Optical-to-X-ray spectral slope α_ox vs. 2500 Å luminosity. Our measurement from J0439+1634 is shown in red, compared with values from quasar samples at similar redhsift and lower redshift. The two red triangles are the α_ox values accounting for the intrinsic absorption based on Γ-fixed (lower value) and Γ-free (upper value) model fitting. For high-redshift comparisons, we show individual measurements at 5.5 < z < 6 from Nanni et al. (2017), 6 < z < 6.5 from Vito et al. (2019), and z > 6.5 from Wang et al. (2021). We also include the new upper limit (black square) from a z = 6.515 quasar in Vito et al. (2021). The low-redshift SDSS quasar sample (gray dots) is generated based on the L_{2–10 keV} from Timlin et al. (2020) and L_{2500 Å} from Wang et al. (2021). The relations in the literature are plotted as black solid line (Nanni et al. 2017), gray solid line (Timlin et al. 2020), dashed line (Lusso & Risaliti 2017), dotted line (Just et al. 2007), and dashed–dotted line (Martocchia et al. 2017).The blue squares are the measurements for low-redshift BAL quasars from Luo et al. (2014). Among the sample in Luo et al. (2014), the quasar PG0043+03 is not detected, and it has been updated in Kollatschny et al. (2016) (blue filled circle). All measurements have been corrected to the cosmology adopted in this work.