Oriented Scintillation Spectrometer Experiment Observations of 57Co in SN 1987A

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory has observed SN 1987A for two 2 week periods during the first 9 months of the mission. Evidence for gamma-ray line and continuum emission from ^57^Co is observed with an intensity of about 10^-4^ gamma cm^-2^ s^-1^. This photon flux between 50 and 136 keV is demonstrated by Monte Carlo calculations to be independent of the radial distribution of ^57^Co for models of low optical depth, viz., models having photoelectric absorption losses of 122 keV photons no greater than several percent. For such models the observed ^57^Co flux indicates that the ratio ^57^Ni/^56^Ni produced in the explosion was about 1.5 times the solar system ratio of ^57^Fe/^56^Fe. When compared with nearly contemporaneous bolometric estimates of the luminosity for SN 1987A, our observations imply that ^57^Co radioactivity does not account for most of the current luminosity of the supernova remnant in low optical depth models. We suggest alternatives, including a large optical depth model that is able to provide the SN 1987A luminosity and is consistent with the OSSE flux. It requires a 57/56 production ratio about twice solar.