Magnetic fields and cosmic rays in clusters of galaxies

TL;DR

This paper links radio and X-ray luminosities in galaxy clusters to secondary electrons from cosmic ray interactions, constraining magnetic fields and cosmic ray origins, with implications for gamma-ray detectability.

Contribution

It presents a model explaining the radio-X-ray correlation in clusters and constrains magnetic fields and cosmic ray properties based on observations.

Findings

CR energy ratio in cluster cores is ~2*10^{-4}

CRs are likely accelerated at cluster accretion shocks

Gamma-ray emission from secondaries is hard to detect with current instruments

Abstract

We argue that the observed correlation between the radio luminosity and the X-ray luminosity in radio emitting galaxy clusters implies that the radio emission is due to secondary electrons that are produced by p-p interactions and lose their energy by emitting synchrotron radiation in a strong magnetic field, B>(8\pi a T_{CMB}^4)^{1/2}\simeq 3\muG. We construct a simple model that naturally explains the correlation, and show that the observations provide stringent constraints on cluster magnetic fields and cosmic rays (CRs): Within the cores of clusters, the ratio beta_{core} between the CR energy (per logarithmic particle energy interval) and the thermal energy is beta_{core}\sim 2*10^{-4}; The source of these CRs is most likely the cluster accretion shock, which is inferred to deposit in CRs ~ 0.1 of the thermal energy it generates; The diffusion time of 100 GeV CRs over scales \gtrsim100 kpc is not short compared to the Hubble time; Cluster magnetic fields are enhanced by mergers to \gtrsim 1 % of equipartition, and decay (to <1 muG) on 1 Gyr time scales. The inferred value of beta_{core} implies that high energy gamma-ray emission from secondaries at cluster cores will be difficult to detect with existing and planned instruments.