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The 511 keV positron annihilation emission remains a mysterious component ofthe high energy emission of our Galaxy. Its study was one of the key scientificobjective of the SPI spectrometer on-board the INTEGRAL satellite. In fact, alot of observing time has been dedicated to the Galactic disk with a particularemphasis on the central region. A crucial issue in such an analysis concernsthe reduction technique used to treat this huge quantity of data, and moreparticularly the background modeling. Our method, after validation through avariety of tests, is based on detector pattern determination per ~6 monthperiods, together with a normalisation variable on a few hour timescale. TheGalactic bulge is detected at a level of ~70 sigma allowing more detailedinvestigations. The main result is that the bulge morphology can be modelledwith two axisymmetric Gaussians of 3.2 deg. and 11.8 deg. FWHM and respectivefluxes of 2.5 and 5.4 x 10^-4 photons/(cm^2.s^1). We found a possible shift ofthe bulge centre towards negative longitude at l=-0.6 +/- 0.2 degrees. Inaddition to the bulge, a more extended structure is detected significantly withflux ranging from 1.7 to 2.9 x10^-3 photons/(cm^2.s^1) depending on its assumedgeometry (pure disk or disk plus halo). The disk emission is also found to besymmetric within the limits of the statistical errors.

On the morphology of the electron-positron annihilation emission as seen by SPI/INTEGRAL