The α_ox gradient as a function of the monochromatic luminosity at rest-frame 2500 Å (upper panel) and the monochromatic luminosity at 2 keV versus the monochromatic luminosity at 2500 Å (lower panel). Black-filled circles indicate the SDSS objects with 0.1 ≤ z ≤ 4.5 from Strateva et al. (2005), while gray-filled circles label the data from Steffen et al. (2006). The dotted line represents the best fit to the sample of Steffen et al. (2006). Further measurements of extreme X-ray weak quasars are indicated with cyan circles (Saez et al. 2012). The positions of PG 0043+039 are highlighted for the years 2005 (red square) and 2013 (black circle).