Broad band spectral analysis of GRB with the GRBM

The joint X and gamma ray emission study of GRB is a crucial diagnostic of theoretical models for the main mechanism responsible of the emission, the most popular of which are Synchrotron Shock Models (SSM, e.g. Tavani 1997, Sari et al., 1998), in which a flux of relativistic particles (originated e.g. by neutron stars coalescence) radiates through thin synchrotron emission in the presence of a weak to moderate magnetic field (Tavani, 1997).
BeppoSAX WFC + GRBM broad band spectral data can put constraints on this kind of models:

• average spectra: the extension to low-energy of average spectra allows to determinate if the nature of the emission is thermal or non-thermal. Pure synchrotron models reproduce very well the average spectral shape of GRBs, which have been found to be well fit by the functional form, a smoothed broken power-law, proposed by Band et al, 1993. The same form seems to fit well also the 2-300 keV Ginga spectra, also if systematically lower values of the spectrum peak energy (i.e. the incident photon energy at which the maximum of the $\nu$F$\nu$ occurs) with respect to BATSE data. Deviations from the Band spectral shape at low energies may be present, due to absorption by the environment surrounding the source, intrinsic synchrotron self-absorption, X-ray excess due to the existence of a quasi-thermal extra component with temperature in the keV range or to effects due to the opacity of the material surrounding the source or to substantial hard to soft evolution of the peak energy. Uncertainties in the incident angles of GRBs detected by Ginga make their low-energy measurements somewhat uncertain;
• spectral evolution: the availability of broad band time resolved spectra is crucial to the study of peak energy evolution towards the end of GRB pulse, when the spectrum generally softens. Moreover, there has been indication from a small fraction of BATSE events of an initial low-energy suppression, thus indicating changes in the radiative environment. Low energy SSM emission below about 50 keV may be temporarily suppressed by relativistic plasma effects, with the radiative environment relaxing from a complex to a thinner medium. Finally, the study of the spectral properties evolution of GRB emission is crucial for the search of connections with the associated X-ray afterglow emission.