cm$^{-3}$. (Abstract abridged) | astro-ph_HE |
The Variable Pulsar Wind Nebula of PSR J1809-1917: PSR J1809-1917 is a young ($\tau=51$ kyr) energetic
($\dot{E}=1.8\times10^{36}$ erg s$^{-1}$) radio pulsar powering a pulsar wind
nebula (PWN). We report on the results of three Chandra X-ray Observatory
observations which show that the PWN consists of a small ($\sim 20''$) bright
compact nebula (CN) and faint extended emission seen up to $2'$ from the
pulsar. The CN is elongated in the northeast-southwest direction and exhibits
morphological and flux variability on a timescale of a few months. We also find
evidence of small arcsecond-scale jets extending from the pulsar along the same
direction, and exhibiting a hard power-law (PL) spectrum with photon index
$\Gamma_{\rm jet}=1.2\pm0.1$. The more extended emission and CN share the same
symmetry axis, which is also aligned with the direction toward the TeV
$\gamma$-ray source HESS J1809--193, supporting their association. The spectrum
of the extended nebula (EN) fits an absorbed PL with about the same slope as
that of the CN, $\Gamma_{\rm CN}\approx\Gamma_{\rm EN}=1.55\pm0.09$; no
spectral changes across the EN's 2 pc extent are seen. The total PWN 0.5-8 keV
luminosity is $L_{\rm PWN}\approx 9\times10^{32}$ erg s$^{-1}$, about half of
which is due to the EN. | astro-ph_HE |
Magnetosphere of an orbiting neutron star: We conduct force-free simulations of a single neutron star undergoing orbital
motion in flat spacetime, mimicking the trajectory of the star about the center
of mass on a compact binary system. Our attention is focused on the kinetic
energy being extracted from the orbit by the acceleration of the magnetic
dipole moment of the neutron star, and particularly, on how this energy gets
distributed within its surrounding magnetosphere. A detailed study of the
resulting magnetospheric configurations in our setting is presented,
incorporating as well the effects due to neutron star spin and the misalignment
of the magnetic and orbital axes. We find many features resembling those of
pulsar magnetospheres for the orbiting neutron star --even in the absence of
spin--, being of particular interest the development of a spiral current sheet
that extends beyond the light cylinder. Then, we use recent advances in pulsar
theory to estimate electromagnetic emissions produced at the reconnection
regions of such current sheets. | astro-ph_HE |
The Redshift Dependence of the Radio Flux of Gamma-Ray Bursts and Their
Host Galaxies: Using multiwavelength observations of radio afterglows, we confirm the
hypothesis that the flux density of gamma-ray bursts (GRBs) at a fixed
observing frequency is invariable when the distance of the GRBs increases,
which means the detection rate will be approximately independent of redshift.
We study this behavior theoretically and find that it can be well explained by
the standard forward shock model involving a thin shell expanding in either a
homogeneous interstellar medium (ISM) or a wind environment. We also found that
short GRBs and supernova-associated GRBs, which are at relatively smaller
distances, marginally match the flux-redshift relationship and they could be
outliers. We rule out the assumption that the medium density evolves with
redshift as $n\propto(1+z)^4$ from the current measurements of $n$ and $z$ for
short and long GRBs. In addition, the possible dependence of host flux on the
redshift is also investigated. We find that a similar redshift independence of
the flux exists for host galaxies as well, which implies that the detection
rate of radio hosts might also be independent of the redshift. It is also
hinted that most radio hosts have the spectral indices ranging from
$\beta_h\simeq-1$ to 2.5 in statistics. Finally, we predict the detection rates
of radio afterglows by the next-generation radio telescopes such as the
Five-hundred meter Aperture Spherical Telescope (FAST) and the Square Kilometer
Array (SKA). | astro-ph_HE |
Searching for TeV Dark Matter in Irregular dwarf galaxies with HAWC
Observatory: We present the results of dark matter (DM) searches in a sample of 31 dwarf
irregular (dIrr) galaxies within the field of view of the HAWC Observatory.
dIrr galaxies are DM dominated objects, which astrophysical gamma-ray emission
is estimated to be negligible with respect to the secondary gamma-ray flux
expected by annihilation or decay of Weakly Interacting Massive Particles
(WIMPs). While we do not see any statistically significant DM signal in dIrr
galaxies, we present the exclusion limits ($95\%~\text{C.L.}$) for annihilation
cross-section and decay lifetime for WIMP candidates with masses between $1$
and $100~\text{TeV}$. Exclusion limits from dIrr galaxies are relevant and
complementary to benchmark dwarf Spheroidal (dSph) galaxies. In fact, dIrr
galaxies are targets kinematically different from benchmark dSph, preserving
the footprints of different evolution histories. We compare the limits from
dIrr galaxies to those from ultrafaint and classical dSph galaxies previously
observed with HAWC. We find that the contraints are comparable to the limits
from classical dSph galaxies and $\thicksim2$ orders of magnitude weaker than
the ultrafaint dSph limits. | astro-ph_HE |
From Observations near the Earth to the Local Interstellar Spectra: Propagation of cosmic rays (CRs) from their sources to the observer is
described mainly as plain diffusion at high energies, while at lower energies
there are other physical processes involved, both in the interstellar space and
in the heliosphere. The latter was a subject of considerable uncertainty until
recently. New data obtained by several CR missions can be used to and the local
interstellar spectra (LIS) of CR species that would significantly reduce the
uncertainties associated with the heliospheric propagation. In this paper we
present the LIS of CR protons and helium outside the heliospheric boundary. The
proposed LIS are tuned to accommodate both, the low energy CR spectra measured
by Voyager 1, and the high energy observations publicly released by BESS,
Pamela, AMS-01 and AMS-02. The proton and helium LIS are derived by combining
CR propagation in the Galaxy, as described by GALPROP, with the heliospheric
modulation computed using the HelMod Monte Carlo Tool. The proposed LIS are
tuned to reproduce the modulated spectra for both, high and low, levels of
solar activity. | astro-ph_HE |
The potential role of binary neutron star merger afterglows in
multimessenger cosmology: Binary neutron star mergers offer a new and independent means of measuring
the Hubble constant $H_0$ by combining the gravitational-wave inferred source
luminosity distance with its redshift obtained from electromagnetic follow-up.
This method is limited by intrinsic degeneracy between the system distance and
orbital inclination in the gravitational-wave signal. Observing the afterglow
counterpart to a merger can further constrain the inclination angle, allowing
this degeneracy to be partially lifted and improving the measurement of $H_0$.
In the case of the binary neutron star merger GW170817, afterglow light-curve
and imagery modeling thus allowed to improve the $H_0$ measurement by a factor
of 3. However, systematic access to afterglow data is far from guaranteed. In
fact, though each one allows a leap in $H_0$ precision, these afterglow
counterparts should prove rare in forthcoming multimessenger campaigns. We
combine models for emission and detection of gravitational-wave and
electromagnetic radiation from binary neutron star mergers with realistic
population models and estimates for afterglow inclination angle constraints.
