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A Search for Fast Optical Transients in the Pan-STARRS1 Medium-Deep
Survey: M Dwarf Flares, Asteroids, Limits on Extragalactic Rates, and
Implications for LSST: [Abridged] We present a search for fast optical transients (~0.5 hr-1 day)
using repeated observations of the Pan-STARRS1 Medium-Deep Survey (PS1/MDS)
fields. Our search takes advantage of the consecutive g/r-band observations
(16.5 min in each filter), by requiring detections in both bands, with
non-detections on preceding and subsequent nights. We identify 19 transients
brighter than 22.5 AB mag (S/N>10). Of these, 11 events exhibit quiescent
counterparts in the deep PS1/MDS templates that we identify as M4-M9 dwarfs.
The remaining 8 transients exhibit a range of properties indicative of
main-belt asteroids near the stationary point of their orbits. With
identifications for all 19 transients, we place an upper limit of
R_FOT(0.5hr)<0.12 deg^-2 d^-1 (95% confidence level) on the sky-projected rate
of extragalactic fast transients at <22.5 mag, a factor of 30-50 times lower
than previous limits; the limit for a timescale of ~day is R_FOT<2.4e-3 deg^-2
d^-1. To convert these sky-projected rates to volumetric rates, we explore the
expected peak luminosities of fast optical transients powered by various
mechanisms, and find that non-relativistic events are limited to M~-10 mag
(M~-14 mag) for a timescale of ~0.5 hr (~day), while relativistic sources
(e.g., GRBs, magnetar-powered transients) can reach much larger luminosities.
The resulting volumetric rates are <13 (M~-10 mag), <0.05 (M~-14 mag) and <1e-6
Mpc^-3 yr^-1 (M~-24 mag), significantly above the nova, supernova, and GRB
rates, respectively, indicating that much larger surveys are required to
provide meaningful constraints. Motivated by the results of our search we
discuss strategies for identifying fast optical transients in the LSST main
survey, and reach the optimistic conclusion that the veil of foreground
contaminants can be lifted with the survey data, without the need for expensive
follow-up observations. | astro-ph_HE |
The Swift/BAT Hard X-ray Transient Monitor: The Swift/Burst Alert Telescope (BAT) hard X-ray transient monitor provides
near real-time coverage of the X-ray sky in the energy range 15-50 keV. The BAT
observes 88% of the sky each day with a detection sensitivity of 5.3 mCrab for
a full-day observation and a time resolution as fine as 64 seconds. The three
main purposes of the monitor are (1) the discovery of new transient X-ray
sources, (2) the detection of outbursts or other changes in the flux of known
X-ray sources, and (3) the generation of light curves of more than 900 sources
spanning over eight years. The primary interface for the BAT transient monitor
is a public web page. Between 2005 February 12 and 2013 April 30, 245 sources
have been detected in the monitor, 146 of them persistent and 99 detected only
in outburst. Among these sources, 17 were previously unknown and were
discovered in the transient monitor. In this paper, we discuss the methodology
and the data processing and filtering for the BAT transient monitor and review
its sensitivity and exposure. We provide a summary of the source detections and
classify them according to the variability of their light curves. Finally, we
review all new BAT monitor discoveries; for the new sources that are previously
unpublished, we present basic data analysis and interpretations. | astro-ph_HE |
The High Time Resolution Universe Pulsar Survey - VII: discovery of five
millisecond pulsars and the different luminosity properties of binary and
isolated recycled pulsars: This paper presents the discovery and timing parameters for five millisecond
pulsars (MSPs), four in binary systems with probable white dwarf companions and
one isolated, found in ongoing processing of the High Time Resolution Universe
Pulsar Survey (HTRU). We also present high quality polarimetric data on four of
them. These further discoveries confirm the high potential of our survey in
finding pulsars with very short spin periods. At least two of these five MSPs
are excellent candidates to be included in the Pulsar Timing Array projects.
Thanks to the wealth of MSP discoveries in the HTRU survey, we revisit the
question of whether the luminosity distributions of isolated and binary MSPs
are different. Using the Cordes and Lazio distance model and our new and
catalogue flux density measurements, we find that 41 of the 42 most luminous
MSPs in the Galactic disk are in binaries and a statistical analysis suggests
that the luminosity functions differ with 99.9% significance. We conclude that
the formation process that leads to solitary MSPs affects their luminosities,
despite their period and period derivatives being similar to those of pulsars
in binary systems. | astro-ph_HE |
Constraints on Proton Synchrotron Origin of Very High Energy Gamma Rays
from the Extended Jet of AP Librae: The multi-wavelength photon spectrum from the BL Lac object AP Librae extends
from radio to TeV gamma rays. The X-ray to very high energy gamma ray emission
from the extended jet of this source has been modeled earlier with inverse
Compton (IC) scattering of relativistic electrons off the CMB photons. The
IC/CMB model requires the kpc scale extended jet to be highly collimated with
bulk Lorentz factor close to 10. Here we discuss the possibility of proton
synchrotron origin of X-rays and gamma-rays from the extended jet with bulk
Lorentz factor 3. This scenario requires extreme energy of protons and high
magnetic field 1 mG of the extended jet with very high jet power in particles
and magnetic field (which is more than 100 times the Eddington luminosity of AP
Librae) to explain the very high energy gamma ray emission. Moreover, we have
shown that X-ray emission from the extended jets of 3C 273 and PKS 0637-752
could be possible by proton synchrotron emission with jet powers comparable to
their Eddington luminosities. | astro-ph_HE |
A falling Corona model for the anomalous behavior of the broad emission
lines in NGC 5548: NGC 5548 has been intensively monitored by the AGN Space Telescope and
Optical Reverberation Mapping collaboration. Approximately after half of the
light curves, the correlation between the broad emission lines and the
lag-corrected ultraviolet continua becomes weak. This anomalous behavior is
accompanied by an increase of soft X-ray emission. We propose a simple model to
understand this anomalous behavior, i.e., the corona might fall down, thereby
increasing the covering fraction of the inner disk. Therefore, X-ray and
extreme ultraviolet emission suffer from spectral variations. The ultraviolet
continua variations are driven by both X-ray and extreme ultraviolet
variations. Consequently, the spectral variability induced by the falling
corona would dilute the correlation between the broad emission lines and the
ultraviolet continua. Our model can explain many additional observational
facts, including the dependence of the anomalous behavior on velocity and
ionization energy. We also show that the time lag and correlation between the
X-ray and the ultraviolet variations change as NGC 5548 displays the anomalous
behavior. The time lag is dramatically longer than the expectation from disk
reprocessing if the anomalous behavior is properly excluded. During the
anomalous state, the time lag approaches the light-travel timescale of disk
reprocessing albeit with a much weaker correlation. We speculate that the time
lag in the normal state is caused by reprocessing of the broad line region gas.
As NGC 5548 enters the abnormal state, the contribution of the broad line
region gas is smaller; the time lag reflects disk reprocessing. We also discuss
alternative scenarios. | astro-ph_HE |
Swift X-ray and ultraviolet observations of the shortest orbital period
double-degenerate system RX J0806.3+1527 (HM Cnc): The system RX J0806.3+1527 (HM Cnc) is a pulsating X-ray source with 100 per
cent modulation on a period of 321.5 s (5.4 min). This period reflects the
orbital motion of a close binary consisting of two interacting white dwarfs.
Here we present a series of simultaneous X-ray (0.2-10 keV) and
near-ultraviolet (2600 angstrom and 1928 angstrom) observations that were
carried out with the Swift satellite. In the near-ultraviolet, the counterpart
of RX J0806.3+1527 was detected at flux densities consistent with a blackbody
with temperature 27E+3 K. We found that the emission at 2600 angstrom is
modulated at the 321.5-s period with the peak ahead of the X-ray one by 0.28
cycles and is coincident within 0.05 cycles with the optical. This phase-shift
measurement confirms that the X-ray hot spot (located on the primary white
dwarf) is at about 80-100 degrees from the direction that connects the two
white dwarfs. Albeit at lower significance, the 321.5-s signature is present
also in the 1928-angstrom data; at this wavelength, however, the pulse peak is
better aligned with that observed at X-rays. We use the constraints on the
source luminosity and the geometry of the emitting regions to discuss the
merits and limits of the main models for RX J0806.3+1527. | astro-ph_HE |
X-ray Spectroscopy of the gamma-ray Brightest Nova V906 Car
(ASASSN-18fv): Shocks in gamma-ray emitting classical novae are expected to produce bright
thermal and non-thermal X-rays. We test this prediction with simultaneous
NuSTAR and Fermi/LAT observations of nova V906 Car, which exhibited the
brightest GeV gamma-ray emission to date. The nova is detected in hard X-rays
while it is still gamma-ray bright, but contrary to simple theoretical
expectations, the detected 3.5-78 keV emission of V906 Car is much weaker than
the simultaneously observed >100 MeV emission. No non-thermal X-ray emission is
detected, and our deep limits imply that the gamma-rays are likely hadronic.
After correcting for substantial absorption (N_H ~ 2 x 10^23 cm^-2), the
thermal X-ray luminosity (from a 9 keV optically-thin plasma) is just ~2% of
the gamma-ray luminosity. We consider possible explanations for the low thermal
X-ray luminosity, including the X-rays being suppressed by corrugated,
radiative shock fronts or the X-rays from the gamma-ray producing shock are
hidden behind an even larger absorbing column (N_H >10^25 cm^-2). Adding
XMM-Newton and Swift/XRT observations to our analysis, we find that the
evolution of the intrinsic X-ray absorption requires the nova shell to be
expelled 24 days after the outburst onset. The X-ray spectra show that the
ejecta are enhanced in nitrogen and oxygen, and the nova occurred on the
surface of a CO-type white dwarf. We see no indication of a distinct super-soft
phase in the X-ray lightcurve, which, after considering the absorption effects,
may point to a low mass of the white dwarf hosting the nova. | astro-ph_HE |
Accretion disc by Roche lobe overflow in the supergiant fast X-ray
transient IGR J08408-4503: Supergiant fast X-ray transients (SFXTs) are X-ray binary systems with a
supergiant companion and likely a neutron star, which show a fast ($\sim 10^3$
s) and high variability with a dynamic range up to $10^{5-6}$. Given their
extreme properties, they are considered among the most valuable laboratories to
test accretion models. Recently, the orbital parameters of a member of this
class, IGR J08408-4503, were obtained from optical observations. We used this
information, together with X-ray observations from previous publications and
new results from X-ray and optical data collected by INTEGRAL and presented in
this work, to study the accretion mechanisms at work in IGR J08408-4503. We
found that the high eccentricity of the compact object orbit and the large size
of the donor star imply Roche lobe overflow (RLO) around the periastron. It is
also likely that a fraction of the outer layers of the photosphere of the donor
star are lost from the Lagrangian point $L_2$ during the periastron passages.
On the basis of these findings, we discuss the flaring variability of IGR
J08408-4503 assuming the presence of an accretion disc. We point out that IGR
J08408-4503 may not be the only SFXT with an accretion disc fueled by RLO.
