Positron annihilation as a cosmic-ray probe

TL;DR

This paper proposes using positron annihilation emissions as a diagnostic tool to distinguish hadronic processes in supernova remnants interacting with molecular clouds, providing a new method to identify cosmic-ray origins in gamma-ray observations.

Contribution

It introduces the idea that positron annihilation emissions from secondary particles can serve as a diagnostic for hadronic cosmic-ray processes in SNRs, linking gamma-ray and X-ray observations.

Findings

Annihilation emission of secondary positrons can be estimated from gamma-ray flux.

Future telescopes like the Advanced Compton Telescope can detect these annihilation lines.

Secondary positron emissions are detectable with upcoming X-ray telescopes such as NuSTAR and ASTRO-H.

Abstract

Recently, the gamma-ray telescopes AGILE and Fermi observed several middle-aged supernova remnants (SNRs) interacting with molecular clouds. A plausible emission mechanism of the gamma rays is the decay of neutral pions produced by cosmic ray (CR) nuclei (hadronic processes). However, observations do not rule out contributions from bremsstrahlung emission due to CR electrons. TeV gamma-ray telescopes also observed many SNRs and discovered many unidentified sources. It is still unclear whether the TeV gamma-ray emission is produced via leptonic processes or hadronic processes. In this Letter, we propose that annihilation emission of secondary positrons produced by CR nuclei is a diagnostic tool of the hadronic processes. We investigate MeV emissions from secondary positrons and electrons produced by CR protons in molecular clouds. The annihilation emission of the secondary positrons from SNRs can be robustly estimated from the observed gamma-ray flux. The expected flux of the annihilation line from SNRs observed by AGILE and Fermi is sufficient for the future Advanced Compton Telescope to detect. Moreover, synchrotron emission from secondary positrons and electrons and bremsstrahlung emission from CR protons can be also observed by the future X-ray telescope NuSTAR and ASTRO-H.