Using these models, we quantify how fast $H_0$ will be narrowed-down by
successive multimessenger events with and without the afterglow. We find that,
because of its rareness and though it greatly refines angle estimates, the
afterglow counterpart should not significantly contribute to the measurement of
$H_0$ in the long run. | astro-ph_HE |
A Wolf-Rayet-like progenitor of supernova SN 2013cu from spectral
observations of a wind: The explosive fate of massive stripped Wolf-Rayet (W-R) stars is a key open
question in stellar physics. An appealing option is that hydrogen-deficient W-R
stars are the progenitors of some H-poor supernova (SN) explosions of Types
IIb, Ib, and Ic. A blue object, having luminosity and colors consistent with
those of some W-R stars, has been recently identified at the location of a
SN~Ib in pre-explosion images but has not yet been conclusively determined to
have been the progenitor. Similar previous works have so far only resulted in
nondetections. Comparison of early photometric observations of Type Ic
supernovae with theoretical models suggests that the progenitor stars had radii
<10^12 cm, as expected for some W-R stars. However, the hallmark signature of
W-R stars, their emission-line spectra, cannot be probed by such studies. Here,
we report the detection of strong emission lines in an early-time spectrum of
SN 2013cu (iPTF13ast; Type IIb) obtained ~15.5 hr after explosion ("flash
spectroscopy"). We identify W-R-like wind signatures suggesting a progenitor of
the WN(h) subclass. The extent of this dense wind may indicate increased mass
loss from the progenitor shortly prior to its explosion, consistent with recent
theoretical predictions. | astro-ph_HE |
Peculiarly Narrow SED of GRB 090926B with MAXI and Fermi/GBM: The monitor of all-sky X-ray image (MAXI) Gas Slit Camera (GSC) on the
International Space Station (ISS) detected a gamma-ray burst (GRB) on 2009,
September 26, GRB\,090926B. This GRB had extremely hard spectra in the X-ray
energy range. Joint spectral fitting with the Gamma-ray Burst Monitor on the
Fermi Gamma-ray Space Telescope shows that this burst has peculiarly narrow
spectral energy distribution and is represented by Comptonized blackbody model.
This spectrum can be interpreted as photospheric emission from the low
baryon-load GRB fireball. Calculating the parameter of fireball, we found the
size of the base of the flow $r_0 = (4.3 \pm 0.9) \times 10^{9} \, Y^{\prime \,
-3/2}$ cm and Lorentz factor of the plasma $\Gamma = (110 \pm 10) \, Y^{\prime
\, 1/4}$, where $Y^{\prime}$ is a ratio between the total fireball energy and
the energy in the blackbody component of the gamma-ray emission. This $r_0$ is
factor of a few larger, and the Lorentz factor of 110 is smaller by also factor
of a few than other bursts that have blackbody components in the spectra. | astro-ph_HE |
Radio Analysis of SN 2004C Reveals an Unusual CSM Density Profile as a
Harbinger of Core Collapse: We present extensive multi-frequency VLA and VLBA observations of the
radio-bright supernova (SN) IIb SN 2004C that span $\sim(40-2793)$ days
post-explosion. We interpret the temporal evolution of the radio spectral
energy distribution (SED) in the context of synchrotron self-absorbed (SSA)
emission from the explosion's forward shock as it expands in the circumstellar
medium (CSM) previously sculpted by the mass-loss history of the stellar
progenitor. VLBA observations and modeling of the VLA data point to a blastwave
with average velocity $\sim0.06c$ that carries an energy of $\sim 10^{49}$ erg.
Our modeling further reveals a flat CSM density profile $\rho_{\rm{CSM}}
\propto R^{-0.03 \pm0.22}$ up to a break radius $R_{br} \approx (1.96 \pm 0.10)
\times 10^{16}$ cm, with a steep density gradient following $\rho_{\rm{CSM}}
\propto R^{-2.3 \pm 0.5}$ at larger radii. We infer that the flat part of the
density profile corresponds to a CSM shell with mass $\sim0.021 M_{\odot}$, and
that the progenitor's effective mass-loss rate varied with time over the range
$(50-500) \times 10^{-5} M_{\odot} \rm{yr}^{-1}$ for an adopted wind velocity
$v_w =1000$ km $s^{-1}$ and shock microphysical parameters ${\epsilon}_e = 0.1,
{\epsilon}_B = 0.01$. These results add to the mounting observational evidence
for departures from the traditional single-wind mass-loss scenarios in evolved,
massive stars in the centuries leading up to core collapse. Potentially viable
scenarios include mass loss powered by gravity waves and/or interaction with a
binary companion. | astro-ph_HE |
Consequences of a Dark Disk for the Fermi and PAMELA Signals in Theories
with a Sommerfeld Enhancement: Much attention has been given to dark matter explanations of the PAMELA
positron fraction and Fermi electronic excesses. For those theories with a
TeV-scale WIMP annihilating through a light force-carrier, the associated
Sommerfeld enhancement provides a natural explanation of the large boost factor
needed to explain the signals, and the light force-carrier naturally gives rise
to hard cosmic ray spectra without excess pi0 gamma rays or anti-protons. The
Sommerfeld enhancement of the annihilation rate, which at low relative
velocities v scales as 1/v, relies on the comparatively low velocity dispersion
of the dark matter particles in the smooth halo. Dark matter substructures in
which the velocity dispersion is smaller than in the smooth halo have even
larger annihilation rates. N-body simulations containing only dark matter
predict the existence of such structures, for example subhalos and caustics,
and the effects of these substructures on dark matter indirect detection
signals have been studied extensively. The addition of baryons into
cosmological simulations of disk-dominated galaxies gives rise to an additional
substructure component, a dark disk. The disk has a lower velocity dispersion
than the spherical halo component by a factor ~6, so the contributions to dark
matter signals from the disk can be more significant in Sommerfeld models than
for WIMPs without such low-velocity enhancements. We consider the consequences
of a dark disk on the observed signals of cosmic rays as measured by Fermi and
PAMELA in models where the WIMP annihilations are into a light boson. We find
that both the PAMELA and Fermi results are easily accomodated by scenarios in
which a disk signal is included with the standard spherical halo signal. Limits
from extrapolations to the center of the galaxy can also be modified. | astro-ph_HE |
Chandra Phase-Resolved Spectroscopy of the High Magnetic Field Pulsar
B1509-58: We report on a timing and spectral analysis of the young, high magnetic field
rotation-powered pulsar (RPP) B1509-58 using Chandra continuous-clocking mode
observation. The pulsar's X-ray light curve can be fit by the two Gaussian
components and the pulsed fraction shows moderate energy dependence over the
Chandra band. The pulsed X-ray spectrum is well described by a power law with a
photon index 1.16(4), which is harder than the values measured with RXTE PCA
and NuSTAR. This result supports the log-parabolic model for the broadband
X-ray spectrum. With the unprecedented angular resolution of Chandra, we
clearly identified off-pulse X-ray emission from the pulsar, and its spectrum
is best fit by a power law plus blackbody model. The latter component has a
temperature of ~0.14 keV with a bolometric luminosity comparable to the
luminosities of other young and high magnetic field RPPs, and it lies between
the temperature of magnetars and typical RPPs. In addition, we found that the
nonthermal X-ray emission of PSR B1509-58 is significantly softer in the
off-pulse phase than in the pulsed phase, with the photon index varying between
