5 Spectra

We can derive information on spectra of well established afterglow sources. The paucity of photons makes difficult fitting data with laws including many parameters. The main feature of the analysed spectra is a power law with an index ranging from 1.5 to 2.3. Within the errors they are consistent with a unique power law index of $1.9\pm0.3$ (with a reduced $\chi ^{2}$ of 0.9). In no case did we find evidence of a cut-off. By jointly fitting power law and photoelectric absorption we derive confidence intervals of the absorption column. Most of these intervals include the galactic value (Owens et al. 1998). A few slightly exceed this value but none with a compelling significance.

In one case only (GRB 970508, Piro et al. 1998) did we find evidence of a spectral variation during the afterglow, correlated with the intensity. The source associated with this GRB rebursted with a hardening of the spectrum, and subsequently decayed with more rapid law and soft spectrum than that of all other afterglow sources. In the very beginning (Piro et al. 1999) we found evidence of an Fe line. This seems much more than a coincidence since the other burst which showed a rebursting behaviour (GRB 970828, Yoshida et al. 1998) shows as well evidence of an Fe line. In the frame of Synchrotron Shock models the re-acceleration of particles is possibly associated with the encounter of a relatively dense cloud of materials, which is also needed to interpret the Fe feature as fluorescence reprocessing of the primary X-rays from a circumburst environment.