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Article overview
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Does the obscured AGN fraction really depend on luminosity? | | Sergey Sazonov
; Eugene Churazov
; Roman Krivonos
; | | Date: |
3 Sep 2015 | | Abstract: | We use a sample of 151 local non-blazar 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 $sim 85$% at $Llesssim 10^{42.5}$ erg/s (17--60 keV),
and decreases to $lesssim 60$% at $Lgtrsim 10^{44}$ erg/s. In terms of the
half-opening angle, $ heta$, of an obscuring torus, this implies that
$ hetalesssim 30$ deg in lower-luminosity AGN, and $ hetagtrsim 45$ deg in
higher-luminosity ones. If, however, the emission from the central SMBH is
collimated as $dL/dOmegaproptocosalpha$, the intrinsic dependence of the
obscured AGN fraction is consistent with a luminosity-independent torus
half-opening angle $ hetasim 30$ deg. | | Source: | arXiv, 1509.1259 | | Services: | Forum | Review | PDF | Favorites | |
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