639 research outputs found
Evidence for anisotropy in the distribution of short-lived gamma-ray bursts
Measurements of the two-point angular correlation function w(\theta) for 407
short gamma-ray bursts collected in the Current BATSE Catalogue reveal a ~2
\sigma deviation from isotropy on angular scales \theta ~ 2-4 degrees. Such an
anisotropy is not observed in the distribution of long gamma-ray bursts and
hints to the presence of repeated bursts for up to ~13% of the sources under
exam. However, the available data cannot exclude the signal as due to the
presence of large-scale structure. Under this assumption, the amplitude of the
observed w(\theta) is compatible with those derived for different populations
of galaxies up to redshifts ~0.5, result that suggests short gamma-ray bursts
to be relatively local sources.Comment: 5 pages, 4 figures, submitted to MNRA
Spectral properties of long and short Gamma-Ray Bursts: comparison between BATSE and Fermi bursts
We compare the spectral properties of 227 Gamma Ray Bursts (GRBs) detected by
the Fermi Gamma Ray Burst Monitor (GBM) up to February 2010 with those of
bursts detected by the CGRO/BATSE instrument. Out of 227 Fermi GRBs, 166 have a
measured peak energy E_peak_obs of their \nuF(\nu) spectrum: of these 146 and
20 belong the long and short class, respectively. Fermi long bursts follow the
correlations defined by BATSE bursts between their E_peak_obs vs fluence and
peak flux: as already shown for the latter ones, these correlations and their
slopes do not originate from instrumental selection effects. Fermi/GBM bursts
extend such correlations toward lower fluence/peak energy values with respect
to BATSE ones whereas no GBM long burst with E_peak_obs exceeding a few MeV is
found, despite the possibility of detecting them. Again as for BATSE, 5%
of long and almost all short GRBs detected by Fermi/GBM are outliers of the
E_peak-isotropic equivalent energy ("Amati") correlation while no outlier
(neither long nor short) of the E_peak-isotropic equivalent luminosity
("Yonetoku") correlation is found. Fermi long bursts have similar typical
values of E_peak_obs but a harder low energy spectral index with respect to all
BATSE events, exacerbating the inconsistency with the limiting slopes of the
simplest synchrotron emission models. Although the short GRBs detected by Fermi
are still only a few, we confirm that their E_peak_obs is greater and the low
energy spectrum is harder than those of long ones. We discuss the robustness of
these results with respect to observational biases induced by the differences
between the GBM and BATSE instruments.Comment: 10 pages, 8 figures, submitted to A&
Constraints on the bulk Lorentz factor in the internal shock scenario for gamma-ray bursts
We investigate, independently of specific emission models, the constraints on
the value of the bulk Lorentz factor Gamma of a fireball. We assume that the
burst emission comes from internal shocks in a region transparent to Thomson
scattering and before deceleration due to the swept up external matter is
effective. We consider the role of Compton drag in decelerating fast moving
shells before they interact with slower ones, thus limiting the possible
differences in bulk Lorentz factor of shells. Tighter constraints on the
possible range of Gamma are derived by requiring that the internal shocks
transform more than a few per cent of the bulk energy into radiation. Efficient
bursts may require a hierarchical scenario, where a shell undergoes multiple
interactions with other shells. We conclude that fireballs with average Lorentz
factors larger than 1000 are unlikely to give rise to the observed bursts.Comment: 5 pages, 3 figures, accepted for publication in MNRAS, pink page
Bulk Comptonization spectra in blazars
We study the time dependent spectra produced via the bulk Compton process by
a cold, relativistic shell of plasma moving (and accelerating) along the jet of
a blazar, scattering on external photons emitted by the accretion disc and
reprocessed in the broad line region. Bulk Comptonization of disc photons is
shown to yield a spectral component contributing in the far UV band, and would
then be currently unobservable. On the contrary, the bulk Comptonization of
broad line photons may yield a significant feature in the soft X-ray band. Such
a feature is time-dependent and transient, and dominates over the non thermal
continuum only when: a) the dissipation occurs close to, but within, the broad
line region; b) other competing processes, like the synchrotron self-Compton
emission, yield a negligible flux in the X-ray band. The presence of a bulk
Compton component may account for the X-ray properties of high redshift blazars
that show a flattening (and possibly a hump) in the soft X-rays, previously
interpreted as due to intrinsic absorption. We discuss why the conditions
leading to a detectable bulk Compton feature might be met only occasionally in
high redshift blazars, concluding that the absence of such a feature in the
spectra of most blazars should not be taken as evidence against
matter--dominated relativistic jets. The detection of such a component carries
key information on the bulk Lorentz factor and kinetic energy associated to
(cold) leptons.Comment: 8 pages; 4 figures; MNRAS, accepte
The peak luminosity - peak energy correlation in GRBs
We derive the peak luminosity - peak energy (L_iso - E_peak) correlation
using 22 long Gamma-Ray Bursts (GRBs) with firm redshift measurements. We find
that its slope is similar to the correlation between the time integrated
isotropic emitted energy E_iso and E_peak (Amati et al. 2002). For the 15 GRBs
in our sample with estimated jet opening angle we compute the collimation
corrected peak luminosity L_gamma, and find that it correlates with E_peak.
This has, however, a scatter larger than the correlation between E_peak and
E_gamma (the time integrated emitted energy, corrected for collimation;
Ghirlanda et al. 2004), which we ascribe to the fact that the opening angle is
estimated through the global energetics. We have then selected a large sample
of 442 GRBs with pseudo--redshifts, derived through the lag-luminosity
relation, to test the existence of the L_iso-E_peak correlation. With this
sample we also explore the possibility of a correlation between time resolved
quantities, namely L_iso,p and the peak energy at the peak of emission
E_peak,p.Comment: 5 pages, 5 figures, 2 tables - MNRAS Letters submitte
A fireworks model for Gamma-Ray Bursts
The energetics of the long duration GRB phenomenon is compared with models of
a rotating Black Hole (BH) in a strong magnetic field generated by an accreting
torus. A rough estimate of the energy extracted from a rotating BH with the
Blandford-Znajek mechanism is obtained with a very simple assumption: an
inelastic collision between the rotating BH and the torus. The GRB energy
emission is attributed to an high magnetic field that breaks down the vacuum
around the BH and gives origin to a e+- fireball. Its subsequent evolution is
hypothesized, in analogy with the in-flight decay of an elementary particle, to
evolve in two distinct phases. The first one occurs close to the engine and is
responsible of energizing and collimating the shells. The second one consists
of a radiation dominated expansion, which correspondingly accelerates the
relativistic photon--particle fluid and ends at the transparency time. This
mechanism simply predicts that the observed Lorentz factor is determined by the
product of the Lorentz factor of the shell close to the engine and the Lorentz
factor derived by the expansion. An anisotropy in the fireball propagation is
thus naturally produced, whose degree depends on the bulk Lorentz factor at the
end of the collimation phase.Comment: Accepted for publication in MNRA
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