astro-grb

Bin-Bin Zhang, Bing Zhang, En-Wei Liang, Xiang-Yu Wang

The apparent spectral evolution observed in the steep decay phase of many GRB
early afterglows raises a great concern of the high-latitude “curvature
effect'' interpretation of this phase. However, previous curvature effect
models only invoked a simple power law spectrum upon the cessation of the
prompt internal emission. We investigate a model that invokes the “curvature
effect'' of a more general non-power-law spectrum and test this model with the
Swift/XRT data of some GRBs. By comparing the simulated lightcurves/spectra
with the observed ones, we show that one can reproduce both the observed
lightcurve and the apparent spectral evolution of several GRBs using a model
invoking a cut-off power-law spectrum. GRB 050814 is presented as an example.
This suggests that at least for some GRBs, the fast spectral evolution in the
steep-decay phase can be explained by the curvature effect invoking a
non-power-law spectrum.

http://arxiv.org/abs/0808.3793

K.-I. Nishikawa, Y. Mizuno, P. Hardee, H. Sol, M. Medvedev, B. Zhang, A. Nordlund, J. T. Frederiksen, G. J. Fishman, R. Preece

Nonthermal radiation observed from astrophysical systems containing
relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active galactic
nuclei (AGNs), and Galactic microquasar systems usually have power-law emission
spectra. Recent PIC simulations of relativistic electron-ion
(electron-positron) jets injected into a stationary medium show that particle
acceleration occurs within the downstream jet. In the presence of relativistic
jets, instabilities such as the Buneman instability, other two-streaming
instability, and the Weibel (filamentation) instability create collisionless
shocks, which are responsible for particle (electron, positron, and ion)
acceleration. The simulation results show that the Weibel instability is
responsible for generating and amplifying highly nonuniform, small-scale
magnetic fields. These magnetic fields contribute to the electron's transverse
deflection behind the jet head. The “jitter'' radiation from deflected
electrons in small-scale magnetic fields has different properties than
synchrotron radiation which is calculated in a uniform magnetic field. This
jitter radiation, a case of diffusive synchrotron radiation, may be important
to understand the complex time evolution and/or spectral structure in gamma-ray
bursts, relativistic jets, and supernova remnants.

http://arxiv.org/abs/0808.3781

R. Hirschi (1,2), U. Frischknecht (3), F.-K. Thielemann (3), M. Pignatari (1), C. Chiappini (4,5), S. Ekstroem (4), G. Meynet (4), A. Maeder (4) ((1) Keele University, UK; (2) IPMU, Tokyo, Japan; (3) University of Basel, CH; (4) Geneva Observatory, CH; (5) Trieste Observatory, Italy)

Massive stars played a key role in the early evolution of the Universe. They
formed with the first halos and started the re-ionisation. It is therefore very
important to understand their evolution. In this paper, we describe the strong
impact of rotation induced mixing and mass loss at very low $Z$. The strong
mixing leads to a significant production of primary nitrogen 14, carbon 13 and
neon 22. Mass loss during the red supergiant stage allows the production of
Wolf-Rayet stars, type Ib,c supernovae and possibly gamma-ray bursts (GRBs)
down to almost Z=0 for stars more massive than 60 solar masses. Galactic
chemical evolution models calculated with models of rotating stars better
reproduce the early evolution of N/O, C/O and C12/C13. We calculated the weak
s-process production induced by the primary neon 22 and obtain overproduction
factors (relative to the initial composition, Z=1.e-6) between 100-1000 in the
mass range 60-90.

