Examples of nontriggered GRBs

In figg. , , , , some time profiles of NTBs discovered by the different SWTCs are shown. In tab.  the main characteristics of the same NTBs are reported. All these events have been also detected by BATSE (the corresponding trigger numbers are reported, as well). In order to better understand why these bursts did not trigger the GRBM on-board logic, the values of the on-board threshold parameters $n$ expressed in $\sigma$ are reported. For each NTB the values of $S/\sigma$ in the two energy bands for the triggered units are provided. The detector unit number is also written within brackets.

In particular, NTB 960723 (fig. ), whose BATSE direction is $\sim$11$^\circ$ off-axis with respect to unit 2, did not trigger the OBTCs, because it had $S_2^{(G)}/\sigma_2^{(G)}\simeq6\ < 8$ (on-board threshold) and $S_2^{(A)}/\sigma_2^{(A)}\simeq3$; therefore, while it just triggered SWTC2, it matched no OBTC in any unit. The NTB 980421 (fig. ), just a little brighter than the previous, did match the OBTC, but only in unit 1, therefore, not enough to trigger the on-board logic; however, it satisfied SWTC2 and SWTC3. The case of NTB 960913 (fig. ) is an example of a burst, that did not trigger on board, only because it happened during the dead time due to a previous event (caused by a high-energy particle), that already started the HTR mode: actually, the burst occurred just 3$^\prime$10 $^{\prime\prime}$ after the previous event; since the minimum dead time is $\sim$ 7 min (tab. ), HTR counts could not have been acquired; nevertheless, the on-board flag indicating the occurrence of a new trigger was set to positive. This bright NTB triggered all the SWTCs.

Table: Example of NTBs caught by the early SWTCs.

GRB	SW Trg	BATSE	GRBM Peak($S/\sigma$)	AC Peak($S/\sigma$)	On-board	$\phi$	$\theta$
	UT	trigger	(SW thr $=6\sigma$)	(SW thr $=3\sigma$)	thr ($\sigma$)	($^\circ$)	($^\circ$)
960723	04:46:06	5551	6 (2)	3 (2)	8	350	+4
960802	22:07:01	5559	7 (1)	5 (1)	8	283	+12
980421	22:44:02	6698	8 (1)	4 (1)	4	238	+47
960913	23:05:20	5604	9 (2)	9 (2)	8	23	-23
			12 (3)	13 (3)	8

Figure: Time history of NTB 960802 (also BATSE trigger 5559) in all the GRBM units, both energy bands. Only unit 1 detected it. Like NTB 960723, also this NTB was caught trough SWTC 2: its BATSE direction was $\sim$20$^\circ$above the Earth limb and $\sim$18$^\circ$off-axis with respect to unit 1.

Figure: Time history of NTB 980421 (also BATSE trigger 6698) in all the GRBM units, both energy bands. The burst can be seen in all the profiles, but it matched the OBTC only in unit 1, because too faint in the other units. It triggered both SWTC 2 and SWTC 3. Its BATSE direction was $\sim$73$^\circ$above the Earth limb and $\sim$55$^\circ$off-axis with respect to unit 1, the one with the best SNR.

Figure: Time history of NTB 960913 (also BATSE trigger 5604) in all the GRBM units, both energy bands. The burst was bright enough in the units 2 and 3 to match the OBTCs, but, since it occurred during the dead time of a previous fake event that triggered the GRBM, there are no HTR counts of it. It triggered all the SWTCs. Its BATSE direction was $\phi\simeq 23$$^\circ$and $\theta\simeq -23$$^\circ$ in the BeppoSAX frame of reference, i.e. bewteen the axes of units 2 and 3: this fully agrees with the GRBM units that indeed detected it.