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 study fossil CRe scattering off ion cyclotron waves generated by a compressing plasma using fully kinetic particle-in-cell (PIC) simulations with test-particle CRe. We present results of 1D simulations that allow ion cyclotron and exclude mirror wave growth. We find that most energy gain comes from magnetic pumping via pitch-angle scattering in gyroresonant regions of CRe phase space, whereas Fermi II-like wave damping is subdominant. We also discuss the scaling from simulation to real parameters and present preliminary results on scattering from both mirror and ion cyclotron waves in 2D.