Emission model of gamma-ray bursts

The emission mechanisms of cosmic gamma-ray bursts are reviewed. In particular, the thermal synchrotron model is discussed as the most viable mechanism for the majority of the continuum emission. Within this framework various information about the source region can be extracted. The picture that emerges is that of a hot (kT = .2 - 1.0 sq mc), thin sheet of dense pair-dominated plasma emitting via cyclo-synchrotron radiation in a strong magnetic field (B approximately one-hundred billion to one trillion gauss). Speculations on the origin and structure of this sheet are attempted. The problem of high-energy photons above pair production threshold escaping from the source is also considered

Confronting Synchrotron Shock and Inverse Comptonization Models with GRB Spectral Evolution

The time-resolved spectra of gamma-ray bursts (GRBs) remain in conflict with many proposed models for these events. After proving that most of the bursts in our sample show evidence for spectral "shape-shifting", we discuss what restrictions that BATSE time-resolved burst spectra place on current models. We find that the synchrotron shock model does not allow for the steep low-energy spectral slope observed in many bursts, including GRB 970111. We also determine that saturated Comptonization with only Thomson thinning fails to explain the observed rise and fall of the low-energy spectral slope seen in GRB 970111 and other bursts. This implies that saturated Comptonization models must include some mechanism which can cause the Thomson depth to increase intially in pulses.Comment: (5 pages, 3 figures, to appear in Proceedings of the Fourth Huntsville Symposium on Gamma-Ray Bursts

Testing the Invariance of Cooling Rate in Gamma-Ray Burst Pulses

Recent studies have found that the spectral evolution of pulses within gamma-ray bursts (GRBs) is consistent with simple radiative cooling. Perhaps more interesting was a report that some bursts may have a single cooling rate for the multiple pulses that occur within it. We determine the probability that the observed "cooling rate invariance" is purely coincidental by sampling values from the observed distribution of cooling rates. We find a 0.1-26% probability that we would randomly observe a similar degree of invariance based on a variety of pulse selection methods and pulse comparison statistics. This probability is sufficiently high to warrant skepticism of any intrinsic invariance in the cooling rate.Comment: 4 pages, 1 figure, to appear in Proceedings of the Fourth Huntsville Symposium on Gamma-Ray Burst

A New Model for the Hard Time Lags in Black Hole X-Ray Binaries

The time-dependent Comptonized output of a cool soft X-ray source drifting inward through an inhomogeneous hot inner disk or corona is numerically simulated. We propose that this scenario can explain from first principles the observed trends in the hard time lags and power spectra of the rapid aperiodic variability of the X-ray emission of Galactic black-hole candidates.Comment: 10 pages, including 2 figures; uses epsf.sty, rotate.sty; accepted for ApJ Letter

J06587-5558 -- A Very Unusual Polarised Radio Source

We have found a peculiar radio source in the field of one of the hottest known clusters of galaxies 1E0657-56. It is slightly extended, highly polarised (54% at 8.8GHz) and has a very steep spectrum, with alpha ~ -1 at 1.3 GHz, steepening to ~ -1.5 at 8.8GHz (S \propto nu^alpha). No extragalactic sources are known with such high integrated polarisation, and sources with spectra as steep as this are rare. In this paper, we report the unusual properties of the source J06587-5558 and speculate on its origin and optical identification.Comment: 5 pages, 6 figures, accepted by MNRAS letter