t_{min}/T $ ($ \ll 1 $) values while for smooth bursts $
\delta t_{min}/T \approx 1 $. Our aim is to explore whether short bursts can be produced by external shocks. However, our analysis is not directed by this motivation and the results concerning the variability of short bursts are valid independently of this motivation.

In the second part of this paper we compare the shortest time scales of long and short bursts. We analyze (using the same algorithm) the high resolution (TTE) data of long bursts. Unfortunately, this data is available only for the first 1-2 seconds of each burst, so we can analyze only a small fraction of each long burst. still we are able to demonstrate that very short time scales (10ms or less) are common.

Our analysis deals with statistically significant individual pulses. We define an observed peak in the light curve as the highest count rate within a series of counts that is statistically significant (more than 4 $\sigma$) above the counts at some time before and some time after it. Each peak corresponds to a pulse. The width of the pulse is determined by the width at a quarter of the maximum, or, rarely, 