On-line detected GRB Mean Rate

From April 20, 2000 (when the on-line quest started) to December 15, 2001, the time elapsed is 605 days; during this period, the on-line quest went off several times for limited time intervals (lasted from days up to more than one month) because of many different reasons: sometimes because of technical problems with the SOC machines devoted to the data handling and transfer; in other cases, it was the satellite itself that went into a Gyro Safe Mode (GSM) , or, more rarely, when the satellite underwent commissioning phases, like ESM2 (October 2001), during which the gyroless attitude control mode, allowing to point the BeppoSAX instruments without the help of any gyroscopes and assuring the required stability during the observations, was successfully loaded on board, after the last gyro had started failing in September 2001.

In order to estimate the mean rate of GRBs that have been on-line detected with the on-ground automatic S/W, based on the late SWTCs, the number of days, the quest indeed worked in, has to account for these periods of inactivity.

During the above time interval, 317 automatic on-line triggers have been detected; out of this, 229 have been classified as true GRBs, while the remaining 88 are due to solar hard x-ray flares, spurious events, and, above all, to short duration burst candidates: these, since they did not trigger the on-board logic, left their counts only in the 1 s ratemeters: unfortunately, from these time histories only the brightest bursts can be surely recognized as true, while the others remain uncertain (some of them could be due to high-energy particles crossing more than one GRBM unit, or simple statistical fluctuations). On the other hand, to date nobody observed any X-ray counterparts of a short burst. This is partially due to the selected GRBM on-board trigger parameters. We would change them to increase the sensitivity in the case of short burst candidates.

If one defines the reliability of the automatic on-line quest as the ratio of the total number of true GRBs automatically identified (229) over the number of on-line triggers (317), it comes out $\sim 0.72 \pm 0.05$, in agreement with a previous estimate ([Guidorzi et al., 2001b]). In other words, at least three quarters of the on-line triggers are indeed cosmic GRBs. Out of the elapsed time (605 days), the on-line quest was not operating for 182 days for the reasons explained above. Therefore, the GRB mean rate turns out to be: 229 GRBs detected in (605-182)=423 days, i.e. $\sim 0.5$ GRB/day (no correction applied for sky exposure). This is lower than in the off-line case (sec. ) for the reasons explained therein (essentially, the on-line quest is more conservative and, therefore, less sensitive as for faint bursts).