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arXiv · 1403.4255

The highest-frequency detection of a radio relic: 16-GHz AMI observations of the `Sausage' cluster

Andra Stroe·Clare Rumsey·Jeremy J. Harwood·Reinout van Weeren·Huub J. A. Röttgering·Richard D. E. Saunders·David Sobral·Yvette C. Perrott·Michel P. Schammel
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Abstract

We observed the cluster CIZA J2242.8+5301 with the Arcminute Microkelvin Imager at $16$ GHz and present the first high radio-frequency detection of diffuse, non-thermal cluster emission. This cluster hosts a variety of bright, extended, steep-spectrum synchrotron-emitting radio sources, associated with the intra-cluster medium, called radio relics. Most notably, the northern, Mpc-wide, narrow relic provides strong evidence for diffusive shock acceleration in clusters. We detect a puzzling, flat-spectrum, diffuse extension of the southern relic, which is not visible in the lower radio-frequency maps. The northern radio relic is unequivocally detected and measures an integrated flux of $1.2\pm0.3$ mJy. While the low-frequency ($<2$ GHz) spectrum of the northern relic is well represented by a power-law, it clearly steepens towards $16$ GHz. This result is inconsistent with diffusive shock acceleration predictions of ageing plasma behind a uniform shock front. The steepening could be caused by an inhomogeneous medium with temperature/density gradients or by lower acceleration efficiencies of high energy electrons. Further modelling is necessary to explain the observed spectrum.

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Andra Stroe, Clare Rumsey, Jeremy J. Harwood, Reinout van Weeren, Huub J. A. Röttgering, Richard D. E. Saunders, David Sobral, Yvette C. Perrott, Michel P. Schammel. 2014-03-17. The highest-frequency detection of a radio relic: 16-GHz AMI observations of the `Sausage' cluster. https://doi.org/10.1093/mnrasl/slu045

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