astro-grb

Jonathan Granot

The exact profile of a gamma-ray burst (GRB) afterglow image on the plane of
the sky can provide important constraints on the underlying physics. In
particular, it can test whether the magnetic field in the emitting shocked
external medium decreases significantly with the distance behind the shock
front, or remains roughly constant. Moreover, it enables more accurate
measurements of the afterglow image size and the expected scintillation
properties. In this work analytic expressions are derived for the afterglow
image in power law segments (PLSs) of the afterglow synchrotron spectrum in
which the emission originates from a very thin layer just behind the shock,
while simple semi-analytic expressions are derived for the remaining PLSs in
which the emission arises from the bulk of the shocked fluid. In all cases the
expressions are for a general power law external density profile, and are
convenient to use in afterglow studies.

http://arxiv.org/abs/0811.3248

A. Pélangeon (LATT), J.-L. Atteia (LATT), Y. E. Nakagawa, K. Hurley (SSL), A. Yoshida, R. Vanderspek, M. Suzuki, N. Kawai, G. Pizzichini (IASF-Bo), M. Boër (OHP), J. Braga, G. Crew, T. Q. Donaghy, J. P. Dezalay (CESR), J. Doty, E. E. Fenimore (LANL), M. Galassi (LANL), C. Graziani, J. G. Jernigan (SSL), D. Q. Lamb, A. Levine, J. Manchanda, F. Martel, M. Matsuoka, J.-F. Olive (CESR), G. Prigozhin, G. R. Ricker, T. Sakamoto (NASA Goddard Space Flight Center, CRESST), Y. Shirasaki, S. Sugita, K. Takagishi, T. Tamagawa, J. Villasenor, S. E. Woosley, M. Yamauchi

Aims: Taking advantage of the forthcoming Catalog of the HETE-2 mission, the
aim of this paper is to evaluate the main properties of HETE-2 GRBs - the
E_peak, the T_90 and the E_iso - in their source frames and to derive their
unbiased distribution. Methods: We first construct a complete sample containing
all the bursts localized by the WXM on-board HETE-2, which are selected with a
uniform criterion and whose observed parameters can be constrained. We then
derive the intrinsic E_peak, T_90 and E_iso distributions using their redshift
when it is available, or their pseudo-redshift otherwise. We finally compute
the number of GRB (N_Vmax) within the visibility volume (V_max) of each GRB, in
order to derive a weight for each detected burst accounting both for the
detection significance and the star formation history of the universe. Results:
The unbiased distributions obtained clearly show the predominence of X-ray
flashes (XRFs) in the global GRB population. We also derive the rate of local
GRBs: R0^H2 > 11 Gpc-3 yr-1, which is intermediate between the local rate
obtained by considering only the high-luminosity bursts (~1 Gpc-3 yr-1) and
that obtained by including the low-luminosity bursts (>200 Gpc-3 yr-1).

http://arxiv.org/abs/0811.3304

T. Sakamoto, G. Sato, L. Barbier, S. D. Barthelmy, J. R. Cummings, E. E. Fenimore, N. Gehrels, D. Hullinger, H. A. Krimm, D. Q. Lamb, C. B. Markwardt, D. M. Palmer, A. M. Parsons, M. Stamatikos, J. Tueller, T. N. Ukwatta

We report a correlation based on a spectral simulation study of the prompt
emission spectra of gamma-ray bursts (GRBs) detected by the Swift Burst Alert
Telescope (BAT). The correlation is between the Epeak energy, which is the peak
energy in the \nu F_\nu spectrum, and the photon index (\Gamma) derived from a
simple power-law model. The Epeak - \Gamma relation, assuming the typical
smoothly broken power-law spectrum of GRBs, is \log Epeak = 3.258 - 0.829\Gamma
(1.3 < \Gamma < 2.3). We take into account not only a range of Epeak energies
and fluences, but also distributions for both the low-energy photon index and
the high-energy photon index in the smoothly broken power-law model. The
distribution of burst durations in the BAT GRB sample is also included in the
simulation. Our correlation is consistent with the index observed by BAT and
Epeak measured by the BAT, and by other GRB instruments. Since about 85% of
GRBs observed by the BAT are acceptably fit with the simple power-law model
because of the relatively narrow energy range of the BAT, this relationship can
be used to estimate Epeak when it is located within the BAT energy range.

