Left panel: distribution of spectroscopically identified objects (optically resolved and unresolved) in the g − i versus EPIC pn HR2 diagram. blue: AGNs with log (L_X) ≥  44, green: 44 ≥ log (L_X) ≥ 42, yellow: galaxies with log (L_X) ≤  42, magenta: Type 2 QSO – filled squares = X-ray selected – filled triangles = optically selected – encircled = Compton Thick. Right panel: the entire SDSS photometric sample. Black: unresolved objects, green: extended objects, magenta: same as in right panel. We only show SDSS entries with a probability of identification with an X-ray source larger than 90%, r magnitudes brighter than 23 and errors on HR2 less than 0.3.