1.0 and 1.8 and anticorrelated with the flux. This is similar to the behavior
of three other young pulsars. We interpreted it as different contributions of
pair-creation processes at different altitudes from the neutron star surface
according to the outer-gap model. | astro-ph_HE |
Evolution of the radio - X-ray coupling throughout an entire outburst of
Aquila X-1: The 2009 November outburst of the neutron star X-ray binary Aquila X-1 was
observed with unprecedented radio coverage and simultaneous pointed X-ray
observations, tracing the radio emission around the full X-ray hysteresis loop
of the outburst for the first time. We use these data to discuss the disc-jet
coupling, finding the radio emission to be consistent with being triggered at
state transitions, both from the hard to the soft spectral state and vice
versa. Our data appear to confirm previous suggestions of radio quenching in
the soft state above a threshold X-ray luminosity of about 10% of the Eddington
luminosity. We also present the first detections of Aql X-1 with Very Long
Baseline Interferometry (VLBI), showing that any extended emission is
relatively diffuse, and consistent with steady jets rather than arising from
discrete, compact knots. In all cases where multi-frequency data were
available, the source radio spectrum is consistent with being flat or slightly
inverted, suggesting that the internal shock mechanism that is believed to
produce optically thin transient radio ejecta in black hole X-ray binaries is
not active in Aql X-1. | astro-ph_HE |
X-ray Pulsations from the region of the Supergiant Fast X-ray Transient
IGR J17544-2619: Phase-targeted RXTE observations have allowed us to detect a transient 71.49
\pm 0.02 s signal that is most likely to be originating from the supergiant
fast X-ray transient IGR J17544-2619. The phase-folded light curve shows a
possible double-peaked structure with a pulsed flux of ~4.8*10^-12 erg cm^-2
s^-1 (3-10 keV). Assuming the signal to indicate the spin period of the neutron
star in the system, the provisional location of IGR J17544-2619 on the Corbet
diagram places the system within the classical wind-fed supergiant XRB region.
Such a result illustrates the growing trend of supergiant fast X-ray transients
to span across both of the original classes of HMXB in Porb - Pspin space. | astro-ph_HE |
StrayCats: A catalog of NuSTAR Stray Light Observations: We present StrayCats: a catalog of NuSTAR stray light observations of X-ray
sources. Stray light observations arise for sources 1--4$^{\circ}$ away from
the telescope pointing direction. At this off-axis angle, X-rays pass through a
gap between optics and aperture stop and so do not interact with the X-ray
optics but, instead, directly illuminate the NuSTAR focal plane. We have
systematically identified and examined over 1400 potential observations
resulting in a catalog of 436 telescope fields and 78 stray light sources that
have been identified. The sources identified include historically known
persistently bright X-ray sources, X-ray binaries in outburst, pulsars, and
Type I X-ray bursters. In this paper we present an overview of the catalog and
how we identified the StrayCats sources and the analysis techniques required to
produce high level science products. Finally, we present a few brief examples
of the science quality of these unique data. | astro-ph_HE |
Spectral and polarization properties of black hole accretion disc
emission: including absorption effects: The study of radiation emitted from black hole accretion discs represents a
crucial way to understand the main physical properties of these sources, and in
particular the black hole spin. Beside spectral analysis, polarimetry is
becoming more and more important, motivated by the development of new
techniques which will soon allow to perform measurements also in the X- and
{\gamma}-rays. Photons emitted from black hole accretion discs in the soft
state are indeed expected to be polarized, with an energy dependence which can
provide an estimate of the black hole spin. Calculations performed so far,
however, considered scattering as the only process to determine the
polarization state of the emitted radiation, implicitly assuming that the
temperatures involved are such that material in the disc is entirely ionized.
In this work we generalize the problem by calculating the ionization structure
of a surface layer of the disc with the public code CLOUDY , and then by
determining the polarization properties of the emerging radiation using the
Monte Carlo code STOKES . This allows us to account for absorption effects
alongside scattering ones. We show that including absorption can deeply modify
the polarization properties of the emerging radiation with respect to what is
obtained in the pure-scattering limit. As a general rule, we find that the
polarization degree is larger when absorption is more important, which occurs
e.g. for low accretion rates and/or spins when the ionization of the matter in
the innermost accretion disc regions is far from complete. | astro-ph_HE |
Modelling Continuum Reverberation in AGN: A Spectral-Timing Analysis of
the UV Variability Through X-ray Reverberation in Fairall 9: Continuum reverberation mapping of AGN can provide new insight into the
nature and geometry of the accretion flow. Some of the X-rays from the central
corona irradiating the disc are absorbed, increasing the local disc
temperature. This gives an additional re-processed contribution to the spectral
energy distribution (SED) which is lagged and smeared relative to the driving
X-ray light-curve. We directly calculate this reverberation from the accretion
disc, creating fully time dependent SEDs for a given X-ray light-curve. We
apply this to recent intensive monitoring data on Fairall 9, and find that it
is not possible to produce the observed UV variability by X-ray reprocessing of
the observed light-curve from the disc. Instead, we find that the majority of
the variability must be intrinsic to the UV emission process, adding to
evidence from changing look AGN that this region has a structure which is quite
unlike a Shakura-Sunyaev disc. We filter out this long timescale variability
and find that reprocessing alone is still insufficient to explain even the fast
variability in our assumed geometry of a central source illuminating a flat
disc. The amplitude of reprocessing can be increased by any vertical structure
such as the BLR and/or an inner disc wind, giving a better match. Fundamentally
though the model is missing the major contributor to the variability, intrinsic
to the UV/EUV emission rather than arising from reprocessing. | astro-ph_HE |
The chemical enrichment of long-GRB nurseries up to z=2: We investigate the existence of a metallicity threshold for the production of
long gamma-ray bursts (LGRBs). We used the host galaxies of the Swift/BAT6
sample of LGRBs. We considered the stellar mass, star formation rate (SFR), and
metallicity determined from the host galaxy photometry and spectroscopy up to z
= 2 and used them to compare the distribution of host galaxies to that of field
galaxies in the mass-metallicity and fundamental metallicity relation plane. We
find that although LGRBs also form in galaxies with relatively large stellar
masses, the large majority of host galaxies have metallicities below
log(O=H)~8.6. The extension to z = 2 results in a good sampling of stellar
masses also above Log(Mstar/Msun)~9.5 and provides evidence that LGRB host
galaxies do not follow the fundamental metallicity relation. As shown by the
comparison with dedicated numerical simulations of LGRB host galaxy population,
these results are naturally explained by the existence of a mild (~0.7 Zsun)
