Dependence of the 2–10 keV photon spectral index Γ of the Comptonized spectra from the sandwich corona (red filled symbols and left axis) and of the coronal temperature kT_e (blue open symbols and right axis) on the ionization parameter for various coronal optical depths. Circles, squares, and triangles correspond to τ_T = 0.2, 0.4, and 0.8, respectively. The shaded area represents the typical observed range for Γ (1.6–1.8) and kT_e (50–120 keV) (Wardziński et al. 2002; Zdziarski & Gierliński 2004; Ibragimov et al. 2005; Torii et al. 2011; Burke et al. 2017).