1999ApJ...519..206C

We have analyzed the evolution of the spectral hardness parameter E_pk (the maximum of the νF_ν spectrum) as a function of fluence in gamma-ray bursts. We fit 41 pulses within 26 bursts with the trend reported by Liang & Kargatis, which found that E_pk decays exponentially with respect to photon fluence Φ(t). We also fit these pulses with a slight modification of this trend, where E_pk 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 lognormal, where the mean of log₁₀ Φ₀ is 1.75±0.07 and the FWHM of log₁₀ Φ₀ is 1.0±0.1. Regarding an earlier reported invariance in Φ₀ among different pulses in a single burst, we found probabilities of 0.49-0.84 (depending on the test used) that such invariance would occur by coincidence, most likely because of the narrow distribution of Φ₀ values among pulses.