astro-grb

A. Shahmoradi, R. J. Nemiroff

GRB satellites are relatively inefficient detectors of dim hard bursts
because they trigger on photon counts, which are number-biased against hard
photons. Therefore, for example, given two bursts of identical peak luminosity
near the detection threshold, a dim soft burst will be preferentially detected
over a dim hard burst. This detector bias can create or skew an apparent
correlation where increasingly hard GRBs appear increasingly bright. Although
such correlations may be obfuscated by a middle step where GRBs need to be
bright enough to have their actual redshifts determined, it is found that the
bias is generally pervasive. This result is derived here through simulations
convolving a wide variety of possible GRB brightnesses and spectra with the
BATSE Large Area Detectors (LAD) detection thresholds. The presented analyses
indicate that the rest-frame $\nu F_{\nu}$ spectrum peak energy of
long-duration GRBs, $\epi$, is not a good cosmological standard candle without
significant corrections for selection effects. Therefore, the appearance of
$\epi$ in seeming correlations such as the Amati ($E_{iso}-\epi$), Ghirlanda
($E_{\gamma}-\epi$), and $L_{iso}-\epi$ relations is statistically real but
strongly influenced by so far uncalibrated GRB detector thresholds.

http://arxiv.org/abs/0904.1464

Alexander Tchekhovskoy (1), Ramesh Narayan (1), Jonathan C. McKinney (2) ((1) Harvard-CfA, (2) Department of Physics and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University)

Achromatic breaks in afterglow light curves of gamma-ray bursts (GRBs) arise
naturally if the product of the jet's Lorentz factor \gamma and opening angle
\Theta_j satisfies (\gamma \Theta_j) >> 1 at the onset of the afterglow phase,
i.e., soon after the conclusion of the prompt emission. Magnetohydrodynamic
(MHD) simulations of collimated GRB jets generally give (\gamma \Theta_j) <~ 1,
suggesting that MHD models may be inconsistent with jet breaks. We work within
the collapsar paradigm and use axisymmetric relativistic MHD simulations to
explore the effect of a finite stellar envelope on the structure of the jet.
Our idealized models treat the jet-envelope interface as a collimating rigid
wall, which opens up outside the star to mimic loss of collimation. We find
that the onset of deconfinement causes a burst of acceleration accompanied by a
slight increase in the opening angle. In our fiducial model with a stellar
radius equal to 10^4.5 times that of the central compact object, the jet
achieves an asymptotic Lorentz factor \gamma ~ 500 far outside the star and an
asymptotic opening angle \Theta_j ~ 0.04 rad ~ 2 deg, giving (\gamma \Theta_j)
~ 20. These values are consistent with observations of typical long-duration
GRBs, and explain the occurrence of jet breaks. We provide approximate analytic
solutions that describe the numerical results well.

http://arxiv.org/abs/0909.0011

A. M. Beloborodov (1), F. Daigne (2), R. Mochkovitch (2), Z. L. Uhm (3) ((1)Columbia University, (2)Institut d'Astrophysique de Paris, (3)Institute for the Early Universe and Research Center of MEMS Space Telescope)

The curvature of a relativistic blast wave implies that its emission arrives
to observers with a spread in time. This effect is believed to wash out any
fast variability in the lightcurves of GRB afterglows. We note that the
spreading effect is reduced if emission is anisotropic in the rest-frame of the
blast wave (i.e. if emission is limb-brightened or limb-darkened). In
particular, synchrotron emission is almost certainly anisotropic, and may be
strongly anisotropic, depending on details of electron acceleration in the
blast wave. Anisotropic afterglows can display fast and strong variability at
high frequencies (above the 'fast-cooling' frequency). This may explain the
existence of bizarre features in the X-ray afterglows of GRBs, such as sudden
drops and flares. We also note that a moderate anisotropy can significantly
delay the 'jet break' in the lightcurve, which makes it harder to detect.

