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J.D. Kurfess (NRL), P.A. Milne (NRC/NRL), M.D. Leising (Clemson), D.D. Dixon (UCR), R.L. Kinzer (NRL)
Positron annihilation radiation from the galactic center region was discovered nearly 30 years ago. Most of the observational results have related to the 511 keV line emission associated with positron annihilation. Early evidence for variability of the galactic center emission suggested that discrete sources were a major component of the emission. However, more recent SMM, CGRO and TGRS/WIND results indicate that the dominant emission is steady and of diffuse origin. The OSSE instrument on CGRO has provided the first maps of the 511 keV emission, and the discovery of a surprising excess at positive galactic latitudes above the center of the Galaxy.
Most of the annihilation emission is in a 3-photon continuum below 500 keV. Mapping this emission is of importance in order to confirm the features in the 511 keV maps and to study the annihilation sites through the ratio of the line to continuum emission. Although the continuum signal is stronger, with the brightest emission from the galactic center region having a positronium fraction near unity, the continuum analysis is more difficult because of complications in the spectral fits from the diffuse emission produced by cosmic rays and the uncertain galactic distribution of this emission. We discuss the first efforts to map the positronium continuum emission and the scientific implications of the initial results.
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