With the setting of the thresholds described above, we detect about 12 triggers per day, that is roughly 0.8 triggers per orbit. These triggers are uniformly distributed along the orbit and there is no evidence for any "hot spot". Most of them are of course false triggers due to the lack of any particle anticoincidence (the GRBM detectors are themselves an active anticoincidence of the PDS experiment). Actually what we experienced is that the false triggers are due to correlated events in two contiguous shields. The physical origin of this kind of events is the same as the spikes, that is high Z particles that excite meta-stable atomic states of the crystals, but they are due to the crossing of two shields by the same particle, and therefore in time coincidence. In fact, what we see in our data is that most of the false triggers are composed by fast events (from few to tens of milliseconds) with slightly different time histories in two shields that are always close by. This characteristic, apart from the statistical considerations given above, almost excludes the possibility that these false triggers can be due to independent spikes in time coincidence on two shields. In this case one would expect they should be equally distributed on the four shields and not always from adjacent pairs. Their outlined characteristics make this kind of false triggers indistinguishable from real, short GRBs and we could reject them only by renouncing to the possibility of detecting very short events. On this regard, an interesting point is that we detect a small amount of short events that has similar characteristics to those described above. They are weak, short, on two adjacent shields (a viewing angle effect on a real GRB could also cause this) in which they show a very similar time profile. Among them we think that there can be some real GRB that can only be confirmed through the simultaneous detection by some other experiment.
In Fig. 4.4 we show few examples of light curves in the GRBM band of the GRBs detected by the GRBM. Some of them were simultaneously detected by other experiments like the BATSE experiment on-board CGRO or the GRB detector on-board the Ulysses. interplanetary probe or the Konus experiment on-board the Wind satellite.
In this figure, GRB start time is zero on the time axis. From the top left: GRB960720 (20/Jul/1996, 11:36:53 UT, also detected by BATSE) as visible from the high resolution time profiles of shield LS1, rebinned at 62.5 ms (data gaps are due to an on-board software bug, later fixed); GRB960805 (05/Aug/1996, 21:55:59 UT, also detected by Ulysses, not detected by BATSE) as visible in the LS3 GRBM ratemeter (1 s time resolution); GRB960912 (12/Sep/1996, 13:57:28, also detected by BATSE) from LS1 GRBM ratemeter; GRB961101 (01/Nov/1996, 16:07:44 UT, also detected by Konus, not detected by BATSE) at 62.5 ms from shield LS2; GRB961122 (22/Nov/1996, 02:17:40 UT, also detected by BATSE) at 7.8 ms from shield LS2; GRB970616 (16/Jun/1997, 18:09:48 UT, also detected by BATSE and Ulysses) at 1 s from shield LS1.
As can be seen from Fig. 4.4, the BeppoSAX GRBM was able to trigger on several different types of GRBs. As an example, GRB960720 (the first GRB detected also in the WFCs) is a FRED (Fast Rise Exponential Decay) type, and lasted for about 4 seconds; GRB961122 is also FRED type, but it lasts about 500 ms; GRB961101 has a small precursor, and GRB970616 has a very complex structure, even more complicated when seen at higher time resolution.