An Iwasawa-Taniguchi Effect for Compton-thick Active Galactic Nuclei
  We present the first study of an Iwasawa-Taniguchi/X-ray Baldwin effect for
Compton-thick active galactic nuclei (AGN). We report a statistically
significant anti-correlation between the rest-frame equivalent width (EW) of
the narrow core of the neutral Fe K$\alpha$ fluorescence emission line,
ubiquitously observed in the reflection spectra of obscured AGN, and the
mid-infrared 12$\,\mu$m continuum luminosity (taken as a proxy for the
bolometric AGN luminosity). Our sample consists of 72 Compton-thick AGN
selected from pointed and deep-field observations covering a redshift range of
$z\sim0.0014-3.7$. We employ a Monte Carlo-based fitting method, which returns
a Spearman's Rank correlation coefficient of $\rho=-0.28\pm0.12$, significant
to 98.7% confidence. The best fit found is ${\rm log}({\rm EW}_{{\rm
Fe\,K}\alpha})\,\propto\,-0.08\pm0.04\,{\rm log}(L_{12\,\mu{\rm m}})$, which is
consistent with multiple studies of the X-ray Baldwin effect for unobscured and
mildly obscured AGN. This is an unexpected result, as the Fe K$\alpha$ line is
conventionally thought to originate from the same region as the underlying
reflection continuum, which together constitute the reflection spectrum. We
discuss the implications this could have if confirmed on larger samples,
including a systematic underestimation of the line of sight X-ray obscuring
column density and hence the intrinsic luminosities and growth rates for the
most luminous AGN.
