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Phase shift of gravitational waves induced by aberration: The velocity of a gravitational wave (GW) source provides crucial information
about its formation and evolution processes. Previous studies considered the
Doppler effect on the phase of GWs as a potential signature of a time-dependent
velocity of the source. However, the Doppler shift only accounts for the time
component of the wave vector, and in principle motion also affects the spatial
components. The latter effect, known as ``aberration'' for light, is analyzed
in this paper for GWs and applied to the waveform modeling of an accelerating
source. We show that the additional aberrational phase shift could be
detectable in two astrophysical scenarios, namely, a recoiling binary black
hole (BBH) due to GW radiation and a BBH in a triple system. Our results
suggest that adding the aberrational phase shift in the waveform templates
could significantly enhance the detectability of moving sources. | astro-ph_HE |
Early Spectral Evolution of the Rapidly Expanding Type Ia SN 2006X: We present optical spectroscopic and photometric observations of Type Ia
supernova (SN) 2006X from --10 to +91 days after the $B$-band maximum. This SN
exhibits one of the highest expansion velocity ever published for SNe Ia. At
premaximum phases, the spectra show strong and broad features of
intermediate-mass elements such as Si, S, Ca, and Mg, while the O{\sc
i}$\lambda$7773 line is weak. The extremely high velocities of Si{\sc ii} and
S{\sc ii} lines and the weak O{\sc i} line suggest that an intense
nucleosynthesis might take place in the outer layers, favoring a delayed
detonation model. Interestingly, Si{\sc ii}$\lambda$5972 feature is quite
shallow, resulting in an unusually low depth ratio of Si{\sc ii}$\lambda$5972
to $\lambda$6355, $\cal R$(Si{\sc ii}). The low $\cal R$(Si{\sc ii}) is usually
interpreted as a high photospheric temperature. However, the weak Si{\sc
iii}$\lambda$4560 line suggests a low temperature, in contradiction to the low
$\cal R$(Si{\sc ii}). This could imply that the Si{\sc ii}$\lambda$5972 line
might be contaminated by underlying emission. We propose that $\cal R$(Si{\sc
ii}) may not be a good temperature indicator for rapidly expanding SNe Ia at
premaximum phases. | astro-ph_HE |
Constraining Low-luminosity Gamma-Ray Bursts as Ultra-high-energy Cosmic
Ray Sources Using GRB 060218 as a Proxy: We study the connection between low-luminosity gamma-ray bursts (llGRBs) and
ultra-high-energy cosmic rays (UHECRs) using the canonical low-luminosity GRB
060218 as a proxy. We focus on the consequential synchrotron emission from
electrons that are coaccelerated in the UHECR acceleration region, comparing
this emission to observations. Both the prompt and afterglow phases are
considered. For the prompt phase, we assume the coaccelerated electrons are
injected with a power law distribution instantaneously (without additional
heating or reacceleration), which results in bright optical-UV emission in
tension with observations. For the afterglow phase, we constrain the total
kinetic energy of the blast wave by comparing electron thermal synchrotron
radiation to available radio data at $\sim~3$ days. Considering mildly
relativistic outflows with bulk Lorentz factor $\Gamma \gtrsim 2$ (slower
trans-relativistic outflows are not treated), we find that the limited
available energy does not allow for GRB 060218-like afterglows to be the main
origin of UHECRs. This analysis independently constrains the prompt phase as a
major UHECR source as well, given that the prompt energy budget is comparable
to that of the afterglow kinetic energy. More generally, our study demonstrates
that synchrotron emission from thermal electrons is a powerful diagnostic of
the physics of mildly relativistic shocks. | astro-ph_HE |
Megaparsec-scale Radio Structure Associated with a Hybrid Blazar SBS
B1646+499: Episodic Jet Activity with Precessing Axis: Here we report on the total-intensity 610 MHz GMRT observations of the
peculiar hybrid blazar SBS B1646+499, which merges the properties of BL
Lacertae objects and flat-spectrum radio quasars. The complex radio structure
of SBS B1646+499, emerging from the archival radio data and our new GMRT
observations, consists of the Mpc-scale, elongated halo, the unilateral
kpc-scale jet, and the nuclear jet extending up to 20pc from the compact core.
The giant halo is characterized by a steep radio spectrum, indicative of the
advanced ageing of the electron population within the lobes. For the
large-scale jet, we detected a spectral gradient along and across the outflow,
and in particular spectral flattening of the radio continuum toward the jet
edges, suggestive of the spine--boundary shear layer morphology. The nuclear
jet displays superluminal knots emerging from the self-absorbed and variable
radio core. We interpret all these findings in the framework of the model of an
episodic jet activity with a precessing jet axis. | astro-ph_HE |
On the nature of the 'hostless' short GRBs: A significant proportion ($\sim30\%$) of the short-duration gamma-ray bursts
(SGRBs) localised by Swift have no detected host galaxy coincident with the
burst location to deep limits, and also no high-likelihood association with
proximate galaxies on the sky. These SGRBs may represent a population at
moderately high redshifts ($z\gtrsim1$), for which the hosts are faint, or a
population where the progenitor has been kicked far from its host or is sited
in an outlying globular cluster. We consider the afterglow and host
observations of three 'hostless' bursts (GRBs 090305A, 091109B and 111020A),
coupled with a new observational diagnostic to aid the association of SGRBs
with putative host galaxies to investigate this issue. Considering the well
localised SGRB sample, 7/25 SGRBs can be classified as 'hostless' by our
diagnostic. Statistically, however, the proximity of these seven SGRBs to
nearby galaxies is higher than is seen for random positions on the sky. This
suggests that the majority of 'hostless' SGRBs have likely been kicked from
proximate galaxies at moderate redshift. Though this result still suggests only
a small proportion of SGRBs will be within the AdLIGO horizon for NS-NS or
NS-BH inspiral detection ($z\sim0.1$), in the particular case of GRB 111020A a
plausible host candidate is at $z=0.02$. | astro-ph_HE |
Hints of a second explosion (a quark nova) in Cassiopeia A Supernova: We show that the explosive transition of the neutron star (NS) to a quark
star (QS) (a Quark Nova) in Cassiopeia A (Cas A) a few days following the SN
proper can account for several of the puzzling kinematic and nucleosynthetic
features observed. The observed decoupling between Fe and 44Ti and the lack of
Fe emission within 44Ti regions is expected in the QN model owing to the
spallation of the inner SN ejecta by the relativistic QN neutrons. Our model
predicts the 44Ti to be more prominent to the NW of the central compact object
(CCO) than in the SE and little of it along the NE-SW jets, in agreement with
NuStar observations. Other intriguing features of Cas A such as the lack of a
pulsar wind nebula (PWN) and the reported a few percent drop of the CCO
temperature over a period of 10 years are also addressed. | astro-ph_HE |
A Study of the Long-term Spectral Variations of 3C 66A Observed with the
Fermi and Kanata Telescopes: 3C 66A is an intermediate-frequency-peaked BL Lac object detected by the
Large Area Telescope onboard the Fermi Gamma-ray Space Telescope.
We present a study of the long-term variations of this blazar seen over 2
years at GeV energies with Fermi and in the optical (flux and polarization) and
near infrared with the Kanata telescope.
In 2008, the first year of the study, we find a correlation between the
gamma-ray flux and the measurements taken with the Kanata telescope.
This is in contrast to the later measurements performed during 2009--2010
which show only a weak correlation along with a gradual increase of the optical
flux. We calculate an external seed photon energy density assuming that the
gamma-ray emission is due to external Compton scattering. The energy density of
the external photons is found to be higher by a factor of two in 2008 compared
to 2009--2010. We conclude that the different behaviors observed between the
first year and the later years might be explained by postulating two different
emission components. | astro-ph_HE |
Searching for Dark Matter Annihilation in M87: Clusters of galaxies, such as the Virgo cluster, host enormous quantities of
dark matter, making them prime targets for efforts in indirect dark matter
detection via potential radiative signatures from annihilation of dark matter
particles and subsequent radiative losses of annihilation products. However, a
careful study of ubiquitous astrophysical backgrounds is mandatory to single
out potential evidence for dark matter annihilation. Here, we construct a
multiwavelength spectral energy distribution for the central radio galaxy in
the Virgo cluster, M87, using a state-of-the-art numerical Synchrotron Self
Compton approach. Fitting recent Chandra, Fermi-LAT and Cherenkov observations,
we probe different dark matter annihilation scenarios including a full
treatment of the inverse Compton losses from electrons and positrons produced
in the annihilation. It is shown that such a template can substantially improve
upon existing dark matter detection limits. | astro-ph_HE |
Calibrating X-ray binary luminosity functions via optical reconnaissance
II. The high-mass XLF and globular cluster population of X-ray binaries in
the low star-forming spiral M81: We characterize the optical counterparts to the compact X-ray source
population within the nearby spiral galaxy M81 using multi-band Hubble Space
Telescope (HST) imaging data. By comparing the optical luminosities and colors
measured for candidate donor stars and host clusters to stellar and cluster
evolutionary models, respectively, we estimate the likely masses and upper age
limits of the field and cluster X-ray binaries. We identify 15 low-mass X-ray
binaries (i.e. donor star mass $\leq$ 3 solar masses) within ancient globular
clusters, as well as 42 candidate high-mass X-ray binaries (i.e. donor star
mass $\geq$ 8 solar masses). To estimate the likelihood of misclassifications,
we inject 4,000 artificial sources into the HST mosaic image and conclude that
our classifications of globular clusters and high-mass X-ray binaries are
reliable at the >90% level. We find that globular clusters that host X-ray
binaries are on average more massive and more compact than globular clusters
that do not. However, there is no apparent correlation between the X-ray
brightness of the clusters and their masses or densities, nor are X-ray binary
hosts more X-ray luminous than the general field population of low-mass X-ray
binaries. This work represents one of the first in-depth analyses of the
population of X-ray binaries within globular clusters in a spiral galaxy. | astro-ph_HE |
Cosmic searches for Lorentz invariance violation: Cosmic messengers (gamma rays, cosmic rays, neutrinos and gravitational
waves) provide a powerful complementary way to search for Lorentz invariance
violating effects to laboratory-based experiments. The long baselines and high
energies involved make Cherenkov telescopes, air-shower arrays, neutrino
telescopes and gravitational wave detectors unique tools to probe the expected
tiny effects that the breaking of Lorentz invariance would cause in the
propagation of these messengers, in comparison with the standard scenario. In
this chapter we explain the expected effects that the mentioned detectors can
measure and summarize current results of searches for Lorentz violation. | astro-ph_HE |
A variable ULX and possible IMBH candidate in M51a: ULX-7, in the northern spiral arm of M51, demonstrates unusual behaviour for
an ultraluminous X-ray source, with a hard X-ray spectrum but very high
short-term variability. This suggests that it is not in a typical ultraluminous
state. We analyse the source using archival data from XMM-Newton, Chandra and
NuSTAR, and by examining optical and radio data from HST and VLA. Our X-ray
spectral analysis shows that the source has a hard power-law spectral shape
with a photon index Gamma~1.5, which persists despite the source's X-ray
luminosity varying by over an order of magnitude. The power spectrum of the
source features a break at 6.5^{+0.5}_{-1.1}x10^-3 Hz, from a low-frequency
spectral index of alpha_1=-0.1^{+0.5}_{-0.2} to a high-frequency spectral index
of alpha_2=0.65^{+0.05}_{-0.14}, making it analogous to the low-frequency break
found in the power spectra of low/hard state black holes (BHs). We can take a
lower frequency limit for a corresponding high-frequency break to calculate a
BH mass upper limit of 1.6x10^3 solar masses. Using the X-ray/radio fundamental
plane we calculate another upper limit to the BH mass of 3.5x10^4 solar masses
for a BH in the low/hard state. The hard spectrum, high rms variability and
mass limits are consistent with ULX-7 being an intermediate-mass BH; however we
cannot exclude other interpretations of this source's interesting behaviour,
most notably a neutron star with an extreme accretion rate. | astro-ph_HE |
The Mergers in Abell 2256: Displaced Gas and its Connection to the
Radio-emitting Plasma: We present the results of deep Chandra and XMM-Newton X-ray imaging and
spatially-resolved spectroscopy of Abell 2256, a nearby (z=0.058) galaxy
cluster experiencing multiple mergers and displaying a rich radio morphology
dominated by a large relic. The X-ray data reveals three subclusters: (i) the
`main cluster'; (ii) the remnant of an older merger in the east of the cluster
with a ~ 600 kpc long tail; (iii) a bright, bullet-like, low-entropy infalling
system, with a large line-of-sight velocity component. The low-entropy system
displays a 250 kpc long cold front with a break and an intriguing surface
brightness decrement. Interestingly, the infalling gas is not co-spatial with
bright galaxies and the radio loud brightest cluster galaxy of the infalling
group appears dissociated from the low entropy plasma by 50 kpc in projection,
to the south of the eastern edge of the cold front. Assuming that the dark
matter follows the galaxy distribution, we predict that it is also
significantly offset from the low-entropy gas. Part of the low frequency radio
emission near the cold front might be revived by magnetic field amplification
due to differential gas motions. Using analytical models and numerical
simulations, we investigate the possibility that the supersonic infall of the
subcluster generates a large scale shock along our line-of-sight, which can be
detected in the X-ray temperature map but is not associated with any clear
features in the surface brightness distribution. | astro-ph_HE |
The polarization of the boundary layer around weakly magnetized neutron
stars in X-ray binaries: X-ray binaries hosting a compact object have been among the main targets of
the Imaging X-ray Polarimetry Explorer (IXPE) since its launch, due to their
high brightness in the 2-8 keV energy band. The spectropolarimetric analysis
performed so far has proved to be of great importance in providing constraints
on the accretion geometry of these systems. However, the data statistics is not
enough to unambiguously disentangle the contribution of the single components
to the net observed polarimetric signal. In this work, we aim to present a
model for computing the polarization degree and polarization angle of the
boundary layer around weakly magnetized neutron stars in low-mass X-ray
binaries in the soft state. The main motivation is to provide strong
theoretical support to data interpretation of observations performed by IXPE or
future satellites for X-ray polarimetry. The results were obtained by modeling
the boundary layer as an equatorial belt around the compact object and locally
approximating it as a plane-parallel scattering atmosphere, for which the
associated radiative transfer equation for polarized radiation in the Thomson
limit was solved. The polarimetric quantities were then transformed from the
comoving frame to the observer frame using the numerical methods formerly
developed for X-ray pulsars. For typical values of the optical depth and
electron temperature of the boundary layer of these systems in a soft state,
the polarization degree was less then 0.5\%, while the polarization angle was
rotated by $\protect \la 5^{\circ}$ with respect to the neutron star spin axis
due to special and general relativistic effects for fast rotation, the amount
progressively decreasing for lower spin frequencies. The derived quantities can
be used to remove degeneracy when multicomponent spectropolarimetry is
performed. | astro-ph_HE |
Status And Highlights Of VERITAS: VERITAS (Very Energetic Radiation Imaging Telescope Array System) is an array
of atmospheric Cherenkov telescopes sensitive to very high energy (VHE)
gamma-rays above 100 GeV. Located at the Fred Lawrence Whipple Observatory in
southern Arizona, USA, the VERITAS array of four 12m-diameter telescopes began
full operation in September 2007. Two major upgrades, the relocation of
telescope 1 in Summer 2009 and the upgrade of the level-2 trigger in Fall 2011,
made VERITAS the most sensitive VHE instrument in the northern hemisphere. The
VERITAS Collaboration consists of scientists from institutions in the USA,
Canada, Germany and Ireland. VERITAS is performing observations that cover a
broad range of science topics, including the study of galactic and
extragalactic astrophysical sources of VHE radiation and the study of particle
astrophysics, such as the indirect search for dark matter in astrophysical
environments. The VERITAS observational campaigns resulted in the detection of
40 VHE sources, including the discovery of 20 new VHE gamma-ray emitting
sources. Here we summarize the current status of the observatory, describe the
recent scientific highlights and outline plans for the future. | astro-ph_HE |
Importance of resolving the spectral support of beam-plasma
instabilities in simulations: Many astrophysical plasmas are prone to beam-plasma instabilities. For
relativistic and dilute beams, the {\it spectral} support of the beam-plasma
instabilities is narrow, i.e., the linearly unstable modes that grow with rates
comparable to the maximum growth rate occupy a narrow range of wave numbers.