threshold for the LGRB formation. The present statistics does not allow us to
discriminate between different shapes of the metallicity cutoff, but the
relatively high metallicity threshold found in this work is somewhat in
disagreement to most of the standard single-star models for LGRB progenitors. | astro-ph_HE |
Fermi Observation of the transitional pulsar binary XSS J12270-4859: Because of the disappearance of its accretion disk since the time period
around 2012 November--December, XSS J12270-4859 has recently been identified
as, in addition to PSR J1023+0038, another transitional millisecond pulsar
binary. We have carried out detailed analysis of the Fermi Large Area Telescope
data for the binary. While both spectra before and after the disk-disappearance
transition are well described by an exponentially cut-off power law, typical
for pulsars' emission in the Fermi's 0.2-300 GeV, a factor of 2 flux decrease
related to the transition is detected. A weak orbital modulation is seen, but
only detectable in the after-transition data, same to that found at X-rays. In
the long-term light curve of the source before the transition, a factor of 3
flux variations are seen. Comparing to the properties of J1023+0038, we disucss
the implications from these results. We suggest that since the modulation is
aligned with that at X-rays in orbital phase, it possibly arises due to the
occultation of the gamma-ray emitting region by the companion. The origin of
the variations in the long-term light curve is not clear, because the source
field also contains unidentified radio or X-ray sources and their contamination
can not be excluded. Multi-wavelength observations of the source field will
help identify the origin of the variations by detecting any related flux
changes from the in-field sources. | astro-ph_HE |
Cosmic ray Transport in Magnetohydrodynamic turbulence: This paper studies cosmic ray (CR) transport in magneto hydrodynamic (MHD)
turbulence. CR transport is strongly dependent on the properties of the
magnetic turbulence. We perform test particle simulations to study the
interactions of CR with both total MHD turbulence and decomposed MHD modes. The
spatial diffusion coefficients and the pitch angle scattering diffusion
coefficients are calculated from the test particle trajectories in turbulence.
Our results confirm that the fast modes dominate the CR propagation, whereas
Alfv\'en and slow modes are much less efficient and have shown similar pitch
angle scattering rates. We investigate the cross field transport on large and
small scales. On large/global scales, normal diffusion is observed and the
diffusion coefficient is suppressed by $M_A^\zeta$ compared to the parallel
diffusion coefficients, with $\zeta$ closer to 4 in Alfv\'en modes than that in
total turbulence as theoretically expected. For the CR transport on scales
smaller than the turbulence injection scale, both the local and global magnetic
reference frames are adopted. Super diffusion is observed on such small scales
in all the cases. Particularly, CR transport in Alfv\'en modes show clear
Richardson diffusion in the local reference frame. Our results have broad
applications to CRs in various astrophysical environments. | astro-ph_HE |
Radio quasars and the link with GAIA: Modeling VLBI ejections of nuclei of extragalactic radio sources, indicates
that their nuclei contain a binary black hole system. One can derive the
distance and the positions of the two black holes in the plane of the sky. We
can also use the RMS of the time series of the ICRF2 survey to obtain an
estimate of the structure and the size of the nuclei. We will discuss the
possible problems to link VLBI observations and GAIA optical observations of
radio quasars if they contain a binary black hole system. | astro-ph_HE |
Observations of the Very Young Type Ia Supernova 2019np with
Early-excess Emission: Early-time radiative signals from type Ia supernovae (SNe Ia) can provide
important constraints on the explosion mechanism and the progenitor system. We
present observations and analysis of SN 2019np, a nearby SN Ia discovered
within 1-2 days after the explosion. Follow-up observations were conducted in
optical, ultraviolet, and near-infrared bands, covering the phases from
$\sim-$16.7 days to $\sim$+367.8 days relative to its $B-$band peak luminosity.
The photometric and spectral evolutions of SN 2019np resembles the average
behavior of normal SNe Ia. The absolute B-band peak magnitude and the post-peak
decline rate are $M_{\rm max}(B)=-19.52 \pm 0.47$mag and $\Delta m_{\rm15}(B)
=1.04 \pm 0.04$mag, respectively. No Hydrogen line has been detected in the
near-infrared and nebular-phase spectra of SN 2019np. Assuming that the
$^{56}$Ni powering the light curve is centrally located, we find that the
bolometric light curve of SN 2019np shows a flux excess up to 5.0% in the early
phase compared to the radiative diffusion model. Such an extra radiation
perhaps suggests the presence of an additional energy source beyond the
radioactive decay of central nickel. Comparing the observed color evolution
with that predicted by different models such as interactions of SN ejecta with
circumstellar matter (CSM)/companion star, a double-detonation explosion from a
sub-Chandrasekhar mass white dwarf (WD), and surface $^{56}$Ni mixing, the
latter one is favored. | astro-ph_HE |
Mass composition of ultra-high-energy cosmic rays with the Telescope
Array Surface Detector Data: The results on ultra-high-energy cosmic rays (UHECR) mass composition
obtained with the Telescope Array surface detector are presented. The analysis
employs the boosted decision tree (BDT) multivariate analysis built upon 14
observables related to both the properties of the shower front and the lateral
distribution function. The multivariate classifier is trained with Monte-Carlo
sets of events induced by the primary protons and iron. An average atomic mass
of UHECR is presented for energies $10^{18.0}-10^{20.0}\ \mbox{eV}$. The
average atomic mass of primary particles shows no significant energy dependence
and corresponds to $\langle \ln A \rangle = 2.0 \pm 0.1 (stat.) \pm 0.44
(syst.)$. The result is compared to the mass composition obtained by the
Telescope Array with $\mbox{X}_{\mbox{max}}$ technique along with the results
of other experiments. Possible systematic errors of the method are discussed. | astro-ph_HE |
A Model for Pair Production Limit Cycles in Pulsar Magnetospheres: It was recently proposed that the electric field oscillation as a result of
self-consistent $e^{\pm}$ pair production may be the source of coherent radio
emission from pulsars. Direct Particle-in-Cell (PIC) simulations of this
process have shown that the screening of the parallel electric field by this
pair cascade manifests as a limit cycle, as the parallel electric field is
recurrently induced when pairs produced in the cascade escape from the gap
region. In this work, we develop a simplified time-dependent kinetic model of
$e^{\pm}$ pair cascades in pulsar magnetospheres that can reproduce the
limit-cycle behavior of pair production and electric field screening. This
model includes the effects of a magnetospheric current, the escape of
$e^{\pm}$, as well as the dynamic dependence of pair production rate on the
plasma density and energy. Using this simple theoretical model, we show that
the power spectrum of electric field oscillations averaged over many limit
cycles is compatible with the observed pulsar radio spectrum. | astro-ph_HE |
Magnetar Activity via the Density-Shear Instability in Hall-MHD: We investigate the density-shear instability in Hall-MHD via numerical
simulation of the full non-linear problem, in the context of magnetar activity.
