astro-grb

Enrico Ramirez-Ruiz, Ken-Ichi Nishikawa, Christian B. Hededal

We investigate here the effects of plasma instabilities driven by rapid
electron/positron pair cascades, which arise in the environment of GRB sources
as a result of back-scattering of a seed fraction of the original spectrum. The
injection of electron/positron pairs induces strong streaming motions in the
ambient medium. One therefore expects the pair-enriched medium ahead of the
forward shock to be strongly sheared on length scales comparable to the
radiation front thickness. Using three-dimensional particle-in-cell
simulations, we show that plasma instabilities driven by these streaming
electron/positron pairs are responsible for the excitation of
near-equipartition, turbulent magnetic fields. Our results reveal the
importance of the electromagnetic filamentation instability in ensuring an
effective coupling between electron/positron pairs and ions, and may help
explain the origin of large upstream fields in GRB shocks.

http://arxiv.org/abs/0707.4381

R. L. C. Starling (1,2), A. J. van der Horst (2), E. Rol (1), R. A. M. J. Wijers (2), C. Kouveliotou (3), K. Wiersema (2), P. A. Curran (2), P. Weltevrede (2) ((1) Leicester, (2) Amsterdam, (3) NASA/MSFC)

We constrain blastwave parameters and the circumburst media of a subsample of
ten BeppoSAX Gamma-Ray Bursts. For this sample we derive the values of the
injected electron energy distribution index, p, and the density structure index
of the circumburst medium, k, from simultaneous spectral fits to their X-ray,
optical and nIR afterglow data. The spectral fits have been done in count space
and include the effects of metallicity, and are compared with the previously
reported optical and X-ray temporal behaviour. Using the blastwave model and
some assumptions which include on-axis viewing and standard jet structure,
constant blastwave energy and no evolution of the microphysical parameters, we
find a mean value of p for the sample as a whole of 2.04 +0.02/-0.03. A
statistical analysis of the distribution demonstrates that the p values in this
sample are inconsistent with a single universal value for p at the 3-sigma
level or greater, which has significant implications for particle acceleration
models. This approach provides us with a measured distribution of circumburst
density structures rather than considering only the cases of k=0 (homogeneous)
and k=2 (wind-like). We find five GRBs for which k can be well constrained, and
in four of these cases the circumburst medium is clearly wind-like. The fifth
source has a value of 0<k<1, consistent with a homogeneous circumburst medium.

http://arxiv.org/abs/0704.3718

Volker Bromm, Abraham Loeb

Current observations are about to open up a direct window into the final
frontier of cosmology: the first billion years in cosmic history when the first
stars and galaxies formed. Even before the launch of the James Webb Space
Telescope, it might be possible to utilize Gamma-ray Bursts (GRBs) as unique
probes of cosmic star formation and the state of the intergalactic medium (IGM)
up to redshifts of several tens, when the first (Population III) stars had
formed. The Swift mission, or future satellites such as EXIST, might be the
first observatories to detect individual Population III stars, provided that
massive metal-free stars were able to trigger GRBs. Spectroscopic follow-up
observations of the GRB afterglow emission would allow to probe the ionization
state and metal enrichment of the IGM as a function of redshift.

