Abstract
Abstract Dedicated searches generally find a decreasing fraction of obscured active galactic nuclei (AGN) with increasing AGN luminosity. This has often been interpreted as evidence for a decrease of the covering factor of the AGN torus with increasing luminosity, the so-called receding torus models. Using a complete flux-limited X-ray selected sample of 199 AGN, from the Bright Ultra-hard XMM-Newton Survey, we determine the intrinsic fraction of optical type-2 AGN at as a function of rest-frame 2–10 keV X-ray luminosity from to . We use the distributions of covering factors of AGN tori derived from CLUMPY torus models. Since these distributions combined over the total AGN population need to match the intrinsic type-2 AGN fraction, we reveal a population of X-ray undetected objects with high-covering factor tori, which are increasingly numerous at higher AGN luminosities. When these “missing” objects are included, we find that Compton-thick AGN account at most for % of the total population. The intrinsic type-2 AGN fraction is 58 ± 4% and has a weak, non-significant (less than 2σ) luminosity dependence. This contradicts the results generally reported by AGN surveys and the expectations from receding torus models. Our findings imply that the majority of luminous rapidly accreting supermassive black holes at reside in highly obscured nuclear environments, but most of them are so deeply embedded that they have so far escaped detection in X-rays in <10 keV wide area surveys.
Highlights
We investigate whether the decrease of f2 with LX, which we observe in the Bright Ultra-hard XMMNewton Survey (BUXS) sample, is a property of the active galactic nuclei (AGN) population
Since the distribution of covering factors needs to match the fraction of optical type-2 AGN, we reveal the existence of a substantial population of X-ray undetected objects with highcovering factor tori, which are increasingly numerous at higher AGN luminosities
We find that the intrinsic type-2 AGN fraction is 58.5-+44..41% and has a weak and non-significant luminosity dependence
Summary
Dedicated searches for active galactic nuclei (AGN) generally find that the fraction of AGN classified either as optical type-2 (obscured) or X-ray absorbed decreases substantially with increasing luminosity 1982; Hasinger et al 2005; Simpson 2005; Della Ceca et al.2008; Burlon et al 2011; Merloni et al 2014; Ueda et al 2014; Buchner et al 2015) To explain these findings, receding torus models have often been adopted (Lawrence 1991; Simpson 2005; Hönig & Beckert 2007). They postulate that the geometry of the material obscuring the AGN nuclear region, the dusty torus (Antonucci 1993; García-Burillo et al 2016), changes with AGN luminosity.
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