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

Abstract

Recently, Boorman et al. (2018) reported on the discovery of the Iwasawa-Taniguchi (I-T) effect (a.k.a. X-ray Baldwin effect) for Compton-thick AGN. They measured a decrease of the 6.4 keV iron line equivalent width with the 12mu 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 which 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 7.5x10^{22} cm^{-2}.