354 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
Spectral Lags and the Lag-Luminosity Relation: An Investigation with Swift BAT Gamma-ray Bursts
Spectral lag, the time difference between the arrival of high-energy and
low-energy photons, is a common feature in Gamma-ray Bursts (GRBs). Norris et
al. 2000 reported a correlation between the spectral lag and the isotropic peak
luminosity of GRBs based on a limited sample. More recently, a number of
authors have provided further support for this correlation using arbitrary
energy bands of various instruments. In this paper we report on a systematic
extraction of spectral lags based on the largest Swift sample to date of 31
GRBs with measured redshifts. We extracted the spectral lags for all
combinations of the standard Swift hard x-ray energy bands: 15-25 keV, 25-50
keV, 50-100 keV and 100-200 keV and plotted the time dilation corrected lag as
a function of isotropic peak luminosity. The mean value of the correlation
coefficient for various channel combinations is -0.68 with a chance probability
of ~ 0.7 x 10^{-3}. In addition, the mean value of the power-law index is 1.4
+/- 0.3. Hence, our study lends support for the existence of a lag-luminosity
correlation, albeit with large scatter.Comment: 19 Pages, 11 Figures and 5 Tables; Accepted to The Astrophysical
Journa
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 statistical study of gamma-ray burst afterglows measured by the Swift Ultra-violet Optical Telescope
We present the first statistical analysis of 27 UVOT optical/ultra-violet
lightcurves of GRB afterglows. We have found, through analysis of the
lightcurves in the observer's frame, that a significant fraction rise in the
first 500s after the GRB trigger, that all lightcurves decay after 500s,
typically as a power-law with a relatively narrow distribution of decay
indices, and that the brightest optical afterglows tend to decay the quickest.
We find that the rise could either be produced physically by the start of the
forward shock, when the jet begins to plough into the external medium, or
geometrically where an off-axis observer sees a rising lightcurve as an
increasing amount of emission enters the observers line of sight, which occurs
as the jet slows. We find that at 99.8% confidence, there is a correlation, in
the observed frame, between the apparent magnitude of the lightcurves at 400s
and the rate of decay after 500s. However, in the rest frame a Spearman Rank
test shows only a weak correlation of low statistical significance between
luminosity and decay rate. A correlation should be expected if the afterglows
were produced by off-axis jets, suggesting that the jet is viewed from within
the half-opening angle theta or within a core of uniform energy density
theta_c. We also produced logarithmic luminosity distributions for three rest
frame epochs. We find no evidence for bimodality in any of the distributions.
Finally, we compare our sample of UVOT lightcurves with the XRT lightcurve
canonical model. The range in decay indices seen in UVOT lightcurves at any
epoch is most similar to the range in decay of the shallow decay segment of the
XRT canonical model. However, in the XRT canonical model there is no indication
of the rising behaviour observed in the UVOT lightcurves.Comment: 16 pages, 9 figures, accepted MNRA
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
Observations of giant outbursts from Cygnus X-1
We present interplanetary network localization, spectral, and time history
information for 7 episodes of exceptionally intense gamma-ray emission from
Cygnus X-1. The outbursts occurred between 1995 and 2003, with durations up to
\~28000 seconds. The observed 15 - 300 keV peak fluxes and fluences reached
3E-7 erg /cm2 s, and 8E-4 erg / cm2 respectively. By combining the
triangulations of these outbursts we derive an ~1700 square arcminute (3 sigma)
error ellipse which contains Cygnus X-1 and no other known high energy sources.
The outbursts reported here occurred both when Cyg X-1 was in the hard state as
well as in the soft one, and at various orbital phases. The spectral data
indicate that these outbursts display the same parameters as those of the
underlying hard and soft states, suggesting that they represent another
manifestation of these states.Comment: 27 pages, 9 figures. Revised version. Accepted for publication in the
Astrophysical Journal, tentatively scheduled for October 20, 2003 Part
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