astro-grb

Li-Xin Li

By dividing a sample of 48 long-duration gamma-ray bursts (GRBs) into four
groups with redshift from low to high and fitting each group with the Amati
relation log Eiso = a + b log Epeak, I find that parameters a and b vary with
the mean redshift of the GRBs in each group systematically and significantly.
The results suggest that GRBs evolve strongly with the cosmic redshift and
hence are not standard candles.

http://arxiv.org/abs/0705.4401

A. Drago, G. Pagliara, J. Schaffner-Bielich

We analyse the occurrence of quiescent times in the temporal structure of the
Gamma-Ray-Bursts (GRBs) light curves. We show that if a long quiescent time is
present, it is possible to divide the total duration of GRBs into three
periods: the pre-quiescence emission, the quiescent time and the
post-quiescence emission. We then discuss a model of the GRBs inner engine
based on the formation of quark phases during the life of an hadronic star.
Within this model the pre-quiescence emission is interpreted as due to the
deconfinement of quark inside an hadronic star and the formation of 2SC quark
matter. The post-quiescence emission is due to the conversion of 2SC into the
Color-Flavor-Locking (CFL) phase. The temporal delay between these two
processes is connected with the nucleation time of the CFL phase in the 2SC
phase and it can be associated with the observed quiescent times in the GRBs
light curves. The stability of CFL cores in compact stars is also discussed.

http://arxiv.org/abs/0705.4418

Hong Li, Meng Su, Zuhui Fan, Zigao Dai, Xinmin Zhang

In this paper we analyze the constraints on the property of dark energy from
cosmological observations. We include 52 long Gamma-Ray Bursts (GRBs) data in
our study. Together with SNe Ia Gold sample, WMAP, SDSS and 2dFGRS data, we
perform global fitting using Markov Chain Monte Carlo (MCMC) technique. Dark
energy perturbations are explicitly considered. We pay particular attention to
the time evolution of the equation of state of dark energy parameterized as
w_{DE}=w_o+w_a(1-a) with a the scale factor of the universe, emphasizing the
complementarity of high redshift GRBs to other cosmological probes. It is found
that the constraints on dark energy become stringent by taking into account
high redshift GRBs, especially for w_a, which delineates the evolution of dark
energy. The best fitting dark energy model has w_0=-1.09 and w_a=0.89. The
cosmological constant with w_0=-1 and w_a=0 is well within 2\sigma range.

http://arxiv.org/abs/astro-ph/0612060

P. A. Price, A. Songaila, L. L. Cowie, J. Bell Burnell, E. Berger, A. Cucchiara, D. B. Fox, I. Hook, S. R. Kulkarni, B. Penprase, K. C. Roth, B. Schmidt

We describe the properties of the host galaxy of the gamma-ray burst
GRB060510B based on a spectrum of the burst afterglow obtained with the Gemini
North 8m telescope. The galaxy lies at a redshift of z = 4.941 making it the
fourth highest spectroscopically identified burst host. However, it is the
second highest redshift galaxy for which the quality of the spectrum permits a
detailed metallicity analysis. The neutral hydrogen column density has a
logarithmic value of 21.0–21.2 cm^-2 and the weak metal lines of Ni, S and Fe
show that the metallicity is in excess of a tenth of solar which is far above
the metallicities in damped Lyman alpha absorbers at high redshift. The
tightest constraint is from the Fe lines which place [Fe/H] in excess of -0.8.
We argue that the results suggest that metallicity bias could be a serious
problem with inferring star formation from the GRB population and consider how
future higher quality measurements could be used to resolve this question.

