Short-Bright GRBs: spectral properties

We study the spectra of short-bright GRBs detected by BATSE and compare them with the average and time resolved spectral properties of long-bright bursts. We confirm that short events are harder than long bursts, as already found from the comparison of their (fluence) hardness ratio, but we find that this difference is mainly due to a harder low energy spectral component present in short bursts, rather than to a (marginally) different peak energy. Moreover, we find that short GRBs are similar to the first 1 sec emission of long bursts. The comparison of the energetic of short and long bursts also suggests that short GRBs do not obey the peak energy--equivalent isotropic energy correlation recently proposed for long events, implying that short GRBs emit lower energy than long ones. Nonetheless, short bursts seem to emit a luminosity similar to long GRBs and under such hypothesis their redshift distribution appears consistent with that observed for long events. These findings might suggest the presence of a common mechanism at the beginning of short and long bursts which operates on different timescales in the these two classes.