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Does the obscured AGN fraction really depend on luminosity?

Sazonov, S. and Churazov, E. and Krivonos, R. (2015) Does the obscured AGN fraction really depend on luminosity? Monthly Notices of the Royal Astronomical Society, 454 (2). pp. 1202-1220. ISSN 0035-8711. doi:10.1093/mnras/stv2069.

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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 ≳ 1044 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/dΩ ∝ cos α, the intrinsic dependence of the obscured AGN fraction is consistent with a luminosity-independent torus half-opening angle θ ∼ 30°.

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Krivonos, R.0000-0003-2737-5673
Additional Information:© 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2015 September 2. Received 2015 September 2; in original form 2015 July 14. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This study was supported by the Russian Foundation for Basic Research (grant 13-02-01365). We thank the referee for useful comments.
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Russian Foundation for Basic Research13-02-01365
Issue or Number:2
Record Number:CaltechAUTHORS:20160131-144240325
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Official Citation:S. Sazonov, E. Churazov, and R. Krivonos Does the obscured AGN fraction really depend on luminosity? MNRAS (December 01, 2015) Vol. 454 1202-1220 doi:10.1093/mnras/stv2069 First published online October 1, 2015
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:64110
Deposited By: Joy Painter
Deposited On:03 Feb 2016 20:29
Last Modified:10 Nov 2021 23:26

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