These findings open a new scenario for accretion mechanisms in SFXTs, since
most of them have so far been based on the assumption of spherically symmetric
accretion. | astro-ph_HE |
What Powered the Kilonova-Like Emission After GRB 230307A in the
Framework of a Neutron Star-White Dwarf Merger?: The second brightest gamma-ray burst, GRB 230307A (with a duration $T_{90}$ ~
40 s), exhibited characteristics indicative of a magnetar engine during the
prompt emission phase. Notably, a suspected kilonova was identified in its
follow-up optical and infrared observations. Here we propose that the origin of
GRB 230307A is a neutron star-white dwarf (NS-WD) merger, as this could
naturally interpret the long duration and the large physical offset from the
center of its host galaxy. In the framework of such a NS-WD merger event, the
late-time kilonova-like emission is very likely to be powered by the spin-down
of the magnetar and the radioactive decay of $^{56}$Ni, rather than by the
decay of r-process elements as these heavy elements may not be easy to be
synthesized in a NS-WD merger. It is demonstrated that the above scenario can
be supported by our fit to the late-time observational data, where a mass of ~
$10^{-3} \ \rm M_{\odot}$ $^{56}$Ni is involved in the ejecta of a mass of ~
$0.1 \ \rm M_{\odot}$. Particularly, the magnetar parameters required by the
fit are consistent with those derived from the early X-ray observation. | astro-ph_HE |
The Knee and the Second Knee of the Cosmic-Ray Energy Spectrum: The cosmic ray flux measured by the Telescope Array Low Energy Extension
(TALE) exhibits three spectral features: the knee, the dip in the $10^{16}$ eV
decade, and the second knee. Here the spectrum has been measured for the first
time using fluorescence telescopes, which provide a calorimetric,
model-independent result. The spectrum appears to be a rigidity-dependent
cutoff sequence, where the knee is made by the hydrogen and helium portions of
the composition, the dip comes from the reduction in composition from helium to
metals, the rise to the second knee occurs due to intermediate range nuclei,
and the second knee is the iron knee. | astro-ph_HE |
Revisit of Local X-ray Luminosity Function of Active Galactic Nuclei
with the MAXI Extragalactic Survey: We construct a new X-ray (2--10 keV) luminosity function of Compton-thin
active galactic nuclei (AGNs) in the local universe, using the first MAXI/GSC
source catalog surveyed in the 4--10 keV band. The sample consists of 37
non-blazar AGNs at $z=0.002-0.2$, whose identification is highly ($>97%$)
complete. We confirm the trend that the fraction of absorbed AGNs with $N_{\rm
H} > 10^{22}$ cm$^{-2}$ rapidly decreases against luminosity ($L_{\rm X}$),
from 0.73$\pm$0.25 at $L_{\rm X} = 10^{42-43.5}$ erg s$^{-1}$ to 0.12$\pm0.09$
at $L_{\rm X} = 10^{43.5-45.5}$ erg s$^{-1}$. The obtained luminosity function
is well fitted with a smoothly connected double power-law model whose indices
are $\gamma_1 = 0.84$ (fixed) and $\gamma_2 = 2.0\pm0.2$ below and above the
break luminosity, $L_{*} = 10^{43.3\pm0.4}$ ergs s$^{-1}$, respectively. While
the result of the MAXI/GSC agrees well with that of HEAO-1 at $L_{\rm X} \gtsim
10^{43.5}$ erg s$^{-1}$, it gives a larger number density at the lower
luminosity range. Comparison between our luminosity function in the 2--10 keV
band and that in the 14--195 keV band obtained from the Swift/BAT survey
indicates that the averaged broad band spectra in the 2--200 keV band should
depend on luminosity, approximated by $\Gamma\sim1.7$ for $L_{\rm X} \ltsim
10^{44}$ erg s$^{-1}$ while $\Gamma\sim 2.0$ for $L_{\rm X} \gtsim 10^{44}$ erg
s$^{-1}$. This trend is confirmed by the correlation between the luminosities
in the 2--10 keV and 14--195 keV bands in our sample. We argue that there is no
contradiction in the luminosity functions between above and below 10 keV once
this effect is taken into account. | astro-ph_HE |
The influence of outflows on the 1/f-like luminosity fluctuations: In accretion systems, outflows may have significant influence on the
luminosity fluctuations. In this paper, following the Lyubarskii's general
scheme, we revisit the power spectral density of luminosity fluctuations by
taking into account the role of outflows. Our analysis is based on the
assumption that the coupling between the local outflow and inflow is weak on
the accretion rate fluctuations. We find that, for the inflow mass accretion
rate $\dot M \propto r^{s}$, the power spectrum of flicker noise component will
present a power-law distribution $p(f) \propto f^{-(1+4s/3)}$ for
advection-dominated flows. We also obtain descriptions of $p(f)$ for both
standard thin discs and neutrino-cooled discs, which show that the power-law
index of a neutrino-cooled disc is generally larger than that of a
photon-cooled disc. Furthermore, the obtained relationship between $p(f)$ and
$s$ indicates the possibility of evaluating the strength of outflows by the
power spectrum in X-ray binaries and gamma-ray bursts. In addition, we discuss
the possible influence of the outflow-inflow coupling on our results. | astro-ph_HE |
Evolution of the X-ray Properties of the Transient Magnetar XTE
J1810-197: We report on X-ray observations of the 5.54 s transient magnetar XTE
J1810-197 using the XMM-Newton and Chandra observatories, analyzing new data
from 2008 through 2014, and re-analyzing data from 2003 through 2007 with the
benefit of these six years of new data. From the discovery of XTE J1810-197
during its 2003 outburst to the most recent 2014 observations, its 0.3-10 keV
X-ray flux has declined by a factor of about 50 from 4.1E-11 to 8.1E-13
erg/cm^2/s. Its X-ray spectrum has now reached a steady state. Pulsations
continue to be detected from a 0.3 keV thermal hot-spot that remains on the
neutron star surface. The luminosity of this hot-spot exceeds XTE J1810-197's
spin down luminosity, indicating continuing magnetar activity. We find that XTE
J1810-197's X-ray spectrum is best described by a multiple component blackbody
model in which the coldest 0.14 keV component likely originates from the entire
neutron star surface, and the thermal hot-spot is, at different epochs, well
described by an either one or two-component blackbody model. A 1.2 keV
absorption line, possibly due to resonant proton scattering, is detected at all
epochs. The X-ray flux of the hot spot decreased by approximately 20% between
2008 March and 2009 March, the same period during which XTE J1810-197 became
radio quiet. | astro-ph_HE |
Resonant scattering in the Perseus Cluster: spectral model for
constraining gas motions with Astro-H: X-ray spectra from cores of galaxy clusters can be strongly distorted by
resonant scattering of line photons, affecting metal abundance and gas velocity
measurements. We introduce simulated spectral models that take into account the
resonant scattering effect, radial variations of thermodynamic properties of
the hot gas, projection effects and small-scale isotropic gas motions. The key
feature of the models is that all these effects are treated self-consistently
for the whole spectrum, rather than for individual lines. The model spectra are
publicly available and can be used for direct comparison with observed
projected spectra. Comparison with the existing XMM-Newton and Chandra data of
the Perseus Cluster shows that even though there is no strong evidence for the
resonant scattering in Perseus, the low energy resolution of the X-ray CCDs is
not sufficient to robustly distinguish spectral distortions due to the resonant
scattering, different metal abundance profiles and different levels of gas
turbulence. Future Astro-H data will resolve most of the problems we are facing
with CCDs. With the help of our models, the resonant scattering analysis can be
done self-consistently using the whole spectral information, constraining the
level of gas turbulence already with a 100 ks observation with Astro-H. | astro-ph_HE |
Constraints on MeV dark matter and primordial black holes: Inverse
Compton signals at the SKA: We investigate the possibilities for probing MeV dark matter (DM) particles
and primordial black holes (PBHs) (for masses $\sim 10^{15}$--$10^{17}$ g) at
the upcoming radio telescope SKA, using photon signals from the Inverse Compton
(IC) effect within a galactic halo. Pair-annihilation or decay of MeV DM
particles (into $e^+ e^-$ pairs) or Hawking radiation from a population of PBHs
generates mildly relativistic $e^{\pm}$ which can lead to radio signals through
the IC scattering on low energy cosmic microwave background (CMB) photons. We
study the ability of SKA to detect such signals coming from nearby ultra-faint
dwarf galaxies Segue I and Ursa Major II as well as the globular cluster
$\omega$-cen and the Coma cluster. We find that with $\sim 100$ hours of
observation, the SKA improves the Planck constraints on the DM
annihilation/decay rate and the PBH abundance for masses in the range $\sim 1$
to few tens of MeV and above $10^{15}$ to $10^{17}$ g, respectively.
Importantly, the SKA limits are independent of the assumed magnetic fields
within the galaxies. Previously allowed regions of diffusion parameters of MeV
electrons inside a dwarf galaxy that give rise to observable signals at the SKA
are also excluded. For objects like dwarf galaxies, predicted SKA constraints
depend on both the DM and diffusion parameters. Independent observations in
different frequency bands, e.g., radio and $\gamma$-ray frequencies, may break
this degeneracy and thus enable one to constrain the combined parameter space
of DM and diffusion. However, the constraints are independent of diffusion
parameters for galaxy clusters such as Coma. | astro-ph_HE |
Three-flavor collective neutrino conversions with multi-azimuthal-angle
instability in an electron-capture supernova model: We investigate the multi-azimuthal angle (MAA) effect on collective neutrino
oscillation by considering the three-dimensional neutrino momentum distribution
in a realistic electron-capture supernova model with an $8.8 M_{\odot}$
progenitor. We find that the MAA effect induces collective flavor conversions
at epochs when it is completely suppressed under the axial-symmetric
approximation. This novel activity is switched on/off by the growth of the MAA
instability and imprints additional time evolution in the expected neutrino
event rate. We validate our results by extending the linear stability analysis
into the three-flavor scheme including mixing angles, and confirm that the
onset of collective neutrino oscillation matches the steep growth of flavor
instability. We discuss how the MAA effect alters neutrino detection at
Super-Kamiokande and DUNE. | astro-ph_HE |
Possible role of magnetic reconnection in the electromagnetic
counterpart of binary black hole merger: We propose a qualitative scenario to interpret the argued association between
the direct measurement of the gravitational wave event GW150914 by Laser
Interferometer Gravitational Wave Observatory (LIGO)-Virgo collaborations and
the hard $X$-ray transient detected by Fermi-Gamma-ray Burst Monitor (GBM)
$0.4$ sec after. In a binary system of two gravitationally collapsing objects
with a non-vanishing electric charge, the compenetration of the two
magnetospheres occurring during the coalescence, through magnetic reconnection,
produces a highly collimated relativistic outflow that becomes optically thin
and shines in the GBM field of view. We propose that this process should be
expected as a commonplace in the future joint gravitational/electromagnetic
detections and, in case of neutron star-neutron star merger event, might lead
to detectable $X$- or $\gamma$-ray precursors to, or transients associated
with, the gravitational bursts. | astro-ph_HE |
The Ultra-Fast Outflow of the Quasar PG 1211+143 as Viewed by
Time-Averaged Chandra Grating Spectroscopy: We present a detailed X-ray spectral study of the quasar PG 1211+143 based on
Chandra High Energy Transmission Grating Spectrometer (HETGS) observations
collected in a multi-wavelength campaign with UV data using the Hubble Space
Telescope Cosmic Origins Spectrograph (HST-COS) and radio bands using the
Jansky Very Large Array (VLA). We constructed a multi-wavelength ionizing
spectral energy distribution using these observations and archival infrared
data to create XSTAR photoionization models specific to the PG 1211+143 flux
behavior during the epoch of our observations. Our analysis of the
Chandra-HETGS spectra yields complex absorption lines from H-like and He-like
ions of Ne, Mg and Si which confirm the presence of an ultra-fast outflow (UFO)
with a velocity ~ $-$17,300 km s$^{-1}$ (outflow redshift $z_{\rm out}$ ~
$-$0.0561) in the rest frame of PG 1211+143. This absorber is well described by
an ionization parameter $\log \xi$ ~ 2.9 erg s$^{-1}$ cm and column density
$\log N_{\rm H}$ ~ 21.5 cm$^{-2}$. This corresponds to a stable region of the
absorber's thermal stability curve, and furthermore its implied neutral
hydrogen column is broadly consistent with a broad Ly$\alpha$ absorption line
at a mean outflow velocity of ~ $-$16,980 km s$^{-1}$ detected by our HST-COS
observations. Our findings represent the first simultaneous detection of a UFO
in both X-ray and UV observations. Our VLA observations provide evidence for an
active jet in PG 1211+143, which may be connected to the X-ray and UV outflows;
this possibility can be evaluated using very-long-baseline interferometric
(VLBI) observations. | astro-ph_HE |
AKARI Near-infrared Spectral Observations of Shocked H2 Gas of the
Supernova Remnant IC 443: We present near-infrared (2.5 - 5.0 um) spectra of shocked H2 gas in the
supernova remnant IC 443, obtained with the satellite AKARI. Three shocked
clumps-known as B, C, and G-and one background region were observed, and only
H2 emission lines were detected. Except the clump B, the extinctioncorrected
level population shows the ortho-to-para ratio of ~ 3.0. From the level
population of the clumps C and G-both AKARI's only and the one extended with
previous mid-infrared observations-we found that the v = 0 levels are more
populated than the v > 0 levels at a fixed level energy, which cannot be
reproduced by any combination of H2 gas in Local Thermodynamic Equilibrium. The
populations are described by the two-density power-law thermal admixture model,
revised to include the collisions with H atoms. We attributed the lower
(n(H2)=10^(2.8-3.8) cm-3) and higher (n(H2)=10^(5.4-5.8) cm-3) density gases to
the shocked H2 gas behind C-type and J-type shocks, respectively, based on
several arguments including the obtained high H I abundance n(H I)/n(H2)=0.01.
Under the hierarchical picture of molecular clouds, the C-type and J-type
shocks likely propagate into "clumps" and "clouds" (interclump media),
respectively. The power-law index b of 1.6 and 3.5, mainly determined by the
lower density gas, is attributed to the shock-velocity diversity, which may be
a natural result during shock-cloud interactions. According to our results, H2
v = 1 - 0 S(1) emission is mainly from J-shocks propagating into interclump
media. The H2 emission was also detected at the background region, and this
diffuse H2 emission may originate from collisional process in addition to the
ultraviolet photon pumping. | astro-ph_HE |
Magnetic-Field Amplification in the Thin X-ray Rims of SN1006: Several young supernova remnants (SNRs), including SN1006, emit synchrotron
X-rays in narrow filaments, hereafter thin rims, along their periphery. The
widths of these rims imply 50 to 100 $\mu$G fields in the region immediately
behind the shock, far larger than expected for the interstellar medium
compressed by unmodified shocks, assuming electron radiative losses limit rim
widths. However, magnetic-field damping could also produce thin rims. Here we
review the literature on rim width calculations, summarizing the case for
magnetic-field amplification. We extend these calculations to include an
arbitrary power-law dependence of the diffusion coefficient on energy, $D
\propto E^{\mu}$. Loss-limited rim widths should shrink with increasing photon
energy, while magnetic-damping models predict widths almost independent of
photon energy. We use these results to analyze Chandra observations of SN 1006,
in particular the southwest limb. We parameterize the full widths at half
maximum (FWHM) in terms of energy as FWHM $\propto E^{m_E}_{\gamma}$. Filament
widths in SN1006 decrease with energy; $m_E \sim -0.3$ to $-0.8$, implying
magnetic field amplification by factors of 10 to 50, above the factor of 4
expected in strong unmodified shocks. For SN 1006, the rapid shrinkage rules
out magnetic damping models. It also favors short mean free paths (small
diffusion coefficients) and strong dependence of $D$ on energy ($\mu \ge 1$). | astro-ph_HE |
Supernova Shock Breakout Through a Wind: The breakout of a supernova shock wave through the progenitor star's outer
envelope is expected to appear as an X-ray flash. However, if the supernova
explodes inside an optically-thick wind, the breakout flash is delayed. We
present a simple model for estimating the conditions at shock breakout in a
wind based on the general observable quantities in the X-ray flash lightcurve:
the total energy E_X, and the diffusion time after the peak, t_diff. We base
the derivation on the self-similar solution for the forward-reverse shock
structure expected for an ejecta plowing through a pre-existing wind at large
distances from the progenitor's surface. We find simple quantitative relations
for the shock radius and velocity at breakout. By relating the ejecta density
profile to the pre-explosion structure of the progenitor, the model can also be
extended to constrain the combination of explosion energy and ejecta mass. For
the observed case of XRO08109/SN2008D, our model provides reasonable
constraints on the breakout radius, explosion energy, and ejecta mass, and
predicts a high shock velocity which naturally accounts for the observed
non-thermal spectrum. | astro-ph_HE |
Observations of High-Energy Gamma-Ray Emission Toward the Galactic
Centre with the Fermi Large Area Telescope: The inner region of the Milky Way is one of the most interesting and complex
regions of the gamma-ray sky. The intense interstellar emission and resolved
point sources, as well as potential contributions by other sources such as
unresolved source populations and dark matter, complicate the interpretation of
the data. In this paper the Fermi LAT team analysis of a 15x15 degree region
about the Galactic centre is described. The methodology for point-source
detection and treatment of the interstellar emission is given. In general, the
bulk of the gamma-ray emission from this region is attributable to a
combination of these two contributions. However, low-intensity residual
emission remains and its characterisation is discussed. | astro-ph_HE |
Constraining black hole spins with low-frequency quasi-periodic
oscillations in soft states: Black hole X-ray transients show a variety of state transitions during their
outburst phases, characterized by changes in their spectral and timing
properties. In particular, power density spectra (PDS) show quasi periodic
oscillations (QPOs) that can be related to the accretion regime of the source.
We looked for type-C QPOs in the disc-dominated state (i.e. the high soft
state) and in the ultra-luminous state in the RXTE archival data of 12
transient black hole X-ray binaries known to show QPOs during their outbursts.
We detected 6 significant QPOs in the soft state that can be classified as
type-C QPOs. Under the assumption that the accretion disc in disc-dominated
states extends down or close to the innermost stable circular orbit (ISCO) and
that type-C QPOs would arise at the inner edge of the accretion flow, we use
the relativistic precession model (RPM) to place constraints on the black hole
spin. We were able to place lower limits on the spin value for all the 12
sources of our sample while we could place also an upper limit on the spin for
5 sources. | astro-ph_HE |
Why not any tau double bang in Icecube, yet?: High Energy Neutrino Astronomy has been revealed by a sudden change in the
flavor composition around maximal energies since three years of recording in
ice km detector. However these discover didn' t led to any clear promised
Neutrino Astronomy land yet. No correlation with hard gamma sources was found.
