astro-grb

Kunihito Ioka (KEK)

We propose that a nearby gamma-ray burst (GRB) about 10^{5-6} years ago may
be responsible for the excesses of cosmic-ray positrons and electrons recently
observed by the PAMELA and ATIC/PPB-BETS experiments. The spectra have a sharp
cutoff that is similar to the dark matter predictions, possibly together with a
line (not similar), since higher energy cosmic-rays cool faster where the
cutoff/line energy marks the source age. The same is true if a source is
GRB-like (old, single and short-lived). An astrophysical source is expected to
have a small but finite spread in the cutoff/line as well as anisotropy in the
cosmic-ray flux, providing a method for the Fermi and future CALET experiments
to discriminate between dark matter and astrophysical origins.

http://arxiv.org/abs/0812.4851

F. Genet, J. Granot

Most gamma-ray bursts (GRBs) observed by the Swift satellite show an early
rapid decay phase (RDP) in their X-ray lightcurve, which is usually a smooth
continuation of the prompt gamma-ray emission, strongly suggesting that it is
its tail. However, the mechanism behind it is still not clear. The most popular
model for this RDP is High Latitude Emission (HLE). While HLE is expected in
many models for the prompt GRB emission, such as the popular internal shocks
model, there are models in which it is not expected, such as sporadic magnetic
reconnection events. Therefore, testing whether the RDP is consistent with HLE
can help distinguish between different prompt emission models. We address this
question by modeling the prompt emission as the sum of its individual pulses
with their HLE tails. Analytic expressions for the observed flux density are
obtained for power-law and Band function emission spectra. For internal shocks
the observed instantaneous spectrum is very close to the emitted one, and
should be well described by a Band function also during the RDP. Our model
naturally produces, the observed spectral softening and steepening of the flux
decay. The observed flux during the RDP is initially dominated by the tail of
the last pulse, but the tails of one or more earlier pulses can become dominant
later on. Moreover, modeling several overlapping pulses as a single wider pulse
would over-predict the emission tail. Thus, one should be very careful when
testing the predictions of HLE and do a combined temporal and spectral fit of
the prompt GRB emission and the RDP.

http://arxiv.org/abs/0812.4677

J. L. Racusin, E. W. Liang, D. N. Burrows, A. Falcone, T. Sakamoto, B. B. Zhang, B. Zhang, P. Evans, J. Osborne

We present a systematic temporal and spectral study of all Swift -XRT
observations of GRB afterglows discovered between 2005 January and 2007
December. After constructing and fitting all light curves and spectra to
power-law models, we classify the components of each afterglow in terms of the
canonical X-ray afterglow and test them against the closure relations of the
forward shock models for a variety of parameter combinations. The closure
relations are used to identify potential jet breaks with characteristics
including the uniform jet model with and without lateral spreading and energy
injection, and a power-law structured jet model, all with a range of
parameters. With this technique, we survey the X-ray afterglows with strong
evidence for jet breaks (~12% of our sample), and reveal cases of potential jet
breaks that do not appear plainly from the light curve alone (another ~30%),
leading to insight into the missing jet break problem. Those X-ray light curves
that do not show breaks or have breaks that are not consistent with one of the
jet models are explored to place limits on the times of unseen jet breaks. The
distribution of jet break times ranges from a few hours to a few weeks with a
median of ~1 day. On average Swift GRBs have lower isotropic equivalent
gamma-ray energies, which in turn results in lower collimation corrected
gamma-ray energies than those of pre-Swift GRBs. Finally, we explore the
implications for GRB jet geometry and energetics.

http://arxiv.org/abs/0812.4780

S. Savaglio (MPE), K. Glazebrook (Swinburne University), D. Le Borgne (CEA/Saclay)

We present the most extensive and complete study of the properties for the
largest sample (46 objects) of gamma-ray burst (GRB) host galaxies. The
redshift interval and the mean redshift of the sample are 0<z<6.3 and z=0.96
(look-back time: 7.2 Gyr), respectively; 89% of the hosts are at z <~ 1.6.
Optical-near-infrared (NIR) photometry and spectroscopy are used to derive
stellar masses, star formation rates (SFRs), dust extinctions and
metallicities. The average stellar mass is 10^9.3 M_sun, with a 1 sigma
dispersion of 0.8 dex. The average metallicity for a subsample of 17 hosts is
about 1/6 solar and the dust extinction in the visual band (for a subsample of
10 hosts) is A_V=0.5. We obtain new relations to derive SFR from [OII] or UV
fluxes, when Balmer emission lines are not available. SFRs, corrected for dust
extinction, aperture-slit loss and stellar Balmer absorption are in the range
0.01-36 M_sun yr^-1. The median SFR per unit stellar mass (specific SFR) is 0.8
Gyr^-1. Equivalently the inverse quantity, the median formation timescale is
1.3 Gyr. Most GRBs are associated with the death of young massive stars, more
common in star-forming galaxies. Therefore GRBs are an effective tool to detect
star-forming galaxies in the universe. Star-forming galaxies at z<1.6 are a
faint and low-mass population, hard to detect by conventional optical-NIR
surveys, unless a GRB event occurs. There is no compelling evidence that GRB
hosts are peculiar galaxies. More data on the subclass of short GRB are
necessary to establish the nature of their hosts.

http://arxiv.org/abs/0803.2718

Xinyu Dai (Univ. of Michigan)

