Emission of Positron Annihilation Line Radiation by Clusters of Galaxies

TL;DR

This paper investigates the production of a narrow positron annihilation line in galaxy clusters due to positrons from AGN jets and cosmic ray interactions, highlighting its detectability with future space missions.

Contribution

It demonstrates that high-temperature cluster environments suppress positronium formation, resulting in a distinct narrow annihilation line potentially observable by upcoming space telescopes.

Findings

Positrons from AGNs and cosmic rays can produce a detectable annihilation line in galaxy clusters.

High cluster temperatures prevent positronium formation, leading to a narrow line.

Future missions like INTEGRAL or EXIST could detect this annihilation line within 100 Mpc.

Abstract

Clusters of galaxies are enriched with positrons from jets of active galactic nuclei (AGNs) or from the interaction of cosmic rays with the intracluster gas. We follow the cooling of these positrons and show that their eventual annihilation with cluster electrons yields a narrow annihilation line. Unlike annihilation in the interstellar medium of galaxies, the line produced in clusters is not smeared by three-photon decay of positronium, because positronium formation is suppressed at the high (>~ 1 keV) temperature of the cluster electrons. We show that if AGN jets are composed of e^+e^- pairs, then the annihilation line from rich clusters within a distance of 100 Mpc might be detectable with future space missions, such as INTEGRAL or EXIST.