A receding torus model for the Iwasawa-Taniguchi effect for Compton-thick AGN

Recently, Boorman et al. reported on the discovery of the Iwasawa-Taniguchi (I-T) effect (also known as X-ray Baldwin effect) for Compton-thick active galactic nucleus (AGN). They measured a decrease of the 6.4 keV iron line equivalent width with the 12 μm luminosity, assumed as a proxy for the intrinsic X-ray luminosity, which in Compton-thick AGN is not directly observable. One of the most popular explanations of the classic I-T effect is the so-called receding torus model, i.e. the decrease of the covering factor of the molecular 'torus' with X-ray luminosity. In this paper, we show that an I-T effect for Compton-thick AGN is indeed expected in the receding torus model, assuming that the torus is funnelling the primary X-ray luminosity that is then scattered in a 'hot mirror'. We found that the observed relation is well reproduced provided that the typical column density of the 'hot mirror' is about 7.5 × 1022 cm-2.