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Article overview
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X-ray spectral properties of the AGN sample in the northern XMM-XXL field | Zhu Liu
; Andrea Merloni
; Antonis Georgakakis
; Marie-Luise Menzel
; Johannes Buchner
; Kirpal Nandra
; Mara Salvato
; Yue Shen
; Marcella Brusa
; Alina Streblyanska
; | Date: |
1 May 2016 | 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 $10^{21.5},
m{cm^{-2}}$, and about
50 per cent of type-2 AGN have an X-ray absorbing column density less than
$10^{21.5},
m{cm^{-2}}$. 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. | Source: | arXiv, 1605.0207 | Services: | Forum | Review | PDF | Favorites |
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