astro-grb

Xiao-Hong Cui (PKU), En-Wei Liang (GXU), Hou-Jun Lv (GXU), Bin-Bin Zhang (UNLV), Ren-Xin Xu (PKU)

We investigate the properties of both the prompt and X-ray afterglows of
gamma-ray bursts (GRBs) in the burst frame with a sample of 33 Swift GRBs.
Assuming that the steep decay segment in the canonical X-ray afterglow
lightcurves is due to the curvature effect, we fit the lightcurves with a
broken power-law to derive the zero time of the last emission epoch of the
prompt emission (t1) and the beginning as well as the end time of the shallow
decay segment (t2 and t3).We show that both the isotropic peak gamma-ray
luminosity and gamma-ray energy are correlated with the isotropic X-ray energy
of the shallow decay phase and the isotropic X-ray luminosity at t2. We infer
the properties of the progenitor stars based on a model proposed by Kumar et
al. who suggested that both the prompt gamma-rays and the X-ray afterglows are
due to the accretions of different layers of materials of the GRB progenitor
star by a central black hole (BH). We find that most of the derived masses of
the core layers are 0.1-5 solar mass with a radius of 10^8-10^10 cm. The
rotation parameter is correlated with the burst duration, being consistent with
the expectation of collapsar models. The estimated radii and the masses of the
fall-back materials for the envelope layers are 10^10-10^12 cm and 10^-3~1
solar mass, respectively. The average accretion rates in the shallow decay
phase are correlated with those in the prompt gamma-ray phase, but they are
much lower. The derived radii of the envelope are smaller than the photospheric
radii of Wolf-Rayet (WR) stars. It is interesting that the assembled mass
density profile for the bursts in our sample is also well consistent with the
simulation for a pre-supernova star with 25 solar mass.

http://arxiv.org/abs/0909.5247

Ming Xu, Yong-Feng Huang, Tan Lu

A long plateau phase and an amazing brightness have been observed in the Xray
afterglow of GRB 060729. This peculiar light curve is likely due to long-term
energy injection in external shock. Here we present a detailed numerical study
on the energy injection process of magnetic dipole radiation from a strongly
magnetized millisecond pulsar and model the multi-band afterglow observations.
It is found that this model can successfully explain the long plateaus in the
observed X-ray and optical afterglow light curves. The sharp break following
the plateaus should be due to the rapid decline of the emission power of the
central pulsar. At an even late time (~5*10^6s), an obvious jet break appears,
which implies a relatively large half opening angle of theta~0.3 for the GRB
ejecta. Due to the energy injection, the Lorentz factor of the outflow is still
larger than two 10^7s post the GRB trigger, making the X-ray afterglow of this
burst detectable by Chandra even 642 days after the burst.

http://arxiv.org/abs/0909.5318

G. V. Lipunova, E. S. Gorbovskoy, A. I. Bogomazov, V. M. Lipunov

We study statistical properties of long gamma-ray bursts (GRBs) produced by
the collapsing cores of WR stars in binary systems. Fast rotation of the cores
enables a two-stage collapse scenario, implying the formation of a spinar-like
object. A burst produced by such a collapse consists of two pulses, whose
energy budget is enough to explain observed GRBs. We calculate models of spinar
evolution using results from a population synthesis of binary systems (done by
the `Scenario Machine') as initial parameters for the rotating massive cores.
Among the resulting bursts, events with the weaker first peak, namely,
precursor, are identified, and the precursor-to-main-pulse time separations
fully agree with the range of the observed values. The calculated fraction of
long GRBs with precursor (about 10 per cent of the total number of long GRBs)
and the durations of the main pulses are also consistent with observations.
Precursors with lead times greater by up to one order of magnitude than those
observed so far are expected to be about twice less numerous. Independently of
a GRB model assumed, we predict the existence of precursors that arrive up to
>~ 10^3 s in advance of the main events of GRBs.

