Average spectra

The study of GRB average spectra is complementary to that on spectral evolution and can help in better classify the events. Extensive works have been done on BATSE GRB average spectra ([Band et al. 1993]) from 20 to 2000 keV. The importance of extending average spectrum measurements to energy as low as $\sim$ 2 keV is underlined by theoreticists and has been demonstrated by the Ginga results. There is an evidence of a systematic lowering of E_peak values when using X and gamma-ray data together, also if Ginga measurements are affected by normalization problems. Thus WFC + GRBM average spectrum measurements can give crucial informations, with the advantage with respect to Ginga data of a much well known X-ray detector response function. Also, with respect to spectral evolution study, average spectra analysis make use of higher statistical quality data, i.e. obtained integrating on entire GRB duration.

**Figure 5.11:** WFC/GRBM Time averaged spectrum of GRB970111. The best fit in the 1.5 to 700 keV energy band is obtained with a Band form (see continuous line) with the following parameters: $\alpha=0.50\pm0.04$ , $\beta=2.13\pm0.03$ , $E_{0} =101\pm1$ keV, ( $\chi^2/195=1.24$ ).

**Figure 5.12:** WFC/GRBM Time averaged spectrum of GRB970228. The best fit in the 1.5 to 700 keV energy band is obtained with a Band form (see continuous line) with the following parameters: $\alpha\,=\,1.35\pm0.07$ , $\beta\,=\,1.95\pm0.05$ , $E_{0}\, =\,13\pm3$ keV, $\chi^2/195\,=\,1.05$

**Figure 5.13:** WFC/GRBM Time averaged spectrum of GRB970402. The best fit in the 1.5 to 700 keV energy band is obtained with a single power law (see continuous line) with photon index $\alpha\,=\,1.35\pm0.08$ ( $\chi^2/199 \,=\,0.96$ ).

In figures 5.11, 5.12, 5.13 results of this spectral analysis for GRB970111, GRB970228 and GRB970402 are shown. Again, XSPEC software package (v. 10.0) was used to derive spectral parameters and their uncertainties (1 $\sigma$ ).

From the fit results, both GRB970111 and GRB970228 spectra are fit with the Band form, while the spectrum of GRB970402 is fit with a single power law. By comparing the break energy of the GRB970111 spectrum with that of the GRB970228 spectrum, we see that it is much higher for GRB97011 than for GRB970228: 101 keV vs. 13 keV. This fact could be a hint that the peak energy of the $\nu F_\nu$ spectrum evolved much more rapidly towards lower energies in the case of GRB970228. A fast evolution of $\nu F_\nu$ was actually observed in GRB970228 (next section, [Frontera et al. 1997b]). How this different evolution can influence the presence or not of an afterglow emission is not clear.