Mon. Not. R. Astron. Soc., 399, 1307-1327 (2009/November-1)
Chandra measurements of non-thermal-like X-ray emission from massive, merging, radio halo clusters.
MILLION E.T. and ALLEN S.W.
Abstract (from CDS):
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 haloes. The emission components can be fitted by power-law models with mean photon indices in the range 1.5 < Γ < 2.0. A control sample of regular, dynamically relaxed clusters, without radio haloes 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 1E0657-56, demonstrate a spatial correlation between the strongest power-law X-ray emission, highest thermal pressure and brightest 1.34 GHz radio halo emission in this cluster. We confirm the presence of a shock front in the 1E0657-56 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 suprathermal electrons energized by Coulomb collisions with an energetic, non-thermal proton population; and synchrotron emission associated with ultrarelativistic 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. Our results have implications for previous discussions of soft excess X-ray emission from 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 these systems.
© 2009 The Authors. Journal compilation © 2009 RAS
magnetic fields - radiation mechanisms: non-thermal - galaxies: clusters: individual (ClG 0657-56, A665, A2163, A2255, A2319, A2744, A2219, A576, A1795, A2204, A478, A2029) - intergalactic medium - X-rays: galaxies: clusters
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