astro-grb

Wei-Hong Gao, Jirong Mao, Dong Xu, Yi-Zhong Fan

We constrain the physical composition of the outflows of GRBs 080916C and
090510 with the prompt emission data and find that the former is likely
magnetic while the latter may be baryonic. The X-ray and optical afterglow
emission of both GRBs can be reasonably fitted using the standard external
shock model but the density profiles of the circum-burst medium are different.
We also propose a simple method to estimate the number of the seed photons
suppose the GeV afterglow photons are due to the inverse Compton radiation of
external forward shock electrons. The seed photons needed in the modeling are
too many to be realistic for both events. The synchrotron radiation of the
forward shock seems able to account for the GeV afterglow data.

http://arxiv.org/abs/0908.3975

Yun-Wei Yu, Xiao-Feng Cao, Xiao-Ping Zheng

The internal-plateau X-ray emission of gamma-ray bursts (GRBs) indicates that
a newly born magnetar could be the central object of some GRBs. The observed
luminosity and duration of the plateaus suggest that, for such a magnetar, a
rapid spin with a sub- or millisecond period is sometimes able to last
thousands of seconds. In this case, the conventional neutron star (NS) model
for the magnetar may be challenged, since the rapid spin of nascent NSs would
be remarkably decelerated within hundreds of seconds due to r-mode instability.
In contrast, the r-modes can be effectively suppressed in nascent strange stars
(SSs). In other words, to a certain extent, only SSs can keep nearly-constant
extremely-rapid spin for a long period of time during the early ages of the
stars. We thus propose that the sample of the GRB rapidly-spinning magnetars
can be used to test the SS hypothesis based on the distinct spin limits of NSs
and SSs.

http://arxiv.org/abs/0910.5291

Rafael da Silva de Souza, Reuven Opher

Various authors have suggested that the gamma-ray burst (GRB) central engine
is a rapidly rotating, strongly magnetized, $(\sim 10^{15}-10^{16}$ G) compact
object. The strong magnetic field can accelerate and collimate the relativistic
flow and the rotation of the compact object can be the energy source of the
GRB. The major problem in this scenario is the difficulty of finding an
astrophysical mechanism for obtaining such intense fields. Whereas, in
principle, a neutron star could maintain such strong fields, it is difficult to
justify a scenario for their creation. If the compact object is a black hole,
the problem is more difficult since, according to general relativity it has “no
hair” (i.e., no magnetic field). Schuster, Blackett, Pauli, and others have
suggested that a rotating neutral body can create a magnetic field by
non-minimal gravitational-electromagnetic coupling (NMGEC). The
Schuster-Blackett form of NMGEC was obtained from M{\o}ller's tetrad theory of
gravitation (MTTG). We call the general theory NMGEC-MTTG.

We investigate here the possible origin of the intense magnetic fields $\sim
10^{15}-10^{16}$ G in GRBs by NMGEC-MTTG. Whereas these fields are difficult to
explain astrophysically, we find that they are easily explained by NMGEC-MTTG.
It not only explains the origin of the $\sim 10^{15}-10^{16}$G fields when the
compact object is a neutron star, but also when it is a black hole.

http://arxiv.org/abs/0910.5258

M. Brassart, J.-P. Luminet

Aims: We investigate the stellar pancake mechanism during which a solar-type
star is tidally flattened within its orbital plane passing close to a 10^6
solar masses black hole. We simulate the relativistic orthogonal compression
process and follow the associated shock waves formation.

Methods: We consider a one-dimensional hydrodynamical stellar model moving in
the relativistic gravitational field of a non-rotating black hole. The model is
numerically solved using a Godunov-type shock-capturing source-splitting method
in order to correctly reproduce the shock waves profiles.

Results: Simulations confirm that the space-time curvature can induce several
successive orthogonal compressions of the star which give rise to several
strong shock waves. The shock waves finally escape from the star and repeatedly
heat up the stellar surface to high energy values. Such a shock-heating could
interestingly provide a direct observational signature of strongly disruptive
star - black hole encounters through the emission of hard X or soft gamma-ray
bursts. Timescales and energies of such a process are consistent with some
observed events such as GRB 970815.

