X-ray spectral properties of the AGN sample in the northern XMM-XXL field

TL;DR

This study presents a comprehensive X-ray spectral catalog of 8445 sources in the XMM-XXL north survey, analyzing AGN properties, spectral features, and their relation to optical classifications, revealing insights into AGN structure and evolution.

Contribution

It provides the first large-scale Bayesian spectral analysis of XMM-XXL AGN, exploring the Iwasawa-Taniguchi effect, torus structure, and absorption characteristics across luminosities and redshifts.

Findings

Confirmed the Iwasawa-Taniguchi effect related to luminosity and torus covering factor.

Found no significant evolution of torus structure with redshift.

Discovered that some optical type-1 AGN show high X-ray absorption, likely due to small-scale gas.

Abstract

In this paper we describe and publicly release a catalogue consisting of 8445 point-like X-ray sources detected in the XMM-XXL north survey. For the 2512 AGN which have reliable spectroscopy, we present the X-ray spectral fitting which has been computed with a Bayesian approach. We have also applied an X-ray spectral stacking method to different sub-samples. We confirm the well-known Iwasawa-Taniguchi effect in our luminosity-redshift sub-samples, and argue that such an effect is due to a decrease in the covering factor of a distant obscuring torus with increasing X-ray luminosity. By comparing the distribution of the reflection fraction, the ratio of the normalization of the reflected component to the direct radiation, we find that the low-luminosity, low-redshift sub-sample had systematically higher reflection fraction values than the high-redshift, high-luminosity one. On the other hand, no significant difference is found between samples having similar luminosity but different redshift, suggesting that the structure of the torus does not evolve strongly with redshift. Contrary to previous works, we do not find evidence for an increasing photon index at high Eddington ratio. This may be an indication that the structure of the accretion disc changes as the Eddington ratio approaches unity. Comparing our X-ray spectral analysis results with the optical spectral classification, we find that $\sim20$ per cent of optical type-1 AGN show an X-ray absorbing column density higher than $10^{21.5}\,\rm{cm^{-2}}$, and about 50 per cent of type-2 AGN have an X-ray absorbing column density less than $10^{21.5}\,\rm{cm^{-2}}$. We suggest that the excess X-ray absorption shown in the high-luminosity optical type-1 AGN can be due to small-scale dust-free gas within (or close to) the broad line region, while in the low-luminosity ones it can be due to a clumpy torus with a large covering factor.