Constraints on Cosmic Ray Transport in Galaxy Clusters from Radio and Gamma Ray Observations

Abstract

The nature of cosmic rays (CRs) and cosmic ray transport in galaxy clusters is probed by a number of observations. Radio observations reveal the synchrotron radiation of cosmic ray electrons (CRe) spiraling around cluster magnetic fields. $\gamma$-ray observations reveal hadronic reactions of cosmic ray protons (CRp) with ambient gas nuclei which produce pions. To date, no such cluster-wide $\gamma$-ray signal has been measured, putting an upper limit on the density of CRp present in clusters. But the presence of CRe implies some source of CRp, and consequently there must be some CRp loss mechanism. In this paper we quantify the observational constraints on this loss mechanism assuming that losses are dominated by CR transport, ultimately deriving a minimum diffusion coefficient of $\sim10^{31}$ cm$^2$ s$^{-1}$ in the Coma cluster. This lower limit on transport may help illuminate some unknown properties of the cluster field topology. Conversely, measurements of cluster field tangling scales can constrain other model parameters, such as the relative acceleration efficiency of protons to electrons. To be consistent with the Coma observations, protons cannot be accelerated more than 15 times more efficiently than electrons of the same energy.