Positron Transport in the Interstellar Medium
Résumé
We seek to understand the propagation mechanisms of positrons in theinterstellar medium (ISM). This understanding is a key to determine whether thespatial distribution of the annihilation emission observed in our Galaxyreflects the spatial distribution of positron sources and, therefore, makes itpossible to place constraints on the origin of positrons. We review thedifferent processes that are likely to affect the transport of positrons in theISM. These processes fall into three broad categories: scattering offmagnetohydrodynamic waves, collisions with particles of the interstellar gasand advection with large-scale fluid motions. We assess the efficiency of eachprocess and describe its impact on the propagation of positrons. We alsodevelop a model of positron propagation, based on Monte-Carlo simulations,which enable us to estimate the distances traveled by positrons in thedifferent phases of the ISM. We find that low-energy (< 10 MeV) positronsgenerally have negligible interactions with magnetohydrodynamic waves, insofaras these waves are heavily damped. Positron propagation is mainly controlled bycollisions with gas particles. Under these circumstances, positrons can travelhuge distances (up to 30 kpc/n_H for 1 MeV positrons) along magnetic fieldlines before annihilating.