astro-grb

Kohta Murase

We investigate neutrino emission from gamma-ray bursts (GRBs) under
alternative scenarios for prompt emission (the photospheric and synchrotron
self-Compton (SSC) scenarios) rather than the classical optically thin
synchrotron scenario. In the former scenario, we find that neutrinos from the
pp reaction can be very important at energies around 10-100 TeV. They may be
detected by IceCube/KM3Net and useful as a probe of baryon acceleration
around/below the photosphere. In the latter scenario, we may expect about EeV p
gamma neutrinos produced by soft photons. Predicted spectra are different from
that in the classical scenario, and neutrinos would be useful as one of the
clues to the nature of GRBs (the jet composition, emission radius, magnetic
field and so on).

http://arxiv.org/abs/0807.0919

T. Bretz (1), D. Dorner (1), R.M. Wagner (2), P. Sawallisch (2) ((1) Universität Würzburg, Würzburg, Germany, (2) Max-Planck-Institut für Physik, Munich, Germany)

The MAGIC telescope is an imaging atmospheric Cherenkov telescope, designed
to observe very high energy gamma-rays while achieving a low energy threshold.
One of the key science goals is fast follow-up of the enigmatic and short lived
gamma-ray bursts. The drive system for the telescope has to meet two basic
demands: (1) During normal observations, the 72-ton telescope has to be
positioned accurately, and has to track a given sky position with high
precision at a typical rotational speed in the order of one revolution per day.
(2) For successfully observing GRB prompt emission and afterglows, it has to be
powerful enough to position to an arbitrary point on the sky within a few ten
seconds and commence normal tracking immediately thereafter. To meet these
requirements, the implementation and realization of the drive system relies
strongly on standard industry components to ensure robustness and reliability.
In this paper, we describe the mechanical setup, the drive control and the
calibration of the pointing, as well as present measurements of the accuracy of
the system. We show that the drive system is mechanically able to operate the
motors with an accuracy even better than the feedback values from the axes. In
the context of future projects, envisaging telescope arrays comprising about
100 individual instruments, the robustness and scalability of the concept is
emphasized.

http://arxiv.org/abs/0810.4593

Letizia Caito, Maria Grazia Bernardini, Carlo Luciano Bianco, Maria Giovanna Dainotti, Roberto Guida, Remo Ruffini

(Shortened) CONTEXT: […] GRB060614 is the first nearby long duration GRB
clearly not associated to a bright Ib/c supernova. Moreover, its duration
(T_{90} ~ 100s) makes it hardly classifiable as a short GRB. It presents strong
similarities with GRB970228, the prototype of the new class of “fake” short
GRBs that appear to originate from the coalescence of binary neutron stars or
white dwarfs spiraled out into the galactic halo. AIMS: Within the “canonical”
GRB scenario based on the “fireshell” model, we test if GRB060614 can be a
“fake”, or, better, “disguised” short GRB. […] METHODS: We fit GRB060614
light curves in Swift's BAT (15-150keV) and XRT (0.2-10keV) energy bands.
Within the fireshell model, light curves are formed by two well defined and
different components: the Proper-GRB (P-GRB), emitted at the fireshell
transparency, and the afterglow, due to the interaction between the leftover
accelerated baryonic shell and the CBM. RESULTS: We determine the two free
parameters describing the GRB source within the fireshell model. […] A small
average CBM density […] is inferred, typical of galactic halos. The first
spikelike emission is identified with the P-GRB and the following prolonged
emission with the peak of the afterglow.[…] CONCLUSIONS: The anomalous
GRB060614 finds a natural interpretation within our canonical GRB scenario: it
is a “disguised” short GRB. […] This result points to an old binary system,
likely formed by a white dwarf and a neutron star, as the progenitor of
GRB060614 and well justify the absence of an associated supernova Ib/c.
Particularly important for further studies of the final merging process are the
temporal structures in the P-GRB down to 0.1s.

