astro-grb

D. Burlon, G. Ghirlanda, G. Ghisellini, J. Greiner, A. Celotti

Aims: Gamma Ray Bursts (GRBs) are sometimes preceded by dimmer emission
episodes, called “precursors”, whose nature is still a puzzle: they could
either have the same origin as the main emission episode or they could be due
to another mechanism. We investigate if precursors have some spectral
distinctive feature with respect to the main GRB episodes. Methods: To this aim
we compare the spectral evolution of the precursor with that of the main GRB
event. We also study if and how the spectral parameters, and in particular the
peak of the vFv spectrum of time resolved spectra, correlates with the flux.
This allows us to test if the spectra of the precursor and of the main event
belong to the same correlation (if any). We searched GRBs with precursor
activity in the complete sample of 2704 bursts detected by BATSE finding that
12% of GRBs have one or more precursors. Among these we considered the bursts
with time resolved spectral analysis performed by Kaneko et al. 2006, selecting
those having at least two time resolved spectra for the precursor. Results: We
find that precursors and main events have very similar spectral properties. The
spectral evolution within precursors has similar trends as the spectral
evolution observed in the subsequent peaks. Also the typical spectral
parameters of the precursors are similar to those of the main GRB events.
Moreover, in several cases we find that within the precursors the peak energy
of the spectrum is correlated with the flux similarly to what happens in the
main GRB event. This strongly favors models in which the precursor is due to
the same fireball physics of the main emission episodes.

http://arxiv.org/abs/0907.5203

Nicola Galante, for the VERITAS Collaboration

During its first cycle of observations, VERITAS observed several GRBs in
response to broadcast alerts from the Gamma-ray bursts Coordinates Network
(GCN). The GRBs were followed up and observed thereafter with typical delays of
2 to 4 minutes from the beginning of the burst and of 92 s in the best case,
searching for a very high energy (VHE) component above 100 GeV. The aim of the
search for VHE emission from GRBs is to understand the behavior, composition
and dynamics of the most accelerated particles in the bursts, as well as to get
a better overall understanding of the model of GRBs. We report on the results
from two years of observations by VERITAS.

http://arxiv.org/abs/0907.4997

Emily M. Levesque, Edo Berger, Lisa J. Kewley, Megan M. Bagley

We present the first observations from a large-scale survey of nearby (z < 1)
long-duration gamma-ray burst (LGRB) host galaxies, which consist of eight
rest-frame optical spectra obtained at Keck and Magellan. Along with two host
galaxy observations from the literature, we use optical emission line
diagnostics to determine metallicities, ionization parameters, young stellar
population ages, and star formation rates. We compare the LGRB host
environments to a variety of local and intermediate-redshift galaxy
populations, as well as the newest grid of stellar population synthesis and
photoionization models generated with the Starburst99/Mappings codes. With
these comparisons we investigate whether the GRB host galaxies are consistent
with the properties of the general galaxy population, and therefore whether
they may be used as reliable tracers of star formation. We find that LGRB host
galaxies generally have low-metallicity ISM environments out to z ~ 1. The ISM
properties of our GRB hosts, including metallicity, ionization parameter, and
young stellar population age, are significantly different from the general
galaxy population, host galaxies of nearby broad-lined Type Ic supernovae, and
nearby metal-poor galaxies.

http://arxiv.org/abs/0907.4988

Jun-Qing Xia

The accelerating expansion of our universe at present could be driven by an
unknown energy component (Dark Energy) or a modification of general relativity
(Modified Gravity). In this note we revisit the constraints on a
phenomenological model which interpolates between the pure \LambdaCDM model and
the Dvali-Gabadadze-Porrati (DGP) braneworld model with an additional parameter
\alpha. Combining the cosmic microwave background (CMB), baryon acoustic
oscillations (BAO) and type Ia supernovae (SNIa), as well as some high-redshift
observations, such as the gamma-ray bursts (GRB) and the measurements of linear
growth factors (LGF), we obtain the tight constraint on the parameter
\alpha=0.254\pm0.153 (68% C.L.), which implies that the flat DGP model is
incompatible with the current observations, while the pure \LambdaCDM model
still fits the data very well. Finally, we simulate the future measurements
with higher precisions and find that the constraint on \alpha can be improved
by a factor two, when compared to the present constraints.

