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On the Shape of Pulse Spectra in Gamma-Ray Bursts

The discovery (by Liang & Kargatis) that the peak energy of time-resolved spectra of gamma-ray burst (GRB) pulses decays exponentially with fluence is analytically shown to imply that the time-integrated photon number spectrum of a pulse should have a unique shape, given by an underlying E-1 behavior. We also show that the asymptotic low-energy normalization of the time-integrated spectrum is equal to the exponential decay constant. We study analytically how this general behavior is modified in more realistic situations and show that diversity is then introduced in the properties of time-integrated GRB pulse spectra. We argue that further diversity will occur in time-integrated multipulse (complex) GRB spectra. The total energy received per cm2 is approximately the decay constant times the maximum peak energy of the pulse. We derive a five-parameter expression for the time-integrated pulse spectrum, where the parameters describe the time-resolved spectra and their time evolution. This result can then be used when studying observed GRB pulse spectra. We illustrate with the bright burst GRB 910807, and comment on GRB 910525 and GRB 921207.