Which radio galaxies can make the highest-energy cosmic rays?

TL;DR

This paper investigates whether radio galaxy lobes can accelerate protons to ultra-high energies, constraining observable properties of candidate sources and discussing implications for cosmic ray origins and future observations.

Contribution

It provides a detailed analysis of stochastic acceleration in radio lobes, establishing constraints on observable properties and energy limits for UHECR production.

Findings

Few radio galaxies satisfy the acceleration constraints within the GZK cutoff.

If UHECR are protons from radio lobes, sources are likely already cataloged.

Maximum achievable energy depends on charge and lobe properties.

Abstract

Numerous authors have suggested that the ultra-high energy cosmic rays (UHECR) detected by the Pierre Auger Observatory and other cosmic-ray telescopes may be accelerated in the nuclei, jets or lobes of radio galaxies. Here I focus on stochastic acceleration in the lobes. I show that the requirement that they accelerate protons to the highest observed energies places constraints on the observable properties of radio lobes that are satisfied by a relatively small number of objects within the Greisen-Zat'sepin-Kuzmin (GZK) cutoff; if UHECR are protons and are accelerated within radio lobes, their sources are probably already known and catalogued radio galaxies. I show that lobe acceleration also implies a (charge-dependent) upper energy limit on the UHECR that can be produced in this way; if lobes are the dominant accelerators in the local universe and if UHECR are predominantly protons, we are unlikely to see cosmic rays much higher in energy than those we have already observed. I comment on the viability of the stochastic acceleration mechanism and the likely composition of cosmic rays accelerated in this way, based on our current understanding of the contents of the large-scale lobes of radio galaxies, and finally discuss the implications of stochastic lobe acceleration for the future of cosmic ray astronomy.