We confirm the development of the instability of a plane-parallel magnetic
field with an appropriate intensity and electron density profile, in accordance
with analytic theory. We find that the instability also appears for a
monotonically decreasing electron number density and magnetic field, a
plane-parallel analogue of an azimuthal or meridional magnetic field in the
crust of a magnetar. The growth rate of the instability depends on the Hall
properties of the field (magnetic field intensity, electron number density and
the corresponding scale-heights), while being insensitive to weak resistivity.
Since the Hall effect is the driving process for the evolution of the crustal
magnetic field of magnetars, we argue that this instability is critical for
systems containing strong meridional or azimuthal fields. We find that this
process mediates the formation of localised structures with much stronger
magnetic field than the average, which can lead to magnetar activity and
accelerate the dissipation of the field and consequently the production of
Ohmic heating. Assuming a $5\times10^{14}$G magnetic field at the base of
crust, we anticipate that magnetic field as strong as $10^{15}$G will easily
develop in regions of typical size of a few $10^{2}$ meters, containing
magnetic energy of $10^{43}$erg, sufficient to power magnetar bursts. These
active regions are more likely to appear in the magnetic equator where the
tangential magnetic field is stronger. | astro-ph_HE |
An Ultra-Fast X-ray Disk Wind in the Neutron Star Binary GX 340+0: We present a spectral analysis of a brief Chandra/HETG observation of the
neutron star low-mass X-ray binary GX~340+0. The high-resolution spectrum
reveals evidence of ionized absorption in the Fe K band. The strongest feature,
an absorption line at approximately 6.9 keV, is required at the 5 sigma level
of confidence via an F-test. Photoionization modeling with XSTAR grids suggests
that the line is the most prominent part of a disk wind with an apparent
outflow speed of v = 0.04c. This interpretation is preferred at the 4 sigma
level over a scenario in which the line is H-like Fe XXVI at a modest
red-shift. The wind may achieve this speed owing to its relatively low
ionization, enabling driving by radiation pressure on lines; in this sense, the
wind in GX 340+0 may be the stellar-mass equivalent of the flows in broad
absorption line quasars (BALQSOs). If the gas has a unity volume filling
factor, the mass ouflow rate in the wind is over 10^-5 Msun/year, and the
kinetic power is nearly 10^39 erg/s (or, 5-6 times the radiative Eddington
limit for a neutron star). However, geometrical considerations - including a
small volume filling factor and low covering factor - likely greatly reduce
these values. | astro-ph_HE |
The blazar TXS 0506+056 associated with a high-energy neutrino: insights
into extragalactic jets and cosmic ray acceleration: A neutrino with energy of $\sim$290 TeV, IceCube-170922A, was detected in
coincidence with the BL Lac object TXS~0506+056 during enhanced gamma-ray
activity, with chance coincidence being rejected at $\sim 3\sigma$ level. We
monitored the object in the very-high-energy (VHE) band with the MAGIC
telescopes for $\sim$41 hours from 1.3 to 40.4 days after the neutrino
detection. Day-timescale variability is clearly resolved. We interpret the
quasi-simultaneous neutrino and broadband electromagnetic observations with a
novel one-zone lepto-hadronic model, based on interactions of electrons and
protons co-accelerated in the jet with external photons originating from a
slow-moving plasma sheath surrounding the faster jet spine. We can reproduce
the multiwavelength spectra of TXS 0506+056 with neutrino rate and energy
compatible with IceCube-170922A, and with plausible values for the jet power of
$\sim 10^{45} - 4 \times 10^{46} {\rm erg \ s^{-1}}$. The steep spectrum
observed by MAGIC is concordant with internal $\gamma\gamma$ absorption above a
few tens of GeV entailed by photohadronic production of a $\sim$290 TeV
neutrino, corroborating a genuine connection between the multi-messenger
signals. In contrast to previous predictions of predominantly hadronic emission
from neutrino sources, the gamma-rays can be mostly ascribed to inverse Compton
up-scattering of external photons by accelerated electrons. The X-ray and VHE
bands provide crucial constraints on the emission from both accelerated
electrons and protons. We infer that the maximum energy of protons in the jet
co-moving frame can be in the range $\sim 10^{14}$ to $10^{18}$ eV. | astro-ph_HE |
Theoretical Delay Time Distributions: We briefly discuss the method of population synthesis to calculate
theoretical delay time distributions of type Ia supernova progenitors. We also
compare the results of the different research groups and conclude that although
one of the main differences in the results for single degenerate progenitors is
the retention efficiency with which accreted hydrogen is added to the white
dwarf core, this cannot explain all the differences. | astro-ph_HE |
Mass Ratios of Merging Double Neutron Stars as Implied by the Milky Way
Population: Of the seven known double neutron stars (DNS) with precisely measure masses
in the Milky Way that will merge within a Hubble time, all but one has a mass
ratio, $q$, close to unity. Recently, precise measurements of three
post-Keplerian parameters in the DNS J1913$+$1102 constrain this system to have
a significantly non-unity mass ratio of 0.78$\pm$0.03. One may be tempted to
conclude that approximately one out of seven (14\%) DNS mergers detected by
gravitational wave observatories will have mass ratios significantly different
from unity. However J1913$+$1102 has a relatively long merger time of 470 Myr.
We show that when merger times and observational biases are taken into account,
the population of Galactic DNSs imply that $\simeq98\%$ of all merging DNSs
will have $q>$0.9. We then apply two separate fitting formulas informed by 3D
hydrodynamic simulations of DNS mergers to our results on Galactic DNS masses,
finding that either $\simeq$0.004 ${M_{\odot}}$ or $\simeq$0.010 ${M_{\odot}}$
of material will be ejected at merger, depending on which formula is used.
