COMPTON GAMMA-RAY OBSERVATORY OSSE OBSERVATIONS OF SN 1991T

The Oriented Scintillation Spectrometer Experiment on the Compton Gamm a Ray Observatory observed SN 1991T on three occasions in 1991. We fin d no evidence for Co-56 gamma-ray line emission from SN 1991T in any o f the three observations. Combining these measurements yields a 99% co nfidence upper limit of 4.1-6.6 x 10(-5) cm(-2) s(-1), depending on ho w the multiple observations are combined, for the 847 keV line flux in the interval 66-79 days post-explosion. The distance to SN 1991T is q uite uncertain, but these limits are inconsistent with some theoretica l models for distances less than or equal to 10 Mpc. If we consider Ty pe Ia supernova models at distances at which they would have apparent magnitude B = 11.6 at blue maximum, as observed for SN 1991T, then mos t reasonable models are consistent with our upper limits. However, if we allow for some extinction, as indicated by other observations of SN 1991T, the implied distance for a given model is smaller, the expecte d gamma-ray flux is higher and many models are no longer consistent wi th the gamma-ray limits. Others are only marginally consistent. These conclusions are significantly strengthened by combining published uppe r limits from the COMPTEL experiment with those reported here. Togethe r these observations indicate that models of SN 1991T that are relativ ely optically bright and gamma-ray faint, perhaps because of low expan sion velocities in part of the ejecta or the presence of an extended e nvelope, should be investigated further.