astro-grb

Susanna D. Vergani (1,2,3), Patrick Petitjean (2), Cedric Ledoux (4), Paul Vreeswijk (5), Alain Smette (4), Evert J.A. Meurs (3) ((1) APC-University Paris 7, (2) University Paris 6 - IAP, (3) School of Physical Sciences and NCPST-Dublin City University, (4) ESO, (5) Dark Cosmology Centre-NIB, Copenhagen)

We analyse the properties of MgII absorption systems detected along the
sightlines toward GRBs using a sample of 10 GRB afterglow spectra obtained with
VLT-UVES over the past six years. The S/N ratio is sufficiently high that we
can extend previous studies to smaller equivalent widths (typically Wr>0.3A).
Over a pathlength of Delta(z)~14 the number of weak absorbers detected is
similar along GRB and QSO lines of sight, while the number of strong systems is
larger along GRB lines of sight with a 2-sigma significance. Using intermediate
and low resolution observations reported in the literature, we increase the
absorption length for strong systems to Delta(z)=31.5 (about twice the path
length of previous studies) and find that the number density of strong MgII
systems is a factor of 2.1+/-0.6 higher (about 3-sigma significance) toward
GRBs as compared to QSOs, about twice smaller however than previously reported.
We divide the sample in three redshift bins and we find that the number density
of strong MgII is larger in the low redshift bins. We investigate in detail the
properties of strong MgII systems observed with UVES. Both the estimated dust
extinction in strong GRB MgII systems and the equivalent width distribution are
consistent with what is observed for standard QSO systems. We find also that
the number density of (sub)-DLAs per unit redshift in the UVES sample is
probably twice larger than what is expected from QSO sightlines which confirms
the peculiarity of GRB lines of sight. These results indicate that neither a
dust extinction bias nor different beam sizes of the sources are viable
explanations for the excess. It is still possible that the current sample of
GRB lines of sight is biased by a subtle gravitational lensing effect. More
data and larger samples are needed to test this hypothesis. (abridged)

http://arxiv.org/abs/0906.3269

L. Izzo, S. Capozziello, G. Covone, M. Capaccioli

A new method to constrain the cosmological equation of state is proposed by
using combined samples of gamma-ray bursts (GRBs) and supernovae (SNeIa). The
Chevallier-Polarski-Linder parameterization is adopted for the equation of
state in order to find out a realistic approach to achieve the
deceleration/acceleration transition phase of dark energy models. As results,
we find that GRBs, calibrated by SNeIa, could be, at least, good distance
indicators capable of discriminating cosmological models with respect to
$\Lambda$CDM at high redshift. Besides, GRBs+SNeIa combined redshift-distance
diagram puts better in evidence the change of slope around redshift $z\sim 0.5$
which is usually addressed as the “signature” of today observed acceleration.
This feature could be interpreted, in more standard way, by the red sequence in
galaxy clusters.

http://arxiv.org/abs/0906.4888

Felix Ryde (KTH, Stockholm), Asaf Pe'er (STScI, Baltimore)

We perform time-resolved spectroscopy on the prompt emission in gamma-ray
bursts (GRBs) and identify a thermal, photospheric component peaking at a
temperature of a few hundreds keV. This peak does not necessarily coincide with
the broad band (keV-GeV) power peak. We show that this thermal component
exhibits a characteristic temporal behavior. We study a sample of 56 long
bursts, all strong enough to allow time-resolved spectroscopy. We analyze the
evolution of both the temperature and flux of the thermal component in 49
individual time-resolved pulses, for which the temporal coverage is sufficient,
and find that the temperature is nearly constant during the first few seconds,
after which it decays as a power law with a sample-averaged index of -0.68. The
thermal flux first rises with an averaged power-law index of 0.63 after which
it decays with an averaged index of -2. The break times are the same to within
errors. We find that the ratio of the observed to the emergent thermal flux
typically exhibits a monotoneous power-law increase during the entire pulse as
well as during complex bursts. Thermal photons carry a significant fraction
($\sim$ 30 % to more than 50%) of the prompt emission energy (in the observed
25-1900 keV energy band), thereby significantly contributing to the high
radiative efficiency. Finally, we show here that the thermal emission can be
used to study the properties of the photosphere, hence the physical parameters
of the GRB fireball.