This places stringent requirements on the box-sizes when simulating the
evolution of the instabilities. We identify the implied lower limits on the box
size imposed by the longitudinal beam plasma instability, i.e., typically the
most stringent condition required to correctly capture the linear evolution of
the instabilities in multidimensional simulations. We find that sizes many
orders of magnitude larger than the resonant wavelength are typically required.
Using one-dimensional particle-in-cell simulations, we show that the failure to
sufficiently resolve the spectral support of the longitudinal instability
yields slower growth and lower levels of saturation, potentially leading to
erroneous physical conclusion. | astro-ph_HE |
Search for Decaying Dark Matter in the Virgo Cluster of Galaxies with
HAWC: The decay or annihilation of dark matter particles may produce a steady flux
of very-high-energy gamma rays detectable above the diffuse background. Nearby
clusters of galaxies provide excellent targets to search for the signatures of
particle dark matter interactions. In particular, the Virgo cluster spans
several degrees across the sky and can be efficiently probed with a wide
field-of-view instrument. The High Altitude Water Cherenkov (HAWC) observatory,
due to its wide field of view and sensitivity to gamma rays at an energy scale
of 300 GeV--100 TeV is well-suited for this search. Using 2141 days of data, we
search for gamma-ray emission from the Virgo cluster, assuming well-motivated
dark matter sub-structure models. Our results provide some of the strongest
constraints on the decay lifetime of dark matter for masses above 10 TeV. | astro-ph_HE |
Hints of the jet composition in gamma-ray bursts from dissipative
photosphere models: We present a model for gamma-ray bursts where a dissipative photosphere
provides the usual spectral peak around MeV energies accompanied by a
subdominant thermal component. We treat the initial acceleration of the jet in
a general way, allowing for magnetic field- and baryon dominated outflows. In
this model, the GeV emission associated with GRBs observed by Fermi LAT, arises
as the interaction of photospheric radiation and the shocked electrons at the
deceleration radius. Through recently discovered correlations between the
thermal and non-thermal peaks within individual bursts, we are able to infer
whether the jet was Poynting flux or baryon dominated. | astro-ph_HE |
Indirect dark matter search in the Galactic Centre with IceCube: It is assumed that dark matter can annihilate or decay into Standard Model
particles which then can produce a neutrino flux detectable at IceCube. Such a
signal can be emitted from the Galactic Center thanks to the high density of
dark matter abundance being gravitationally captured. This analysis aims at
searching for neutrino signals from dark matter annihilation and decay in the
Galactic Center using $\sim$9 years of IceCube-DeepCore data with an optimized
selection for low energy. In this contribution, we present the sensitivities on
the thermally averaged dark matter self-annihilation cross-section for dark
matter masses ranging from 5 GeV up to 8 TeV. | astro-ph_HE |
The interstellar environment in the outer Galaxy as seen in gamma rays
by Fermi: Gamma-ray emission produced by interactions between cosmic rays (CRs) and
interstellar gas traces the product of their densities throughout the Milky
Way. The outer Galaxy is a privileged target of investigation to separate
interstellar structures seen along the line of sight. Recent observations by
the Fermi Large Area Telescope (LAT) shed light on open questions of the EGRET
era about the distribution of CR densities and the census of the interstellar
medium. The gradient of gamma-ray emissivities measured in the outer Galaxy is
significantly flatter than predictions from widely used CR propagation models
given the rapid decline of putative CR sources beyond the solar circle. Large
propagation volumes, with halo heights up to 20 kpc, or a flat CR source
distribution are required to match the data. Other viable possibilities include
non-uniform CR diffusion properties or more gas than accounted for by the
radio/mm-wave data. Gamma-ray data constrain the evolution of the
Xco=N(H2)/W(CO) ratio within a few kpc from the Sun. There is a significant
increase by a factor 2 from nearby clouds in the Gould Belt to the local spur.
No further significant variations are measured from the local spur to the
Perseus spiral arm. At the level of statistical accuracy provided by the LAT
data, the most important source of uncertainty, often overlooked so far, is due
to the optical depth correction applied to derive the column densities of H I.
Reliable determinations of the amount of atomic gas in the plane are key to
better probe the properties of CRs in the Galaxy. | astro-ph_HE |
Blueshifted absorption lines from X-ray reflection in IRAS 13224-3809: We explore a disc origin for the highly-blueshifted, variable absorption
lines seen in the X-ray spectrum of the Narrow Line Seyfert 1 galaxy
IRAS13224-3809. The blueshift corresponds to a velocity of about 0.25c. Such
features in other Active Galactic Nuclei are often interpreted as UltraFast
Outflows (UFOs). The velocity is of course present in the orbital motions of
the inner disk. The absorption lines in IRAS13224-3809 are best seen when the
flux is low and the reflection component of the disk is strong relative to the
power-law continuum. The spectra are consistent with a model in which the
reflection component passes through a thin, highly-ionized absorbing layer at
the surface of the inner disc, the blue-shifted side of which dominates the
flux due to relativistic aberration (the disc inclination is about 70 deg). No
fast outflow need occur beyond the disc. | astro-ph_HE |
Lorentz Factor-Isotropic Luminosity/Energy Correlations of GRBS and
Their Interpretation: The bulk Lorentz factor of the gamma-ray burst (GRB) ejecta (Gamma_0) is a
key parameter to understand the GRB physics. Liang et al. have discovered a
correlation between Gamma_0 and isotropic gamma-ray energy:
Gamma_0\proptoE_{gamma,iso,52}^{0.25}. By including more GRBs with updated data
and more methods to derive Gamma_0, we confirm this correlation and obtain
Gamma_0~91E_{gamma,iso,52}^{0.29}. Evaluating the mean isotropic gamma-ray
luminosities L_{gamma,iso} of the GRBs in the same sample, we discover an even
tighter correlation Gamma_0~249 L_{gamma,iso,52}^{0.30}. We propose an
interpretation to this later correlation. Invoking a neutrino-cooled
hyperaccretion disk around a stellar mass black hole as the central engine of
GRBs, we derive jet luminosity powered by neutrino annihilation and baryon
loading from a neutrino-driven wind. Applying beaming correction, we finally
derive Gamma_0\proptoL_{gamma,iso}^{0.22}, which is well consistent with the
data. This suggests that the central engine of long GRBs is likely a stellar
mass black hole surrounded by a hyper-accreting disk. | astro-ph_HE |
Waves in pulsar winds: The radio, optical, X-ray and gamma-ray nebulae that surround many pulsars
are thought to arise from synchrotron and inverse Compton emission. The energy
powering this emission, as well as the magnetic fields and relativistic
particles, are supplied by a "wind" driven by the central object. The inner
parts of the wind can be described using the equations of MHD, but these break
down in the outer parts, when the density of charge carriers drops below a
critical value. This paper reviews the wave properties of the inner part
(striped wind), and uses a relativistic two-fluid model (cold electrons and
positrons) to re-examine the nonlinear electromagnetic modes that propagate in
the outer parts. It is shown that in a radial wind, two solutions exist for
circularly polarised electromagnetic modes. At large distances one of them
turns into a freely expanding flow containing a vacuum wave, whereas the other
decelerates, corresponding to a confined flow. | astro-ph_HE |
Ultra High Energy Neutrinos: Absorption, Thermal Effects and Signatures: We study absorption of ultra high energy neutrinos by the cosmic neutrino
background, with full inclusion of the effect of the thermal distribution of
the background on the resonant annihilation channel. For a hierarchical
neutrino mass spectrum (with at least one neutrino with mass below $\sim
10^{-2}$ eV), thermal effects are important for ultra high energy neutrino
sources at $z \gtrsim 16$. The neutrino transmission probability shows no more
than two separate suppression dips since the two lightest mass eigenstates
contribute as a single species when thermal effects are included. Results are
applied to a number of models of ultra high energy neutrino emission.
Suppression effects are strong for sources that extend beyond $z \sim 10$,
which can be realized for certain top down scenarios, such as superheavy dark
matter decays, cosmic strings and cosmic necklaces. For these, a broad
suppression valley should affect the neutrino spectrum at least in the energy
interval $10^{12} - 10^{13}$ GeV -- which therefore is disfavored for ultra
high energy neutrino searches -- with only a mild dependence on the neutrino
mass spectrum and hierarchy. The observation of absorption effects would
indicate a population of sources beyond $z \sim 10$, and favor top-down
mechanisms; it would also be an interesting probe of the physics of the relic
neutrino background in the unexplored redshift interval $z \sim 10 -100$. | astro-ph_HE |
A Photosphere-Internal Shock Model of Gamma-Ray Bursts: Case Studies of
Fermi/LAT Bursts: Radially inhomogeneous gamma-ray burst (GRB) jets release variable
photospheric emission and can have internal shocks occurring above the
photosphere. We generically formulate a photospheric emission model of GRBs
including Compton up-scattered photospheric (UP) emission off the electrons
(and positrons) in the internal shocks, and find that the photospheric emission
may correspond to the traditional (Band) component at <~1 MeV and the UP
emission to the high-energy emission observed by Fermi/LAT for some GRBs at >~
10 MeV. The two components can be separate in the spectrum in some cases or can
mimic a smooth broad Band spectrum in other cases. We apply our formulation to
the well-studied long and short LAT GRBs, GRB 080916C, GRB 090902B, and GRB
090510, and typically find reasonable parameters for fitting the time-binned
spectra, although fine tuning of several parameters is required. The observed
delays of the high-energy emission with respect to the MeV emission which are
large compared to the variability times are unlikely to be due to simple
kinematic effects of a non-evolving jet. These delays may be attributed to the
temporal evolution of the physical parameters of the jet, and thus the delay
timescales could provide a potential tool for investigating the structures of
GRB jets themselves and their progenitors. The difference of the delay
timescales of long and short GRBs inferred from the Fermi data might be due to
the differences in the progenitors of long and short GRBs. Some other
properties and consequences of this model are discussed, including temporal
correlations among the prompt optical, the soft X-ray, and the distinct
high-energy component as well as the Band component. | astro-ph_HE |
Equations of state in the Hartle-Thorne model of neutron stars selecting
acceptable variants of the resonant switch model of twin HF QPOs in the atoll
source 4U 1636-53: The Resonant Switch (RS) model of twin high-frequency quasi-periodic
oscillations (HF QPOs) observed in neutron star binary systems, based on switch
of the twin oscillations at a resonant point, has been applied to the atoll
source 4U 1636-53 under assumption that the neutron star exterior can be
approximated by the Kerr geometry. Strong restrictions of the neutron star
parameters M (mass) and a (spin) arise due to fitting the frequency pairs
admitted by the RS model to the observed data in the regions related to the
resonant points. The most precise variants of the RS model are those combining
the relativistic precession frequency relations with their modifications. Here,
the neutron star mass and spin estimates given by the RS model are confronted
with a variety of equations of state (EoS) governing structure of neutron stars
in the framework of the Hartle-Thorne theory of rotating neutron stars applied
for the observationally given rotation frequency f_rot~580 Hz (or alternatively
f_rot~290 Hz) of the neutron star at 4U 1636-53. It is shown that only two
variants of the RS model based on the Kerr approximation are compatible with
two EoS applied in the Hartle-Thorne theory for f_rot~580 Hz, while no variant
of the RS model is compatible for f_rot~290 Hz. The two compatible variants of
the RS model are those giving the best fits of the observational data. However,
a self-consistency test by fitting the observational data to the RS model with
oscillation frequencies governed by the Hartle-Thorne geometry described by
three spacetime parameters M, a and (quadrupole moment) q related by the two
available EoS puts strong restrictions. The test admits only one variant of the
RS model of twin HF QPOs for the Hartle-Thorne theory with the Gandolfi et al.
(2010) EoS predicting the parameters of the neutron star $M \sim 2.10
\mathrm{M}_{\odot}$, $a \sim 0.208$, and $q/a^2 \sim 1.77$. | astro-ph_HE |
The XMM-Newton survey of the Small Magellanic Cloud: Although numerous archival XMM-Newton observations existed towards the Small
Magellanic Cloud (SMC) before 2009, only a fraction of the whole galaxy was
covered. Between May 2009 and March 2010 we carried out an XMM-Newton survey of
the SMC, in order to obtain a complete overage of both its bar and wing.
Thirty-three observations of 30 different fields with a total exposure of about
ne Ms filled the missing parts. We systematically processed all available SMC
data from the European Photon Imaging Camera. After rejecting observations with
very high background we included 53 archival and the 33 survey observations. We
produced images in five different energy bands. We applied astrometric
boresight corrections using secure identifications of X-ray sources and combine
all the images to produce a mosaic, which covers the main body of the SMC. We
present an overview of the XMM-Newton observations, describe their analysis and
summarise first results which will be presented in follow-up papers in detail.
