Maximum disk temperature (kT_max) as a function of (a) the bolometric luminosity (color-coded with black hole mass), and (b) the Eddington ratio. The solid black line shows the best-fit relation (Sect. 7.2.1, Table 2) and the shaded gray region is the one sigma confidence interval. The dashed red line is the theoretical relation between (a) kT_max and L_bol ($T_{\max }\propto L_{\mathrm{bol}}^{1/4}$) and (b) kT_max and λ_Edd ($T_{\max }\propto {\lambda }_{\mathrm{Edd}}^{1/4}$), as is expected from the standard Shakura & Sunyaev (1973) disk model for a fixed black hole mass.