http://arxiv.org/abs/1003.1265

B. Gendre (1), A. Klotz (2,3), E. Palazzi (4), T. Kruhler (5,6), S. Covino (7), P. Afonso (5), L.A. Antonelli (8), J.L. Atteia (9), P. D'Avanzo (7,10), M. Boer (2), J. Greiner (5), S. Klose (11) ((1) LAM, (2) OHP, (3) CESR, (4) IASF-Bologna/INAF, (5) MPE, (6) Technische Universitat Munchen, (7) Osservatorio Astronomico di Brera/INAF, (8) Osservatorio Astronomico di Roma/INAF, (9) LATT, (10) Universita dell'Insubria, (11) Thuringer Landessternwarte Tautenburg)

We present the observations of the afterglow of gamma-ray burst GRB 090102.
Optical data taken by the TAROT, REM, GROND, together with publicly available
data from Palomar, IAC and NOT telescopes, and X-ray data taken by the XRT
instrument on board the Swift spacecraft were used. This event features an
unusual light curve. In X-rays, it presents a constant decrease with no hint of
temporal break from 0.005 to 6 days after the burst. In the optical, the light
curve presents a flattening after 1 ks. Before this break, the optical light
curve is steeper than that of the X-ray. In the optical, no further break is
observed up to 10 days after the burst. We failed to explain these observations
in light of the standard fireball model. Several other models, including the
cannonball model were investigated. The explanation of the broad band data by
any model requires some fine tuning when taking into account both optical and
X-ray bands.

http://arxiv.org/abs/0909.1167

Ivan Zalamea, Andrei M. Beloborodov (Columbia University)

Hyper-accretion discs around black holes emit copious neutrinos and
anti-neutrinos. A fraction of the emitted neutrinos convert to
electron-positron plasma above the disc through the annihilation reaction
$\nu\bar\nu\to e^+e^-$. This process may drive relativistic jets associated
with GRB explosions. We calculate the efficiency of energy deposition by
neutrinos. Our calculation is fully relativistic and based on a
geodesic-tracing method. We find that the efficiency of neutrino heating is a
well-defined function of (i) accretion rate and (ii) spin of the black hole. It
is practically independent of the details of neutrino transport in the opaque
zone of the disc. The results help identify accretion discs whose neutrino
emission can power GRBs.

http://arxiv.org/abs/1003.0710

The LIGO Scientific Collaboration, the Virgo Collaboration: J. Abadie, B. P. Abbott, R. Abbott, T. Accadia, F. Acernese, R. Adhikari, P. Ajith, B. Allen, G. Allen, E. Amador Ceron, R. S. Amin, S. B. Anderson, W. G. Anderson, F. Antonucci, S. Aoudia, M. A. Arain, M. Araya, K. G. Arun, Y. Aso, S. Aston, P. Astone, P. Aufmuth, C. Aulbert, S. Babak, P. Baker, G. Ballardin, S. Ballmer, D. Barker, F. Barone, B. Barr, P. Barriga, L. Barsotti, M. Barsuglia, M.A. Barton, I. Bartos, R. Bassiri, M. Bastarrika, Th. S. Bauer, B. Behnke, M.G. Beker, A. Belletoile, M. Benacquista, J. Betzwieser, P. T. Beyersdorf, S. Bigotta, I. A. Bilenko, G. Billingsley, S. Birindelli, R. Biswas, M. A. Bizouard, E. Black, J. K. Blackburn, L. Blackburn, D. Blair, B. Bland, M. Blom, C. Boccara, O. Bock, T. P. Bodiya, R. Bondarescu, et al. (606 additional authors not shown)

Progenitor scenarios for short gamma-ray bursts (short GRBs) include
coalescenses of two neutron stars or a neutron star and black hole, which would
necessarily be accompanied by the emission of strong gravitational waves. We
present a search for these known gravitational-wave signatures in temporal and
directional coincidence with 22 GRBs that had sufficient gravitational-wave
data available in multiple instruments during LIGO's fifth science run, S5, and
Virgo's first science run, VSR1. We find no statistically significant
gravitational-wave candidates within a [-5, +1) s window around the trigger
time of any GRB. Using the Wilcoxon-Mann-Whitney U test, we find no evidence
for an excess of weak gravitational-wave signals in our sample of GRBs. We
exclude neutron star-black hole progenitors to a median 90% CL exclusion
distance of 6.7 Mpc.

