astro-grb

Erik Strahler, for the IceCube Collaboration

IceCube is a km^3 scale neutrino detector being constructed deep in the
Antarctic ice. When complete, IceCube will consist of 4800 optical modules
deployed on 80 strings between 1450 and 2450 m of depth. During the 2007-2008
data taking season, 22 strings were operational. This configuration is already
much larger than previous neutrino telescopes and provides better sensitivity
to point sources of high energy (>1 TeV) neutrino emission. Such astrophysical
objects are leading candidates for the acceleration of cosmic rays. We describe
the IceCube detector and present the methods and results of several recent
searches for steady (e.g. AGN) and transient (GRB) point sources.

http://arxiv.org/abs/0905.4705

M. Nysewander, A.S. Fruchter, A. Pe'er

We present a comparative study of the observed properties of the optical and
X-ray afterglows of short- and long-duration gamma-ray bursts (GRBs). Using a
large sample of 37 short and 421 long GRBs, we find a strong correlation
between the afterglow brightness measured after 11 hours and the observed
fluence of the prompt emission. Both the optical (R band) and X-ray flux
densities (F_R and F_X) scale with the gamma-ray fluence, F_gamma. For bursts
with a known redshift, a tight correlation exists between the afterglow flux
densities at 11 hours (rest-frame) and the total isotropic gamma-ray energy,
Egi: F_{R,X} ~ Egi^{alpha}, with alpha ~ 1. The constant of proportionality is
nearly identical for long and short bursts, when Egi is obtained from the Swift
data. Additionally, we find that for short busts with F_gamma >10^{-7} erg
cm^{-2}, optical afterglows are nearly always detected by reasonably deep early
observations. Finally, we show that the ratio F_R/F_X has very similar values
for short and long bursts. These results are difficult to explain in the
framework of the standard scenario, since they require that either (1) the
number density of the surrounding medium of short bursts is typically
comparable to, or even larger than the number density of long bursts; (2) short
bursts explode into a density profile, n(r)\alpha r^{-2} or (3) the prompt
gamma-ray fluence depends on the density of the external medium. We therefore
find it likely that either basic assumptions on the properties of the
circumburst environment of short GRBs or else the standard models of GRB
emission must be re-examined. We believe that the most likely solution is that
the ambient density surrounding typical short bursts is higher than has
generally been expected: a typical value of ~1 per cm^{-3} is indicated.

http://arxiv.org/abs/0806.3607

A. Meli, J.K. Becker, J.J Quenby

Diffusive shock acceleration is invoked to explain non-thermal particle
acceleration in Supernova Remnants, Active Galactic Nuclei (AGN) Jets, Gamma
ray Bursts (GRBs) and various large scale cosmic structures. The importance of
achieving the highest observed particle energies by such a mechanism in a given
astrophysical situation is a recurring theme. In this work, shock acceleration
in relativistic shocks is discussed, mostly focusing on a numerical study
concerning proton acceleration efficiency by subluminal and superluminal
shocks, emphasising on the dependence of the scattering model, bulk Lorentz
factor and the angle between the magnetic field and the shock flow. We
developed a diffuse cosmic ray model based on the study of different shock
boost factors, which shows that spectra from AGN fit current observations of
ultra high energy cosmic rays, above 5.7 x 10^10 GeV, much better than GRBs,
indicating that AGN are the primary candidates to explain the UHECR flux.
Recent Fermi observations of GRB090816c indicate very flat spectra which are
expected within our model predictions and support evidence that GRB particle
spectra can be flat, when the shock Lorentz factor is of order ~1000.

http://arxiv.org/abs/0905.4466

Maxim V. Barkov, Anton N. Baushev

We present numerical simulations of the axisymmetric accretion of a massive
magnetized plasma torus on a rotating black hole. We use a realistic equation
of state, which takes into account neutrino cooling and energy loss due to
nucleus dissociations. We simulated various magnetic field configurations and
torus models, both optically thick and thin for neutrinos. It is shown that the
neutrino cooling does not significantly change either the structure of the
accretion flow or the total energy release of the system. The calculations
evidence heating of the wind surrounding the collapsar by the shock waves
generated at the jet-wind border. This mechanism can give rise to a hot corona
around the binary system like SS433.

