samples considered in our analysis. We describe the results in section 3 and we discuss their implications in section 4. Our algorithm is described in the appendix.

The data samples
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We examine bright short and long bursts from BATSE 4B-catalog. We use two BATSE data formats: the 64ms concatenate data and the TTE data (see Scargle (1998) for a detailed review). The 64ms concatenate data includes the photon counts of each burst, in a 64ms time bins, from a few seconds before the burst trigger till a few hundred of seconds after the trigger. The concatenate data includes also very early and very late data of the burst in a 1024ms resolution. We use only the 64ms resolution data. The TTE (Time Tagged Events) data includes the arrival time of each photon in a $ 2\mu sec $ resolution. This data contains only records of the first 1-2 seconds of each burst. Hence it contains whole short bursts, but only a fraction of long bursts. Both data formats have four energy channels. We use the sum of all the channels (in both formats), that is photon energy E $ > $ 25Kev.

We consider several samples. We consider a 