un 2 00 1 Comparative Studies of Line and Continuum Positron Annihilation Radiation

Positron annihilation radiation from the Galaxy has been observed by the OSSE, SMM and TGRS instruments. Improved spectral modeling of OSSE observations has allowed studies of the distribution of both positron annihilation radiation components, the narrow line emission at 511 keV and the positronium continuum emission. The results derived for each individual annihilation component are then compared with each other. These comparisons reveal approximate agreement between the distribution of these two emissions. In certain regions of the sky (notably in the vicinity of the previously reported positive latitude enhancement), the distribution of the emissions differ. We discuss these differences and the methods currently being employed to understand whether the differences are physical or a systematic error in the present analysis. Nine years of observations made with the Oriented Scintillation Spectrometer Experiment (OSSE) on-board NASA's COMPTON observatory (1991-2000) [1], eight years of observations made with the Gamma-Ray Spectrometer on-board the Solar Maximum Mission (SMM) (1980-1989) [2], and two years of observations made with the Transient Gamma-Ray Spectrometer (TGRS) on-board the WIND mission (1995-1997) [3] have been utilized to study the galactic distribution of positron annihilation radiation. The OSSE instrument featured a 3.8 • x 11.4 • FWHM FoV, a ∼3 x 10 −5 photons cm −2 s −1 line sensitivity (10 6 s on-source time), and a 45 keV energy resolution at 511 keV. These detector attributes have permitted the first detailed studies of the distribution of annihilation radiation in the inner radian of the Galaxy. The annihilation of positrons with electrons gives rise to two spectral features, a line emission at 511 keV and a positronium continuum emission (which increases in intensity with energy roughly as a power law up to 511 keV and falls abruptly to zero above 511 keV) [4]. The TGRS instrument, which featured a