astro-grb

Shi Qi, Tan Lu, Fa-Yin Wang

Motivated by the fact that both SNe Ia and GRBs seem to prefer a dark energy
EOS greater than -1 at redshifts $z \gtrsim 0.5$, we perform a careful
investigation on this situation. We find that the deviation of dark energy from
the cosmological constant at redshifts $z \gtrsim 0.5$ is large enough that we
should pay close attention to it with future observational data. Such a
deviation may arise from some biasing systematic errors in the handling of SNe
Ia and/or GRBs or more interestingly from the nature of the dark energy itself.

http://arxiv.org/abs/0904.2832

J. I. Katz

I consider how physical processes scale over eight orders of magnitude in
black hole mass, from stellar masses in gamma-ray bursts (GRB) and black-hole
X-ray binaries (BHXRB) to supermassive active galactic nuclei (AGN). Accretion
rates onto stellar mass black holes range over more than sixteen orders of
magnitude, from the lower luminosity BHXRB to GRB. These enormous parameter
ranges correspond to qualitative as well as quantitative differences in
behavior. The fundamental questions involve the balance between nonequilibrium
and thermalized plasmas. When energy fluxes exceed a critical value $\sim
10^{29}$ erg/cm$^2$s, as in GRB, a black-body equilibrium pair plasma forms. At
the lower fluxes found in AGN, BHXRB and microquasars, accretion power
electrodynamically accelerates a small number of very energetic particles,
explaining their non-thermal spectra and the high energy gamma-ray emission of
blazars. Ultra-high energy cosmic rays may be accelerated by massive black
holes, otherwise undetectable, with very low thermal luminosities. New-born
fast high-field pulsars may be in the black-body equilibrium regime, resembling
SGR in permanent outburst. I also consider the question, significant for the
acceleration of nonthermal particles in GRB outflows, of whether collisionless
plasmas interpenetrate rather than forming hydrodynamic shocks, and propose
this as an alternative to internal shock models of GRB.

http://arxiv.org/abs/astro-ph/0603772

Xinyu Dai (Univ. of Michigan)

Using the sample of long Gamma-ray bursts (GRBs) detected by Swift-BAT before
June 2007, we measure the cumulative distribution of the peak photon fluxes
(log N - log P) of the Swift bursts. Compared with the BATSE sample, we find
that the two distributions are consistent after correcting the band pass
difference, suggesting that the two instruments are sampling the same
population of bursts. We also compare the log N - log P distributions for
sub-samples of the Swift bursts, and find evidence for a deficit (99.75%
confident) of dark bursts without optical counterparts at high peak flux
levels, suggesting different redshift or Gamma-ray luminosity distributions for
these bursts. The consistency between the log N - log P distributions for the
optically detected bursts with and without redshift measurements indicates that
the current sample of the Swift bursts with redshift measurements, although
selected heterogeneously, represents a fair sample of the non-dark bursts. We
calculate the luminosity functions of this sample in two redshift bins (z<1 and
z>1), and find a broken power-law is needed to fit the low redshift bin, where
dN/dL \propto L^{-1.27\pm0.06} for high luminosities (L_{peak} > 5e48 erg/s)
and dN/dL \propto L^{-2.3\pm0.3} at for low luminosities, confirming the
results of several studies for a population of low luminosity GRBs.

http://arxiv.org/abs/0812.4466

Vladimir Sudilovsky (MPE & Guilford College), Sandra Savaglio (MPE), Donald Smith (Guilford College)

There is nearly a factor of four difference in the number density of
intervening MgII absorbers as determined from gamma-ray burst (GRB) and quasar
lines of sight. We use a Monte-Carlo simulation to test if a dust extinction
bias can account for this discrepancy. We apply an empirically determined
relationship between dust column density and MgII rest equivalent width to
simulated quasar sight-lines and model the underlying number of quasars that
must be present to explain the published magnitude distribution of SDSS
quasars. We find that an input MgII number density dn/dz of 0.273 +- 0.002 over
the range 0.4 <= z <= 2.0 and with MgII equivalent width W_0 >= 1.0 angstroms
accurately reproduces observed distributions. From this value, we conclude that
a dust obstruction bias cannot be the sole cause of the observed discrepancy
between GRB and quasar sight-lines: this bias is likely to reduce the
discrepancy only by ~10%.

