astro-grb

R. O'Shaughnessy (1), V. Kalogera (1), K. Belczynski (2) ((1) Northwestern University, (2) New Mexico State University)

To test whether the short GRB rates, redshift distribution and host galaxies
are consistent with current theoretical predictions, we use avery large
database of population synthesis calculations to examine BH-NS and NS-NS merger
rates in the universe, factoring in (i) the star formation history of the
universe, (ii) a heterogeneous population of star-forming galaxies, including
spirals and ellipticals, and (iii) a simple flux-limited selection model for
short GRB detection. When we require our models reproduce the known short GRB
rates and redshift measurements (and, for NS-NS, the merger rates extrapolated
from binary pulsars in the Galaxy), a small fraction of models reproduce all
observations, both when we assume a NS-NS and a BH-NS origin for bursts. Most
commonly models produce mergers preferentially in spiral galaxies if short GRBs
arise from NS-NS mergers alone. Model universes where present-day binary
mergers occur preferentially in elliptical galaxies necessarily include a
significant fraction of binaries with long delay times between birth and merger
(often $O(10{\rm Gyr})$). Though long delays occur, almost all of our models
predict that a higher proportion of short GRBs should occur at moderate to high
redshift (e.g., $z>1$) than has presently been observed, in agreement with
recent observations which suggest a selection bias towards successful follow-up
of low-redshift short GRBs. Finally, if only a fraction of BH-NS mergers have
the right combination of masses and spins to make GRBs, then at best only a
small fraction of BH-NS models could be consistent with all {\em current}
available data. (Abridged)

http://arxiv.org/abs/0706.4139

E. Howell, D. Coward, R. Burman, D. Blair

It has been shown that the observed temporal distribution of transient events
in the cosmos can be used to constrain their rate density. Here we show that
the peak flux–observation time relation takes the form of a power law that is
invariant to the luminosity distribution of the sources, and that the method
can be greatly improved by invoking time reversal invariance and the temporal
cosmological principle. We demonstrate how the method can be used to constrain
distributions of transient events, by applying it to Swift gamma-ray burst data
and show that the peak flux–observation time relation is in good agreement
with recent estimates of source parameters. We additionally show that the
intrinsic time dependence allows the method to be used as a predictive tool.
Within the next year of Swift observation, we find a 50% chance of obtaining a
peak flux greater than that of GRB 060017 — the highest Swift peak flux to
date — and the same probability of detecting a burst with peak flux > 100
photons s^{-1} cm^{-2} within 6 years.

http://arxiv.org/abs/0706.4136

M. F. Bietenholz, N. Bartel

We report on our second-epoch VLBI and VLA observations of the Type Ib/c
supernova 2001em, five years after the explosion. It was suggested that SN
2001em might be a jet-driven gamma ray burst (GRB), with the jet oriented near
the plane of the sky, which would entail relativistic expansion or motion. Our
VLBI image shows that SN 2001em is still unresolved five years after the
explosion. For a distance of 83 Mpc (H_0 = 70 km/s/Mpc), the nominal expansion
velocity is 5800 +/- 10,000 km/s, and the proper motion is 33,000 +/- 34,000
km/s. Our values are inconsistent with either relativistic expansion or motion,
but are consistent with the non-relativistic expansion speeds and small proper
motions seen in other supernovae. In particular these values are consistent
with radio emission from SN 2001em being due to normal, non-relativistic
supernova ejecta interacting with the circumstellar medium. Our VLA
observations show a power-law decay in flux density since the time of the peak
in the 8.4 GHz radio lightcurve in ~2003.

http://arxiv.org/abs/0706.3344

D. A. Perley, J. S. Bloom, N. R. Butler, L. K. Pollack, J. Holtzman, C. H. Blake, D. Kocevski, W. T. Vestrand, W. Li, R. J. Foley, E. Bellm, H.-W. Chen, J. X. Prochaska, D. Starr, A. V. Filippenko, E. E. Falco, A. H. Szentgyorgyi, J. Wren, P. R. Wozniak, R. White, J. Pergande

