Cosmic ray streaming in clusters of galaxies

Abstract

The observed bimodality in radio luminosity in galaxy clusters is puzzling. We investigate the possibility that cosmic-ray (CR) streaming in the intra-cluster medium can 'switch off' hadronically induced radio and gamma-ray emission. For self-confined CRs, this depends on the source of MHD wave damping: if only non-linear Landau damping operates, then CRs stream on the slow Alfvénic timescale, but if turbulent wave damping operates, super-Alfvénic streaming is possible. As turbulence increases, it promotes outward streaming more than it enables inward turbulent advection. Curiously, the CR flux is independent of ∇f (as long as it is non-zero) and depends only on plasma parameters; this enables radio halos with flat inferred CR profiles to turn off. We perform 1D time-dependent calculations of a radio mini-halo (Perseus) and giant radio halo (Coma) and find that both diminish in radio luminosity by an order of magnitude in several hundred Myr, given plausible estimates for the magnetic field in the outskirts of the cluster. Due to the energy dependence of CR streaming, spectral curvature develops, and radio halos turn off more slowly at low frequencies -properties consistent with observations. Similarly, CR streaming rapidly turns off gamma-ray emission at the high-energies probed by Cherenkov telescopes, but not at the low energies probed by Fermi. CR mediated wave-heating of the ICM is unaffected, as it is dominated by ∼GeV CRs which stream Alfvénically.

Cosmic-Ray Feedback Heating of the Intracluster Medium

“…For conditions representative of the cluster cool cores, damping mechanisms can lead to moderately super-Alfvénic speeds for the following reasons. Wiener et al (2013) consider turbulent and nonlinear Landau damping mechanisms. In the turbulent damping case, the effective streaming speed is is the CR number density, = ( ) L L 10 kpc mhd,10 mhd is the length scale at which turbulence is driven at the Alfvén speed u A , γ 3 = γ/3 is the average CR Lorentz factor, and n > 4 is the slope of the CR distribution function in momentum (approximately n = 4.6).…”

Testing Secondary Models for the Origin of Radio Mini-Halos in Galaxy Clusters

“…Momentum-diffusion of CRs that is mediated by the scattering of particles with MHD waves in the plasma is a different process that is more difficult to model due to the uncertainties in the physics of (small-scale) turbulence in these environments. In addition, Enßlin et al (2011) and Wiener et al (2013) proposed that super-Alfvénic CR streaming is possible in the ICM. In particular, Wiener et al (2013) calculated the suppression of the streaming instability in a turbulent flow showing that under these conditions the self-generated waves do not limit the particle drift velocity to the Alfvén speed.…”

“…In addition, Enßlin et al (2011) and Wiener et al (2013) proposed that super-Alfvénic CR streaming is possible in the ICM. In particular, Wiener et al (2013) calculated the suppression of the streaming instability in a turbulent flow showing that under these conditions the self-generated waves do not limit the particle drift velocity to the Alfvén speed. However, this does not automatically imply that streaming is efficient, because the background turbulence (necessary to suppress the instability) still provides a source of scattering that may make the transport of CRs diffusive and potentially inefficient (see Brunetti & Jones 2014, for a discussion).…”

“…In our conditions, momentumdependent spatial transport of CRs could change the spectrum of CRp. For example, Wiener et al (2013) discussed the possibility of a steepening of the CRp spectral distribution resulting from an efficient momentum-dependent diffusion. This is also not included in our simulations.…”

An overview of jet-mode AGN feedback and the prospects for studying its cosmic evolution with LOFAR

“…Additionally, the outbursts create weak shocks (Wise et al 2007), with energies comparable to the cavity enthalpies (Brüggen et al 2007); sound waves ; and heat through cosmic rays (CRs) injected at the tips of the jets (Böhringer & Morfill 1988;Loewenstein et al 1991;Enßlin et al 1997;Guo & Oh 2008;Enßlin et al 2011;Wiener et al 2013;Pfrommer 2013), all of which produce a convective core and provide heating isotropically (Tabor & Binney 1993;Chandran & Rasera 2007;Sharma et al 2009). AGN heating can also come from radiative-mode AGN such as quasars (Ciotti & Ostriker 1997 or through primordial CRs from blazars (Pfrommer et al 2012).…”