Moreover the astrophysical spectra expected at Fermi value, seem to converge to
a softer value, also required to avoid any Glashow resonant neutrino signal. We
suggest a main solution within a composite flux ruled by prompt atmospheric
neutrinos. Nevertheless the recent discover of twentyone crossing muons at
hundreds TeVs had shown a first narrow overlapped doublet and a correlated
track with a peculiar hard UHECR event clustering pointing to a well known
microjet in bynary precession; these first connections hint for a non
negligible astrophysical component making neutrino astronomy anyway already
more than a hope. | astro-ph_HE |
Vertical Structure of Neutrino Dominated Accretion Disks and Neutrino
Transport in the disks: We investigate the vertical structure of neutrino dominated accretion disks
by self-consistently considering the detailed microphysics, such as the
neutrino transport, vertical hydrostatic equilibrium, the conservation of
lepton number, as well as the balance between neutrino cooling, advection
cooling and viscosity heating. After obtaining the emitting spectra of
neutrinos and antineutrinos by solving the one dimensional Boltzmann equation
of neutrino and antineutrino transport in the disk, we calculate the
neutrino/antineutrino luminosity and their annihilation luminosity. We find
that the total neutrino and antineutrino luminosity is about $10^{54}$ ergs/s
and their annihilation luminosity is about $5\times10^{51}$ ergs/s with an
extreme accretion rate $10 M_{\rm {sun}}$/s and an alpha viscosity
$\alpha=0.1$. In addition, we find that the annihilation luminosity is
sensitive to the accretion rate and will not exceed $10^{50}$ ergs/s which is
not sufficient to power the most fireball of GRBs, if the accretion rate is
lower than $1 M_{\rm {sun}}$/s. Therefore, the effects of the spin of black
hole or/and the magnetic field in the accretion flow might be introduced to
power the central engine of GRBs. | astro-ph_HE |
Optical Afterglows and IGM Attenuation: Because of their cosmological origin, gamma-ray burst (GRB) optical
afterglows are attenuated when they pass intergalactic absorbers in the GRB
line-of-sight. Without the knowledge of the number of absorbers and their
physical properties, the effect of absorption on the observed magnitudes can
not be determined precisely. Different methods have been applied in order to
correct for this effect statistically, either using semi-analytical
calculations or numerical simulations. We follow these works and present the
expected magnitude corrections as a function of redshift for a set of filters
most commonly used in the scientific community. The results are publically
available on the web (http://igmac.fmf.uni-lj.si). | astro-ph_HE |
First combined studies on Lorentz Invariance Violation from observations
of astrophysical sources: Imaging Atmospheric Cherenkov Telescopes study the highest energy (up to tens
of TeV) photon emission coming from nearby and distant astrophysical sources,
thus providing valuable results from searches for Lorentz Invariance Violation
(LIV) effects. Highly variable, energetic and distant sources such as Pulsars
and AGNs are the best targets for the Time-of-Flight LIV studies. However, the
limited number of observations of AGN flares or of high-energy pulsed emission
greatly restricts the potential of such studies, especially any potential LIV
effects as a function of redshift. To address these issues, an inter-experiment
working group has been established by the three major collaborations taking
data with Imaging Atmospheric Cherenkov Telescopes (H.E.S.S., MAGIC and
VERITAS) with the aim to increase sensitivity to any effects of LIV, together
with an improved control of systematic uncertainties, by sharing data samples
and developing joint analysis methods. This will allow an increase in the
number of available sources and to perform a sensitive search for redshift
dependencies. This presentation reviews the first combined maximum likelihood
method analyses using simu- lations of published source observations done in
the past with H.E.S.S., MAGIC and VERITAS. The results from analyses based on
combined maximum likelihood methods, the strategies to deal with data from
different types of sources and instruments, as well as future plans will be
presented. | astro-ph_HE |
Interpreting the relation between the gamma-ray and infrared
luminosities of star-forming galaxies: It has been found that there is a quasi-linear scaling relationship between
the gamma-ray luminosity in GeV energies and the total infrared luminosity of
star-forming galaxies, i.e. $L_{\gamma}\propto L_{\rm IR}^{\alpha}$ with
$\alpha\simeq 1$. However, the origin of this linear slope is not well
understood. Although extreme starburst galaxies can be regarded as calorimeters
for hadronic cosmic ray interaction and thus a quasi-linear scaling may hold,
it may not be the case for low star-formation-rate (SFR) galaxies, as the
majority of cosmic rays in these galaxies are expected to escape. We calculate
the gamma-ray production efficiency in star-forming galaxies by considering
realistic galaxy properties, such as the gas density and galactic wind velocity
in star-forming galaxies. We find that the slope for the relation between
gamma-ray luminosity and the infrared luminosity gets steeper for low infrared
luminosity galaxies, i.e. $\alpha\rightarrow 1.6$, due to increasingly lower
efficiency for the production of gamma-ray emission. We further find that the
measured data of the gamma-ray luminosity is compatible with such a steepening.
The steepening in the slope suggests that cosmic-ray escape is very important
in low-SFR galaxies. | astro-ph_HE |
A search for fast radio burst-like emission from Fermi gamma-ray bursts: We report the results of the rapid follow-up observations of gamma-ray bursts
(GRBs) detected by the Fermi satellite to search for associated fast radio
bursts. The observations were conducted with the Australian Square Kilometre
Array Pathfinder at frequencies from 1.2-1.4 GHz. A set of 20 bursts, of which
four were short GRBs, were followed up with a typical latency of about one
minute, for a duration of up to 11 hours after the burst. The data was searched
using 4096 dispersion measure trials up to a maximum dispersion measure of 3763
pc cm$^{-3}$, and for pulse widths $w$ over a range of duration from 1.256 to
40.48 ms. No associated pulsed radio emission was observed above $26 {\rm Jy
ms} (w/1 {\rm ms})^{-1/2}$ for any of the 20 GRBs. | astro-ph_HE |
Observational signatures of stellar explosions driven by relativistic
jets: The role of relativistic jets in unbinding the stellar envelope during a
supernova (SN) associated with a gamma-ray burst (GRB) is unclear. To study
that, we explore observational signatures of stellar explosions that are driven
by jets. We focus on the final velocity distribution of the outflow in such
explosions and compare its observational imprints to SN/GRB data. We find that
jet driven explosions produce an outflow with a flat distribution of energy per
logarithmic scale of proper velocity. The flat distribution seems to be
universal as it is independent of the jet and the progenitor properties that we
explored. The velocity range of the flat distribution for typical GRB
parameters is $\gamma\beta \approx 0.03-3$, where $\gamma$ is the outflow
Lorentz factor and $\beta$ is its dimensionless velocity. A flat distribution
is seen also for collimated choked jets where the highest outflow velocity
decreases with the depth at which the jet is choked. Comparison to observations
of SN/GRBs rules out jets as the sole explosion source in these events.
Instead, in SN/GRB the collapsing star must deposit its energy into two
channels - a quasi-spherical (or wide angle) channel and a narrowly collimated
one. The former carries most of the energy and is responsible for the SN
sub-relativistic ejecta while the latter carries 0.01-0.1 of the total outflow
energy and is the source of the GRB. Intriguingly, the same two channels, with
a similar energy ratio, were seen in the binary neutron star merger GW170817,
suggesting that similar engines are at work in both phenomena. | astro-ph_HE |
The luminosity function and the rate of Swift's Gamma Ray Bursts: We invert directly the redshift - luminosity distribution of observed long
Swift GRBs to obtain their rate and luminosity function. Our best fit rate is
described by a broken power law that rises like (1+z)^2.1{+0.5-0.6} for 0<z<3
and decrease like (1+z)^-1.4{+2.4-1.0} for z>3. The local rate is
1.3^{+0.6-0.7} [Gpc^-3 yr^-1]. The luminosity function is well described by a
broken power law with a break at L* = 10^52.5{+-0.2}[erg/sec] and with indices
alpha = 0.2^{+0.2-0.1} and beta = 1.4^{+0.3-0.6}. The recently detected GRB
090423, with redshift ~8, fits nicely into the model's prediction, verifying
that we are allowed to extend our results to high redshifts. While there is a
possible agreement with the star formation rate (SFR) for z<3, the high
redshift slope is shallower than the steep decline in the SFR for 4<z. However
we cannot rule out a GRB rate that follows one of the recent SFR models. | astro-ph_HE |
The optical noise monitoring systems of Lake Baikal environment for the
Baikal-GVD telescope: We present data on the luminescence of the Baikal water medium collected with
the Baikal-GVD neutrino telescope. This three-dimensional array of light
sensors allows the observation of time and spatial variations of the ambient
light field. We report on observation of an increase of luminescence activity
in 2016 and 2018. On the contrary, we observed practically constant optical
noise in 2017. An agreement has been found between two independent optical
noise data sets. These are data collected with online monitoring system and the
trigger system of the cluster. | astro-ph_HE |
Deexcitation nuclear gamma-ray line emission from low-energy cosmic rays
in the inner Galaxy: Recent observations of high ionization rates of molecular hydrogen in diffuse
interstellar clouds point to a distinct low-energy cosmic-ray component.
Supposing that this component is made of nuclei, two models for the origin of
such particles are explored and low-energy cosmic-ray spectra are calculated
which, added to the standard cosmic ray spectra, produce the observed
ionization rates. The clearest evidence of the presence of such low-energy
nuclei between a few MeV per nucleon and several hundred MeV per nucleon in the
interstellar medium would be a detection of nuclear \gamma-ray line emission in
the range E_ 0.1 - 10 MeV, which is strongly produced in their collisions with
the interstellar gas and dust. Using a recent \gamma-ray cross section
compilation for nuclear collisions, \gamma-ray line emission spectra are
calculated alongside with the high-energy \gamma-ray emission due to {\pi} 0
decay, the latter providing normalization of the absolute fluxes by comparison
with Fermi-LAT observations of the diffuse emission above E \gamma = 0.1 GeV.
Our predicted fluxes of strong nuclear \gamma-ray lines from the inner Galaxy
are well below the detection sensitivies of INTEGRAL, but a detection,
especially of the 4.4-MeV line, seems possible with new-generation \gamma-ray
telescopes based on available technology. We predict also strong \gamma-ray
continuum emission in the 1-8 MeV range, which in a large part of our model
space for low-energy cosmic rays exceeds considerably estimated instrument
sensitivities of future telescopes. | astro-ph_HE |
Revealing Double White Dwarf Mergers with Multi-messenger Signals: A significant number of double white dwarfs (DWDs) are believed to merge
within the Hubble time due to the gravitational wave (GW) emission during their
inspiraling phase. The outcome of a DWD system is either a type Ia Supernova as
the double-degenerate model, or a massive, long-lasting merger remnant.
Expected multi-messenger signals of these events will help us to distinguish
detailed merging physical processes. In this work, we aim to provide a generic
scenario of DWD merging, investigate the emission of all major messengers, with
a focus on GWs and neutrinos. Our goal is to provide some guidance for current
and future (collaborative) efforts of multi-messenger observations. Throughout
the merging evolution of a DWD system, different messengers (GW, neutrino and
electromagnetic wave) will dominate at different times. In this work, we show
that DWD merger events located at the distance of 1 kpc can indeed produce
detectable signals of GWs and neutrinos. The GW frequency are in 0.3-0.6 Hz
band around 10 days before tidal disruption begin. We estimate that in
optimistic situations, the neutrino number detected by upcoming detectors such
as JUNO and Hyper-Kamiokande can reach O(1) for a DWD merging event at $\sim$ 1
kpc. | astro-ph_HE |
Swift Observations of Hard X-ray Emitting White Dwarfs in Symbiotic
Stars: The X-ray emission from most accreting white dwarfs (WDs) in symbiotic binary
stars is quite soft. Several symbiotic WDs, however, produce strong X-ray
emission at energies greater than ~20 keV. The Swift BAT instrument has
detected hard X-ray emission from 4 such accreting WDs in symbiotic stars: RT
Cru, T CrB, CD -57 3057, and CH Cyg. In one case (RT Cru), Swift detected
X-rays out to greater than 50 keV at a > 5 sigma confidence level. Combining
data from the XRT and BAT detectors, we find that the 0.3-150 keV spectra of RT
Cru, T CrB, and CD -57 3057 are well described by emission from a
single-temperature, optically thin thermal plasma, plus an unresolved 6.4-6.9
keV Fe line complex. The X-ray spectrum of CH Cyg contains an additional bright
soft component. For all 4 systems, the spectra suffer high levels of absorption
from material that both fully and partially covers the source of hard X-rays.
The XRT data did not show any of the rapid, periodic variations that one would
expect if the X-ray emission were due to accretion onto a rotating, highly
magnetized WD. The X-rays were thus more likely from the accretion-disk
boundary layer around a massive, non-magnetic WD in each binary. The X-ray
emission from RT Cru varied on timescales of a few days. This variability is
consistent with being due to changes in the absorber that partially covers the
source, suggesting localized absorption from a clumpy medium moving into the
line of sight. The X-ray emission from CD -57 3057 and T CrB also varied during
the 9 months of Swift observations, in a manner that was also consistent with
variable absorption. | astro-ph_HE |
Supernova 2013by: A Type IIL Supernova with a IIP-like light curve drop: We present multi-band ultraviolet and optical light curves, as well as
visual-wavelength and near-infrared spectroscopy of the Type II linear (IIL)
supernova (SN) 2013by. We show that SN 2013by and other SNe IIL in the
literature, after their linear decline phase that start after maximum, have a
sharp light curve decline similar to that seen in Type II plateau (IIP)
supernovae. This light curve feature has rarely been observed in other SNe IIL
due to their relative rarity and the intrinsic faintness of this particular
phase of the light curve. We suggest that the presence of this drop could be
used as a physical parameter to distinguish between subclasses of SNe II,
rather than their light curve decline rate shortly after peak. Close inspection
of the spectra of SN 2013by indicate asymmetric line profiles and signatures of
high-velocity hydrogen. Late (less than 90 days after explosion) near-infrared
spectra of SN 2013by exhibit oxygen lines, indicating significant mixing within
the ejecta. From the late-time light curve, we estimate that 0.029 solar mass
of 56Ni was synthesized during the explosion. It is also shown that the V -band
light curve slope is responsible for part of the scatter in the luminosity (V
magnitude 50 days after explosion) vs. 56Ni relation. Our observations of SN
2013by and other SNe IIL through the onset of the nebular phase indicate that
their progenitors are similar to those of SNe IIP. | astro-ph_HE |
Signatures of the disk-jet coupling in the Broad-line Radio Quasar
4C+74.26: Here we explore the disk-jet connection in the broad-line radio quasar
4C+74.26, utilizing the results of the multiwavelength monitoring of the
source. The target is unique in that its radiative output at radio wavelengths
is dominated by a moderately-beamed nuclear jet, at optical frequencies by the
accretion disk, and in the hard X-ray range by the disk corona. Our analysis
reveals a correlation (local and global significance of 96\% and 98\%,
respectively) between the optical and radio bands, with the disk lagging behind
the jet by $250 \pm 42$ days. We discuss the possible explanation for this,
speculating that the observed disk and the jet flux changes are generated by
magnetic fluctuations originating within the innermost parts of a truncated
disk, and that the lag is related to a delayed radiative response of the disk
when compared with the propagation timescale of magnetic perturbations along
relativistic outflow. This scenario is supported by the re-analysis of the
NuSTAR data, modelled in terms of a relativistic reflection from the disk
illuminated by the coronal emission, which returns the inner disk radius
$R_{\rm in}/R_{\rm ISCO} =35^{+40}_{-16}$. We discuss the global energetics in
the system, arguing that while the accretion proceeds at the Eddington rate,
with the accretion-related bolometric luminosity $L_{\rm bol} \sim 9 \times
10^{46}$ erg s$^{-1}$ $\sim 0.2 L_{\rm Edd}$, the jet total kinetic energy
$L_\textrm{j} \sim 4 \times 10^{44}$ erg s$^{-1}$, inferred from the dynamical
modelling of the giant radio lobes in the source, constitutes only a small
fraction of the available accretion power. | astro-ph_HE |
Status of the Lunar Detection Mode for Cosmic Particles of LOFAR: Cosmic particles hitting Earth's moon produce radio emission via the Askaryan
effect. If the resulting radio ns-pulse can be detected by radio telescopes,
this technique potentially increases the available collective area for ZeV
scale particles by several orders of magnitude compared to current experiments.
