Observation of the Starburst Galaxy Ngc 253 with the Osse Instrument

Gamma-ray observations of the nearby starburst galaxy NGC 253 over the energy range 0.06–10 MeV have been obtained with the OSSE spectrometer. The source was detected up to 200 keV with a total significance of 4.2σ. When attributed to NGC 253 this corresponds to an estimated luminosity of 3 × 1040 ergs s-1. The spectrum is fit by a power law of photon index ~ 2.5. A search for 56Ni → 56Co → 56Fe supernova gamma-ray lines yielded no significant detection: the 3σ upper limits at 0.158, 0.847 and 1.238 MeV are 4 × 10-5 , 8 × 10-5 and 9 × 10-5 ph cm-2 s-1 , respectively. We find that inverse Compton scattering is insufficient to explain the observed continuum radiation. Bremsstrahlung and discrete sources may account for the flux. We also consider the possibility that the detected emission may result from low energy continuum from scattered gamma-ray lines produced by a very recent Type Ia or Ib supernova outburst in NGC 253.