1 Introduction

The BeppoSAX instrumentation was not specifically designed to search for prompt and delayed X-ray counterparts to GRBs. This could be done because the BeppoSAX payload is rather flexible and complete, but still it suffers from some instrumental limitations for GRB investigations, mainly due to the fact that the Gamma Ray Burst Monitor (GRBM, [Frontera et al. 1997]; [Feroci et al. 1997]; [Costa et al. 1998]) is a secondary function of the anticoincidence shields of the PDS experiment and to the limited ($\sim$5%) sky coverage by the Wide Field Cameras (WFC, [Jager et al. 1997]). These limitations imply that some selection effects can be expected in the sample of GRBs localized by the WFC. On the basis of the instrumentation design and of the procedures used so far for localizing GRBs we can expect the following GRB characteristics to be possibly biased: coordinates, gamma-ray intensity, X-ray intensity, duration. This is made clear by considering the three basic procedures used so far:

(a) Upon a GRBM trigger, the WFC 1-s ratemeters are searched for a possible counting excess at the time of the trigger. The integration time to detect a count rate increase that can determine a GRBM trigger is currently set at 1 s. This implies that GRBs shorter than 1 s are selected against just by the triggering and screening procedure. Also, the GRBM detectors are individually very sensitive but, as explained in Sect. 4, the trigger condition could cause a selection on the gamma-ray intensity of the event.

(b) The WFC 8-s ratemeters are continuously searched for counting excesses, independently of the GRBM triggers. Given the 8-s integration time this operates a selection both on duration and on X-ray intensity.

(c) When the nominal position of a BATSE/LOCBURST trigger ([Kippen et al. 1997]; [Paciesas et al. 1999]) is within or close to the WFC field of view, the 8-s ratemeters are searched for counting excess. Therefore, in addition to the same effect on duration and X-ray intensity as above, since the BATSE localization capabilities improve at higher intensities this procedure implies a selection on gamma-ray intensity.