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.
