2002MNRAS.332..799R

We present detailed computations of the vertical structure of an accretion disc illuminated by hard X-ray radiation with the code titan-noar, which is suitable for Compton-thick media. The energy generated via accretion is dissipated partially in the cold disc as well as in the X-ray source. We study the differences between the case where the X-ray source is in the form of a lamp-post above the accretion disc and the case of a heavy corona. We consider radiative heating via Comptonization together with heating via photo-absorption on numerous heavy elements such as carbon, oxygen, silicon and iron. The transfer in lines is precisely calculated. A better description of the heating/cooling through the inclusion of line transfer, a correct description of the temperature in the deeper layers, and a correct description of the entire disc vertical structure, as well as the study of the possible coronal pressure effect, constitute an improvement in comparison to previous works.

We show that exact calculations of hydrostatic equilibrium and determination of the disc thickness have a crucial impact on the optical depth of the hot illuminated zone. We study the lamp-post model for a low (m{dot} =0.03) and high (m{dot} =0.3) accretion rate. In both cases we assume a moderate illumination where the viscous flux equals the X-ray radiation flux. A highly ionized skin is created in the lamp-post model, with the outgoing spectrum containing many emission lines and ionization edges in emission or absorption in the soft X-ray domain, as well as an iron line at ∼7keV consisting of a blend of a low-ionization line from the deepest layers and hydrogen- and helium-like resonance lines from the upper layers, and almost no absorption edge, contrary to the case of a slab of constant density. A full heavy corona completely suppresses the highly ionized zone on the top of the accretion disc and in such a case the spectrum is featureless.