An Iwasawa-Taniguchi Effect for Compton-thick Active Galactic Nuclei

Here we present the first study of an Iwasawa-Taniguchi/‘X-ray Baldwin’ e↵ect forCompton-thick active galactic nuclei (AGN). We report a statistically significant anticorrelationbetween the rest-frame equivalent width (EW) of the narrow core of theneutral FeK↵ fluorescence emission line, ubiquitously observed in the reflection spectraof obscured AGN, and the mid-infrared 12 μm continuum luminosity (taken as aproxy for the bolometric AGN luminosity). Our sample consists of 68 Compton-thickAGN selected from pointed and deep-field observations covering a redshift range ofz ⇠ 0.0014 − 3.7. We employ a Monte Carlo-based fitting method, which returns aSpearman’s Rank correlation coefficient of ⇢ = −0.26 +0.11−0.13, significant to 98% confidence.The best fit found is log(EWFeK↵) / −0.10+0.04−0.04 log(L12 μm), which is consistentwith multiple studies of the X-ray Baldwin e↵ect for unobscured to mildly obscuredAGN. This is an unexpected result, as the FeK↵ line is conventionally thought tooriginate from the same region as the underlying reflection continuum, which togethermake up the reflection spectrum. Our result potentially decouples the neutral FeK↵line from the ‘composite’ reflection spectrum for Compton-thick AGN. We discuss furtherimplications this could have if confirmed on larger samples, including a systematicunder-estimation of the X-ray obscuration column density along the line of sight andhence the intrinsic luminosities and growth rates for the most infrared-luminous AGN.