Here, we mainly focus on extended X-ray sources like supernova remnants (SNRs)
and clusters of galaxies which are seen in our X-ray images. The XMM-Newton
survey represents the deepest complete survey of the SMC in the 0.15-12.0 keV
X-ray band. We propose three new SNRs with low surface brightness of a few
10^-14 erg s^-1 cm^-2 arcmin^-2 and large extent. Also several known remnants
appear larger than previously measured from X-rays or other wavelengths
extending the size distribution of SMC SNRs to larger values. | astro-ph_HE |
The destruction and recreation of the X-ray corona in a changing-look
Active Galactic Nucleus: We present the drastic transformation of the X-ray properties of the active
galactic nucleus 1ES 1927+654, following a changing-look event. After the
optical/UV outburst the power-law component, produced in the X-ray corona,
disappeared, and the spectrum of 1ES 1927+65 instead became dominated by a
blackbody component ($kT\sim 80-120$ eV). This implies that the X-ray corona,
ubiquitously found in AGN, was destroyed in the event. Our dense $\sim 450$ day
long X-ray monitoring shows that the source is extremely variable in the X-ray
band. On long time scales the source varies up to $\sim 4$ dex in $\sim 100$
days, while on short timescales up to $\sim2$ dex in $\sim 8$ hours. The
luminosity of the source is found to first show a strong dip down to $\sim
10^{40}\rm\,erg\,s^{-1}$, and then a constant increase in luminosity to levels
exceeding the pre-outburst level $\gtrsim $300 days after the optical event
detection, rising up asymptotically to $\sim 2\times10^{44}\rm\,erg\,s^{-1}$.
As the X-ray luminosity of the source increases, the X-ray corona is recreated,
and a very steep power-law component ($\Gamma\simeq 3$) reappears, and
dominates the emission for 0.3-2 keV luminosities $\gtrsim
10^{43.7}\rm\,erg\,s^{-1}$, $\sim 300$ days after the beginning of the event.
We discuss possible origins of this event, and speculate that our observations
could be explained by the interaction between the accretion flow and debris
from a tidally disrupted star. Our results show that changing-look events can
be associated with dramatic and rapid transformations of the innermost regions
of accreting SMBHs. | astro-ph_HE |
Lorentz invariance violation from GRB221009A: The Large High Altitude Air Shower Observatory~(LHAASO) reported observation
of photons with energies above 10~TeV from gamma ray burst GRB221009A. A
suggestion was proposed that this result may contradict with our knowledge of
special relativity~(SR) and the standard model~(SM), according to which photons
of about 10~TeV from such a distant object should be severely suppressed
because of the absorption by extragalactic background light. As a result, a
number of mechanisms have been proposed to solve this potential puzzle,
including Lorentz invariance violation~(LIV). In this work, we perform a
detailed numerical calculation and show the feasibility to constrain LIV of
photons from the LHAASO observation of GRB221009A quantitatively. | astro-ph_HE |
In-medium enhancement of the modified Urca neutrino reaction rates: We calculate modified Urca neutrino emission rates in the dense nuclear
matter in neutron star cores. We find that these rates are strongly enhanced in
the beta-stable matter in regions of the core close to the direct Urca process
threshold. This enhancement can be tracked to the use of the in-medium nucleon
spectrum in the virtual nucleon propagator. We describe the in-medium nucleon
scattering in the non-relativistic Bruckner-Hartree-Fock framework taking into
account two-body as well as the effective three-body forces, although the
proposed enhancement does not rely on a particular way of the nucleon
interaction treatment. Finally we suggest a simple approximate expression for
the emissivity of the n-branch of the modified Urca process that can be used in
the neutron stars cooling simulations with any nucleon equation of state of
dense matter. | astro-ph_HE |
Revealing the variation mechanism of ON 231 via the two-components
shock-in-jet model: The variation mechanism of blazars is a long-standing unresolved problem. In
this work, we present a scenario to explain diverse variation phenomena for ON
231, where the jet emissions are composed of the flaring and the less variable
components (most probably from the post-flaring blobs), and the variation is
dominated by shock-in-jet instead of the Doppler effect. We perform correlation
analysis for the multiwavelength light curves and find no significant
correlations. For optical band, ON 231 exhibits a harder when brighter (HWB)
trend, and the trend seems to shift at different periods. Correspondingly, the
correlation between polarization degree and flux exhibits a V-shaped behavior,
and a similar translation relation during different periods is also found.
These phenomena could be understood via the superposition of the flaring
component and slowly varying background component. We also find that the slopes
of HWB trend become smaller at higher flux levels, which indicates the
energy-dependent acceleration processes of the radiative particles. For X-ray,
we discover a trend transition from HWB to softer when brighter (SWB) to HWB.
We consider that the X-ray emission is composed of both the synchrotron tail
and the Synchrotron Self-Compton components, which could be described by two
log-parabolic functions. By varying the peak frequency, we reproduce the
observed trend transition in a quantitative manner. For $\gamma$-ray, we find
the SWB trend, which could be explained naturally if a very-high-energy
$\gamma$-ray background component exists. Our study elucidates the variation
mechanism of intermediate synchrotron-peaked BL Lac objects. | astro-ph_HE |
Exact analytical expression for the synchrotron radiation spectrum in
the Gaussian turbulent magnetic field: We demonstrate that the exact solution for the spectrum of synchrotron
radiation from an isotropic population of mono-energetic electrons in turbulent
magnetic field with Gaussian distribution of local field strengths can be
expressed in the simple analytic form: $\left( \frac{{\rm d} \dot{N}}{{\rm d}
\omega} \right)_t
= \frac{\alpha}{3} \frac{1}{\gamma^2} \left( 1 + \frac{1}{x^{2/3}} \right)
\exp \left( - 2 x^{2/3} \right)$, where $x = \frac{\omega}{\omega_0}\, ;
\omega_0 = \frac{4}{3} \gamma^2 \frac{eB_0}{m_e c}\, .$ We use this expression
to find approximate synchrotron spectra for power-law electron distributions
with $\propto \exp\left( -\left[ \gamma/\gamma_0 \right]^\beta\right)$ type
high-energy cut-off; the resulting synchrotron spectrum has the exponential
cut-off factor with frequency raised to $2\beta/(3\beta+4)$ power in the
exponent. For the power-law electron distribution without high-energy cut-off,
we find the coefficient $a_m$ as a function of the power-law index, which
results in exact expression for the synchrotron spectrum when using
monochromatic (i.e., each electron radiates at frequency $\omega_m = a_m
\gamma^2 \, \frac{e B_0}{m_e c}$) approximation. | astro-ph_HE |
Magnetically Confined Mountains on Accreting Neutron Stars in General
Relativity: The general relativistic formulation of the problem of magnetically confined
mountains on neutron stars is presented, and the resulting equations are solved
numerically, generalising previous Newtonian calculations. The hydromagnetic
structure of the accreted matter and the subsequent magnetic burial of the
star's magnetic dipole moment are computed. Overall, it is observed that
relativistic corrections reduce the hydromagnetic deformation associated with
the mountain. The magnetic field lines are curved more gently than in previous
calculations, and the screening of the dipole moment is reduced.
Quantitatively, it is found that the dimensionless dipole moment ($m_{\rm d}$)
depends on the accreted mass ($M_{\rm a}$) as $m_{\rm d} =
-3.2\times10^{3}M_{\rm a}/M_\odot + 1.0$, implying approximately three times
less screening compared to the Newtonian theory. Additionally, the
characteristic scale height of the mountain, governing the gradients of
quantities like pressure, density, and magnetic field strength, reduces by
approximately $40\%$ for an isothermal equation of state. | astro-ph_HE |
Very high energy gamma-ray follow-up observations of novae and dwarf
novae with the MAGIC telescopes: In the last few years the Fermi-LAT instrument has detected GeV gamma-ray
emission from several novae. Such GeV emission can be interpreted in terms of
inverse Compton emission from electrons accelerated in the shock or in terms of
emission from hadrons accelerated in the same conditions. The latter might
reach much higher energies and could produce a second component in the
gamma-ray spectrum at TeV energies. We perform follow-up observations of
selected novae and dwarf novae in search of the second component in TeV energy
gamma rays. This can shed light on the acceleration process of leptons and
hadrons in nova explosions. We have performed observations with the MAGIC
telescopes of 3 sources, a symbiotic nova YY Her, a dwarf nova ASASSN-13ax and
a classical nova V339 Del, shortly after their outbursts. We did not detect TeV
gamma-ray emission from any of the objects observed. The TeV upper limits from
MAGIC observations and the GeV detection by Fermi constrain the acceleration
parameters for electrons and hadrons. | astro-ph_HE |
Test of the string loop oscillation model using kHz quasiperiodic
oscillations in a neutron star binary: The model of current-carrying string loop oscillations is tested to explain
the special set of frequencies related to the high-frequency quasiperiodic
oscillations (HF QPOs) observed recently in the low-mass X-ray binary XTE
J1701-407 containing a neutron star. The external geometry of the neutron star
is approximated by the Kerr geometry, introducing errors not exceeding $10~\%$
for slowly rotating massive neutron stars. The frequencies of the radial and
vertical string loop oscillations are then governed by the mass $M$ and
dimensionless spin $a$ of the neutron star, and by the dimensionless parameter
$\omega$ describing combined effects of the string loop tension and its angular
momentum. It is explicitly demonstrated that the string-loop oscillation model
can explain the observed kHz frequencies for the neutron star parameters
restricted to the intervals ${0.2<a<0.4}$ and ${2.1<M/{\rm M}_{\odot}<2.5}$.
However, the stringy parameter $\omega$ cannot be the same for all the three HF
QPO observations in the XTE J1701-407 source; the limits on the acceptable
values of $\omega$ are given in dependence on the spacetime parameters $M$ and
$a$. | astro-ph_HE |
SN 2019zrk, a bright SN 2009ip analog with a precursor: We present photometric and spectroscopic observations of the Type IIn
supernova SN 2019zrk (also known as ZTF20aacbyec). The SN shows a $\gtrsim$ 100
day precursor, with a slow rise, followed by a rapid rise to M $\sim -19.2$ in
the $r$ and $g$ bands. The post-peak light-curve decline is well fit with an
exponential decay with a timescale of $\sim 39$ days, but it shows prominent
undulations, with an amplitude of $\sim 1$ mag. Both the light curve and
spectra are dominated by an interaction with a dense circumstellar medium
(CSM), probably from previous mass ejections. The spectra evolve from a
scattering-dominated Type IIn spectrum to a spectrum with strong P-Cygni
absorptions. The expansion velocity is high, $\sim 16,000$ km s$^{-1}$, even in
the last spectra. The last spectrum $\sim 110$ days after the main eruption
reveals no evidence for advanced nucleosynthesis. From analysis of the spectra
and light curves, we estimate the mass-loss rate to be $\sim 4 \times 10^{-2}$
M$_\odot$ yr$^{-1}$ for a CSM velocity of 100 km s$^{-1}$, and a CSM mass of
$\gtrsim 1$ M$_\odot$. We find strong similarities for both the precursor,
general light curve, and spectral evolution with SN 2009ip and similar SNe,
although SN 2019zrk displays a brighter peak magnitude. Different scenarios for
the nature of the 09ip-class of SNe, based on pulsational pair instability
eruptions, wave heating, and mergers, are discussed. } | astro-ph_HE |
NuSTAR Hard X-ray Survey of the Galactic Center Region I: Hard X-ray
Morphology and Spectroscopy of the Diffuse Emission: We present the first sub-arcminute images of the Galactic Center above 10
keV, obtained with NuSTAR. NuSTAR resolves the hard X-ray source IGR
J17456-2901 into non-thermal X-ray filaments, molecular clouds, point sources
and a previously unknown central component of hard X-ray emission (CHXE).
NuSTAR detects four non-thermal X-ray filaments, extending the detection of
their power-law spectra with $\Gamma\sim1.3$-$2.3$ up to ~50 keV. A
morphological and spectral study of the filaments suggests that their origin
may be heterogeneous, where previous studies suggested a common origin in young
pulsar wind nebulae (PWNe). NuSTAR detects non-thermal X-ray continuum emission
spatially correlated with the 6.4 keV Fe K$\alpha$ fluorescence line emission
associated with two Sgr A molecular clouds: MC1 and the Bridge. Broad-band
X-ray spectral analysis with a Monte-Carlo based X-ray reflection model
self-consistently determined their intrinsic column density ($\sim10^{23}$
cm$^{-2}$), primary X-ray spectra (power-laws with $\Gamma\sim2$) and set a
lower limit of the X-ray luminosity of Sgr A* flare illuminating the Sgr A
clouds to $L_X \stackrel{>}{\sim} 10^{38}$ erg s$^{-1}$. Above ~20 keV, hard
X-ray emission in the central 10 pc region around Sgr A* consists of the
candidate PWN G359.95-0.04 and the CHXE, possibly resulting from an unresolved
population of massive CVs with white dwarf masses $M_{\rm WD} \sim 0.9
M_{\odot}$. Spectral energy distribution analysis suggests that G359.95-0.04 is
likely the hard X-ray counterpart of the ultra-high gamma-ray source HESS
J1745-290, strongly favoring a leptonic origin of the GC TeV emission. | astro-ph_HE |
No evidence for a low-mass black hole in Swift J1753.5-0127: We present high-resolution, time-resolved optical spectroscopy of the black
hole X-ray transient Swift J1753.5-0127. Our optical spectra do not show
features that we can associate with the companion star. However we do observe
broad, double-peaked emission lines, typical of an accretion disc. We show that
the mass of the compact object is likely $>7.4\pm1.2M_{\odot}$, much higher
than previous suggestions of a low-mass ($<5M_{\odot}$) black hole. | astro-ph_HE |
Variable mass accretion and failed wind explain changing look phenomena
in NGC 1365: Changing look active galactic nuclei (CLAGNs) show complex nature in their
X-ray spectral shape and line of sight column density variation. The physical
mechanisms responsible for these variations are unclear. Here, we study the
spectral properties of a CLAGN, NGC\,1365 using combined {\it XMM-Newton} and
{\it NuSTAR} observations to understand the CL behavior. The model fitted mass
accretion rate varied between $0.003\pm 0.001$ and $0.009\pm0.002$ $\dot M_{\rm
Edd}$ and the dynamic corona changed from $28\pm 3$ to $10\pm1$ $r_g$. We found
that the variable absorption column density correlates with the mass accretion
rate and the geometry of the corona. The derived wind velocity was sufficiently
low compared to the escape velocity to drive the wind away from the disc for
the epochs when column densities were high. This suggests that the high and
variable absorption can be due to failed winds from the disc. Our estimated
ratio of mass outflow to inflow rate from the inner region of the disc lies
between $0.019\pm0.006$ and $0.12\pm0.04$. From spectral fitting of the
combined data, we found the mass of the central black hole to be constant
$4.38\pm0.34 - 4.51\pm0.29 \times10^{6} M_\odot$, consistent with earlier
findings. The confidence contours of $N_H$ with other model parameters show
that the model fitted parameters are robust and non-degenerate. Our study
construed that the changing accretion rate, which is a fundamental physical
quantity and the geometry of the corona driving the CL phenomena in NGC\,1365.
