astro-grb

Shlomo Dado, Arnon Dar, A. De Rujula

Many gamma ray bursts (GRBs) have been observed with the Burst-Alert and
X-Ray telescopes of the SWIFT satellite. The successive `pulses' of these GRBs
end with a fast decline and a fast spectral softening, until they are overtaken
by another pulse, or the last pulse's decline is overtaken by a less
rapidly-varying `afterglow'. The fast decline-phase has been attributed, in the
standard fireball model of GRBs, to `high-latitude' synchrotron emission from a
collision of two conical shells. This interpretation does not agree with the
observed spectral softening. The temporal behaviour and the spectral evolution
during the fast-decline phase agree with the predictions of the cannonball
model of GRBs.

http://arxiv.org/abs/0709.4307

Warren R. Brown (1), Lisa J. Kewley (2), Margaret J. Geller (1) ((1) Smithsonian Astrophysical Observatory (2) University of Hawaii)

We demonstrate a successful strategy for identifying extremely metal poor
galaxies. Our preliminary survey of 24 candidates contains 10 metal poor
galaxies of which 4 have 12+log(O/H)<7.65, some of the lowest metallicity blue
compact galaxies known to date. Interestingly, our sample of metal poor
galaxies have systematically lower metallicity for their luminosity than
comparable samples of blue compact galaxies, dIrrs, and normal star-forming
galaxies. Our metal poor galaxies share very similar properties, however, with
the host galaxies of nearby long-duration gamma-ray bursts (GRBs), including
similar metallicity, stellar ages, and star formation rates. We use H\beta to
measure the number of OB stars present in our galaxies and estimate a
core-collapse supernova rate of ~10^-3 yr^-1. A larger sample of metal poor
galaxies may provide new clues into the environment where GRBs form and may
provide a list of potential GRB hosts.

http://arxiv.org/abs/0709.4400

N. Bucciantini (1), E. Quataert (1), J. Arons (1), B. D. Metzger (1), Todd A. Thompson (2) ((1)Astronomy Department, UC Berkeley, (2)Department of Astrophysical Sciences, Princeton)

We present time-dependent axisymmetric magnetohydrodynamic simulations of the
interaction of a relativistic magnetized wind produced by a proto-magnetar with
a surrounding stellar envelope, in the first $\sim 10$ seconds after core
collapse. We inject a super-magnetosonic wind with $\dot E = 10^{51}$ ergs
s$^{-1}$ into a cavity created by an outgoing supernova shock. A strong
toroidal magnetic field builds up in the bubble of plasma and magnetic field
that is at first inertially confined by the progenitor star. This drives a jet
out along the polar axis of the star, even though the star and the magnetar
wind are each spherically symmetric. The jet has the properties needed to
produce a long-duration gamma-ray burst (GRB). At $\sim 5$ s after core bounce,
the jet has escaped the host star and the Lorentz factor of the material in the
jet at large radii $\sim 10^{11}$ cm is similar to that in the magnetar wind
near the source. Most of the spindown power of the central magnetar escapes via
the relativistic jet. There are fluctuations in the Lorentz factor and energy
flux in the jet on $\sim 0.01-0.1$ second timescale. These may contribute to
variability in GRB emission (e.g., via internal shocks).

http://arxiv.org/abs/0707.2100

Raphael Lamon, Nicolas Produit, Frank Steiner

We search for possible time lags caused by quantum gravitational (QG) effects
using gamma-ray bursts (GRBs) detected by INTEGRAL. The advantage of this
satellite is that we have at our disposal the energy and arrival time of every
detected single photon, which enhances the precision of the time resolution. We
present a new method for seeking time lags in unbinned data using a maximum
likelihood method and support our conclusions with Monte Carlo simulations. The
analysis of the data yields a mass scale well below the Planck mass whose value
may however increase if better statistics of GRBs were available. Furthermore,
we disagree with previous studies in which a non-monotonic function of the
redshift was used to perform a linear fit.

http://arxiv.org/abs/0706.4039

D. Horan, for the VERITAS Collaboration

Many authors have predicted very-high-energy (VHE; E > 100 GeV) emission from
gamma-ray bursts (GRBs) both during the prompt phase and during the
multi-component afterglow. To date, however, there has been no definitive
detection of such emission. Recently, the Swift Satellite made the exciting
discovery that almost 50% of GRBs are accompanied by one or more X-ray flares,
which are found to occur from several seconds to many hours after the prompt
emission. The discovery of this phenomenon and the many predictions that VHE
emission should accompany these flares increases the already strong motivation
for making immediate follow-up VHE observations of GRBs. Observations of GRBs
have high priority at VERITAS, preempting any observations that may be in
progress. GRB alerts are received from the GCN via a socket connection. This is
interfaced to the VERITAS Tracking Software to minimize the time between a
notification arriving and the telescope being slewed to the GRB. We report here
on GRB observations with VERITAS and with the Whipple Telescope from 2005
through 2007.

http://arxiv.org/abs/0709.3830

Atish Kamble, L. Resmi, Kuntal Misra

The collapse of a massive star is believed to be the most probable progenitor
of a long GRB. Such a star is expected to modify its environment by stellar
wind. The effect of such a circum-stellar wind medium is expected to be seen in
the evolution of a GRB afterglow, but has so far not been conclusively found.
We claim that a signature of wind to constant density medium transition of
circum-burst medium is visible in the afterglow of GRB 050319. Along with the
optical observations of the afterglow of GRB 050319 we present a model for the
multiband afterglow of GRB 050319. We show that the break seen in optical light
curve at $\sim$ 0.02 day could be explained as being due to wind to constant
density medium transition of circum-burst medium, in which case, to our
knowledge, this could be the first ever detection of such a transition at any
given frequency band. Detection of such a transition could also serve as a
confirmation of massive star collapse scenario for GRB progenitors, independent
of supernova signatures.

