BATSE OBSERVATIONS OF GAMMA-RAY BURST SPECTRA - IV - TIME-RESOLVED HIGH-ENERGY SPECTROSCOPY

We report on the temporal behavior of the high-energy power-law contin uum component of gamma-ray burst spectra with data obtained by the Bur st and Transient Source Experiment. We have selected 126 high-fluence and high-flux bursts from the beginning of the mission up until the pr esent. Much of the data were obtained with the Large Area Detectors, w hich have nearly all-sky coverage, excellent sensitivity over 2 decade s of energy, and moderate energy resolution, ideal for continuum spect ra studies of a large sample of bursts at high time resolution. At lea st eight spectra from each burst were fitted with a spectral form that consisted of a low-energy power law, a spectral break at middle energ ies, and a high-energy continuum. In most bursts (122), the high-energ y continuum was consistent with a power law. The evolution of the fitt ed high-energy power-law index over the selected spectra for each burs t is inconsistent with a constant for 34% of the total sample. The sam ple distribution of the average value for the index from each burst is fairly narrow, centered on -2.12. A linear trend in time is ruled out for only 20% of the bursts, with hard-to-soft evolution dominating th e sample (100 events). The distribution for the total change in the po wer-law index over the duration of a burst peaks at the value -0.37 an d is characterized by a median absolute deviation of 0.39, arguing tha t a single physical process is involved. We present analyses of the co rrelation of the power-law index with time, burst intensity, and low-e nergy time evolution. In general, we confirm the general hard-to-soft spectral evolution observed in the low-energy component of the continu um, while presenting evidence that this evolution is different in natu re from that of the rest of the continuum.