The Ep distribution of long-duration γ-ray bursts observed by the PHEBUS experiment

We present a distribution in Ep, the break energy of γ-ray burst (GRB) spectra, obtained from a spectral analysis of 91 long-duration, high fluence, GRBs recorded by the PHEBUS experiment. From spectral fits above 0.1 MeV of 279 time slices during these 91 GRB events, the centroid of the Ep distribution is found to be ∼0.41 MeV while the full width at half maximum (FWHM) is ∼0.60 MeV. These values are significantly higher than those derived from BATSE data (viz. ∼0.25 and ∼0.30 MeV, respectively). Nevertheless, we confirm that the Ep distribution is narrow with an upper-energy limit at about 2 MeV.