astro-grb

Ruth Lazkoz, Vincenzo Salzano, Irene Sendra

Model independent reconstructions of dark energy have received some
attention. The approach that addresses the reconstruction of the dimensionless
coordinate distance and its two first derivatives using a polynomial fit in
different redshift windows is well developed
\cite{DalyDjorgovski1,DalyDjorgovski2,DalyDjorgovski3}. In this work we offer
new insights into the problem by focusing on two types of observational probes:
SNeIa and GRBs. Our results allow to highlight some of the intrinsic weaknesses
of the method. One of the directions we follow is to consider updated
observational samples. Our results indicate than conclusions on the main dark
energy features as drawn from this method are intimately related to the
features of the samples themselves (which are not quite ideal). This is
particularly true of GRBs, which manifest themselves as poor performers in this
context. In contrast to original works, we conclude they cannot be used for
cosmological purposes, and the state of the art does not allow to regard them
on the same quality basis as SNeIa. The next direction we contribute to is the
question of how the adjusting of some parameters (window width, overlap,
selection criteria) affect the results. We find again there is a considerable
sensitivity to these features. Then, we try to establish what is the current
redshift range for which one can make solid predictions on dark energy
evolution. Finally, we strengthen the former view that this model is modest in
the sense it provides only a picture of the global trend. But, on the other
hand, we believe it offers an interesting complement to other approaches given
that it works on minimal assumptions.

http://arxiv.org/abs/1202.4689

P. Eger, W. Domainko

Recently, diffuse and extended sources in TeV gamma-rays as well as in X-rays
have been detected in the direction of the Galactic globular cluster (GC)
Terzan 5. Remarkably, this is among the brightest GCs detected in the GeV
regime. The nature of both the TeV and the diffuse X-ray signal from Terzan 5
is not settled yet. These emissions most likely indicate the presence of
several non-thermal radiation processes in addition to these giving rise to the
GeV signal.

The aim of this work is to search for diffuse X-ray emission from the GeV
detected GCs M 62, NGC 6388, NGC 6541, M 28, M 80 and NGC 6139 to compare the
obtained results with the signal detected from Terzan 5. This study will help
to determine whether Terzan 5 stands out amongst other GC or whether a whole
population of globular clusters feature similar properties.

None of the six GCs show significant diffuse X-ray emission on similar scales
as observed from Terzan 5 above the particle and diffuse galactic X-ray
background components. The derived upper limits allow to assess the validity of
different models that were discussed in the interpretation of the
multi-wavelength data of Terzan 5. A scenario based on synchrotron emission
from relativistic leptons provided by the millisecond pulsar population can not
be securely rejected if a comparable magnetic field strength as in Terzan 5 is
assumed for every GC. However, such a scenario seems to be unlikely for NGC
6388 and M 62. An inverse-Compton scenario relying on the presence of a
putative GRB remnant with the same properties as the one proposed for Terzan 5
can be ruled out for all of the six GCs. Finally, the assumption that each GC
hosts a source with the same luminosity as in Terzan 5 is ruled out for all GCs
but NGC 6139. (abridged)

http://arxiv.org/abs/1202.2749

L. Izzo, R. Ruffini, A. V. Penacchioni, C. L. Bianco, L. Caito, S. K. Chakrabarti, Jorge A. Rueda, A. Nandi, B. Patricelli

The joint X and gamma-ray observations of GRB 090618 by a large number of
satellites offer an unprecedented possibility of testing crucial aspects of
theoretical models. In particular, it allows us to test (a) the formation of an
optically thick e+e- baryon plasma self-accelerating to Lorentz factors in the
range 200 < g < 3000; (b) its transparency condition with the emission of a
component of 10^{53-54} baryons in the TeV region and (c) the collision of
these baryons with the circumburst medium clouds, characterized by dimensions
of 10^{15-16} cm. In addition, these observations offer the possibility of
testing a new understanding of the thermal and power-law components in the
early phase of this GRB. We test the fireshell model of GRBs in one of the
closest (z = 0.54) and most energetic (Eiso = 2.90 x 10^{53} ergs) GRBs, namely
GRB 090618. We analyze its emission using several spectral models, with special
attention to the thermal and power-law components. We determine the fundamental
parameters of a canonical GRB within the context of the fireshell model. We
find evidences of the existence of two different episodes in GRB 090618. The
first episode lasts 50 s and is characterized by a spectrum consisting of
thermal component, which evolves between kT = 54 keV and kT = 12 keV. The
second episode, which lasts for \sim 100s, behaves as a canonical long GRB with
a Lorentz gamma factor at transparency of g = 495, a temperature at
transparency of 29.22 keV and with characteristic masses of the surrounding
clouds of \sim 10^{22-24} g. We support the recently proposed two-component
nature of GRB 090618 by using specific theoretical analysis and illustrate that
the episode 1 cannot be considered to be either a GRB or a part of a GRB event,
but it appears to be related to the progenitor of the collapsing bare core
leading to the formation of the black hole which we call a proto-black hole.

