Stochastic particle acceleration in the lobes of giant radio galaxies

TL;DR

This paper explores whether stochastic particle acceleration in the lobes of giant radio galaxies, especially Centaurus A, can produce ultra-high energy cosmic rays within current physical constraints.

Contribution

It provides a detailed analysis of second-order Fermi acceleration in radio galaxy lobes, assessing its viability for generating ultra-high energy cosmic rays.

Findings

Quasilinear theory approximations are valid at low turbulence levels.

Pure stochastic acceleration requires baryon densities below current upper limits.

Centaurus A's lobes could accelerate particles to ultra-high energies under specific conditions.

Abstract

We investigate the acceleration of particles via the second-order Fermi process in the lobes of giant radio galaxies. Such sites are candidates for the accelerators of ultra-high energy cosmic rays. We focus on the nearby FR I radio galaxy Centaurus A. This is motivated by the coincidence of its position with the arrival direction of several of the highest energy Auger events. The conditions necessary for consistency with the acceleration timescales predicted by quasi-linear theory are reviewed. Test particle calculations are performed in fields which guarantee electric fields with no component parallel to the local magnetic field. The results of quasilinear theory are, to order of magnitude, found to be accurate at low turbulence levels for non-relativistic Alfven waves and at both low and high turbulence levels in the mildly relativistic case. We conclude that for pure stochastic acceleration to be plausible as the generator of ultra-high energy cosmic rays in Centaurus A, the baryon number density would need to be several orders of magnitude below currently held upper-limits.