67 research outputs found
Gamma Ray Bursts Spectral--Energy correlations: recent results
The correlations between the rest frame peak of the EF_E spectrum of GRBs
Epeak and their isotropic energy (E_iso) or luminosity (L_iso) could have
several implications for the understanding of the GRB prompt emission. These
correlations are presently founded on the time-averaged spectral properties of
a sample of 95 bursts, with measured redshifts, collected by different
instruments in the last 13 years (pre-Fermi). One still open issue is wether
these correlations have a physical origin or are due to instrumental selection
effects. By studying 10 long and 14 short GRBs detected by Fermi we find that a
strong time-resolved correlation between E_peak and the luminosity L_iso is
present within individual GRBs and that it is consistent with the
time-integrated correlation. This result is a direct proof of the existence in
both short and long GRBs of a similar physical link between the hardness and
the luminosity which is not due to instrumental selection effects. The origin
of the E_peak-L_iso correlation should be searched in the radiation mechanism
of the prompt emission.Comment: 5 pages, 3 figures, to appear in the proceedings of the 275 IAU Symp:
"Jets at all scales", Buenos Aires, Sept. 201
SGR giant flares in the BATSE short GRB catalogue: constraints from spectroscopy
The giant flare observed on Dec. 27th 2004 from SGR 1806-20 has revived the
idea that a fraction of short (<2 s) Gamma Ray Bursts (GRBs) is due to giant
flares from Soft Gamma Ray Repeaters located in nearby galaxies. One of the
distinguishing characteristics of these events is the thermal (black body)
spectrum with temperatures ranging from ~50 to ~180 keV, with the highest
temperature observed for the initial 0.2 s spike of the Dec. 27th 2004 event.
We analyzed the spectra of a complete sample of short GRBs with peak fluxes
greater than 4 photon s^(-1) cm^(-2) detected by BATSE. Of the 115 short GRBs
so selected only 76 had sufficient signal to noise to allow the spectral
analysis. We find only 3 short GRBs with a spectrum well fitted by a black
body, with 601
sec) and a more complex light curve than the Dec. 27th 2004 event. This implies
a stringent limit on the rate of extragalactic SGR giant flares with spectral
properties analogous to the Dec. 27th flare. We conclude that up to 4 per cent
of the short GRBs could be associated to giant flares (2 sigma confidence).
This implies that either the distance to SGR 1806-20 is smaller than 15 kpc or
the rate of Galactic giant flares is lower than the estimated 0.033 per year.Comment: 5 pages, 3 figures, MNRAS in pres
Jet-driven and jet-less fireballs from compact binary mergers
During a compact binary merger involving at least one neutron star, a small
fraction of the gravitational energy could be liberated in such a way to
accelerate a small fraction (~ 10^-6) of the neutron star mass in an isotropic
or quasi-isotropic way. In presence of certain conditions, a pair-loaded
fireball can form, which undergoes accelerated expansion reaching relativistic
velocities. As in the standard fireball scenario, internal energy is partly
transformed into kinetic energy. At the photospheric radius, the internal
radiation can escape, giving rise to a pulse that lasts for a time equal to the
delay time since the merger. The subsequent interaction with the interstellar
medium can then convert part of the remaining kinetic energy back into
radiation in a weak isotropic afterglow at all wavelengths. This scenario does
not require the presence of a jet: the associated isotropic prompt and
afterglow emission should be visible for all NS-NS and BH-NS mergers within 90
Mpc, independent of their inclination. The prompt emission is similar to that
expected from an off-axis jet, either structured or much slower than usually
assumed (Gamma ~ 10), or from the jet cocoon. The predicted afterglow emission
properties can discriminate among these scenarios.Comment: 5 pages, 1 figure, revised version submitted to MNRAS Letter
The luminosity--volume test for cosmological Fast Radio Bursts
We perform the luminosity--volume test, also known as , to Fast Radio Bursts (FRBs). We compare the 23 FRBs,
recently discovered by ASKAP, with 20 of the FRBs found by Parkes. These
samples have different flux limits and correspond to different explored
volumes. We assume that their dispersion measure indicates their redshift and
apply the appropriate cosmological corrections to the spectrum and rate in
order to compute the for the ASKAP and Parkes
samples. For a radio spectrum of FRBs , we find
for the ASKAP sample, that includes
FRBs up to , and 0.52 for Parkes, that extends up to .
The ASKAP value suggests that the population of FRB progenitors evolves faster
than the star formation rate, while the Parkes value is consistent with it.
