235 research outputs found
Epeak estimator for Gamma-Ray Bursts Observed by the Swift Burst Alert Telescope
We report a correlation based on a spectral simulation study of the prompt
emission spectra of gamma-ray bursts (GRBs) detected by the Swift Burst Alert
Telescope (BAT). The correlation is between the Epeak energy, which is the peak
energy in the \nu F_\nu spectrum, and the photon index (\Gamma) derived from a
simple power-law model. The Epeak - \Gamma relation, assuming the typical
smoothly broken power-law spectrum of GRBs, is \log Epeak = 3.258 - 0.829\Gamma
(1.3 < \Gamma < 2.3). We take into account not only a range of Epeak energies
and fluences, but also distributions for both the low-energy photon index and
the high-energy photon index in the smoothly broken power-law model. The
distribution of burst durations in the BAT GRB sample is also included in the
simulation. Our correlation is consistent with the index observed by BAT and
Epeak measured by the BAT, and by other GRB instruments. Since about 85% of
GRBs observed by the BAT are acceptably fit with the simple power-law model
because of the relatively narrow energy range of the BAT, this relationship can
be used to estimate Epeak when it is located within the BAT energy range.Comment: 27 pages, 31 figures, accepted for publication in Ap
Peak energy of the prompt emission of long Gamma Ray Bursts vs their fluence and peak flux
The spectral-energy and (luminosity) correlations in long GRBs are being
hotly debated to establish, first of all, their reality against possible
selection effects. These are best studied in the observer planes, namely the
peak energy E_peak_obs vs the fluence F or the peak flux P. In a recent paper
we started to attack this problem considering all GRBs with known z and
spectral properties. Here we consider instead all bursts with known E_peak_obs,
irrespective of z, adding to those a sample of 100 faint BATSE bursts
representative of a larger population. This allows us to construct a complete,
fluence limited, sample, to study the selection/instrumental effects. We found
that fainter bursts have smaller E_peak_obs than those of bright events. As a
consequence, the E_peak_obs of these bursts is correlated with the fluence,
though with a slope flatter than that defined by bursts with z. Selection
effects, which are present, are shown not to be responsible for the existence
of such a correlation. About 6% of these bursts are surely outliers of the
E_peak-E_iso correlation (updated to include 83 bursts), since they are
inconsistent with it for any z. E_peak_obs correlates also with P, with a slope
similar to the E_peak-L_iso correlation.In this case there is only one sure
outlier.The scatter of the E_peak_obs-P correlation defined by the BATSE bursts
of our sample is smaller than the E_peak_obs-F correlation of the same bursts,
while for the bursts with known z the E_peak-E_iso correlation is tighter than
the E_peak-L_iso one. Once a very large number of bursts with E_peak_obs and z
will be available, we thus expect that the E_peak-L_iso correlation will be
similar to that currently found, whereas it is likely that the E_peak-E_iso
correlation will become flatter and with a larger scatter.Comment: 17 pages, 8 figures. Accepted for publication in MNRA
Testing the Epeak - Eiso relation for GRBs detected by Swift and Suzaku-WAM
One of the most prominent, yet controversial associations derived from the
ensemble of prompt-phase observations of gamma-ray bursts (GRBs) is the
apparent correlation in the source frame between the peak energy Epeak) of the
nu-F(nu) spectrum and the isotropic radiated energy, Eiso. Since most gamma-ray
bursts (GRBs) have Epeak above the energy range (15-150 keV) of the Burst Alert
Telescope (BAT) on Swift, determining accurate Epeak values for large numbers
of Swift bursts has been difficult. However, by combining data from Swift/BAT
and the Suzaku Wide-band All-Sky Monitor (WAM), which covers the energy range
from 50-5000 keV, for bursts which are simultaneously detected, one can
accurately fit Epeak and Eiso and test the relationship between them for the
Swift sample. Between the launch of Suzaku in July 2005 and the end of April
