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

Temporal Evolution of the Pulse Width in GRBs

Many cosmological models of GRBs envision the energy source to be a cataclysmic stellar event leading to a relativistically expanding fireball. Particles are thought to be accelerated at shocks and produce nonthermal radiation. The highly variable temporal structure observed in most GRBs has significantly constrained models. By using different methods of statistical analysis in the time domain we show that the width of the pulses in GRBs time histories remain remarkably constant throughout the classic GRB phase. Peaks at the end of a burst have the same average duration to within a few percent as the peaks at the start of the burst. For emission sites that lie on a relativistically expanding shell, peaks should grow in duration because of deceleration. We find no deceleration over at least 2/3 of the burst duration. For emission sites that occupy a spread of angles on a shell, the curvature should cause the later peaks to grow in duration. Since we see no such growth, we can limit the total angular size of the shell to be substantially smaller than \Gamma^{-1} where \Gamma is the bulk Lorentz factor. This lack of temporal evolution of the pulse width should be explained by any fireball shock scenario