"ON THE SPECTRA OF GAMMA-RAY BURSTS AT HIGH ENERGIES" by STEVEN MICHAEL MATZ

Date of Award

Spring 1986

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

Abstract

Between 1980 February and 1983 August the Gamma-Ray Spectrometer (GRS) on the Solar Maximum Mission satellite (SMM) observed 71 gamma-ray bursts. These events form a representative subset of the class of classical gamma-ray bursts. Since their discovery more than 15 years ago, hundreds of gamma-ray bursts have been detected; however, most observations have been limited to an energy range of roughly 30 keV-1MeV. The large sensitive area and spectral range of the GRS allow, for the first time, an investigation of the high-energy (>1 MeV) behavior of a substantial number of gamma-ray bursts.

It is found that high-energy emission is seen in a large fraction of all events and that the data are consistent with all bursts emitting to at least 5 MeV with no cut-offs. Further, no burst spectrum measured by GRS has a clear high-energy cut-off. The high-energy emission can be a significant part of the total burst energy; on the average about 30% of the observed energy above 30 keV is contained in the >1 MeV photons.

Tests of spectral models yield mixed results. Neither a power law nor a thermal model can adequately explain all of the observed spectra. Some GRS spectra show clear curvature and cannot be well-fit by a power law. However, a number of spectra are clearly power laws, and the power-law model is consistent with more events ((TURN)80%) than either thermal synchrotron or optically-thin thermal bremsstrahlung. In addition, the two thermal models are generally too soft to explain the observed high-energy emission.

The fact that the observations are consistent with the presence of high-energy emission in all events implies a limit on the preferential beaming of high-energy photons, from any mechanism. Single-photon pair-production in a strong magnetic field produces such beaming; assuming that the low-energy emission is isotropic, the data imply an upper limit of 1 x 10('12) G on the typical magnetic field at burst radiation sites.

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