Characteristics of gamma-ray bursts at MeV energies measured by COMPTEL

We present results from the spectral analysis of 29 gamma-ray bursts measured by the COMPTELTelescope instrument (0.75–30 MeV) during the period from April 1991 through May 1995. The time-averaged spectra of these events are consistent with a simple power law model with spectral index in the range 1.5–3.5, whereas simple thermal models are statistically inconsistent with the burst sample. We find good agreement between burst spectra measured simultaneously by BATSE, COMPTEL and EGRET, which typically show a spectral transition or “break” in the BATSE energy range around a few hundred keV followed by simple power law emission extending to hundreds of MeV. Observations of rapid intensity variations up to a few MeV in the stronger bursts are used to investigate the importance of photon-photon opacity and to place limits on the source distance and bulk Lorentz factor