These ejecta masses have implications for both the peak bolometric luminosities
of electromagnetic counterparts (which we find to be $\sim$10$^{41}$ erg
s$^{-1}$) as well as the $r$-process enrichment of the Milky Way. | astro-ph_HE |
Obscured $pp$-channel neutrino sources: We explore the possibility that the astrophysical neutrinos are produced in
$pp$-interactions with a gas cloud near the source acting as a beam dump, which
is sufficiently dense to significantly attenuate the associated gamma-ray flux
through pair-production on this gas. In this way, such sources could
potentially supply the astrophysical neutrino flux whilst avoiding the existing
constraints on the non-blazar contribution to the extragalactic gamma-ray
background. After defining our model, we implement a Monte Carlo simulation and
apply this to different scenarios. First, we investigate a set of active
galaxies which exhibit signs of obscuration. We find that, currently, the
expected neutrino flux from these objects in our model is below the existing
exclusion limits, but can already constrain the amount of protons accelerated
in such sources. Second, we investigate the diffuse neutrino flux generated by
a population of obscured sources. We find that such a population can indeed
alleviate the tension with the extragalactic background light. We discuss the
possibility that ultra-luminous infrared galaxies represent such a source
class. | astro-ph_HE |
Wide band observations of the X-ray burster GS 1826-238: GS 1826-238 is a well-studied X-ray bursting neutron star in a low mass
binary system. Thermal Comptonisation by a hot electron cloud is a widely
accepted mechanism accounting for its high energy emission, while the nature of
most of its soft X-ray output is not completely understood. A further low
energy component is typically needed to model the observed spectra: pure
blackbody and Comptonisation-modified blackbody radiation by a lower
temperature (a few keV) electron plasma were suggested to explain the low
energy data. We studied the steady emission of GS 1826-238 by means of broad
band (X to soft Gamma-rays) measurements obtained by the INTEGRAL observatory
in 2003 and 2006. The newly developed, up-to-date Comptonisation model CompTB
is applied for the first time to study effectively the low-hard state
variability of a low-luminosity neutron star in a low-mass X-ray binary system.
We confirm that the 3-200 keV emission of \GS is characterised by
Comptonisation of soft seed photons by a hot electron plasma. A single spectral
component is sufficient to model the observed spectra. At lower energies, no
direct blackbody emission is observed and there is no need to postulate a low
temperature Compton region. Compared to the 2003 measurements, the plasma
temperature decreased from 20 to 14 keV in 2006, together with the seed photons
temperature. The source intensity was also found to be 30% lower in 2006,
whilst the average recurrence frequency of the X-ray bursts significantly
increased. Possible explanations for this apparent deviation from the typical
limit-cycle behaviour of this burster are discussed. | astro-ph_HE |
High Energy Neutrinos from the Gravitational Wave event GW150914
possibly associated with a short Gamma-Ray Burst: High-energy neutrino (HEN) and gravitational wave (GW) can probe
astrophysical sources in addition to electromagnetic observations.
Multimessenger studies can reveal nature of the sources which may not be
discerned from one type of signal alone. We discuss HEN emission in connection
with the Advanced Laser Interferometer Gravitational-wave Observatory (ALIGO)
event GW150914 which could be associated with a short gamma-ray burst (GRB)
detected by the $Fermi$ Gamma-ray Burst Monitor (GBM) 0.4 s after the GW event
and within localization uncertainty of the GW event. We calculate HEN flux from
this short GRB, GW150914-GBM, and show that non-detection of a high-energy
starting event (HESE) by the IceCube Neutrino Observatory can constrain the
total isotropic-equivalent jet energy of this short burst to be less than
$3\times 10^{52}$ erg. | astro-ph_HE |
Simulations of Magnetic Fields in Tidally-Disrupted Stars: We perform the first magnetohydrodynamical simulations of tidal disruptions
of stars by supermassive black holes. We consider stars with both tangled and
ordered magnetic fields, for both grazing and deeply disruptive encounters.
When the star survives disruption, we find its magnetic field amplifies by a
factor of up to twenty, but see no evidence for the a self-sustaining dynamo
that would yield arbitrary field growth. For stars that do not survive, and
within the tidal debris streams produced in partial disruptions, we find that
the component of the magnetic field parallel to the direction of stretching
along the debris stream only decreases slightly with time, eventually resulting
in a stream where the magnetic pressure is in equipartition with the gas. Our
results suggest that the returning gas in most (if not all) stellar tidal
disruptions is already highly magnetized by the time it returns to the black
hole. | astro-ph_HE |
Spectro-Timing Analysis of a highly variable narrow-line Seyfert 1
galaxy NGC 4748 with AstroSat and XMM-Newton: We present a detailed timing and spectral study of an extremely variable
narrow-line Seyfert~1 galaxy NGC 4748 using observations in the year 2017 and
2014 performed with AstroSat and XMM-Newton, respectively. Both observations
show extremely variable soft and hard X-ray emission that are correlated with
each other. In the 2014 data set, the source retains its general behaviour of
"softer when brighter" while the 2017 observation exhibits a "harder when
brighter" nature. Such changing behaviour is rare in AGNs and is usually
observed in the black hole binary systems. The "harder when brighter" is
confirmed with the anti-correlation between the photon index and the 0.3-10 keV
power-law flux. This suggests a possible change in the accretion mode from
standard to the advection-dominated flow. Additionally, both the observations
show soft X-ray excess below 2 keV over the power-law continuum. This excess
was fitted with a single or multiple blackbody component(s). The origin of soft
excess during the 2017 observation is likely due to the cool Comptonization as
the photon index changes with time. On the other hand, the broad iron line and
delayed UV emission during the 2014 observation strongly suggest that X-ray
illumination onto the accretion disk and reflection and reprocessing play a
significant role in this AGN. | astro-ph_HE |
The Konus-Wind catalog of gamma-ray bursts with known redshifts. II.
Waiting mode bursts simultaneously detected by Swift/BAT: In the Second part of The Konus-Wind Catalog of Gamma-Ray Bursts with Known
Redshifts (first part: Tsvetkova et al. 2017; T17), we present the results of a
systematic study of gamma-ray bursts (GRBs) with reliable redshift estimates
detected simultaneously by the Konus-Wind (KW) experiment (in the waiting mode)
and by the Swift/BAT (BAT) telescope during the period from 2005 January to the
end of 2018. By taking advantage of the high sensitivity of BAT and the wide
spectral band of KW we were able to constrain the peak spectral energies, the
broadband energy fluences, and the peak fluxes for the joint KW-BAT sample of
167 weak, relatively soft GRBs (including four short bursts). Based on the GRB
redshifts, which span the range $0.04 \leq z \leq 9.4$, we estimate the
rest-frame, isotropic-equivalent energy, and peak luminosity. For 14 GRBs with
reasonably constrained jet breaks, we provide the collimation-corrected values
of the energetics. This work extends the sample of KW GRBs with known redshifts
to 338 GRBs, the largest set of cosmological GRBs studied to date over a broad
energy band. With the full KW sample, accounting for the instrumental bias, we
explore GRB rest-frame properties, including hardness-intensity correlations,
GRB luminosity evolution, luminosity and isotropic-energy functions, and the
evolution of the GRB formation rate, which we find to be in general agreement
with those reported in T17 and other previous studies. | astro-ph_HE |
Massive non-thermal radio emitters: new data and their modelling: During recent years some non-thermal radio emitting OB stars have been
discovered to be binary, or multiple systems. The non-thermal emission is due
to synchrotron radiation that is emitted by electrons accelerated up to high
energies. The electron acceleration occurs at the strong shocks created by the
collision of radiatively-driven winds. Here we summarize the available radio
data and more recent observations for the binary Cyg OB2 No. 9. We also show a
new emission model which is being developed to compare the theoretical total
radio flux and the spectral index with the observed radio light curves. This
comparison will be useful in order to solve fundamental questions, such as the
determination of the stellar mass loss rates, which are perturbed by clumping. | astro-ph_HE |
Puffy accretion disks: sub-Eddington, optically thick, and stable: We report on a new class of solutions of black hole accretion disks that we
have found through three-dimensional, global, radiative magnetohydrodynamic
simulations in general relativity.
