2 Instrument calibration and response functions

The GRBM is located at the center of the BeppoSAX payload and thus its response as a function of incoming photons energy and direction is very complex. In order to derive it, in addition to the standard calibrations at experiment level, a campaign of on-ground calibrations when the payload was integrated in the satellite (about 8 months before launch) has been performed (Amati et al. 1997; Costa et al. 1998). Moreover, a very detailed model of the entire satellite has been developed using the Los Alamos MCNP Monte Carlo code (Rapisarda et al. 1997). Both calibrations and model show that LS1 and LS3, the two detectors which are co-aligned with the WFCs (Jager et al. 1997), have rather clean and uniform field of views. Particular efforts have been made to reconstruct the response functions of these units, in order to be able to perform broad band (WFC+GRBM, 1.5-700 keV) spectral analysis of GRB detected by both experiments. The effective area in the GRBM and AC bands as a function of energy and photon direction was described with an analytical model partly based on physical and geometrical considerations and partly empirical (Amati et al. 1997). This allowed the construction of a two-channels (40-100 and 100-700 keV) response matrix that can be used to derive flux and spectral evolution from the 1 s ratemeters in the two bands under the assumption of a simple input model (e.g. power-law, thermal bremsstrahlung). The response matrices for the 240 channels PHA spectra have been built on the basis of the calibration spectra by modeling the relevant components typical of gamma-ray spectra of radioactive sources (photo-peak, escape peak, Compton continuum, back-scattering peak) (Amati et al. 1997). Preliminary results of the Monte Carlo simulations have been used to better determine detector efficiency above 200 keV and to estimate the effects of the environment on calibration measurements.

  

Figure 1: LS1 on-axis CRAB nebula spectrum (during this measurement the GRBM low energy threshold was set to $\sim 20$ keV)