Chandra measurements of non-thermal X-ray emission from massive, merging, radio-halo clusters

We report the discovery of spatially-extended, non-thermal or hot, quasi-thermal emission components in Chandra X-ray spectra for five of a sample of seven massive, merging galaxy clusters with powerful radio halos: Abell 665, 2163, 2255, 2319, and 1E 065756. The emission components can be fitted by power-law models with mean photon indices in the range 1.4 < Γ < 2.0, or by bremsstrahlung emission from hot gas with kT ∼> 20 keV. A control sample of regular, dynamically relaxed clusters without radio halos but with comparable thermal temperatures and luminosities shows no evidence for similar components in their Chandra spectra. Detailed X-ray spectral mapping reveals the complex thermodynamic states of the radio halo clusters. We report the discovery of a clear, large-scale shock front in Abell 2219. Our deepest observations, of the Bullet Cluster 1E 0657-56, demonstrate a spatial correlation between the strongest power law X-ray emission, highest thermal pressure, and brightest 1.34GHz radio halo emission in this cluster. The integrated flux and mean spectral index of the power law X-ray emission from 1E 0657-56 are in good agreement with constraints at harder X-ray energies from the Rossi X-ray Timing Explorer. We explore possible origins for the power law X-ray components which include inverse Compton scattering of cosmic microwave background photons by relativistic electrons in the clusters; quasi-thermal bremsstrahlung from supra-thermal electrons energized by Coulomb collisions with an energetic, non-thermal proton population; synchrotron emission associated with ultra-relativistic electrons; and spatially unresolved temperature structure e.g. due to hot gas associated with shocks driven by the ongoing merger activity. Our results have implications for the interpretation of some previous claims of ‘soft-excess’ X-ray emission from galaxy clusters and highlight the importance of further deep X-ray and radio mapping, coupled with new hard X-ray, γ-ray and TeV observations, for improving our understanding of the non-thermal particle populations in galaxy clusters.