Values of α_OXas a function of extinction-corrected 2500 Å monochromatic luminosities. Symbols for WISSH quasars as in Fig. 4. Blue triangles indicate the broad-line AGN sample from the XMM-Newton-COSMOS survey by Lusso et al. (2010); the yellow squares indicate PG quasars. Green symbols represent BAL AGN. The dashed line represents the relation found by Lusso et al. (2010), the dash-dotted line represents the Just et al. (2007) relation, and the black solid line indicates our linear fit. In the inset on the lower left corner of the figure, the open pentagons indicate absorption-corrected α_OXvalues; the gray open circles indicate α_OXvalues when X-ray absorption is not taken into account. The asterisks with error bars indicate α_OXvalues for the undetected X-WISSH sources, calculated with the N_Hmedian value of the sample, i.e., N_H = 8 × 10²¹ cm^-2. The error bars were calculated using N_H= 0 and N_H = 4 × 10²³ cm^-2, i.e., the maximum value of the sample.