2009, there were 48 gamma-ray bursts (GRBs) which triggered both Swift/BAT and
WAM and an additional 48 bursts which triggered Swift and were detected by WAM,
but did not trigger. A BAT-WAM team has cross-calibrated the two instruments
using GRBs, and we are now able to perform joint fits on these bursts to
determine their spectral parameters. For those bursts with spectroscopic
redshifts, we can also calculate the isotropic energy. Here we present the
results of joint Swift/BAT-Suzaku/WAM spectral fits for 91 of the bursts
detected by the two instruments. We show that the distribution of spectral fit
parameters is consistent with distributions from earlier missions and confirm
that Swift bursts are consistent with earlier reported relationships between
Epeak and isotropic energy. We show through time-resolved spectroscopy that
individual burst pulses are also consistent with this relationship.Comment: Accepted for publication in the Astrophysical Journa
The first GRB survey of the IBIS/PICsIT archive
The multi-purpose INTEGRAL mission is continuously contributing to Gamma Ray
Burst (GRB) science, thanks to the performances of its two main instruments,
IBIS and SPI, operating in the hard X-ray/soft gamma-ray domain. We investigate
the possibilities offered to the study of GRBs by PICsIT, the high-energy
detector of the IBIS instrument. We searched for transient episodes in the
PICsIT light curves archive from May 2006 to August 2009, using stringent
criteria optimized for the detection of long events. In the time interval under
examination PICsIT provides an energy coverage from 208 to 2600 keV, resolved
in eight energy channels, combined with a fine time resolution of 16 ms. PICsIT
successfully observes GRBs in the 260-2600 keV energy range with an incoming
direction spread over half the sky for the brightest events. We compiled a list
of 39 bursts, most of which are confirmed GRBs or simultaneous to triggers from
other satellites/instruments. We produced light curves with a time sampling
down to 0.25 s in three energy intervals for all events. Because an adequate
response matrix is not yet available for the PICsIT burst sample, we obtained a
calibration coefficient in three selected energy bands by comparing
instrumental counts with physical fluences inferred from observations with
different satellites. The good time resolution provided by the PICsIT data
allows a spectral variability study of our sample through the hardness ratio.Comment: 20 pages, 48 figures. Accepted for publication in A&
Extended Emission from Short Gamma-Ray Bursts Detected with SPI-ACS/INTEGRAL
The short duration (T90 < 2 s) gamma-ray bursts (GRBs) detected in the
SPI-ACS experiment onboard the INTEGRAL observatory are investigated. Averaged
light curves have been constructed for various groups of events, including
short GRBs and unidentified short events. Extended emission has been found in
the averaged light curves of both short GRBs and unidentified short events. It
is shown that the fraction of the short GRBs in the total number of SPI-ACS
GRBs can range from 30 to 45%, which is considerably larger than has been
thought previously.Comment: 27 pages, 10 figure
A general scheme for modeling gamma-ray burst prompt emission
We describe a general method for modeling gamma-ray burst prompt emission. We
find that for the burst to be produced via the synchrotron process unphysical
conditions are required -- the distance of the source from the center of the
explosion () must be larger than cm and the source
Lorentz factor \gta 10^3; for such a high Lorentz factor the deceleration
radius () is less than even if the number density of particles
in the surrounding medium is as small as cm. The result,
, is in contradiction with the early x-ray and optical
afterglow data. The synchrotron-self-Compton (SSC) process fares much better.
There is a large solution space for a typical GRB prompt emission to be
produced via the SSC process. The prompt optical emission accompanying the
burst is found to be very bright (\lta 14 mag; for ) in the SSC
model, which exceeds the observed flux (or upper limit) for most GRBs.
