astro-grb

S. McGlynn (1,2), S. Foley (2), B. McBreen (2), L. Hanlon (2), S. McBreen (2,3), D. J. Clark (4), A. J. Dean (4), A. Martin-Carrillo (2), R. O'Connor (2) ((1) Royal Institute of Technology (KTH), Stockholm (2) University College Dublin, (3) MPE Garching, (4) University of Southampton)

(Abridged) GRB 061122 is one of the brightest GRBs detected within INTEGRAL's
field of view to date. The two gamma-ray detectors on INTEGRAL were used to
investigate the spectral characteristics of GRB 061122. A search for linear
polarisation in the prompt emission was carried out using the SPI multiple
event data in the energy range 100 keV-1 MeV.

The prompt spectrum was best fit by a combination of a blackbody and a
power–law model (the quasithermal model), with evidence for high energy
emission continuing above 8 MeV. A pseudo-redshift value of pz = 0.95 +/- 0.18
was determined using the spectral fit parameters. The isotropic energy at this
pseudo-redshift is 8.5 x 10^{52} erg. The jet opening angle was estimated to be
smaller than 2.8 deg or larger than 11.9 deg from the X-ray lightcurve. An
upper limit of 60% polarisation was determined for the prompt emission of GRB
061122, using the multiple event data.

The high energy emission observed in the spectrum may be due to the reverse
shock interacting with the GRB ejecta when it is decelerated by the circumburst
medium. This behaviour has been observed in a small fraction of GRBs to date,
but is expected to be more commonly observed by the Fermi Gamma-ray Space
Telescope. The conditions for polarisation are met if the jet opening angle is
less than 2.8 deg, but further constraints on the level of polarisation are not
possible.

http://arxiv.org/abs/0903.5218

L.G. Balazs, I. Horvath, Z. Bagoly, A. Meszaros, P. Veres

Using the discriminant analysis of the multivariate statistical analysis we
compared the distribution of the physical quantities of the optically dark and
bright GRBs, detected by the BAT and XRT on board of the Swift Satellite. We
found that the GRBs having detected optical transients (OT) have systematically
higher peak fluxes and lower HI column densities than those without OT.

http://arxiv.org/abs/0903.5275

Nayantara Gupta (IIT Mumbai)

Air shower experiments have detected cosmic ray events of energies upto 300
EeV. Most likely these cosmic rays have originated from compact objects. Their
exact sources are yet to be identified. It has been suggested before that gamma
ray bursts are possible sources of ultra-high energy cosmic rays. The two
models of gamma ray burst emissions most often discussed are the internal and
external shock models. We have calculated the proton spectrum above 60EeV from
all gamma ray bursts distributed upto a redshift of 0.02 in the internal shock
model assuming redshift and luminosity distributions consistent with
observations, log normal distributions for their values of Lorentz factors,
variability times and duration of bursts. Within the external shock model we
have calculated the proton flux above 60EeV from all nearby gamma ray bursts
assuming log normal distributions in their values of total energies, Lorentz
factors at the deceleration epoch and compared with the observed data. We find
that gamma ray bursts can produce cosmic ray proton flux comparable to the flux
observed by the Pierre Auger experiment both within the internal and external
shock models. We have also studied the dependence of the maximum proton
energies and the cooling breaks in the proton spectrum on the various
parameters like Lorentz factor, energy of the GRB fireball, variability time
(in case of internal shocks), ambient particle density (in case of external
shocks). Our results are important to understand how the various observable
parameters determine which mechanism e.g. $p\gamma$ interactions, synchrotron
cooling of protons will dominate over one another inside these sources.

http://arxiv.org/abs/0903.4733

Brian C. Thomas (Washburn U.)

