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

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  • Andra Stroe
  • Clare Rumsey
  • Jeremy J. Harwood
  • Reinout J. Van weeren
  • Huub J. A. Röttgering
  • Richard D. E. Saunders
  • David Sobral
  • Yvette C. Perrott
  • Michel P. Schammel
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Article numberslu045
<mark>Journal publication date</mark>11/06/2014
<mark>Journal</mark>Monthly Notices of the Royal Astronomical Society: Letters
Issue number1
Volume441
Number of pages4
Pages (from-to)L41-L45
Publication StatusPublished
Early online date15/04/14
<mark>Original language</mark>English

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 intracluster 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 ± 0.3 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.