2012MNRAS.422..637C

The radiation from the central regions of active galactic nuclei, including that from the accretion disc surrounding the black hole, is likely to peak in the extreme-ultraviolet ∼ 13-100 eV. However, due to Galactic absorption, we are limited to constrain the physical properties, i.e. the black hole mass and the accretion rate, from what observations we have below ∼ 10 eV or above ∼ 100 eV. In this paper, we predict the thermal and ionization states of warm absorbers as a function of the shape of the unobservable continuum. In particular we model an accretion disc at kT_in ∼ 10 eV and a soft excess at kT_se ∼ 150 eV. The warm absorber, which is the highly ionized gas along the line of sight to the continuum, shows signatures in the ∼ 0.3-2 keV energy range consisting of numerous absorption lines and edges of various ions, some of the prominent ones being H- and He-like oxygen, neon, magnesium and silicon. We find that the properties of the warm absorber are significantly influenced by the changes in the temperature of the accretion disc, as well as by the strength of the soft excess, as they affect the optical depth particularly for iron and oxygen. These trends may help develop a method of characterizing the shape of the unobservable continuum and the occurrence of warm absorbers.