astro-grb

Shi Qi, Fa-Yin Wang, Tan Lu

We explore the gamma-ray bursts' (GRBs') contributions in constraining the
dark energy equation of state (EOS) at high ($1.8 < z < 7$) and at middle
redshifts ($0.5 < z < 1.8$) and estimate how many GRBs are needed to get
substantial constraints at high redshifts. We estimate the constraints with
mock GRBs and mock type Ia supernovae (SNe Ia) for comparisons. When
constraining the dark energy EOS in a certain redshift range, we allow the dark
energy EOS parameter to vary only in that redshift bin and fix EOS parameters
elsewhere to -1. We find that it is difficult to constrain the dark energy EOS
beyond the redshifts of SNe Ia with GRBs unless some new luminosity relations
for GRBs with smaller scatters are discovered. However, at middle redshifts,
GRBs have comparable contributions with SNe Ia in constraining the dark energy
EOS.

http://arxiv.org/abs/0807.4594

A.Gomboc, S.Kobayashi, C.Guidorzi, A.Melandri, V.Mangano, B.Sbarufatti, C.G.Mundell, P.Schady, R.J.Smith, A.C.Updike, D.A.Kann, K.Misra, E.Rol, A.Pozanenko, A.J.Castro-Tirado, G.C.Anupama, D.Bersier, M.F.Bode, D.Carter, P.Curran, A.Fruchter, J.Graham, D.H.Hartmann, M.Ibrahimov, A.Levan, A. Monfardini, C.J.Mottram, P.T.O'Brien, P.Prema, D.K.Sahu, I.A.Steele, N.R.Tanvir, K.Wiersema

We present a detailed study of the prompt and afterglow emission from Swift
GRB 061126 using BAT, XRT, UVOT data and multi-color optical imaging from ten
ground-based telescopes. GRB 061126 was a long burst (T_90=191 s) with four
overlapping peaks in its gamma-ray light curve. The X-ray afterglow, observed
from 26 min to 20 days after the burst, shows a simple power-law decay with
alpha_X=1.290 \pm 0.008. Optical observations presented here cover the time
range from 258 s (Faulkes Telescope North) to 15 days (Gemini North) after the
burst; the decay rate of the optical afterglow shows a steep-to-shallow
transition (from alpha_1=1.48 \pm 0.06 to alpha_2=0.88 \pm 0.03) approximately
13 min after the burst. We suggest the early, steep component is due to a
reverse shock and show that the magnetic energy density in the ejecta,
expressed as a fraction of the equipartion value, is a few ten times larger
than in the forward shock in the early afterglow phase. The ejecta might be
endowed with primordial magnetic fields at the central engine. The optical
light curve implies a late-time break at about 1.5 days after the burst, while
there is no evidence of the simultaneous break in the X-ray light curve. We
model the broad band emission and show that some afterglow characteristics (the
steeper decay in X-ray and the shallow spectral index from optical to X-ray)
are difficult to explain in the framework of the standard fireball model. This
might imply that the X-ray afterglow is due to an additional emission process,
such as late time central engine activity rather than blast-wave shock
emission. The possible chromatic break at 1.5 days after the burst would give
support to the additional emission scenario.

http://arxiv.org/abs/0804.1727

Eric C. Bellm, Kevin Hurley, Valentin Pal'shin, Kazutaka Yamaoka, Mark S. Bandstra, Steven E. Boggs, Soojing Hong, Natsuki Kodaka, A. S. Kozyrev, M. L. Litvak, I. G. Mitrofanov, Yujin E. Nakagawa, Masanori Ohno, Kaori Onda, A. B. Sanin, Satoshi Sugita, Makoto Tashiro, V. I. Tretyakov, Yuji Urata, Claudia Wigger

