The Global Implications of the Hard Excess II: Analysis of the Local population of Radio Quiet AGN

TL;DR

This study demonstrates that the diverse X-ray spectral features of local radio-quiet AGN can be explained by a unified model involving reprocessing by circumnuclear clouds, incorporating absorption and scattering effects.

Contribution

It extends previous work by including type 2 AGN and shows a unified scheme can describe the entire local radio-quiet AGN population's X-ray spectra.

Findings

Spectral hardness ratio and Fe Kα EW vary continuously across AGN types.

A simple cloud ensemble model with absorption and 3D Compton scattering reproduces observed spectra.

Variation in cloud parameters like column density is not necessary to explain spectral diversity.

Abstract

Active galactic nuclei (AGN) show evidence for reprocessing gas, outflowing from the accreting black hole. The combined effects of absorption and scattering from the circumnuclear material likely explains the `hard excess' of X-ray emission above 20 keV, compared with extrapolation of spectra from lower X-ray energies. In a recent {\it Suzaku} study, we established the ubiquitous hard excess in hard X-ray-selected, radio-quiet type\,1 AGNs to be consistent with reprocessing of the X-ray continuum an ensemble of clouds, located tens to hundreds of gravitational radii from the nuclear black hole. Here we add hard X-ray-selected, type\,2 AGN to extend our original study and show that the gross X-ray spectral properties of the entire local population of radio quiet AGN may be described by a simple unified scheme. We find a broad, continuous distribution of spectral hardness ratio and Fe\,K$\alpha$ equivalent width across all AGN types, which can be reproduced by varying the observer's sightline through a single, simple model cloud ensemble, provided the radiative transfer through the model cloud distribution includes not only photoelectric absorption but also 3D Compton scattering. Variation in other parameters of the cloud distribution, such as column density or ionisation, should be expected between AGN, but such variation is not required to explain the gross X-ray spectral properties.