PHEBUS burst spectra between 0.1 and 10 MeV

We analyze spectra from the 20 most intense GRB events detected by PHEBUS above 0.1 MeV. This burst sample includes both short (T90<2 s) and long events. The spectral data from the two most brightly illuminated detectors are fit jointly for more reliable results. The continuum is found to be well represented by a photon model which makes the curvature energy E0 and the break energy Eb independent. We report evidence in favor of the existence of two sharp breaks at typical energies of 1 and 2 MeV. These findings strongly support the hypothesis of photon-magnetic field interactions at the source.