The long, bright gamma-ray burst GRB 070125 was localized by the
Interplanetary Network. We present light curves of the prompt gamma-ray
emission as observed by Konus-WIND, RHESSI, Suzaku-WAM, and \textit{Swift}-BAT.
We detail the results of joint spectral fits with Konus and RHESSI data. The
burst shows moderate hard-to-soft evolution in its multi-peaked emission over a
period of about one minute. The total burst fluence as observed by Konus is
$1.79 \times 10^{-4}$ erg/cm$^2$ (20 keV–10 MeV). Using the spectroscopic
redshift $z=1.548$, we find that the burst is consistent with the “Amati''
$E_{peak,i}-E_{iso}$ correlation. Assuming a jet opening angle derived from
broadband modeling of the burst afterglow, GRB 070125 is a significant outlier
to the “Ghirlanda'' $E_{peak,i}-E_\gamma$ correlation. Its
collimation-corrected energy release $E_\gamma = 2.5 \times 10^{52}$ ergs is
the largest yet observed.

http://arxiv.org/abs/0710.4590

Elena M. Rossi (JILA), Rosalba Perna (JILA), Frédéric Daigne (IAP)

The paucity of reliable achromatic breaks in Gamma-Ray Burst afterglow light
curves motivates independent measurements of the jet aperture. Serendipitous
searches of afterglows, especially at radio wavelengths, have long been the
classic alternative. These survey data have been interpreted assuming a
uniformly emitting jet with sharp edges (“top-hat'' jet), in which case the
ratio of weakly relativistically beamed afterglows to GRBs scales with the jet
solid angle. In this paper, we consider, instead, a very wide outflow with a
luminosity that decreases across the emitting surface. In particular, we adopt
the universal structured jet (USJ) model, that is an alternative to the top-hat
model for the structure of the jet. However, the interpretation of the survey
data is very different: in the USJ model we only observe the emission within
the jet aperture and the observed ratio of prompt emission rate to afterglow
rate should solely depend on selection effects. We compute the number and rate
of afterglows expected in all-sky snapshot observations as a function of the
survey sensitivity. We find that the current (negative) results for OA searches
are in agreement with our expectations. In radio and X-ray bands this was
mainly due to the low sensitivity of the surveys, while in the optical band the
sky-coverage was not sufficient. In general we find that X-ray surveys are poor
tools for OA searches, if the jet is structured. On the other hand, the FIRST
radio survey and future instruments like the Allen Telescope Array (in the
radio band) and especially GAIA, Pan-Starrs and LSST (in the optical band) will
have chances to detect afterglows.

http://arxiv.org/abs/0711.4096

Rhaana L.C. Starling (University of Leicester, UK)

The effects that gamma-ray bursts have on their environments is an important
and outstanding issue. Dust destruction in particular has long been predicted
while observational evidence is difficult to obtain. We examine the evidence
for dust destruction by GRB 980703, in which various inconsistent measurements
of the host galaxy extinction have been made using the GRB afterglow emission.
We construct a spectral energy distribution from nIR to X-ray to measure the
extinction at early times and compare this with previous findings. We also
construct nIR/optical SEDs at intermediate epochs to examine a previously
reported decrease in extinction. The extinction is very high for a GRB host
galaxy. The earliest extinction measurement is likely to be lower than
previously estimated, and consistent with most later measurements. In a series
of SEDs we do not find any evidence of variable extinction. We therefore
conclude that there is no clear evidence of dust destruction in this case.

http://arxiv.org/abs/0807.4144

B.E. Zhilyaev, D.L. Dubinovska

Gamma-ray bursts (GRB) are the most powerful transient phenomena in the
Universe. Nowadays dozens of speculations on the origin of GRB were undertaken,
but so far a single model for the origin of, in particular, short GRBs does not
exist. The black hole (BH) - neutron star (NS) coalescence is a promising
candidate source for short GRBs. Most of binary mergers numerical simulations
were carried out with the purpose of investigating the emission of
gravitational waves. Such a scenario consists of an inspiral, merging and
ringdown phase. In this paper we present the comparison of the observational
results and analytical predictions for a test particle in a quasicircular orbit
around the BH. The emission of gravitational waves causes a rapid decrease of
the orbital radius and a rise of a {\it chirp} of radiation. Matter orbiting
the black hole would be expected to produce high-frequency oscillations (HFO).
Timescales of the coalescence process are of the order of milliseconds and
oscillation frequencies of hundreds Hz for a system with a solar mass BH
companion. We report on the detection of HFO in two short gamma-ray bursts in
this paper. The frequencies and durations of the oscillations are in agreement
with the predicted values. A {\it chirp} phenomenon is identified also. We
therefore argue in favor of BH-NS mergers as a scenario for the production of
short gamma-ray bursts.

