610 research outputs found
Limits on Expanding Relativistic Shells from Gamma-Ray Burst Temporal Structure
We calculate the expected envelope of emission for relativistic shells under
the assumption of local spherical symmetry. Gamma-Ray Burst envelopes rarely
conform to the expected shape, which is similar to a FRED; a fast rise and
exponential decay. The fast rise is determined by the time that the
relativistic shell prodcues gamma rays. The decay has the form of a power law
and arises from the curvature of the shell. The amount of curvature comes from
the overall size of the shell so the duration of the decay phase is related to
the time the shell expands before converting its energy to gamma rays. From the
envelope of emission, one can estimate when the central explosion occurred and,
thus, the energy required for the shell to sweep up the ISM. The energy greatly
exceeds 10^{53} erg unless the bulk Lorentz factor is less than 75. This puts
extreme limits on the "external" shock models. However, the alternative,
"internal" shocks from a central engine, has a problem: the entire long complex
time history lasting hundreds of seconds must be postulated at the central
site.Comment: to appear in Proc. 18-th Texas Symposium on Relativistic
Astrophysics, eds. A. Olinto, J. Frieman, and D. Schram
Class of near-perfect coded apertures
Coded aperture imaging of gamma ray sources has long promised an improvement in the sensitivity of various detector systems. The promise has remained largely unfulfilled, however, for either one of two reasons. First, the encoding/decoding method produces artifacts, which even in the absence of quantum noise, restrict the quality of the reconstructed image. This is true of most correlation-type methods. Second, if the decoding procedure is of the deconvolution variety, small terms in the transfer function of the aperture can lead to excessive noise in the reconstructed image. It is proposed to circumvent both of these problems by use of a uniformly redundant array (URA) as the coded aperture in conjunction with a special correlation decoding method
The Unique Signature of Shell Curvature in Gamma-Ray Bursts
As a result of spherical kinematics, temporal evolution of received gamma-ray
emission should demonstrate signatures of curvature from the emitting shell.
Specifically, the shape of the pulse decay must bear a strict dependence on the
degree of curvature of the gamma-ray emitting surface. We compare the spectral
evolution of the decay of individual GRB pulses to the evolution as expected
from curvature. In particular, we examine the relationship between photon flux
intensity (I) and the peak of the \nu F\nu distribution (E_{peak}) as predicted
by colliding shells. Kinematics necessitate that E_{peak} demonstrate a
power-law relationship with I described roughly as: I=E_{peak}^{(1-\zeta)}
where \zeta represents a weighted average of the low and high energy spectral
indices. Data analyses of 24 BATSE gamma-ray burst pulses provide evidence that
there exists a robust relationship between E_{peak} and I in the decay phase.
Simulation results, however, show that a sizable fraction of observed pulses
evolve faster than kinematics allow. Regardless of kinematic parameters, we
found that the existence of curvature demands that the I - E_{peak} function
decay be defined by \sim (1-\zeta). Efforts were employed to break this
curvature dependency within simulations through a number of scenarios such as
anisotropic emission (jets) with angular dependencies, thickness values for the
colliding shells, and various cooling mechanisms. Of these, the only method
successful in dominating curvature effects was a slow cooling model. As a
result, GRB models must confront the fact that observed pulses do not evolve in
the manner which curvature demands.Comment: 3 pages, To appear in Proc. from the 2nd Workshop on Gamma-Ray Bursts
in the Afterglow Er
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
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