Positron transport in the interstellar medium

TL;DR

This paper reviews and models the transport mechanisms of positrons in the interstellar medium, highlighting that collisions with gas particles primarily control positron propagation, allowing them to travel large distances before annihilation.

Contribution

It provides a comprehensive review of positron transport processes and introduces a Monte-Carlo simulation model to estimate positron travel distances in different ISM phases.

Findings

Positrons mainly interact via collisions with gas particles.

Low-energy positrons can travel up to 30 kpc/n_H before annihilation.

Magnetohydrodynamic waves have negligible effects on positron propagation.

Abstract

We seek to understand the propagation mechanisms of positrons in the interstellar medium (ISM). This understanding is a key to determine whether the spatial distribution of the annihilation emission observed in our Galaxy reflects the spatial distribution of positron sources and, therefore, makes it possible to place constraints on the origin of positrons. We review the different processes that are likely to affect the transport of positrons in the ISM. These processes fall into three broad categories: scattering off magnetohydrodynamic waves, collisions with particles of the interstellar gas and advection with large-scale fluid motions. We assess the efficiency of each process and describe its impact on the propagation of positrons. We also develop a model of positron propagation, based on Monte-Carlo simulations, which enable us to estimate the distances traveled by positrons in the different phases of the ISM. 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 these circumstances, positrons can travel huge distances (up to 30 kpc/n_H for 1 MeV positrons) along magnetic field lines before annihilating.