Electron Re-acceleration via Ion Cyclotron Waves in the Intracluster Medium

TL;DR

This study investigates how ion cyclotron waves in galaxy cluster environments can re-accelerate cosmic-ray electrons, potentially explaining observed radio emissions through kinetic simulations of wave-particle interactions.

Contribution

It demonstrates that ion cyclotron waves can induce significant energy gain in cosmic-ray electrons via magnetic pumping during compression cycles.

Findings

Electrons can gain 10-30% energy per compression cycle.

Magnetic field amplification of 3-6x enhances re-acceleration.

Pitch-angle scattering facilitates energy transfer through magnetic pumping.

Abstract

In galaxy clusters, the intracluster medium (ICM) is expected to host a diffuse, long-lived, and invisible population of "fossil" cosmic-ray electrons (CRe) with 1-100 MeV energies. These CRe, if re-accelerated by 100x in energy, can contribute synchrotron luminosity to cluster radio halos, relics, and phoenices. Re-acceleration may be aided by CRe scattering upon the ion-Larmor-scale waves that spawn when ICM is compressed, dilated, or sheared. We study CRe scattering and energy gain due to ion cyclotron (IC) waves generated by continuously-driven compression in 1D fully kinetic particle-in-cell simulations. We find that pitch-angle scattering of CRe by IC waves induces energy gain via magnetic pumping. In an optimal range of IC-resonant momenta, CRe may gain up to ~10-30% of their initial energy in one compress/dilate cycle with magnetic field amplification ~3-6x, assuming adiabatic decompression without further scattering and averaging over initial pitch angle.