Contours of the corona optical depth τ (in red), the amplification factor A (in blue) andthe minimal fraction (in %) of disk intrinsic emission (in green) in the corona spectral parameter Γ-kT plane. A slab geometry (i.e., g = 1 in Eq. (21)) is assumed here. Smaller values of g (i.e., a patchy corona) would move the green contours down. The magenta rectangle corresponds to the Γ and temperature kT_e ranges obtained by P13 for the soft excess in the spectral analysis of the Mkn 509 campaign.