The physical picture considered in this work connects both variable continuum
and variable absorbing medium scenarios. | astro-ph_HE |
Probing dark matter annihilation in the Galaxy with antiprotons and
gamma rays: A possible hint of dark matter annihilation has been found in Cuoco,
Korsmeier and Kr\"amer (2017) from an analysis of recent cosmic-ray antiproton
data from AMS-02 and taking into account cosmic-ray propagation uncertainties
by fitting at the same time dark matter and propagation parameters. Here, we
extend this analysis to a wider class of annihilation channels. We find
consistent hints of a dark matter signal with an annihilation cross-section
close to the thermal value and with masses in range between 40 and 130 GeV
depending on the annihilation channel. Furthermore, we investigate in how far
the possible signal is compatible with the Galactic center gamma-ray excess and
recent observation of dwarf satellite galaxies by performing a joint global fit
including uncertainties in the dark matter density profile. As an example, we
interpret our results in the framework of the Higgs portal model. | astro-ph_HE |
High-energy neutrinos from Galactic sources: Even 100 years after the discovery of cosmic rays their origin remains a
mystery. In recent years, TeV gamma-ray detectors have discovered and
investigated many Galactic sources where particles are accelerated up to
energies of 100 TeV. However, it has not been possible up to now to identify
these sites unambiguously as sources of hadronic acceleration. The observation
of cosmic high-energy neutrinos from these or other sources will be a
smoking-gun evidence for the sites of the acceleration of cosmic rays. | astro-ph_HE |
Update on the GRB universal scaling
E$_{\rm{X,iso}}$-E$_{\rm{γ,iso}}$-E$_{\rm{pk}}$ with ten years of
$Swift$ data: From a comprehensive statistical analysis of $Swift$ X-ray light-curves of
gamma-ray bursts (GRBs) collected from December 2004 to the end of 2010, we
found a three-parameter correlation between the isotropic energy emitted in the
rest frame 1-10$^4$ keV energy band during the prompt emission
(E$_{\rm{\gamma,iso}}$), the rest frame peak of the prompt emission energy
spectrum (E$_{\rm{pk}}$), and the X-ray energy emitted in the rest frame 0.3-30
keV observed energy band (E$_{\rm{X,iso}}$), computed excluding the
contribution of the flares. In this paper, we update this correlation with the
data collected until June 2014, expanding the sample size with $\sim$35% more
objects, where the number of short GRBs doubled. With this larger sample we
confirm the existence of a universal correlation that connects the prompt and
afterglow properties of long and short GRBs. We show that this correlation does
not depend on the X-ray light-curve morphology and that further analysis is
necessary to firmly exclude possible biases derived by redshift measurements.
In addition we discuss about the behavior of the peculiar objects as ultra-long
GRBs and we propose the existence of an intermediate group between long and
short GRBs. Interestingly, two GRBs with uncertain classification fall into
this category. Finally, we discuss the physics underlying this correlation, in
the contest of the efficiency of conversion of the prompt $\gamma$-ray emission
energy into the kinetic energy of the afterglow, the photosferic model, and the
cannonball model. | astro-ph_HE |
Modelling of blazar SEDs with the nonlinear SSC cooling process: Observations of blazar flaring states reveal remarkably different variability
time scales. Especially rapid flares with flux doubling time scales of the
order of minutes have been puzzling for quite some time. Many modeling attempts
use the well known linear relations for the cooling and emission processes in
the jet in a steady-state scenario, albeit the obvious strongly time-dependent
nature of flares. Due to the feedback of self-produced radiation with
additional scattering by relativistic electrons, the synchrotron-self Compton
(SSC) effect is inherently time-dependent. Although this feedback is usually
implemented in numerical treatments, only recently an analytical analysis of
the effects of this nonlinear behaviour has been performed. Here, we report our
results concerning the effect of the time-dependent SSC on the spectral energy
distribution (SED) of blazars. We calculated analytically the synchrotron and
the SSC component, giving remarkably different spectral features compared to
the standard linear approach. Adding an external photon field to the original
setting, we could implement quite easily the effect of an additional external
Compton (EC) cooling, since such strong external photon fields are observed in
flat spectrum radio quasars (FSRQ), a subclass of blazars. Calculating the
resulting flux due to the EC cooling, we were able to show that the resulting
inverse Compton component strongly depends on the free parameters, and that SSC
could potentially have a strong effect in FSRQs, contrary to what is usually
assumed. | astro-ph_HE |
The Soft State of the Black Hole Transient Source MAXI J1820+070:
Emission from the Edge of the Plunge Region?: The Galactic black hole X-ray binary MAXI J1820+070 had a bright outburst in
2018 when it became the second brightest X-ray source in the Sky. It was too
bright for X-ray CCD instruments such as XMM-Newton and Chandra, but was well
observed by photon-counting instruments such as NICER and NuSTAR. We report
here on the discovery of an excess emission component during the soft state. It
is best modelled with a blackbody spectrum in addition to the regular disk
emission, modelled either as diskbb or kerrbb. Its temperature varies from
about 0.9 to 1.1 keV which is about 30 to 80 per cent higher than the inner
disc temperature of diskbb. Its flux varies between 4 and 12 percent of the
disc flux. Simulations of magnetised accretion discs have predicted the
possibility of excess emission associated with a non-zero torque at the
Innermost Stable Circular Orbit (ISCO) about the black hole, which from other
NuSTAR studies lies at about 5 gravitational radii or about 60 km (for a black
hole mass is 8 M). In this case the emitting region at the ISCO has a width
varying between 1.3 and 4.6 km and would encompass the start of the plunge
region where matter begins to fall freely into the black hole. | astro-ph_HE |
New Measurement of Muon Neutrino Disappearance from the IceCube
Experiment: The IceCube Neutrino Observatory is a Cherenkov detector located at the South
Pole. Its main component consists of an in-ice array of optical modules
instrumenting one cubic kilometer of deep Glacial ice. The DeepCore
sub-detector is a denser in-fill array with a lower energy threshold, allowing
us to study atmospheric neutrinos oscillations with energy below 100 GeV
arriving through the Earth. We present preliminary results of an atmospheric
muon neutrino disappearance analysis using data from 2012 to 2021 and employing
convolutional neural networks (CNNs) for precise and fast event
reconstructions. | astro-ph_HE |
The Origin of the Prompt Emission for Short GRB 170817A: Photosphere
Emission or Synchrotron Emission?: The first gravitational-wave event from the merger of a binary neutron star
system (GW170817) was detected recently. The associated short gamma-ray burst
(GRB 170817A) has a low isotropic luminosity ($\sim 10^{47}$ erg s$ ^{-1}$) and
a peak energy $E_{p}\sim 145$ keV during the initial main emission between -0.3
and 0.4 s. The origin of this short GRB is still under debate, but a plausible
interpretation is that it is due to the off-axis emission from a structured
jet. We consider two possibilities. First, since the best-fit spectral model
for the main pulse of GRB 170817A is a cutoff power law with a hard low-energy
photon index ($\alpha =-0.62_{-0.54}^{+0.49} $), we consider an off-axis
photosphere model. We develop a theory of photosphere emission in a structured
jet and find that such a model can reproduce a low-energy photon index that is
softer than a blackbody through enhancing high-latitude emission. The model can
naturally account for the observed spectrum. The best-fit Lorentz factor along
the line of sight is $\sim 20$, which demands that there is a significant delay
between the merger and jet launching. Alternatively, we consider that the
emission is produced via synchrotron radiation in an optically thin region in
an expanding jet with decreasing magnetic fields. This model does not require a
delay of jet launching but demands a larger bulk Lorentz factor along the line
of sight. We perform Markov Chain Monte Carlo fitting to the data within the
framework of both models and obtain good fitting results in both cases. | astro-ph_HE |
First search for GeV neutrinos from bright gamma-ray solar flares using
the IceCube Neutrino Observatory: In response to a reported increase in the total neutrino flux in the
Homestake experiment in coincidence with solar flares at the end of the
eighties, solar neutrino detectors have searched for solar flare signals. Solar
flares convert magnetic energy into thermal energy of plasma and kinetic energy
of charged particles such as protons. As a consequence of magnetic
reconnection, protons are injected downwards from the coronal acceleration
region and can interact with dense plasma in the lower solar atmosphere,
producing mesons that will subsequently decay into gamma rays and neutrinos at
O(MeV-GeV) energies. The main motivation to search for solar flare neutrinos
comes from their hadronic origin. As inherent products of high-energy proton
collisions with the chromosphere, they are a direct probe of the proton
accelerated towards the chromosphere. Using a multi-messenger approach, it is
therefore possible to constrain the proton acceleration taking place in the
solar flares, including the spectral index of the accelerated flux and its
shape. We present the results of the first search for GeV neutrinos emitted
during solar flares carried out with the IceCube Neutrino Observatory. We
present a new approach which allows us to strongly lower the energy threshold
of IceCube, originally designed to detect 10 GeV - PeV neutrinos. We compare
the results with theoretical estimates of the corresponding flux. | astro-ph_HE |
SN 2023emq: a flash-ionised Ibn supernova with possible CIII emissio: SN 2023emq is a fast-evolving transient initially classified as a rare Type
Icn supernova (SN), interacting with a H- and He-free circumstellar medium
(CSM) around maximum light. Subsequent spectroscopy revealed the unambiguous
emergence of narrow He lines, confidently placing SN 2023emq in the more common
Type Ibn class. Photometrically SN 2023emq has several uncommon properties
regardless of its class, including its extreme initial decay (faster than > 90%
of Ibn/Icn SNe) and sharp transition in the decline rate from 0.20 mag/d to
0.07 mag/d at +20 d. The bolometric light curve can be modelled as CSM
interaction with 0.32M_Sun of ejecta and 0.12M_Sun of CSM, with 0.006M_Sun of
nickel, as expected of fast interacting SNe. Furthermore, broad-band
polarimetry at +8.7 days (P = 0.55 +/- 0.30%) is consistent with spherical
symmetry. A discovery of a transitional Icn/Ibn SN would be unprecedented and
would give valuable insights into the nature of mass loss suffered by the
progenitor just before death, but we favour an interpretation that SN 2023emq
is a type Ibn SN that exhibited flash-ionised features in the earliest
spectrum, as the features are not an exact match with other SNe Icn to date.
However, the feature at 5700{\AA}, in the region of C III and N II emission, is
significantly stronger in SN 2023emq than in the few other flash-ionised Type
Ibn SNe, and if it is related to C III, it possibly implies a continuum of
properties between the two classes. | astro-ph_HE |
Testing the rotating hot spot model using X-ray burst oscillations from
4U 1636-536: Precise and accurate measurements of neutron star masses and radii would
provide valuable information about the still uncertain properties of cold
matter at supranuclear densities. One promising approach to making such
measurements involves analysis of the X-ray flux oscillations often seen during
thermonuclear (type 1) X-ray bursts. These oscillations are almost certainly
produced by emission from hotter regions on the stellar surface modulated by
the rotation of the star. One consequence of the rotation is that the
oscillation should appear earlier at higher photon energies than at lower
energies. Ford (1999) found compelling evidence for such a hard lead in the
tail oscillations of one type 1 burst from Aql X-1. We have therefore analyzed
individually the oscillations observed in the tails of the four type 1 bursts
from 4U 1636-536 that, when averaged, provided the strongest evidence for a
soft lead in the analysis by Muno et al. (2003). We have also analyzed the
oscillation observed during the superburst from this star. We find that the
data from these five bursts, treated both individually and jointly, are fully
consistent with a rotating hot spot model. Unfortunately, the uncertainties in
these data are too large to provide interesting constraints on the mass and
radius of this star. | astro-ph_HE |
Supernova remnants in the very-high-energy sky: prospects for the
Cherenkov Telescope Array: The Cherenkov Telescope Array is expected to lead to the detection of many
new supernova remnants in the TeV and multi-TeV range. In addition to the
individual study of each SNR, the study of these objects as a population can
help constraining the parameters describing the acceleration of particles and
increasing our understanding of the mechanisms involved. We present Monte Carlo
simulations of the population of Galactic SNRs emitting TeV gamma rays. We also
discuss how the simulated population can be confronted with future observations
to provide a novel test for the SNR hypothesis of cosmic ray origins. | astro-ph_HE |
Neutron stars as sources of gravitational waves: The global network of ground-based gravitational-wave detectors (the Advanced
LIGO and the Advanced Virgo) is sensitive at the frequency range corresponding
to relativistic stellar-mass compact objects. Among the promising types of
gravitational-wave sources are binary systems and rotating, deformed neutron
stars. I will describe these sources and present predictions of how their
observations will contribute to modern astrophysics in the near future. | astro-ph_HE |
Gamma-ray burst spectra and spectral correlations from sub-photospheric
Comptonization: One of the most important unresolved issues in gamma-ray burst physics is the
origin of the prompt gamma-ray spectrum. Its general non-thermal character and
the softness in the X-ray band remain unexplained. We tackle these issues by
performing Monte Carlo simulations of radiation-matter interactions in a
scattering dominated photon-lepton plasma. The plasma -- initially in
equilibrium -- is driven to non-equilibrium conditions by a sudden energy
injection in the lepton population, mimicking the effect of a shock wave or the
dissipation of magnetic energy. Equilibrium restoration occurs due to energy
exchange between the photons and leptons. While the initial and final
equilibrium spectra are thermal, the transitional photon spectra are
characterized by non-thermal features such as power-law tails, high energy
bumps, and multiple components. Such non-thermal features are observed at
infinity if the dissipation occurs at small to moderate optical depths, and the
spectrum is released before thermalization is complete. We model the synthetic
spectra with a Band function and show that the resulting spectral parameters
are similar to observations for a frequency range of 2-3 orders of magnitude
around the peak. In addition, our model predicts correlations between the
low-frequency photon index and the peak frequency as well as between the low-
and high-frequency indices. We explore baryon and pair dominated fireballs and
reach the conclusion that baryonic fireballs are a better model for explaining
the observed features of gamma-ray burst spectra. | astro-ph_HE |
Search for fingerprints of disoriented chiral condensates in cosmic ray
showers: Although the generation of disoriented chiral condensates (DCCs), where the
order parameter for chiral symmetry breaking is misaligned with respect to the
vacuum direction in isospin state, is quite natural in the theory of strong
interactions, they have so far eluded experiments in accelerators and cosmic
rays. If DCCs are formed in high-energy nuclear collisions, the relevant
outcome are very large event-by-event fluctuations in the neutral-to-charged
pion fraction. In this note we search for fingerprints of DCC formation in
observables of ultra-high energy cosmic ray showers. We present simulation
results for the depth of the maximum ($X_{max}$) and number of muons on the
ground, evaluating their sensitivity to the neutral-to-charged pion fraction
asymmetry produced in the primary interaction. | astro-ph_HE |
Astrophysical signatures of leptonium: More than 10^43 positrons annihilate every second in the centre of our Galaxy
yet, despite four decades of observations, their origin is still unknown. Many
candidates have been proposed, such as supernovae and low mass X-ray binaries.
