On the Hardness-Intensity Correlation in Gamma-Ray Burst Pulses

Abstract

We study the hardness-intensity correlation (HIC) in gamma-ray bursts (GRBs). In particular, we analyze the decay phase of pulse structures in their light curves. The study comprises a sample of 82 long pulses selected from 66 long bursts observed by BATSE on the Compton Gamma-Ray Observatory. We find that at least 57% of these pulses have HICs that can be well described by a power law. The distribution of the power law indices, obtained by modeling the HIC of pulses from different bursts, is broad with a mean of 1.9 and a standard deviation of 0.7. We also compare indices among pulses from the same bursts and find that their distribution is significantly narrower. The probability of a random coincidence is shown to be very small. In most cases, the indices are equal to within the uncertainties. This is particularly relevant when comparing the external versus the internal shock models. In our analysis, we also use a new method for studying the HIC, in which the intensity is represented by the peak value of the E F_E spectrum. This new method gives stronger correlations and is useful in the study of various aspects of the HIC. In particular, it produces a better agreement between indices of different pulses within the same burst. Also, we find that some pulses exhibit a "track jump" in their HICs, in which the correlation jumps between two power laws with the same index. We discuss the possibility that the "track jump" is caused by strongly overlapping pulses. Based on our findings, the constancy of the index is proposed to be used as a tool for pulse identification in overlapping pulses.