Photon index as function of redshift. We report the individual best-fitting values for sources with > 30 total net counts (gray symbols), the results derived from joint spectral analysis of QSOs with > 30, < 30 net counts, and of z >  6.5 QSOs (red, blue, and cyan circles, respectively, plotted at the median redshift of each subsample), and the average photon indices derived by Piconcelli et al. (2005), Vignali et al. (2005), Shemmer et al. (2006b), Just et al. (2007), and Nanni et al. (2017) for optically selected luminous QSOs at different redshifts. Errors are at the 68% confidence level.