http://arxiv.org/abs/0808.3723

Enrico Ramirez-Ruiz, Andrew I. MacFadyen

This paper reports on the results of a numerical investigation designed to
address how the initially anisotropic appearance of a GRB remnant is modified
by the character of the circumburst medium and by the possible presence of an
accompanying supernova (SN). Axisymmetric hydrodynamical calculations of light,
impulsive jets propagating in both uniform and inhomogeneous external media are
presented, which show that the resulting dynamics of their remnants since the
onset of the non-relativistic phase is different from the standard self-similar
solutions. Because massive star progenitors are expected to have their close-in
surroundings modified by the progenitor winds, we consider both free winds and
shocked winds as possible external media for GRB remnant evolution. Abundant
confirmation is provided here of the important notion that the morphology and
visibility of GRB remnants are determined largely by their circumstellar
environments. For this reason, their detectability is highly biased in favor of
those with massive star progenitors; although, in this class of models, the
beamed component may be difficult to identify because the GRB ejecta is
eventually swept up by the accompanying SN. The number density of asymmetric
GRB remnants in the local Universe could be, however, far larger if they expand
in a tenuous interstellar medium, as expected for some short GRB progenitor
models. In these sources, the late size of the observable, asymmetric remnant
could extend over a wide, possibly resolvable angle and may be easier to
constrain directly.

http://arxiv.org/abs/0808.3448

Yi Cao, Yu-Qing Lou

During the gravitational core collapse of a massive progenitor star which may
give rise to at least a class of gamma-ray bursts (GRBs) associated with
supernovae, a stellar core rapidly passes through a short yet important phase
of neutronization, producing a huge amount of energetic neutrinos and photons
which contribute to the total pressure within the progenitor core. The
collection of neutrinos, photons and gas materials together may be approximated
as a fluid with a polytropic index $\gamma=4/3$ under the action of
self-gravity. With a substantial generalization and using analytical and
numerical methods (Lou & Cao 2008), we recently constructed and examined
various self-similar solutions to describe collapses, rebound shocks and flows
systematically in a $\gamma=4/3$ polytropic gas mixture with spherical
symmetry, and compare our results with those of Goldreich & Weber (1980). It is
also possible to construct central void solutions without or with shocks.
Various features and characteristics of this nonlinear relativistically hot gas
dynamics, including asymptotic and exact solutions, are presented. This more
general polytropic model analysis provides the dynamic basis of understanding
the evolution of rebound shocks in supernovae (SNs) and the results may be also
utilized to benchmark hydrodynamic simulations.

http://arxiv.org/abs/0808.3523

Paul O'Brien (1), Richard Willingale (1) ((1) Department of Physics & Astronomy, University of Leicester)

We summarise the X-ray temporal and spectral variability properties of GRBs
as observed using the Swift satellite. Despite much individual complexity, the
flux and spectral variability can be reasonably well described by a combination
of two components - which we denote as the prompt and the afterglow. The first,
prompt component consists of the burst and its initial decay while the second,
afterglow component fits the X-ray plateau phase and subsequent decline
observed in the majority of GRBs. When strong spectral variability occurs it is
associated with the prompt component while the X-ray plateau and later emission
shows little if any spectral variability. We briefly compare the observations
with some of the proposed models. Any model for the early or late emission must
explain the differences in both temporal and spectral behaviour.

http://arxiv.org/abs/0808.3492

Nozomu Tominaga

Jet-induced supernovae (SNe) have been suggested to occur in gamma-ray bursts
(GRBs) and highly-energetic SNe (hypernovae). I investigate hydrodynamical and
nucleosynthetic properties of the jet-induced explosion of a population III
$40_\odot$ star with a two-dimensional special relativistic hydrodynamical
code. The abundance distribution after the explosion and the angular dependence
of the yield are obtained for the models with high and low energy deposition
rates $\dot{E}_{\rm dep}=120\times10^{51} {\rm ergs s^{-1}}$ and
$1.5\times10^{51} {\rm ergs s^{-1}}$. The ejection of Fe-peak products and the
fallback of unprocessed materials in the jet-induced SNe account for the
abundance patterns of the extremely metal-poor (EMP) stars. It is also found
that the peculiar abundance pattern of a Si-deficient metal-poor star HE
1424–0241 is reproduced by the angle-delimited yield for
$\theta=30^\circ-35^\circ$ of the model with $\dot{E}_{\rm
dep}=120\times10^{51} {\rm ergs s^{-1}}$. Furthermore, I compare the yield of
the jet-induced explosion with that of the spherical explosion and confirm the
ejection and fallback in the jet-induced explosion is almost equivalent to the
“mixing-fallback” in spherical explosions. In contrast to the spherical models,
however, the high-entropy environment is realized in the jet-induced explosion
and thus [(Sc, Ti, V, Cr, Co, Zn)/Fe] are enhanced. The enhancements of [Sc/Fe]
and [Ti/Fe] improve agreements with the abundance patterns of the EMP stars.

