Can photo-ionization explain the decreasing fraction of X-ray obscured AGN with luminosity?

TL;DR

This study investigates whether photo-ionisation of the obscuring material by AGN radiation can explain the observed decrease in obscured AGN fraction with increasing luminosity, using spectral simulations and simple models.

Contribution

The paper demonstrates through simulations that photo-ionisation can account for the luminosity-dependent obscured fraction in AGN, aligning with observed X-ray Baldwin effects.

Findings

Obscured AGN fraction decreases with luminosity as L^{-0.16}

Photo-ionisation can explain the decreasing obscuration fraction

Results are consistent with the X-ray Baldwin effect

Abstract

Chandra and XMM surveys show that the fraction of obscured AGN decreases rapidly with increasing luminosity. Although this is usually explained by assuming that the covering factor of the central engine is much smaller at luminous QSOs, the exact origin of this effect remains unknown. We perform toy simulations to test whether photo-ionisation of the obscuring screen in the presence of a strong radiation field can reproduce this effect. In particular, we create X-ray spectral simulations using a warm absorber model assuming a range of input column densities and ionization parameters. We fit instead the simulated spectra with a simple cold absorption power-law model that is the standard practice in X-ray surveys. We find that the fraction of absorbed AGN should fall with luminosity as $L^{-0.16\pm0.03}$ in rough agreement with the observations. Furthermore, this apparent decrease in the obscuring material is consistent with the dependence of the FeK$\alpha$ narrow-line equivalent width on luminosity, ie. the X-ray Baldwin effect.