Pair production with capture by energetic cosmic ray nuclei in a photon background

TL;DR

This paper studies how energetic cosmic ray nuclei interact with photon backgrounds, focusing on pair production with electron capture, and finds that ionization dominates during cosmological propagation, but equilibrium can occur near sources affecting acceleration limits.

Contribution

It introduces the process of pair production with electron capture in cosmic ray nuclei and analyzes its impact on ionization states in different astrophysical environments.

Findings

Ionization dominates over PPC during cosmic ray propagation in the CMB.

Equilibrium between ionization and PPC can occur near sources at high energies.

This process limits the acceleration of high-Z nuclei in photon-rich environments.

Abstract

We investigate the ionization state of very energetic cosmic ray nuclei in photon fields, such as the cosmic microwave background (CMB) in extragalactic propagation and the environment surrounding the acceleration site in astrophysical sources. We focus on the process of pair production with electron capture (PPC), where the interaction of a single photon with a nucleus produces an $e^\pm$ pair (similar to Bethe-Heitler process) with the subsequent capture of electron by the nucleus. This process effectively reduces the nucleus charge by one unit and counteracts the photo-ionization process. We show that during cosmological propagation, where the ultra-high energy cosmic rays interact predominantly with the CMB, ionization dominates over PPC for all the cases of practical interest. However, within the source environment and at sufficiently high energies, ionization and PPC processes can reach an equilibrium, leading to a significant fraction of dressed heavy nuclei. This provides a further limitation to the acceleration of high-$Z$ nuclei to very high energies in environments with hot and dense photon fields.