Distribution of the 0.5 − 2 keV versus 1.4 GHz flux limit for the ten radio and X-ray fields considered in this work. The reported flux limits correspond to the faintest sources’ flux in the 0.5 − 2 keV and in the 1.4 GHz catalogs, respectively. Each field marker size is proportional to the area of the overlapping region between the radio and X-ray observations. The three dashed lines correspond to level curves for different ratios between the X-ray and radio fluxes.