Even a delayed (as a power law or gaussian) star formation rate cannot
reproduce the of both samples. If FRBs do not evolve
in luminosity, the values of ASKAP and Parkes sample
are consistent with a population of progenitors whose density strongly evolves
with redshift as up to . We discuss possible
scenarios accounting for our results.Comment: 10 pages, 5 figures, 3 tables, accepted by A&A on 2019/04/0
Interpreting GRB170817A as a giant flare from a jet-less double neutron-star merger
We show that the delay between GRB170817A and GW170817 is incompatible with
de-beamed emission from an off-axis relativistic jet. The prompt emission and
the subsequent radio and X-ray observations can instead be interpreted within a
giant-flare-like scenario, being the result of a relativistic outflow driven by
the ultra-strong magnetic field produced by magnetohydrodynamic amplification
during the merger of the progenitor double neutron-star binary. Within such
picture, the data indicate that the outflow must be endowed with a steep
velocity profile, with a relatively fast tail extending to Gamma~8. Since the
conditions for the launch of such an outflow are quite general, and the
presence of a velocity profile is a natural expectation of the acceleration
process, most neutron star binary mergers should feature this quasi-isotropic,
hard X-ray emission component, that can be a powerful guide to the discovery of
additional kilonovae associated to relatively nearby gravitational wave events.Comment: 6 pages, 2 figures, accepted by Astronomy and Astrophysic
Light curves and spectra from off-axis gamma-ray bursts
If gamma-ray burst prompt emission originates at a typical radius, and if
material producing the emission moves at relativistic speed, then the
variability of the resulting light curve depends on the viewing angle. This is
due to the fact that the pulse evolution time scale is Doppler contracted,
while the pulse separation is not. For off-axis viewing angles , the pulse broadening
significantly smears out the light curve variability. This is largely
independent of geometry and emission processes. To explore a specific case, we
set up a simple model of a single pulse under the assumption that the pulse
rise and decay are dominated by the shell curvature effect. We show that such a
pulse observed off-axis is (i) broader, (ii) softer and (iii) displays a
different hardness-intensity correlation with respect to the same pulse seen
on-axis. For each of these effects, we provide an intuitive physical
explanation. We then show how a synthetic light curve made by a superposition
of pulses changes with increasing viewing angle. We find that a highly variable
light curve, (as seen on-axis) becomes smooth and apparently single-pulsed
(when seen off-axis) because of pulse overlap. To test the relevance of this
fact, we estimate the fraction of off-axis gamma-ray bursts detectable by
\textit{Swift} as a function of redshift, finding that a sizable fraction
(between 10\% and 80\%) of nearby () bursts are observed with
. Based on these
results, we argue that low luminosity gamma-ray bursts are consistent with
being ordinary bursts seen off-axis.Comment: 13 pages, 17 figures, submitted to MNRAS main journal; updated
estimate of the fraction of off-axis grbs seen by Swif
Orphan afterglows: the off-axis view of gamma ray bursts
GRBs detected in the γ–ray band are those “seen" within their jets (which have typical opening angles θjet ∼ 2 − 10 degrees). When seen off–axis, the high relativistic beaming (due to bulk Lorentz factors Γ > 100) prevents the detection of the high energy γ–ray emission. However, the slow down of the outflow (due to the interaction with the interstellar matter) makes the afterglow potentially observable a few days after the GRB itself. Deep wide field radio surveys have the. potential to detect the off-axis population of Gamma Ray Bursts (dubbed Orphan Afterglows - OA) as relatively slow (days to months timescales) transients (at a flux density level of 10-100 µJy
at GHz frequencies) with a detection rate of few events per year. The detection rate of OA should increase from the the VLASS (∼16 yr−1) through the VAST-Wide (∼25–45 yr−1) to the SKA1 (∼220 yr−1) survey. On the other hand, low frequency (MHz) surveys have less chances to detect these transients. Spectroscopy and multi wavelength follow up will be necessary to distinguish OA from extragalactic transients (SN, TDE and AGN flares) with similar temporal properties and brightness. Detection and study of the OA population in the radio band is of primary importance for the potential support in the identification of the counterparts of gravitational wave events
Selection effects on GRB spectral-energy correlations
Instrumental selection effects can act upon the estimates of the peak energy
Ep, the fluence F and the peak flux P of GRBs. If this were the case, then the
correlations involving the corresponding rest frame quantities (i.e. Ep, Eiso
and the peak luminosity Liso) would be questioned. We estimated, as a function
of Ep, the minimum peak flux necessary to trigger a GRB and the minimum fluence
a burst must have to determine the value of Ep by considering different
instruments (BATSE, Swift, BeppoSAX). We find that the latter dominates over
the former. We then study the Ep-fluence (and flux) correlation in the observer
plane. GRBs with redshift show well defined Ep-F and Ep-P correlations: in this
planes the selection effects are present, but do not determine the found
orrelations. This is not true for Swift GRBs with redshift, for which the
spectral analysis threshold does affect their distribution in the observer
planes. Extending the sample to GRBs without z, we still find a significant
Ep-F correlation, although with a larger scatter than that defined by GRBs with
redshift. We find that 6% are outliers of the Amati correlation. The Ep-P
correlation of GRBs with or without redshift is the same and no outlier is
found among bursts without redshift.Comment: 6 pages, 3 figures. Contributed to the Proceedings of the Sixth
Huntsville GRB Symposiu
On the orientation of Narrow Line Seyfert I
We study a sample of Narrow-line Seyfert 1 galaxies (NLS1) in their optical
and radio features, in order to understand the differences between their radio
silent, radio-loud and radio-quiet subclasses. We first show that the different
redshift and mass distributions of radio-loud and -quiet NLS1s could be
ascribed to observational biases. We then present a geometrical model according
to which most of the different observational features of radio-loud and
radio-quiet NLS1s are ascribed to the orientation of an intrinsically
structured jet. We estimate the fraction of intrinsically jetted sources among
NLS1s that justifies the observed radio-detected population. Noticeably, under
the assumptions of the geometrical model, we derive a fraction of jetted
sources significantly larger than in standard AGN.Comment: 6 pages, 3 figures, accepted for publication on A&
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