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

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 100x in energy, can contribute to diffuse low-frequency radio synchrotron emission. We study CRe scattering and energy gain due to ion cyclotron (IC) waves generated by continuously-driven compression in 1D 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.