Angular momentum of the accreting matter defines the time scale of the
accretion. Due to the absence of the magnetic dynamo in our calculations, the
initial strength and topology of the magnetic field determines magnetization of
the black hole, jet formation properties and the total energy yield. We
estimated the total energy transformed to jets as $1.3\times 10^{52}$ {ergs}
which was sufficient to explain hypernova explosions like GRB 980425 or GRB
030329.

http://arxiv.org/abs/0905.4440

Lado Samushia, Bharat Ratra

We use the cosmology-independent gamma-ray burst (GRB) distances of Y. Wang
(2008) to constrain dark energy cosmological model parameters. Current GRB data
alone can not tightly constrain cosmological parameters and allow for a wide
range of dark energy models. When used jointly with current Type Ia supernovae
data and baryon acoustic peak measurements, the GRB data favor slightly lower
values of nonrelativistic matter energy density. We show that with a future
factor of 2 reduction in the GRB distance errors, GRBs can give very tight
constraints on cosmological parameters.

http://arxiv.org/abs/0905.3836

Nicolas Tejos, Sebastian Lopez, J. Xavier Prochaska, Joshua S. Bloom, Hsiao-Wen Chen, Miroslava Dessauges-Zavadsky, Maria J. Maureira

We revisit echelle spectra (spectral resolution R ~ 40000) of 8 Gamma-Ray
Burst afterglows to obtain the incidence (dN/dz) of weak intervening Mg II
systems at a mean redshift of <z> = 1.5. We show that dN/dz of systems having
restframe equivalent widths 0.07 A < W_r(MgII) < 1 A toward GRBs is
statistically consistent with the incidence toward QSOs. Our result is in
contrast to the results for Mg II systems having W_r > 1 A, where dN/dz toward
GRBs has been found to be larger than toward QSOs by a factor of ~ 4. We
confirm the overdensity albeit at a factor of ~ 3 only. This suggests that any
explanation for the GRB/QSO discrepancy, be it intrinsic to the absorbers or a
selection effect, should be inherent only to the galaxies that host strong
absorbers in the line-of-sight to GRBs. We argue that, of all scenarios that
have been proposed, lensing amplification is the one that could explain the
strong Mg II enhancement while allowing for no significant enhancement in the
weak absorbers.

http://arxiv.org/abs/0905.3768

Monika Moscibrodzka, Daniel Proga

We study how axisymmetric magnetohydrodynamical (MHD) accretion flows depend
on gamma adiabatic index in the polytropic equation of state. This work is an
extension of Moscibrodzka & Proga (2008), where we investigated the gamma
dependence of 2-D Bondi-like accretion flows in the hydrodynamical (HD) limit.
Our main goal is to study if simulations for various gamma can give us insights
into to the problem of various modes of accretion observed in several types of
accretion systems such as black hole binaries (BHB), active galactic nuclei
(AGN), and gamma-ray bursts (GRBs). We find that for gamma >~ 4/3, the fast
rotating flow forms a thick torus that is supported by rotation and gas
pressure. As shown before for gamma=5/3, such a torus produces a strong,
persistent bipolar outflow that can significantly reduce the polar funnel
accretion of a slowly rotating flow. For low gamma, close to 1, the torus is
thin and is supported by rotation. The thin torus produces an unsteady outflow
which is too weak to propagate throughout the polar funnel inflow. Compared to
their HD counterparts, the MHD simulations show that the magnetized torus can
produce an outflow and does not exhibit regular oscillations. Generally, our
simulations demonstrate how the torus thickness affects the outflow production.
They also support the notion that the geometrical thickness of the torus
correlates with the power of the torus outflow. Our results, applied to
observations, suggest that the torus ability to radiatively cool and become
thin can correspond to a suppression of a jet as observed in the BHB during a
transition from a hard/low to soft/high spectral state and a transition from a
quiescent to hard/low state in AGN.

