17,120 research outputs found
GRB 030226 in a Density-Jump Medium
We present an explanation for the unusual temporal feature of the GRB 030226
afterglow. The R-band afterglow of this burst faded as ~ t^{-1.2} in ~ 0.2 days
after the burst, rebrightened during the period of ~ 0.2 - 0.5 days, and then
declined with ~ t^{-2.0}. To fit such a light curve, we consider an
ultrarelativistic jetted blast wave expanding in a density-jump medium. The
interaction of the blast wave with a large density jump produces relativistic
reverse and forward shocks. In this model, the observed rebrightening is due to
emissions from these newly forming shocks, and the late-time afterglow is
caused by sideways expansion of the jet. Our fitting implies that the
progenitor star of GRB 030226 could have produced a stellar wind with a large
density jump prior to the GRB onset.Comment: 9 pages, 1 figure, accepted for publication in ApJ Letter
X-Ray Flares from Postmerger Millisecond Pulsars
Recent observations support the suggestion that short-duration gamma-ray
bursts are produced by compact star mergers. The X-ray flares discovered in two
short gamma-ray bursts last much longer than the previously proposed postmerger
energy release time scales. Here we show that they can be produced by
differentially rotating, millisecond pulsars after the mergers of binary
neutron stars. The differential rotation leads to windup of interior poloidal
magnetic fields and the resulting toroidal fields are strong enough to float up
and break through the stellar surface. Magnetic reconnection--driven explosive
events then occur, leading to multiple X-ray flares minutes after the original
gamma-ray burst.Comment: 10 pages, published in Scienc
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
Intrinsic Parameters of GRB990123 from Its Prompt Optical Flash and Afterglow
We have constrained the intrinsic parameters, such as the magnetic energy
density fraction (), the electron energy density fraction
(), the initial Lorentz factor () and the Lorentz factor
of the reverse external shock (), of GRB990123, in terms of the
afterglow information (forward shock model) and the optical flash information
(reverse shock model). Our result shows: 1) the inferred values of
and are consistent with the suggestion that they may be universal
parameters, comparing to those inferred for GRB970508; 2) the reverse external
shock may have become relativistic before it passed through the ejecta shell.
Other instrinsic parameters of GRB990123, such as energy contained in the
forward shock and the ambient density are also determined and discussed
in this paper.Comment: 5 pages, MN LaTeX style, a few changes made according to referee's
suggestions, references up dated, MNRAS accepte
Optical Flashes and Very Early Afterglows in Wind Environments
The interaction of a relativistic fireball with its ambient medium is
described through two shocks: a reverse shock that propagates into the
fireball, and a forward shock that propagates into the medium. The observed
optical flash of GRB 990123 has been considered to be the emission from such a
reverse shock. The observational properties of afterglows suggest that the
progenitors of some GRBs may be massive stars and their surrounding media may
be stellar winds. We here study very early afterglows from the reverse and
forward shocks in winds. An optical flash mainly arises from the relativistic
reverse shock while a radio flare is produced by the forward shock. The peak
flux densities of optical flashes are larger than 1 Jy for typical parameters,
if we do not take into account some appropriate dust obscuration along the line
of sight. The radio flare always has a long lasting constant flux, which will
not be covered up by interstellar scintillation. The non-detections of optical
flashes brighter than about 9th magnitude may constrain the GRBs isotropic
energies to be no more than a few ergs and wind intensities to be
relatively weak.Comment: 21 pages, 6 figures, accepted by MNRAS on March 7, 200
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