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

We find and study a variety of the spectral-temporal behavior during the decay phase of the light curve of long and bright pulse structures in gamma-ray bursts (GRBs). 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 is consistent with a power law in time, where the photon flux ∝ 1/time. This decay behavior is a consequence of the validity of both a power law correlation between the hardness and the intensity and an exponential correlation between the hardness and the time-integrated intensity, the fluence. Here, we study a complete sample of 25 pulses (having a peak flux in 1 s time resolution of more than 5 photons s cm 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). For a few of the 1/(time) decays, the two correlations, the hardness-intensity correlation and the hardness-fluence correlation, are constrained and found to be consistent with the light curve decay behavior. For these cases, the hardness-intensity correlation is still a power law while the hardness-fluence correlation is described by a generalized function. We study and describe these behaviors analytically and examine actual burst data from the complete catalog of the Burst and Transient Source Experiment on the Compton Gamma Ray Observatory. Finally, we briefly discuss our results in a physical context. Subject headings: gamma rays: bursts Center for Space Science and Astrophysics, Stanford University, Stanford, CA 94305 SCFAB, Stockholm Observatory, SE-106 91 Stockholm, Sweden