COMPTEL upper limits for the 56Co gamma-ray emission from SN1998bu

Abstract

Supernova 1998bu in the galaxy M96 was observed by COMPTEL for a total of 88 days starting 17 days after the explosion. We searched for a signal in the 847 keV and 1238 keV lines of radioactive 56Co from this type Ia supernova. Using several different analysis methods, we did not detect SN1998bu. Our measurements should have been sensitive enough to detect 60Co gamma-rays as predicted from supernova models. Our 2-sigma flux limit is 2.3 10^{-5} photons cm^{-2} s^{-1}; this would correspond to 0.35 solar mass of ejected 56Ni, if SN1998bu were at a distance of 11.3 Mpc and transparent to MeV gamma rays for the period of our measurements. We discuss our measurements in the context of common supernova models, and conclude disfavoring a supernova event with large mixing and major parts of the freshly-generated radioactivity in outer layers.