GRB Rate

The final set of 944 GRBs has been extracted within the time interval July 3, 1996 - October 3, 2001 (sec. ); the total elapsed time amounts to 1919 days, i.e. $\sim 1.66 \times 10^8$ s; the time covered by the 1 s ratemeters that have been scanned during the off-line quest, amounts to $\sim 1.17 \times 10^8$ s, i.e. $\sim$ 70% out of the total elapsed time (so far, the total time spent by the spacecraft over the SAGA, i.e. when no GRBM data are acquired, has not been taken into account yet). If one excludes the time spent over the SAGA, amounting to $\sim$ 18% on average (sec. ), it comes out that the total scanned time amounts to $\sim 9.6 \times 10^7$ s, i.e. $\sim$ 1111 days. In order to take into account the sky exposure owing to the only Earth-blocking, the sky fraction, that is always hidden by the Earth, amounting to $\sim$ 33%, has to be considered. Furthermore, since not all the 1 s bins have been scanned by the off-line quest algorithms, also the number of detected GRBs must be corrected by the off-line quest coverage factor, $f_{\mbox{\small {cov}}}$ (eq. ). The final mean rate GRB $<r>$ turns out to be the following:

$$<r> \ = \ \frac{\mbox{\char93 GRBs} \, / \, f_{\mbox{\small {cov... ... \frac{(899 \, \mbox{GRBs}\, / \, 0.956)}{(1111 \ \mbox{days}) \times 0.66} \ =$$

$$\ = \ 1.28 \pm 0.04 \ \mbox{GRBs/day}$$ (39)

In estimating the mean GRB rate, the value of $N=899$ GRBs has been adopted in eq. , instead of 944, because there have excluded both the 35 bursts taken from the RAW data (the Scanned Time used, does not cover the time periods including these bursts) and the 10 GRBs detected by means of the early SWTCs, and coming from the gaps, that are already accounted for by means of the coverage factor $f_{\mbox{\small {cov}}}$ (see table ).

The GRB rate $<r>$ as measured from eq.  does depend on the GRBM sensitivity; thus, it expresses the mean GRB rate as measured with the GRBM, and should not be confused with the intrinsic cosmic GRB rate. Nevertheless, by comparing this mean rate with that of BATSE, it gives an insight into the GRBM sensitivity in combination with the quest algorithms used to extract the present GRB catalog.