GAMMA-RAY OBSERVATIONS OF NGC-253 AND M82 WITH OSSE

Gamma-ray observations of the nearby starburst galaxies NGC 253 and M8 2 over the energy range (0.05-10) MeV have been obtained with the OSSE spectrometer on the Compton Gamma-Ray Observatory. The priority of th ese galaxies as OSSE targets had been established on the grounds that the average supernova rate may be high in starbursts as indicated by i nfrared and radio observations, and at distances of similar to 3 MPc a significant chance of supernova gamma-ray line detection exists. NGC 253 was detected in continuum emission up to 165 keV with a total sign ificance of 4.4 sigma and an estimated luminosity of 3 x 10(40) ergs s (-1). The spectrum is best fit by a power law of photon index similar to 2.5. We consider the possible contribution of different emission me chanisms, including inverse Compton scattering, bremsstralung, discret e sources, and Type Ia/Ib supernova continuum to the measured flux. No significant continuum flux was observed from M82. A search for the ga mma-ray line from the decay of the most abundant radioactive element p roduced in supernovae (Ni-56 --> (CO)-C-56 --> Fe-56) yielded no signi ficant detection: the 3 sigma upper limits to the line fluxes at 0.158 , 0.812, 0.847, and 1.238 MeV for both galaxies are obtained.