However, these models are difficult to reconcile with the distribution of
positrons, which are highly concentrated in the Galactic bulge, and therefore
require specific propagation of the positrons through the interstellar medium.
Alternative sources include dark matter decay, or the supermassive black hole,
both of which would have a naturally high bulge-to-disc ratio.
The chief difficulty in reconciling models with the observations is the
intrinsically poor angular resolution of gamma-ray observations, which cannot
resolve point sources. Essentially all of the positrons annihilate via the
formation of positronium. This gives rise to the possibility of observing
recombination lines of positronium emitted before the atom annihilates. These
emission lines would be in the UV and the NIR, giving an increase in angular
resolution of a factor of 10^4 compared to gamma ray observations, and allowing
the discrimination between point sources and truly diffuse emission.
Analogously to the formation of positronium, it is possible to form atoms of
true muonium and true tauonium. Since muons and tauons are intrinsically
unstable, the formation of such leptonium atoms will be localised to their
places of origin. Thus observations of true muonium or true tauonium can
provide another way to distinguish between truly diffuse sources such as dark
matter decay, and an unresolved distribution of point sources. | astro-ph_HE |
Study of Asymptotic Velocity in the Bondi-Hoyle Accretion Flows in the
Domain of Kerr and 4-D Einstein-Gauss-Bonnet Gravities: Understanding the physical structures of the accreated matter very close to
the black hole in quasars and active galactic nucleus (AGNs) is an important
milestone to constrain the activities occurring in their centers. In this
paper, we numerically investigate the effects of the asymptotic velocities on
the physical structures of the accretion disk around the Kerr and
Einstein-Gauss-Bonnet (EGB) rapidly rotating black holes. The Bondi-Hoyle
accretion is considered with a falling gas towards the black hole in upstream
region of the computational domain. The shock cones are naturally produced in
the downstream part of the flow around both black holes. It is found that the
structure of the cones and the amount of the accreated matter depend on
asymptotic velocity $V_{\infty}$ (Mach number) and the types of the gravities
(Kerr or EGB). Increasing the Mach number of the inflowing matter in the
supersonic region causes the shock opening angle and accretion rates getting
smaller because of the rapidly falling gas towards the black hole. The EGB
gravity leads to an increase in the shock opening angle of the shock cones
while the mass accretion rates $\dot{M}$ are decreasing in EGB gravity with a
Gauss-Bonnet (GB) coupling constant $\alpha$. It is also confirmed that
accretion rates and drag forces are significantly altered in the EGB gravity.
Our numerical simulation results could be used to identify the accreation
mechanism and physical properties of the accretion disk and black hole in the
observed $X-$ rays such as NGC $1313$ $X-1$ and $1313$ $X-2$ and MAXI
$J1803-298$. | astro-ph_HE |
Neutrino cooling and spin-down of rapidly rotating compact stars: The gravitational-wave instability of r-modes in rapidly rotating compact
stars is believed to spin them down to angular frequencies of about a tenth of
the Kepler frequency soon after their birth in a Supernova. We point out that
the r-mode perturbation also impacts the neutrino cooling and viscosity in hot
compact stars via processes that restore weak equilibrium. We illustrate this
fact with a simple model of spin-down due to gravitational wave emission in
compact stars composed entirely of three-flavor degenerate quark matter (a
strange quark star). Non-equilibrium neutrino cooling of this oscillating fluid
matter is quantified. Our results imply that a consistent treatment of thermal
and spin-frequency evolution of a young and hot compact star is a requisite in
estimating the persistence of gravitational waves from such a source. | astro-ph_HE |
Efficiency of Thin Magnetically-Arrested Disks Around Black Holes: The radiative and jet efficiencies of thin magnetized accretion disks around
black holes (BHs) are affected by BH spin and the presence of a magnetic field
that, when strong, could lead to large deviations from Novikov-Thorne (NT) thin
disk theory. To seek the maximum deviations, we perform general relativistic
magnetohydrodynamic (GRMHD) simulations of radiatively efficient thin
(half-height $H$ to radius $R$ of $H/R\approx 0.10$) disks around moderately
rotating BHs with $a/M=0.5$. First, our simulations, each evolved for more than
$70,000r_g/c$ (gravitational radius $r_g$ and speed of light $c$), show that
large-scale magnetic field readily accretes inward even through our thin disk
and builds-up to the magnetically-arrested disk (MAD) state. Second, our
simulations of thin MADs show the disk achieves a radiative efficiency of
$\eta_{\rm r}\approx 15\%$ (after estimating photon capture), which is about
twice the NT value of $\eta_{\rm r}\sim 8\%$ for $a/M=0.5$ and gives the same
luminosity as a NT disk with $a/M\approx 0.9$. Compared to prior simulations
with $\lesssim 10\%$ deviations, our result of an $\approx 80\%$ deviation sets
a new benchmark. Building on prior work, we are now able to complete an
important scaling law which suggest that observed jet quenching in the
high-soft state in BH X-ray binaries is consistent with an ever-present MAD
state with a weak yet sustained jet. | astro-ph_HE |
Radio and X-ray observations of Five TeV SNRs: We briefly summarize recent results of five TeV SNRs from radio and X-ray
observations. We focus on remeasuring kinematic distances of 5 TeV SNRs, i.e.
HESS J1732-347/SNR G353.6-0.7 (3.2 kpc), HESS J1834-087/G23.3-0.3 (also W41,
4.0 kpc), HESS J1833-105/G21.5-0.9 (4.8 kpc), HESS J1846-029/G29.7-0.3 (Kes 75,
6.3 kpc) and TeV SNR G54.1-0.3 (6.5 kpc), and studying non-thermal X-ray
emissions from two old SNRs (G353.6-0.7 and W41). These not only allow
constraining the TeV SNR basic physical properties, but also help reveal
acceleration mechanisms of TeV Gamma-rays in the SNRs which are either related
with the SNRs or the pulsar wind nebulae. | astro-ph_HE |
Evidence for quark-matter cores in massive neutron stars: The theory governing the strong nuclear force, Quantum Chromodynamics,
predicts that at sufficiently high energy densities hadronic nuclear matter
undergoes a deconfinement transition to a new phase of quarks and gluons.
Although this has been observed in ultrarelativistic heavy-ion collisions, it
is currently an open question whether quark matter exists inside neutron stars.
By combining astrophysical observations and theoretical ab-initio calculations
in a model-independent way, we find that the inferred properties of matter in
the cores of neutron stars with mass corresponding to 1.4 solar masses are
compatible with nuclear model calculations. However, the matter in the interior
of maximally massive, stable neutron stars exhibits characteristics of the
deconfined phase, which we interpret as evidence for the presence of
quark-matter cores. For the heaviest reliably observed neutron stars with
masses of about two solar masses, the presence of quark matter is found to be
linked to the behaviour of the speed of sound c_s in strongly interacting
matter. If the conformal bound (c_s)^2 < 1/3 is not strongly violated, massive
neutron stars are predicted to have sizable quark-matter cores. This finding
has important implications for the phenomenology of neutron stars, and affects
the dynamics of neutron star mergers with at least one sufficiently massive
participant. | astro-ph_HE |
The Correlation between Dispersion Measure and X-ray Column Density from
Radio Pulsars: Pulsars are remarkable objects that emit across the entire electromagnetic
spectrum, providing a powerful probe of the interstellar medium. In this study,
we investigate the relation between dispersion measure (DM) and X-ray
absorption column density NH using 68 radio pulsars detected at X-ray energies
with the Chandra X-ray Observatory or XMM-Newton. We find a best-fit empirical
linear relation of NH (10^20 cm^-2)= 0.30+0.13-0.09 DM (pc cm^-3), which
corresponds to an average ionization of 10+4-3%, confirming the ratio of one
free electron per ten neutral hydrogen atoms commonly assumed in the
literature. We also compare different NH estimates and note that some NH values
obtained from X-ray observations are higher than the total Galactic HI column
density along the same line of sight, while the optical extinction generally
gives the best NH predictions. | astro-ph_HE |
On the simultaneous generation of radio and soft X-ray emission by AXP
4U 0142+61: In the present paper we study the possibility of a simultaneous generation of
radio waves and soft $X$-rays by means of the quasi-linear diffusion (QLD) in
the anomalous pulsar AXP 4U 0142+61. Considering the magnetosphere composed of
the so-called beam component and the plasma component respectively, we argue
that the frozen-in condition will inevitably lead to the generation of the
unstable cyclotron waves. These waves, via the QLD, will in turn influence the
particle distribution function, leading to certain values of the pitch angles,
thus to an efficient synchrotron mechanism, producing soft $X$-ray photons. We
show that for physically reasonable parameters of magnetospheric plasmas, the
QLD can provide generation of radio waves in the following interval $40$
MHz-$111$ MHz connected to soft $X$-rays for the domain $0.3$keV-$1.4$keV. | astro-ph_HE |
GN-z11-flash in the context of Gamma-Ray Burst Afterglows: The recently discovered rapid transient GN-z11-flash has been suggested to be
the prompt-emission ultraviolet flash associated with a gamma-ray burst
serendipitously exploding in the ultra-high-$z$ galaxy GN-z11. We here place
the flash into the context of the early ultraviolet emission of gamma-ray
bursts, and find it is in agreement with the luminosity distribution of these
events. | astro-ph_HE |
Extreme photometric and polarimetric variability of blazar S4 0954+65 at
its maximum optical and $γ$-ray brightness levels: In 2022 the BL Lac object S4 0954+65 underwent a major variability phase,
reaching its historical maximum brightness in the optical and $\gamma$-ray
bands. We present optical photometric and polarimetric data acquired by the
Whole Earth Blazar Telescope (WEBT) Collaboration from 2022 April 6 to July 6.
Many episodes of unprecedented fast variability were detected, implying an
upper limit to the size of the emitting region as low as $10^{-4}$ parsec. The
WEBT data show rapid variability in both the degree and angle of polarization.
We analyse different models to explain the polarization behaviour in the
framework of a twisting jet model, which assumes that the long-term trend of
the flux is produced by variations in the emitting region viewing angle. All
the models can reproduce the average trend of the polarization degree, and can
account for its general anticorrelation with the flux, but the dispersion of
the data requires the presence of intrinsic mechanisms, such as turbulence,
shocks, or magnetic reconnection. The WEBT optical data are compared to
$\gamma$-ray data from the Fermi satellite. These are analysed with both fixed
and adaptive binning procedures. We show that the strong correlation between
optical and $\gamma$-ray data without measurable delay assumes different slopes
in faint and high brightness states, and this is compatible with a scenario
where in faint states we mainly see the imprint of the geometrical effects,
while in bright states the synchrotron self-Compton process dominates. | astro-ph_HE |
Swift: the science across the rainbow. Mission Overview and Highlights
of Results: I present an overview of the Swift mission, which was launched on November
20, 2004 to discover and observe the most energetic of astrophysical phenomena,
gamma-ray bursts (GRBs). After almost 6 years in space the Observatory is in
excellent shape, with all systems and instruments performing nominally and in
burst chasing mode for an average of 97% of the time. Swift is also a
multi-purpose multi-frequency mission with the observing time evolving from
mostly GRB targets, to mainly secondary science ones such as supernovae,
cataclysmic variables and novae, active galactic nuclei, Galactic transients,
active stars and comets. I present the most recent science highlights. | astro-ph_HE |
Highly Ionized Fe-K Absorption Line from Cygnus X-1 in the High/Soft
State Observed with Suzaku: We present observations of a transient He-like Fe K alpha absorption line in
Suzaku observations of the black hole binary Cygnus X-1 on 2011 October 5 near
superior conjunction during the high/soft state, which enable us to map the
full evolution from the start and the end of the episodic accretion phenomena
or dips for the first time. We model the X-ray spectra during the event and
trace their evolution. The absorption line is rather weak in the first half of
the observation, but instantly deepens for ~10 ks, and weakens thereafter. The
overall change in equivalent width is a factor of ~3, peaking at an orbital
phase of ~0.08. This is evidence that the companion stellar wind feeding the
black hole is clumpy. By analyzing the line with a Voigt profile, it is found
to be consistent with a slightly redshifted Fe XXV transition, or possibly a
mixture of several species less ionized than Fe XXV. The data may be explained
by a clump located at a distance of ~10^(10-12) cm with a density of
~10^((-13)-(-11)) g cm^-3, which accretes onto and/or transits the
line-of-sight to the black hole, causing an instant decrease in the observed
degree of the ionization and/or an increase in density of the accreting matter.
Continued monitoring for individual events with future X-ray calorimeter
missions such as ASTRO-H and AXSIO will allow us to map out the accretion
environment in detail and how it changes between the various accretion states. | astro-ph_HE |
Broad band polarimetric follow-up of Type IIP SN 2012aw: We present the results based on R-band polarimetric follow-up observations of
the nearby (~10 Mpc) Type II-plateau SN 2012aw. Starting from ~10 days after
the SN explosion, these polarimetric observations cover ~90 days (during the
plateau phase) and are distributed over 9 epochs. To characterize the Milky Way
interstellar polarization (ISP_MW ), we have observed 14 field stars lying in a
radius of 10 degree around the SN. We have also tried to subtract the host
galaxy dust polarization component assuming that the dust properties in the
host galaxy are similar to that observed for Galactic dust and the general
magnetic field follow the large scale structure of the spiral arms of a galaxy.