http://arxiv.org/abs/1202.4374

C. Guidorzi, R. Margutti, L. Amati, S. Campana, M. Orlandini, P. Romano, M. Stamatikos, G. Tagliaferri

We calculate the average power density spectra (PDS) of 244 long gamma-ray
bursts detected with the Swift Burst Alert Telescope in the 15-150 keV band
from January 2005 to August 2011. For the first time we derived the average PDS
in the source rest frame of 97 GRBs with known redshift. For 49 of them an
average PDS was also obtained in a common source-frame energy band to account
for the dependence of time profiles on energy. Previous results obtained on
BATSE GRBs with unknown redshift showed that the average spectrum in the
25-2000 keV band could be modelled with a power-law with a 5/3 index over
nearly two decades of frequency with a break at ~1 Hz. Depending on the
normalisation and on the subset of GRBs considered, our results show analogous
to steeper slopes (between 1.7 and 2.0) of the power-law. However, no clear
evidence for the break at ~1 Hz was found, although the softer energy band of
BAT compared with BATSE might account for that. We instead find a break at
lower frequency corresponding to a typical source rest frame characteristic
time of a few seconds. We furthermore find no significant differences between
observer and source rest frames. Notably, no distinctive PDS features are found
for GRBs with different intrinsic properties of the prompt emission either.
Finally, the average PDS of GRBs at higher redshifts shows possibly shallower
power-law indices than that of low-z GRBs. It is not clear whether this is due
to an evolution with z of the average PDS.

http://arxiv.org/abs/1202.3443

S. Campana, R. Salvaterra, A. Melandri, S. D. Vergani, S. Covino, P. D'Avanzo, D. Fugazza, G. Ghisellini, B. Sbarufatti, G. Tagliaferri

A complete sample of bright Swift Gamma-ray Bursts (GRBs) has been recently
selected by Salvaterra et al. (2011). The sample has a high level of
completeness in redshift (91%). We derive here the intrinsic absorbing X-ray
column densities of these GRBs making use of the Swift X-ray Telescope data.
This distribution has a mean value of log(NH/cm-2)=21.7+-0.5. This value is
consistent with the distribution of the column densities derived from the total
sample of GRBs with redshift. We find a mild increase of the intrinsic column
density with redshift. This can be interpreted as due to the contribution of
intervening systems along the line of sight. Making use of the spectral index
connecting optical and X-ray fluxes at 11 hr (beta_OX), we investigate the
relation of the intrinsic column density and the GRB `darkness'. We find that
there is a very tight correlation between dark GRBs and high X-ray column
densities. This clearly indicates that the dark GRBs are formed in a metal-rich
environment where dust must be present.

http://arxiv.org/abs/1112.5111

Ariadna Montiel, Nora Bretón

We study the range of consistency of a model based on a nonlinear scalar
field Dirac-Born-Infeld action for the unification of dark matter and dark
energy using Gamma-Ray Bursts at high-redshifts. We use the sample of 59
high-redshift GRBs reported by Wei (2010), calibrated at low redshifts with the
Union 2 sample of SNe Ia, thus avoiding the circularity problem. In this
analysis, we also include the CMB7-year data and the baryonic acoustic peak
BAO. Besides, it is calculated the parameter of the equation of state $w$, the
deceleration parameter $q_0$ and the redshift of the transition to the
decelerate-accelerated phase $z_t$.

http://arxiv.org/abs/1202.2970

Rupal Basak, A. R. Rao

We find a strong correlation between the peak energy at zero fluence ($\rm
E_{peak,0}$) and the isotropic energy ($\rm E_{\gamma,iso}$) of the 22 pulses
of 9 Gamma Ray Bursts (GRB) detected by the Fermi satellite. The correlation
holds for the individual pulses of each GRB, which shows the reality of the
correlation. The derived correlation (Spearman correlation coefficient, $r$, is
0.96) is much stronger compared to the correlations using $\rm E_{peak}$ (in
place of $\rm E_{peak,0}$) determined from the time-integrated spectrum ($r$ =
0.8), or the time-resolved spectrum not accounting for broad pulse structures
($r$ = 0.37), or the pulse-wise spectrum ($r$ = 0.89). Though the improvement
in the $\rm E_{peak}$ - $\rm E_{\gamma,iso}$ relation (the Amati relation) for
a pulse-wise analysis is known earlier, this is the first time a parameter
derived from a joint spectral and timing fit to the data is shown to improve
the correlation. We suggest that $\rm E_{peak,0}$, rather than $\rm E_{peak}$,
is intrinsic to a GRB pulse and a natural choice as the parameter in the
pulse-wise correlation studies.