http://arxiv.org/abs/0905.3920

J. K. Cannizzo, N. Gehrels

We present a new way of looking at the very long term evolution of GRBs in
which the disk of material surrounding the putative black hole powering the GRB
jet modulates the mass flow, and hence the efficacy of the process that
extracts rotational energy from the black hole and inner accretion disk. The
pre-Swift paradigm of achromatic, shallow-to-steep “breaks” in the long term
GRB light curves has not been borne out by detailed Swift data amassed in the
past several years. We argue that, given the initial existence of a fall-back
disk near the progenitor, an unavoidable consequence will be the formation of
an “external disk” whose outer edge continually moves to larger radii due to
angular momentum transport and lack of a confining torque. The mass reservoir
at large radii moves outward with time and gives a natural power law decay to
the GRB light curves. In this model, the different canonical power law decay
segments in the GRB identified by Zhang et al. and Nousek et al. represent
different physical states of the accretion disk. We identify a physical disk
state with each power law segment.

http://arxiv.org/abs/0901.3564

Maurice H.P.M. van Putten

Ultra-high energy cosmic rays (UHECRs) and gamma-ray bursts (GRBs) are the
most exceptional nonthermal transient events, that appear to be associated with
black holes. Here, we describe radiation mechanisms induced by turbulent flows
around rapidly rotating black holes: high-energy emissions from a relativistic
capillary effect along the black hole spin-axis and low-energy emissions by
catalytic conversion of spin-energy. High-energy emissions arise, concurrently,
in photons and, upstream of an outgoing Alfv\'en front, in ionic contaminants
by linear acceleration. The latter develop into ultra-high energy cosmic rays
(UHECRs) about the Greisen-Zatsepin-Kuzmin (GZK) threshold in low-luminosity,
intermittent active galactic nuclei. These may include Seyfert galaxies and Cen
A suggested by detections of UHECRs by the Pierre Auger Observatory and, for
the latter, also of Very High Energy (VHE) gamma-rays by the High Energy
Stereoscopic System (HESS). Nearly complete spin-down of stellar mass black
holes is common to collapsars and mergers of neutron stars with another neutron
star or companion black hole. Thus, long GRBs from rotating black holes explain
events with and without supernovae and a diversity in their X-ray afterglows.
Their intrinsic exponential decay is remarkably consistent with the average of
600 light curves of long GRBs, whose total output agrees with observed peak and
true energies in gamma-rays. We conclude that long GRBs are spin-powered.
Gravitational radiation from turbulent flows in SgrA* might be of interest to
the planned Laser Interferometric Space Antenna (LISA) and, for stellar mass
black holes in GRBs, should be detectable by LIGO-Virgo. Long GRBs from naked
inner engines produced in mergers produce long-duration radio-burst that may be
seen in all-sky surveys by the Low Frequency Array (LOFAR).

http://arxiv.org/abs/0905.3367

Truong Le, Charles D. Dermer

Results of a phenomenological model to estimate the GRB detection rate by the
Fermi Gamma ray Space Telescope are reported. This estimate is based on the
BATSE 4B GRB fluence distribution, the mean ratio of fluences measured at 100
MeV - 5 GeV with EGRET and at 20 keV - 2 MeV with BATSE, and the mean EGRET GRB
spectrum for the 5 EGRET spark-chamber GRBs. For a 10% fluence ratio and a
number spectral index alpha_1 = -2 at 100 MeV - 5 GeV energies, we estimate a
rate of ~ 20 and 4 GRBs per yr in the Fermi Large Area Telescope field of view
with at least 5 photons with energy E > 100 MeV and E > 1 GeV, respectively. We
also estimate ~ 1.5 GRBs per yr in the Fermi FoV where at least 1 photon with
energy E > 10 GeV is detected. For these parameters, we estimate = 1 - 2 GRBs
per year detected with the Fermi telescope with more than 100 gamma rays with E
>~ 100$ MeV. Comparison predictions for alpha_1 = -2.2, different fluence
ratios, and the AGILE gamma-ray satellite are made. Searches for different
classes of GRBs using a diagram plotting 100 MeV - 10 GeV fluence vs. 20 keV -
20 MeV fluence is considered as a way to search for separate classes of GRBs
and, specifically, spectral differences between the short-hard and long
duration GRB classes, and for hard components in GRBs.

http://arxiv.org/abs/0807.0355