X-ray-to-optical flux ratio (X/O) as a function of the 0.5−4.5 keV X-ray luminosity for extragalactic sources. The horizontal dotted lines mark the level of X/O = 10, 1 and 0.1 from top to bottom. Left panel: BLAGN. Right panel: NELGs and ALGs. The dashed diagonal line is the best linear regression between log (X/O) and log (L_{0.5−4.5 keV}) for NELGs and ALGs with L_{0.5 − 4.5 keV} > 10⁴² erg s^-1(see text for details).