Left: aperture diameter used to measure the radio halo flux as a function of the observing frequency; asteriscs mark single-dish observations. The dashed line marks the representative aperture radius, R_H = 0.48   R₅₀₀, used in our first approach to estimate the SZ-decrement. (Right) Full azimuthal flux profile derived from the Brown & Rudnick (2011) WSRT data (solid, blue) without the quadrant in the west, which is significantly contaminated by the tailed radio source NGC 4869. Errors are dominated by statistical noise at small radii and by the uncertainty in the “zero level” at large radii. The red square marks the total flux and size of the halo from Venturi et al. (1990), while the solid red lines show the integrated flux profile obtained by Govoni et al. (2001) rederived from these same data. The cyan profile is derived from Deiss et al. (1997) at 1.4 GHz and the magenta profile from Pizzo (2010) at 139 MHz. Fluxes at 139 MHz and 1400 MHz are scaled at 330 MHz using a spectrum α = 1.22.