2010ApJ...720.1772B

The 511 keV positron annihilation emission remains a mysterious component of the high energy emission of our Galaxy. Its study was one of the key scientific objectives of the SPI spectrometer on board the International Gamma-Ray Astrophysics Laboratory satellite. In fact, a lot of observing time has been dedicated to the Galactic disk with a particular emphasis on the central region. A crucial issue in such an analysis concerns the reduction technique used to treat this huge quantity of data, and more particularly the background modeling. Our method, after validation through a variety of tests, is based on detector pattern determination per ∼6 month period, together with a normalization variable on a few hour timescale. The Galactic bulge is detected at a level of ∼70σ, allowing more detailed investigations. The main result is that the bulge morphology can be modeled with two axisymmetric Gaussians of 3{fdg}2 and 11{fdg}8 FWHM and respective fluxes of 2.5 and 5.4 ×10^-4  photons  cm^-2  s^-1. We found a possible shift of the bulge center toward negative longitude at l = -0{fdg}6±0{fdg}2. In addition to the bulge, a more extended structure is detected significantly with flux ranging from 1.7 to 2.9 ×10^-3  photons  cm^-2  s^-1 depending on its assumed geometry (pure disk or disk plus halo). The disk emission is also found to be symmetric within the limits of the statistical errors.