365 research outputs found
Neutrinos From Individual Gamma-Ray Bursts in the BATSE Catalog
We calculate the neutrino emission from individual gamma-ray bursts observed
by the BATSE detector on the Compton Gamma-Ray Observatory. Neutrinos are
produced by photoproduction of pions when protons interact with photons in the
region where the kinetic energy of the relativistic fireball is dissipated
allowing the acceleration of electrons and protons. We also consider models
where neutrinos are predominantly produced on the radiation surrounding the
newly formed black hole. From the observed redshift and photon flux of each
individual burst, we compute the neutrino flux in a variety of models based on
the assumption that equal kinetic energy is dissipated into electrons and
protons. Where not measured, the redshift is estimated by other methods. Unlike
previous calculations of the universal diffuse neutrino flux produced by all
gamma-ray bursts, the individual fluxes (compiled at
http://www.arcetri.astro.it/~dafne/grb/) can be directly compared with
coincident observations by the AMANDA telescope at the South Pole. Because of
its large statistics, our predictions are likely to be representative for
future observations with larger neutrino telescopes.Comment: 49 pages, 7 figures. Accepted for publication in Astroparticle
Physic
Analysis of X-ray flares in GRBs
We present a detailed study of the spectral and temporal properties of the
X-ray flares emission of several GRBs. We select a sample of GRBs which X-ray
light curve exhibits large amplitude variations with several rebrightenings
superposed on the underlying three-segment broken powerlaw that is often seen
in Swift GRBs. We try to understand the origin of these fluctuations giving
some diagnostic in order to discriminate between refreshed shocks and late
internal shocks. For some bursts our time-resolved spectral analysis supports
the interpretation of a long-lived central engine, with rebrightenings
consistent with energy injection in refreshed shocks as slower shells generated
in the central engine prompt phase catch up with the afterglow shock at later
times.Comment: 9 pages, 3 figures. Invited talk at the Swift-Venice 2006 meeting to
be published by "Il Nuovo Cimento
Do long-duration GRBs follow star formation?
We compare the luminosity function and rate inferred from the BATSE long
bursts peak flux distribution with those inferred from the Swift peak flux
distribution. We find that both the BATSE and the Swift peak fluxes can be
fitted by the same luminosity function and the two samples are compatible with
a population that follows the star formation rate. The estimated local long GRB
rate (without beaming corrections) varies by a factor of five from 0.05
Gpc^(-3)yr^(-1) for a rate function that has a large fraction of high redshift
bursts to 0.27 Gpc^(-3)yr^(-1) for a rate function that has many local ones. We
then turn to compare the BeppoSax/HETE2 and the Swift observed redshift
distributions and compare them with the predictions of the luminosity function
found. We find that the discrepancy between the BeppoSax/HETE2 and Swift
observed redshift distributions is only partially explained by the different
thresholds of the detectors and it may indicate strong selection effects. After
trying different forms of the star formation rate (SFR) we find that the
observed Swift redshift distribution, with more observed high redshift bursts
than expected, is inconsistent with a GRB rate that simply follows current
models for the SFR. We show that this can be explained by GRB evolution beyond
the SFR (more high redshift bursts). Alternatively this can also arise if the
luminosity function evolves and earlier bursts were more luminous or if strong
selection effects affect the redshift determination.Comment: 15 pages, 8 figures, accepted for publication in JCA
Extragalactic gamma-ray background from AGN winds and star-forming galaxies in cosmological galaxy formation models
We derive the contribution to the extragalactic gamma-ray background (EGB)
from AGN winds and star-forming galaxies by including a physical model for the
gamma-ray emission produced by relativistic protons accelerated by AGN-driven
and supernova-driven shocks into a state-of-the-art semi-analytic model of
galaxy formation. This is based on galaxy interactions as triggers of AGN
accretion and starburst activity and on expanding blast wave as the mechanism
to communicate outwards the energy injected into the interstellar medium by the
active nucleus. We compare the model predictions with the latest measurement of
the EGB spectrum performed by the Fermi-LAT in the range between 100 MeV and
820 GeV. We find that AGN winds can provide ~3515% of the observed EGB in
the energy interval E_{\gamma}=0.1-1 GeV, for ~7315% at E_{\gamma}=1-10
GeV, and for ~6020% at E_{\gamma}>10 GeV. The AGN wind contribution to the
EGB is predicted to be larger by a factor of 3-5 than that provided by
star-forming galaxies (quiescent plus starburst) in the hierarchical clustering
scenario. The cumulative gamma-ray emission from AGN winds and blazars can
account for the amplitude and spectral shape of the EGB, assuming the standard
acceleration theory, and AGN wind parameters that agree with observations. We
also compare the model prediction for the cumulative neutrino background from
AGN winds with the most recent IceCube data. We find that for AGN winds with
accelerated proton spectral index p=2.2-2.3, and taking into account internal
absorption of gamma-rays, the Fermi-LAT and IceCube data could be reproduced
simultaneously.Comment: 12 pages, 8 figures, accepted for publication in A&
