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

TL;DR

This study reports the detection of extended non-thermal X-ray emission in merging galaxy clusters with radio halos, correlating with thermal and radio features, and explores potential origins including inverse Compton scattering and complex temperature structures.

Contribution

First detection of spatially-extended non-thermal X-ray components in merging clusters with radio halos, linking X-ray, radio, and thermal properties, and identifying new shock fronts.

Findings

Non-thermal X-ray components found in 5 of 7 merging clusters with radio halos.

Spatial correlation between non-thermal X-ray emission, thermal pressure, and radio halos.

Discovery of a new large-scale shock front in Abell 2219.

Abstract

We report the discovery of spatially-extended, non-thermal-like emission components in Chandra X-ray spectra for five of a sample of seven massive, merging galaxy clusters with powerful radio halos. The emission components can be fitted by power-law models with mean photon indices in the range 1.5 < Gamma < 2.0. A control sample of regular, dynamically relaxed clusters, without radio halos but with comparable mean thermal temperatures and luminosities, shows no compelling evidence for similar components. Detailed X-ray spectral mapping reveals the complex thermodynamic states of the radio halo clusters. 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. We confirm the presence of a shock front in the 1E 0657-56 and report the discovery of a new, large-scale shock front in Abell 2219. We explore possible origins for the power-law X-ray components. These include inverse Compton scattering of cosmic microwave background photons by relativistic electrons in the clusters; bremsstrahlung from supra-thermal electrons energized by Coulomb collisions with an energetic, nonthermal proton population; and synchrotron emission associated with ultra-relativistic electrons. Interestingly, we show that the power-law signatures may also be due to complex temperature and/or metallicity structure in clusters particularly in the presence of metallicity gradients. In this case, an important distinguishing characteristic between the radio halo clusters and control sample of predominantly cool-core clusters is the relatively low central X-ray surface brightness of the former.