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The positron emissivity of the Galactic bulge and disk, resulting fromradioactivity of SNIa, is reassessed in the light of a recent evaluation of theSNIa rate. It is found that the disk may supply more positrons than required byrecent SPI/INTEGRAL observations, but the bulge (where the characteristicpositron annihilation line at 511 keV is in fact observed) only about 10%. Itis argued that a large fraction of the disk positrons may be transported viathe regular magnetic field of the Galaxy into the bulge, where they annihilate.This would increase both the bulge emissivity and the bulge/disk ratio,alleviating considerably the constraints imposed by INTEGRAL data analysis. Weargue that the bulge/disk positron emissivity ratio can be considerably smallerthan the values derived by the recent analysis of Knoedlseder et al. (2005), ifthe disk positrons diffuse sufficiently away from their sources, as required byour model; this possibility could be tested in the future, as data areaccumulated in the SPI detectors. The success of the proposed scenario dependscritically upon the, very poorly known at present, properties of the galacticmagnetic field and of the propagation of low energy positrons in it.Comment: 11 pages, 7 figures, to appear in Astronomy and Astrophysic

On the intensity and spatial morphology of the 511 keV emission in the Milky Way