We report on observations of a gamma-ray burst (GRB 061126) with an extremely
bright (R ~ 12 mag at peak) early-time optical afterglow. The optical afterglow
is already fading as a power law 22 seconds after the trigger, with no
detectable prompt contribution in our first exposure, which was coincident with
a large prompt-emission gamma-ray pulse. The optical–infrared photometric
spectral energy distribution is an excellent fit to a power law, but it
exhibits a moderate red-to-blue evolution in the spectral index at about 500 s
after the burst. This color change is contemporaneous with a switch from a
relatively fast decay to slower decay. The rapidly decaying early afterglow is
broadly consistent with synchrotron emission from a reverse shock, but a bright
forward-shock component predicted by the intermediate- to late-time X-ray
observations under the assumptions of standard afterglow models is not
observed. Indeed, despite its remarkable early-time brightness, this burst
would qualify as a dark burst at later times on the basis of its nearly flat
optical-to-X-ray spectral index. Our photometric spectral energy distribution
provides no evidence of host-galaxy extinction, requiring either large
quantities of grey dust in the host system (at redshift 1.1588 +/- 0.0006,
based upon our late-time Keck spectroscopy) or separate physical origins for
the X-ray and optical afterglows.

http://arxiv.org/abs/astro-ph/0703538

Y. W. Yu, X. W. Liu, Z. G. Dai

The widely existing shallow decay phase of the X-ray afterglows of gamma-ray
bursts (GRBs) is generally accepted to be due to long-lasting energy injection.
The outflows carrying the injecting energy, based on their composition, could
correspond to two types: baryon-dominated and lepton-dominated ones. The
former-type outflows could be lagging materials in a wide GRB ejecta with a
radial distribution of Lorentz factor, while the latter could be an
electron-positron-pair wind that is driven by the post-burst central engine. We
here provide a unified description for the dynamics of the two models of energy
injection, and calculate the corresponding high-energy photon emission by
considering the synchrotron radiation and inverse Compton scattering (including
synchrotron self-Compton and combined inverse-Compton). We find that, for both
models, there is a plateau (even a hump) in high-energy light curves during the
X-ray shallow decay phase. In particular, a considerable fraction of the
injecting energy in the leptonic-component-dominated model can be shared by the
long-lasting reverse shock since it is relativistic. Furthermore, almost all of
the energy of the reverse shock is carried by leptons and thus enhances the
inverse-Compton emission dramatically. Therefore, this model predicts more
significant high-energy afterglow emission than the
baryonic-component-dominated model. We argue that these observational
signatures would be used to discriminate between different energy-injection
models in the upcoming {\em Gamma-ray Large Area Space Telescope} (GLAST) era.

http://arxiv.org/abs/0706.3741

F. Y. Wang, Z. G. Dai, Zong-Hong Zhu

It has been widely shown that the cosmological parameters and dark energy can
be constrained by using data from type-Ia supernovae (SNe Ia), the cosmic
microwave background (CMB) anisotropy, the baryon acoustic oscillation (BAO)
peak from Sloan Digital Sky Survey (SDSS), the X-ray gas mass fraction in
clusters, and the linear growth rate of perturbations at z=0.15 as obtained
from the 2dF Galaxy Redshift Survey. Recently, gamma-ray bursts (GRBs) have
also been argued to be promising standard candles for cosmography. In this
paper, we present constraints on the cosmological parameters and dark energy by
combining a recent GRB sample including 69 events with the other cosmological
probes. First, we find that for the LambdaCDM cosmology this combination makes
the constraints stringent and the best fit is close to the flat universe.
Second, we fit the flat Cardassian expansion model and find that this model is
consistent with the LambdaCDM cosmology. Third, we present constraints on
several two-parameter dark energy models and find that these models are also
consistent with the LambdaCDM cosmology. Finally, we reconstruct the dark
energy equation-of-state parameter w(z) and the deceleration parameter q(z). We
see that the acceleration could have started at a redshift from
z_T=0.40_{-0.08}^{+0.14} to z_T=0.65_{-0.05}^{+0.10}. This difference in the
transition redshift is due to different dark energy models that we adopt. The
most stringent constraint on w(z) lies in the redshift range z\sim 0.3-0.6.

http://arxiv.org/abs/0706.0938

V. Lipunov (1,2,3), E. Gorbovskoy (1,2,3) (1-Sternberg Astronomical Institute, 2-Dept. of Physics of Moscow State University,3-Moscow Union “Optic'')

