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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 399; no. 3; pp. 1307 - 1327
• Main Authors: Million, E. T., Allen, S. W.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01-11-2009; Wiley-Blackwell; Oxford University Press
• Subjects: A1795; A2029; A2163; A2204; A2219; A2255; A2319; A2744; A478; A576; A665; Astronomy; Astrophysics; Earth, ocean, space; Exact sciences and technology; galaxies: clusters: individual: 1E 0657−56; galaxies: clusters: individual: 1E 0657−56, A665, A2163, A2255, A2319, A2744, A2219, A576, A1795, A2204, A478, A2029; intergalactic medium; magnetic fields; radiation mechanisms: non-thermal; Radio astronomy; Spectrum analysis; Thermodynamics; X-ray astronomy; X-rays: galaxies: clusters; radiation mechanisms: non-thermal; galaxies: clusters: individual: 1E 0657−56, A665, A2163, A2255, A2319, A2744, A2219, A576, A1795, A2204, A478, A2029; intergalactic medium; magnetic fields; X-rays: galaxies: clusters; Protons; Merging galaxies; Intergalactic matter; A2319; A2219; Soft X radiation; Spatial correlation; Cosmic background radiation; Luminosity; Relativistic electron; A2255; A665; Thermodynamics; Radio map; Radio halo; Ultrarelativistic electron; A2163; Surface brightness; galaxies: clusters: individual: 1E 0657―56; Power law; Synchrotron radiation; Energetics; X ray cluster of galaxies; Hard x radiation; Bremsstrahlung; A1795; X-ray spectra; A478; A2029; A2744; A576; Shock front; A2204; Galaxy clusters; Radio emission; Radiation mechanism; Metallicity; Inverse Compton scattering; Mapping manifolds; Models; Magnetic fields; X ray emission; Suprathermal particle
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2009.15359.x

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 1E 0657−56, 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 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 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.