Positron annihilation radiation from the inner Galaxy

The OSSE instrument on the Compton Gamma Ray Observatory (CGRO) was used to measure the one-dimensional latitudinal and longitudinal distributions of positron annihilation radiation (annihilation line and positronium continuu m components) along the inner Galactic ridge. Intensity measurements near t he Galactic center (differential relative to background fields offset by 9 degrees -12 degrees) show a symmetrical and spatially narrow bulge around t he center (effective FWHMs of 4.degrees9 +/-0.degrees7 in Galactic latitude and 6.degrees3 +/-1.degrees5 in longitude) with significant disk contribut ions to at least 35 degrees in longitude. The latitude width of the Galacti c plane emission away from the Galactic center is only weakly constrained i n the current analysis. The longitudinal distribution is well fitted by eit her a model comprising a narrow (similar to6.degrees3 FWHM) Gaussian bulge plus similar to 35 degrees FWHM Gaussian and CO-like disk components, or by a center-truncated R-1/4 spheroid plus exponential disk model. For the lat ter model, the observed narrow latitudinal distribution near the Galactic c enter suggests that the bulge is ellipsoidal, although the fits are consist ent with a spherical bulge as well. The positronium fraction estimate from the Galactic center direction is 0.93 +/-0.04. This, combined with the exis ting narrow line width measurements for the 511 keV annihilation line, sugg ests that the bulk of the positron annihilation from the Galactic center di rection occurs after positronium formation in a warm environment. The inner Galaxy total annihilation radiation flux intensities of similar to 10.6 x 10(-3) to similar to 13.7 x 10(-3) cm(-2) s(-1) implied by the fitted R-1/4 ellipsoid plus exponential disk model (narrow and broad disks, respectivel y) are in agreement with intensities implied by the 511 keV line flux obser ved by the broad-field SMM instrument. The double-Gaussian plus CO disk mod el gives similar agreement if a broad (similar to 12 degrees FWHM) disk com ponent is assumed.