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2 The GRBM onboard BeppoSAX

The Gamma Ray Burst Monitor (e.g. [Frontera et al. 1997]; [Feroci et al. 1997]) is composed of the four veto shields of the high energy PDS experiment, which is located in the inner part of the spacecraft (see e.g. [Frontera et al. 1997]; [Costa et al. 1998]). This location causes a partial obscuration of the instrument's field of view by other parts of the satellite. Moreover, each detector is affected by different elements of the scientific payload, causing a different energy and direction dependence of the response for the four detectors. Detectors GRBM1 and GRBM3 are affected by the Low and Medium Energy Concentrator Spectrometers, GRBM2 by the HPGSPC and GRBM4 by the Wide Field Cameras. On-ground calibrations performed after the full integration of the spacecraft have been analized, corrected for environmental effects, interpolated and fitted with physical and semiempirical functions ([Amati et al. 1997]) to provide the response of each detector as a function of the incoming direction and the spectrum of GRBs. To evaluate the accuracy of this positioning we started an extended check of the response of GRBM to GRBs located by BATSE.

Table 1: BATSE/GRBM relative trigger efficiency from May, 23$^{\rm rd}$ 1997 to June, 6$^{\rm th}$ 1998

Number of days & 374 \\ BATSE triggers & 322 \\ GRBM...
Common triggers & 32\% \\ Common detections & 46\% \\ \hline\end{tabular}

To test the capability of GRBM of detecting GRBs ([Feroci et al. 1997]), we have considered a sample of events simultaneosly detected by BATSE and GRBM and studied their characteristics of time duration, intensity and spectral properties. A deep study carried out over about one year has given the results listed in Table 1 where with "Hardware OFF" we indicate that the instrument is switched off, and this can occur for example during the crossing of the South Atlantic Geomagnetic Anomaly, or in case of satellite maintenaince. With "Software OFF", instead, we indicate that the instrument is on, but the software is not enabled: data are stored and can be analized, but the trigger function is inactive. A particular care was devoted to the study of possible selection effects on the GRBM trigger efficiency (see also [Feroci et al. 1999]), showing a clear selection on duration (only 2 events shorter than 1 s), a partial correlation between intensity and incoming direction (weaker events are preferably triggered at directions close to the satellite's equatorial plane). The angular distribution of these GRBs in the satellite rest frame reveals the pair of detectors GRBM2 and GRBM3 as the most efficient in triggering GRBs (we remind here that a valid GRB is recognized when at least 2 detectors trigger on the same event).


\includegraphics [width=8cm,clip]{}

\vspace*{-2mm}\vspace*{-4mm}\end{figure} Figure 1: Dependence of the expected GRBM2/GRBM3 relative intensity on the azimuthal angle $\phi$ with respect to the GRBM3 axis (computed for altitude $\theta=6.8\hbox{$^\circ$}$,spectral index $\alpha=0.9$)

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