Positron transport in the interstellar medium

摘要

Aims. We seek to understand the propagation mechanisms of positrons in the interstellar medium (ISM). This understanding is a keyto determine whether the spatial distribution of the annihilation emission observed in our Galaxy reflects the spatial distribution ofpositron sources and, therefore, makes it possible to place constraints on the origin of positrons.Methods. We review the different processes that are likely to affect the transport of positrons in the ISM. These processes fall intothree broad categories: scattering off magnetohydrodynamic waves, collisions with particles of the interstellar gas, and advection withlarge-scale fluid motions. We assess the efficiency of each process 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 bypositrons in the different phases of the ISM.Results. We find that low-energy (≤10 MeV) positrons generally have negligible interactions with magnetohydrodynamic waves,insofar as these waves are heavily damped. Positron propagation is mainly controlled by collisions with gas particles. Under thesecircumstances, positrons can travel very large distances (up to –30 kpc/rill,,,,-3 for 1 MeV positrons) along magnetic field lines beforeannihilating.