Production of Positrons in the Galactic Center by Subrelativistic Protons and Nuclei

Abstract

We propose alternative processes of generation of positrons producing 511 keV annihilation line in the Galactic bulge by cosmic rays. Since relativistic cosmic rays produce both positrons and gamma rays and the flux of the latter is limited by observations, we consider sub-relativistic particles. Particles with energies below the threshold of charged pions production can generate positrons in two processes: direct pair production in electromagnetic interactions (ultraperipheral collisions) and by production of unstable isotopes by spallation and proton capture. Cross-sections of these processes are very small at non-relativistic energies and rapidly increase with particles velocities. Optimal protons velocities are about \(0.1{-}0.3\) of luminal velocity. In this velocity range, the cross-sections are high enough while gamma rays production is low. Thus, to produce positrons a special class of cosmic ray sources is necessary. These sources should produce large amount of sub-relativistic particles in the specified velocity range. We consider fast blue optical transients and stellar tidal disruptions by central black hole as these type of sources. Characteristic outflow velocities in these objects are of order of tenth of luminal velocity. If acceleration of relativistic particles in these outflows is low enough, they can produce the required amount of positrons. We also show that energy produced by stellar tidal disruptions events is large enough to produce the required amount of positrons.