Diffuse radio emission in/around the Coma cluster: Beyond simple accretion

We report on new 1.41-GHz Green Bank Telescope (GBT) and 352-MHz Westerbork Synthesis Radio Telescope observations of the Coma cluster and its environs. At 1.41 GHz, we tentatively detect an extension to the Coma cluster radio relic source 1253+275 which makes its total extent ∼2 Mpc. This extended relic is linearly polarized as seen in our GBT data, the NRAO VLA Sky Survey, and archival images, strengthening a shock interpretation. The extended relic borders a previously undetected 'wall' of galaxies in the infall region of the Coma cluster. We suggest that the radio relic is an infall shock, as opposed to the outgoing merger shocks believed responsible for other radio relics. We also find a sharp edge, or 'front', on the western side of the 352-MHz radio halo. This front is coincident with a similar discontinuity in the X-ray surface brightness and temperature in its southern half, suggesting a primary shock-acceleration origin for the local synchrotron emitting electrons. The northern half of the synchrotron front is less well correlated with the X-ray properties, perhaps due to projection effects. We confirm the global pixel-to-pixel power-law correlation between the 352-MHz radio brightness and X-ray brightness with a slope that is inconsistent with predictions of either primary shock acceleration or secondary production of relativistic electrons in giant radio haloes, but is allowable in the framework of the turbulent re-acceleration of relic plasma. The failure of these first-order models and the need for a more comprehensive view of the intracluster medium energization are also highlighted by the very different shapes of the diffuse radio and X-ray emission. We note the puzzling correspondence between the shape of the brighter regions of the radio halo and the surface mass density derived from weak lensing.