Energy Spectra of Cosmic Gamma-Ray Bursts

We report spectral measurements of six cosmic gamma-ray bursts in the energy region 0.1-1.2 MeV, made using a semi-omnidirectional X-ray detector on IMP-6. These measurements confirm the hard X-ray or gamma-ray nature of the bursts, as inferred from the original observations by KIebesadel et at., and show that their maximum energy release is in this several-hundred-keV region. Each burst consists of several 1- or 2-second pulses each with the characteristic spectrum of a 150-keV exponential, followed by a softer decay. There is no evidence of line structure in this energy region, or for a marked change in the energy spectrum within a given pulse. Event size spectra are estimated for galactic and extragalactic models; the total emission is consistent with present measurements of the diffuse bacground, and is unlikely to account for any spectral feature in the few-MeV region. Subject heading: gamma rays