http://arxiv.org/abs/0705.4101

Zhuo Li, Xiang-Yu Wang, Eli Waxman, Peter Meszaros

The predicted thermal flash from SN shock breakout might have been detected
for the first time by Swift in GRB 060218/SN 2006aj. The detected thermal X-ray
emission in this event implies emergence of a trans-relativistic (TR) SN shock
with kinetic energy of E_k>1E49 erg. During TRSN shock breakout, the thermal
photons could be “accelerated” by the shock through repeated bulk Compton
scattering, forming a nonthermal gamma/X-ray component with dominant energy
over thermal one. This mechanism of “photon acceleration” at TRSN shock
breakout might also account for gamma-rays in the other similar low-luminosity
GRBs, implying that they are atypical GRBs with only TR outflows. TRSNe form a
peculiar type of SNe with large kinetic energy, >1E49 erg, in TR ejecta,
\Gamma\beta ~2.

http://arxiv.org/abs/0705.3958

{Tanuka Chattopadhyay (Dinobundhoo Coll.), Ranjeev Misra (IUCAA), Asis Kumar Chattopadhyay (Dept. of Statistics, Calcutta Univ.), Malay Naskar (NIRJAIT)

Two different multivariate clustering techniques, the K-means partitioning
method and the Dirichlet process of mixture modeling, have been applied to the
BATSE Gamma-ray burst (GRB) catalog, to obtain the optimum number of coherent
groups. In the standard paradigm, GRB are classified in only two groups, the
long and short bursts. However, for both the clustering techniques, the optimal
number of classes was found to be three, a result which is consistent with
previous statistical analysis. In this classification, the long bursts are
further divided into two groups which are primarily differentiated by their
total fluence and duration and hence are named low and high fluence GRB.
Analysis of GRB with known red-shifts and spectral parameters suggests that low
fluence GRB have nearly constant isotropic energy output of 10^{52} ergs while
for the high fluence ones, the energy output ranges from 10^{52} to 10^{54}
ergs. It is speculated that the three kinds of GRB reflect three different
origins which may be mergers of neutron star systems, white dwarf with neutron
stars and collapse of massive stars.

http://arxiv.org/abs/0705.4020

John Middleditch

Early measurements of SN 1987A indicate a beam/jet (BJ) which hit polar
ejecta (PE) to produce the “Mystery Spot” (MS). The SN flash takes an extra 8 d
to hit the MS, and this was confirmed at 2e39 ergs/s in the optical at day 8. A
ramp in luminosity starting near day 10 indicates particles from the BJ hitting
the PE, with the fastest particles traveling at 0.8 c, and an upper limit for
the optical luminosity of the MS of 5e40 ergs/s at day 20. The details of SN
1987A suggest that it resulted from a merger of two stellar cores of a common
envelope (CE) binary, i.e. a DD-initiated SN, and is thus the Rosetta Stone for
99% of MSPs in the non-core-collapsed globular clusters, GRBs, and SNe,
including all recent nearby SNe except SN 1986J and the more distant SN 2006gy.
Without having to blast through the CE of Sk -69 202, it is likely that the BJ
would have caused a full, long-soft gamma-ray burst (lGRB) upon hitting the PE,
thus DD is a mechanism which can produce lGRBs. The typical 0.5 deg collimation
of a GRB, over the 22 l-d from SN 1987A to its PE, produces ~100 s of delay,
MATCHING the observed delay of the non-prompt parts of lGRBs. Because DD must
be the dominant SN mechanism in elliptical galaxies, where only short, hard
GRBs (sGRBs) have been observed, DD without CE or PE must also produce sGRBs,
and thus the initial photon spectrum of 99% of ALL GRBs is KNOWN, and neutron
star (NS)-NS mergers may not make GRBs as we know them. Observational details
of Ia's strongly suggest that these are also DD, implying another systematic
effect in Ia Cosmology, as Ia's will appear to be Ic's when viewed from their
DD merger poles, given sufficient matter above that lost to core-collapse
(otherwise it would just beg the question of what ELSE they could possibly be).
There is no need to invent exotica to account for GRBs.