http://arxiv.org/abs/0807.3754

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

L. Christensen, P. M. Vreeswijk, J. Sollerman, C. C. Thoene, E. Le Floc'h, K. Wiersema

The collapsar model predicts that the progenitors of Gamma-ray Bursts (GRBs)
are metal poor in Fe group elements. The existence of low metallicity stellar
populations could manifest itself through special characteristics of the
immediate environment of the GRB site in the host galaxy. We analyse the strong
emission lines from the sub-luminous host galaxy of GRB 980425, which showed
the first connection with a supernova explosion (SN 1998bw). The host has a
sufficient size to allow detailed resolved spectroscopy of individual HII
regions and to look for regions with special properties close to the the GRB
site. Using integral field spectroscopy with VIMOS we cover most of the high
surface brightness part of the host including the HII region where the
supernova and GRB occurred. The star formation rate, reddening, equivalent
width and stellar mass in the GRB region is similar to other HII regions in the
host. Extreme values arise in the only region that shows emission lines from
Wolf-Rayet stars; a region that lies 800 pc in projection from the GRB site.
Strong emission line diagnostics of all HII regions imply oxygen abundances
between 0.3 and 0.8 solar with the lowest values arising in the WR and GRB
regions. Comparing the measured emission line ratios for a sample of low
redshift GRB hosts to theoretical models and to observations of field galaxies,
we find that GRBs arise in a range of metallicity environments while the
regions are consistently very young. The similar line ratios of GRB hosts
compared to the WR region can arise in spatially unresolved galaxies with
bright HII regions near the GRB location. (Abridged)

http://arxiv.org/abs/0807.3554

Elena M. Rossi, Philip J. Armitage, Kristen Menou

Hyper-accreting discs occur in compact-object mergers and collapsars, and may
power gamma-ray bursts (GRBs). We calculate the microscopic viscosity and
resistivity of plasma in these discs, and discuss the implications for their
global structure and evolution. In the neutrino-cooled innermost regions, the
viscosity is provided mainly by mildly degenerate electrons, while the
resistivity is modified from the Spitzer value due to the effects of both
relativity and degeneracy. The plasma behaves as an almost ideal MHD fluid.
Among the non-ideal MHD effects the Hall term is relatively the most important,
while the magnetic Prandtl number, Pr (the ratio of viscosity to resistivity),
is typically larger than unity: 10 < Pr < 6000. The outer radiatively
inefficient regions also display high Pr. Numerical simulations of the
magneto-rotational instability indicate that the saturation level and angular
momentum transport efficiency may be greatly enhanced at high Pr. If this
behaviour persists in the presence of a strong Hall effect we would expect that
hyper-accreting discs should be strongly magnetised and highly variable. The
expulsion of magnetic field that cannot be dissipated at small scales may also
favour a magnetic outflow. We note the similaries between the Prandtl number in
hyper-accreting discs and X-ray binary discs, which suggests that a comparison
between late-time activity in GRBs and X-ray binary accretion states may be
fruitful. Our results imply that the behavior of high Prandtl number MHD flows
needs to be considered in studies of hyper-accreting discs.

http://arxiv.org/abs/0807.3547

S. Capozziello, L. Izzo

Relations connecting GRB quantities can be used to constrain cosmographic
parameters of the Hubble law at medium-high redshifts. We consider a sample of
64 GRBs to construct the luminosity distance - redshift relation and derive the
values of the parameters q_0$, j_0 and s_0. The results, calibrated by SNeIa
data, agree with the $\Lambda$CDM model.

http://arxiv.org/abs/0806.1120