On the Variety of the Spectral and Temporal Behaviors of Long Gamma-Ray Burst Pulses

Abstract

We find and study a variety of the spectral-temporal behavior during the decay phase of long and bright GRB pulses. Even though only a small fraction of observed bursts exhibit such pulses, these are of interest to study as they reflect individual emission episodes during the burst. We have previously found that for about half of these decays, the instantaneous photon flux ~ 1/time, which is a consequence of the validity of both a power-law hardness-intensity correlation (HIC) and an exponential hardness-fluence correlation (HFC). Here, we study a complete sample of 25 BATSE pulses (having a peak flux in 1 s resolution of > 5 photons/s/cm^2 and a S/N of 30 in at least 8 time bins) and, specifically, search for other types of decay behaviors. First, we find that a power law gives a better description of the pulse decays than a stretched exponential, the most commonly assumed pulse shape so far. Then we find that about half of the decays behave approximately as 1/time, and the other half approximately as 1/time^3. For a few of the latter decays, the two correlations, the HIC and the HFC, are constrained and found to be consistent with the light curve behavior. For these cases, the HIC is still a power law while the HFC is described by a generalized function. Finally, we briefly discuss our results in a physical context.