Dynamics of rising magnetized cavities and UHECR acceleration in clusters of galaxies

TL;DR

This paper models the expansion of magnetized cavities in galaxy clusters, showing how magnetic reconnection within these structures can accelerate cosmic rays to ultra-high energies, potentially explaining observed cosmic ray fluxes.

Contribution

It introduces self-similar solutions for expanding electromagnetic cavities with no surface currents, highlighting their role in UHECR acceleration via magnetic reconnection.

Findings

Magnetic cavities can expand stably with no surface currents.

Large-scale current sheets form during cavity expansion for certain plasma conditions.

Magnetic reconnection in these sheets can accelerate cosmic rays to ultra-high energies.

Abstract

We study the expansion of low density cavities produced by Active Galactic Nuclei jets in clusters of galaxies. The long term stability of these cavities requires the presence of linked magnetic fields. We find solutions describing the self-similar expansion of structures containing large-scale electromagnetic fields. Unlike the force-free spheromak-like configurations, these solutions have no surface currents and, thus, are less susceptible to resistive decay. The cavities are internally confined by external pressure, with zero gradient at the surface. If the adiabatic index of the plasma within the cavity is $\Gamma>4/3$, the expansion ultimately leads to the formation of large-scale current sheets. The resulting dissipation of the magnetic field can only partially offset the adiabatic and radiative losses of radio emitting electrons. We demonstrate that if the formation of large-scale current sheets is accompanied by explosive reconnection of the magnetic field, the resulting reconnection layer can accelerate cosmic rays to ultra high energies. We speculate that the enhanced flux of UHECRs towards Centaurus A originates at the cavities due to magnetic reconnection.