http://arxiv.org/abs/0910.5362

Christopher C. Lindner, Milos Milosavljevic, Sean M. Couch, Pawan Kumar

We present axisymmetric hydrodynamical simulations of the long-term accretion
of a rotating GRB progenitor star, a “collapsar,” onto the central compact
object. The simulations were carried out with the adaptive mesh refinement code
FLASH in two spatial dimensions and with an explicit shear viscosity. The
evolution of the central accretion rate exhibits phases reminiscent of the long
GRB gamma-ray and X-ray light curve, which lends support to the proposal that
the luminosity is modulated by the central accretion rate. After a few tens of
seconds, an accretion shock sweeps outward through the star. The formation and
outward expansion of the accretion shock is accompanied with a sudden and rapid
power-law decline in the central accretion rate Mdot ~ t^{-2.8}, which
resembles the L_X ~ t^{-3} decline observed in the X-ray light curves. The
collapsed, shock-heated stellar envelope settles into a thick, low-mass
equatorial disk embedded within a massive, pressure-supported atmosphere. After
a few hundred seconds, the inflow of low-angular-momentum material in the axial
funnel reverses into an outflow from the surface of the thick disk. Meanwhile,
the rapid decline of the accretion rate slows down, or even settles a in steady
state with Mdot ~ 5×10^{-5} Msun/s, which resembles the “plateau” phase in the
X-ray light curve. While the duration of the “prompt” phase depends on the
resolution in our simulations, we provide an analytical model taking into
account neutrino losses that estimates the duration to be ~20 s. The model
suggests that the steep decline in GRB X-ray light curves is triggered by the
circularization of the infalling stellar envelope at radii where the virial
temperature is below ~10^{10} K, such that neutrino cooling shuts off and an
outward expansion of the accretion shock becomes imminent.

http://arxiv.org/abs/0910.4989

Limin Xiao, Bradley E. Schaefer

We are constructing a program to estimate the redshifts for GRBs from the
original Swift light curves and spectra, aiming to get redshifts for the Swift
bursts \textit{without} spectroscopic or photometric redshifts. We derive the
luminosity indicators from the light curves and spectra of each burst,
including the lag time between low and high photon energy light curves, the
variability of the light curve, the peak energy of the spectrum, the number of
peaks in the light curve, and the minimum rise time of the peaks. These
luminosity indicators can each be related directly to the luminosity, and we
combine their independent luminosities into one weighted average. Then with our
combined luminosity value, the observed burst peak brightness, and the
concordance redshift-distance relation, we can derive the redshift for each
burst. In this paper, we test the accuracy of our method on 107 bursts with
known spectroscopic redshift. The reduced $\chi^2$ of our best redshifts
($z_{best}$) compared with known spectroscopic redshifts ($z_{spec}$) is 0.86,
and the average value of $log_{10}(z_{best}/z_{spec})$ is 0.01, with this
indicating that our error bars are good and our estimates are not biased. The
RMS scatter of $log_{10}(z_{best}/z_{spec})$ is 0.26. For Swift bursts measured
over a relatively narrow energy band, the uncertainty in determining the peak
energy is one of the main restrictions on our accuracy. Although the accuracy
of our $z_{best}$ values are not as good as that of spectroscopic redshifts, it
is very useful for demographic studies, as our sample is nearly complete and
the redshifts do not have the severe selection effects associated with optical
spectroscopy.

http://arxiv.org/abs/0910.4945

The Fermi/GBM collaboration, The Fermi/LAT Collaborations, The Swift Team

We report on the observation of the bright, long gamma-ray burst, GRB
090902B, by the Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT)
instruments on-board the Fermi observatory. This was one of the brightest GRBs
to have been observed by the LAT, which detected several hundred photons during
the prompt phase. With a redshift of z = 1.822, this burst is among the most
luminous detected by Fermi. Time-resolved spectral analysis reveals a
significant power-law component in the LAT data that is distinct from the usual
Band model emission that is seen in the sub-MeV energy range. This power-law
component appears to extrapolate from the GeV range to the lowest energies and
is more intense than the Band component both below $\sim$ 50 keV and above 100
MeV. The Band component undergoes substantial spectral evolution over the
entire course of the burst, while the photon index of the power-law component
remains constant for most of the prompt phase, then hardens significantly
towards the end. After the prompt phase, power-law emission persists in the LAT
data as late as 1 ks post-trigger, with its flux declining as $t^{-1.5}$. The
LAT detected a photon with the highest energy so far measured from a GRB,
$33.4_{-3.5}^{+2.7}$ GeV. This event arrived 82 seconds after the GBM trigger
and $\sim$ 50 seconds after the prompt phase emission had ended in the GBM
band. We discuss the implications of these results for models of GRB emission
and for constraints on models of the Extragalactic Background Light.

