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
2 keV is underlined by theoreticists and has been demonstrated by
the Ginga results. There is an evidence of a systematic
lowering of Epeak 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.
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 ).
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 spectrum
evolved much more rapidly towards lower energies in the case of GRB970228.
A fast evolution of
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.