Selection effects shaping the Gamma Ray Burst redshift distributions
Long Gamma Ray Bursts hold the promise of probing star-formation and metal
enrichment up to very high redshifts. The present GRB samples with redshift
determination are largely incomplete and therefore a careful analysis of
selection effects is mandatory before any conclusion can be drawn from the
observed GRB redshift distribution. We study and compare three well defined
samples of long GRBs detected by Swift, HETE2 and BeppoSAX. We find that Swift
GRBs are slighly fainter and harder than BeppoSAX and HETE2 GRBs, as expected
due to the higher energy range in which Swift GRBs are detected and localized,
compared to BeppoSAX and HETE2. Gas and dust obscuration plays a role in
shaping the GRB samples and the present samples of GRBs with redshift. We argue
that the majority of the bright Swift GRBs without redshift might actually be
z<~2 events therefore the present Swift GRB sample with redshift is biased
against low-z GRBs. On the other hand, the detection of bright UV rest-frame
afterglows from high-z GRBs, and even from those with large X-ray obscuration,
implies a dust amount lower than in nearby GRBs,and/or a different dust
composition. If this is the case, the Swift sample of GRBs with redshifts is
probably a fair sample of the real high-z GRB population. The absence of high-z
GRBs in the BeppoSAX and HETE2 samples of GRBs with redshifts is probably due
to the fact at the time of BeppoSAX and HETE2 follow-up faint afterglows of
high redshift GRBs will have weaken below the spectroscopic capabilities of
even 10m class telescopes. The redshift distribution of a subsample of Swift
GRBs with distributions of peak-fluxes, X-ray obscuration and optical magnitude
at a fixed observing time similar to those of the BeppoSAX and HETE2 samples,
is roughly consistent with BeppoSAX+HETE2 redshift distribution.Comment: 9 pages, back to A&A after referee repor
Precursor Plerionic Activity and High Energy Gamma-Ray Emission in the Supranova Model of Gamma-Ray Bursts
The supranova model of gamma-ray bursts (GRBs), in which the GRB event is
preceded by a supernova (SN) explosion by a few months to years, has recently
gained support from Fe line detections in X-ray afterglows. A crucial
ingredient of this model yet to be studied is the fast-rotating pulsar that
should be active during the time interval between the SN and the GRB, driving a
powerful wind and a luminous plerionic nebula. We discuss some observational
consequences of this precursor plerion, which should provide important tests
for the supranova model: 1) the fragmentation of the outlying SN ejecta
material by the plerion and its implications for Fe line emission; and 2) the
effect of inverse Compton cooling and emission in the GRB external shock due to
the plerion radiation field. The plerion-induced inverse Compton emission can
dominate in the GeV-TeV energy range during the afterglow, being detectable by
GLAST from redshifts and distinguishable from self-Compton
emission by its spectrum and light curve. The prospects for direct detection
and identification of the precursor plerion emission are also briefly
considered.Comment: ApJ vol.583, in pres
On the generation of UHECRs in GRBs: a reappraisal
We re-examine critically the arguments raised against the theory that Ultra
High Energy Cosmic Rays observed at Earth are produced in Gamma Ray Bursts.
These include the limitations to the highest energy attainable by protons
around the bursts' shocks, the spectral slope at the highest energies, the
total energy released in non--thermal particles, the occurrence of doublets and
triplets in the data reported by AGASA. We show that, to within the
uncertainties in our current knowledge of GRBs, none of these objections is
really fatal to the scenario. In particular, we show that the total energy
budget of GRBs easily accounts for the energy injection rate necessary to
account for UHECRs as observed at Earth. We also compute the expected particle
spectrum at Earth, showing that it fits the HiRes and AGASA data to within
statistical uncertainties. We consider the existence of multiplets in AGASA'
data. To this end, we present a Langevin--like treatment for the motion of a
charged particle in the IGM magnetic field, which allows us to estimate both
the average and the rms timedelay for particles of given energy; we discuss
when particles of identical energies reach the Earth in bunches, or spread over
the rms timedelay, showing that multiplets pose no problem for an explosive
model for the sources of UHECRs. We compare our model with a scenario where the
particles are accelerated at internal shocks, underlining differences and
advantages of particle acceleration at external shocks.Comment: Accepted for publication in the Astrophysical Journal; minor change
On the detectability of gravitational waves background produced by gamma ray bursts
In this paper we discuss a new strategy for the detection of gravitational
radiation likely emitted by cosmological gamma ray burst. Robust and
conservative estimates lead to the conclusion that the uncorrelated
superimposition of bursts of gravitational waves can be detected by
interferometric detectors like VIRGO or LIGO. The expected signal is predicted
to carry two very distinctive signatures: the cosmological dipole anisotropy
and a characteristic time scale in the auto correlation spectrum, which might
be exploited, perhaps with ad hoc modifications and/or upgrading of the planned
experiments, to confirm the non-instrumental origin of the signal.Comment: 9 pages, 2 figures, LATEX2e, Accepted for pubblications as a Letter
to the Editor in Journal of Physics G: Nuclear and Particle Physic
- …