682 research outputs found
The X-ray Spectrum of Soft Gamma Repeater 1806-20
Soft Gamma Repeaters (SGRs) are a class of rare, high-energy galactic
transients that have episodes of short (~0.1 sec), soft (~30 keV), intense
(~100 Crab), gamma-ray bursts. We report an analysis of the x-ray emission from
95 SGR1806-20 events observed by the International Cometary Explorer. The
spectral shape remains remarkably constant for bursts that differ in intensity
by a range of 50. Below 15 keV the number spectrum falls off rapidly such that
we can estimate the total intensity of the events. Assuming that SGR1806-20 is
associated with the supernova remnant G10.0-0.3 (Kulkarni and Frail, Murakami
\etal), the brightest events had a total luminosity of ~1.8 x 10^42 erg sec^-1,
a factor of 2 x 10^4 above the Eddington limit. A third of the emission was
above 30 keV. There are at least three processes that are consistent with the
spectral rollover below 15 keV. (1)The rollover is consistent with some forms
of self absorption. Typical thermal temperatures are ~20 keV and require an
emitting surface with a radius between 10 and 50 km. The lack of spectral
variability implies that only the size of the emitting surface varies between
events. If the process is thermal synchrotron the required magnetic field might
be too small to confine the plasma against the super Eddington flux. (2)The low
energy rollover could be due to photoelectric absorption by ~10^24 Hydrogen
atoms cm^-2 of neutral material with a cosmic abundance assuming a continuum
similar to TB with T= ~22 keV. (3) Emission in the two lowest harmonics from a
1.3 x 10^12 Gauss field would appear as Doppler broadened lines and fall off
rapidly below 15 keV.Comment: TeX: 32 pg+ 8 appended postscript figures, in press ApJ(9/94
Log N-log S in inconclusive
The log N-log S data acquired by the Pioneer Venus Orbiter Gamma Burst Detector (PVO) are presented and compared to similar data from the Soviet KONUS experiment. Although the PVO data are consistent with and suggestive of a -3/2 power law distribution, the results are not adequate at this state of observations to differentiate between a -3/2 and a -1 power law slope
Constraints on the Gamma-ray Burst Luminosity Function from PVO and BATSE
We examine the width of the gamma-ray burst luminosity function through the
distribution of GRB peak fluxes as detected by the Pioneer Venus Orbiter (PVO)
and the Burst and Transient Source Experiment (BATSE). The strength of the
analysis is greatly enhanced by using a merged catalog of peak fluxes from both
instruments with good cross-calibration of their sensitivities. The range of
peak fluxes is increased by approximately a factor of 20 relative to the BATSE
catalog. Thus, more sensitive investigations of the
distribution are possible. We place constraints on the width of the luminosity
function of gamma-ray bursts brighter than the BATSE completeness limit by
comparing the intensity distribution in the merged catalog with those produced
by a variety of spatial density and luminosity functions. For the models
examined, of the {\em detectable\/} bursts have peak luminosities within
a range of 10, indicating that the peak luminosities of gamma-ray bursts span a
markedly less wide range of values than many other of their measurable
properties. We also discuss for which slopes of a power-law luminosity function
the observed width is at the upper end of the constrained range. This is
important in determining the power-law slopes for which luminosity-duration
correlations could be important.Comment: 10 pages latex + 2 uuencoded figures; APJL accepte
SVOM pointing strategy: how to optimize the redshift measurements?
The Sino-French SVOM mission (Space-based multi-band astronomical Variable
Objects Monitor) has been designed to detect all known types of gamma-ray
bursts (GRBs) and to provide fast and reliable GRB positions. In this study we
present the SVOM pointing strategy which should ensure the largest number of
localized bursts allowing a redshift measurement. The redshift measurement can
only be performed by large telescopes located on Earth. The best scientific
return will be achieved if we are able to combine constraints from both space
segment (platform and payload) and ground telescopes (visibility).Comment: Proceedings of Gamma-Ray Bursts 2007 conference, Santa Fe, USA, 5-9
November 2007. Published in AIP conf. proc. 1000, 585-588 (2008
X-ray and low energy gamma-ray observations of the 16 February 1984 solar flare
The February 16, 1984 (0900 UT) solar flare was very energetic and produced a variety of emissions. The X-ray and gamma ray continuum measurement, made aboard the International Cometary Explorer (ICE) and the Pioneer Venus Orbiter (PVO), are briefly described
Predictions for The Very Early Afterglow and The Optical Flash
According to the internal-external shocks model for -ray bursts
(GRBs), the GRB is produced by internal shocks within a relativistic flow while
the afterglow is produced by external shocks with the ISM. We explore the early
afterglow emission. For short GRBs the peak of the afterglow will be delayed,
typically, by few dozens of seconds after the burst. For long GRBs the early
afterglow emission will overlap the GRB signal. We calculate the expected
spectrum and the light curves of the early afterglow in the optical, X-ray and
-ray bands. These characteristics provide a way to discriminate
between late internal shocks emission (part of the GRB) and the early afterglow
signal. If such a delayed emission, with the characteristics of the early
afterglow, will be detected it can be used both to prove the internal shock
scenario as producing the GRB, as well as to measure the initial Lorentz factor
of the relativistic flow. The reverse shock, at its peak, contains energy which
is comparable to that of the GRB itself, but has a much lower temperature than
that of the forward shock so it radiates at considerably lower frequencies. The
reverse shock dominates the early optical emission, and an optical flash
brighter than 15th magnitude, is expected together with the forward shock peak
at x-rays or -rays. If this optical flash is not observed, strong
limitations can be put on the baryonic contents of the relativistic shell
deriving the GRBs, leading to a magnetically dominated energy density.Comment: 23 pages including 4 figure
Quiescent times in gamma-ray bursts: II. Dormant periods in the central engine?
Within the framework of the internal-external shocks model for gamma-ray
bursts, we study the various mechanisms that can give rise to quiescent times
in the observed gamma-ray light-curves. In particular, we look for the
signatures that can provide us with evidence as to whether or not the central
engine goes dormant for a period of time comparable to the duration of the
gaps. We show that the properties of the prompt gamma-ray and X-ray emission
can in principle determine whether the quiescent episodes are due to a
modulated relativistic wind or a switching off of the central engine. We
suggest that detailed observations of the prompt afterglow emission from the
reverse shock will strongly constrain the possible mechanisms for the
production of quiescent times in gamma-ray bursts.Comment: 12 pages, 8 figures, with final revisions, MNRAS in pres
Resolving The ISM Surrounding GRBs with Afterglow Spectroscopy
We review current research related to spectroscopy of gamma-ray burst (GRB)
afterglows with particular emphasis on the interstellar medium (ISM) of the
galaxies hosting these high redshift events. These studies reveal the physical
conditions of star-forming galaxies and yield clues to the nature of the GRB
progenitor. We offer a pedagogical review of the experimental design and review
current results. The majority of sightlines are characterized by large HI
column densities, negligible molecular fraction, the ubiquitous detection of UV
pumped fine-structure transitions, and metallicities ranging from 1/100 to
nearly solar abundance.Comment: Conference procedings for Gamma Ray Bursts 2007 November 5-9, 2007
Santa Fe, New Mexico (8 pages, 4 figures
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