After correcting the IS_PMW , our analysis infer that SN 2012aw has maximum
polarization of 0.85% +- 0.08% but polarization angle does not show much
variation with a weighted mean value of ~138 degree. However, if both ISP_MW
and host galaxy polarization (ISP_HG ) components are subtracted from the
observed polarization values of SN, maximum polarization of the SN becomes
0.68% +- 0.08%. The distribution of Q and U parameters appears to follow a loop
like structure. The evolution of polarimetric light curve (PLC) properties of
this event is also compared with other well studied core-collapse supernovae of
similar type. | astro-ph_HE |
Astrophysical Implications of the Binary Black-Hole Merger GW150914: The discovery of the gravitational-wave source GW150914 with the Advanced
LIGO detectors provides the first observational evidence for the existence of
binary black-hole systems that inspiral and merge within the age of the
Universe. Such black-hole mergers have been predicted in two main types of
formation models, involving isolated binaries in galactic fields or dynamical
interactions in young and old dense stellar environments. The measured masses
robustly demonstrate that relatively "heavy" black holes ($\gtrsim 25\,
M_\odot$) can form in nature. This discovery implies relatively weak
massive-star winds and thus the formation of GW150914 in an environment with
metallicity lower than $\sim 1/2$ of the solar value. The rate of binary
black-hole mergers inferred from the observation of GW150914 is consistent with
the higher end of rate predictions ($\gtrsim 1 \, \mathrm{Gpc}^{-3} \,
\mathrm{yr}^{-1}$) from both types of formation models. The low measured
redshift ($z \sim 0.1$) of GW150914 and the low inferred metallicity of the
stellar progenitor imply either binary black-hole formation in a low-mass
galaxy in the local Universe and a prompt merger, or formation at high redshift
with a time delay between formation and merger of several Gyr. This discovery
motivates further studies of binary-black-hole formation astrophysics. It also
has implications for future detections and studies by Advanced LIGO and
Advanced Virgo, and gravitational-wave detectors in space. | astro-ph_HE |
Massive protostars as gamma-ray sources: Massive protostars have associated bipolar outflows with velocities of
hundreds of km s$^{-1}$. Such outflows can produce strong shocks when interact
with the ambient medium leading to regions of non-thermal radio emission. We
aim at exploring under which conditions relativistic particles are accelerated
at the terminal shocks of the protostellar jets and can produce significant
gamma-ray emission. We estimate the conditions necessary for particle
acceleration up to very high energies and gamma-ray production in the
non-thermal hot spots of jets associated with massive protostars embedded in
dense molecular clouds. We show that relativistic Bremsstrahlung and
proton-proton collisions can make molecular clouds with massive young stellar
objects detectable by the {\it Fermi}{} satellite at MeV-GeV energies and by
Cherenkov telescope arrays in the GeV-TeV range. Gamma-ray astronomy can be
used to probe the physical conditions in star forming regions and particle
acceleration processes in the complex environment of massive molecular clouds. | astro-ph_HE |
Impact of Massive Binary Star and Cosmic Evolution on Gravitational Wave
Observations II: Double Compact Object Rates and Properties: Making the most of the rapidly increasing population of gravitational-wave
detections of black hole (BH) and neutron star (NS) mergers requires comparing
observations with population synthesis predictions. In this work we investigate
the combined impact from the key uncertainties in population synthesis
modelling of the isolated binary evolution channel: the physical processes in
massive binary-star evolution and the star formation history as a function of
metallicity, $Z$, and redshift $z, \mathcal{S}(Z,z)$. Considering these
uncertainties we create 560 different publicly available model realizations and
calculate the rate and distribution characteristics of detectable BHBH, BHNS,
and NSNS mergers. We find that our stellar evolution and $\mathcal{S}(Z,z)$
variations can impact the predicted intrinsic and detectable merger rates by
factors $10^2$-$10^4$. We find that BHBH rates are dominantly impacted by
$\mathcal{S}(Z,z)$ variations, NSNS rates by stellar evolution variations and
BHNS rates by both. We then consider the combined impact from all uncertainties
considered in this work on the detectable mass distribution shapes (chirp mass,
individual masses and mass ratio). We find that the BHNS mass distributions are
predominantly impacted by massive binary-star evolution changes. For BHBH and
NSNS we find that both uncertainties are important. We also find that the shape
of the delay time and birth metallicity distributions are typically dominated
by the choice of $\mathcal{S}(Z,z)$ for BHBH, BHNS and NSNS. We identify
several examples of robust features in the mass distributions predicted by all
560 models, such that we expect more than 95% of BHBH detections to contain a
BH $\gtrsim 8\,\rm{M}_{\odot}$ and have mass ratios $\lesssim 4$. Our work
demonstrates that it is essential to consider a wide range of allowed models to
study double compact object merger rates and properties. | astro-ph_HE |
Reverse Shock Emission Driven By Post-Merger Millisecond Magnetar Winds:
Effects of the Magnetization Parameter: The study of short-duration gamma-ray bursts provides growing evidence that a
good fraction of double neutron star mergers lead to the formation of stable
millisecond magnetars. The launch of Poynting flux by the millisecond magnetars
could leave distinct electromagnetic signatures that reveal the energy
dissipation processes in the magnetar wind. In previous studies (Wang & Dai
2013b; Wang et al. 2015), we assume that the magnetar wind becomes completely
lepton-dominated so that electrons/positrons in the magnetar wind are
accelerated by a diffusive shock. However, theoretical modeling of pulsar wind
nebulae shows that in many cases the magnetic field energy in the pulsar wind
may be strong enough to suppress diffusive shock acceleration. In this paper,
we investigate the reverse shock emission as well as the forward shock emission
with an arbitrary magnetization parameter $\sigma$ of a magnetar wind. We find
that the reverse shock emission strongly depends on $\sigma$, and in
particular, $\sigma \sim 0.3$ leads to the strongest reverse shock emission.
Future observations would be helpful to diagnose the composition of the
magnetar wind. | astro-ph_HE |
Connections Between Local and Global Turbulence in Accretion Disks: We analyze a suite of global magnetohydrodynamic (MHD) accretion disk
simulations in order to determine whether scaling laws for turbulence driven by
the magnetorotational instability, discovered via local shearing box studies,
are globally robust. The simulations model geometrically-thin disks with zero
net magnetic flux and no explicit resistivity or viscosity. We show that the
local Maxwell stress is correlated with the self-generated local vertical
magnetic field in a manner that is similar to that found in local simulations.
Moreover, local patches of vertical field are strong enough to stimulate and
control the strength of angular momentum transport across much of the disk. We
demonstrate the importance of magnetic linkages (through the low-density
corona) between different regions of the disk in determining the local field,
and suggest a new convergence requirement for global simulations -- the
vertical extent of the corona must be fully captured and resolved. Finally, we
examine the temporal convergence of the average stress, and show that an
initial long-term secular drift in the local flux-stress relation dies away on
a time scale that is consistent with turbulent mixing of the initial magnetic
field. | astro-ph_HE |
Dynamical Modeling of CXOGBS J175553.2-281633: A 10 Hour Long Orbital
Period Cataclysmic Variable: We present modeling of the long-term optical light curve and radial velocity
curve of the binary stellar system CXOGBS J175553.2-281633, first detected in
X-rays in the \textit{Chandra} Galactic Bulge Survey. We analyzed 7 years of
optical I-band photometry from OGLE and found long-term variations from year to
year. These long-term variations can most likely be explained with by either
variations in the luminosity of the accretion disk or a spotted secondary star.
The phased light curve has a sinusoidal shape, which we interpret as being due
to ellipsoidal modulations. We improve the orbital period to be $P = 10.34488
\pm 0.00006$ h with a time of inferior conjunction of the secondary star $T_0 =
{\rm HJD\ } 2455260.8204 \pm 0.0008$. Moreover, we collected 37 spectra over 6
non-consecutive nights. The spectra show evidence for an evolved K7 secondary
donor star, from which we obtain a semi-amplitude for the radial velocity curve
of $K_2 = 161 \pm 6 $ km s$^{-1}$. Using the light curve synthesis code {\tt
XRbinary}, we derive the most likely orbital inclination for the binary of $i =
63.0\pm0.7$ deg, a primary mass of $M_1 = 0.83 \pm 0.06$ M$_\odot$, consistent
with a white dwarf accretor, and a secondary donor mass of $M_2 = 0.65 \pm
0.07$ M$_\odot$, consistent with the spectral classification. Therefore, we
identify the source as a long orbital period cataclysmic variable star. | astro-ph_HE |
Interesting Clues to Detect Hidden Tidal Disruption Events in Active
Galactic Nuclei: In the manuscript, effects of Tidal Disruption Events (TDEs) are estimated on
long-term AGN variability, to provide interesting clues to detect probable
hidden TDEs in normal broad line AGN with apparent intrinsic variability which
overwhelm the TDEs expected variability features, after considering the unique
TDEs expected variability patterns. Based on theoretical TDEs expected
variability plus AGN intrinsic variability randomly simulated by Continuous
AutoRegressive process, long-term variability properties with and without TDEs
contributions are well analyzed in AGN. Then, interesting effects of TDEs can
be determined on long-term observed variability of AGN. First, more massive
BHs, especially masses larger than $10^7{\rm M_\odot}$, can lead to more
sensitive and positive dependence of $\tau_{TN}$ on $R_{TN}$, with $\tau_{TN}$
as variability timescale ratio of light curves with TDEs contributions to
intrinsic light curves without TDEs contributions, and $R_{TN}$ as ratio of
peak intensity of TDEs expected variability to the mean intensity of intrinsic
AGN variability without TDEs contributions. Second, stronger TDEs contributions
$R_{TN}$ can lead to $\tau_{TN}$ quite larger than 5. Third, for intrinsic AGN
variability having longer variability timescales, TDEs contributions will lead
$\tau_{TN}$ to be increased more slowly. The results actually provide an
interesting forward-looking method to detect probable hidden TDEs in normal
broad line AGN, due to quite different variability properties, especially
different DRW/CAR process expected variability timescales, in different epochs,
especially in normal broad line AGN with shorter intrinsic variability
timescales and with BH masses larger than $10^7{\rm M_\odot}$. | astro-ph_HE |
Gamma Ray Burst Prompt correlations: The mechanism responsible for the prompt emission of gamma-ray bursts (GRBs)
is still a debated issue. The prompt phase-related GRB correlations can allow
to discriminate among the most plausible theoretical models explaining this
emission. We present an overview of the observational two-parameter
correlations, their physical interpretations, their use as redshift estimators
and possibly as cosmological tools. The nowadays challenge is to make GRBs, the
farthest stellar-scaled objects observed (up to redshift $z=9.4$), standard
candles through well established and robust correlations. However, GRBs
spanning several orders of magnitude in their energetics are far from being
standard candles. We describe the advances in the prompt correlation research
in the past decades, with particular focus paid to the discoveries in the last
20 years. | astro-ph_HE |
Compact Object Modeling in the Globular Cluster 47 Tucanae: The globular cluster 47~Tucanae (47~Tuc) is one of the most massive star
clusters in the Milky Way and is exceptionally rich in exotic stellar
populations. For several decades it has been a favorite target of observers,
and yet it is computationally very challenging to model because of its large
number of stars ($N\gtrsim 10^6$) and high density. Here we present detailed
and self-consistent 47~Tuc models computed with the \texttt{Cluster Monte
Carlo} code (\texttt{CMC}). The models include all relevant dynamical
interactions coupled to stellar and binary evolution, and reproduce various
observations, including the surface brightness and velocity dispersion
profiles, pulsar accelerations, and numbers of compact objects. We show that
the present properties of 47~Tuc are best reproduced by adopting an initial
stellar mass function that is both bottom-heavy and top-light relative to
standard assumptions \citep[as in, e.g.,][]{Kroupa2001}, and an initial Elson
profile \citep{Elson1987} that is overfilling the cluster's tidal radius. We
include new prescriptions in \texttt{CMC} for the formation of binaries through
giant star collisions and tidal captures, and we show that these mechanisms
play a crucial role in the formation of neutron star binaries and millisecond
pulsars in 47~Tuc; our best-fit model contains $\sim 50$ millisecond pulsars,
$70\%$ of which are formed through giant collisions and tidal captures. Our
models also suggest that 47~Tuc presently contains up to $\sim 200$
stellar-mass black holes, $\sim 5$ binary black holes, $\sim 15$ low-mass X-ray
binaries, and $\sim 300$ cataclysmic variables. | astro-ph_HE |
Neutron stars - thermal emitters: Confronting theoretical models with observations of thermal radiation emitted
by neutron stars is one of the most important ways to understand the properties
of both, superdense matter in the interiors of the neutron stars and dense
magnetized plasmas in their outer layers. Here we review the theory of thermal
emission from the surface layers of strongly magnetized neutron stars, and the
main properties of the observational data. In particular, we focus on the
nearby sources for which a clear thermal component has been detected, without
being contaminated by other emission processes (magnetosphere, accretion,
nebulae). We also discuss the applications of the modern theoretical models of
the formation of spectra of strongly magnetized neutron stars to the observed
thermally emitting objects. | astro-ph_HE |
Approximating new ice models with B-splines for improved IceCube event
reconstruction: application to cascades and tracks: Event signatures in IceCube are complex, modulated by both particle physics
and properties of the ice and detector. Event reconstruction thus requires
accurate modeling of ice properties and detector effects to fit for physics
parameters, such as energy and direction. Here, we highlight how improvements
in calibration can translate into substantially improving the angular
resolution of electromagnetic showers. Since showers are also used to model
stochastic energy losses of tracks, we further show how improved ice modeling,
along with other track-specific optimizations, leads to more meaningful
directional likelihood spaces for high-energy muons. The median angular
resolution for showers is improved by a factor of two over an older B-spline
model, and accurate directional contours for tracks can be obtained with Wilks'
theorem. | astro-ph_HE |
Inferring spin tilts at formation from gravitational wave observations
of binary black holes: Interfacing precession-averaged and orbit-averaged
spin evolution: Two important parameters inferred from the gravitational wave signals of
binaries of precessing black holes are the spin tilt angles, i.e., the angles
at which the black holes' spin axes are inclined with respect to the binary's
orbital angular momentum. The LIGO-Virgo parameter estimation analyses
currently provide spin tilts at a fiducial reference frequency, often the
lowest frequency used in the data analysis. However, the most astrophysically
interesting quantities are the spin tilts when the binary was formed, which can
be significantly different from those at the reference frequency for strongly
precessing binaries. The spin tilts at formally infinite separation are a good
approximation to the tilts at formation in many formation channels and can be
computed efficiently for binary black holes using precession-averaged
evolution. Here, we present a new code for computing the tilts at infinity that
combines the precession-averaged evolution with orbit-averaged evolution at
high frequencies and illustrate its application to GW190521 and other binary
black hole detections from O3a. We have empirically determined the transition
frequency between the orbit-averaged and precession-averaged evolution to
produce tilts at infinity with a given accuracy. We also have regularized the
precession-averaged equations in order to obtain good accuracy for the very
close-to-equal-mass binary parameters encountered in practice. This
additionally allows us to investigate the singular equal-mass limit of the
precession-averaged expressions, where we find an approximate scaling of $1/(1
- q)$ with the mass ratio $q$. | astro-ph_HE |
Discovery of a 26.2 day period in the long-term X-ray light curve of SXP
1323: a very short orbital period for a long spin period pulsar: Context. About 120 Be/X-ray binaries (BeXBs) are known in the Small
Magellanic Cloud (SMC); about half of them are pulsating with periods from a
few to hundreds of seconds. SXP 1323 is one of the longest-period pulsars known
in this galaxy. Aims. SXP 1323 is in the field of view of a large set of
calibration observations that we analyse systematically, focusing on the time
analysis, in search of periodic signals. Methods. We analyse all available
X-ray observations of SXP 1323 from Suzaku, XMM-Newton, and Chandra, in the
time range from 1999 to the end of 2016. We perform a Lomb-Scargle periodogram
search in the band 2.5-10 keV on all observations to detect the neutron star
spin period and constrain its long-term evolution. We also perform an orbital
period search on the long-term light curve, merging all datasets. Results. We
report the discovery of a 26.188+-0.045 d period analysing data from Suzaku,
XMM-Newton, and Chandra, which confirms the optical period derived from the
Optical Gravitational Lensing Experiment (OGLE) data. If this corresponds to
the orbital period, this would be very short with respect to what is expected
from the spin/orbital period relationship. We furthermore report on the spin
period evolution in the last years. The source is spinning-up with an average
rate of Pdot/P of 0.018 yr-1, decreasing from 1340 to 1100 s, in the period
from 2006 to the end of 2016, which is also extreme with respect to the other
Be/X-ray pulsars. From 2010 to the end of 2014, the pulse period is not clearly
detectable, although the source was still bright. Conclusions. SXP 1323 is a
peculiar BeXB due to its long pulse period, rapid spin-up for several years,
and short orbital period. A continuous monitoring of the source in the next
years is necessary to establish the long-term behaviour of the spin period. | astro-ph_HE |
Magnetic Deflections of Ultra-High Energy Cosmic Rays from Centaurus A: We present the results of a study that simulates trajectories of ultra-high
energy cosmic rays from Centaurus A to Earth, for particle rigidities from $E/Z
= 2$ EV to 100 EV, i.e., covering the possibility of primary particles as heavy
as Fe nuclei with energies exceeding 50 EeV. The Galactic magnetic field is
modeled using the recent work of Jansson and Farrar (JF12) which fitted its
parameters to match extragalactic Faraday rotation measures and WMAP7
synchrotron emission maps. We include the random component of the GMF using the
JF12 3D model for $B_{\rm rand}(\vec{r})$ and explore the impact of different
random realizations, coherence length and other features on cosmic ray
deflections. Gross aspects of the arrival direction distribution such as mean
deflection and the RMS dispersion depend mainly on rigidity and differ
relatively little from one realization to another. However different
realizations exhibit non-trivial substructure whose specific features vary
considerably from one realization to another, especially for lower rigidities.
