ACCRETION DISCS WITH ACCRETING CORONAE IN AGN .1. SOLUTIONS IN HYDROSTATIC EQUILIBRIUM

We discuss a model for the central regions of radio-quiet AGN in which the coronal accretion is the source of energy for X-ray emission. We consider a two-layer geometry and we construct solutions taking into a ccount radiative interaction and pressure balance between the disc and the corona, We define three models of the disc-corona transition regi on. We use two descriptions of the angular momentum transport: either the angular momentum is transported locally both in the disc and in th e corona or all the angular momentum is transported within the disc on ly, We employ the a-description of viscosity within the two layers but we also discuss models parametrized by a fraction of energy dissipate d in the corona. Both layers are treated using vertical averages but w ith a clear distinction between the values of the pressure and other r elevant functions at the disc surface and the mean values inside the d isc. Both descriptions of the angular momentum transport lead to simil ar conclusions about the fraction of energy liberated in the disc alth ough they predict very different properties of the disc interior. The coupled disc/corona system has one family of solutions at low accretio n rates, two families of solutions at intermediate accretion rates and no solution for high accretion rates. Of the two families of solution s, the first one has weakly dissipating coronae and the second one has coronae which dominate the system energetically. The two solutions me rge at sub-Eddington accretion rates because a corona in hydrostatic e quilibrium can dissipate only a limited amount of energy, which depend s on the viscosity parameter. At higher accretion rates an outflow fro m the corona in the vertical direction should be taken into account.