Ultra-high energy cosmic rays from shocks in the lobes of powerful radio galaxies

TL;DR

This paper proposes that shocks in the backflowing material of radio galaxy lobes can accelerate particles to ultra-high energies via diffusive shock acceleration, offering a plausible source for observed UHECRs.

Contribution

It demonstrates through simulations and physical arguments that non-relativistic or mildly relativistic shocks in radio galaxy lobes can produce UHECRs, a novel acceleration site compared to previous models.

Findings

Shocks in radio galaxy lobes can reach energies of 10^{19-20} eV.

Approximately 10% of jet energy flux passes through these shocks.

Multiple shocks can enhance particle acceleration to ultra-high energies.

Abstract

The origin of ultra-high energy cosmic rays (UHECRs) has been an open question for decades. Here, we use a combination of hydrodynamic simulations and general physical arguments to demonstrate that UHECRs can in principle be produced by diffusive shock acceleration (DSA) in shocks in the backflowing material of radio galaxy lobes. These shocks occur after the jet material has passed through the relativistic termination shock. Recently, several authors have demonstrated that highly relativistic shocks are not effective in accelerating UHECRs. The shocks in our proposed model have a range of non-relativistic or mildly relativistic shock velocities more conducive to UHECR acceleration, with shock sizes in the range 1-10kpc. Approximately 10% of the jet's energy flux is focused through a shock in the backflow of $M>3$. Although the shock velocities can be low enough that acceleration to high energy via DSA is still efficient, they are also high enough for the Hillas energy to approach $10^{19-20}$eV, particularly for heavier CR composition and in cases where fluid elements pass through multiple shocks. We discuss some of the more general considerations for acceleration of particles to ultra-high energy with reference to giant-lobed radio galaxies such as Centaurus A and Fornax A, a class of sources which may be responsible for the observed anisotropies from UHECR observatories.