http://arxiv.org/abs/0811.4135

L. K. Pollack (1), H.-W. Chen (2), J. X. Prochaska (1,3), J. S. Bloom (4) ((1) UC Santa Cruz, (2) KICP/UChicago, (3) UCO/Lick, (4) UC Berkeley)

We present results from an imaging and spectroscopic study of four strong
MgII absorbers of W(2796) >~ 1 Ang revealed by the afterglow of GRB060418 at
z_GRB=1.491. These absorbers, at z=0.603,0.656,1.107 and z_GRB, exhibit large
ion abundances that suggest neutral gas column densities characteristic of
damped Lya systems. The imaging data include optical images obtained using LRIS
on the Keck I telescope and using ACS on board HST, and near-infrared H-band
images obtained using PANIC on the Magellan Baade Telescope and K'-band images
obtained using NIRC2 with LGSAO on the Keck II telescope. These images reveal
six distinct objects at <~ 3.5'' of the afterglow's position, two of which
exhibit well-resolved mature disk morphology, one shows red colors, and three
are blue compact sources. Follow-up spectroscopic observations using LRIS
confirm that one of the disk galaxies coincides with the MgII absorber at
z=0.656. The observed broad-band spectral energy distributions of the second
disk galaxy and the red source indicate that they are associated with the
absorbers at z=0.603 and z=1.107, respectively. These results show that strong
MgII absorbers identified in GRB afterglow spectra are associated with typical
galaxies of luminosity ~ (0.1-1) L* at impact parameter <~ 10 h^-1 kpc. The
close angular separation would preclude easy detections toward a bright quasar.
Finally, we associate the remaining three blue compact sources with the GRB
host galaxy, noting that they are likely star-forming knots located at
projected distances 2-12 h^-1 kpc from the afterglow. At the afterglow's
position, we derive a 2-sigma upper limit to the underlying SFR intensity of
0.0074 M_sun yr^-1 kpc^-2.

http://arxiv.org/abs/0902.3661

Monica Capone, Vincenzo Fabrizio Cardone, Matteo Luca Ruggiero

In an attempt to look for a viable mechanism leading to a present day
accelerated expansion, we investigate the possibility that the observed cosmic
speed up may be recovered in the framework of the Rastall's theory, relying on
the non - conservativity of the stress - energy tensor, i.e. $T^{\mu}_{\nu ;
\mu} \neq 0$. We derive the modified Friedmann equations and show that they
correspond to Cardassian-like equations. We also show that, under suitable
assumptions on the equation of state of the matter term sourcing the
gravitational field, it is indeed possible to get an accelerated expansion, in
agreement with the Hubble diagram of both Type Ia Supernovae (SNeIa) and Gamma
Ray Bursts (GRBs). Unfortunately, to achieve such a result one has to postulate
a matter density parameter much larger than the typical $\Omega_M \simeq 0.3$
value inferred from cluster gas mass fraction data. As a further issue, we then
discuss the possibility to retrieve the Rastall's theory from a Palatini
variational principle approach to $f(R)$ gravity. However, such an attempt
turns out to be unsuccessful.

http://arxiv.org/abs/0906.4139

C. Bartolini, G. Greco, A. Guarnieri, A. Piccioni, G. Beskin, S. Bondar, S. Karpov, E. Molinari

We present the observed/intrinsic optical parameters and the variability
analysis of the Naked-Eye Burst, GRB 080319B, observed by the TORTORA
wide-field optical monitoring system. The event is extreme not only in observed
properties but also intrinsically: it is the most luminous event ever recorded
at optical wavelengths. The temporal properties suggest short-lived periodic
activities of the internal engine. This is the fastest optically variable
source detected at cosmological distances.

http://arxiv.org/abs/0906.4144

Z. Bagoly, P. Veres

The measured redshifts of gamma-ray bursts (GRBs), which were first detected
by the Swift satellite, seem to be bigger on average than the redshifts of GRBs
detected by other satellites. We analyzed the redshift distribution of GRBs
triggered and observed by different satellites (Swift, HETE2, BeppoSax,
Ulyssses). After considering the possible biases {significant difference was
found at the p=95.70% level in the redshift distributions of GRBs measured by
HETE and the Swift.

