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 \pm 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 .