http://arxiv.org/abs/0711.4815

D. A. Kann

The precise localization of short/hard (Type I) gamma-ray bursts (GRBs) in
recent years has answered many questions but raised even more. I present some
results of a systematic study of the optical afterglows of long/soft (Type II)
and short/hard (Type I) GRBs, focusing on the optical luminosity as another
puzzle piece in the classification of GRBs.

http://arxiv.org/abs/0808.3221

N. Gehrels, S. D. Barthelmy, D. N. Burrows, J. K. Cannizzo, G. Chincarini, E. Fenimore, C. Kouveliotou, P. O'Brien, D. M. Palmer, J. Racusin, P. W. A. Roming, T. Sakamoto, J. Tueller, R. A. M. J. Wijers, B. Zhang

Correlation studies of prompt and afterglow emissions from gamma-ray bursts
(GRBs) between different spectral bands has been difficult to do in the past
because few bursts had comprehensive and intercomparable afterglow
measurements. In this paper we present a large and uniform data set for
correlation analysis based on bursts detected by the Swift mission. For the
first time, short and long bursts can be analyzed and compared. It is found for
both classes that the optical, X-ray and gamma-ray emissions are linearly
correlated, but with a large spread about the correlation line; stronger bursts
tend to have brighter afterglows, and bursts with brighter X-ray afterglow tend
to have brighter optical afterglow. Short bursts are, on average, weaker in
both prompt and afterglow emissions. No short bursts are seen with extremely
low optical to X-ray ratio as occurs for “dark” long bursts. Although
statistics are still poor for short bursts, there is no evidence yet for a
subgroup of short bursts with high extinction as there is for long bursts. Long
bursts are detected in the dark category at the same fraction as for pre-Swift
bursts. Interesting cases are discovered of long bursts that are detected in
the optical, and yet have low enough optical to X-ray ratio to be classified as
dark. For the prompt emission, short and long bursts have different average
tracks on flux vs fluence plots. In Swift, GRB detections tend to be fluence
limited for short bursts and flux limited for long events.

http://arxiv.org/abs/0808.3391

Sandra Savaglio (Max-Planck Institute for Extraterrestrial Physics)

Gamma-ray bursts (GRBs) are cosmologically distributed, very energetic and
very transient sources detected in the gamma-ray domain. The identification of
their x-ray and optical afterglows allowed so far the redshift measurement of
150 events, from z = 0.01 to z = 6.29. For about half of them, we have some
knowledge of the properties of the parent galaxy. At high redshift (z > 2),
absorption lines in the afterglow spectra give information on the cold
interstellar medium in the host. At low redshift (z < 1.0) multi-band
optical-NIR photometry and integrated spectroscopy reveal the GRB host general
properties. A redshift evolution of metallicity is not noticeable in the whole
sample. The typical value is a few times lower than solar. The mean host
stellar mass is similar to that of the Large Magellanic Cloud, but the mean
star formation rate is five times higher. GRBs are discovered with gamma-ray,
not optical or NIR, instruments. Their hosts do not suffer from the same
selection biases of typical galaxy surveys. Therefore, they might represent a
fair sample of the most common galaxies that existed in the past history of the
universe, and can be used to better understand galaxy formation and evolution.

http://arxiv.org/abs/0808.2917