http://arxiv.org/abs/0907.4860

The ARGO-YBJ Collaboration: G. Aielli, et al

The ARGO-YBJ (Astrophysical Radiation Ground-based Observatory at YangBaJing)
experiment is designed for very high energy $\gamma$-astronomy and cosmic ray
researches. Due to the full coverage of a large area ($5600 m^2$) with
resistive plate chambers at a very high altitude (4300 m a.s.l.), the ARGO-YBJ
detector is used to search for transient phenomena, such as Gamma-ray bursts
(GRBs). Because the ARGO-YBJ detector has a large field of view ($\sim$2 sr)
and is operated with a high duty cycle ($>$90%), it is well suited for GRB
surveying and can be operated in searches for high energy GRBs following alarms
set by satellite-borne observations at lower energies. In this paper, the
sensitivity of the ARGO-YBJ detector for GRB detection is estimated. Upper
limits to fluence with 99% confidence level for 26 GRBs inside the field of
view from June 2006 to January 2009 are set in the two energy ranges 10$-$100
GeV and 10 GeV$-$1 TeV.

http://arxiv.org/abs/0903.0119

K.L. Page (1), R. Willingale (1), E. Bissaldi (2), A. de Ugarte Postigo (3), S.T. Holland (4,5,6), S. McBreen (7,2), P.T. O'Brien (1), J.P. Osborne (1), J.X. Prochaska (8), E. Rol (1,9), E.S. Rykoff (10), R.L.C. Starling (1), N.R. Tanvir (1), A.J. van der Horst (11,12), K. Wiersema (1), B. Zhang (13), F.J. Aceituno (14), C. Akerlof (15), A.P. Beardmore (1), M.S. Briggs (16), D.N. Burrows (17), A.J. Castro-Tirado (14), V. Connaughton (16), P.A. Evans (1), J.P.U. Fynbo (18), N. Gehrels (4), C. Guidorzi (19,20), A.W. Howard (21), J.A. Kennea (17), C. Kouveliotou (11), C. Pagani (17), R. Preece (16), D. Perley (21), F. Yuan (15) ((1) University of Leicester, (2) MPE, (3) ESO, (4) GSFC, (5) USRA, (6) CRESST, (7) UCD, (8) UCO/Lick Observatory, (9) Astronomical Institute Anton Pannekoek, University of Amsterdam, (10) University of California at Santa Barbara, (11) MSFC, (12) NASA Postdoctoral Program Fellow, (13) University of Nevada, (14) CSIC, (15) University of Michigan, (16) University of Alabama in Huntsville, (17) PSU, (18) Dark Cosmology Centre, (19) University of Ferrara, (20) Liverpool John Moores University, (21) University of California, Berkeley)

GRB 080810 was one of the first bursts to trigger both Swift and the Fermi
Gamma-ray Space Telescope. It was subsequently monitored over the X-ray and
UV/optical bands by Swift, in the optical by ROTSE and a host of other
telescopes and was detected in the radio by the VLA. The redshift of z= 3.355
+/- 0.005 was determined by Keck/HIRES and confirmed by RTT150 and NOT. The
prompt gamma/X-ray emission, detected over 0.3-10^3 keV, systematically softens
over time, with E_peak moving from ~600 keV at the start to ~40 keV around 100
s after the trigger; alternatively, this spectral evolution could be identified
with the blackbody temperature of a quasithermal model shifting from ~60 keV to
~3 keV over the same time interval. The first optical detection was made at 38
s, but the smooth, featureless profile of the full optical coverage implies
that this originated from the afterglow component, not the pulsed/flaring
prompt emission.

Broadband optical and X-ray coverage of the afterglow at the start of the
final X-ray decay (~8 ks) reveals a spectral break between the optical and
X-ray bands in the range 10^15 - 2×10^16 Hz. The decay profiles of the X-ray
and optical bands show that this break initially migrates blueward to this
frequency and then subsequently drifts redward to below the optical band by
~3×10^5 s. GRB 080810 was very energetic, with an isotropic energy output for
the prompt component of 3×10^53 erg and 1.6×10^52 erg for the afterglow; there
is no evidence for a jet break in the afterglow up to six days following the
burst.

http://arxiv.org/abs/0907.4578

R.L.C. Starling (1), E. Rol (1,2), A.J. van der Horst (3), S.-C. Yoon (4), V. Pal'shin, C. Ledoux, K.L. Page, J.P.U. Fynbo, K. Wiersema, N.R. Tanvir, P. Jakobsson, C. Guidorzi, P.A. Curran, A.J. Levan, P.T. O'Brien, J.P. Osborne, D. Svinkin, A. de Ugarte Postigo, T. Oosting, I.D. Howarth ((1) University of Leicester, (2) University of Amsterdam, (3) NSSTC, (4) UCO/Lick)

