2011MNRAS.418L.109G

We study the spectral evolution on second and subsecond time-scales in 11 long and 12 short gamma-ray bursts (GRBs) with peak flux >8.5 {x} 10^–6 erg/cm2 s (8 keV–35 MeV) detected by the Fermi satellite. The peak flux correlates with the time-averaged peak energy in both classes of bursts. The peak energy evolution, as a function of time, tracks the evolution of the flux on short time-scales in both short and long GRBs. We do not find evidence of a hard-to-soft spectral evolution. While short GRBs have observed peak energies larger than few MeV during most of their evolution, long GRBs can start with a softer peak energy (of few hundreds keV) and become as hard as short ones (i.e. with E^obs_peak larger than few MeV) at the peak of their light curve. Six GRBs in our sample have a measured redshift. In these few cases we find that their correlations between the rest frame E_peak and the luminosity L_isoare less scattered than their correlations in the observer frame between the peak energy E^obs_peak and the flux P. We find that the rest frame E_peak of long bursts can be as high or even larger than that of short GRBs and that short and long GRBs follow the same E_peak(t)–L_iso(t) correlation, despite the fact that they likely have different progenitors.