http://arxiv.org/abs/0810.4855

Pablo Cerdá-Durán, José A. Font, Luis Antón, Ewald Müller

We present a new numerical code which solves the general relativistic
magneto-hydrodynamics (GRMHD) equations coupled to the Einstein equations for
the evolution of a dynamical spacetime within the conformally-flat
approximation. This code has been developed with the main objective of studying
astrophysical scenarios in which both, high magnetic fields and strong
gravitational fields appear, such as the magneto-rotational collapse of stellar
cores, the collapsar model of GRBs, and the evolution of neutron stars. The
code is based on an existing and thoroughly tested purely hydrodynamics code
and on its extension to accommodate weakly magnetized fluids (passive magnetic
field approximation). The numerical code we present here is based on
high-resolution shock-capturing schemes to solve the GRMHD equations together
with the flux constraint transport method to ensure the solenoidal condition of
the magnetic field. Since the astrophysical applications envisaged do not
deviate much from spherical symmetry, the conformal flatness condition
approximation is used for the formulation of the Einstein equations. In
addition, the code can handle several equations of state, from simple
analytical expressions to microphysical tabulated ones. In this paper we
present stringent tests of our new GRMHD numerical code, which show its ability
to handle all aspects appearing in the astrophysical scenarios for which the
code is intended, namely relativistic shocks, highly magnetized fluids, and
equilibrium configurations of magnetized neutron stars. As an application,
magneto-rotational core collapse simulations of a realistic progenitor are
presented, comparing the results with our previous finding in the passive
magnetic field approximation.

http://arxiv.org/abs/0804.4572

Y.-P. Qin (1,2), A. C. Gupta (3,1), J. H. Fan (1), R.-J. Lu (2) ((1) Center for Astrophysics, Guangzhou University, China (2) Guangxi University, Nanning, China (3) ARIES, Nainital, India)

The softening process observed in the steep decay phase of early X-ray
afterglows of Swift bursts has remained a puzzle since its discovery. The
softening process can also be observed in the later phase of the bursts and its
cause has also been unknown. Recently, it was suggested that, influenced by the
curvature effect, emission from high latitudes would shift the Band function
spectrum from higher energy band to lower band, and this would give rise to the
observed softening process accompanied by a steep decay of the flux density.
The curvature effect scenario predicts that the terminating time of the
softening process would be correlated with the duration of the process. In this
paper, based on the data from the UNLV GRB group web-site, we found an obvious
correlation between the two quantities. In addition, we found that the
softening process can be divided into two classes: the early type softening
($t_{s,max}\leq “4000″s$) and the late type softening ($t_{s,max} > “4000″s$).
The two types of softening show different behaviors in the duration vs.
terminating time plot. In the relation between the variation rates of the flux
density and spectral index during the softening process, a discrepancy between
the two types of softening is also observed. According to their time scales and
the discrepancy between them, we propose that the two types are of different
origins: the early type is of internal shock origin and the late type is of
external shock origin. The early softening is referred to the steep decay just
following the prompt emission, whereas the late decay typically conceives the
transition from flat decay to late afterglow decay. We suspect that there might
be a great difference of the Lorentz factor in two classes which is responsible
for the observed discrepancy.

http://arxiv.org/abs/0810.4379

M. Nysewander, D. E. Reichart, J. A. Crain, A. Foster, J. Haislip, K. Ivarsen, A. Lacluyze, A. Trotter

PROMPT (Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes)
observed the early-time optical afterglow of GRB 060607A and obtained a densely
sampled multiwavelength light curve that begins only tens of seconds after the
GRB. Located at Cerro Tololo Inter-American Observatory in Chile, PROMPT is
designed to observe the afterglows of gamma-ray bursts using multiple automated
0.4-m telescopes that image simultaneously in many filters when the afterglow
is bright and may be highly variable. The data span the interval from 44
seconds after the GRB trigger to 3.3 hours in the Bgri filters. We observe an
initial peak in the light curve at approximately three minutes, followed by
rebrightenings peaking around 40 minutes and again at 66 minutes. Although our
data overlap with the early Swift gamma-ray and x-ray light curves, we do not
see a correlation between the optical and high-energy flares. We do not find
evidence for spectral evolution throughout the observations. We model the
variations in the light curves and find that the most likely cause of the
rebrightening episodes is a refreshment of the forward shock preceded by a
rapidly fading reverse shock component, although other explanations are
plausible.

http://arxiv.org/abs/0708.3444

Mikhail V. Medvedev (KU), Anatoly Spitkovsky (Princeton)

