Directions of GRBs with the GRBM

Although the GRBM is an all-sky monitor without imaging capabilities, and, therefore, it could not determine the arrival direction of a GRB, nevertheless the ratios of the counts detected by each single detector unit due to the same burst strongly depend on this direction; thus, in priciple this relationship can be exploited to reconstruct the arrival direction of a burst and to convert its counts into physical units: the total counts into fluence ( $\rm erg\ \rm cm2$), the peak count rate into peak flux ( $\rm erg\ \rm cm2\ \rm s1$): in other words, a proper response matrix can be obtained. The GRBM response matrix, exploiting this property, is based on a Monte Carlo model of the BeppoSAX payload and it has been initially developed by [Rapisarda et al., 1997] and completed by [Calura et al., 2000] (see also [Montanari et al., 2000]). Below a brief description of the payload model and of the localization technique are given. In the next chapters, the localizing capabilities and the spectral reconstruction performances of the GRBM response matrices derived from this model and finally tested with on-flight data are widely discussed. The importance of such matrices is obvious: as it will be shown, thanks to them, it has been possible to estimate the incoming directions of about two hundreds bursts among those that were detected only by the GRBM with an accuracy of about 20-40$\rm ^{\circ}$. Morevoer, in the hope of an automatic localization of on-line detected bursts, this could help some of the current robotic searches for optical flashes, like the Robotic Optical Transient Search Experiment (ROTSE) ([Akerlof et al., 2000], [Akerlof et al., 1999]), or the Burst Observer and Optical Transient Exploring System (BOOTES) ([Castro Ceron et al., 2001]).