On-board Trigger Efficiency

Out of 944 GRBs detected, only 606 triggered the GRBM on-board logic too. Therefore, if the on-board trigger efficiency $\epsilon_{\mbox{\small {onb}}}$ is defined as the ratio between the number of on-board triggered bursts and the total number of detected GRBs, then it results:

$$\epsilon_{\mbox{\small {onb}}} \ = \ \frac{\mbox{\char93 on... ...triggered GRBs}} \ = \ \frac{606}{944} \ \simeq \ (64 \pm 3)\%$$ (35)

From the above estimate it comes out that, on average, 2/3 of the bursts, that can be detected within the GRBM 1 s ratemeters data, are recognized by the on-board logic as well. In other words, this also gives an estimate of how many GRBs are recovered thanks to the SWTCs. Obviously, the one third missed by the on-board logic, mainly consists of faint bursts, while a smaller fraction includes those GRBs that, in despite of their brightness, occurred during GRBM dead times.

During the time interval scanned by the off-line quest, i.e. July 3, 1996 - October 3, 2001, the on-board logic was triggered by 15,857 events; out of them, after this off-line quest, only 693 could not have been scanned, because of the proximity to data gaps, mainly connected with the SAGA (this number, i.e. 693, is smaller than the 1032 taken in sec. , because that was for the 1 s quest only: in this case, the number is smaller because several on-board triggers missed by one particular quest, have been scanned with other rebin times, instead; therefore, the final number of 693 unscanned on-board triggers refer to those events, that could not have been scanned by any quest).

Thus, if one takes into account only those on-board triggers that have been scanned by at least one kind of off-line quest (1 s, 2 s, 4 s, or 8 s, resp.), i.e. 15,857 - 693 = 15,164 triggers, the reliability $R_{\mbox{\small {onb}}}$ of the on-board trigger logic can be defined as the ratio of the number of true GRBs that triggered the GRBM on board, over the number of scanned on-board triggers:

$$R_{\mbox{\small {onb}}} \ = \ \frac{\mbox{on-board triggered... ...riggers}} \ = \ \frac{606}{15,164} \ \simeq \ (4.0 \pm 0.2) \%$$ (36)

In summary, while 2 every 3 GRBs that can be detected within the GRBM data, usually trigger the on-board logic, on the other side the probability that a given on-board trigger is really due to a cosmic GRB is very low, $\sim$ 4%.

The estimate coming out from eq.  relies on the assumption that all the on-board triggers that have been automatically scanned by the off-line quest and that have not been classified as GRBs, cannot be GRBs. On the other hand, from table , it is known that there are at least 5 exceptions (4 in common with BATSE and 1 with one WFC); however, in the next sections, when dealing with the common sample with BATSE, it will be shown that this fraction still remains negligible, so that the above assumption is still reasonable.