http://arxiv.org/abs/0903.3169

E. Thrane, for the Super-Kamiokande Collaboration

It has been hypothesized that large fluxes of neutrinos may be created in
astrophysical “cosmic accelerators.” The primary background for a search for
astrophysical neutrinos comes from atmospheric neutrinos, which do not exhibit
the pointlike directional clustering that characterizes a distant astrophysical
signal. We perform a search for neutrino point sources using the upward-going
muon data from three phases of operation (SK-I, SK-II, and SK-III) spanning
2623 days of live time taken from April 1, 1996 to August 11, 2007. The search
looks for signals from suspected galactic and extragalactic sources, transient
sources, and unexpected sources. We find interesting signatures from two
objects–RX J1713.7-3946 (97.5% CL) and GRB 991004D (95.3% CL)–but the
signatures lack compelling statistical significance given trial factors. We set
limits on the flux and fluence of neutrino point sources above energies of 1.6
GeV.

http://arxiv.org/abs/0907.1594

Ehud Nakar, Shin'ichiro Ando, Re'em Sari

We present analytic approximations to the optically thin synchrotron and
synchrotron self-Compton (SSC) spectra when Klein-Nishina (KN) effects are
important and pair production and external radiation fields can be neglected.
This theory is useful for analytical treatment of radiation from astrophysical
sources, such as gamma-ray bursts (GRBs), active galactic nuclei and pulsar
wind nebula, where KN effects may be important. We consider a source with a
continuous injection of relativistic electrons with a power-law energy
distribution above some typical injection energy. We find that the
synchrotron-SSC spectra can be described by a broken power-law, and provide
analytic estimates for the break frequencies and power-law indices. In general,
we show that the dependence of the KN cross-section on the energy of the
upscattering electron results in a hardening of the energy distribution of fast
cooling electrons and therefore in a hardening of the observed synchrotron
spectrum. As a result the synchrotron spectrum of fast cooling electrons, below
the typical injection energy, can be as hard as $F_\nu \propto \nu^0$, instead
of the classical $\nu^{-1/2}$ when KN effects are neglected. The synchrotron
energy output can be dominated by electrons with energy above the typical
injection energy. We solve self-consistently for the cooling frequency and find
that the transition between synchrotron and SSC cooling can result in a
discontinuous variations of the cooling frequency and the synchrotron and SSC
spectra. We demonstrate the application of our results to theory by applying
them to prompt and afterglow emission models of GRBs.

http://arxiv.org/abs/0903.2557

L. Amati, F. Frontera, C. Guidorzi

The extremely energetic Fermi GRBs 080916C, with its Eiso of ~ 10^{55} erg in
1 keV - 10 GeV and intense GeV emission, and 090323 give us a unique
opportunity to test the reliability and extension of spectral energy
correlations. Based on Konus/WIND and Fermi spectral measurements, we find that
both events are fully consistent with the updated (95 events as of April 2009)
Ep,i - Eiso correlation, thus further confirming and extending it and pointing
against a possible flattening or increased dispersion at very high energies.
This also suggests that the physics behind the emission of peculiarly bright
and hard GRBs is the same as for softer and weaker ones. In addition, we find
that the normalization of the correlation obtained by considering these two
GRBs and the other long ones for which Ep,i was measured with high accuracy by
the Fermi/GBM are fully consistent with those obtained by other instruments
(e.g., BeppoSAX, Swift, Konus-WIND), thus indicating that the correlation is
not affected significantly by detectors limited thresholds and energy bands.
Prompted by the extension of the spectrum of GRB 080916C up to several GeVs
without any excess or cut-off, we also investigated if the evaluation of Eiso
in the commonly adopted 1 keV - 10 MeV energy band may bias the Ep,i - Eiso
correlation contributing to its scatter. By computing Eiso from 1 keV to 10
GeV, the slope of the correlation becomes slightly flatter, while its
dispersion does not change significantly. Finally, we find that GRB 080916C is
also consistent with most of the other spectral energy correlations derived
from it, with the possible exception of the Ep,i - Eiso - tb correlation.

