Positron transport in the interstellar medium

¹ CESR, Université de Toulouse, CNRS, INSU: 9 avenue du colonel Roche, BP 44346, 31028 Toulouse, France e-mail: Pierre.Jean@cesr.fr
² KTH, Department of Physics, AlbaNova University Centre, 10691 Stockholm, Sweden
³ The Oskar Klein Centre for Cosmo Particle Physics, AlbaNova, 10691 Stockholm, Sweden
⁴ LPTA, CNRS, Université Montpellier II, 34095 Montpellier Cedex 5, France
⁵ LATT, Université de Toulouse, CNRS: 14 avenue Édouard Belin, 31400 Toulouse, France

Received: 22 March 2008
Accepted: 22 September 2009

Abstract

Aims. 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.

Methods. 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.

Results. We find that low-energy () 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 very large distances (up to for 1 MeV positrons) along magnetic field lines before annihilating.