Is the Sunyaev-Zeldovich effect responsible for the observed steepening in the spectrum of the Coma radio halo?

Aims. The radio halo in the Coma cluster is unique in that its spectrum has been measured over almost two decades in frequency. The current radio data show a steepening of the spectrum at higher frequencies, which has implications for models of the radio halo origin. There is an on-going debate on the possibility that the observed steepening of the spectrum and the apparent shrinking of the halo-size at higher frequencies is not intrinsic to the emitted radiation, but is instead caused by the Sunyaev-Zeldovich (SZ) effect.