It combines features of the canonical thin, slim and thick disk models but
differs in crucial respects from each of them. We expect these new solutions to
provide a more realistic description of black hole disks than the slim disk
model. We are presenting a disk solution for a non-spinning black hole at a
sub-Eddington mass accretion rate, $\dot M=0.6\,\dot M_{\rm Edd}$. By the
density scale-height measure the disk appears to be thin, having a high density
core near the equatorial plane of height $h_{\rho} \sim 0.1 \,r$, but most of
the inflow occurs through a highly advective, turbulent, optically thick,
Keplerian region that sandwiches the core and has a substantial geometrical
thickness comparable to the radius, $H \sim r$. The accreting fluid is
supported above the midplane in large part by the magnetic field, with the gas
and radiation to magnetic pressure ratio $\beta \sim 1$, this makes the disk
thermally stable, even though the radiation pressure strongly dominates over
gas pressure. A significant part of the radiation emerging from the disk is
captured by the black hole, so the disk is less luminous than a thin disk would
be at the same accretion rate. | astro-ph_HE |
Cosmic ray Transport in Magnetohydrodynamic turbulence: This paper studies cosmic ray (CR) transport in magneto hydrodynamic (MHD)
turbulence. CR transport is strongly dependent on the properties of the
magnetic turbulence. We perform test particle simulations to study the
interactions of CR with both total MHD turbulence and decomposed MHD modes. The
spatial diffusion coefficients and the pitch angle scattering diffusion
coefficients are calculated from the test particle trajectories in turbulence.
Our results confirm that the fast modes dominate the CR propagation, whereas
Alfv\'en and slow modes are much less efficient and have shown similar pitch
angle scattering rates. We investigate the cross field transport on large and
small scales. On large/global scales, normal diffusion is observed and the
diffusion coefficient is suppressed by $M_A^\zeta$ compared to the parallel
diffusion coefficients, with $\zeta$ closer to 4 in Alfv\'en modes than that in
total turbulence as theoretically expected. For the CR transport on scales
smaller than the turbulence injection scale, both the local and global magnetic
reference frames are adopted. Super diffusion is observed on such small scales
in all the cases. Particularly, CR transport in Alfv\'en modes show clear
Richardson diffusion in the local reference frame. Our results have broad
applications to CRs in various astrophysical environments. | astro-ph_HE |
The Spin of New Black Hole Candidate: MAXI J1803-298 Observed by NuSTAR
and NICER: MAXI J1803-298, a newly-discovered Galactic transient and black hole
candidate, was first detected by \emph{MAXI}/GSC on May 1st, 2021. In this
paper, we present a detailed spectral analysis of MAXI J1803-298. Utilizing the
X-ray reflection fitting method, we perform a joint fit to the spectra of MAXI
J1803-298, respectively, observed by \emph{NuSTAR} and \emph{NICER}/XTI on the
same day over the energy range between 0.7-79.0 keV, and found its spin (and
the inclination angle i) can be constrained to be close to an extreme value,
0.991 ($i\sim$ $70 ^{\circ}$), at 68\% confidence interval. The results suggest
that MAXI J1803-298 may be a fast-rotating black hole with a large inclination
angle. | astro-ph_HE |
Discovery of a recurrent spectral evolutionary cycle in the
ultra-luminous X-ray sources Holmberg II X-1 and NGC 5204 X-1: Most ultra-luminous X-ray sources (ULXs) are now thought to be powered by
stellar-mass compact objects accreting at super-Eddington rates. While the
discovery of evolutionary cycles have marked a breakthrough in our
understanding of the accretion flow changes in the sub-Eddington regime in
Galactic Black Hole Binaries, their evidence in the super-Eddington regime
remained elusive. However, recent circumstantial evidence had hinted the
presence of a recurrent evolutionary cycle in two archetypal ULXs: Holmberg II
X-1 and NGC 5204 X-1. Here we build on our previous work and exploit the
long-term high-cadence monitoring of Swift-XRT in order to provide evidence of
the evolutionary cycle in these two sources and investigate the main physical
parameters inducing their spectral transitions. We study the long-term
evolution of both sources using hardness-intensity diagrams (HID) and by means
of Lomb-Scargle periodograms and Gaussian processes modelling to look for
periodic variability. We show that both sources follow a recurrent evolutionary
pattern in the HID that can be characterized by the hard ultraluminous (HUL)
and soft ultraluminous (SUL) spectral regimes, and a third state with
characteristics similar to the supersoft ultraluminous (SSUL) state. The
transitions between the soft states seem aperiodic, as revealed by timing
analysis of the light curve of Holmberg II X-1, albeit further investigation is
warranted. The light curve of NGC 5204 X-1 shows a periodicity of $\sim$ 200
days, possibly associated with the duration of the evolutionary cycle. We
support a scenario in which the spectral changes from HUL to SUL are due to a
periodic increase of the mass-transfer rate and subsequent narrowing of the
opening angle of the supercritical funnel. The narrower funnel, combined with
stochastic variability imprinted by the wind, might explain the SUL--SSUL
spectral changes. | astro-ph_HE |
Accretion Properties and Estimation of Spin of Galactic Black Hole
Candidate Swift J1728.9-3613 with NuSTAR during its 2019 outburst: Black hole X-ray binaries (BHXRBs) play a crucial role in understanding the
accretion of matter onto a black hole. Here, we focus on exploring the
transient BHXRB \source~discovered by Swift/BAT and MAXI/GSC during its January
2019 outburst. We present measurements on its accretion properties, long
time-scale variability, and spin. To probe these properties we make use of
several NICER observations and an unexplored data set from NuSTAR, as well as
long term light curves from MAXI/GSC. In our timing analysis we provide
estimates of the cross-correlation functions between light curves in various
energy bands. In our spectral analysis we employ numerous phenomenological
models to constrain the parameters of the system, including flavours of the
relativistic reflection model Relxill to model the Fe K$\alpha$ line and the
$>15$ keV reflection hump. Our analysis reveals that: (i) Over the course of
the outburst the total energy released was $\sim 5.2 \times 10^{44}$~ergs,
corresponding to roughly 90\% the mass of Mars being devoured. (ii) We find a
continuum lag of $8.4 \pm 1.9$ days between light curves in the $2-4$ keV and
$10-20$ keV bands which could be related to the viscous inflow time-scale of
matter in the standard disc. (iii) Spectral analysis reveals a spin parameter
of $\sim 0.6 - 0.7$ with an inclination angle of $\sim 45^{\circ}-70^{\circ}$,
and an accretion rate during the NuSTAR observation of $\sim 17\% ~L_{\rm
Edd}$. | astro-ph_HE |
Dark Matter Density Spikes around Primordial Black Holes: We show that density spikes begin to form from dark matter particles around
primordial black holes immediately after their formation at the
