Compton Gamma Ray Observatory OSSE Observations of SN 1991T

Abstract

The Oriented Scintillation Spectrometer Experiment on the Compton Gamma Ray Observatory observed SN 1991T on three occasions in 1991. We find no evidence for <SUP>56</SUP>Co γ-ray line emission from SN 1991T in any of the three observations. Combining these measurements yields a 99% confidence upper limit of 4.1- 6.6 × 10<SUP>-5</SUP> cm<SUP>-2</SUP> s<SUP>-1</SUP>, depending on how 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 quite uncertain, but these limits are inconsistent with some theoretical models for distances ≤ 10 Mpc. If we consider Type Ia supernova models at distances at which they would have apparent magnitude B = 11.6 at blue maximum, as observed for SN 1991T, then most 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 expected γ-ray flux is higher and many models are no longer consistent with the γ-ray limits. Others are only marginally consistent. These conclusions are significantly strengthened by combining published upper limits from the COMPTEL experiment with those reported here. Together these observations indicate that models of SN 1991T that are relatively optically bright and γ-ray faint, perhaps because of low expansion velocities in part of the ejecta or the presence of an extended envelope, should be investigated further.