Using the sample of long Gamma-ray bursts detected by Swift-BAT before June
2007, we measure the Log N - Log P distribution of the Swift bursts. Compared
with the BATSE sample, we find that the two distributions are consistent after
correcting the bandpass 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 at high peak flux levels suggesting
different redshift or Gamma-ray luminosity distributions. 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 fare sample of the none-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^{-1.30\pm0.06} at the high luminosity range (L_{peak} > 5E48 erg/s) and dN/dL
\propto L^{-2.5\pm0.3} at the low luminosity end confirming the existence of a
population of low luminosity GRBs. For the high redshift bin, the normalization
of the luminosity function is not higher than the low redshift counterpart
challenging the hypothesis that GRB rate follows the star formation rate.

http://arxiv.org/abs/0812.4466

Ivan Zalamea, Andrei M. Beloborodov

A fraction of neutrino emission from GRB accretion disks annihilates above
the disk, creating e+- plasma that can drive GRB explosions. We calculate the
efficiency of this annihilation using the recent detailed model of
hyper-accretion disks around Kerr black holes. Our calculation is fully
relativistic and based on a geodesic-tracing method. We find that the
efficiency is a well-defined function of (1) accretion rate and (2) spin of the
black hole. It is practically independent of the details of neutrino transport
in the opaque zone of the disk. The results help identify the accretion disks
whose neutrino emission can power GRBs.

http://arxiv.org/abs/0812.4041

Aristotle Socrates, Enrico Ramirez-Ruiz

All accretion models of gamma-ray bursts share a common assumption: accretion
power and gravitational binding energy is released and then dissipated locally,
with the mass of its origin. This is equivalent to the Shakura-Sunyaev 1973
(SS73) prescription for the dissipation of accretion power and subsequent
conversion into radiate output. Since their seminal paper, broadband
observations of quasars and black hole X-ray binaries insist that the SS73
prescription cannot wholly describe their behavior. In particular, optically
thick black hole accretion flows are almost universally accompanied by coronae
whose relative power by far exceeds anything seen in studies of stellar
chromospheric and coronal activity. In this note, we briefly discuss the
possible repercussions of freeing accretion models of GRBs from the SS73
prescription. Our main conclusion is that the efficiency of converting
gravitational binding energy into a GRB power can be increased by an order of
magnitude or more.

http://arxiv.org/abs/0812.3917

Shi Qi, Fa-Yin Wang, Tan Lu

The behavior of the dark energy equation of state (EOS) is crucial in
distinguishing different cosmological models. With a model independent
approach, we constrain the possible evolution of the dark energy EOS. Gamma-ray
bursts (GRBs) of redshifts up to $z>6$ are used, in addition to type Ia
supernovae (SNe Ia). We separate the redshifts into 4 bins and assume a
constant EOS parameter for dark energy in each bin. The EOS parameters are
decorrelated by diagonalizing the covariance matrix. And the evolution of dark
energy is estimated out of the uncorrelated EOS parameters. By including GRB
luminosity data, we significantly reduce the confidence interval of the
uncorrelated EOS parameter whose contribution mostly comes from the redshift
bin of $0.5<z<1.8$. At high redshift where we only have GRBs, the constraints
on the dark energy EOS are still very weak. However, we can see an obvious cut
at about zero in the probability plot of the EOS parameter, from which we can
infer that the ratio of dark energy to matter most probably continues to
decrease beyond redshift 1.8. We carried out analyses with and without
including the latest BAO measurements, which themselves favor a dark energy EOS
of $w<-1$. If they are included, the results show some evidence of an evolving
dark energy EOS. If not included, however, the results are consistent with the
cosmological constant within $1 \sigma$ for redshift $0<z \lesssim 0.5$ and $2
\sigma$ for $0.5 \lesssim z<1.8$.

http://arxiv.org/abs/0803.4304

Shlomo Dado, Arnon Dar

The two brightest and so far the best studied gamma ray bursts (GRBs), 990123
and 080319B, were ordinary, highly collimated GRBs produced in a core collapse
supernova explosion within a high-density wind environment and observed from a
very near-axis viewing angle. Inverse Compton scattering (ICS) and synchrotron
radiation (SR), the two dominant radiation mechanisms in the cannonball (CB)
model of GRBs, together with the burst environment, provide a very simple and
sufficiently accurate description of the multiwavelength lightcurves of their
prompt and afterglow emissions.

http://arxiv.org/abs/0812.3340

N.P.M. Kuin, W. Landsman, M.J. Page, P. Schady, M. Still, A.A. Breeveld, M. De Pasquale, P.J. Brown, M.Carter, C. James, P.A. Curran, A. Cucciara, C. Gronwall, S.T. Holland, E. Hoversten, S. Hunsberger, T. Kennedy, S. Koch, H. Lamoureux, F.E. Marshall, S.R. Oates, A. Parsons, D. Palmer, P. Roming, P.J. Smith

We present the earliest ever ultraviolet spectrum of a gamma-ray burst (GRB)
as observed with the Swift-UVOT. The spectrum of GRB 081203A was observed for
50 seconds with the UV grism starting 251 seconds after the Swift-BAT trigger
when the GRB was of u ~13.4 mag and still rising to its peak optical
brightness. The UV grism spectrum shows a damped Ly-alpha line, Ly-beta, and
the Lyman continuum break at a redshift z = 2.05 +/- 0.01. A model fit to the
Lyman absorption implies log N(HI) = 22.0 +/- 0.2 cm-2, which is typical for
GRB host galaxies with damped Ly-alpha absorbers. This observation of GRB
081203A demonstrates that for GRBs brighter than v ~14 mag and with 0.5 < z <
3.5 the UVOT will be able to provide redshifts, and probe for damped Ly-alpha
absorbers within 4-6 minutes from the time of the Swift-BAT trigger.

http://arxiv.org/abs/0812.2943