radiation-dominated cosmological stage. This follows from the fact that in the
thermal velocity distribution of particles there are particles with low
velocities that remain in finite orbits around black holes and are not involved
in the cosmological expansion. The accumulation of such particles near black
holes gives rise to density spikes. These spikes are considerably denser than
those that are formed later by the mechanism of secondary accretion. The
density spikes must be bright gamma-ray sources. Comparison of the calculated
signal from particle annihilation with the Fermi-LAT data constrains the
present-day cosmological density parameter for primordial black holes with
masses $M_{\rm BH}\geq10^{-8}M_\odot$ from above by values from $\Omega_{\rm
BH}\leq1$ to $\Omega_{\rm BH}\leq10^{-8}$, depending on $M_{\rm BH}$. These
constraints are several orders of magnitude more stringent than other known
constraints. | astro-ph_HE |
Early optical emission in support of synchrotron radiation in
$γ$-ray bursts: The origin of prompt emission in $\gamma$-ray bursts (GRBs) is highly debated
topic. The observed spectra are supposed to play a crucial role in constraining
the location of the emitting region, the strength of the magnetic field and the
distribution of the accelerated particles. The apparent inconsistency of the
prompt emission spectra with the synchrotron radiation scenario has resulted in
considering more complex models. The inclusion of the soft X-ray data (down to
0.5 keV) in GRB spectra have led to the discovery of low-energy breaks in their
spectra. More importantly, the distribution of spectral slopes has been shifted
towards the prediction of the synchrotron radiation scenario if the break is
associated with the synchrotron cooling frequency. We discuss the recent study
that systematically extend the range of investigation down to the optical
domain. It was shown that the optical-to-gamma-rays spectra are consistent with
the synchrotron model. In addition, widely used empirical model made of thermal
and non-thermal components has been tested. We conclude that most of the
spectra are consistent with the synchrotron scenario while the two-component
model faces difficulties to account for the optical radiation in
presence/absence of the contaminating afterglow emission. We comment on the
parameter space of GRB emitting region derived from the best fit parameters of
the synchrotron model. In a basic one-shot particle acceleration model it
corresponds to the quite contrived solutions for the magnetic field strength
($\sim$ 10 G) and for the radius of the emitting region ($R_\gamma \ge 10^{16}$
cm). Possible modifications of the basic model would be necessary to have a
fully consistent picture. | astro-ph_HE |
A NICER Discovery of a Low-Frequency Quasi-Periodic Oscillation in the
Soft-Intermediate State of MAXI J1535-571: We present the discovery of a low-frequency $\approx 5.7$ Hz quasi-periodic
oscillation (QPO) feature in observations of the black hole X-ray binary MAXI
J1535-571 in its soft-intermediate state, obtained in September-October 2017 by
the Neutron Star Interior Composition Explorer (NICER). The feature is
relatively broad (compared to other low-frequency QPOs; quality factor
$Q\approx 2$) and weak (1.9% rms in 3-10 keV), and is accompanied by a weak
harmonic and low-amplitude broadband noise. These characteristics identify it
as a weak Type A/B QPO, similar to ones previously identified in the
soft-intermediate state of the transient black hole X-ray binary XTE J1550-564.
The lag-energy spectrum of the QPO shows increasing soft lags towards lower
energies, approaching 50 ms at 1 keV (with respect to a 3-10 keV continuum).
This large phase shift has similar amplitude but opposite sign to that seen in
Rossi X-ray Timing Explorer data for a Type B QPO from the transient black hole
X-ray binary GX 339-4. Previous phase-resolved spectroscopy analysis of the
Type B QPO in GX 339-4 pointed towards a precessing jet-like corona
illuminating the accretion disk as the origin of the QPO signal. We suggest
that this QPO in MAXI J1535-571 may have the same origin, with the different
lag sign depending on the scale height of the emitting region and the observer
inclination angle. | astro-ph_HE |
Spectral evolution responsible for the transition from positive lags to
negative lags in Gamma-ray Bursts: It was well known that most of gamma-ray bursts (GRBs) are dominated by
positive spectral lags, while a small fraction of GRBs show negative lags.
However, Wei et al. firstly identified a well-defined transition from positive
lags to negative lags in GRB 160625B, and then got robust limits on possible
violation of Lorentz Invariance (LIV) based on the observation. Recently, such
a transition has been found in three different emission episodes in \thisgrb by
Gunapati et al., which provides us a great opportunity to investigate whether
the transition results from LIV-induced observed spectral lags. Our analysis
shows that the LIV model can not be compatible with the current observations,
whereas, only the spectral evolution induced spectral lags could responsible
for the transition. So, spectral evolution can also explain the positive to
negative lag in GRB 190530A. | astro-ph_HE |
Gravitational-wave Signals From Three-dimensional Supernova Simulations
With Different Neutrino-Transport Methods: We compare gravitational-wave (GW) signals from eight three-dimensional
simulations of core-collapse supernovae, using two different progenitors with
zero-age main sequence masses of 9 and 20 solar masses. The collapse of each
progenitor was simulated four times, at two different grid resolutions and with
two different neutrino transport methods, using the Aenus-Alcar code. The main
goal of this study is to assess the validity of recent concerns that the
so-called "Ray-by-Ray+" (RbR+) approximation is problematic in core-collapse
simulations and can adversely affect theoretical GW predictions. Therefore,
signals from simulations using RbR+ are compared to signals from corresponding
simulations using a fully multidimensional (FMD) transport scheme. The 9
solar-mass progenitor successfully explodes, whereas the 20 solar-mass model
does not. Both the standing accretion shock instability and hot-bubble
convection develop in the postshock layer of the non-exploding models. In the
exploding models, neutrino-driven convection in the postshock flow is
established around 100 ms after core bounce and lasts until the onset of shock
revival. We can, therefore, judge the impact of the numerical resolution and
neutrino transport under all conditions typically seen in non-rotating
core-collapse simulations. We find excellent qualitative agreement in all GW
features. We find minor quantitative differences between simulations, but find
no systematic differences between simulations using different transport
schemes. Resolution-dependent differences in the hydrodynamic behaviour of
low-resolution and high-resolution models have a greater impact on the GW
signals than consequences of the different transport methods. Furthermore,
increasing the resolution decreases the discrepancies between models with
different neutrino transport. | astro-ph_HE |
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