The structure of the boundary layer between an accretion disk and a corona

We have studied the influence of X-ray illumination by external source on the vertical structure of accretion disks in active galactic nuclei (AGN). We have shown that besides comptonization also atomic cooling and heating processes are important because of strong density gradient in the vertical direction. We have demonstrated that the condition of hydrostatic equilibrium leads to the existence of narrow two-phase medium zones between an accretion disk and a corona. We discuss the structure of such a two-medium zone and the possible consequences of its existence. Our results can be adopted to accretion disks in galactic X-ray binaries.