The LOw Frequency ARray (LOFAR) is the largest radio telescope operating in the
optimum frequency regime for this technique. In this contribution, we report on
the status of the implementation of the lunar detection mode at LOFAR. | astro-ph_HE |
On the Polarized Absorption Lines in Gamma-ray Burst Optical Afterglows: Spectropolarimetric measurements of gamma-ray burst (GRB) optical afterglows
contain polarization information for both continuum and absorption lines. Based
on the Zeeman effect, an absorption line in a strong magnetic field is
polarized and split into a triplet. In this paper, we solve the polarization
radiative transfer equations of the absorption lines, and obtain the degree of
linear polarization of the absorption lines as a function of the optical depth.
In order to effectively measure the degree of linear polarization for the
absorption lines, a magnetic field strength of at least $10^3$ G is required.
The metal elements that produce the polarized absorption lines should be
sufficiently abundant and have large oscillation strengths or Einstein
absorption coefficients. We encourage both polarization measurements and
high-dispersion observations of the absorption lines in order to detect the
triplet structure in early GRB optical afterglows. | astro-ph_HE |
Observation of the Crab Nebula with the HAWC Gamma-Ray Observatory: The Crab Nebula is the brightest TeV gamma-ray source in the sky and has been
used for the past 25 years as a reference source in TeV astronomy, for
calibration and verification of new TeV instruments. The High Altitude Water
Cherenkov Observatory (HAWC), completed in early 2015, has been used to observe
the Crab Nebula at high significance across nearly the full spectrum of
energies to which HAWC is sensitive. HAWC is unique for its wide field-of-view,
nearly 2 sr at any instant, and its high-energy reach, up to 100 TeV. HAWC's
sensitivity improves with the gamma-ray energy. Above $\sim$1 TeV the
sensitivity is driven by the best background rejection and angular resolution
ever achieved for a wide-field ground array.
We present a time-integrated analysis of the Crab using 507 live days of HAWC
data from 2014 November to 2016 June. The spectrum of the Crab is fit to a
function of the form $\phi(E) = \phi_0 (E/E_{0})^{-\alpha
-\beta\cdot{\rm{ln}}(E/E_{0})}$. The data is well-fit with values of
$\alpha=2.63\pm0.03$, $\beta=0.15\pm0.03$, and
log$_{10}(\phi_0~{\rm{cm}^2}~{\rm{s}}~{\rm{TeV}})=-12.60\pm0.02$ when $E_{0}$
is fixed at 7 TeV and the fit applies between 1 and 37 TeV. Study of the
systematic errors in this HAWC measurement is discussed and estimated to be
$\pm$50\% in the photon flux between 1 and 37 TeV.
Confirmation of the Crab flux serves to establish the HAWC instrument's
sensitivity for surveys of the sky. The HAWC survey will exceed sensitivity of
current-generation observatories and open a new view of 2/3 of the sky above 10
TeV. | astro-ph_HE |
The Intrinsic Properties of Multiwavelength Energy Spectra for Fermi
Teraelectronvolt Blazars: In this paper, we have selected a sample of 64 teraelectronvolt blazars, with
redshift, from those classified in the fourth Fermi Large Area Telescope source
catalog\footnote{\url{https://fermi.gsfc.nasa.gov/ssc/data/access/lat/8yr_catalog/}}.
We have obtained the values of the relevant physical parameters by performing a
log-parabolic fitting of the average-state multiwavelength spectral energy
distributions. We estimate the range of the radiation zone parameters, such as
the Doppler factor (${D}$), the magnetic field strength ($B$), the radiative
zone radius ($R$) and the peak Lorentz factor (${\gamma _{\rm p}}$) of
nonthermal electrons. Here, we show that (1) there is a strong linear positive
correlation between the intrinsic synchrotron peak frequency and the intrinsic
inverse Compton scattering (ICs) peak frequency among different types of
blazars; (2) if radio bands are excluded, the spectral index of each band is
negatively correlated with the intrinsic peak frequency; (3) there is a strong
linear negative correlation between the curvature at the peak and the intrinsic
peak frequency of the synchrotron bump, and a weak positive correlation between
the curvature at the peak and the intrinsic peak frequency of the ICs bump; (4)
there is a strong linear positive correlation between the intrinsic ICs peak
luminosity and intrinsic $\gamma$-ray luminosity and between the intrinsic ICs
peak frequency and peak Lorentz factor; (5) there is a strong negative linear
correlation between $\rm log{\;B}$ and $\rm log{\;\gamma_{p}}$; and (6) there
is no correlation between $\rm log{\;R}$ and $\rm log{\;\gamma_{p}}$. | astro-ph_HE |
Bayesian Analysis of Hybrid EoS based on Astrophysical Observational
Data: We perform a Bayesian analysis of probability measures for compact star
equations of state using new, disjunct constraints for mass and radius. The
analysis uses a simple parametrization for hybrid equations of state to
investigate the possibility of a first order deconfinement transition in
compact stars. The latter question is relevant for the possible existence of a
critical endpoint in the QCD phase diagram under scrutiny in heavy-ion
collisions. | 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 |
X-ray nova MAXI J1828-249. Evolution of the broadband spectrum during
its 2013-2014 outburst: Based on data from the SWIFT, INTEGRAL, MAXI/ISS orbital observatories, and
the ground-based RTT-150 telescope, we have investigated the broadband (from
the optical to the hard X-ray bands) spectrum of the X-ray nova MAXI J1828-249
and its evolution during the outburst of the source in 2013-2014. The optical
and infrared emissions from the nova are shown to be largely determined by the
extension of the power-law component responsible for the hard X-ray emission.
The contribution from the outer cold regions of the accretion disk, even if the
X-ray heating of its surface is taken into account, turns out to be moderate
during the source's "high" state (when a soft blackbody emission component is
observed in the X-ray spectrum) and is virtually absent during its "low"
("hard") state. This result suggests that much of the optical and infrared
emissions from such systems originates in the same region of main energy
release where their hard X-ray emission is formed. This can be the Compton or
synchro-Compton radiation from a high-temperature plasma in the central
accretion disk region puffed up by instabilities, the synchrotron radiation
from a hot corona above the disk, or the synchrotron radiation from its
relativistic jets. | astro-ph_HE |
X-rays from the episodic dust maker WR 137: We present an analysis of the XMM-Newton observation of the episodic dust
maker WR 137. Global spectral fits show that its X-ray spectrum is well matched
by a two-temperature optically-thin plasma emission (kT_1 ~ 0.4 keV and kT_2 ~
2.2 keV). If we adopt the colliding stellar wind (CSW) picture for this wide
WR+O binary, the theoretical CSW spectra match well the shape of the observed
X-ray spectrum of WR 137 but they overestimate the observed flux (emission
measure) by about two orders of magnitude. To reconcile the model predictions
with observations, the mass loss of \WR must be reduced considerably (by about
an order of magnitude) with respect to its currently accepted value. In all the
spectral fits, the derived X-ray absorption is consistent with the optical
extinction to WR 137. | astro-ph_HE |
The evaporating primordial black hole fraction in cool-core galaxy
clusters: Recent studies of gamma-ray, cosmic-ray and radio data put stringent
constraints on the fraction of primordial black holes (PBHs) in our universe.
In this article, we propose a new indirect method in using the X-ray luminosity
data of cool-core clusters to constrain the evaporating PBH fraction for the
monochromatic, log-normal and power-law mass distributions. The present results
show that the amount of evaporating PBHs only constitutes a minor component of
dark matter for a large parameter space. The constraints are consistent with
and close to that obtained from other cosmic-ray and multi-wavelength
observations. | astro-ph_HE |
The GMRT High Resolution Southern Sky Survey for pulsars and transients
-- VI: Discovery of nulling, localisation and timing of PSR J1244-4708: Many pulsars in the known population exhibit nulling, which is characterised
by a sudden cessation and subsequent restoration of radio emission. In this
work, we present the localization, timing, and emission properties of a GHRSS
discovered pulsar J1244-4708. Moreover, we find that this pulsar shows nulling
with a nulling fraction close to 60%. A quasi-periodicity is also seen in the
nulling from this pulsar with two timescales. We demonstrate the broadband
nature of nulling in this pulsar using simultaneous observations in band-3
(300-500 MHz) and band-4 (550-750 MHz) with the uGMRT. We also present a
comparison of the efficiency of various search approaches such as single pulse
search, Fast Folding Algorithm (FFA) based search, and Fast Fourier Transform
(FFT) based search to search for nulling pulsars. We demonstrated that the FFA
search is advantageous for detecting extreme nulling pulsars, which is also
confirmed with multiple epochs of observations for the nulling pulsars using
the GMRT. | astro-ph_HE |
Constraints on UHECR sources and extragalactic magnetic fields from
directional anisotropies: A dipole anisotropy in ultra-high-energy cosmic ray (UHECR) arrival
directions, of extragalactic origin, is now firmly established at energies E >
8 EeV. Furthermore, the UHECR angular power spectrum shows no power at smaller
angular scales than the dipole, apart from hints of possible individual hot or
warm spots for energy thresholds $\gtrsim$40 EeV. Here, we exploit the
magnitude of the dipole and the limits on smaller-scale anisotropies to place
constraints on two quantities: the extragalactic magnetic field (EGMF) and the
number density of UHECR sources or the volumetric event rate if UHECR sources
are transient. We also vary the bias between the extragalactic matter and the
UHECR source densities, reflecting whether UHECR sources are preferentially
found in over- or under-dense regions, and find that little or no bias is
favored. We follow Ding et al. (2021) in using the Cosmic Flows 2 density
distribution of the local universe as our baseline distribution of UHECR
sources, but we improve and extend that work by employing an accurate and
self-consistent treatment of interactions and energy losses during propagation.
Deflections in the Galactic magnetic field are treated using both the full JF12
magnetic field model, with random as well as coherent components, or just the
coherent part, to bracket the impact of the GMF on the dipole anisotropy. This
Large Scale Structure (LSS) model gives good agreement with both the direction
and magnitude of the measured dipole anisotropy and forms the basis for
simulations of discrete sources and the inclusion of EGMF effects. | astro-ph_HE |
Data-driven extrapolation schemes of Fermi-LAT spectra to the VHE: After 10 years of operations of the Large Area Telescope (LAT), a high-energy
pair-creation telescope onboard the Fermi satellite, the Fermi Collaboration
has produced two major catalogs: the 4FGL and the 3FHL. These catalogs
represent the best sample of potential very high energy (VHE) emitters that may
be studied by Imaging Atmospheric Cherenkov Telescopes (IACTs). Several methods
are used to extrapolate the Fermi-LAT spectra to TeV energies, generally using
simple analytical functions. The recent success of IACTs has motivated the
creation of catalogs listing the discoveries of these experiments. Among these
initiatives, gamma-cat excels as an open-access tool to archive high-level
results in the VHE field, such as catalogs, spectra and light curves. By using
these resources, we present a data-driven methodology to test the reliability
of different VHE extrapolation schemes used in the literature and evaluate
their accuracy reproducing real VHE observations. | astro-ph_HE |
Evidence of a Hadronic Origin for the TeV Source J1834-087: We report on the discovery of compact, narrow OH line emission from the
hydroxl molecule at 1720 MHz toward the extended TeV source J1834-087. The
origin of this high energy emission is unknown; it could be powered by one or
more candidate neutron stars (leptonic) or by cosmic rays interacting with
dense gas (hadronic). The OH emission is detected near the center of J1834-087,
coincident with the radio continuum of the supernova remnant W41, and the
radial velocity of the line is the same velocity as a giant molecular cloud
along the line of sight. We argue that the OH is maser emission stimulated by
the interaction of the W41 shock with the molecular cloud. The known
correlation between gamma-ray bright supernova remnants and OH masers favors a
hadronic interpretation for this high energy emission. | astro-ph_HE |
An XMM-Newton view of the dipping low-mass X-ray binary XTE J1710-281: (abridged) We analyzed the archived XMM-Newton observation of the poorly
studied low-mass X-ray binary XTE J1710-281 performed in 2004 that covered one
orbital period of the system (3.8 hr). The source shows dips as well as
eclipses, hence it is viewed close to edge-on. We modeled the spectral changes
between persistent and dips in the framework of the partial covering model and
the ionized absorber approach. The persistent spectrum can be fit by a power
law with a photon index of 1.94(+-0.02) affected by absorption from cool
material with a hydrogen column density of 0.401(+-0.007)*10^22 cm^-2. The
spectral changes from persistent to deep-dipping intervals are consistent with
the partial covering of the power-law emission, with the covering fraction
increasing from 26% during shallow dipping to 78% during deep dipping. We do
not detect any absorption lines from highly ionized species such as FeXXV. The
upper-limits we derive on their equivalent width (EW) are not constraining.
