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

¹ Université de Toulouse, UPS-OMP, IRAP, 31400 Toulouse, France
e-mail: aalexis@irap.omp.eu
² CNRS, IRAP, 9, avenue du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France

Received: 30 July 2013
Accepted: 23 February 2014

Abstract

Aims. We want to estimate whether the positrons produced by the β⁺-decay of ²⁶Al, ⁴⁴Ti, and ⁵⁶Ni synthesised in massive stars and supernovae are sufficient to explain the 511 keV annihilation emission observed in our Galaxy. Such a possibility has often been put forward in the past. In a previous study, we showed that nucleosynthesis positrons cannot explain the full annihilation emission. Here, we extend this work using an improved propagation model.

Methods. We developed a Monte Carlo Galactic propagation code for ~MeV positrons in which the Galactic interstellar medium, the Galactic magnetic field, and the propagation are finely described. This code allows us to simulate the spatial distribution of the 511 keV annihilation emission. We tested several Galactic magnetic fields models and several positron escape fractions from type-Ia supernova for ⁵⁶Ni positrons to account for the large uncertainties in these two parameters. We considered the collisional/ballistic transport mode and then compared the simulated 511 keV intensity spatial distributions to the INTEGRAL/SPI data.

Results. Regardless of the Galactic magnetic field configuration and the escape fraction chosen for ⁵⁶Ni positrons, the 511 keV intensity distributions are very similar. The main reason is that ~MeV positrons 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 Galactic magnetic field configuration and the escape fraction for ⁵⁶Ni positrons. In any case, nucleosynthesis positrons produced in steady state cannot explain the full annihilation emission. The comparison to the data shows that (a) the annihilation emission from the Galactic disk can be accounted for; (b) the strongly peaked annihilation emission from the inner Galactic bulge can be explained by positrons annihilating in the central molecular zone, but this seems to require more positron sources than the population of massive stars and type Ia supernovae usually assumed for this region; (c) the more extended emission from the Galactic bulge cannot be explained. We show that a delayed 511 keV emission from a transient source, such as a starburst episode or a recent activity of Sgr A*, occurring between 0.3 and 10 Myr ago and producing between 10⁵⁷ and 10⁶⁰ sub-MeV positrons could explain this extended component, and potentially contribute to the inner bulge signal.