It is almost impossible to define a general morphology of the GRBs. It changes from one to the other, passing from single pulse events to multiple pulses, through a wide variety of shapes and time-scales. The single pulse events can last from few ms to hundreds of seconds. The multiple events are usually longer than a few seconds. Often the multiple structure of an event is found only at very high time resolution (of the order of few ms). In figure 4.4 an example of the zoo of the GRBs detected by the GRBM is shown.
Thanks to its high detection efficiency, the BATSE experiment has built a duration distribution of the GRBs. The duration of a GRB is defined as the time needed to go from the 5% to the 95% of the total counts detected from a given event. This distribution has a bi-modal shape, with the two broad peaks at about 0.3 and 30 s.