http://arxiv.org/abs/0709.3561

A. Biland (1), M. Garczarczyk (2), H. Anderhub (1), V. Danielyan (3), D. Hakobyan (3), E. Lorenz (1), R. Mirzoyan (2) for the MAGIC Collaboration ((1) ETH Zurich, Switzerland; (2) MPI fuer Physik, Munich, Germany; (3) Yerevan Physics Institute, Armenia)

One of the main design goals of the MAGIC telescopes is the very fast
repositioning in case of Gamma Ray Burst (GRB) alarms, implying a low weight of
the telescope dish. This is accomplished by using a space frame made of carbon
fiber epoxy tubes, resulting in a strong but not very rigid support structure.
Therefore it is necessary to readjust the individual mirror tiles to correct
for deformations of the dish under varying gravitational load while tracking an
object. We present the concept of the Active Mirror Control (AMC) as
implemented in the MAGIC telescopes and the actual performance reached.
Additionally we show that also telescopes using a stiff structure can benefit
from using an AMC.

http://arxiv.org/abs/0709.1574

S. S. Komissarov, M. V. Barkov

In this paper we report on the early evolution of core-collapse supernova
explosion following the birth of a magnetar with the dipolar magnetic field of
B=10^{15}G and the rotational period of 2ms, which was studied by means of
axisymmetric general relativistic MHD simulations. The numerical models exhibit
highly collimated magnetically-driven jets very early on. The jets are
super-Alfvenic but remain sub-fast until the end of the simulations (t=0.2s).
The power released in the jets is about 3×10^{50}erg/s which implies the
spin-down time of ~37s. The total rotational energy of the magnetar,
E~10^{52}erg, is sufficient to drive hypernova but it is not clear as to how
large a fraction of this energy can be transfered to the stellar envelope.
Given the observed propagation speed of the jets, v_p~0.17c, they are expected
to traverse the progenitor in few seconds and after this most of the released
rotational energy would be simply carried away by these jets into the
surrounding space. Our results provide the first more or less self-consistent
numerical model of a central engine capable of producing, in the supernova
setting and on a long-term basis, collimated jets with sufficient power to
explain long duration GRBs and their afterglows. Although the flow speed of our
jets is relatively low, v_j~0.5c$, the cooling of proto-neutron star will
eventually result in much higher magnetization of its magnetospheres and
ultra-relativistic asymptotic speeds of the jets. Given the relatively long
cooling time-scale we still expect the jets to be only weakly relativistic by
the time of break out. This leads to a model of GRB jets with systematic
longitudinal variation of Lorentz factor which may have specific observational
signatures both in the prompt and the afterglow emission.

http://arxiv.org/abs/0707.0264

Kenji Toma, Kunihito Ioka, Takashi Nakamura

The late-time optical/radio afterglows of $\gamma$-ray bursts (GRBs) are
believed to be synchrotron emission of electrons accelerated in relativistic
collisionless shocks propagating in the ambient medium of the sources. However,
the fraction $f$ of electrons that are injected into the shock acceleration
remains unclear and a large number of non-accelerated thermal electrons may be
left behind. If $f<1$, the true explosion energies of GRBs are $f^{-1}$ times
larger than those commonly estimated with $f=1$. Thus the value of $f$ gives an
important constraint on the nature of the central engine of GRBs and the
physics of collisionless shocks. Although early-time radio observations can
probe the thermal electrons, they are difficult at present. We show that the
Faraday rotation effects of the thermal electrons may suppress the linear
polarization of the afterglow at frequencies higher than the absorption
frequency in the late time, if the magnetic field is ordered at least in parts,
and that $f$ can be constrained through the observation of the effects. We find
that those effects may be detected with late-time, > 1 day, polarimetry with
ALMA for a burst occurring within 1 Gpc (i.e., z ~ 0.2), if $f \sim 10^{-1}$.

http://arxiv.org/abs/0709.3332

Athina Meli (1,2), Julia K. Becker (2), John J. Quenby (3) ((1) Physics Department, National University of Athens, Greece (2) Institut fuer Physik, Universitaet Dortmund, Germany (3) Blackett Laboratory, Imperial College London, UK)

The flux of Ultra High Energy Cosmic Rays (UHECRs) at E>10^{18.5} eV is
believed to arise in plasma shock environments in extragalactic sources.
Galactic sources are not able to generate these high energies and the
distribution of charged particles is too isotropic to originate from the
galaxy. The best candidates for particle acceleration up to 10^{21} eV are
believed to be the Active Galactic Nuclei (AGN) and the Gamma Ray Bursts (GRBs)
environments. In these source types, relativistic shocks prevail. While shock
fronts in AGN jets have boost factors of Gamma ~ 10, GRBs reach Lorentz factors
of 100< Gamma <1000. In this paper, Monte Carlo studies of particle
acceleration in oblique, relativistic shocks are presented for a wide range of
Gamma and shock inclination angles. Both relativistic superluminal and
subluminal conditions are investigated. It turns out that only subluminal
shocks are efficient enough and able in accelerating particles up to 10^{21}
eV, while superluminal shocks are only effective up to ~10^{5}GeV. In the case
of subluminal shocks, it is found that although these shocks are very efficient
to the highest energies, the resulting downstream particle spectra flatten with
increasing $\Gamma$. This leads to differential spectra as flat as ~ E^{-1.5}
for boost factors of 1000, supporting latest observational evidence. Comparison
with observation suggests AGN are the more likely source of UHECR. Since AGN
outflows are less relativistic than those from GRB, their particle spectra are
close to E^-2 and the summation of their output fits the observed CR spectrum
well.

http://arxiv.org/abs/0709.3031