Top panel: column density and photon index as a function of the absorbed flux (errors quoted at 1σ c.l.). Theplotted values correspond to those listed in Table 3. The anti-correlation Γ vs. flux is emphasized by the dot-dashed line, which shows the best linear fit (Γ vs. log ₁₀ F_x). Bottom panel: confidence contours at 68%, 95%, and 99% in the N_H −Γ plane of the Chandra spectrum at the lowest luminosity level and the XMM-Newton spectra.