http://arxiv.org/abs/0907.0384

A. Giuliani, F. Fuschino, G. Vianello, M. Marisaldi, S. Mereghetti, M. Tavani, S. Cutini, G. Barbiellini, F. Longo, E. Moretti, M. Feroci, E. Del Monte, A. Argan, A. Bulgarelli, P. Caraveo, P. W. Cattaneo, A. W. Chen, T. Contessi, F. D'Ammando, E. Costa, G. De Paris, G. Di Cocco, I. Donnarumma, Y. Evangelista, A. Ferrari, M. Fiorini, M. Galli, F. Gianotti, C. Labanti, I. Lapshov, F. Lazzarotto, P. Lipari, A. Morselli, L. Pacciani, A. Pellizzoni, F. Perotti, G. Piano, P. Picozza, M. Pilia, G. Pucella, M. Prest, M. Rapisarda, A. Rappoldi, A. Rubini, S. Sabatini, E. Scalise, E. Striani, P. Soffitta, M. Trifoglio, A. Trois, E. Vallazza, S. Vercellone, V. Vittorini, A. Zambra, D. Zanello, C. Pittori, F. Verrecchia, P. Santolamazza, P. Giommi, S. Colafrancesco, L.A. Antonelli, L. Salotti

Short gamma-ray bursts (GRBs), typically lasting less than 2 s, are a special
class of GRBs of great interest. We report the detection by the AGILE satellite
of the short GRB 090510 which shows two clearly distinct emission phases: a
prompt phase lasting ~ 200 msec and a second phase lasting tens of seconds. The
prompt phase is relatively intense in the 0.3-10 MeV range with a spectrum
characterized by a large peak/cutoff energy near 3 MeV, in this phase, no
significant high-energy gamma-ray emission is detected. At the end of the
prompt phase, intense gamma-ray emission above 30 MeV is detected showing a
power-law time decay of the flux of the type t^-1.3 and a broad-band spectrum
remarkably different from that of the prompt phase. It extends from sub-MeV to
hundreds of MeV energies with a photon index alpha ~ 1.5. GRB 090510 provides
the first case of a short GRB with delayed gamma-ray emission. We present the
timing and spectral data of GRB 090510 and briefly discuss its remarkable
properties within the current models of gamma-ray emission of short GRBs.

http://arxiv.org/abs/0908.1908

Matthew D. Kistler, Hasan Yuksel, John F. Beacom, Andrew M. Hopkins, J. Stuart B. Wyithe

High-redshift gamma-ray bursts (GRBs) offer an extraordinary opportunity to
study aspects of the early Universe, including the cosmic star formation rate
(SFR). Motivated by the two recent highest-z GRBs, GRB 080913 at z = 6.7 and
GRB 090423 at z = 8.1, and more than four years of Swift observations, we first
confirm that the GRB rate does not trace the SFR in an unbiased way. Correcting
for this, we find that the implied SFR to beyond z = 8 is consistent with
LBG-based measurements after accounting for unseen galaxies at the faint end of
the UV luminosity function. We show that this provides support for the
integrated star formation in the range 6 < z < 8 to have been alone sufficient
to reionize the Universe.

http://arxiv.org/abs/0906.0590

W. Zheng, J. Deng, J. Wang

We compare the properties of optically dark GRBs, defined by the
optical-to-X-ray spectral index beta_OX<0.5, and normal ones discovered by the
Swift satellite before the year 2008 in a statistical way, using data collected
from the literature and online databases. Our sample include 200 long bursts,
19 short bursts, and 10 with measured high redshifts (z>=4). The ratio of dark
bursts is found to be about 10-20%, and is similar between long bursts, short
ones, and the high-z sub-sample. The result for long bursts is consistent with
both the pre-Swift sample and studies by other authors on smaller Swift
samples. The existence of dark short GRBs is pointed out for the first time.
The X-ray derived hydrogen column densities of dark GRBs clearly prefer large
values compared with those of normal bursts. This supports the dust extinction
scenario as the main cause of dark GRBs. Other possibilities like very high
redshifts and non-standard emission mechanisms are less likely although not
fully excluded.

http://arxiv.org/abs/0906.2244

Joeri van Leeuwen, The LOFAR Transients Key Science Project

LOFAR, the Low Frequency Array, is an innovative new radio telescope
currently under construction in the Netherlands. With its continuous monitoring
of the radio sky we expect LOFAR will detect many new transient events,
including GRB afterglows and pulsating/single-burst neutron stars. We here
describe all-sky surveys ranging from a time resolution of microseconds to a
cadence span of years.

http://arxiv.org/abs/0909.3212