α_ox as a function of the rest-frame monochromatic luminosity at (a) 2500 Å (UV) and (b) 2 keV (X-ray). We find a tight anticorrelation between α_ox and L_2500 Å (shown as the solid black line in the left panel) but no significant relation with L_2 keV (shown as a solid black line in the right panel). For comparison, we also plot the best-fit relations from Steffen et al. (2006; dash-dotted blue line), Green et al. (2009; thick dotted green line), Lusso et al. (2010; dashed red line), and Stalin et al. (2010; dotted magenta line). The shaded gray region shows the one sigma confidence interval for our best-fit linear regression.