5 Selection on the GRB duration

The best evidence for a duration selection in the BeppoSAX/WFC GRB sample is provided by the fact that out of 14 events detected till November 1998, the shortest is about 6 s long. From the BATSE duration distribution, one would have expected about $20-30\%$ of them to be shorter than 1 s. A further indication is given by a search of the BATSE GRBs in the GRBM triggers database over a 1-year period. This work ([Preger et al. 1999]) shows that only few GRBs out of $\sim$60 have a T₉₀ duration shorter than 1 s (mainly because of the GRBM 1-s integration time for the trigger).