A 15.5 GHz detection of the galaxy cluster minihalo in RXJ1720.1+2638

RXJ1720.1+2638 is a cool-core, `relaxed-appearing' cluster with a minihalo previously detected up to 8.4 GHz, confined by X-ray-detected cold fronts. We present observations of the minihalo at 13--18 GHz with the Arcminute Microkelvin Imager telescope, simultaneously modelling the Sunyaev--Zel'dovich signal of the cluster in conjunction with Planck and Chandra data in order to disentangle the non-thermal emission of the minihalo. We show that the previously-reported steepening of the minihalo emission at 8.4 GHz is not supported by the AMI data and that the spectrum is consistent with a single power-law up to 18 GHz. We also show the presence of a larger-scale component of the minihalo extending beyond the cold fronts. Both of these observations could be explained by the `hadronic' or `secondary' mechanism for the production of relativistic electrons, rather than the currently-favoured `re-acceleration' mechanism and/or multiple episodes of jet activity from the active galactic nucleus in the brightest cluster galaxy