P oS(Integral08)028 Positron Propagation and the INTEGRAL/SPI 511 keV Bulge/Disk Ratio

The Galactic bulge/disk ratio of 511 keV positron annihilation radiation measured by INTE-GRAL/SPI and the bulge/disk ratio of Galactic supernovae differ significantly. We show, however, that this difference can be understood in the context of a Galactic supernova origin of positrons from decay of nucleosynthetic 56Ni, 44Ti, and 26Al, if the detailed propagation of these MeV positrons in the various phases of the interstellar medium is also taken into consideration. These relativistic positrons must first slow down to energies <10 eV before they can annihilate, and therefore can travel significant distances before annihilating. We show that about 72 % of the positrons resulting from supernovae nucleosynthesis are born in the hot (106K) tenuous phase of the interstellar medium, because of its large filling factor. However, they do not annihilate there because they stream out of this medium and either escape into the overlying halo, or slow down and annihilate in the much denser, warm (104K) ionized and neutral phases — predominately the outer envelopes of molecular clouds. This propagation explains the observed shape of the 511 keV positron annihilation line and the ratio of the line to positronium continuum fluxes. Such propagation also explains the observed bulge/disk ratio of the annihilation radiation, since more than half of the positrons born in the disk escape into the bulge and halo. Further, we predict tha