Gamma-ray bursts (GRBs) are likely to have made a number of significant
impacts on the Earth during the last billion years. The gamma radiation from a
burst within a few kiloparsecs would quickly deplete much of the Earth's
protective ozone layer, allowing an increase in solar ultraviolet radiation
reaching the surface. This radiation is harmful to life, damaging DNA and
causing sunburn. In addition, NO2 produced in the atmosphere would cause a
decrease in visible sunlight reaching the surface and could cause global
cooling. Nitric acid rain could stress portions of the biosphere, but the
increased nitrate deposition could be helpful to land plants. We have used a
two-dimensional atmospheric model to investigate the effects on the Earth's
atmosphere of GRBs delivering a range of fluences, at various latitudes, at the
equinoxes and solstices, and at different times of day. We have estimated DNA
damage levels caused by increased solar UVB radiation, reduction in solar
visible light due to NO2 opacity, and deposition of nitrates through rainout of
HNO3. In this paper I give a concise review of this work and discuss current
and future work on extending and improving our estimates of the terrestrial
impact of a GRB.

http://arxiv.org/abs/0903.4710

Zhi-Ping Jin, Dong Xu, Yi-Zhong Fan, Da-Ming Wei

We present the prompt BAT and afterglow XRT data of Swift-discovered
GRB081109A up to ~5\times10^5 sec after the trigger. The ground-based optical
afterglow follow-ups are rare for this event. The temporal and spectral indices
of the X-ray afterglow emission change remarkably, which are interpreted as the
GRB outflow firstly expands into the stellar-wind-like (WIND) medium
surrounding the progenitor (for t\lesssim 300 sec after the burst), and lately
expands into the homogeneous interstellar-medium like (ISM) medium. Numerical
fit of the afterglow data indicates that for this burst the parameter of the
WIND medium, $A_*$, is ~0.01, the particle density, $n$, of the ISM-like medium
is ~0.1 cm^-3, and the WIND-ISM transition radius, $R_t$, occurs at
~3.3\times10^17 cm.

http://arxiv.org/abs/0903.4476

Weiqun Zhang (NYU), Andrew MacFadyen (NYU)

Direct multi-dimensional numerical simulation is the most reliable approach
for calculating the fluid dynamics and observational signatures of relativistic
jets in gamma-ray bursts (GRBs). We present a two-dimensional relativistic
hydrodynamic simulation of a GRB outflow during the afterglow phase, which uses
the fifth-order weighted essentially non-oscillatory scheme and adaptive mesh
refinement. Initially, the jet has a Lorentz factor of 20. We have followed its
evolution up to 150 years. Using the hydrodynamic data, we calculate
synchrotron radiation based upon standard afterglow models and compare our
results with previous analytic work. We find that the sideways expansion of a
relativistic GRB jet is a very slow process and previous analytic works have
overestimated its rate. In our computed lightcurves, a very sharp jet break is
seen and the post-break lightcurves are steeper than analytic predictions. We
find that the jet break in GRB afterglow lightcurves is mainly caused by the
missing flux when the edge of the jet is observed. The outflow becomes
nonrelativistic at the end of the Blandford-McKee phase. But it is still highly
nonspherical, and it takes a rather long time for it to become a spherical
Sedov-von Neumann-Taylor blast wave. We find that the late-time afterglows
become increasingly flatter over time. But we disagree with the common notion
that there is a sudden flattening in lightcurves due to the transition into the
Sedov-von Neumann-Taylor solution. We have also found that there is a bump in
lightcurves at very late times ($\sim 1000$ days) due to radiation from the
counter jet. We speculate that such a counter jet bump might have already been
observed in GRB 980703.

