Observation of gamma-ray bursts from 10 keV to 9 MeV

The γ-ray spectrometer on the Solar Maximum Mission (SMM) satellite has detected at least 14 confirmed cosmic γ-ray bursts during its first 14 months in orbit. Individual peaks observed in some of the bursts differ signficantly in hardness from one another. Similarity of the time profiles in different energy bands suggests that photons spanning two decades in energy are produced by the same mechanism. All of the bursts were detected to energies >~1 MeV. Detection of photons into the MeV region can be used to set limits on the distance to γ-ray burst sources17. Spectra of four events falling within the instrument's aperture are presented. Two appear to be well fit by single power laws, but the indices are strikingly different. The other two require either two power laws or an exponential function. A thermal bremsstrahlung fit would require temperatures >~1 MeV. No clear evidence has been found for the existence of narrow line features in any of the bursts. No evidence for broadened line features is seen in any of the four spectra presented.