Structure function parameters as functions of the X-ray luminosity. Upper panel: slope b; the dotted line indicates the average ⟨b⟩ = 0.115; the dashed line indicates the fixed value b = 0.12, adopting the same dependence as in the general SF of Fig. 7. Lower panel: the intercept A. The open squares represent the values derived by the SFs of Fig. 9, with free A and b. The crosses are the values derived with fixed b = 0.12. The corresponding fits are shown with dotted lines (free b, A = (6.55 ± 1.42)−(0.17 ± 0.03)log L_X) and dashed lines (fixed b, A = (7.24 ± 0.81)−(0.19 ± 0.02)log L_X).