Thermal Conduction and Thermal Instability in the Transition Layer between an Accretion Disc and a Corona in AGN

Abstract

We study the vertical structure of the transition layer between an accretion disc and corona in the context of the existence of two-phase medium in thermally unstable regions. The disc is illuminated by hard X-ray radiation and satisfies the condition of hydrostatic equilibrium. We take into account the energy exchange between hot corona ($\sim 10^8$ K) and cool disc ($\sim 10^4$ K) through the radiative processes and due to thermal conduction. We make local stability analysis of the case with conductivity and we conclude that thermal conduction does not suppress thermal instability. In spite of continuous temperature profile $T(\tau)$ there are regions with strong temperature gradient where spontaneous perturbations can lead to cloud condensation in the transition layer. We determine the minimum size $\lambda_{TC}$ of such a perturbation.