Continuous acceleration of electrons can significantly reduce the optical flux
and bring it down to the observed limits. (Abridged)Comment: Published in MNRAS Jan 2008, 56 page
Testing the E_p,i - L_p,iso - T_0.45 correlation on a BeppoSAX and Swift sample of gamma-ray bursts
Using a sample of 14 BeppoSAX and 74 Swift GRBs with measured redshift we
tested the correlation between the intrinsic peak energy of the time-integrated
spectrum, E_p,i, the isotropic-equivalent peak luminosity, L_p,iso, and the
duration of the most intense parts of the GRB computed as T_0.45 ("Firmani
correlation"). For 41 out of 88 GRBs we could estimate all of the three
required properties. Apart from 980425, which appears to be a definite outlier
and notoriously peculiar in many respects, we used 40 GRBs to fit the
correlation with the maximum likelihood method discussed by D'Agostini,
suitable to account for the extrinsic scatter in addition to the intrinsic
uncertainties affecting every single GRB. We confirm the correlation. However,
unlike the results by Firmani et al., we found that the correlation does have a
logarithmic scatter comparable with that of the E_p,i-E_iso ("Amati")
correlation. We also find that the slope of the product L_p,iso T_0.45 is equal
to ~0.5, which is consistent with the hypothesis that the E_p,i-L_p,iso-T_0.45
correlation is equivalent to the E_p,i-E_iso correlation (slope ~0.5). We
conclude that, based on presently available data, there is no clear evidence
that the E_p,i-L_p,iso-T_0.45 correlation is different (both in terms of slope
and dispersion) from the E_p,i-E_iso correlation.Comment: 9 pages, 4 figures, revised version submitted to MNRA
Strong bursts from the anomalous X-ray pulsar 1E 1547.0-5408 observed with the INTEGRAL/SPI Anti-Coincidence Shield
In January 2009, multiple short bursts of soft gamma-rays were detected from
the direction of the anomalous X-ray pulsar 1E 1547.0-5408 by different
satellites. Here we report on the observations obtained with the INTEGRAL
SPI-ACS detector during the period with the strongest bursting activity. More
than 200 bursts were detected at energies above 80 keV in a few hours on
January 22. Among these, two remarkably bright events showed pulsating tails
lasting several seconds and modulated at the 2.1 s spin period of 1E
1547.0-5408. The energy released in the brightest of these bursts was of a few
10^43 erg, for an assumed distance of 10 kpc. This is smaller than that of the
three giant flares seen from soft gamma-ray repeaters, but higher than that of
typical bursts from soft gamma-ray repeaters and anomalous X-ray pulsars.Comment: Accepted for publication on The Astrophysical Journal Letter
A giant, periodic flare from the soft gamma repeater SGR1900+14
Soft gamma repeaters are high-energy transient sources associated with
neutron stars in young supernova remnants. They emit sporadic, short (~ 0.1 s)
bursts with soft energy spectra during periods of intense activity. The event
of March 5, 1979 was the most intense and the only clearly periodic one to
date. Here we report on an even more intense burst on August 27, 1998, from a
different soft gamma repeater, which displayed a hard energy spectrum at its
peak, and was followed by a ~300 s long tail with a soft energy spectrum and a
dramatic 5.16 s period. Its peak and time integrated energy fluxes at Earth are
the largest yet observed from any cosmic source. This event was probably
initiated by a massive disruption of the neutron star crust, followed by an
outflow of energetic particles rotating with the period of the star. Comparison
of these two bursts supports the idea that magnetic energy plays an important
role, and that such giant flares, while rare, are not unique, and may occur at
any time in the neutron star's activity cycle.Comment: Accepted for publication in Natur
X-Ray Light Curves of Gamma-ray Bursts Detected with the All-Sky Monitor on RXTE
We present X-ray light curves (1.5-12 keV) for fifteen gamma-ray bursts
(GRBs) detected by the All-Sky Monitor on the Rossi X-ray Timing Explorer. We
compare these soft X-ray light curves with count rate histories obtained by the
high-energy (>12 keV) experiments BATSE, Konus-Wind, the BeppoSAX Gamma-Ray
Burst Monitor, and the burst monitor on Ulysses. We discuss these light curves
within the context of a simple relativistic fireball and synchrotron shock
paradigm, and we address the possibility of having observed the transition
between a GRB and its afterglow. The light curves show diverse morphologies,
with striking differences between energy bands. In several bursts, intervals of
significant emission are evident in the ASM energy range with little or no
corresponding emission apparent in the high-energy light curves. For example,
the final peak of GRB 970815 as recorded by the ASM is only detected in the
softest BATSE energy bands. We also study the duration of bursts as a function
of energy. Simple, singly-peaked bursts seem consistent with the E^{-0.5} power
law expected from an origin in synchrotron radiation, but durations of bursts
that exhibit complex temporal structure are not consistent with this
prediction. Bursts such as GRB 970828 that show many short spikes of emission
at high energies last significantly longer at low energies than the synchrotron
cooling law would predict.Comment: 15 pages with 20 figures and 2 tables. In emulateapj format. Accepted
by ApJ
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