X-ray versus C IV emission-line properties for the 38 SDSS spectra. In the first two rows from the top, the continuum photon index and the 2−10 keV flux (in erg s⁻¹ cm⁻²) are plotted against the C IV FWHM (in km s⁻¹), EW (in Å), the UV continuum slope (α_ν), and the total C IV emission-line flux (in erg s⁻¹ cm⁻²), the distributions of which are shown in the third row. The solid red line at Γ_X = 1.7 represents the minimum value adopted by RL19 to select the clean quasar sample for cosmology. The dot-dashed red line in the α_ν panel marks an UV continuum slope of −0.5, typical for z > 2 quasars (Lusso et al. 2015). Only F_{2 − 10 keV} shows a significant correlation with F_C IV, where the solid blue and dotted red lines represent the best-fit regression solutions for N and N+W+w sources (radio-bright, BALs, and red quasars are excluded; see Sect. 2.1). In the fourth and fifth rows, Γ_X and F_{2 − 10 keV} are plotted as a function of the C IV peak velocity υ_peak (with respect to the nominal wavelength), the velocity shift of the line centroid, the velocity dispersion σ, and the asymmetry index (AS). Redward (blueward) asymmetries correspond to AS values > 1 (< 1), where the dot-dashed line marks AS = 1 (as generally found at lower UV luminosity and redshift) and the red shaded area the typical dispersion on this parameter of 0.22 dex.