Exploration of bi‐modality in gamma‐ray burst duration and hardness distributions

A bimodal burst duration distribution with the minimum near 1–2 s, previously reported by several investigators, is now confirmed by Compton’s Burst and Transient Source Experiment (BATSE). The burst subgroups are also distinguished by their integral spectral hardness ratios—shorter events tend to be harder. After fitting 280 pulses in 35 long, bright bursts, we find that the interval distribution exhibits a single wide mode centered near 1 s. Analysis of a sample of 12 short, bright bursts indicates that they tend to have considerably fewer pulses, while their pulse width and interval distributions are peaked on the shortward end of those distributions for long bursts. Simulations patterned after these measured attributes of pulse distributions reproduce fairly well the shortward mode and minimum in the duration distribution. The average trend of spectral evolution for long, bright bursts suggests that the difference in hardness ratios might be connected with the tendency of these bursts to soften. No ev...