νFνSpectra of Intense Short Gamma‐Ray Bursts Peak at About 1 MeV

Abstract

We compare the spectral parameter distributions of short- and long-duration gamma-ray bursts (GRBs) recorded by the PHEBUS experiment. Spectral parameters are those of standard photon models. They are derived from fits to fluence and peak flux spectra over a typical 0.1-17 MeV energy range. A striking result of this study is the significant difference between the Ep distributions of short and long GRBs, for both fluence and peak flux spectra. For long GRBs, the most probable value of Ep is seen to be ~0.4 MeV for fluence spectra and ~0.5 MeV for peak flux spectra. These values exceed those reported for BATSE by 0.2-0.3 MeV. At least in part, the reason for this is the larger proportion of hard events detected by PHEBUS above 0.1 MeV. However, for 30 long events common to both experiments, we find that the logarithmic mean of Ep values from 0.25 s peak flux spectra of PHEBUS is higher than that of BATSE by ~0.1 MeV with an increasing disagreement in Ep when the peak energy is greater than 0.4 MeV. Bright short bursts are found to have a peak energy at ~1 MeV on average for both fluence and peak flux spectra. We present evidence for an additional spectral break at about 0.2-0.3 MeV from the analysis of average spectra formed by combining the count-rate spectra of the 19 short PHEBUS events. This low-energy break might mistakenly be interpreted as the peak energy for those hard short events detected with poor statistics above a few hundred keV by experiments with limited high-energy sensitivity.