There is a short gamma-ray burst prompt phase at the beginning of each long one

Abstract

We compare the prompt intrinsic spectral properties of a sample of short gamma-ray bursts (GRBs) with the first 0.3 s (rest frame) of long GRBs observed by Fermi/GBM (Gamma Burst Monitor). We find that short GRBs and the first part of long GRBs lie on the same Ep-Eiso correlation, that is parallel to the relation for the time-averaged spectra of long GRBs. Moreover, they are indistinguishable in the Ep-Liso plane. This suggests that the emission mechanism is the same for short and for the beginning of long events, and both short and long GRBs are very similar phenomena, occurring on different time-scales. If the central engine of a long GRB would stop after ~0.3 × (1 + z) s, the resulting event would be spectrally indistinguishable from a short GRB.