Annihilation radiation from the galactic center - Positrons in dust?

HEAO 3 autumn 1979 gamma-ray observations of the galactic center indicate that the 2 gamma 0.511 MeV annihilation line contains - as compared with the observed 3 gamma continuum, which is also emitted in the annihilation process - more than 2 times as many photons as can be accounted for by standard models, in which positron annihilation takes place in an ionized interstellar gas. Positron annihilation in the interstellar medium is discussed, and it is shown that a significant increase of the line intensity, sufficient to explain the observed spectrum, may arise from positron annihilation on grains of interstellar dust. Dust, in spite of its small abundance, is bound to play a significant role in positron annihilation because its projected surface area, about 10 to the -21st sq cm per gas atom, is three orders of magnitude more than the Thomson cross section, relevant annihilation rates on ionized gas. Approximate analytic expressions for the rates of various annihilation processes occurring in the interstellar medium are given.