SPECTRAL EVOLUTION OF A SUBCLASS OF GAMMA-RAY BURSTS OBSERVED BY BATSE

Among the gamma-ray bursts observed by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory we defin e a subclass of bursts based on similar morphology: a sharp rise follo wed by a longer decay time. About 7% of all the gamma-ray bursts obser ved by BATSE fall into this subclass. We study the spectral evolution of these bursts by fitting models to time-segmented burst spectra and find no clear distinction between the spectral evolutionary properties of this subclass and those of other bursts. Further, we study the hig h time resolution spectral evolution of this subclass of GRBs using th eir spectral hardness ratios. A majority of the bursts show hardness r atio leading the counting rate and also display a continuous hard to s oft evolution. The time lag between the counting rate and the hardness ratio is found to be directly correlated with the rise time of the co unting rate profile. We also find, for the first time, evidence for sp ectral variation in a timescale of 64 ms.