We follow the bright, highly energetic afterglow of Swift-discovered GRB
080721 at z=2.591 out to 36 days or 3e6 s since the trigger in the optical and
X-ray bands. We do not detect a break in the late-time light curve inferring a
limit on the opening angle of theta_j >= 7.3 deg and setting tight constraints
on the total energy budget of the burst of E_gamma >= 9.9e51 erg within the
fireball model. To obey the fireball model closure relations the GRB jet must
be expanding into a homogeneous surrounding medium and likely lies behind a
significant column of dust. The energy constraint we derive can be used as
observational input for models of the progenitors of long gamma-ray bursts: we
discuss how such high collimation-corrected energies could be accommodated with
certain parameters of the standard massive star core-collapse models. We can,
however, most probably rule out a magnetar progenitor for this GRB which would
require 100% efficiency to reach the observed total energy.

http://arxiv.org/abs/0812.2490

V.F. Cardone, S. Capozziello, M.G. Dainotti

Gamma ray bursts (GRBs) have recently attracted much attention as a possible
way to extend the Hubble diagram to very high redshift. To this aim, the
luminosity (or isotropic emitted energy) of a GRB at redshift z must be
evaluated from a correlation with a distance independent quantity so that one
can then solve for the luminosity distance D_L(z) and hence the distance
modulus mu(z). Averaging over five different two parameters correlations and
using a fiducial cosmological model to calibrate them, Schaefer (2007) has
compiled a sample of 69 GRBs with measured mu(z) which has since then been
widely used to constrain cosmological parameters. We update here that sample by
many aspects. First, we add a recently found correlation for the X - ray
afterglow and use a Bayesian inspired fitting method to calibrate the different
GRBs correlations known insofar assuming a fiducial LCDM model in agreement
with the recent WMAP5 data. Averaging over six correlations, we end with a new
GRBs Hubble diagram comprising 83 objects. We also extensively explore the
impact of varying the fiducial cosmological model considering how the estimated
mu(z) change as a function of the $(\Omega_M, w_0, w_a)$ parameters of the
Chevallier - Polarski - Linder phenomenological dark energy equation of state.
In order to avoid the need of assuming an {\it a priori} cosmological model, we
present a new calibration procedure based on a model independent local
regression estimate of mu(z) using the Union SNeIa sample to calibrate the GRBs
correlations. This finally gives us a GRBs Hubble diagram made out of 69 GRBs
whose estimated distance modulus mu(z) is almost independent on the underlying
cosmological model.

http://arxiv.org/abs/0901.3194

M. Nardini, G. Ghisellini, G. Ghirlanda, A. Celotti

The optical and X-ray light-curves of long Gamma Ray Bursts (GRBs) often show
a complex evolution and in most cases do not track each other. This behaviour
can not be easily explained by the simplest standard afterglow models. A
possible interpretation is to consider the observed optical and X-ray
light-curves as the sum of two separate components. This scenario requires the
presence of a spectral break between these bands. One of the aims of this work
is to test whether such a break is present within the observed Swift XRT energy
range. We analyse the X-ray afterglow spectra of a sample of 33 long GRBs with
known redshift, good optical photometry and published estimate of the host
galaxy dust absorption A_V(host). We find that indeed in 7 bright events a
broken power-law provides a fit to the data that is better than a single
power-law model. For 8 events, instead, the X-ray spectrum is better fitted by
a single power-law. We discuss the role of these breaks in connection to the
relation between the host hydrogen column density N_H(host) and A_V(host) and
check the consistency of the X-ray spectral breaks with the optical bands
photometry. We analyse the optical to X-ray spectral energy distributions at
different times and find again consistency with two components interpretation.

http://arxiv.org/abs/0907.4157

R. R. Xue, P. H. Tam, S. J. Wagner, B. Behera, Y. Z. Fan, D. M. Wei

The synchrotron self-Compton (SSC) emission from Gamma-ray Burst (GRB)
forward shock can extend to the very-high-energy (VHE; $E_\gamma > $100 GeV)
range. Such high energy photons are rare and are attenuated by the cosmic
infrared background before reaching us. In this work, we discuss the prospect
to detect these VHE photons using the current ground-based Cherenkov detectors.
Our calculated results are consistent with the upper limits obtained with
several Cherenkov detectors for GRB 030329, GRB 050509B, and GRB 060505 during
the afterglow phase. For 5 bursts in our nearby GRB sample (except for GRB
030329), current ground-based Cherenkov detectors would not be expected to
detect the modeled VHE signal. Only for those very bright and nearby bursts
like GRB 030329, detection of VHE photons is possible under favorable observing
conditions and a delayed observation time of $\la$10 hours.

http://arxiv.org/abs/0907.4014