At the lowest rigidity of 2 EV, the distribution is broad enough that it might
be compatible with a scenario in which Cen A is the principle source of all
UHECRs. No attempt is made here to formulate a robust test of this possibility,
although some challenges to such a scenario are noted. | astro-ph_HE |
The pulsar synchrotron: coherent radio emission: We propose a simple physical picture for the generation of coherent radio
emission in the axisymmetric pulsar magnetosphere that is quite different from
the canonical paradigm of radio emission coming from the magnetic polar caps.
In this first paper we consider only the axisymmetric case of an aligned
rotator. Our picture capitalizes on an important element of the MHD
representation of the magnetosphere, namely the separatrix between the
corotating closed-line region (the `dead zone') and the open field lines that
originate in the polar caps. Along the separatrix flows the return current that
corresponds to the main magnetospheric electric current emanating from the
polar caps. Across the separatrix, both the toroidal and poloidal components of
the magnetic field change discontinuously. The poloidal component discontinuity
requires the presence of a significant annular electric current which has up to
now been unaccounted for. We estimate the position and thickness of this
annular current at the tip of the closed line region, and show that it consists
of electrons (positrons) corotating with Lorentz factors on the order of 10^5,
emitting incoherent synchrotron radiation that peaks in the hard X-rays. These
particles stay in the region of highest annular current close to the equator
for a path-length of the order of one meter. We propose that, at wavelengths
comparable to that path-length, the particles emit coherent radiation, with
radiated power proportional to N^2, where N is the population of particles in
the above path-length. We calculate the total radio power in this wavelength
regime and its scaling with pulsar period and stellar magnetic field and show
that it is consistent with estimates of radio luminosity based on observations. | astro-ph_HE |
GRB 090510: a short burst from a massive star ?: GRB afterglow 090510 is (so far) the best-monitored afterglow in the optical,
X-ray, and above 100 MeV, measurements covering 2-3 decades in time at each
frequency. Owing to its power-law temporal decay and power-law spectrum, it
seems very likely that the highest energy emission is from the forward-shock
energizing the ambient medium (the standard blast-wave model for GRB
afterglows), the GeV flux and its decay rate being consistent with that model's
expectations. However, the synchrotron emission from a collimated outflow (the
standard jet model) has difficulties in accounting for the lower-energy
afterglow emission, where a simultaneous break occurs at 2 ks in the optical
and X-ray light-curves, but with the optical flux decay (before and after the
break) being much slower than in the X-rays (at same time). The measured X-ray
and GeV fluxes are incompatible with the higher-energy afterglow emission being
from same spectral component as the lower-energy afterglow emission, which
suggests a synchrotron self-Compton model for this afterglow. Cessation of
energy injection in the blast-wave and an ambient medium with a wind-like n ~
r^{-2} density can explain all features of the optical and X-ray light-curves
of GRB afterglow 090510. Such an ambient medium radial structure is
incompatible with this short-GRB originating from the merger of two compact
stars. | astro-ph_HE |
A joint spectro-imaging analysis of the XMM-Newton and HESS observations
of the supernova remnant RX J1713.7-3946: The supernova remnant (SNR) RX J1713.7-3946 (also known as G347.3-0.5) is
part of the class of remnants dominated by synchrotron emission in X-rays. It
is also one of the few shell-type SNRs observed at TeV energies allowing to
investigate particle acceleration at SNRs shock.
Our goal is to compare spatial and spectral properties of the remnant in X-
and gamma-rays to understand the nature of the TeV emission. This requires to
study the remnant at the same spatial scale at both energies. To complement the
non-thermal spectrum of the remnant, we attempt to provide a reliable estimate
for the radio flux density. In radio, we revisited ATCA data and used HI and
mid-infrared observations to disentangle the thermal from the non-thermal
emission. In X-rays, we produced a new mosaic of the remnant and degraded the
spatial resolution of the X-ray data to the resolution of the HESS instrument
to perform spatially resolved spectroscopy at the same spatial scale in X- and
gamma-rays. Radial profiles were obtained to investigate the extension of the
emission at both energies.
We found that part of the radio emission within the SNR contours is thermal
in nature. Taking this into account, we provide new lower and upper limits for
the integrated synchrotron flux of the remnant at 1.4 GHz of 22 Jy and 26 Jy
respectively. In X-rays, we obtained the first full coverage of RX J1713.7-3946
with XMM-Newton. The spatial variation of the photon index seen at small scale
in X-rays is smeared out at HESS resolution. A non-linear correlation between
the X- and gamma-ray fluxes of the type Fx \propto Fg^2.41 is found. | astro-ph_HE |
Spectral and morphological analysis of the remnant of Supernova 1987A
with ALMA & ATCA: We present a comprehensive spectral and morphological analysis of the remnant
of Supernova (SN) 1987A with the Australia Telescope Compact Array (ATCA) and
the Atacama Large Millimeter/submillimeter Array (ALMA). The non-thermal and
thermal components of the radio emission are investigated in images from 94 to
672 GHz ($\lambda$ 3.2 mm to 450 $\mu$m), with the assistance of a
high-resolution 44 GHz synchrotron template from the ATCA, and a dust template
from ALMA observations at 672 GHz. An analysis of the emission distribution
over the equatorial ring in images from 44 to 345 GHz highlights a gradual
decrease of the east-to-west asymmetry ratio with frequency. We attribute this
to the shorter synchrotron lifetime at high frequencies. Across the transition
from radio to far infrared, both the synchrotron/dust-subtracted images and the
spectral energy distribution (SED) suggest additional emission beside the main
synchrotron component ($S_{\nu}\propto\nu^{-0.73}$) and the thermal component
originating from dust grains at $T\sim22$ K. This excess could be due to
free-free flux or emission from grains of colder dust. However, a second
flat-spectrum synchrotron component appears to better fit the SED, implying
that the emission could be attributed to a pulsar wind nebula (PWN). The
residual emission is mainly localised west of the SN site, as the spectral
analysis yields $-0.4\lesssim\alpha\lesssim-0.1$ across the western regions,
with $\alpha\sim0$ around the central region. If there is a PWN in the remnant
interior, these data suggest that the pulsar may be offset westward from the SN
position. | astro-ph_HE |
Gravitational wave astronomy of single sources with a pulsar timing
array: Abbreviated:
We investigate the potential of detecting the gravitational wave from
individual binary black hole systems using pulsar timing arrays (PTAs) and
calculate the accuracy for determining the GW properties. This is done in a
consistent analysis, which at the same time accounts for the measurement of the
pulsar distances via the timing parallax.
We find that, at low redshift, a PTA is able to detect the nano-Hertz GW from
super massive black hole binary systems with masses of $\sim10^8 -
10^{10}\,M_{\sun}$ less than $\sim10^5$\,years before the final merger, and
those with less than $\sim10^3 - 10^4$ years before merger may allow us to
detect the evolution of binaries.
We derive an analytical expression to describe the accuracy of a pulsar
distance measurement via timing parallax. We consider five years of bi-weekly
observations at a precision of 15\,ns for close-by ($\sim 0.5 - 1$\,kpc)
pulsars. Timing twenty pulsars would allow us to detect a GW source with an
amplitude larger than $5\times 10^{-17}$. We calculate the corresponding GW and
binary orbital parameters and their measurement precision. The accuracy of
measuring the binary orbital inclination angle, the sky position, and the GW
frequency are calculated as functions of the GW amplitude. We note that the
"pulsar term", which is commonly regarded as noise, is essential for obtaining
an accurate measurement for the GW source location.
We also show that utilizing the information encoded in the GW signal passing
the Earth also increases the accuracy of pulsar distance measurements. If the
gravitational wave is strong enough, one can achieve sub-parsec distance
measurements for nearby pulsars with distance less than $\sim 0.5 - 1$\,kpc. | astro-ph_HE |
Optical and Radio Variability of the Blazar S4 0954+658: We present an optical-to-radio study of the BL Lac object S4 0954+658
observations during 1998-2023. The measurements were obtained with the SAO RAS
Zeiss-1000 1-m and AS-500/2 0.5-m telescopes in 2003-2023, with the RATAN-600
radio telescope at 1.25 (0.96, 1.1), 2.3, 4.7 (3.7, 3.9), 8.2 (7.7), 11.2, 22.3
(21.7) GHz in 1998-2023, with the IAA RAS RT-32 Zelenchukskaya and Badary
telescopes at 5.05 and 8.63 GHz in 2020--2023, and with the RT-22 single-dish
telescope of CrAO RAS at 36.8 GHz in 2009-2023. In this period the blazar had
been showing extremely high broadband activity with the variability amplitude
of flux densities up to 70-100% both in the optical and radio domains. In the
period of 2014-2023 the blazar had been showing the historically highest
activity in the radio wavelengths, and we detected multiple radio flares of
varying amplitude and duration. The large flares last on average from 0.3 to 1
year at 22-36.8 GHz and slightly longer at 5-11.2 GHz. The optical flares are
shorter and last 7-50 days. In the most active epoch of 2018-2023 the
characteristic time scale $\tau$ of variation at 5-22 GHz is about 100 days and
about 1000 days for the state with lower activity in 2009-2014. We found a
general correlation between the optical, radio, and $\gamma$-ray flux
variations, which suggests that we observe the same photon population from
different emission regions. We estimated linear size of this region as 0.5-2 pc
for different epochs. A broadband two components radio spectrum of S4 0954+658
jet was modelled by using both electrons and protons as emitting particles. It
is shown that the synchrotron radio waves in this AGN may be generated by
relativistic protons. | astro-ph_HE |
Discovery of the Optical Counterparts to Four Energetic Fermi
Millisecond Pulsars: In the last few years, over 43 millisecond radio pulsars have been discovered
by targeted searches of unidentified gamma-ray sources found by the Fermi
Gamma-Ray Space Telescope. A large fraction of these millisecond pulsars are in
compact binaries with low-mass companions. These systems often show eclipses of
the pulsar signal and are commonly known as black widows and redbacks because
the pulsar is gradually destroying its companion. In this paper, we report on
the optical discovery of four strongly irradiated millisecond pulsar
companions. All four sources show modulations of their color and luminosity at
the known orbital periods from radio timing. Light curve modelling of our
exploratory data shows that the equilibrium temperature reached on the
companion's dayside with respect to their nightside is consistent with about
10-30% of the available spin-down energy from the pulsar being reprocessed to
increase the companion's dayside temperature. This value compares well with the
range observed in other irradiated pulsar binaries and offers insights about
the energetics of the pulsar wind and the production of gamma-ray emission. In
addition, this provides a simple way of estimating the brightness of irradiated
pulsar companions given the pulsar spin-down luminosity. Our analysis also
suggests that two of the four new irradiated pulsar companions are only
partially filling their Roche lobe. Some of these sources are relatively bright
and represent good targets for spectroscopic follow-up. These measurements
could enable, among other things, mass determination of the neutron stars in
these systems. | astro-ph_HE |
Search for Gravitational Waves Associated with Fast Radio Bursts
Detected by CHIME/FRB During the LIGO--Virgo Observing Run O3a: We search for gravitational-wave transients associated with fast radio bursts
(FRBs) detected by the Canadian Hydrogen Intensity Mapping Experiment Fast
Radio Burst Project (CHIME/FRB), during the first part of the third observing
run of Advanced LIGO and Advanced Virgo (1 April 2019 15:00 UTC-1 Oct 2019
15:00 UTC). Triggers from 22 FRBs were analyzed with a search that targets
compact binary coalescences with at least one neutron star component. A
targeted search for generic gravitational-wave transients was conducted on 40
FRBs. We find no significant evidence for a gravitational-wave association in
either search. Given the large uncertainties in the distances of the FRBs
inferred from the dispersion measures in our sample, however, this does not
conclusively exclude any progenitor models that include emission of a
gravitational wave of the types searched for from any of these FRB events. We
report $90\%$ confidence lower bounds on the distance to each FRB for a range
of gravitational-wave progenitor models. By combining the inferred maximum
distance information for each FRB with the sensitivity of the
gravitational-wave searches, we set upper limits on the energy emitted through
gravitational waves for a range of emission scenarios. We find values of order
$10^{51}$-$10^{57}$ erg for a range of different emission models with central
gravitational wave frequencies in the range 70-3560 Hz. Finally, we also found
no significant coincident detection of gravitational waves with the repeater,
FRB 20200120E, which is the closest known extragalactic FRB. | astro-ph_HE |
The nova-like nebular optical spectrum of V404 Cygni at the beginning of
the 2015 outburst decay: We report on FORS2 optical spectroscopy of the black hole X-ray binary V404
Cygni, performed at the very beginning of its 2015 outburst decay, complemented
by quasi-simultaneous $Swift$ X-ray and ultra-violet as well as REM
near-infrared observations. Its peculiar spectrum is dominated by a wealth of
emission signatures of HI, HeI, and higher ionisation species, in particular
FeII. The spectral features are divided between broad red-shifted and narrow
stationary varieties, the latter being emitted in the outer regions. Continuum
and line variability at short time scale is high and we find Baldwin
effect-like anti-correlations between the full-widths at half-maximum and
equivalent widths of the broad lines with their local continua. The Balmer
decrement H{\alpha}/H{\beta} is also abnormally large at $4.61\pm0.62$. We
argue that these properties hint at the broad lines being optically thick and
arising within a circumbinary component in which shocks between faster
