ON THE MORPHOLOGY OF THE ELECTRON-POSITRON ANNIHILATION EMISSION AS SEEN BY SPI/INTEGRAL

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 3.02 and 11.08 FWHM and respective fluxes of 2.5 and 5.4 x10-4 photons cm-2 s-1. We found a possible shift of the bulge center toward negative longitude at l = -0.06 ± 0.02. In addition to the bulge, a more extended structure is detected significantly with flux ranging from 1.7 to 2.9 x10-3 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.