Central panel: distribution of the estimated rest-frame soft X-ray luminosity (0.5–2 keV) vs. redshift for the final sample. The cleaned sample of 22 sources with z <  5.3 are indicated in solid black; in dark grey we report those selected at z >  5.3 (9 out of 53); the remaining (22/53) in light grey indicate those not included in the analysis because of the selection criteria applied (e.g. X-ray flux upper limits or too steep or flat X-ray spectral slope). Top panel and right panel: redshift and soft X-ray luminosity distribution of the final quasar sample, respectively. The colour coding adopted is the same used in the central panel except for the hatched black filling instead of solid.