Spectral Hardness Decay with Respect to Fluence in BATSE Gamma-Ray Bursts

We have analyzed the evolution of the spectral hardness parameter, Epk (the maximum of the νFν spectrum) as a function of fluence in gamma-ray bursts. We fit 41 pulses within 26 bursts with the trend reported by Liang & Kargatis (1996) which found that Epk decays exponentially with respect to photon fluence Φ(t). We also fit these pulses with a slight modification of this trend, where Epk decays linearly with energy fluence. In both cases, we found the set of 41 pulses to be consistent with the trend. For the latter trend, which we believe to be more physical, the distribution of the decay constant Φ0 is roughly log-normal, where the mean of log10Φ0 is 1.75± 0.07 and the FWHM of log10Φ0 is 1.0± 0.1. Regarding an earlier reported invariance in Φ0 among different pulses in a single burst, we found probabilities of 0.49 to 0.84 (depending on the test used) that such invariance would occur by coincidence, most likely due to the narrow distribution of Φ0 values among pulses. Subject headings: gamma-ray: bursts, observations