Iron K$\alpha$ emission in type-I and type-II Active Galactic Nuclei

TL;DR

This study analyzes the properties of the narrow Fe Kα emission line in different types of active galactic nuclei, revealing differences explained by orientation and obscuration effects, and providing insights into the structure of AGN and their contribution to the cosmic X-ray background.

Contribution

It offers a comprehensive analysis of Fe Kα line characteristics across AGN types, incorporating physical torus models and revealing the X-ray Baldwin effect in Seyfert 2s.

Findings

Seyfert 2s have lower Fe Kα luminosities than Seyfert 1s at the same continuum luminosity.

Differences in Fe Kα emission are explained by inclination angles and torus geometry.

Evidence of absorption in the Fe Kα line flux in Compton-thick AGN.

Abstract

The narrow Fe K$\alpha$ line is one of the main signatures of the reprocessing of X-ray radiation from the material surrounding supermassive black holes, and it has been found to be omnipresent in the X-ray spectra of active galactic nuclei (AGN). In this work we study the characteristics of the narrow Fe K$\alpha$ line in different types of AGN. Using the results of a large Suzaku study we find that Seyfert 2s have on average lower Fe K$\alpha$ luminosities than Seyfert 1s for the same 10-50 keV continuum luminosity. Simulating dummy Sy1s and Sy2s populations using physical torus models of X-ray reflected emission, we find that this difference can be explained by means of different average inclination angles with respect to the torus, as predicted by the unified model. Alternative explanations include differences in the intensities of Compton humps or in the photon index distributions. We show that the ratio between the flux of the broad and narrow Fe K$\alpha$ line in the 6.35-6.45 keV range depends on the torus geometry considered, and is on average $<25\%$ and $<15\%$ for type I and type II AGN, respectively. We find evidence of absorption of the narrow Fe K$\alpha$ line flux in Compton-thick AGN, which suggests that part of the reflecting material is obscured. We estimate that on average in obscured AGN the reflected radiation from neutral material is seen through a column density which is 1/4 of that absorbing the primary X-ray emission. This should be taken into account in synthesis models of the CXB and when studying the luminosity function of heavily obscured AGN. We detect the first evidence of the X-ray Baldwin effect in Seyfert 2s, with the same slope as that found for Seyfert 1s, which suggests that the mechanism responsible for the decrease of the equivalent width with the continuum luminosity is the same in the two classes of objects.