http://arxiv.org/abs/1001.0165

Vincenzo Vitagliano (SISSA and INFN, Trieste), Jun-Qing Xia (SISSA, Trieste), Stefano Liberati (SISSA and INFN, Trieste), Matteo Viel (INFN and INAF-Osservatorio Astronomico di Trieste)

We constrain the parameters describing the kinematical state of the universe
using a cosmographic approach, which is fundamental in that it requires a very
minimal set of assumptions (namely to specify a metric) and does not rely on
the dynamical equations for gravity. On the data side, we consider the most
recent compilations of Supernovae and Gamma Ray Bursts catalogues. This allows
to further extend the cosmographic fit up to $z = 6.6$, i.e. up to redshift for
which one could start to resolve the low z degeneracy among competing
cosmological models. In order to reliably control the cosmographic approach at
high redshifts, we adopt the expansion in the improved parameter $y = z/(1+z)$.
This series has the great advantage to hold also for $z > 1$ and hence it is
the appropriate tool for handling data including non-nearby distance
indicators. We find that Gamma Ray Bursts, probing higher redshifts than
Supernovae, have constraining power and do require (and statistically allow) a
cosmographic expansion at higher order than Supernovae alone. Exploiting the
set of data from Union and GRBs catalogues, we show (for the first time in a
purely cosmographic approach parametrized by deceleration $q_0$, jerk $j_0$,
snap $s_0$) a definitively negative deceleration parameter $q_0$ up to the
3$\sigma$ confidence level. We present also forecasts for realistic data sets
that are likely to be obtained in the next few years.

http://arxiv.org/abs/0911.1249

Wei Deng, Yong-Feng Huang, Si-Wei Kong (Nanjing University)

Multiple rebrightenings have been observed in the multiband afterglow of GRB
030329. Especially, a marked and quick rebrightening occurred at about t ~ 1.2
* 10^5 s. Energy injection from late shells seems to be the best interpretation
for these rebrightenings. Usually it is assumed that the energy is injected
into the whole external shock. However, in the case of GRB 030329, the
rebrightenings are so quick that the usual consideration fails to give a
satisfactory fit to the observed light curves. Actually, since these late
shells coast freely in the wake of the external shock, they should be cold and
may not expand laterally. The energy injection then should only occur at the
central region of the external shock. Considering this effect, we numerically
re-fit the quick rebrightenings observed in GRB 030329. By doing this, we were
able to derive the beaming angle of the energy injection process. Our result,
with a relative residual of only 5% - 10% during the major rebrightening, is
better than any previous modeling. The derived energy injection angle is about
0.027. We suggest that this angle should just be the beaming angle of the
prompt gamma-ray emission. Our study gives a novel method to measure the
beaming angle of gamma-ray bursts.

http://arxiv.org/abs/1003.0099

T. N. Ukwatta, K. S. Dhuga, M. Stamatikos, T. Sakamoto, W. C. Parke, S. D. Barthelmy, N. Gehrels

We have investigated the empirical lag-luminosity relation in the Gamma-ray
Burst (GRB) source-frame. We selected two energy bands (100-200 keV and 300-400
keV) in the GRB source-frame, which after redshift correction, lie in the
observer-frame energy range of the Swift Burst Alert Telescope (BAT). The
spectral lags between these energy channels are then presented as a function of
the isotropic peak luminosity of the GRBs in the sample.

http://arxiv.org/abs/1003.0229

Antoine Calvez, Alexander Kusenko

Gamma-ray bursts (GRB) have been invoked to explain both the 511 keV emission
from the galactic bulge and the high-energy positron excess inferred from the
PAMELA and Fermi data. While independent explanations can be responsible for
these phenomena, we explore the possibility of their common GRB-related origin
by modeling the GRB distribution and estimating the rates. For an expected
Milky Way GRB rate, neither of the two signals is generic, but each requires a
10-20% coincidence with respect to the timing of the latest GRB, and
simultaneous explanation requires a 2% coincidence. Considering the large
number of statistical “trials” created by multiple searches for new physics,
the coincidences of a few per cent cannot be dismissed as unlikely.
Alternatively, both phenomena can be explained by GRB if the galactic rate is
higher than expected.

http://arxiv.org/abs/1003.0045