Does the obscured AGN fraction really depend on luminosity?

Abstract

We use a sample of 151 local non-blazar active galactic nuclei (AGN) selected from the INTEGRAL all-sky hard X-ray survey to investigate if the observed declining trend of the fraction of obscured (i.e. showing X-ray absorption) AGN with increasing luminosity is mostly an intrinsic or selection effect. Using a torus-obscuration model, we demonstrate that in addition to negative bias, due to absorption in the torus, in finding obscured AGN in hard X-ray flux-limited surveys, there is also positive bias in finding unobscured AGN, due to Compton reflection in the torus. These biases can be even stronger taking into account plausible intrinsic collimation of hard X-ray emission along the axis of the obscuring torus. Given the AGN luminosity function, which steepens at high luminosities, these observational biases lead to a decreasing observed fraction of obscured AGN with increasing luminosity even if this fraction has no intrinsic luminosity dependence. We find that if the central hard X-ray source in AGN is isotropic, the intrinsic (i.e. corrected for biases) obscured AGN fraction still shows a declining trend with luminosity, although the intrinsic obscured fraction is significantly larger than the observed one: the actual fraction is larger than ∼85 per cent at L 10 42.5 erg s −1 (17-60 keV), and decreases to 60 per cent at L 10 44 erg s −1 . In terms of the half-opening angle θ of an obscuring torus, this implies that θ 30 • in lower luminosity AGN, and θ 45 • in higher luminosity ones. If, however, the emission from the central supermassive black hole is collimated as dL/d ∝ cos α, the intrinsic dependence of the obscured AGN fraction is consistent with a luminosity-independent torus half-opening angle θ ∼ 30 • .

X-Ray Spectral Analyses of AGNs from the 7Ms Chandra Deep Field-South Survey: The Distribution, Variability, and Evolutions of AGN Obscuration

“…Apparently, this obscurationluminosity correlation suggests different LFs of obscured and unobscured AGNs. However, it does not necessarily indicate different LF shapes at different N H ; it could also be explained as a result of anisotropy of AGN X-ray emission (Lawrence & Elvis 2010;Burlon et al 2011;Liu et al 2014;Sazonov et al 2015), that is, different LF normalizations rather than shapes at different N H . Therefore, the assumption of an N H -independent LF shape does not contradictour results; we do not discuss more complex corrections based on N H -dependent LFs.…”

“…This behavior can be explained by a decreased covering factor of the obscuring material at high luminosity (Lawrence 1991; Lamastra et al 2006;Maiolino et al 2007), or 2 as a result of higher intrinsic luminosities in unobscured than in obscured AGNs (Lawrence & Elvis 2010;Burlon et al 2011;Liu et al 2014;Sazonov et al 2015). However, other studies suggest that the relation between intrinsic absorption and luminosity is more complex, and may be non-monotonic.…”

Polarized radiative transfer modeling of warped and clumpy dusty tori

“…The outer torus radius corresponds to the typical extent of compact and optically thick tori in nearby AGN, ranging from 1 to 10 pc according to simulations (Pier & Krolik 1992;Siebenmorgen et al 2015) and observations (Tristram et al 2007;Kishimoto et al 2009b). The torus half-opening angle was fixed to 30 • from the equatorial plane, a value chosen to agree with observed half-opening angles of the equatorial AGN region recently estimated by Sazonov et al (2015) and Marin & Antonucci (2016). The toroidal structure was then filled with constant-density spheres with radius 0.11 pc (Stalevski et al 2012).…”

X-RAY ABSORPTION, NUCLEAR INFRARED EMISSION, AND DUST COVERING FACTORS OF AGNs: TESTING UNIFICATION SCHEMES

“…This model postulates that the luminosity-dependence of the type 2 AGN fraction is directly associated with the geometry of the torus in the sense that the covering factor of the torus (the fraction of the sky as seen by the source obscured by dust) decreases with increasing AGN luminosity (Lawrence 1991). Unfortunately, it is not yet fully understood how the AGN accretion power can influence the physical extent of the torus, as there is substantial quantitative disagreement between published luminosity trends (e.g., Lawrence & Elvis 2010;Sazonov et al 2015).…”