OBSERVATION OF THE 0.511 MEV ANNIHILATION LINE FROM THE INNER GALAXY WITH THE FIGARO-II EXPERIMENT

We report a positive detection of the 0.511 MeV annihilation line from the inner Galaxy. The observation has been performed with the FIGARO II experiment (field of view 77°) during a balloon flight from Charleville, Australia on 1988 November 25-26. Assuming a single point source we find a flux of (2.24 ± 0.41)10-3 photons cm-2 s-1, while for an extended, disk source we obtain a differential flux of (1.91 ± 0.32)10-3 photons cm-2S-1 rad-1 at the Galactic center. Assuming a two-component model (point plus diffuse) and using also a nearly simultaneous, narrow field-of-view observation (Leventhal et al.), we obtain the value (1.23 ± 0.24) 10-3 photons cm-2 s-1 rad-1 for the intensity of the extended component and (0.86 ± 0.28)10-3 photons cm-2 s-1 for the point source.