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We want to estimate whether the positrons produced by the beta plus decay of26Al, 44Ti and 56Ni synthesised in massive stars and supernovae are sufficientto explain the 511 keV annihilation emission observed in our Galaxy. Such apossibility has often been put forward in the past. We developed a Monte CarloGalactic propagation code for ~MeV positrons in which the Galactic interstellarmedium, the Galactic magnetic field (GMF) and the propagation are finelydescribed. This code allows us to simulate the spatial distribution of the 511keV annihilation emission. We test several GMFs models and several positronescape fractions from type-Ia supernova for 56Ni positrons. We consider thecollisional ballistic transport mode and then compare the simulated 511 keVintensity spatial distributions to the INTEGRAL/SPI data. Whatever the GMFconfiguration and the escape fraction chosen for 56Ni positrons, the 511 keVintensity distributions are very similar. The main reason is that ~MeVpositrons do not propagate very far away from their birth sites in our model.The direct comparison to the data does not allow us to constrain the GMFconfiguration and the escape fraction for 56Ni positrons. In any case,nucleosynthesis positrons produced in steady state cannot explain the fullannihilation emission. The comparison to the data shows that: (a) theannihilation emission from the Galactic disk can be accounted for; (b) thestrongly peaked annihilation emission from the inner Galactic bulge can beexplained by positrons annihilating in the central molecular zone but thisseems to require more positron sources than the population of massive stars andtype Ia supernovae usually assumed for this region; (c) the more extendedemission from the Galactic bulge cannot be explained. We show that a transientsource at the GC, such as a starburst episode, could explain this extendedcomponent.

Monte Carlo modelling of the propagation and annihilation of nucleosynthesis positrons in the Galaxy