Observations of galactic gamma-radiation with the SMM spectrometer

Preliminary results from the SMM γ-ray spectrometer indicate the detection of a constant source of 0.511 MeV annihilation radiation from the Galaxy. This source was observed in each of 5 years as the region of the Galactic center passed through the instrument's ∼120° field of view. Any year-to-year variability appears to be less than 30%. The measured intensity of the source is model dependent: for a point source at the center the average flux is (1.6 - 2.9) × 10⁻³ γ cm⁻² s⁻¹; for a distributed source following the Galactic CO emission the flux is (1.4 - 2.7) × 10⁻³ γ cm⁻² s⁻¹ rad⁻¹ (uncertainty is due primarily to systematic errors). It is likely that the radiation comes from a diffuse source and is not associated with the reported compact source at the Galactic center. We have no new information to report on the distribution of ²⁶Al γ-rays. Upper limits of 1.5 × 10⁻³ γ cm⁻² s⁻¹ are placed on Doppler-shifted lines from SS433.