Ratio between RASS flux, computed in an aperture of five arcmin in radius centred on the Planck position, and MCXC value for Planck candidates identified with MCXC clusters. The fluxes are computed in the [0.1–2.4] keV band at Earth and corrected for absorption. S₅₀₀ is the flux corresponding to the luminosity within R₅₀₀ published in the MCXC catalogue. Left panel: the ratio is plotted as a function of distance between the Planck and X-ray positions; middle panel: same, as a function of cluster redshift, for distances smaller than five arcmin; right panel: same as middle panel, for RASS flux within R₅₀₀ derived from the aperture flux, using the MCXC iterative procedure based on the L₅₀₀–M₅₀₀ relation and the REXCESS gas density profile (Piffaretti et al. 2011). The red line is the median ratio in distance or redshift bins with the grey area corresponding to ± 1σ standard deviation in each bin.