Despite not detecting any signatures of a warm absorber, we show that the
spectral changes are consistent with an increase in column density
(4.3(-0.5;+0.4)*10^22 cm^-2 during shallow dipping to 11.6(-0.6;+0.4)*10^22
cm^-2 during deep dipping) and a decrease in ionization state of a
highly-ionized absorber (10^2.52 during shallow dipping to 10^2.29 erg.s^-1.cm
during deep dipping), associated with a slight increase in the column density
of a neutral absorber. The parameters of the ionized absorber are not
constrained during persistent emission. The warm absorber model better accounts
for the ~1 keV depression visible in the pn dipping spectra, and naturally
explains it as a blend of lines and edges unresolved by pn. A deeper
observation of XTE J1710-281 would enable this interpretation to be confirmed. | astro-ph_HE |
The Morphology and Dynamics of Relativistic Jets with Relativistic
Equation of State: We study the effect of plasma composition on the dynamics and morphology of
the relativistic astrophysical jets. Our work is based on a relativistic total
variation diminishing (TVD) simulation code. We use a relativistic equation of
state in the simulation code which accounts for the thermodynamics of a
multispecies plasma which is a mixture of electrons, positrons, and protons. To
study the effect of plasma composition we consider various jet models. These
models are characterized by the same injection parameters, same jet kinetic
luminosity, and the same Mach numbers. The evolution of these models shows that
the plasma composition affects the jet head propagation speed, the structure of
the jet head, and the morphology despite fixing the initial parameters. We
conclude that the electron-positron jets are the slowest and show more
pronounced turbulent structures in comparison to other plasma compositions. The
area and locations of the hot-spots also depend on the composition of jet
plasma. Our results also show that boosting mechanisms are also an important
aspect of multi-dimensional simulations which are also influenced by the change
in composition. | astro-ph_HE |
Sensitivity of the correlation between the depth of shower maximum and
the muon shower size to the cosmic ray composition: The composition of ultra-high energy cosmic rays is an important issue in
astroparticle physics research, and additional experimental results are
required for further progress. Here we investigate what can be learned from the
statistical correlation factor r between the depth of shower maximum and the
muon shower size, when these observables are measured simultaneously for a set
of air showers. The correlation factor r contains the lowest-order moment of a
two-dimensional distribution taking both observables into account, and it is
independent of systematic uncertainties of the absolute scales of the two
observables. We find that, assuming realistic measurement uncertainties, the
value of r can provide a measure of the spread of masses in the primary beam.
Particularly, one can differentiate between a well-mixed composition (i.e., a
beam that contains large fractions of both light and heavy primaries) and a
relatively pure composition (i.e., a beam that contains species all of a
similar mass). The number of events required for a statistically significant
differentiation is ~ 200. This differentiation, though diluted, is maintained
to a significant extent in the presence of uncertainties in the phenomenology
of high energy hadronic interactions. Testing whether the beam is pure or
well-mixed is well motivated by recent measurements of the depth of shower
maximum. | astro-ph_HE |
Kinematics of Parsec-Scale Jets of Gamma-Ray Bright Blazars at 43 GHz
during Ten Years of the VLBA-BU-BLAZAR Program: We analyze the parsec-scale jet kinematics from 2007 June to 2018 December of
a sample of $\gamma$-ray bright blazars monitored roughly monthly with the Very
Long Baseline Array at 43 GHz under the VLBA-BU-BLAZAR program. We implement a
novel piece-wise linear fitting method to derive the kinematics of 521 distinct
emission knots from a total of 3705 total intensity images in 22 quasars, 13 BL
Lacertae objects, and 3 radio galaxies. Apparent speeds of these components
range from $0.01c$ to $78c$, and 18.6\% of knots (other than the "core") are
quasi-stationary. One-fifth of moving knots exhibit non-ballistic motion, with
acceleration along the jet within 5 pc of the core (projected) and deceleration
farther out. These accelerations occur mainly at locations coincident with
quasi-stationary features. We calculate the physical parameters of 273 knots
with statistically significant motion, including their Doppler factors, Lorentz
factors, and viewing angles. We determine the typical values of these
parameters for each jet and the average for each subclass of active galactic
nuclei. We investigate the variability of the position angle of each jet over
the ten years of monitoring. The fluctuations in position of the
quasi-stationary components in radio galaxies tend to be parallel to the jet,
while no directional preference is seen in the components of quasars and BL
Lacertae objects. We find a connection between $\gamma$-ray states of blazars
and their parsec-scale jet properties, with blazars with brighter 43 GHz cores
typically reaching higher $\gamma$-ray maxima during flares. | astro-ph_HE |
Late Afterglow Bump/Plateau around the Jet Break: Signature of a
free-to-shocked wind Environment in Gamma-ray Burst: A number of gamma-ray bursts (GRBs) exhibit the late simultaneous bumps in
their optical and Xray afterglows around the jet break. Its origin is unclear.
Based on the following two facts, we suggest that this feature may sound a
transition of circum-burst environment from a free-wind medium to a homogeneous
medium. (I) The late bump followed by a steep decay is strongly reminiscent of
the afterglows of GRB 170817A, which is attributed to an off-axis observed
external-forward shock (eFS) propagating in an interstellar medium. (II)
Observations seem to feature a long shallow decay before the late optical bump,
which is different from the afterglow of GRB 170817A. In this paper, we study
the emission of an eFS propagating in a free-to-shocked wind for on/off-axis
observers, where the mass density in the shocked-wind is almost constant. The
late simultaneous bumps/plateaux in the optical and X-ray afterglows are really
found around the jet break for high-viewing-angle observers. Moreover, there is
a long plateau or shallow decay before the late bump in the theoretical
light-curves, which is formed during the eFS propagating in the free-wind. For
low-viewing-angle observers, the above bumps appear only in the situation that
the structured jet has a low characteristic angle and the deceleration radius
of the on-axis jet flow is at around or beyond the free-wind boundary. As
examples, the X-ray and optical afterglows of GRBs 120326A, 120404A, and
100814A are fitted. We find that an off-axis observed eFS in a free-to-shocked
wind can well explain the afterglows in these bursts. | astro-ph_HE |
An Occultation Event in Centaurus A and the Clumpy Torus Model: We have analyzed 16 months of sustained monitoring observations of Cen A from
the Rossi X-ray Timing Explorer to search for changes in the absorbing column
in the line of sight to the central nucleus. We present time-resolved
spectroscopy which indicates that a discrete clump of material transited the
line of sight to the central illuminating source over the course of ~170 days
between 2010 August and 2011 February with a maximum increase in the column
density of about 8.4 x 10^22 cm^-2. This is the best quality data of such an
event that has ever been analyzed with the shape of the ingress and egress
clearly seen. Modeling the clump of material as roughly spherical with a
linearly decreasing density profile and assuming a distance from the central
nucleus commensurate with the dusty torus we found that the clump would have a
diameter of 1.4-2.4 x 10^15 cm with a central number density of n_H = 1.8-3.0 x
10^7 cm^-3. This is consistent with previous results for a similar (though
possibly much longer) occultation event inferred in this source in 2003-2004
and supports models of the molecular torus as a clumpy medium. | astro-ph_HE |
Cosmic Evolution of Stellar-mass Black Hole Merger Rate in Active
Galactic Nuclei: Binary black hole mergers encode information about their environment and the
astrophysical processes that led to their formation. Measuring the redshift
dependence of their merger rate will help probe the formation and evolution of
galaxies and the evolution of the star formation rate. Here we compute the
cosmic evolution of the merger rate for stellar-mass binaries in the disks of
Active Galactic Nuclei (AGNs). We focus on recent evolution out to redshift
$z=2$, covering the accessible range of current Earth-based gravitational-wave
observatories. On this scale, the AGN population density is the main
contributor to redshift-dependence. We find that the AGN-assisted merger rate
does not meaningfully evolve with redshift, differentiating this channel from
field binaries and some other dynamical formation scenarios. | astro-ph_HE |
Reapproaching the Spin Estimate of GX 339-4: We systematically reanalyze two previous observations of the black hole (BH)
GX 339-4 in the very high and intermediate state taken with $\emph{XMM-Newton}$
and $\emph{Suzaku}$. We utilize up-to-date data reduction procedures and
implement the recently developed, self-consistent model for X-ray reflection
and relativistic ray tracing, {\sc relxill}. In the very high and intermediate
state, the rate of accretion is high and thus the disk remains close to the
innermost stable circular orbit (ISCO). We require a common spin parameter and
inclination when fitting the two observations since these parameters should
remain constant across all states. This allows for the most accurate
determination of the spin parameter of this galactic black hole binary from
fitting the Fe K$\alpha$ emission line and provides a chance to test previous
estimates. We find GX 339-4 to be consistent with a near maximally spinning
black hole with a spin parameter $a_{*}$ $>0.97$ with an inclination of $36 \pm
4$ degrees. This spin value is consistent with previous high estimates for this
object. Further, if the inner disk is aligned with the binary inclination, this
modest inclination returns a high black hole mass, but they need not be
aligned. Additionally, we explore how the spin is correlated with the power of
the jet emitted but find no correlation between the two. | astro-ph_HE |
Unraveling the Geometry of the Crab Nebula's "Inner Ring": Chandra images of the Crab Nebula resolve the detailed structure of its
"inner ring", possibly a termination shock where pulsar-accelerated
relativistic particles begin to emit X radiation. Analysis of these images
finds that the center of the ellipse-presumably a circular ring in
projection-lies about 0.9" (10 light-days at 2 kpc) from the pulsar's image, at
a position angle of about 300{\deg} (East of North). This analysis also
measures properties of the ellipse: The position angle of the semi-major axis
is about 210{\deg} (East of North); the aspect ratio, 0.49.
In a simple-albeit, not unique-de-projection of the observed geometry, a
circular ring is centered on the axis of symmetry of the pulsar wind nebula.
This ring is not equatorial but rather lies near +4.5{\deg} latitude in
pulsar-centered coordinates. Alternative geometries are briefly discussed. | astro-ph_HE |
Low metallicity natal environments and black hole masses in
Ultraluminous X-ray Sources: We review the available estimates of the masses of the compact object in
Ultraluminous X-ray Sources (ULXs) and critically reconsider the stellar-mass
versus intermediate-mass black hole interpretations. Black holes of several
hundreds to thousands of $M_\odot$ are not required for the majority of ULXs,
although they might be present in the handful of known hyper-luminous ($\sim
10^{41}$ erg s$^{-1}$) objects and/or some sources showing timing features in
their power density spectra. At the same time, however, stellar mass BHs may be
quite a reasonable explanation for ULXs below $\sim 10^{40}$ erg s$^{-1}$, but
they need super-Eddington accretion and some suitable dependence of the beaming
factor on the accretion rate in order to account for ULXs above this
(isotropic) luminosity. We investigate in detail a 'third way' in which a
proportion of ULXs contain $\approx 30-90 M_\odot$ black holes formed in a low
metallicity environment and accreting in a slightly critical regime and find
that it can consistently account for the properties of bright ULXs. Surveys of
ULX locations looking for a statistically meaningful relationship between ULX
position, average luminosity and local metallicity will provide a definitive
test of our proposal. | astro-ph_HE |
Overview of MHz air shower radio experiments and results: In this paper, I present a review of the main results obtained in the last 10
years in the field of radio-detection of cosmic-ray air showers in the MHz
range. All results from all experiments cannot be reported here so that I will
focus on the results more than on the experiments themselves. Modern
experiments started in 2003 with CODALEMA and LOPES. In 2006, small-size
autonomous prototypes setup were installed at the Pierre Auger Observatory
site, to help the design of the Auger Engineering Radio Array (AERA). We will
discuss the principal aspects of the radio data analysis and the determination
of the primary cosmic ray characteristics: the arrival direction, the lateral
distribution of the electric field, the correlation with the primary energy,
the emission mechanisms and the sensitivity to the composition of the cosmic
rays. | astro-ph_HE |
Neutron Stars with realistic EoS in f(R) theories of gravity: In this paper, we examine neutron star structure in perturbative $f(R)$
gravity models with realistic equation of state. We obtain mass-radius
relations in two gravity models of the form $f_{1}(R)=R+ \alpha
R(e^{-R/R_0}-1)$ and $f_{2}(R)=R+\alpha R^2$. For this purpose, we consider NS
with several nucleonic as well as strange EoSs generated in the framework of
relativistic mean field models. The strange particles in the core of NS are in
the form of $\Lambda$ hyperons and quarks, in addition to the nucleons and
leptons. The M-R relation of the chosen EoSs lies well within the observational
limit in the case of GR. We show that these EoSs provide the most stringent
constraint on the perturbative parameter $\alpha$ and therefore can be
considered as important experimental probe for modified gravity at
astrophysical level. | astro-ph_HE |
Light Curve Modeling of Superluminous Supernova 2006gy: Collision
between Supernova Ejecta and Dense Circumstellar Medium: We show model light curves of superluminous supernova 2006gy on the
assumption that the supernova is powered by the collision of supernova ejecta
and its dense circumstellar medium. The initial conditions are constructed
based on the shock breakout condition, assuming that the circumstellar medium
is dense enough to cause the shock breakout within it. We perform a set of
numerical light curve calculations by using a one-dimensional multigroup
radiation hydrodynamics code STELLA. We succeeded in reproducing the overall
features of the early light curve of SN 2006gy with the circumstellar medium
whose mass is about 15 Msun (the average mass-loss rate ~ 0.1 Msun/yr). Thus,
the progenitor of SN 2006gy is likely a very massive star. The density profile
of the circumstellar medium is not well constrained by the light curve modeling
only, but our modeling disfavors the circumstellar medium formed by steady mass
loss. The ejecta mass is estimated to be comparable to or less than 15 Msun and
the explosion energy is expected to be more than 4e51 erg. No 56Ni is required
to explain the early light curve. We find that the multidimensional effect,
e.g., the Rayleigh-Taylor instability, which is expected to take place in the
cool dense shell between the supernova ejecta and the dense circumstellar
medium, is important in understanding supernovae powered by the shock
interaction. We also show the evolution of the optical and near-infrared model
light curves of high-redshift superluminous supernovae. They can be potentially
used to identify SN 2006gy-like superluminous supernovae in the future optical
and near-infrared transient surveys. | astro-ph_HE |
diffSph: a Python tool to compute diffuse signals from dwarf spheroidal
galaxies: So far no diffuse emissions in dwarf spheroidal satellites of the Milky Way
have ever been observed. Given that dwarf galaxies are predominantly composed
of Dark Matter, the discovery of these signals could offer valuable insights
into understanding the nature of Dark Matter. We present "diffSph", a Python
tool which in its present version provides fast predictions of such diffuse
signals in radio frequencies. It also features a very comprehensive module for
the computation of "J" and "D" factors that are relevant for indirect Dark
Matter detection using gamma rays. Routines are coupled to parton-shower
algorithms and Dark Matter halo mass functions from state-of-the-art kinematic
fits. This code is also useful for testing generic hypotheses (not necessarily
associated with any Dark Matter candidate) about the cosmic-ray
electron/positron sources in the dwarf galaxies. The diffSph tool has already
been employed in searches for diffuse signals from dwarf spheroidal galaxies
using the LOw Frequency ARray (LOFAR). | astro-ph_HE |
A Comparative Study of Long and Short GRBs. I. Overlapping Properties: Gamma ray bursts (GRBs) are classified into long and short categories based
on their durations. Broad band studies suggest that these two categories of
objects roughly correspond to two different classes of progenitor systems, i.e.
compact star mergers (Type I) vs. massive star core collapse (Type II).