http://arxiv.org/abs/1202.3089

R. Salvaterra, S. Campana, S. D. Vergani, S. Covino, P. D'Avanzo, D. Fugazza, G. Ghirlanda, G. Ghisellini, A. Melandri, L. Nava, B. Sbarufatti, H. Flores, S. Piranomonte, G. Tagliaferri

We present a carefully selected sub-sample of Swift Long Gamma-ray Bursts
(GRBs), that is complete in redshift. The sample is constructed by considering
only bursts with favorable observing conditions for ground-based follow-up
searches, that are bright in the 15-150 keV Swift/BAT band, i.e. with 1-s peak
photon fluxes in excess to 2.6 ph s^-1 cm^-2. The sample is composed by 58
bursts, 52 of them with redshift for a completeness level of 90%, while another
two have a redshift constraint, reaching a completeness level of 95%. For only
three bursts we have no constraint on the redshift. The high level of redshift
completeness allows us for the first time to constrain the GRB luminosity
function and its evolution with cosmic times in a unbiased way. We find that
strong evolution in luminosity (d_l=2.3\pm 0.6) or in density (d_d=1.7\pm 0.5)
is required in order to account for the observations. The derived redshift
distribution in the two scenarios are consistent with each other, in spite of
their different intrinsic redshift distribution. This calls for other
indicators to distinguish among different evolution models. Complete samples
are at the base of any population studies. In future works we will use this
unique sample of Swift bright GRBs to study the properties of the population of
long GRBs.

http://arxiv.org/abs/1112.1700

L. Nava (1), R. Salvaterra (2), G. Ghirlanda (3), G. Ghisellini (3), S. Campana (3), S. Covino (3), G. Cusumano (4), P. D'Avanzo (3), V. D'Elia (5,6), D. Fugazza (3), A. Melandri (3), B. Sbarufatti (3), S. D. Vergani (3), G. Tagliaferri (3) ((1) SISSA-ISAS, (2) INAF-IASF Milano, (3) INAF-Oss. Astron. Brera, (4) INAF-IASF Palermo, (5) ASDC Rome, (6) INAF-Oss. Astron. Roma)

We use a nearly complete sample of Gamma Ray Bursts (GRBs) detected by the
Swift satellite to study the correlations between the spectral peak energy Ep
of the prompt emission, the isotropic energetics Eiso and the isotropic
luminosity Liso. This GRB sample is characterized by a high level of
completeness in redshift (90%). This allows us to probe in an unbiased way the
issue related to the physical origin of these correlations against selection
effects. We find that one burst, GRB 061021, is an outlier to the Ep-Eiso
correlation. Despite this case, we find strong Ep-Eiso and Ep-Liso correlations
for the bursts of the complete sample. Their slopes, normalisations and
dispersions are consistent with those found with the whole sample of bursts
with measured redshift and Ep. This means that the biases present in the total
sample commonly used to study these correlations do not affect their
properties. Finally, we also find no evolution with redshift of the Ep-Eiso and
Ep-Liso correlations.

http://arxiv.org/abs/1112.4470

S. Campana, R. Salvaterra, A. Melandri, S. D. Vergani, S. Covino, P. D'Avanzo, D. Fugazza, G. Ghisellini, B. Sbarufatti, G. Tagliaferri

A complete sample of bright Swift Gamma-ray Bursts (GRBs) has been recently
selected by Salvaterra et al. (2011). The sample has a high level of
completeness in redshift (91%). We derive here the intrinsic absorbing X-ray
column densities of these GRBs making use of the Swift X-ray Telescope data.
This distribution has a mean value of log(NH/cm-2)=21.7+-0.5. This value is
consistent with the distribution of the column densities derived from the total
sample of GRBs with redshift. We find a mild increase of the intrinsic column
density with redshift. This can be interpreted as due to the contribution of
intervening systems along the line of sight. Making use of the spectral index
connecting optical and X-ray fluxes at 11 hr (beta_OX), we investigate the
relation of the intrinsic column density and the GRB `darkness'. We find that
there is a very tight correlation between dark GRBs and high X-ray column
densities. This clearly indicates that the dark GRBs are formed in a metal-rich
environment where dust must be present.

http://arxiv.org/abs/1112.5111