X-ray photon index as a function of Δ(α_OX) for the sources with ≥20 counts. (a) Γ_obs derived using a power law model modified by Galactic absorption. (b) Cold absorption component included in the spectral model used to derive Γ (see Section 3.1.1). The BAL and non-BAL QSOs are represented as indigo and purple dots, respectively. The grey-shaded areas highlight the locus of X-ray-weak sources.