http://arxiv.org/abs/0811.3401

Yuan-Chuan Zou, Yi-Zhong Fan, Tsvi Piran

We calculate the high energy (sub-GeV to TeV) prompt and afterglow emission
of GRB 080319B that was distinguished by a naked-eye optical flash and by an
unusual strong early X-ray afterglow. There are three possible sources for high
energy emission: the prompt optical and $\gamma$-ray photons IC scattered by
the internal shock electrons, the prompt photons IC scattered by the early
external reverse-forward shock electrons, and the higher band of the
synchrotron and the synchrotron self-Compton emission of the external shock.
There should have been in total $\sim 500$ high energy photons detectable for
the Large Area Telescope (LAT) onboard the Fermi satellite, and $> 30$ photons
of those with energy $> 10$ GeV. The $> 10$ GeV emission had a duration about
twice that of the soft $\gamma$-rays. AGILE could have observed these energetic
signals if it was not occulted by the Earth at that moment. The physical
origins of the high energy emission detected in GRB 080514B, GRB 080916C and
GRB 081024B are also discussed. These observations can be reasonably
interpreted by available high energy emission models based on our current
understanding of GRBs and afterglows.

http://arxiv.org/abs/0811.2997

Z. Bosnjak (1), F. Daigne (1 and 2), G. Dubus (3 and 1) ((1) Institut d'Astrophysique de Paris, (2) Institut Universitaire de France, (3) Laboratoire d'Astrophysique de Grenoble)

The prompt GRB emission is thought to arise from electrons accelerated in
internal shocks propagating within a highly relativistic outflow. The launch of
Fermi offers the prospect of observations with unprecedented sensitivity in
high-energy (>100 MeV) gamma-rays. The aim is to explore the predictions for HE
emission from internal shocks, taking into account both dynamical and radiative
aspects, and to deduce how HE observations constrain the properties of the
relativistic outflow. The emission is modeled by combining a time-dependent
radiative code with a dynamical code giving the evolution of the physical
conditions in the shocked regions.Synthetic lightcurves and spectra are
compared to observations. The HE emission deviates significantly from
analytical estimates, which tend to overpredict the IC component, when the time
dependence and full cross-sections are included. The exploration of the
parameter space favors the case where the dominant process in the BATSE range
is synchrotron emission. The HE component becomes stronger for weaker magnetic
fields. The HE lightcurve can display a prolonged pulse duration due to IC
emission, or even a delayed peak compared to the BATSE range.Alternatively,
having dominant IC emission in the BATSE range requires most electrons to be
accelerated into a steep power-law distribution and implies strong 2nd order IC
scattering. In this case, the BATSE and HE lightcurves are very similar. The
combined dynamical and radiative approach allows a firm appraisal of GRB HE
prompt emission. A diagnostic procedure is presented to identify from
observations the dominant emission process and derive constrains on the bulk
Lorentz factor, particle density and magnetic field of the outflow.

http://arxiv.org/abs/0811.2956

Jonathan Granot

The long lived afterglow emission that follows gamma-ray bursts (GRBs) was
predicted prior to its detection in 1997, in the X-rays, optical and radio. It
is thought to arise from the shock that is driven into the external medium as
the latter decelerates the relativistic outflow that drives the GRB, and
persists well after most of the energy in the outflow is transferred to the
shocked external medium. As the blast wave decelerates, the typical emission
frequency shifts to longer wavelength. Recent observations following the launch
of the Swift satellite challenge the traditional afterglow modeling and call
into questions some of the basic underlying concepts. This brief review
outlines some of the major strengths and weaknesses of the standard afterglow
model, as well as some of the challenges that it faces in explaining recent
data, and potential directions for future study that may eventually help
overcome some of the current difficulties.

http://arxiv.org/abs/0811.1657

A. Panaitescu

We investigate the possibility that the E_p propto E_gamma^{1/2} relation
between the peak energy E_p of the nuF_nu spectrum and energy output E_gamma
for long-duration GRBs arises from the external shock produced by the
interaction of a relativistic outflow with the ambient medium. To that aim, we
take into account the dependence of all parameters which determine E_p and
E_gamma on the radial distribution of the ambient medium density and find that
the E_p-E_gamma relation can be explained if the medium around GRBs has a
universal radial stratification. For various combinations of GRB radiative
process (synchrotron or inverse-Compton) and dissipation mechanism (reverse or
forward shock), we find that the circumburst medium must have a particle
density with a radial distribution different than the R^{-2} expected for
constant mass-loss rate and terminal speed.

