Date of Award

Spring 1986

Project Type


Program or Major


Degree Name

Doctor of Philosophy


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.