However, the duration criterion sometimes leads to mis-identification of the
progenitor systems. We perform a comprehensive multi-wavelength comparative
study between duration-defined long GRBs and short GRBs as well as the
so-called "consensus" long GRBs and short GRBs (which are believed to be more
closely related to the two types of progenitor systems). The parameters we
study include two parts: the prompt emission properties including duration
($T_{90}$), spectral peak energy ($E_{\rm p}$), low energy photon index
($\alpha$), isotropic $\gamma$-ray energy ($E_{\rm \gamma, iso}$), isotropic
peak luminosity ($L_{\rm p,iso}$), and the amplitude parameters ($f$ and
$f_{\rm eff}$); and the host galaxy properties including stellar mass ($M_*$),
star formation rate (SFR), metallicity ([X/H]), half light radius ($R_{50}$),
angular and physical ($R_{\rm off}$) offset of the afterglow from the center of
the host galaxy, the normalized offset ($r_{\rm off}=R_{\rm off}/R_{50}$), and
the brightness fraction $F_{\rm light}$. For most parameters, we find
interesting overlapping properties between the two populations in both 1D and
2D distribution plots. The three best parameters for the classification purpose
are $T_{90}$, $f_{\rm eff}$, and $F_{\rm light}$. However, no single parameter
alone is good enough to place a particular burst into the right physical
category, suggesting a need of multiple criteria for physical classification. | astro-ph_HE |
A Bayesian test for periodic signals in red noise: Many astrophysical sources, especially compact accreting sources, show
strong, random brightness fluctuations with broad power spectra in addition to
periodic or quasi-periodic oscillations (QPOs) that have narrower spectra. The
random nature of the dominant source of variance greatly complicates the
process of searching for possible weak periodic signals. We have addressed this
problem using the tools of Bayesian statistics; in particular using Markov
chain Monte Carlo techniques to approximate the posterior distribution of model
parameters, and posterior predictive model checking to assess model fits and
search for periodogram outliers that may represent periodic signals. The
methods developed are applied to two example datasets, both long XMM-Newton
observations of highly variable Seyfert 1 galaxies: RE J1034+396 and Mrk 766.
In both cases a bend (or break) in the power spectrum is evident. In the case
of RE J1034+396 the previously reported QPO is found but with somewhat weaker
statistical significance than reported in previous analyses. The difference is
due partly to the improved continuum modelling, better treatment of nuisance
parameters, and partly to different data selection methods. | astro-ph_HE |
The puzzling symbiotic X-ray system 4U1700+24: Symbiotic X-ray binaries form a subclass of low-mass X-ray binary systems
consisting of a neutron star accreting material from a red giant donor star via
stellar wind or Roche lobe overflow. Only a few confirmed members are currently
known; 4U 1700+24 is a good candidate as it is a relatively bright X-ray
object, possibly associated with the late-type star V934 Her. We analysed the
archive {\it XMM}-Newton and Swift/XRT observations of 4U 1700+24 in order to
have a uniform high-energy ($0.3-10$ keV) view of the source. We confirmed the
existence of a red-shifted O VIII Ly-$\alpha$ transition (already observed in
the 2002 {\it XMM}-Newton data) in the high-resolution spectra collected via
the RGS instruments. The red-shift of the line is found in all the analysed
observations and, on average, it was estimated to be $\simeq 0.009$. We also
observed a modulation of the centroid energy of the line on short time scales
(a few days) and discuss the observations in the framework of different
scenarios. If the modulation is due to the gravitational red-shift of the
neutron star, it might arise from a sudden re-organization of the emitting
$X$-ray matter on the scale of a few hundreds of km. Alternatively, we are
witnessing a uni-polar jet of matter (with typical velocity of $1000-4000$ km
s$^{-1}$) possibly emitted by the neutron star in an almost face-on system. The
second possibility seems to be required by the apparent lack of any modulation
in the observed $X$-ray light curve. We also note also that the low-resolution
spectra (both {\it XMM}-Newton and Swift/XRT in the $0.3-10$ keV band) show the
existence of a black body radiation emitted by a region (possibly associated
with the neutron star polar cap) with typical size from a few tens to hundreds
of meters. The size of this spot-like region reduces as the overall luminosity
of 4U 1700+24 decreases. | astro-ph_HE |
The Swift Serendipitous Survey in deep XRT GRB fields (SwiftFT) I. The
X-ray catalog and number counts: We searched for X-ray serendipitous sources in over 370 Swift-XRT fields
centered on gamma ray bursts detected between 2004 and 2008 and observed with
total exposures ranging from 10 ks to over 1 Ms. This defines the Swift
Serendipitous Survey in deep XRT GRB fields, which is quite broad compared to
existing surveys (~33 square degrees) and medium depth, with a faintest flux
limit of 7.2e-16 erg cm^-2 s^-1 in the 0.5 to 2 keV energy range. The survey
has a high degree of uniformity thanks to the stable point spread function and
small vignetting correction factors of the XRT, moreover is completely random
on the sky as GRBs explode in totally unrelated parts of the sky. In this paper
we present the sample and the X-ray number counts of the high Galactic-latitude
sample, estimated with high statistics over a wide flux range (i.e., 7.2e-16 to
~5e-13 erg cm^-2 s^-1 in the 0.5-2 keV band and 3.4e-15 to ~6e-13 erg cm^-2
s^-1 in the 2-10 keV band). We detect 9387 point-like sources, while 7071
point-like sources are found at high Galactic-latitudes (i.e. >=20 deg). The
large number of detected sources resulting from the combination of large area
and deep flux limits make this survey a new important tool for investigating
the evolution of AGN. In particular, the large area permits finding rare
high-luminosity objects like QSO2, which are poorly sampled by other surveys,
adding precious information for the luminosity function bright end. The high
Galactic-latitude logN-logS relation is well determined over all the flux
coverage, and it is nicely consistent with previous results at 1 sigma
confidence level. By the hard X-ray color analysis, we find that the Swift
Serendipitous Survey in deep XRT GRB fields samples relatively unobscured and
mildly obscured AGN, with a fraction of obscured sources of ~37% (~15%) in the
2-10 (0.3-3 keV) band. | astro-ph_HE |
Solar Wind Charge Exchange contribution to the ROSAT All Sky Survey Maps: DXL (Diffuse X-ray emission from the Local Galaxy) is a sounding rocket
mission designed to estimate the contribution of Solar Wind Charge eXchange
(SWCX) to the Diffuse X-ray Background (DXB) and to help determine the
properties of the Local Hot Bubble (LHB). The detectors are large-area
thin-window proportional counters with a spectral response similar to that of
the PSPC used in the ROSAT All Sky Survey (RASS). A direct comparison of DXL
and RASS data for the same part of the sky viewed from quite different vantage
points in the Solar system and the assumption of approximate isotropy for the
Solar wind allowed us to quantify the SWCX contribution to all 6 RASS bands
(R1-R7, excepting R3). We find that the SWCX contribution at l=140 deg, b=0
deg, where the DXL path crosses the Galactic plane is 33%+-6%
(statistical)+-12%(systematic) for R1, 44%+-\%+-5% for R2, 18%+-12%+-11% for
R4, 14%+-11%+-9% for R5, and negligible for R6 and R7 bands. Reliable models
for the distribution of neutral H and He in the Solar system permit estimation
of the contribution of interplanetary SWCX emission over the the whole sky and
correction of the RASS maps. We find that the average SWCX contribution in the
whole sky is 26%+-6%+-13% for R1, 30%+-4%+-4% for R2, 8%+-5%+-5% for R4,
6%+-4%+-4% for R5, and negligible for R6 and R7. | astro-ph_HE |
Modeling the oxygen K absorption in the interstellar medium: an {\it
XMM}-Newton view of Sco X-1: We investigate the absorption structure of the oxygen in the interstellar
medium by analyzing {\it XMM}-Newton observations of the low mass X-ray binary
Sco X-1. We use simple models based on the O {\sc i} atomic cross section from
different sources to fit the data and evaluate the impact of the atomic data in
the interpretation of astrophysical observations. We show that relatively small
differences in the atomic calculations can yield spurious results. We also show
that the most complete and accurate set of atomic cross sections successfully
reproduce the observed data in the $21-24.5$ {\AA} wavelength region of the
spectrum. Our fits indicate that the absorption is mainly due to neutral gas
with an ionization parameter of $\xi=10^{-4}$ erg cm s$^{-1}$, and an oxygen
column density of $N_{\mathrm{O}}\approx 8-10\times 10^{17}$ cm$^{-2}$. Our
models are able to reproduce both the K edge and the K$\alpha$ absorption line
from O {\sc i}, which are the two main features in this region. We find no
conclusive evidence for absorption by other than atomic oxygen. | astro-ph_HE |
VERITAS Observations of the Unidentified Point Source HESS J1943+213: The H.E.S.S. Galactic plane scan has revealed a large population of Galactic
very high energy (VHE; E > 100 GeV) emitters. The majority of the galactic
sources are extended and can typically be associated with pulsar wind nebulae
(35%) and supernova remnants (21%), while some of the sources remain
unidentified (31%). A much smaller fraction of point-like sources (5 in total,
corresponding to 4%) are identified as gamma-ray binaries. Active galactic
nuclei located behind the Galactic plane are also a potential source class. An
active galaxy could be identified in the VHE regime by a point-like appearance,
a high variability amplitude (up to a factor of 100) and a typically soft
spectrum (due to absorption by the extra-galactic background light). Here we
report on VERITAS observations of HESS J1943+213, an unidentified point source
discovered to emit above 470 GeV during the extended H.E.S.S. Galactic plane
scan. This source is thought to be a distant BL Lac object behind the Galactic
plane and, though it exhibits a steep spectrum it is a weak GeV source, only
recently detected using 5 years of Fermi-LAT data. Deep VERITAS observations at
high elevations result in the most significant VHE detection of this object so
far, with an excess above 200 GeV of more than 18 standard deviations. We use
variability and spectral analyses of VERITAS data on HESS J1943+213 in a
multi-wavelength context to address the source classification. | astro-ph_HE |
Gravitational waves from the Papaloizou-Pringle instability in black
hole-torus systems: Black hole (BH)--torus systems are promising candidates for the central
engine of gamma-ray bursts (GRBs), and also possible outcomes of the collapse
of supermassive stars to supermassive black holes (SMBHs). By three-dimensional
general relativistic numerical simulations, we show that an $m=1$
nonaxisymmetric instability grows for a wide range of self-gravitating tori
orbiting BHs. The resulting nonaxisymmetric structure persists for a timescale
much longer than the dynamical one, becoming a strong emitter of large
amplitude, quasiperiodic gravitational waves. Our results indicate that both,
the central engine of GRBs and newly formed SMBHs, can be strong gravitational
wave sources observable by forthcoming ground-based and spacecraft detectors. | astro-ph_HE |
2D radiation-hydrodynamic simulations of supernova ejecta with a central
power source: We present the results of two-dimensional radiation-hydrodynamic simulations
of expanding supernova ejecta with a central energy source. As suggested in
previous multi-dimensional hydrodynamic simulations, a sufficiently powerful
central energy source can blow away the expanding supernova ejecta, leading to
efficient mixing of stratified layers in the ejecta. We assume that the energy
injection is realized in the form of non-thermal radiation from the wind nebula
embedded at the center of the ejecta. We found that the multi-dimensional
mixing in the ejecta assists the injected non-thermal radiation escaping from
the ejecta. When the non-thermal radiation is absorbed by the ejecta, it is
converted into bright thermal radiation or is consumed as the kinetic energy of
the supernova ejecta. We found that central energy sources with the injection
timescale similar to the photon diffusion timescale realize an efficient
conversion of the injected energy into thermal radiation. On the other hand, a
rapid energy injection ends up accelerating the ejecta rather than giving rise
to bright thermal emission. This remarkable difference potentially explains the
diversity of energetic supernovae including broad-lined Ic and superluminous
supernovae. | astro-ph_HE |
Neutrino and cosmic-ray emission from multiple internal shocks in
gamma-ray bursts: Gamma-ray bursts are short-lived, luminous explosions at cosmological
distances, thought to originate from relativistic jets launched at the deaths
of massive stars. They are among the prime candidates to produce the observed
cosmic rays at the highest energies. Recent neutrino data have, however,
started to constrain this possibility in the simplest models with only one
emission zone. In the classical theory of gamma-ray bursts, it is expected that
particles are accelerated at mildly relativistic shocks generated by the
collisions of material ejected from a central engine. We consider neutrino and
cosmic-ray emission from multiple emission regions since these internal
collisions must occur at very different radii, from below the photosphere all
the way out to the circumburst medium, as a consequence of the efficient
dissipation of kinetic energy. We demonstrate that the different messengers
originate from different collision radii, which means that multi-messenger
observations open windows for revealing the evolving GRB outflows. | astro-ph_HE |
Uncloaking hidden repeating fast radio bursts with unsupervised machine
learning: The origins of fast radio bursts (FRBs), astronomical transients with
millisecond timescales, remain unknown. One of the difficulties stems from the
possibility that observed FRBs could be heterogeneous in origin; as some of
them have been observed to repeat, and others have not. Due to limited
observing periods and telescope sensitivities, some bursts may be misclassified
as non-repeaters. Therefore, it is important to clearly distinguish FRBs into
repeaters and non-repeaters, to better understand their origins. In this work,
we classify repeaters and non-repeaters using unsupervised machine learning,
without relying on expensive monitoring observations. We present a repeating
FRB recognition method based on the Uniform Manifold Approximation and
Projection (UMAP). The main goals of this work are to: (i) show that the
unsupervised UMAP can classify repeating FRB population without any prior
knowledge about their repetition, (ii) evaluate the assumption that
non-repeating FRBs are contaminated by repeating FRBs, and (iii) recognise the
FRB repeater candidates without monitoring observations and release a
corresponding catalogue. We apply our method to the Canadian Hydrogen Intensity
Mapping Experiment Fast Radio Burst (CHIME/FRB) database. We found that the
unsupervised UMAP classification provides a repeating FRB completeness of 95
per cent and identifies 188 FRB repeater source candidates from 474
non-repeater sources. This work paves the way to a new classification of
repeaters and non-repeaters based on a single epoch observation of FRBs. | astro-ph_HE |
A statistical analysis of the "heartbeat" behaviour of GRS 1915+105: GRS 1915+105 has been active for more than 26 years since it was discovered
in 1992. There are hundreds of RXTE pointed observations on this source, and
the quasi-regular flares with a slow rise and a sharp decrease (i.e. the
"heartbeat" state) were recorded in more than 200 observations. The connections
among the disk/corona, jet, and the disk wind at the heartbeat state have been
extensively studied. In this work, we firstly perform a statistical analysis of
the light curves and the X-ray spectra to investigate this peculiar state. We
calculate the parameters for heartbeat cycles, including the recurrence time,
the maximum and the minimum count rate, the flare amplitude, and the cumulative
radiation for each cycle. The recurrence time has a bimodal distribution
ranging from $\sim 20$ to $\sim 200$ s. The minimum count rate increases with
increasing recurrence time; while the maximum count rate remains nearly
constant around 2 Crab. Fitting the averaged spectrum for each observation, we
find the strong correlations among the recurrence time, the apparent inner
radius of the accretion disk (or the color correction factor), and the
(nonthermal) X-ray luminosity. We suggest that the true inner edge of the
accretion disk might always extend to the marginally stable orbit, while the
change in corona size should result in the observed correlations. | astro-ph_HE |
Subphotospheric emission from short gamma-ray bursts: Protons mold the
multi-messenger signals: The origin of the observed Band-like photon spectrum in short gamma-ray
bursts (sGRBs) is a long-standing mystery. We carry out the first general
relativistic magnetohydrodynamic simulation of a sGRB jet with initial
magnetization $\sigma_0 = 150$ in dynamical ejecta from a binary merger. From
this simulation, we identify regions along the jet of efficient energy
dissipation due to magnetic reconnection and collisionless sub-shocks. Taking
into account electron and proton acceleration processes, we solve for the first
time the coupled transport equations for photons, electrons, protons,
neutrinos, and intermediate particles species up to close to the photosphere
(i.e., up to $1 \times 10^{12}$ cm), accounting for all relevant radiative and
cooling processes. We find that the subphotospheric multi-messenger signals
carry strong signatures of the hadronic interactions and their resulting
particle cascades. Importantly, the spectral energy distribution of photons is
significantly distorted with respect to the Wien one, commonly assumed below
the photosphere. Our findings suggest that the bulk of the non-thermal photon
spectrum observed in sGRBs can stem from hadronic processes, occurring below
the photosphere and previously neglected, with an accompanying energy flux of
neutrinos peaking in the GeV energy range. | astro-ph_HE |
TRAPUM discovery of thirteen new pulsars in NGC 1851 using MeerKAT: We report the discovery of 13 new pulsars in the globular cluster NGC 1851 by
the TRAPUM Large Survey Project using the MeerKAT radio telescope. The
discoveries consist of six isolated millisecond pulsars (MSPs) and seven binary
pulsars, of which six are MSPs and one is mildly recycled. For all the pulsars,
we present the basic kinematic, astrometric, and orbital parameters, where
applicable, as well as their polarimetric properties, when these are
measurable. Two of the binary MSPs (PSR J0514-4002D and PSR J0514-4002E) are in
wide and extremely eccentric (e > 0.7) orbits with a heavy white dwarf and a
neutron star as their companion, respectively. With these discoveries, NGC 1851
is now tied with M28 as the cluster with the third largest number of known
pulsars (14). Its pulsar population shows remarkable similarities with that of
M28, Terzan 5 and other clusters with comparable structural parameters. The
newly-found pulsars are all located in the innermost regions of NGC 1851 and
will likely enable, among other things, detailed studies of the cluster
structure and dynamics. | astro-ph_HE |
Disc-corona interaction in the heartbeat state of GRS 1915+105: Timing analysis provides information about the dynamics of matter accreting
on to neutron stars and black holes, and hence is crucial for studying the
physics of the accretion flow around these objects. It is difficult, however,
to associate the different variability components with each of the spectral
components of the accretion flow. We apply several new methods to two Rossi
X-ray Timing Explorer observations of the black hole binary GRS 1915+105 during
its heartbeat state to explore the origin of the X-ray variability and the
interactions of the accretion-flow components. We offer a promising window into
the disc--corona interaction through analysing the formation regions of the
disc aperiodic variabilities with different time-scales via comparing the
corresponding transition energies of the amplitude-ratio spectra. In a previous
paper, we analysed the Fourier power density as a function of energy and
frequency to study the origin of the aperiodic variability, and combined that
analysis with the phase lag as a function of frequency to derive a picture of
the disc--corona interaction in this source. We here, for the first time,
investigate the phase lag as a function of energy and frequency, and display
some interesting details of the disc--corona interaction. Besides, the results
from the shape of amplitude-ratio spectrum and from several other aspects
suggest that the quasi-periodic oscillation originates from the corona. | astro-ph_HE |
Evidence for gravitational lensing of GRB 200716C: Observationally, there is a small fraction of Gamma-ray bursts (GRBs) with
prompt emission observed by Fermi/GBM that are composed of two pulses.
