The E_peak distribution of long gamma-ray bursts observed by the PHEBUS experiment.

The Phebus experiment on board the Granat satellite has recorded ∼165 gamma-ray bursts in the nominal 0.1-100 MeV energy range between January 1990 and February 1994. About 130 of them are long events characterized by an emission time τ₅₀ longer than 0.5 second. In this paper, we present a study of the distribution of E_peak, the peak energy of the ν F_ν spectrum, obtained from a time-resolved spectral analysis of 70 long events selected for their high fluence and/or peak flux. We find that E_peak has possibly a bimodal distribution which consists of: 1) a main component below ∼1 MeV that peaks at an energy twice as high as that derived from Batse data. However, the full width at half maximum is comparable to that of Batse (i.e. a change in E_peak within a factor of ∼3). 2) a weaker, secondary component at energies between ∼1 and ∼2 MeV. No value of E_peak is observed above 2 MeV. We show how E_peak varies within the course of bursts and discuss how E_peak depends on the fluence.