Evidence for two distinct morphological classes of gamma-ray bursts from their short time scale variability

We have analyzed the 241 bursts for which peak counts exist in the publicly available Burst and Transient Source Experiment (BATSE) catalog. Introducing peak counts in 1024 ms as a measure of burst brightness and the ratio of peak counts in 64 and 1024 ms as a measure of short timescale variability , we find a statistically significant correlation between the brightness and the short timescale variability of -ray bursts. The bursts which are smoother on short timescales are both faint and bright, while the bursts which are variable on short timescales are faint only, suggesting the existence of two distinct morphological classes of bursts.