Spectral Properties of Gamma-Ray Bursts Observed by the Burst and Transient Source Experiment

Lyle A. Ford

A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Physics

Thesis committee chairman: Professor Laurence Peterson
Advisor: Dr. David Band


This thesis studies Gamma-Ray Burst (GRB) continuum spectra as a way to understand the physical processes which occur in these events. The continuum analyses in this work concern evolution of spectra within a GRB, the likelihood that optical counterparts can be observed if the optical spectrum can be extrapolated from the gamma-ray spectrum, and a search for short-lived black body spectra within a GRB. Spectral features are discussed, but their rarity prevents direct physical interpretation.

The spectral evolution study confirmed previous claims that GRB spectra tend to evolve from hard to soft though there does appear to be a complicated dependence on intensity. These results might imply that the emitting region is being continuously flooded with radiating particles which reduce the average energy of the observed radiation or it could suggest an efficient cooling process. In either case, electron-positron pair processes are not directly observable.

By extrapolating GRB spectra to optical wavelengths, it is possible to determine whether telescopes designed to detect flaring optical emission from GRBs are likely to discover anything. It is found that unless there is more emission at lower energies than is implied by a simple extrapolation of GRB spectra, flaring counterparts will be difficult to observe.

Finally, a new statistical method is used to determine whether short duration black bodies compose a significant fraction of GRB spectra. This is an important question because thermal emission appears to be a natural consequence of GRB models from distant sources. The method determines the likelihood that a given pair of counts came from photons of similar energies. If short-lived black body emission is important in GRBs, then pairs of counts separated by small times should be more correlated. In most bursts, no evidence for this type of emission is found though there are a few exceptions which are likely a consequence of extremely complex spectral evolution. Therefore, short time scale thermal emission appears incompatible with GRBs.

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