In-flight performances of the BeppoSAX gamma-ray burst monitor

The Italian-Dutch satellite for X-ray Astronomy BeppoSAX is successfully operating on a 600 km equatorial orbit since May 1996. We present here the in-flight performances of the Gamma Ray Burst Monitor experiment during its first year of operation. The GRBM is the secondary function of the four CsI(Na) slabs primarily operating as an active anticoincidence of the PDS hard X-ray experiment. It has a geometric area of about 4000 cm(2) but, due to its location in the core of the satellite its effective area is dependent on the energy and direction of the impinging photons. A dedicated electronics allows to trigger on cosmic gamma-ray bursts. When the trigger condition is satisfied the light curve of the event is recorded from 8 s before to 98 s after the trigger time, with a maximum time resolution of 0.48 ms, in an energy band of 40-700 keV. As an instrument housekeeping the 1 s event ratemeter of the same detectors in the same energy band is stored regardless the trigger condition, allowing for an off-line detection of non-triggered events. Finally, the onboard software collects the event count rate that is used as anticoincidence, i.e. the events above a given energy threshold, typically kept at 100 keV. The flight-data screening is in progress, in order to extract real Gamma Ray Bursts from the many sources of background. Already many results have been obtained, as those GRBs detected simultaneously with the Wide Field Cameras onboard BeppoSAX itself.