We address the question of whether numerical particle-in-cell (PIC)
simulations and laboratory laser-plasma experiments can (or will be able to, in
the near future) model realistic gamma-ray burst (GRB) shocks. For this, we
compare the radiative cooling time, t_cool, of relativistic electrons in the
shock magnetic fields to the microscopic dynamical time of collisionless
relativistic shocks — the inverse plasma frequency of protons, omega_pp^{-1}.
We obtain that for t_cool*omega_pp^{-1}\lesssim ~few hundred, the electrons
cool efficiently at or near the shock jump and are capable of emitiing away a
large fraction of the shock energy. Such shocks are well-resolved in existing
PIC simulations; therefore, the microscopic structure can be studied in detail.
Since most of the emission in such shocks would be coming from the vicinity of
the shock, the spectral power of the emitted radiation can be directly obtained
from finite-length simulations and compared with observational data. Such
radiative shocks correspond to the internal baryon-dominated GRB shocks for the
conventional range of ejecta parameters. Fermi acceleration of electrons in
such shocks is limited by electron cooling, hence the emitted spectrum should
be lacking a non-thermal tail, whereas its peak likely falls in the multi-MeV
range. Incidentally, the conditions in internal shocks are almost identical to
those in laser-produced plasmas; thus, such GRB-like plasmas can be created and
studied in laboratory experiments using the presently available Petawatt-scale
laser facilities. An analysis of the external shocks shows that only the highly
relativistic shocks, corresponding to the extremely early afterglow phase, can
have efficient electron cooling in the shock transition. We emphasize the
importance of radiative PIC simulations for further studies.

http://arxiv.org/abs/0810.4014

P. H. Tam, R. R. Xue, S. J. Wagner, B. Behera, Y. Z. Fan, D. M. Wei

Gamma-ray Bursts (GRBs) are among the potential extragalactic sources of
very-high-energy (VHE) gamma-rays. We discuss the prospects of detecting VHE
gamma-rays with current ground-based Cherenkov instruments during the afterglow
phase. Using the fireball model, we calculate the synchrotron self-Compton
(SSC) emission from forward-shock electrons. The modeled results are compared
with the observational afterglow data taken with and/or the sensitivity level
of ground-based VHE instruments (e.g. STACEE, H.E.S.S., MAGIC, VERITAS, and
Whipple). We find that modeled SSC emission from bright and nearby bursts such
as GRB 030329 are detectable by these instruments even with a delayed
observation time of ~10 hours.

http://arxiv.org/abs/0810.3894

V. D'Elia, F. Fiore, R. Perna, Y. Krongold, S. Covino, D. Fugazza, D. Lazzati, F. Nicastro, L.A. Antonelli, S. Campana, G. Chincarini, P. D'Avanzo, M. Della Valle, P. Goldoni, D. Guetta, C. Guidorzi, E.J.A. Meurs, F. Mirabel E. Molinari, L. Norci, S. Piranomonte, L. Stella, G. Stratta, G. Tagliaferri, P. Ward

GRB080319B reached 5th optical magnitude during the burst prompt emission.
Thanks to the VLT/UVES rapid response mode, we observed its afterglow just
8m:30s after the GRB onset when the magnitude was R ~ 12. This allowed us to
obtain the best signal-to-noise, high resolution spectrum of a GRB afterglow
ever (S/N per resolution element ~ 50). The spectrum is rich of absorption
features belonging to the main system at z=0.937, divided in at least six
components spanning a total velocity range of 100 km/s. The VLT/UVES
observations caught the absorbing gas in a highly excited state, producing the
strongest Fe II fine structure lines ever observed in a GRB. A few hours later
the optical depth of these lines was reduced by a factor of 4-20, and the
optical/UV flux by a factor of ~ 60. This proves that the excitation of the
observed fine structure lines is due to “pumping” by the GRB UV photons. A
comparison of the observed ratio between the number of photons absorbed by the
excited state and those in the Fe II ground state suggests that the six
absorbers are ~ 2-6 kpc from the GRB site, with component I ~ 3 times closer to
the GRB site than components III to VI. Component I is characterized also by
the lack of Mg I absorption, unlike all other components. This may be due both
to a closer distance and a lower density, suggesting a structured ISM in this
galaxy complex.

http://arxiv.org/abs/0804.2141

P. H. Tam (1), S. J. Wagner (1), G. Pühlhofer (1), the HESS Collaboration ((1) Landessternwarte-Heidelberg, Germany)

H.E.S.S. (High Energy Stereoscopic System), which is designed to detect TeV
gamma-rays, is a system of four Imaging Atmospheric Cherenkov Telescopes
situated in Namibia. The system has been shown to be very successful in
detecting and observing galactic and extra-galactic TeV sources. In order to
explore the highest energy end of GRB spectra, a GRB observing program has been
established in the H.E.S.S. collaboration. Here we introduce our GRB observing
program and report on its current status.

http://arxiv.org/abs/0810.3639