the observed bimodality.

Discussion
==========

We have shown that most short bursts as well as most long bursts are multi-peaked and highly variable with $ \delta t_{min}/T\ll 1
$. These statistically significant variations involve generally a change of more than a factor of two in the count rate. 30% of the short bursts have a single pulse for which $ \delta
t_{min} $ is the same as the observed duration - $ \delta
t_{min}\approx T $. These are smooth bursts. However, a comparison with the *’noisy long’* sample, shows that this might be an artifact of the low S/N in the short bursts sample. We also find significant variability on very short time scale in long GRBs. This reduces the values of the variability parameter $ \delta t_{min}/ T $ to $10^{-3}-10^{-4}$ in long bursts.

External shocks (at least simple models of external shocks) cannot produce variable bursts (Sari & Piran, 1997; Fenimore, Madras & Nayakshin, 1996). Our result suggests that most short bursts are produced via internal shocks. 30% of the short bursts are smooth. We cannot rule out the possibility that these bursts are produced by external shock. The observed very short time scales in long GRBs strengthen 