Session 58 - Gamma Rays & Cosmic Rays.
Oral session, Tuesday, January 16
Salon del Rey South, Hilton

## [58.04] Observations of Galactic Positron Annihilation Radiation

W. R. Purcell, D. A. Grabelsky, S. M. Matz (Northwestern U.), J. G. Skibo (NRL/NRC), D. D. Dixon (U.C. Riverside), W. N. Johnson, R. L. Kinzer, J. D. Kurfess (NRL)

\def\Al#1#2^#1#2\negthinspace Al

Positron annihilation radiation has been detected from the direction of the galactic center for over 20 years. Early observations indicated that the emission was time variable, suggesting that the positrons were produced by a compact source. By the late 1980's it became clear that there was a significant diffuse source of positron annihilation radiation, in addition to a possible compact source. Observations by the OSSE instrument on NASA's Gamma Ray Observatory\/ have greatly increased our understanding of the source and distribution of galactic positrons. The annihilation radiation is found to follow a 2-component, nova-like distribution consisting of a galactic disk and a nuclear bulge component. The disk component, which represents only \sim 15-20% of the total flux, may be completely due to the radioactive decay of \Al26; however, the source of the bulge component is uncertain. The same 2-component distribution is also found to be consistent with most of the earlier observations by other instruments, suggesting that the emission may be entirely diffuse in origin. A description of the distribution of galactic positron annihilation radiation, limits to variability of the emission and a discussion of the possible sources of the bulge positrons will be presented.