X-ray–to–optical index (α_OX) as a function of Log(L_{2500#x2004;Å}). WISSH QSOs are compared to literature samples; the symbols are the same as in Figure 5. The black dashed, solid, and dotted lines are the best fits from Martocchia et al. (2017), L10, and Just et al. (2007), respectively. In particular, L10 fit XMM-COSMOS data only; Just et al. (2007) consider 34 QSOs of their core sample, 332 sources from Steffen et al. (2006), and 14 from Shemmer et al. (2006); and Martocchia et al. (2017) consider XMM-COSMOS objects, 23 optically selected QSOs from the Palomar-Green (PG) Bright QSO Survey of the complete sample by Laor et al. (1994) and the 41 WISSH QSOs with available X-ray data at the time of the publication.