http://arxiv.org/abs/0706.2445

Bin-Bin Zhang, En-Wei Liang, Bing Zhang

An early steep decay component following the prompt GRBs is commonly observed
in {\em Swift} XRT light curves, which is regarded as the tail emission of the
prompt gamma-rays. Prompted by the observed strong spectral evolution in the
tails of GRBs 060218 and 060614, we present a systematic time-resolved spectral
analysis for the {\em Swift} GRB tails detected between 2005 February and 2007
January. We select a sample of 44 tails that are bright enough to perform
time-resolved spectral analyses. Among them 11 tails are smooth and without
superimposing significant flares, and their spectra have no significant
temporal evolution. We suggest that these tails are dominated by the curvature
effect of the prompt gamma-rays due to delay of propagation of photons from
large angles with respect to the line of sight . More interestingly, 33 tails
show clear hard-to-soft spectral evolution, with 16 of them being smooth tails
directly following the prompt GRBs,while the others being superimposed with
large flares. We focus on the 16 clean, smooth tails and consider three toy
models to interpret the spectral evolution. The curvature effect of a
structured jet and a model invoking superposition of the curvature effect tail
and a putative underlying soft emission component cannot explain all the data.
The third model, which invokes an evolving exponential spectrum, seems to
reproduce both the lightcurve and the spectral evolution of all the bursts,
including GRBs 060218 and 060614. More detailed physical models are called for
to understand the apparent evolution effect.

http://arxiv.org/abs/astro-ph/0612246

L. Piro, J.W. den Herder, T. Ohashi, et al

How structures on various scales formed and evolved from the early Universe
up to present time is a fundamental question of astrophysical cosmology. EDGE
will trace the cosmic history of the baryons from the early generations of
massive star by Gamma-Ray Burst (GRB) explosions, through the period of cluster
formation, down to very low redshifts, when between a third and one half of the
baryons are expected to reside in cosmic filaments undergoing gravitational
collapse by dark matter (Warm Hot Intragalactic Medium: WHIM). In addition
EDGE, with its unprecedented observational capabilities, will provide key
results on several other topics. The science is feasible with a medium class
mission using existing technology combined with innovative instrumental and
observational capabilities on a single satellite by: a) observing with fast
reaction Gamma-Ray Bursts with a high spectral resolution (R ~ 500). This
enables the study of their (star-forming) environment from the Dark to the
local Universe and the use of GRB as back light of large scale cosmological
structures b) Observing and surveying extended sources (clusters, WHIM) with
high sensitivity using two wide field of view X-ray telescopes (one with a high
angular resolution and the other with a high spectral resolution).

http://arxiv.org/abs/0707.4103

G. E. Brown, C. -H. Lee, E. Moreno Mendez

We show that Nova Sco (GRO J1655-40), Il Lupi (4U 1543-47), XTE J1550-564 and
GS 2023+338 are relics of gamma-ray burst (GRB) and Hypernova explosions. They
had more than enough rotational energy to power themselves. In fact, they had
so much energy that they would have disrupted the accretion disk of the black
hole that powered them by the communicated rotational energy, so that the
energy delivery was self limiting. The most important feature in producing high
rotational energy in the binary is low donor (secondary star) mass. We suggest
that V4641 Sgr (XTE J1819-254) and GRS 1915+105 underwent less energetic
explosions; because of their large donor masses. These explosions were one or
two orders of magnitude lower in energy than that of Nova Sco. None the less
their rotational energy should have been enough to power GRBs and Hypernovae
without dismantling the black hole disk. Cyg X-1 (1956+350) had an even less
energetic explosion, because of an even larger donor mass.

http://arxiv.org/abs/0707.4008

En-Wei Liang, Bin-Bin Zhang, Bing Zhang

We analyze the properties of the shallow decay segment with a sample of 53
long Swift GRBs. We show that the distributions of its characteristics are
log-normal or normal, and its isotropic X-ray energy (E_{iso,X}) is linearly
correlated with the prompt gamma-ray energy, but with a steeper photon spectrum
except for some X-ray flashes. No significant spectral evolution is observed
from this phase to the follow-up phase, and the follow-up phase is usually
consistent with the external shock models, implying that this segment likely
due to a refreshed external shock. Within this scenario, the data are generally
consistent with a roughly constant injection luminosity up to the end of this
phase $t_b$. A positive correlation between E_{iso, X} and t_b also favors this
scenario. Among the 13 bursts that have well-sampled optical light curves, 6
have an optical break around t_b and the breaks are consistent with being
achromatic. However, the other 7 bursts either do not show an optical break or
have a break at a different epoch than t_b. This raises a concern to the energy
injection scenario, suggesting that the optical and X-ray emissions may not be
the same component at least for a fraction of bursts. There are 4 significant
outliers in the sample, and this shallow decau phase in these bursts is
immediately followed by a very steep decay after t_b, which is inconsistent
with any external shock model. The optical data of these bursts evolve
independently from the X-ray data. These X-ray plateaus likely have an internal
origin and demand continuous operation of a long-term GRB central engine. We
conclude that the observed shallow decay phase likely has diverse physical
origins(Abridged).