http://arxiv.org/abs/0902.2396

Yi-Zhong Fan

The hypernovae (HNe) associated with Gamma-ray Bursts (GRBs) may have a
fairly steep energy-velocity distribution, i.e., $E(\geq \beta)\propto
\beta^{-q}$ for $q<2$ and $\beta\geq \beta_o$, where $\beta$ is the velocity of
the material and $\beta_o \sim 0.1$ is the velocity of the slowest ejecta of
the HN explosion, both in units of the speed of light $(c)$. The cosmic ray
protons above the second knee but below the ankle may be accelerated by the HN
shocks in the velocity range of $\beta \sim \beta_o - 4\beta_o$. When $\beta
\leq 4\beta_o$, the radius of the shock front to the central engine is very
large and the medium decelerating the HN outflow is very likely to be
homogeneous. With this argument, we show that for $q\sim 1.7$, as inferred from
the optical modelling of SN 2003lw, the stochastic gyroresonant acceleration
model can account for the spectrum change of high energy protons around the
second knee. The self-magnetized shock acceleration model, however, yields a
too much steep spectrum that is inconsistent with the observation unless, the
medium surrounding the HN is a free wind holding up to a (unrealistic large)
radius $\sim 1-10 {\rm kpc}$ or alternatively the particle acceleration mainly
occurs in a narrow “dense” shell that terminates the free wind at a radius
$\sim 10^{19}$ cm.

http://arxiv.org/abs/0807.0029

Á. Elíasdóttir (1), J. P. U. Fynbo (1), J. Hjorth (1), C. Ledoux (2), D. Watson (1), A. C. Andersen (1), D. Malesani (1), P. M. Vreeswijk (1), J. X. Prochaska (3), J. Sollerman (1,4), A. O. Jaunsen (5) ((1) DARK, (2) ESO, (3) UCSC, (4) Stockholm, (5) U. Oslo)

We report the clear detection of the 2175A dust absorption feature in the
optical afterglow spectrum of the gamma-ray burst (GRB) GRB070802 at a redshift
of z=2.45. This is the highest redshift for a detected 2175A dust bump to date,
and it is the first clear detection of the 2175A bump in a GRB host galaxy,
while several tens of optical afterglow spectra without the bump have been
recorded in the past decade. The derived extinction curve gives A_V=0.8-1.5
depending on the assumed intrinsic slope. Of the three local extinction laws,
an LMC type extinction gives the best fit to the extinction curve of the host
of GRB070802. Besides the 2175A bump we find that the spectrum of GRB070802 is
characterized by unusually strong low-ionization metal lines and possibly a
high metallicity for a GRB sightline ([Si/H]=-0.46+/-0.38,
[Zn/H]=-0.50+/-0.68). In particular, the spectrum of GRB070802 is unique for a
GRB spectrum in that it shows clear CI absorption features, leading us to
propose a correlation between the presence of the bump and CI. The gas to dust
ratio for the host galaxy is found to be significantly lower than that of other
GRB hosts with N(HI)/A_V=(2.4+/-1.0)x10^21 cm^-2 mag^-1, which lies between
typical MW and LMC values. Our results are in agreement with the tentative
conclusion reached by Gordon et al. 2003 that the shape of the extinction
curve, in particular the presence of the bump, is affected by the UV flux
density in the environment of the dust.

http://arxiv.org/abs/0810.2897

Plamen P. Fiziev, Denitsa R. Staicova

Despite all the already existing observational data, current models still
cannot explain completely the excessive energy output and the time variability
of GRB. One of the reasons for this is the lack of a good model of the central
engine of GRB. A major problem in the proposed models with a black hole (BH) in
the center is that they don't explain the observed evidences of late time
activity of the central engine.

In this paper we are starting the search for a possible model of that central
engine as a rotating compact body of still unknown nature. The formation of
jets in the new model lies entirely on the fundamental Teukolsky Master
Equation. We demonstrate that this general model can describe the formation of
collimated GRB-jets of various forms. Some preliminary results are presented.

http://arxiv.org/abs/0902.2408

P. P. Fiziev, D. R. Staicova

We present new developments of the simple model of the central engine of GRB,
proposed recently. The model is based on minimal assumptions: some rotating
compact relativistic object at the center and stable perturbations of its
rotating gravitational field, described by Teukolsky Master Equation. We show
that using nonstandard polynomial solutions to the angular Teukolsky equation
we can describe the formation of collimated jets of various forms. Appearance
of imaginary part of the superradiance-like frequency is established for the
first time for pure vacuum black hole jet solutions of Teukolsky equation.

http://arxiv.org/abs/0902.2411