astro-grb

J. Greiner, T. Krühler, S. McBreen, M. Ajello, D. Giannos, R. Schwarz, S. Savaglio, A. Küpcü Yoldas, C. Clemens, A. Stefanescu, G. Sala, F. Bertoldi, G. Szokoly, S. Klose

We report on GROND observations of a 40 sec duration (rest-frame) optical
flare from GRB 080129 at redshift 4.349. The rise- and decay time follow a
power law with indices +12 and -8, respectively, inconsistent with a reverse
shock and a factor 10$^5$ faster than variability caused by ISM interaction.
While optical flares have been seen in the past (e.g. GRB 990123, 041219B,
060111B and 080319B), for the first time, our observations not only resolve the
optical flare into sub-components, but also provide a spectral energy
distribution from the optical to the near-infrared once every minute. The delay
of the flare relative to the GRB, its spectral energy distribution as well as
the ratio of pulse widths suggest it to arise from residual collisions in GRB
outflows \cite{liw08}.If this interpretation is correct and can be supported by
more detailed modelling or observation in further GRBs, the delay measurement
provides an independent, determination of the Lorentz factor of the outflow.

http://arxiv.org/abs/0811.4291

Felix Ryde (KTH, Stockholm), Asaf Pe'er (STScI, Baltimore)

We identify the spectral peak in the prompt emission of gamma-ray bursts
(GRBs) with a quasi blackbody component. We show that thermal photons carry a
significant fraction (~30 % to more than 50 %) of the prompt emission energy,
thereby significantly contributing to the high radiative efficiency. We study a
sample of 56 long bursts, all strong enough to allow time-resolved spectroscopy
and show that it is possible to model the spectra with a Planck function
combined with a single power-law, with the latter describing the non-thermal
component in the observed 20-2000 keV range. We analyze the evolution of both
the temperature and flux of the thermal component in 49 individual
time-resolved pulses, for which the temporal coverage is sufficient, and find a
recurring broken power-law behavior: the temperature is nearly constant during
the first few seconds, after which it decays as a power law with a
sample-averaged index of -0.68. The thermal flux first rises with an averaged
index of 0.63 after which it decays with an averaged index of -2. The break
times are the same to within errors. The ratio of the observed to the emergent
flux typically exhibits a monotoneous power-law increase during the entire
pulse and during complex bursts. We show here that the thermal emission can be
used to study the properties of the photosphere, hence the physical parameters
of the GRB fireball.

http://arxiv.org/abs/0811.4135

J. Fynbo, D. Malesani (Dark Cosmology Centre)

Long-duration gamma-ray bursts are the manifestations of massive stellar
death. Due to the immense energy release they are detectable from most of the
observable universe. In this way they allow us to study the deaths of single
(or binary) massive stars possibly throughout the full timespan massive stars
have existed in the Universe. GRBs provide a means to infer information about
the environments and typical galaxies in which massive stars are formed. Two
main obstacles remain to be crossed before the full potential of GRBs as probes
of massive stars can be harvested: i) we need to build more complete and well
understood samples in order not to be fooled by biases, and ii) we need to
understand to which extent GRBs may be intrinsically biased in the sense that
they are only formed by a limited subset of massive stars defined by most
likely a restricted metallicity interval. We describe the status of an ongoing
effort to build a more complete sample of long-duration GRBs with measured
redshifts. Already now we can conclude that the environments of GRB progenitors
are very diverse with metallicities ranging from solar to a hundredth solar and
extinction ranging from none to A_V>5 mag. We have also identified a sightline
with significant escape of Lyman continuum photons and another with a clear
2175AA extinction bump.

http://arxiv.org/abs/0811.3897

J. Fynbo, D. Malesani (Dark Cosmology Centre)

Long-duration gamma-ray bursts are the manifestations of massive stellar
death. Due to the immense energy release they are detectable from most of the
observable universe. In this way they allow us to study the deaths of single
(or binary) massive stars possibly throughout the full timespan massive stars
have existed in the Universe. GRBs provide a means to infer information about
the environments and typical galaxies in which massive stars are formed. Two
main obstacles remain to be crossed before the full potential of GRBs as probes
of massive stars can be harvested: i) we need to build more complete and well
understood samples in order not to be fooled by biases, and ii) we need to
understand to which extent GRBs may be intrinsically biased in the sense that
they are only formed by a limited subset of massive stars defined by most
likely a restricted metallicity interval. We describe the status of an ongoing
effort to build a more complete sample of long-duration GRBs with measured
redshifts. Already now we can conclude that the environments of GRB progenitors
are very diverse with metallicities ranging from solar to a hundredth solar and
extinction ranging from none to A_V>5 mag. We have also identified a sightline
with significant escape of Lyman continuum photons and another with a clear
2175AA extinction bump.

http://arxiv.org/abs/0811.3897

Weiqun Zhang, Andrew MacFadyen (NYU), Peng Wang (KIPAC, Stanford)

Magnetic field strengths inferred for relativistic outflows including
gamma-ray bursts (GRB) and active galactic nuclei (AGN) are larger than naively
expected by orders of magnitude. We present three-dimensional relativistic
magnetohydrodynamics (MHD) simulations demonstrating amplification and
saturation of magnetic field by a macroscopic turbulent dynamo triggered by the
Kelvin-Helmholtz shear instability. We find rapid growth of electromagnetic
energy due to the stretching and folding of field lines in the turbulent
velocity field resulting from non-linear development of the instability. Using
conditions relevant for GRB internal shocks and late phases of GRB afterglow,
we obtain amplification of the electromagnetic energy fraction to $\epsilon_B
\sim 5 \times 10^{-3}$. This value decays slowly after the shear is dissipated
and appears to be largely independent of the initial field strength. The
conditions required for operation of the dynamo are the presence of velocity
shear and some seed magnetization both of which are expected to be commonplace.
We also find that the turbulent kinetic energy spectrum for the case studied
obeys Kolmogorov's 5/3 law and that the electromagnetic energy spectrum is
essentially flat with the bulk of the electromagnetic energy at small scales.

http://arxiv.org/abs/0811.3638

D.M. Coward

We show how the observed gamma ray burst (GRB) optical afterglow (OA) and
redshift distributions are changing in time from selection effects. For a
subset of {\it Swift} triggered long duration bursts, we show that the mean
time taken to acquire spectroscopic redshifts for a GRB OA has evolved to
shorter times. We identify a strong correlation between the mean time taken to
acquire a spectroscopic redshift and the measured redshift. This correlation
reveals that shorter response times favour smaller redshift bursts. This is
compelling evidence for a selection effect that biases longer response times
with relatively brighter high redshift bursts. Conversely, for shorter response
times, optically fainter bursts that are relatively closer are bright enough
for spectroscopic redshifts to be acquired. This selection effect could explain
why the average redshift, $<z>\approx2.8$ measured in 2005, has evolved to
$<z>\approx2$, by mid 2008. Understanding these selection effects provides an
important tool for separating the contributions of intrinsically faint bursts,
those obscured by host galaxy dust and bursts not seen in the optical because
their OAs are observed at late times. The study highlights the importance of
rapid response telescopes capable of spectroscopy, and identifies a new
redshift selection effect that has not been considered previously, namely the
response time to measure the redshift.

http://arxiv.org/abs/0811.3443

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