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

The intracluster medium (ICM) is expected to host a diffuse, long-lived population of "fossil" cosmic-ray electrons (CRe). Fossil CRe have MeV–GeV energies, too low to emit MHz-GHz radio synchrotron, but a 10–1000x gain in energy can re-accelerate fossil CRe to radio-emitting energies. We propose a mechanism for CRe re-acceleration via scattering from ICM plasma waves. The ICM, a high-beta plasma, sprouts mirror, firehose, and ion cyclotron waves in response to compression or expansion; these waves may transfer energy directly from large-scale ICM motions to fossil CRe, bypassing the scale-by-scale turbulent cascade. We focus upon ion cyclotron waves generated by a compressing plasma using fully kinetic particle-in-cell (PIC) simulations. Pitch-angle scattering on ion cyclotron waves drives most CRe energy gain via magnetic pumping. We discuss how the energy gain may be scaled to real parameters.