http://arxiv.org/abs/0705.3846

Luc Dessart, Adam Burrows, Eli Livne, Christian Ott

We present 2D multi-group flux-limited diffusion magnetohydrodynamics (MHD)
simulations of the Accretion-Induced Collapse (AIC) of a rapidly-rotating white
dwarf. We focus on the dynamical role of MHD processes after the formation of a
millisecond-period protoneutron star. We find that including magnetic fields
and stresses can lead to a powerful explosion with an energy of a few Bethe,
rather than a weak one of at most 0.1 Bethe, with an associated ejecta mass of
~0.1Msun, instead of a few 0.001Msun. The core is spun down by ~30% within
500ms after bounce, and the rotational energy extracted from the core is
channeled into magnetic energy that generates a strong magnetically-driven
wind, rather than a weak neutrino-driven wind. Baryon loading of the ejecta,
while this wind prevails, precludes it from becoming relativistic. This
suggests that a GRB is not expected to emerge from such AICs during the early
protoneutron star phase, except in the unlikely event that the massive white
dwarf has sufficient mass to lead to black hole formation. In addition, we
predict both negligible 56Ni-production (that should result in an
optically-dark, adiabatically-cooled explosion) and the ejection of 0.1Msun of
material with an electron fraction of 0.1-0.2. Such pollution by neutron-rich
nuclei puts strong constraints on the possible rate of such AICs. Moreover,
being free from “fallback,'' such highly-magnetized millisecond-period
protoneutron stars may later become magnetars, and the magnetically-driven
winds may later transition to Poynting-flux-dominated, relativistic winds,
eventually detectable as GRBs at cosmological distances. However, the low
expected event rate of AICs will constrain them to be, at best, a small subset
of GRB and/or magnetar progenitors.

http://arxiv.org/abs/0705.3678

Dafne Guetta, Tsvi Piran

We compare the luminosity function and rate inferred from the BATSE long
bursts peak flux distribution with those inferred from the Swift peak flux
distribution. We find that both the BATSE and the Swift peak fluxes can be
fitted by the same luminosity function and the two samples are compatible with
a population that follows the star formation rate. The estimated local long GRB
rate (without beaming corrections) varies by a factor of five from 0.05
Gpc^(-3)yr^(-1) for a rate function that has a large fraction of high redshift
bursts to 0.27 Gpc^(-3)yr^(-1) for a rate function that has many local ones. We
then turn to compare the BeppoSax/HETE2 and the Swift observed redshift
distributions and compare them with the predictions of the luminosity function
found. We find that the discrepancy between the BeppoSax/HETE2 and Swift
observed redshift distributions is only partially explained by the different
thresholds of the detectors and it may indicate strong selection effects. After
trying different forms of the star formation rate (SFR) we find that the
observed Swift redshift distribution, with more observed high redshift bursts
than expected, is inconsistent with a GRB rate that simply follows current
models for the SFR. We show that this can be explained by GRB evolution beyond
the SFR (more high redshift bursts). Alternatively this can also arise if the
luminosity function evolves and earlier bursts were more luminous or if strong
selection effects affect the redshift determination.

http://arxiv.org/abs/astro-ph/0701194

Katsuaki Asano, Susumu Inoue

In the framework of the internal shock scenario, we model the broadband
prompt emission of gamma-ray bursts (GRBs) with emphasis on the GeV-TeV bands,
utilizing Monte Carlo simulations that include various processes associated
with electrons and protons accelerated to high energies. While inverse Compton
emission from primary electrons is often dominant, different proton-induced
mechanisms can also give rise to distinct high-energy components, such as
synchrotron emission from protons, muons or secondary electron-positron pairs
injected via photomeson interactions. In some cases, they give rise to double
spectral breaks that can serve as unique signatures of ultra-high-energy
protons. We discuss the conditions favorable for such emission, and how they
are related to the production of ultra-high-energy cosmic rays and neutrinos in
internal shocks. Ongoing and upcoming observations by {\it GLAST}, atmospheric
Cerenkov telescopes and other facilities will test these expectations and
provide important information on the physical conditions in GRB outflows.

http://arxiv.org/abs/0705.2910