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Fermi/GBM and BATSE Gamma-Ray Bursts: comparison of the spectral properties

The Gamma-ray Burst Monitor (GBM) on board Fermi allows to study the spectra of Gamma Ray Bursts (GRBs) over an unprecedented wide energy range (8 keV - 35 MeV). We compare the spectral properties of short and long GRBs detected by the GBM (up to March 2010) with those of GRBs detected by the BATSE instrument on board the CGRO. GBM and BATSE long bursts have similar distributions of fluence (F), Epeak and peak flux (P) but GBM bursts have a slightly harder low-energy spectral index αwith respect to BATSE GRBs. GBM and BATSE short bursts have similar distributions of fluence, αand peak flux, with GBM bursts having slightly larger Epeak. We discuss these properties in light of the found correlations between Epeak and the fluence and the peak flux. GBM bursts confirm that these correlations are not determined by instrumental selection effects. Indeed, GBM bursts extend the Epeak-Fluence and Epeak-P correlations both in fluence/peak flux and in peak energy. No GBM long burst with Epeak exceeding a few MeV is found, despite the possibility of detecting it. Similarly to what found with BATSE, there are 3% of GBM long bursts (and almost all short ones) that are outliers at more than 3σof the Epeak-Eiso correlation. Instead there is no outlier of the Epeak-Lp,iso correlation, for both long and short GBM bursts.