Attributes of Pulses in Long Bright Gamma-Ray Bursts

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Abstract

We examine the temporal profiles of bright gamma-ray bursts detected by the Burst and Transient Source Experiment on the Compton Gamma Ray Observatory. We deconvolve these profiles into pulses using a model-dependent, least-squares pulse-fitting algorithm. The algorithm has been applied to 41 bright gamma-ray bursts with durations longer than 1.5 s, resulting in more than 400 fitted pulses. Temporal profiles with 64 ms resolution are fitted in four energy bands covering 25 keV to >1 MeV. We use a pulse model with separate rise and decay time constants and a peakedness parameter. Several trends in pulse shape as a function of energy and time of occurrence are apparent: Pulse width distributions are energy dependent, the mode shifting from 600 ms at low energy to 200 at high energy. The distribution of intervals between pulses exhibits a broad maximum near 1 s, presumably a characteristic emission timescale in long bursts. The most frequently occurring pulse shape is intermediate between exponential and Gaussian. Pulses show a tendency to self-similarity across energy bands. We find that average raw pulse shape dependence on energy is approximately power law, with an index of -0.40, consistent with the autocorrelation analysis of Fenimore et al. Burst asymmetry on short time scales results from the tendency for most ( 90%) pulses to rise more quickly than they decay, the majority having rise-to-decay ratios of 0.3-0.5, independent of energy. The dominant trend of spectral softening seen in most pulses arises partially from faster onsets at higher energy and longer decays at lower energies, although in addition, pulses sometimes peak earlier in the higher energy bands. We make an important connection between pulse asymmetry, width, and spectral softening: Among pulses for which shape is relatively well determined, the rise-to-decay ratio is unity or less; as this ratio decreases, pulses tend to be wider, the pulse centroid is more shifted to later times at lower energies, and pulses tend to be spectrally softer.

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