Spectral evolution in gamma-ray bursts

Abstract

The Hard X-ray Burst Spectrometer (HXRBS) and the Gamma-Ray Spectrometer (GRS) on NASA's Solar Maximum Mission satellite (SMM) have independently monitored cosmic gamma-ray bursts since launch in February 1980. Several bursts with relatively simple pulse structure and sufficient intensity have been analyzed for evidence of spectral variability on timescales shorter than the pulse durations. In many of these bursts we find pulse structures, ranging in duration from 1 to 10 seconds, which exhibit a trend of hard-to-soft spectral evolution. No significant evidence for soft-to-hard evolution has been found, although the possibility of weak, extended low-energy emission is suggested in a few bursts. The HXRBS data above 100 keV and the GRS data above 1 MeV indicate that the spectral evolution generally is not due to time-varying absorption features at energies below 100 keV.