Ultra-High-Energy Cosmic Rays from the Radio Lobes of AGNs

TL;DR

This paper investigates how ultra-high-energy cosmic rays are accelerated in AGN radio lobes, presenting a first-principles stochastic acceleration model that reaches energies around 10^20 eV within a million years.

Contribution

It provides the first detailed stochastic acceleration model from first principles for particles in AGN radio lobes, estimating maximum energies achievable.

Findings

Protons can reach ~10^20 eV in about 10^6 years.

The high-energy cutoff may be due to either propagation effects or the acceleration limit.

The model challenges previous assumptions about the origin of the cutoff energy.

Abstract

In the past year, the HiRes and Auger collaborations have reported the discovery of a high-energy cutoff in the ultra-high energy cosmic-ray (UHECR) spectrum, and an apparent clustering of the highest energy events towards nearby active galactic nuclei (AGNs). Consensus is building that such $\sim 10^{19}$--$10^{20}$ eV particles are accelerated within the radio-bright lobes of these sources, but it is not yet clear how this actually happens. In this paper, we report (to our knowledge) the first treatment of stochastic particle acceleration in such environments from first principles, showing that energies $\sim 10^{20}$ eV are reached in $\sim 10^6$ years for protons. However, our findings reopen the question regarding whether the high-energy cutoff is due solely to propagation effects, or whether it does in fact represent the maximum energy permitted by the acceleration process itself.