http://arxiv.org/abs/0906.4283

D. Gotz, J.Paul, S. Basa, J. Wei, S. N. Zhang, J.-L. Atteia, D. Barret, B. Cordier, A. Claret, J. Deng, X. Fan, J.Y. Hu, M. Huang, P. Mandrou, S. Mereghetti, Y. Qiu, B. Wu

We present the SVOM (Space-based multi-band astronomical Variable Object
Monitor) mission, that is being developed in cooperation between the Chinese
National Space Agency (CNSA), the Chinese Academy of Science (CAS) and the
French Space Agency (CNES). Its scientific objectives include the study of the
GRB phenomenon, GRB physics and progenitors, cosmology, and fundamental
physics. SVOM is designed to detect all known types of Gamma-Ray Bursts (GRBs),
to provide fast and reliable GRB positions, to measure the broadband spectral
characteristics and temporal properties of the GRB prompt emission. This will
be obtained in first place thanks to a set of four space flown instruments. A
wide field (~2 sr) coded mask telescope (ECLAIRs), operating in the 4-250 keV
energy range, will provide the triggers and localizations, while a gamma-ray
non-imaging spectrometer (GRM), sensitive in the 50 keV-5 MeV domain, will
extend the prompt emission energy coverage. After a satellite slew, in order to
place the GRB direction within field of view of the two narrow field
instruments - a soft X-ray (XIAO), and a visible telescope (VT) - the GRB
position will be refined and the study of the early phases of the GRB afterglow
will be possible. A set of three ground based dedicated instruments, two
robotic telescopes (GFTs) and a wide angle optical monitor (GWAC), will
complement the space borne instruments. Thanks to the low energy trigger
threshold (~4 keV) of the ECLAIRs, SVOM is ideally suited for the detection of
soft, hence potentially most distant, GRBs. Its observing strategy is optimized
to facilitate follow-up observations from the largest ground based facilities.

http://arxiv.org/abs/0906.4195

Giovanni Amelino-Camelia, Lee Smolin

We discuss the prospects for bounding and perhaps even measuring quantum
gravity effects on the dispersion of light using the highest energy photons
produced in gamma ray bursts measured by the Fermi telescope. These prospects
are brigher than might have been expected as in the first 10 months of
operation Fermi has reported so far eight events with photons over 100 MeV seen
by its Large Area Telescope (LAT). We review features of these events which may
bear on Planck scale phenomenology and we discuss the possible implications for
the alternative scenarios for in-vacua dispersion coming from breaking or
deforming of Poincare invariance. Among these are semi-conservative bounds,
which rely on some relatively weak assumptions about the sources, on subluminal
and superluminal in-vacuo dispersion. We also propose that it may be possible
to look for the arrival of still higher energy photons and neutrinos from GRB's
with energies in the range 10^14 - 10^17 eV. In some cases the quantum gravity
dispersion effect would predict these arrivals to be delayed or advanced by
days to months from the GRB, giving a clean separation of astrophysical source
and spacetime propagation effects.

http://arxiv.org/abs/0906.3731

Hao Wei, Shuang Nan Zhang

Recently, Gamma-Ray Bursts (GRBs) were proposed to be a complementary
cosmological probe to type Ia supernovae (SNIa). GRBs have been advocated to be
standard candles since several empirical GRB luminosity relations were proposed
as distance indicators. However, there is a so-called circularity problem in
the direct use of GRBs. Recently, a new idea to calibrate GRBs in a completely
cosmology independent manner has been proposed, and the circularity problem can
be solved. In the present work, following the method proposed by Liang {\it et
al.}, we calibrate 70 GRBs with the Amati relation using 307 SNIa. Then,
following the method proposed by Shafieloo {\it et al.}, we smoothly
reconstruct the cosmic expansion history up to redshift $z=6.29$ with the
calibrated GRBs. We find some new features in the reconstructed results.

http://arxiv.org/abs/0808.2240