http://arxiv.org/abs/0909.2470

En-Wei Liang, Hou-Jun Lv, Shu-Jin Hou, Bin-Bin Zhang, Bing Zhang

By analyzing the Swift/XRT lightcurves detected before 2009 July, we find 19
cases that monotonously decay as a single power law (SPL) with an index of 1 ~
1.7 from tens (or hundreds) to ~ 10^5 seconds post the GRB trigger, apparently
different from the canonical lightcurves characterized by a shallow-to-normal
decay transition. No statistical difference is found in their prompt
gamma-rays, and the X-ray properties of the two samples are also similar,
although the SPL sample tend to have a slightly lower NH value of the host
galaxies and larger energy release compared with the canonical sample. The SPL
XRT lightcurves in the burst frame gradually merge into a conflux.The normal
decay segment for the canonical sample has the same feature. Similar to the
normal decay segment, the SPL lightcurves satisfy the closure relations of
external shock models. If the X-rays are the afterglow of the GRB fireball, our
results indicate that the shallow decay would be due to energy injection and
the total energy budget after injection for both samples of GRBs is comparable.
More intriguing, we find that a prior X-ray emission model proposed by Yamazaki
is more straightforward to interpret the data. We show that the zero times (T0)
of the X-rays atisfy a log-normal distribution, and the negligible T0's of the
SPL sample are consistent with being the tail of T0 distributions at low end.
Referenced to T0, the canonical XRT lightcurves well trace the SPL lightcurves.
The T0's of the canonical lightcurves in our analysis are usually much larger
than the offsets of the known precursors from the main GRBs, indicating the
X-rays would be an emission component from external shocks prior to GRB
trigger. The lack of detection of a jet-like break in most XRT lightcurves
implies that the opening angle of its jet would be usually large.

http://arxiv.org/abs/0902.3504

Poonam Chandra, Dale A. Frail, Derek Fox, Shrinivas Kulkarni, Edo BErger, S. Bradley Cenko, Douglas C.-J. Bock, Fiona Harrison, Mansi Kasliwal

We report the discovery of radio afterglow emission from the gamma-ray burst
GRB 090423, which exploded at a redshift of 8.3, making it the object with the
highest known redshift in the Universe. By combining our radio measurements
with existing X-ray and infrared observations, we estimate the kinetic energy
of the afterglow, the geometry of the outflow and the density of the
circumburst medium. Our best fit model is a quasi-spherical, high-energy
explosion in a low, constant-density medium. \event had a similar energy
release to the other well-studied high redshift GRB 050904 ($z=6.26$), but
their circumburst densities differ by two orders of magnitude. We compare the
properties of \event with a sample of GRBs at moderate redshifts. We find that
the high energy and afterglow properties of \event are not sufficiently
different from other GRBs to suggest a different kind of progenitor, such as a
Population III star. However, we argue that it is not clear that the afterglow
properties alone can provide convincing identification of Population III
progenitors. We suggest that the millimeter and centimeter radio detections of
\event at early times contained emission from a reverse shock component. This
has important implications for the detection of high redshift GRBs by the next
generation of radio facilities.

http://arxiv.org/abs/0910.4367

P. Ubertini, A. Corsi, S. Foley, S. McGlynn, G. De Cesare, A. Bazzano (on behalf of the IBIS Team)

After more than six and half years in orbit, the ESA space observatory
INTEGRAL has provided new, exciting results in the soft gamma-ray energy range
(from a few keV to a few MeV). With the discovery of about 700 hard X-Ray
sources, it has changed our previous view of a sky composed of peculiar and
“monster'' sources. The new high energy sky is in fact full of a large variety
of normal, very energetic emitters, characterized by new accretion and
acceleration processes (see also IBIS cat4, Bird et al. 2009). If compared to
previous IBIS/ISGRI surveys it is clear that there is a continual increase in
the rate of discoveries of HMXB and AGN, including a variety of distant QSOs.
This is basically due to increased exposure away from the Galactic Plane, while
the percentage of sources without an identification has remained constant. At
the same time, about one GRB/month is detected and imaged by the two main
gamma-ray instruments on board: IBIS and SPI. INTEGRAL, after six and half
years of observations, has completed the Core Programme phase and is now fully
open to the scientific community for Open Time and Key Programme observations,
with AO7 recently announced by ESA. In this paper we review the major
achievements of the INTEGRAL Observatory in the field of Gamma Ray Bursts.

http://arxiv.org/abs/0910.4346