optically thick and slower optically thin regions may occur. We associate it to
a nova-like nebula formed by the cooling remnant of strong accretion disc winds
that turned off when the mass-accretion rate dropped following the last major
flare. The FeII lines likely arise from the overlap region between this nebula
and the companion star winds, whereas we favour the shocks within the nebula as
responsible for the optical continuum via self-absorbed optically thin
bremsstrahlung. The presence of a near-infrared excess also points towards the
contribution of a strongly variable compact jet or a dusty component. | astro-ph_HE |
Time-dependent neutrino emission from Mrk 421 during flares and
predictions for IceCube: Blazars are prime candidate sources for the high energy neutrinos recently
detected by IceCube. Being one of the brightest sources in the extragalactic
X-ray and $\gamma$-ray sky as well as one of the nearest blazars to Earth, Mrk
421 is an excellent source for testing the scenario of the blazar-neutrino
connection. Here, we model the spectral energy distribution of Mrk 421 during a
13-day flare in 2010 with unprecedented multi-wavelength coverage, and
calculate the respective neutrino flux. We find a correlation between the $>1$
PeV neutrino and photon fluxes, in all energy bands. Using typical IceCube
through-going muon event samples with good angular resolution and high
statistics, we derive the mean event rate above 100 TeV ($\sim0.57$ evt/yr) and
show that it is comparable to that expected from a four-month quiescent period
in 2009. Due to the short duration of the flare, an accumulation of similar
flares over several years would be necessary to produce a meaningful signal for
IceCube. To better assess this, we apply the correlation between the neutrino
and $\gamma$-ray fluxes to the 6.9 yr Fermi-LAT light curve of Mrk 421. We find
that the mean event count above 1 PeV for the full IceCube detector livetime is
$3.59\pm0.60$ ($2.73\pm0.38$) $\nu_\mu+\bar{\nu}_\mu$ with (without) major
flares included in our analysis. This estimate exceeds, within the
uncertainties, the $95\%$ ($90\%$) threshold value for the detection of one or
more muon (anti-)neutrinos. Meanwhile, the most conservative scenario, where no
correlation of $\gamma$-rays and neutrinos is assumed, predicts $1.60\pm0.16$
$\nu_\mu+\bar{\nu}_\mu$ events. We conclude that a non-detection of high-energy
neutrinos by IceCube would probe the neutrino/$\gamma$-ray flux correlation
during major flares or/and the hadronic contribution to the blazar emission. | astro-ph_HE |
QPO-jet relation in X-ray binaries: In the past years, a clear picture of the evolution of outbursts of
black-hole X-ray binaries has emerged. While the X-ray properties can be
classified into our distinct states, based on spectral and timing properties,
the observations in the radio band have shown strong links between accretion
and ejection properties. Here I briefly outline the association between X-ray
timing and jet properties. | astro-ph_HE |
Detection of diffuse gamma-ray emission near the young massive cluster
NGC 3603: We report the Fermi Large Area Telescope's detection of extended gamma-ray
emission towards the direction of the young massive star cluster NGC 3603. The
emission shows a hard spectrum with a photon index of 2.3 from 1 GeV to 250
GeV. The large size and high luminosity of this structure make it unlikely a
pulsar wind nebular. On the other hand the spatial correlation with the ionised
gas indicate a hadronic origin. The total cosmic ray (CR) protons energy are
estimated to be of the order $10^{50} ~\rm erg$ assuming the gamma-ray are
produced in the interaction of CRs with ambient gas . The environment and
spectral features show significant similarity with the Cygnus cocoon. It
reveals that the young star clusters may be a gamma-ray source population and
they can potentially accelerate a significant fraction of the Galactic cosmic
rays. | astro-ph_HE |
Insight-HXMT, NuSTAR and INTEGRAL Data Show Disk Truncation in the Hard
State of the Black-Hole X-Ray Binary MAXI J1820+070: We study X-ray and soft gamma-ray spectra from the hard state of the
accreting black-hole binary MAXI J1820+070. We perform analysis of joint
spectra from HXMT, NuSTAR and INTEGRAL. We find an overall agreement between
the spectra from all three satellites. Satisfactory fits to the data require
substantial spectral complexity, with our models including two Comptonization
regions and their associated disk reflection, a disk blackbody and a narrow Fe
K$\alpha$ line. Our fits confirm the presence of the truncation of the
reflecting optically-thick disk at least at $>$10 gravitational radii. However,
we find that the HXMT data alone cannot significantly constrain the disk inner
radii. | astro-ph_HE |
Constraining cosmic-ray acceleration in the magnetospheric gaps of Sgr
A*: Sagittarius A* (Sgr A*) is a potential VHE gamma-ray and cosmic-ray source.
We examine limits to gap-type particle acceleration in the magnetosphere of Sgr
A*, showing that in the current phase of activity proton acceleration to PeV
energies is possible, with injection powers into the environment usually
limited to several 10^{36} erg/s. Compton upscattering of ambient soft photons
by gap-accelerated electrons could yield TeV emission compatible with the
detected VHE points source. We explore the dependency of the results on changes
in the accretion rate showing that higher stages in the past are unlikely to
increase the power output unless the inner accretion flows itself changed its
configuration. | astro-ph_HE |
Is the energy generation rate of nuclear reactions in hot accretion
flows important?: The temperature of hot accretion flows around black holes is sufficiently
high for the ignition of nuclear reactions. This is potentially an important
nucleosynthesis mechanism in the universe. As the first step in studying this
problem, we need to measure physical quantities such as density and temperature
of the accretion flow. In usual studies of the hot accretion flow, viscous
dissipation is considered to be the only heating mechanism, while the heating
caused by nuclear reactions is not considered. In this paper, we investigate
whether the energy generation rate of nuclear reaction is important compared to
the viscous heating. Our calculation indicates that the former is at most one
percent of the latter and thus is not important. The dynamics of accretion flow
can be therefore calculated in the usual way, without the need to consider
heating due to nuclear reactions. | astro-ph_HE |
High-energy electromagnetic offline follow-up of LIGO-Virgo
gravitational-wave binary coalescence candidate events: We present two different search methods for electromagnetic counterparts to
gravitational-wave (GW) events from ground-based detectors using archival NASA
high-energy data from the Fermi-GBM and RXTE-ASM instruments. To demonstrate
the methods, we use a limited number of representative GW background noise
events produced by a search for binary neutron star coalescence over the last
two months of the LIGO-Virgo S6/VSR3 joint science run. Time and sky location
provided by the GW data trigger a targeted search in the high-energy photon
data. We use two custom pipelines: one to search for prompt gamma-ray
counterparts in GBM, and the other to search for a variety of X-ray afterglow
model signals in ASM. We measure the efficiency of the joint pipelines to weak
gamma-ray burst counterparts, and a family of model X-ray afterglows. By
requiring a detectable signal in either electromagnetic instrument coincident
with a GW event, we are able to reject a large majority of GW candidates. This
reduces the signal-to-noise of the loudest surviving GW background event by
around 15-20%. | astro-ph_HE |
The power-law component of the X-ray emissions from pulsar wind nebulae
and their pulsars: To look for possible phenomenological connections between pulsar's timing
properties and emissions from pulsar wind nebulae and their pulsars, we studied
the power-law component of the X-ray emissions from 35 pulsar wind nebulae
which have a detected pulsar in X-rays. Our major results are in the following:
(1) The power-law component of the X-ray luminosities, in the energy range from
0.5 keV to 8 keV, of the nebulae and of the pulsar both show a strong
correlation with the pulsar spin-down power ($\dot{E}$), consistent with
earlier studies. However, equally significant correlations with the magnetic
field strength at the light cylinder ($B_{\rm lc}$) are also found. The similar
significance level of the correlations with $\dot{E}$ and with $B_{\rm lc}$
suggests that not only $\dot{E}$ but also $B_{\rm lc}$ plays an important role
in understanding these power-law emissions. (2) Thermal X-ray emissions are
detected in 12 pulsars among the 35 samples. With derived temperature as one
additional variable, we found that the photon indices of pulsar's non-thermal
X-ray power-law spectra can be well described by a linear function of $\log P$,
$\log\dot{P}$ and temperature logarithm $\log T$. It indicates that the surface
temperature of neutron stars plays an important role in determining the energy
distribution of the radiating pair plasma in pulsar's magnetospheres. | astro-ph_HE |
Evidence for a change in the radiation mechanism in the hard state of
GRO J1655-40. Hysteresis in the broad-band noise components: We have analysed RXTE archival data to study the aperiodic variability of the
black-hole binary GRO J1655-40 during the hard state of the 2005 outburst. We
computed the 0.008-64 Hz power spectral density during the rise and decay of
the 2005 outburst, and we found that they were reasonably well modelled by the
sum of two, broad Lorenztian functions in most cases (plus a narrow QPO) which
correspond to three different variability components. Our aim is to study the
evolution of the timing properties of the source during the outburst, by
studying the correlation between the characteristics of the broad-band noise
components in the power spectra and the source luminosity. Our results suggest
that the whole power spectrum shifts to high (low) frequencies as the source
luminosity increases (decreases), in agreement with previous studies of other
black hole binaries. However, we also detect a strong "hysteresis" pattern in
the "frequency-luminosity" plots, and show that the "critical" luminosity
limit, above which the timing properties of the source change, is different
during the rise and the decay phase of the outburst. We discuss the general
implications of these results in the context of the truncated disc model. | astro-ph_HE |
Constraints on the X-ray Luminosity Function of AGN at z=5.7-6.4 with
the Extragalactic Serendipitous Swift Survey: X-ray luminosity functions (XLFs) of Active Galactic Nuclei (AGN) trace the
growth and evolution of supermassive black hole populations across cosmic time,
however, current XLF models are poorly constrained at redshifts of z>6, with a
lack of spectroscopic constraints at these high redshifts. In this work we
\redit{place limits} on the bright-end of the XLF at z=5.7-6.4 using
high-redshift AGN identified within the Extragalactic Serendipitous Swift
Survey (ExSeSS) catalogue. Within ExSeSS we find one serendipitously X-ray
detected z>6 AGN, ATLAS J025.6821-33.4627, with an X-ray luminosity of
$L_\mathrm{X}=8.47^{+3.40}_{-3.13}\times10^{44}\mathrm{erg.s^{-1}}$ and
$z=6.31\pm0.03$ making it the highest redshift, spectroscopically confirmed,
serendipitously X-ray detected quasar known to date. We also calculate an upper
limit on the space density at higher luminosities where no additional sources
are found, enabling us to place constraints on the shape of the XLF. Our
results are consistent with the rapid decline in the space densities of
high-luminosity AGN toward high redshift as predicted by extrapolations of
existing parametric models of the XLF. We also find that our X-ray based
measurements are consistent with estimates of the bolometric quasar luminosity
function based on UV measurements at $z\gtrsim6$, although they require a large
X-ray to bolometric correction factor at these high luminosities. | astro-ph_HE |
A Luminous Gamma-ray Binary in the Large Magellanic Cloud: Gamma-ray binaries consist of a neutron star or a black hole interacting with
a normal star to produce gamma-ray emission that dominates the radiative output
of the system. Only a handful of such systems have been previously discovered,
all within our Galaxy. Here we report the discovery with the Fermi Large Area
Telescope (LAT) of a luminous gamma-ray binary in the Large Magellanic Cloud
from a search for periodic modulation in all sources in the third Fermi LAT
catalog. This is the first such system to be found outside the Milky Way. The
system has an orbital period of 10.3 days and is associated with a massive
O5III star located in the supernova remnant DEM L241, previously identified as
the candidate high-mass X-ray binary (HMXB) CXOU J053600.0-673507. X-ray and
radio emission are also modulated on the 10.3 day period, but are in anti-phase
with the gamma-ray modulation. Optical radial velocity measurements suggest
that the system contains a neutron star. The source is significantly more
luminous than similar sources in the Milky Way at radio, optical, X-ray and
gamma-ray wavelengths. The detection of this extra-galactic system, but no new
Galactic systems raises the possibility that the predicted number of gamma-ray
binaries in our Galaxy has been overestimated, and that HMXBs may be born
containing relatively slowly rotating neutron stars. | astro-ph_HE |
Search for transitions between states in redbacks and black widows using
seven years of Fermi-LAT observations: Considering about seven years of Fermi-Large Area Telescope (LAT) data, we
present a systematic search for variability possibly related to transitions
between states in redbacks and black widow systems. Transitions are
characterized by sudden and significant changes in the gamma-ray flux that
persist on a timescale much larger than the orbital period. This phenomenology
was already detected in the case of two redback systems, PSR J1023+0038 and PSR
J1227-4853, for which we present here a dedicated study. We show the existence
of only one transition for each of these systems over the past seven years. We
determine their spectra, establishing high-energy cutoffs at a few GeV for the
high gamma-ray state of PSR J1023+0038 and for both states of PSR J1227-4853.
The surveying capability of the Fermi-LAT allows studying whether similar
phenomenology has occurred in other sources. Although we have not found any
hint for a state transition for most of the studied pulsars, we note two
black-widow systems, PSR J2234+0944 and PSR J1446-4701, whose apparent
variability is reminiscent of the transitions in PSR J1023+0038 and PSR
J1227-4853. For the other systems we set limits on potential transitions in
their measured gamma-ray light curves. | astro-ph_HE |
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