Monochromatic luminosity at 2500 Åvs. the monochromatic luminosity at 2 keV. The two best-fit relations (Eqs. (4) and (5)) and the final bisector regression from our work (Eq. (6)) are shown as dashed and solid black lines, respectively. We also show the relation reported by Lusso et al. (2010; for their photometrically selected sample) and Marchese et al. (2012) as a dashed red line and a dash-dotted blue line, respectively. The typical uncertainties on L_2 keV are less than 5% and those on L_2500 Å are shown in the bottom right corner (also valid for subsequent plots including L_2 keV and $L_{2500\mathrm{\AA }}$).