20,637 research outputs found
A Reverse-Shock Model for the Early Afterglow of GRB 050525A
The prompt localization of gamma-ray burst (GRB) 050525A by {\em Swift}
allowed the rapid follow-up of the afterglow. The observations revealed that
the optical afterglow had a major rebrightening starting at days
and ending at days, which was followed by an initial power-law
decay. Here we show that this early emission feature can be interpreted as the
reverse shock emission superposed by the forward shock emission in an
interstellar medium environment. By fitting the observed data, we further
constrain some parameters of the standard fireball-shock model: the initial
Lorentz factor of the ejecta , the magnetic energy fraction
, and the medium density . These
limits are consistent with those from the other very-early optical afterglows
observed so far. In principle, a wind environment for GRB 050525A is
disfavored.Comment: 11 pages, 1 figure, accepted for publication in Ap
Behavior of X-Ray Dust Scattering and Implications for X-Ray Afterglows of Gamma-Ray Bursts
The afterglows of gamma-ray bursts (GRBs) have commonly been assumed to be
due to shocks sweeping up the circum-stellar medium. However, most GRBs have
been found in dense star-forming regions where a significant fraction of the
prompt X-ray emission can be scattered by dust grains. Here we revisit the
behavior of dust scattering of X-rays in GRBs. We find that the features of
some X-ray afterglows from minutes to days after the gamma-ray triggers are
consistent with the scattering of prompt X-ray emission from GRBs off host dust
grains. This implies that some of the observed X-ray afterglows (especially
those without sharp rising and decaying flares) could be understood with a
dust-scattering--driven emission model.Comment: ApJ, in pres
Strong GeV Emission Accompanying TeV Blazar H1426+428
For High frequency BL Lac objects (HBLs) like H1426+428, a significant
fraction of their TeV gamma-rays emitted are likely to be absorbed in
interactions with the diffuse IR background, yielding pairs. The
resulting pairs generate one hitherto undiscovered GeV emission by
inverse Compton scattering with the cosmic microwave background photons
(CMBPs). We study such emission by taking the 1998-2000 CAT data, the
reanalyzed 1999 & 2000 HEGRA data and the corresponding intrinsic spectra
proposed by Aharonian et al. (2003a). We numerically calculate the scattered
photon spectra for different intergalactic magnetic field (IGMF) strengths. If
the IGMF is about or weaker, there comes very strong GeV
emission, whose flux is far above the detection sensitivity of the upcoming
satellite GLAST! Considered its relatively high redshift (), the
detected GeV emission in turn provides us a valuable chance to calibrate the
poor known spectral energy distribution of the intergalactic infrared
background, or provides us some reliable constraints on the poorly known IGMF
strength.Comment: 5 pages, 1 figure. A&A in Pres
Hyperaccretion Disks around Neutron Stars
(Abridged) We here study the structure of a hyperaccretion disk around a
neutron star. We consider a steady-state hyperaccretion disk around a neutron
star, and as a reasonable approximation, divide the disk into two regions,
which are called inner and outer disks. The outer disk is similar to that of a
black hole and the inner disk has a self-similar structure. In order to study
physical properties of the entire disk clearly, we first adopt a simple model,
in which some microphysical processes in the disk are simplified, following
Popham et al. and Narayan et al. Based on these simplifications, we
analytically and numerically investigate the size of the inner disk, the
efficiency of neutrino cooling, and the radial distributions of the disk
density, temperature and pressure. We see that, compared with the black-hole
disk, the neutron star disk can cool more efficiently and produce a much higher
neutrino luminosity. Finally, we consider an elaborate model with more physical
considerations about the thermodynamics and microphysics in the neutron star
disk (as recently developed in studying the neutrino-cooled disk of a black
hole), and compare this elaborate model with our simple model. We find that
most of the results from these two models are basically consistent with each
other.Comment: 44 pages, 10 figures, improved version following the referees'
comments, main conclusions unchanged, accepted for publication in Ap
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