http://arxiv.org/abs/0811.2163

Serguei Komissarov, Nektarios Vlahakis, Arieh Konigl, Maxim Barkov

We present a relativistic-MHD numerical study of axisymmetric, magnetically
driven jets with parameters applicable to gamma-ray burst (GRB) flows. We also
present analytic expressions for the asymptotic jet shape and other flow
parameters that agree very well with the numerical results. All
current-carrying outflows exhibit self-collimation and consequent acceleration
near the rotation axis, but unconfined outflows lose causal connectivity across
the jet and therefore do not collimate or accelerate efficiently in their outer
regions. Magnetically accelerated jets confined by an external pressure that
varies with distance with a power-law index < 2 assume a paraboloidal shape and
have an acceleration efficiency > 50%. They attain Lorentz factors > 30 on
scales 10^9-3×10^10 cm, consistent with the possibility that short/hard GRB
jets are accelerated on scales where they can be confined by moderately
relativistic winds from accretion discs, and > 100 on scales 10^10-10^12 cm,
consistent with the possibility that long/soft GRB jets are accelerated within
the envelopes of collapsing massive stars. We also find that the Lorentz factor
of a magnetically accelerated jet is approximately inversely proportional to
the opening half-angle of the poloidal streamlines. This implies that the
gamma-ray emitting components of GRB outflows are very narrow, with a
half-angle < 1 degree in regions where the Lorentz factor exceeds 100, and that
the afterglow light curves of these components would either exhibit a very
early jet break or show no jet break at all.

http://arxiv.org/abs/0811.1467

S.L. Liang, Aigen Li

We explore the extinction properties of the dust in the distant universe
through the afterglows of high-redshifted GRBs based on the “Drude” model
which, unlike previous studies, does not require a prior assumption of template
extinction laws. We select GRB070802 at z~2.45 (which shows clear evidence for
the 2175\AA extinction bump) and GRB050904 at z~6.29, the 2nd most distant GRB
observed to date. We fit their afterglow spectra to determine the extinction of
their host galaxies. We find that (1) their extinction curves differ
substantially from that of the Milky Way, the Small and Large Magellanic Clouds
(which were widely adopted as template extinction laws in literature); (2) the
2175\AA extinction feature appears to be also present in GRB050904 at z~6.29;
and (3) there does not appear to show strong evidence for a dependence of dust
extinction on redshifts. The inferred extinction curves are closely reproduced
in terms of a mixture of amorphous silicate and graphite, both of which are
expected supernova condensates and have been identified in primitive meteorites
as presolar grains originating from supernovae (which are considered as the
main source of dust at high-z).

http://arxiv.org/abs/0811.1867

A. Cucchiara, T. Jones, J. C. Charlton, D. B. Fox, D. Einsig, A. Narayanan

The startling discovery of Prochter et al. (2006) that the frequency of very
strong (W_r(2796)>1 A) MgII absorbers along gamma-ray burst (GRB) lines of
sight ([dN/dz]_{GRB} = 0.90) is more than three times the frequency along
quasar lines of sight ([dN/dz]_{QSO} = 0.24), over similar redshift ranges, has
yet to be understood. We reconsider the possibility that the excess of very
strong MgII absorbers toward GRBs is intrinsic either to the GRBs themselves or
to their immediate environment, and associated with bulk outflows with
velocities as large as v_{max} ~ 0.3c. In order to examine this hypothesis, we
accumulate a sample of 27 W_r(2796) > 1 A absorption systems found toward 81
quasars, and compare their properties to those of 9 W_r(2796)>1 A absorption
systems found toward 6 GRBs; all systems have been observed at high spectral
resolution (R = 45,000) using the Ultraviolet and Visual Echelle Spectrograph
on the Very Large Telescope. We make multiple comparisons of the absorber
properties across the two populations, testing for differences in metallicity,
ionization state, abundance patterns, dust abundance, kinematics, and phase
structure. We find no significant differences between the two absorber
populations using any of these metrics, implying that, if the excess absorbers
toward GRB lines of sight are indeed intrinsic, they must be produced in a
process which has strong similarities to the processes yielding strong MgII
systems in association with intervening galaxies. Although this may seem a
priori unlikely, given the high outflow velocities required for any intrinsic
model, we note that the same conclusion was reached, recently, with respect to
the narrow absorption line systems seen in some quasars.

http://arxiv.org/abs/0811.1382