Top panel: photonionization cross section as a function of the wavelength from the PINTofALE library (Kashyap & Drake 2000), assuming a heavy element abundance of 0.5 (black line) and 0.8 (red line). Bottom panel: mean free path as a function of the energy, for the two densities used in our models (solid line for n_acc = 10¹¹ cm^-3, and dashed lines for n_acc = 5 × 10¹¹ cm^-3). The dotted horizontal line (L_s) indicates the characteristic length of the accretion stream equal to its section radius.