Thermal instabilities in Active Galactic Nuclei: the case of thin vs. thick ionized media

Abstract

We have studied the thermal instabilities in the context of Active Galactic Nuclei (AGN), addressing the cases of thin and thick (stratified) X-ray illuminated media. For that, we have compared the behaviour of different models in pressure equilibrium. Ionized gas in pressure equilibrium show temperature discontinuities which, however difficult to tackle, can be modelled with the transfer-photoionization code TITAN. Our code allows to choose between the hot and the cold stable solutions in the multi-branch regime of the S-curve, and then to compute a fully consistent photoionisation model with the chosen solution. For the first time, it is now possible to compare the true stable solution models with the approximate solution model normally used. Our studies may be applied to media in any pressure equilibrium conditions, e.g. constant gas pressure, constant total pressure, or hydrostatic pressure equilibrium; they can be used to model irradiated accretion discs in AGN, the Warm Absorber in type 1 AGN, or the X-ray line-emitting gas in type 2 AGN.