The hardness evolution of gamma-ray burst pulses

We have analyzed the evolution of the spectral hardness parameter, E-pk (th e maximum of the nu F-nu spectrum) as a function of fluence in gamma-ray bu rsts. We fit 41 pulses within 26 bursts with the trend reported by Liang & Kargatis (1996) which found that E-pk decays exponentially with respect to photon fluence Phi(t). We also fit these pulses with a slight modification of this trend, where E-pk decays linearly with energy fluence. In both case s, we found the set of 41 pulses to be consistent with the trend.