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

Abstract

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°.2 and 11.°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 1 = -0.°6 ± 0.°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. © 2010. The American Astronomical Society.