Occasionally, the distance to a GRB may be lensed when a high mass
astrophysical object resides in the path between the GRB source and observer.
In this paper, we describe GRB 200716C, which has a two-pulse emission and
duration of a few seconds. We present a Bayesian analysis identifying
gravitational lensing in both temporal and spectral properties, and calculate
the time delay ($\Delta t\sim 1.92$ s) and magnification ($\gamma\sim 1.5$)
between those two pulses based on the temporal fits. One can roughly estimate
the lens mass to be about $2.4\times 10^{5}~M_{\odot}$ in the rest frame. We
also calculate the false alarm probability for this detection to be about
0.07\% with trial factors, and a present-day number density of about $808
\rm~Mpc^{-3}$ with an energy density $\Omega\sim 1.4\times 10^{-3}$. If the
first pulse of this GRB near the trigger time is indeed gravitationally echoed
by a second pulse, GRB 200716C may be a short GRB candidate with extended
emission. | astro-ph_HE |
An outflow powers the optical rise of the nearby, fast-evolving tidal
disruption event AT2019qiz: At 66 Mpc, AT2019qiz is the closest optical tidal disruption event (TDE) to
date, with a luminosity intermediate between the bulk of the population and
iPTF16fnl. Its proximity allowed a very early detection and triggering of
multiwavelength and spectroscopic follow-up well before maximum light. The
velocity dispersion of the host galaxy and fits to the TDE light curve indicate
a black hole mass $\approx 10^6$ M$_\odot$, disrupting a star of $\approx 1$
M$_\odot$. Comprehensive UV, optical and X-ray data shows that the early
optical emission is dominated by an outflow, with a luminosity evolution $L
\propto t^2$, consistent with a photosphere expanding at constant velocity
($\gtrsim 2000$ km s$^{-1}$), and a line-forming region producing initially
blueshifted H and He II profiles with $v=3000-10000$ km s$^{-1}$. The fastest
optical ejecta approach the velocity inferred from radio detections (modelled
in a forthcoming companion paper from K.~D.~Alexander et al.), thus the same
outflow may be responsible for both the fast optical rise and the radio
emission -- the first time this connection has been observed in a TDE. The
light curve rise begins $29 \pm 2$ days before maximum light, peaking when the
photosphere reaches the radius where optical photons can escape. The
photosphere then undergoes a sudden transition, first cooling at constant
radius then contracting at constant temperature. At the same time, the
blueshifts disappear from the spectrum and Bowen fluorescence lines (N III)
become prominent, implying a source of far-UV photons, while the X-ray light
curve peaks at $\approx 10^{41}$ erg s$^{-1}$. Assuming that these X-rays are
from prompt accretion, the size and mass of the outflow are consistent with the
reprocessing layer needed to explain the large optical to X-ray ratio in this
and other optical TDEs, possibly favouring accretion-powered over
collision-powered outflow models. | astro-ph_HE |
Light curves and spectra from off-axis gamma-ray bursts: If gamma-ray burst prompt emission originates at a typical radius, and if
material producing the emission moves at relativistic speed, then the
variability of the resulting light curve depends on the viewing angle. This is
due to the fact that the pulse evolution time scale is Doppler contracted,
while the pulse separation is not. For off-axis viewing angles $\theta_{\rm
view} \gtrsim \theta_{\rm jet} + \Gamma^{-1}$, the pulse broadening
significantly smears out the light curve variability. This is largely
independent of geometry and emission processes. To explore a specific case, we
set up a simple model of a single pulse under the assumption that the pulse
rise and decay are dominated by the shell curvature effect. We show that such a
pulse observed off-axis is (i) broader, (ii) softer and (iii) displays a
different hardness-intensity correlation with respect to the same pulse seen
on-axis. For each of these effects, we provide an intuitive physical
explanation. We then show how a synthetic light curve made by a superposition
of pulses changes with increasing viewing angle. We find that a highly variable
light curve, (as seen on-axis) becomes smooth and apparently single-pulsed
(when seen off-axis) because of pulse overlap. To test the relevance of this
fact, we estimate the fraction of off-axis gamma-ray bursts detectable by
\textit{Swift} as a function of redshift, finding that a sizable fraction
(between 10\% and 80\%) of nearby ($z<0.1$) bursts are observed with
$\theta_{\rm view} \gtrsim \theta_{\rm jet} + \Gamma^{-1}$. Based on these
results, we argue that low luminosity gamma-ray bursts are consistent with
being ordinary bursts seen off-axis. | astro-ph_HE |
Seven Years with the Swift Supergiant Fast X-ray Transients Project: Supergiant Fast X-ray Transients (SFXTs) are HMXBs with OB supergiant
companions. I review the results of the Swift SFXT Project, which since 2007
has been exploiting Swift's capabilities in a systematic study of SFXTs and
supergiant X-ray binaries (SGXBs) by combining follow-ups of outbursts, when
detailed broad-band spectroscopy is possible, with long-term monitoring
campaigns, when the out-of-outburst fainter states can be observed. This
strategy has led us to measure their duty cycles as a function of luminosity,
to extract their differential luminosity distributions in the soft X-ray
domain, and to compare, with unprecedented detail, the X-ray variability in
these different classes of sources. I also discuss the "seventh year crisis",
the challenges that the recent Swift observations are making to the prevailing
models attempting to explain the SFXT behaviour. | astro-ph_HE |
A Genetic Algorithm for Astroparticle Physics Studies: Precision measurements of charged cosmic rays have recently been carried out
by space-born (e.g. AMS-02), or ground experiments (e.g. HESS). These measured
data are important for the studies of astro-physical phenomena, including
supernova remnants, cosmic ray propagation, solar physics and dark matter.
Those scenarios usually contain a number of free parameters that need to be
adjusted by observed data. Some techniques, such as Markov Chain Monte Carlo
and MultiNest, are developed in order to solve the above problem. However, it
is usually required a computing farm to apply those tools. In this paper, a
genetic algorithm for finding the optimum parameters for cosmic ray injection
and propagation is presented. We find that this algorithm gives us the same
best fit results as the Markov Chain Monte Carlo but consuming less computing
power by nearly 2 orders of magnitudes. | astro-ph_HE |
Optical Counterparts of ULXs and Their Host Environments in NGC
4490/4485: We report the identification of the possible optical counterparts of five out
of seven Ultraluminous X-ray Sources (ULXs) in NGC 4490/4485 galaxy pair. Using
archival Hubble Space Telescope ({\it HST}) imaging data, we identified a
single optical candidate for two ULXs (X-4 and X-7) and multiple optical
candidates for the other three ULXs (X-2, X-3 and X-6) within $\sim$ $0\farcs2$
error radius at the 90\% confidence level. Of the two remaining ULXs, X-1 has
no {\it HST} imaging data and photometry could not be performed due to the
position of X-5 in NGC4490. Absolute magnitudes ($M_{V}$) of the optical
candidates lie between $-5.7$ and $-3.8$. Color-Magnitude Diagrams (CMDs) have
been used to investigate the properties of counterparts and their environments.
The locations of the counterparts of X-2, X-4, and X-6 suggest possible
association with nearby group of stars while others have no association with a
star cluster or group of stars. For comparison purposes, we analyzed previously
unused three archival XMM-Newton observations. The long-term X-ray light curves
of the sources (except transient X-7) show variability by a factor of three in
a time scale more than a decade. The use of disk blackbody model for the mass
of the compact objects indicates that these objects might have masses most
likely in the range 10$-$15 $M_{\sun}$. | astro-ph_HE |
Radio WISSH: tuning on the most luminous quasars in the Universe: In the past years, the results obtained by the WISSH quasar project provided
a novel general picture on the distinctive multi-band properties of
hyper-luminous ($L_{bol}>10^{47}$ erg/s) quasars at high redshift (z$\sim$2-4),
unveiling interesting relations among active galactic nuclei, winds and
interstellar medium, in these powerful sources at cosmic noon. Since 2022, we
are performing a systematic and statistically-significant VLA study of the
radio properties of WISSH. We carried out high-resolution VLA observations
aiming at: 1) identifying young radio source from the broad-band spectral shape
of these objects; 2) sample an unexplored high redshift/high luminosity regime,
tracking possible evolutionary effects on the radio-loud/radio-quiet dichotomy;
3) quantifying orientation effects on the observed winds/outflows properties. | astro-ph_HE |
Contribution to the Extragalactic Gamma-ray Background from the Cascades
of Very-high Energy Gamma Rays: As very-high--energy photons propagate through the extragalactic background
light (EBL), they interact with the soft photons and initiate electromagnetic
cascades of lower energy photons and electrons. The collective intensity of a
cosmological population emitting at very-high energies (VHE) will be attenuated
at the highest energies through interactions with the EBL and enhanced at lower
energies by the resulting cascade. We calculate the cascade radiation created
by VHE photons produced by blazars and investigate the effects of cascades on
the collective intensity of blazars and the resulting effects on the
extragalactic gamma-ray background. We find that cascade radiation greatly
enhances the collective intensity from blazars at high energies before turning
over due to attenuation. The prominence of the resulting features depends on
the blazar gamma-ray luminosity function, spectral index distribution, and the
model of the EBL. We additionally calculate the cascade radiation from the
distinct spectral sub-populations of blazars, BL Lacertae objects (BL Lacs) and
flat-spectrum radio quasars (FSRQs), finding that the collective intensity of
BL Lacs is considerably more enhanced by cascade radiation than that of the
FSRQs due to their harder spectra. As such, studies of the blazar contribution
to the EGRB by Fermi will have profound implications for the nature of the EBL,
the evolution of blazars, and blazar spectra. | astro-ph_HE |
A young SNR illuminating nearby Molecular Clouds with cosmic rays: The Supernova Remnant (SNR) HESS J1731-347 displays strong non-thermal TeV
gamma-ray and X-ray emission, thus the object is at present time accelerating
particles to very high energies. A distinctive feature of this young SNR is the
nearby (~30 pc in projection) extended source HESS J1729-345, which is
currently unidentified but is in spatial projection coinciding with known
molecular clouds (MC). We model the SNR evolution to explore if the TeV
emission from HESS J1729-345 can be explained as emission from runaway hadronic
cosmic rays (CRs) that are illuminating these MCs. The observational data of
HESS J1729-345 and HESS J1731-347 can be reproduced using core-collapse SN
models for HESS J1731-347. Starting with different progenitor stars and their
pre-supernova environment, we model potential SNR evolution histories along
with the CR acceleration in the SNR and the diffusion of the CRs. A simplified
3-dimensional structure of the MCs is introduced based on 12CO data, adopting a
distance of 3.2 kpc to the source. A Monte Carlo-based diffusion model for the
escaping CRs is developed to deal with the inhomogeneous environment. The fast
SNR forward shock speed as implied from the X-ray data can easily be explained
when employing scenarios with progenitor star masses between 20 and 25 solar
masses, where the SNR shock is still expanding inside the main sequence
(MS)-bubble at present time. The TeV spectrum of HESS J1729-345 is
satisfactorily fitted by the emission from the highest-energy CRs that have
escaped the SNR, using a standard galactic CR diffusion coefficient in the
inter-clump medium. The TeV image of HESS J1729-345 can be explained with a
reasonable 3-dimensional structure of MCs. The TeV emission from the SNR itself
is dominated by leptonic emission in this model. We also explore scenarios
where the shock is starting to encounter the dense MS progenitor wind bubble
shell. | astro-ph_HE |
SN 2015an: a normal luminosity type II supernova with low expansion
velocity at early phases: We present the photometry and spectroscopy of SN 2015an, a Type II Supernova
(SN) in IC 2367. The recombination phase of the SN lasts up to $\sim$120 d,
with a decline rate of 1.24 mag/100d, higher than the typical SNe IIP. The SN
exhibits bluer colours than most SNe II, indicating higher ejecta temperatures.
