Left: AB magnitude vs. X-ray flux. The symbols are the same as in Fig. 2 (right); also shown are the X-ray upper limit fluxes for a sample of AGN candidates selected from mid-IR colours (red triangles for AGN1 candidates and blue triangles for AGN2 candidates). The dashed line indicates an X-ray–to–optical flux ratio of f_XO = 1. Right: as in the left figure, but with the CatWISE2020 W1 magnitude on the x-axis. Here the dashed line shows the empirical discrimination between AGN and other objects defined in Salvato et al. (2018).