A Search for Hard-spectrum Gamma-ray Bursts Using Smm

We have searched over 9 years of data from the Solar Maximum Mission (SMM) Gamma Ray Spectrometer for gamma-ray bursts (GRBs) with abnormally hard spectra which may have been missed in previous searches in the 0.35{0.8 MeV energy band. For this purpose, we searched the higher 0.8{10 MeV band. We have found only two new GRBs in this search, compared with 177 GRBs found in the 0.35{0.8 MeV band. We have made a careful examination of the threshold conditions for identifying the 0.8{10 MeV events, which connrms that there is no undiscovered population of hard-spectrum GRBs which have escaped detection by SMM , BATSE and other instruments triggered by low-energy-ray emission. This conclusion does not apply to GRBs with duration < 2 s, nor to those which are much fainter than the general population. Our results constrain models of GRBs which require or allow the peak in energy emission to be above 1 MeV.