http://arxiv.org/abs/0904.3227

Vladimir Sudilovsky

There is nearly a factor of four difference in the number density of
intervening MgII absorbers as determined from gamma-ray burst (GRB) and quasar
lines of sight. We use a Monte-Carlo simulation to test if a dust extinction
bias can account for this discrepancy. We apply an empirically determined
relationship between dust column density and MgII rest equivalent width to
simulated quasar sight-lines and model the underlying number of quasars that
must be present to explain the published magnitude distribution of SDSS
quasars. We find that an input MgII number density dn/dz of 0.273 +- 0.002 over
the range 0.4 <= z <= 2.0 and with MgII equivalent width W_0 >= 1.0 angstroms
accurately reproduces observed distributions. From this value, we conclude that
a dust obstruction bias cannot be the sole cause of the observed discrepancy
between GRB and quasar sight-lines: this bias is likely to reduce the
discrepancy only by ~10%

http://arxiv.org/abs/0904.3227

Shi Qi, Tan Lu, Fa-Yin Wang

Motivated by the fact that both SNe Ia and GRBs seem to prefer a dark energy
EOS greater than -1 at redshifts $z \gtrsim 0.5$, we perform a careful
investigation on this situation. We find that the deviation of dark energy from
the cosmological constant at redshifts $z \gtrsim 0.5$ is large enough that we
should pay close attention to it with future observational data. Such a
deviation may arise from some biasing systematic errors in the handling of SNe
Ia and/or GRBs or more interestingly from the nature of the dark energy itself.

http://arxiv.org/abs/0904.2832

Bing Zhang (UNLV), Asaf Pe'er (STScI)

The composition of gamma-ray burst (GRB) ejecta is still a mystery. The
standard model invokes an initially hot “fireball” composed of baryonic matter.
Here we analyze the broad band spectra of GRB 080916C detected by the Fermi
satellite. The featureless Band-spectrum of all five epochs as well as the
detections of >~ 10 GeV photons in this burst place a strong constraint on the
prompt emission radius >~ 10^{15} cm, independent on the details of the
emission process. The lack of the detection of a thermal component as predicted
by the baryonic models strongly suggests that a significant fraction of the
outflow energy is initially not in the “fireball” form, but is likely in a
Poynting flux entrained with the baryonic matter. The ratio between the
Poynting and the baryonic flux is at least ~(15-20).

http://arxiv.org/abs/0904.2943

Xiang-Yu Wang (NJU), Zhuo Li (PKU), Zi-Gao Dai (NJU), Peter Meszaros (PSU)

Fermi observations of high-energy gamma-ray emission from GRB 080916C shows
that its spectrum is consistent with the Band function from MeV to tens of GeV.
Assuming one single emission mechanism dominates in the whole energy range, we
show that this spectrum is consistent with synchrotron origin by
shock-accelerated electrons. The simple electron inverse-Compton model and the
hadronic model are found to be less viable. In the synchrotron scenario, the
synchrotron self-Compton scattering is likely to be in the Klein-Nishina regime
and therefore the resulting high-energy emission is subdominant, even though
the magnetic field energy density is lower than that in relativistic electrons.
The Klein-Nishina inverse-Compton cooling may also affect the low-energy
electron number distribution and hence results in a low-energy synchrotron
photon spectrum $n(\nu)\propto\nu^{-1}$ below the peak energy. Under the
framework of the electron synchrotron interpretation, we constrain the shock
microphysical parameters and derive a lower limit of the upstream magnetic
fields. The detection of synchrotron emission extending to about 70 GeV in the
source frame in GRB 080916C favors the Bohm diffusive shock acceleration if the
bulk Lorentz factor of the relativistic outflow is not significantly greater
than thousands.

http://arxiv.org/abs/0903.2086

Michael Stamatikos

Gamma-ray Bursts (GRBs) are relativistic cosmological beacons of transient
high energy radiation whose afterglows span the electromagnetic spectrum.
Theoretical expectations of correlated neutrino emission position GRBs at an
astrophysical nexus for a metamorphosis in our understanding of the Cosmos.
This new dawn in the era of experimental (particle) astrophysics and cosmology
is afforded by current facilities enabling the novel astronomy of high energy
neutrinos, in concert with unprecedented electromagnetic coverage. In that
regard, GRBs represent a compelling scientific theme that may facilitate
fundamental breakthroughs in the context of Swift, Fermi and IceCube.
Scientific synergy will be achieved by leveraging the combined sensitivity of
contemporaneous ground-based and satellite observatories, thus optimizing their
collective discovery potential. Hence, the advent of GRB multi-messenger
astronomy may cement an explicit connection to fundamental physics, via nascent
cosmic windows, throughout the next decade.

http://arxiv.org/abs/0904.2755

Limin Xiao, Bradley E. Schaefer

A sample of 18 long-lag (tau_{lag} > 1 s) Gamma-Ray Bursts (GRBs) has been
drawn from our catalog of all Swift long GRBs. Four different tests are done on
this sample to test the prediction that a large fraction of long-lag GRBs are
from our Local Supercluster. The results of these four tests come out that: (1)
the distribution of these GRBs shows no tendency towards the Supergalactic
plane; (2) the distribution shows no tendency towards the Virgo or Coma
Cluster; (3) no associated bright host galaxies (m <=15) in the Local
Supercluster are found for any of the 18 GRBs; (4) 17 of these 18 GRBs have
redshifts of z>0.5, which are too far to be in the Local Supercluster. All
these results disproved the hypothesis that any significant fraction of
long-lag GRBs are from Local Supercluster. Hence these long-lag GRBs can not be
counted in the calculation of LIGO detection rates. An explanation of why we
can detect long-lag GRBs at high redshift is presented.

http://arxiv.org/abs/0904.2566