http://arxiv.org/abs/0705.1373

David Eichler, Hadar Manis

Gamma ray bursts (GRB's) often feature subpulses that have a distinctively
asymmetric profile — they rise quickly and decay much more slowly, while their
spectrum softens slightly with observer time. It is suggested that these
subpulses are caused by slow baryonic clouds embedded within a primary
$\gamma$-ray beam, which scatter the $\gamma$-radiation into our line of sight
as they accelerate. Good quantitative agreement is obtained with observed light
curves and spectral evolution. The kinetic energy that the baryonic component
of GRB jets receives from the primary $\gamma$-radiation is predicted to be
about equal to the amount of $\gamma$-radiation that is scattered, consistent
with observations of afterglow. Several other observational consequences are
briefly discussed. The possibility is raised that the time scale of short GRB
is established by radiative acceleration and/or baryon injection rather than
the time scale of the central engine.

http://arxiv.org/abs/0707.3635

Enwei Liang, Bing Zhang, Francisco Virgili, Z. G. Dai

Swift/BAT has detected ~ 200 long-duration GRBs, with redshift measurements
for ~50 of them. We derive the luminosity function (Phi^{HL}) and the local
event rate (rho_0^{HL}) of the conventional high luminosity (HL) GRBs by using
the z-known Swift GRBs. Our results are generally consistent with that derived
from the CGRO/BATSE data. However, the fact that Swift detected a low
luminosity (LL) GRB, GRB 060218, at z=0.033 within ~ 2 year of operation,
together with the previous detection of the nearby GRB 980425, suggests a much
higher local rate for these LL-GRBs. We explore the possibility that LL-GRBs as
a distinct GRB population from the HL-GRBs. We find that rho_0^{LL} is
325_{-177}^{+352} Gpc^{-3} yr^{-1}, which is much higher than
rho_0^{HL}(1.12_{-0.20}^{+0.43} Gpc^{-3} yr^{-1}). This rate is ~ 0.7% of the
local Type Ib/c SNe. Our results, together with the finding that less than 10%
of Type Ib/c SNe are associated with off-beam GRBs, suggest that LL-GRBs have a
beaming factor typically less than 14, or a jet angle typically wider than
31^{o}. The high local GRB rate, the small beaming factor, and low luminosity
make the LL-GRBs distinct from the HL-GRBs. Although the current data could not
fully rule out the possibility that both HL- and LL-GRBs are the same
population, our results suggest that LL-GRBs are likely a unique GRB population
and the observed low redshift GRB sample is dominated by the LL-GRBs.

http://arxiv.org/abs/astro-ph/0605200

H. J. M. Cuesta, H. Dumet M., C. Furlanetto, H. G. Khachatryan, S. Mirzoyan, G. Yegorian

Gamma-ray bursters being the most luminous among known cosmic objects carry
an essential potential for cosmological studies if properly used as standard
candles. This appears to be possible due to the use of several empirical
relations obtained for gamma-ray burst (GRBs) observables which, after a
consistent calibration for a specific model, enables one to probe current
cosmological models. In this paper we test the Gurzadyan-Xue cosmological
models of dark energy and show their compatibility to the GRBs observational
data up to redshift about 6. The exceptional fit of GX cosmology with
parameters suggests that it is a prospective one to explain the late-time
acceleration of the Universe. The analysis also confirms that GRBs can contain
more decisive information about cosmological models and parameters than
supernovae, mainly because of the huge increase in the traced cosmic scale
distance

http://arxiv.org/abs/0707.1297