The recently discovered gamma-ray burst GRB 070110 displayed an extraordinary
x-ray afterglow with x-ray radiation — x-ray plateau — observed for 20000
seconds. We show that the observed properties of the plateau can be naturally
interpreted in terms of the model with a spinar — a quasi-equilibrium
collapsing object whose equilibrium is maintained by the balance of centrifugal
and gravitational forces and whose evolution is determined by its magnetic
field. If this model is true, then for one hour Swift x-ray telescopes recorded
radiation from an object with size smaller than the Schwarzschild radius!

http://arxiv.org/abs/0705.1648

J.I. Cabrera (1), C. Firmani (2,1), V. Avila-Reese (1), G. Ghirlanda (2), G. Ghisellini (2), L. Nava (2,3) ((1)U.N.A.M., (2)INAF-OAB, (3)Univ. Insubria)

We study the spectral and energetics properties of 47 long-duration gamma-ray
bursts (GRBs) with known redshift, all of them detected by the Swift satellite.
Due to the narrow energy range (15-150 keV) of the Swift-BAT detector, the
spectral fitting is reliable only for fitting models with 2 or 3 parameters. As
high uncertainty and correlation among the errors is expected, a careful
analysis of the errors is necessary. We fit both the power law (PL, 2
parameters) and cut–off power law (CPL, 3 parameters) models to the
time-integrated spectra of the 47 bursts, and present the corresponding
parameters, their uncertainties, and the correlations among the uncertainties.
The CPL model is reliable only for 29 bursts for which we estimate the nuf_nu
peak energy Epk. For these GRBs, we calculate the energy fluence and the rest-
frame isotropic-equivalent radiated energy, Eiso, as well as the propagated
uncertainties and correlations among them. We explore the distribution of our
homogeneous sample of GRBs on the rest-frame diagram E'pk vs Eiso. We confirm a
significant correlation between these two quantities (the “Amati” relation) and
we verify that, within the uncertainty limits, no outliers are present. We also
fit the spectra to a Band model with the high energy power law index frozen to
-2.3, obtaining a rather good agreement with the “Amati” relation of non-Swift
GRBs.

http://arxiv.org/abs/0704.0791

Nathaniel R. Butler, Daniel Kocevski

The early, highly time-variable X-ray emission immediately following GRBs
exhibits strong spectral variations that are unlike the temporally smoother
emission which dominates after $t\sim 10^3$ s. The ratio of hard channel
(1.3-10.0 keV) to soft channel (0.3-1.3 keV) counts in the Swift X-ray
telescope provides a new measure delineating the end time of this emission. We
define $T_{H}$ as the time at which this transition takes place and measure for
59 events a range of transition times that span $10^2$ s to $10^{4}$ s, on
average 5 times longer than the prompt $T_{90}$ duration observed in the
Gamma-ray band. It is very likely that the mysterious light curve plateau phase
and the later powerlaw temporal evolution, both of which typically occur at
times greater than $T_{H}$ and hence exhibit very little hardness ratio
evolution, are both produced by external shocking of the surrounding medium and
not by the internal shocks thought responsible for the earlier emission. We use
the apparent lack of spectral evolution to discriminate against proposed models
for the plateau phase emission. We favor energy injection scenarios with a
roughly linearly increasing input energy versus time for six well sampled
events with nearly flat light curves at $t\approx 10^3-10^4$s. Also, using the
transition time $T_{H}$ as the delineation between the GRB and afterglow
emission, we calculate that the kinetic energy in the afterglow shock is
typically a factor of 10 lower than that released in the GRB. Lack of hardness
variations in these three events may be due to a very highly relatavistic
outflow or due to a very dense circumburst medium. There are a handful of rare
cases of very late time $t>10^4$s hardness evolution, which may point to
residual central engine activity at very late time.

http://arxiv.org/abs/astro-ph/0702638

C. Dermer, E. Ramirez-Ruiz, T. Le

Relativistic black-hole jet sources are leading candidates for high energy
(>> TeV) neutrino production. The relations defining (a) efficient photopion
losses of cosmic-ray protons on target photons and (b) gamma-gamma opacity of
gamma rays through that same target photon field imply clear multiwavelength
predictions for when and at what energies blazars and GRBs should be most
neutrino bright and gamma-ray dim. The use of multiwavelength observations to
test the standard relativistic jet model for these source is illustrated.

http://arxiv.org/abs/astro-ph/0703219