Logarithm of the net cooling rate Λ_rad (Eq. (17)), plotted in the n_H – T parameter plane ranging from blue (low rate) to red (high rate). The points of constant pressure are connected with black dash-dotted lines. The red dotted contours show the contribution of the Compton cooling to the total cooling. The solid white line shows where the cooling function is constant in isobaric perturbation, and separates the area of unstable solutions where the gradient of the cooling function ℒ_rad <  0. Dark blue lines of increasing width represent the vertical structure for three accretion rates 0.035, 0.050, and 0.072.