Shocks and Non-thermal Particles in Clusters of Galaxies

Abstract

Galaxy clusters grow by gas accretion, mostly from mergers of substructures, which release powerful shock waves into cosmic plasmas and convert a fraction of kinetic energy into thermal energy, amplification of magnetic fields and into the acceleration of energetic particles. The modeling of the radio signature of cosmic shocks, combined with the lack of detected γ-rays from cosmic ray (CR) protons, poses challenges to our understanding of how cosmic rays get accelerated and stored in the intracluster medium. Here we review the injection of CRs by cosmic shocks of different strengths, combining the detailed "microscopic" view of collisionless processes governing the creation of non-thermal distributions of electrons and protons in cluster shocks (based on analytic theory and particle-in-cell simulations), with the "macroscopic" view of the large-scale distribution of cosmic rays, suggested by modern cosmological simulations. Time dependent non-linear kinetic models of particle acceleration by multiple internal shocks with large scale compressible motions of plasma with soft CR spectra containing a noticable energy density in the superthermal protons of energies below a few GeV which is difficult to constrain by Fermi

Tempestuous life beyond R500: X-ray view on the Coma cluster with SRG/eROSITA

“…Since in this scenario the western shock is the "secondary shock" (or "mini-accretion shock"), the gas upstream of the shock has already passed through the primary shock soon after the NGC4839 group passage through the core about a Gyr ago. We assume that the primary shock was strong enough to accelerate (or reaccelerate) particles that can potentially emit synchrotron radiation (see, e.g., Bykov et al 2019, for a review). The question arises of whether these particles survive for a gigayear and eventually been compressed (and re-accelerated) by the secondary shock.…”

Simulations and observational tests of primordial magnetic fields from Cosmic Microwave Background constraints

“…Protons and/or electrons should undergo different kinds of shock acceleration as a function of plasma parameters as well as of the topology of up-stream magnetic field (e.g. Bykov et al 2019, and references therein for a recent review). While CR protons should be efficiently accelerated by strong shocks with a quasi-parallel geometry between the shock normal and the upstream magnetic field via diffusive shock acceleration (DSA), CR electrons may be accelerated in a two-phase fashion, in which they first gain energy via shock-drift acceleration if shocks are quasiperpendicular, and are later suitable for acceleration by DSA (e.g.…”

Limiting the shock acceleration of cosmic ray protons in the ICM

“…A likely explanation for relics is diffusive shock acceleration (DSA; e.g. Bykov et al 2019, and references therein). Yet, several questions for the complete understanding of relics remain.…”