2.2. Temporal Structure and Variability

The bursts have a complicated and irregular time profiles which vary drastically from one burst to another. Several time profiles, selected from the second BATSE catalog, are shown in Fig. 1. In most bursts, the typical variation takes place on a time-scale T significantly smaller than the total duration of the burst, T. In a minority of the bursts there is only one peak with no substructure and in this case T ~ T. It turns out that the observed variability provides an interesting clue to the nature of GRBs. We discuss this in section 7. We define the ratio T / T which is a measure of the variability. Fig. 2 depicts the total observed counts (at E > 25 keV) from GRB1676. The bursts lasted T ~ 100 sec and it had peaks of width T ~ 1 sec, leading to = 100.

Figure 1. Total number of counts vs. time for several bursts from the BATSE Catalogue. Note the large diversity of temporal structure observed.

Figure 2. Counts vs. time for BATSE burst 1676. The bursts lasted T ~ 60 sec and it had peaks of width T ~ 1 sec, leading to 60.