Modeling the interaction between ICM and relativistic plasma in cooling flows: The case of the Perseus cluster

Abstract

Abstract. We argue that the presence of diffuse synchrotron emission forming the so-called radio mini-halos in some cooling flow clusters can be explained by re-acceleration of relativistic electrons, the necessary energetics being supplied by the cooling flows themselves. In particular, the re-acceleration due to MHD turbulence has the correct radial dependence on the parameters to naturally balance the radiative losses. As an application we show that the main properties of the radio mini-halo in the Perseus cluster (brightness profile, total radio spectrum and radial spectral steepening) can be accounted for by the synchrotron radiation from relic relativistic electrons in the cluster, which are efficiently re-accelerated by MHD turbulence via Fermi-like processes due to the compression of the cluster magnetic field in the cooling flow region. Since the presence of an observable radio mini-halo in a cooling flow region critically depends on the combination of several physical parameters, we suggest that the rarity of radio mini-halos found in cooling flow clusters is due to the fact that the physical conditions of the ICM are intermediate between those which lead to the formation of extended radio halos and those holding in cooling flows without radio halos. The basic results of our model remain unchanged even if the cooling flow is stopped somewhere in the innermost region of the cluster.

A radio minihalo in the extreme cool-core galaxy cluster RXC J1504.1–0248

“…Thus, the discovery of a new one in an extreme cool-core cluster provides useful information on their origin and the minihalo/cool-core connection. Gitti et al (2002) propose that minihalos result from reacceleration of pre-existing relativistic electrons in the ICM by turbulence in the cooling flow. More recenlty, based on the observed connection between radio minihalos and X-ray cold fronts in several cool-core clusters, Mazzotta & Giacintucci (2008) have proposed that, instead, the gas sloshing that creates those cold fronts may be responsible for generating the turbulence needed to reaccelerate the fossil relativistic electrons.…”

“…One possibility is given by reacceleration of pre-exisiting, relativistic electrons in the intracluster medium (ICM) by turbulence in the cooling flow (Gitti et al 2002). Buoyant bubbles of relativistic plasma, inflated by the central AGN and disrupted by the ICM motion, may provide the relativistic seed electrons that rapidly cool to energies where they do not emit in the observable radio band.…”

GALAXY CLUSTERS IN THESWIFT/BURST ALERT TELESCOPE ERA: HARD X-RAYS IN THE INTRACLUSTER MEDIUM

“…Nevalainen et al (2004) used BeppoSAX and previous Rossi X-Ray Timing Explorer (RXTE) measurements to prove that this nonthermal component is variable and must therefore be connected to the central bright AGN. Sanders & Fabian (2007) reported, using Chandra, the presence of nonthermal X-ray emission in the core of Perseus in correspondence to the radio mini-halo (Gisler & Miley 1979;Gitti et al 2002). This nonthermal emission, which displays a power-law behavior with a photon index of 2.0, seems to exceed the flux of NGC 1275 by a factor of ∼3 (Sanders et al 2004).…”

How unusual is the cool-core radio halo cluster CL1821+643?

“…The origin of mini-haloes has been proposed to be a combination of relativistic electrons produced in hadronic in-⋆ E-mail: ruta@ncra.tifr.res.in teractions and reaccelerated via MHD turbulence (e. g. Gitti, Brunetti & Setti 2002;ZuHone et al 2013ZuHone et al , 2015.…”