Cosmic ray transport in galaxy clusters: implications for radio halos, gamma-ray signatures, and cool core heating

By Torsten A. Ensslin, C. Pfrommer, Francesco Miniati, K. Subramanian. 2010

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We investigate the interplay of cosmic ray (CR) propagation and advection in galaxy clusters. Propagation in form of CR diffusion and streaming tends to drive the CR radial profiles towards being flat, with equal CR number density everywhere. Advection of CR by the turbulent gas motions tends to produce centrally enhanced profiles. We assume that the CR streaming velocity is of the order of the sound velocity. This is motivated by plasma physical arguments. The CR streaming is then usually larger than typical advection velocities and becomes comparable or lower than this only for periods with trans- and super-sonic cluster turbulence. As a consequence a bimodality of the CR spatial distribution results. Strongly turbulent, merging clusters should have a more centrally concentrated CR energy density profile with respect to relaxed ones with very subsonic turbulence. This translates into a bimodality of the expected diffuse radio and gamma-ray emission of clusters, since more centrally concentrated CR will find higher target densities for hadronic CR proton interactions, higher plasma wave energy densities for CR electron and proton re-acceleration, and stronger magnetic fields. Thus, the observed bimodality of cluster radio halos appears to be a natural consequence of the interplay of CR transport processes, independent of the model of radio halo formation, be it hadronic interactions of CR protons or re-acceleration of low-energy CR electrons. Energy dependence of the CR propagation should lead to spectral steepening of dying radio halos. Furthermore, we show that the interplay of CR diffusion with advection implies first order CR re-acceleration in the pressure-stratified atmospheres of galaxy clusters. Finally, we argue that CR streaming could be important in turbulent cool cores of galaxy clusters since it heats preferentially the central gas with highest cooling rate.

Subfields: Astronomy and Astrophysics, Instrumentation, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/47d55fa596f8780f2330cd7d9f73e699570a776c

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Citations in the corpus

Galactic winds driven by cosmic ray streaming (2012, methodology): “Consequently, a number of authors have worked on the problem of CR streaming in high-beta plasmas (Holman, Ionson & Scott 1979; Achterberg 1981; Felice & Kulsrud 2001; Yan & Lazarian 2008), and a summary of these works can be found in Enßlin et al. (2011).”
SEARCH FOR COSMIC-RAY-INDUCED GAMMA-RAY EMISSION IN GALAXY CLUSTERS (2013, background): “…& Dursi 2010; Ruszkowski et al. 2011, for evidence of radial bias of the magnetic geometry), CR streaming could either be a perturbation to the peaked advection-dominated CR distribution or cause a substantial flattening by a substantial net outward CR flux (Enßlin et al. 2011; Wiener et al. 2013).”