The absolute $V$-band magnitude of SN 2015an at 50 d is $-$16.83$\pm$0.04 mag,
pretty typical for SNe II. However, the $^{56}$Ni mass yield, estimated from
the tail $V$-band light curve to be 0.021$\pm$0.010 M$_\odot$, is comparatively
low. The spectral properties of SN 2015an are atypical, with low H$\alpha$
expansion velocity and presence of high velocity component of H$\alpha$ at
early phases. Moreover, the continuum exhibits excess blue flux up to $\sim$50
d, which is interpreted as a progenitor metallicity effect. The high velocity
feature indicates ejecta-circumstellar material interaction at early phases.
The semi-analytical modelling of the bolometric light curve yields a total
ejected mass of $\sim$12 M$_\odot$, a pre-supernova radius of
$\sim$388~R$_\odot$ and explosion energy of $\sim$1.8 foe. | astro-ph_HE |
Early-time VLA observations and broad-band afterglow analysis of the
Fermi-LAT detected GRB 130907A: We present multi-wavelength observations of the hyper-energetic gamma-ray
burst (GRB) 130907A, a Swift-discovered burst with early radio observations
starting at $\approx 4$ hr after the $\gamma$-ray trigger. GRB 130907A was also
detected by the Fermi/LAT instrument and, at late times, showed a strong
spectral evolution in X-rays. We focus on the early-time radio observations,
especially at $>10 $ GHz, to attempt identifying reverse shock signatures.
While our radio follow-up of GRB 130907A ranks among the earliest observations
of a GRB with the Karl G. Jansky Very Large Array (VLA), we did not see an
unambiguous signature of a reverse shock. While a model with both reverse and
forward-shock can correctly describe the observations, the data is not
constraining enough to decide upon the presence of the reverse-shock component.
We model the broad-band data using a simple forward-shock synchrotron scenario
with a transition from a wind environment to a constant density interstellar
medium (ISM) in order to account for the observed features. Within the confines
of this model, we also derive the underlying physical parameters of the
fireball, which are within typical ranges except for the wind density parameter
($A_*$), which is higher than those for bursts with wind-ISM transition, but
typical for the general population of bursts. We note the importance of
early-time radio observations of the afterglow (and of well sampled light
curves) to unambiguously identify the potential contribution of the reverse
shock. | astro-ph_HE |
Fornax A, Centaurus A and other radio galaxies as sources of ultra-high
energy cosmic rays: The origin of ultra-high energy cosmic rays (UHECRs) is still unknown. It has
recently been proposed that UHECR anisotropies can be attributed to starburst
galaxies or active galactic nuclei. We suggest that the latter is more likely
and that giant-lobed radio galaxies such as Centaurus A and Fornax A can
explain the data. | astro-ph_HE |
Gamma Ray Signal from the Pulsar Wind in the Binary Pulsar system PSR
B1259-63/LS2883: Binary pulsar systems emit potentially detectable components of gamma ray
emission due to Comptonization of the optical radiation of the companion star
by relativistic electrons of the pulsar wind, both before and after termination
of the wind. The recent optical observations of binary pulsar system PSR
B1259-63/LS 2883 revealed radiation properties of the companion star which
differ significantly from previous measurements. In this paper we study the
implications of these observations for the interaction rate of the unshocked
pulsar wind with the stellar photons and the related consequences for fluxes of
high energy (HE) and very high energy (VHE) gamma rays. We show that the signal
should be strong enough to be detected with Fermi close to the periastron
passage, unless the pulsar wind is strongly anisotropic or the Lorentz factor
of the wind is smaller than $10^3$ or larger that $10^5$. The higher luminosity
of the optical star also has two important implications: (i) attenuation of
gamma rays due to photon-photon pair production, and (ii) Compton drag of the
unshocked wind. While the first effect has an impact on the lightcurve of VHE
gamma rays, the second effect may significantly decrease the energy available
for particle acceleration after termination of the wind. | astro-ph_HE |
First observation of PeV-energy neutrinos with IceCube: We report on the observation of two neutrino-induced events which have an
estimated deposited energy in the IceCube detector of 1.04 $\pm$ 0.16 and 1.14
$\pm$ 0.17 PeV, respectively, the highest neutrino energies observed so far.
These events are consistent with fully contained particle showers induced by
neutral-current $\nu_{e,\mu,\tau}$ ($\bar\nu_{e,\mu,\tau}$) or charged-current
$\nu_{e}$ ($\bar\nu_{e}$) interactions within the IceCube detector. The events
were discovered in a search for ultra-high energy neutrinos using data
corresponding to 615.9 days effective livetime. The expected number of
atmospheric background is $0.082 \pm 0.004 \text{(stat)}^{+0.041}_{-0.057}
\text{(syst)}$. The probability to observe two or more candidate events under
the atmospheric background-only hypothesis is $2.9\times10^{-3}$ ($2.8\sigma$)
taking into account the uncertainty on the expected number of background
events. These two events could be a first indication of an astrophysical
neutrino flux, the moderate significance, however, does not permit a definitive
conclusion at this time. | astro-ph_HE |
Properties of neutrino transfer in a deformed remnant of neutron star
merger: We study properties of neutrino transfer in a remnant of neutron star merger,
consisting of a massive neutron star and a surrounding torus. We perform
numerical simulations of the neutrino transfer by solving the Boltzmann
equation with momentum-space angles and energies of neutrinos for snapshots of
the merger remnant having elongated shapes. The evaluation of the neutrino
distributions in the multi-dimensions enable us to provide the detailed
information of angle and energy spectra and neutrino reaction rates. We
demonstrate features of asymmetric neutrino fluxes from the deformed remnant
and investigate the neutrino emission region by determining the neutrinosphere
for each energy. We examine the emission and absorption of neutrinos to
identify important ingredients of heating rates through neutrino irradiation.
We show that the contributions of $\mu$- and $\tau$-types neutrinos are
important for the heating in the region above the massive neutron star. We also
examine the angle moments and the Eddington tensor calculated directly by the
neutrino distribution functions and compare them with those obtained by a
moment closure approach, which is often used in the study of neutrino-radiation
hydrodynamics. We show that the components of the Eddington tensor have
non-monotonic behaviors and the approximation of the closure relation may
become inaccurate for high energy neutrinos, whose fluxes are highly aspherical
due to the extended merger remnant. | astro-ph_HE |
Investigation of the particle-particle interaction effects in the cosmic
Zevatron based on cyclotron auto-resonance by particle-in-cell simulations: Cyclotron autoresonance acceleration has been recently advanced as a
potential mechanism for accelerating nuclei to ZeV energies (1 ZeV = $10^{21}$
eV). All results have been based on single- and many-particle calculations
employing analytic solutions to the relativistic equations of motion in the
combined magnetic and radiation fields, excluding effects related to the
particle-particle interactions. Here, results from many-particle calculations
and Particle-In-Cell (PIC) simulations, are presented which lend support to the
single-particle investigations. Each single-particle result is found to lie
well within one standard deviation about the ensemble average obtained from the
corresponding many-particle calculation. The PIC simulations show that, even
for number densities far exceeding those employed in the non-interacting case,
the energy gain drops markedly due to the particle-particle interactions, over
the first $\sim 8~ mm$ of the acceleration length. Together with the
substantial attenuation, this finding supports the conclusion that the
particle-particle interaction effects can be negligibly small over acceleration
lengths of typically many kilometers. | astro-ph_HE |
Search for Tens of MeV Neutrinos associated with Gamma-Ray Bursts in
Super-Kamiokande: A search for neutrinos produced in coincidence with Gamma-Ray Bursts(GRB) was
conducted with the Super-Kamiokande (SK) detector. Between December 2008 and
March 2017, the Gamma-ray Coordinates Network recorded 2208 GRBs that occurred
during normal SK operation. Several time windows around each GRB were used to
search for coincident neutrino events. No statistically significant signal in
excess of the estimated backgrounds was detected. The $\bar\nu_e$ fluence in
the range from 8 MeV to 100 MeV in positron total energy for
$\bar\nu_e+p\rightarrow e^{+}+n$ was found to be less than $\rm 5.07\times10^5$
cm$^{-2}$ per GRB in 90\% C.L. Upper bounds on the fluence as a function of
neutrino energy were also obtained. | astro-ph_HE |
On the Particle Acceleration Mechanisms in a Double Radio Relic Galaxy
Cluster, Abell 1240: We present a 368 ks deep Chandra observation of Abell~1240, a binary merging
galaxy cluster at a redshift of 0.195 with two Brightest Cluster Galaxies
(BCGs) may have passed each other 0.3 Gyr ago. Building upon previous
investigations involving GMRT, VLA, and LOFAR data, our study focuses on two
prominent extended radio relics at the north-west (NW) and south-east (SE) of
the cluster core. By leveraging the high-resolution Chandra imaging, we have
identified two distinct surface brightness edges at $\sim$ 1 Mpc and 1.2 Mpc NW
and SE of the cluster center, respectively, coinciding with the outer edges of
both relics. Our temperature measurements hint the edges to be shock front
edges. The Mach numbers, derived from the gas density jumps, yield
$\cal{M}_{\rm SE}$ = 1.49$^{+0.22}_{-0.24}$ for the South Eastern shock and
$\cal{M}_{\rm NW}$ = 1.41$^{+0.17}_{-0.19}$ for the North Western shock. Our
estimated Mach numbers are remarkably smaller compared to those derived from
radio observations ($\cal{M}_{\rm SE}$ = 2.3 and $\cal{M}_{\rm NW}$ = 2.4),
highlighting the prevalence of a re-acceleration scenario over direct
acceleration of electrons from the thermal pool. Furthermore, we compare the
observed temperature profiles across both shocks with that of predictions from
collisional vs. collisionless models. Both shocks favor the Coulomb collisional
model, but we could not rule out a purely collisionless model due to pre-shock
temperature uncertainties. | astro-ph_HE |
Different behaviors of wavelet results for type-B and type-C QPOs of
MAXI J1535-571 based on NICER data: Wavelet analysis, in addition to power density spectra, is another method to
study the quasi-periodic signals in the light curves, but has been rarely used
in black hole X-ray transients. We performed wavelet analysis of X-ray timing
features and quasi-periodic oscillations (QPOs) based on NICER observations of
the black hole candidate MAXI J1535-571 in this paper. Separating the light
curves by the confidence level of wavelet results, we find significant
differences exist in the PDS, hardness ratio and mean count between light curve
segments above and below the confidence level. The S-factor, which is defined
as the ratio of the effective oscillation time and the total time, demonstrates
distinct values between type-C and type-B QPOs. Based on our results, the
S-factor for type-B QPO is very close or equal to 0, no matter the confidence
level is set as 95\% or 68\%, while the S-factor of type-C QPO is significantly
higher, especially in the 68\% confidence level case. We discuss the
implications of the wavelet results on resolving type-B and type-C QPOs in
black hole X-ray binaries. | astro-ph_HE |
Non-Maxwellian electron distributions in clusters of galaxies: Thermal X-ray spectra of clusters of galaxies and other sources are commonly
calculated assuming Maxwellian electron distributions. There are situations
where this approximation is not valid, for instance near interfaces of hot and
cold gas and near shocks. The presence of non-thermal electrons affects the
X-ray spectrum. To study the role of these electrons in clusters and other
environments, an efficient algorithm to calculate the X-ray spectra is needed.
We approximate an arbitrary electron distribution by the sum of Maxwellian
components. The decomposition is done using either a genetic algorithm or an
analytical approximation. The X-ray spectrum is then evaluated using a linear
combination of those Maxwellian components. Our method is fast and leads to an
accurate evaluation of the spectrum. The use of Maxwellian components allows to
use the standard collisional rates that are available in plasma codes such as
SPEX. We give an example of a spectrum for the supra-thermal electron
distribution behind a shock in a cluster of galaxies. The relative intensities
of the satellite lines in such a spectrum are sensitive to the presence of the
supra-thermal electrons. These lines can only be investigated with high
spectral resolution. We show that the instruments on future missions like
Astro-H and IXO will be able to demonstrate the presence or absence of these
supra-thermal electrons. | astro-ph_HE |
A Compton-thin Solution for the Suzaku X-ray Spectrum of the Seyfert 2
Galaxy Mkn 3: Mkn 3 is a Seyfert 2 galaxy that is widely regarded as an exemplary
Compton-thick AGN. We study the Suzaku X-ray spectrum using models of the X-ray
reprocessor that self-consistently account for the Fe K$\alpha$ fluorescent
emission line and the associated Compton-scattered, or reflection, continuum.
We find a solution in which the average global column density,
$0.234^{+0.012}_{-0.010} \times 10^{24} \ \rm cm^{-2}$, is very different to
the line-of-sight column density, $0.902^{+0.012}_{-0.013} \times 10^{24} \ \rm
cm^{-2}$. The global column density is $\sim 5$ times smaller than that
required for the matter distribution to be Compton-thick. Our model accounts
for the profiles of the Fe K$\alpha$ and Fe K$\beta$ lines, and the Fe K edge
remarkably well, with a solar abundance of Fe. The matter distribution could
consist of a clumpy medium with a line-of-sight column density higher than the
global average. A uniform, spherically-symmetric distribution alone cannot
simultaneously produce the correct fluorescent line spectrum and reflection
continuum. Previous works on Mkn 3, and other AGN, that assumed a reflection
continuum from matter with an infinite column density could therefore lead to
erroneous or "puzzling" conclusions if the matter out of the line-of-sight is
really Compton-thin. Whereas studies of samples of AGN have generally only
probed the line-of-sight column density, with simplistic, one-dimensional
models, it is important now to establish the global column densities in AGN. It
is the global properties that affect the energy budget in terms of reprocessing
of X-rays into infrared emission, and that constrain population synthesis
models of the cosmic X-ray background. | astro-ph_HE |
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