Emission of Positron Annihilation Line Radiation by Clusters of Galaxies

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 ee 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. Subject headings: galaxies: jets – X-rays: galaxies: clusters – galaxies: clusters – cosmic rays