Shock Waves and Cosmic Ray Acceleration in the Outskirts of Galaxy Clusters

Abstract

The outskirts of galaxy clusters are continuously disturbed by mergers and gas infall along filaments, which in turn induce turbulent flow motions and shock waves. We examine the properties of shocks that form within r 200 in sample galaxy clusters from structure formation simulations. While most of these shocks are weak and inefficient accelerators of cosmic rays (CRs), there are a number of strong, energetic shocks which can produce large amounts of CR protons via diffusive shock acceleration. We show that the energetic shocks reside mostly in the outskirts and a substantial fraction of them are induced by infall of the warm-hot intergalactic medium from filaments. As a result, the radial profile of the CR pressure in the intracluster medium is expected to be broad, dropping off more slowly than that of the gas pressure, and might be even temporarily inverted, peaking in the outskirts. The volume-integrated momentum spectrum of CR protons inside r 200 has the power-law slope of 4.25-4.5, indicating that the average Mach number of the shocks of main CR production is in the range of M s CR ≈ 3-4. We suggest that some radio relics with relatively flat radio spectrum could be explained by primary electrons accelerated by energetic infall shocks with M s 3 induced in the cluster outskirts.

Evidence for a pressure discontinuity at the position of the Coma relic from Planck Sunyaev–Zel'dovich effect data

“…The best fit pressure ratio suggests a shock with Mach number M ∼ 3, as expected for radio emitting merger shocks near cluster virial radii (e.g. Hong et al 2014). Our results are tied to the assumption of the pressure tube geometry, which we base on the new 2.3 GHz radio measurement of the Coma relic.…”

“…The shock waves can amplify particle energies through the Fermi-I process as well as amplify the magnetic fields, resulting in the strong synchrotron emissions seen as radio relics (e.g. Enßlin et al 1998;Miniati et al 2001;Hong et al 2014; see also Guo, Sironi & Narayan 2014). However, there are several shortcomings to this accepted picture and the relevant details are not well understood.…”

Simulating the shocks in the dissociative galaxy cluster Abell 1758N

“…They are diffuse radio emission, sometimes in the form of arcs, in the periphery of merging clusters and are interpreted as the result of reacceleration of relativistic electrons in the presence of amplified magnetic fields (see Brüggen et al 2012;Feretti et al 2012, and references therein). As pointed out by Hong et al (2014), in the cases where shocks are detected both from radio and X-ray data, estimates of shock velocity tend to disagree. Indeed, Mach numbers derived from X-ray data seem to be systematically underestimated with respect to the spectral analysis of radio relics.…”

“…In fact, strong shocks are expected mostly on the periphery of clusters (e.g. Hong et al 2014), but their detection is made difficult by the low X-ray flux is those regions.…”

Another shock for the Bullet cluster, and the source of seed electrons for radio relics

“…Unfortunately, in the cluster outskirts, where more strong shocks are expected (e.g. Hong et al 2014), the X-ray flux is low and the X-ray identification of shocks is difficult (see e.g. Ogrean et al 2013a).…”