J/A+A/564/A125 AGN Torus model comparison of AGN in the CDFS (Buchner+, 2014)
X-ray spectral modelling of the AGN obscuring region in the CDFS:
Bayesian model selection and catalogue.
Buchner J., Georgakakis A., Nandra K., Hsu L., Rangel C., Brightman M.,
Merloni A., Salvato M., Donley J., Kocevski D.
<Astron. Astrophys. 564, A125 (2014)>
=2014A&A...564A.125B 2014A&A...564A.125B
ADC_Keywords: X-ray sources ; Active gal. nuclei ; Redshifts ;
Keywords: accretion, accretion disks - methods: data analysis -
methods: statistical - galaxies: nuclei - X-rays: galaxies -
galaxies: high-redshift
Abstract:
Active Galactic Nuclei are known to have complex X-ray spectra that
depend on both the properties of the accreting supermassive black hole
(e.g. mass, accretion rate) and the distribution of obscuring material
in its vicinity (i.e. the "torus"). Often however, simple and even
unphysical models are adopted to represent the X-ray spectra of AGN,
which do not capture the complexity and diversity of the observations.
In the case of blank field surveys in particular, this should have an
impact on e.g. the determination of the AGN luminosity function, the
inferred accretion history of the Universe and also on our
understanding of the relation between AGN and their host galaxies.
We develop a Bayesian framework for model comparison and parameter
estimation of X-ray spectra. We take into account uncertainties
associated with both the Poisson nature of X-ray data and the
determination of source redshift using photometric methods. We also
demonstrate how Bayesian model comparison can be used to select among
ten different physically motivated X-ray spectral models the one that
provides a better representation of the observations. This methodology
is applied to X-ray AGN in the 4 Ms Chandra Deep Field South.
For the ∼350 AGN in that field, our analysis identifies four
components needed to represent the diversity of the observed X-ray
spectra: (1) an intrinsic power law, (2) a cold obscurer which
reprocesses the radiation due to photo-electric absorption, Compton
scattering and Fe-K fluorescence, (3) an unabsorbed power law
associated with Thomson scattering off ionised clouds, and (4) Compton
reflection, most noticeable from a stronger-than-expected Fe-K line.
Simpler models, such as a photo-electrically absorbed power law with a
Thomson scattering component, are ruled out with decisive evidence
(B>100). We also find that ignoring the Thomson scattering component
results in underestimation of the inferred column density, NH, of
the obscurer. Regarding the geometry of the obscurer, there is strong
evidence against both a completely closed (e.g. sphere), or entirely
open (e.g. blob of material along the line of sight), toroidal
geometry in favour of an intermediate case.
Despite the use of low-count spectra, our methodology is able to draw
strong inferences on the geometry of the torus. Simpler models are
ruled out in favour of a geometrically extended structure with
significant Compton scattering. We confirm the presence of a soft
component, possibly associated with Thomson scattering off ionised
clouds in the opening angle of the torus. The additional Compton
reflection required by data over that predicted by toroidal geometry
models, may be a sign of a density gradient in the torus or reflection
off the accretion disk. Finally, we release a catalogue of AGN in the
CDFS with estimated parameters such as the accretion luminosity in the
2-10keV band and the column density, NH, of the obscurer.
Description:
We present the Bayesian parameter estimation results derived using the
torus+pexmon+scattering model.
All parameters are shown with their posterior uncertainty, which is
summarised using the 1-σ equivalent quantiles.
The prior used on the Photon index was a normal distribution with mean
1.95 and standard deviation 0.15, so if no information was gained this
value remains. The KL column measures the information gain measured
from the NH posterior in bans. As a reference, the narrowing of a
Gaussian from prior to posterior by a factor of 2 corresponds to
0.13ban, and thus values higher than that correspond to significant
discriminatory information in the data.
Annotations:
S when fscat>3%, s when fscat>0.5% with ≥90% probability
R when R>0.3 with ≥90% probability, i.e. strong additional pexmon
reflection
Compton-thick (CT) if N_H>1024cm-2,
Compton-thin (O) if 1022cm-2<N_H<1024cm-2,
Unobscured (U, N_H<1022cm-2, each with ≥50% probability.
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
catalog.dat 141 334 Catalog of the derived quantities for each
source in the Chandra Deep Field South (CDFS)
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See also:
II/253 : Chandra Deep Field South: multi-colour data (Wolf+, 2008)
J/ApJS/155/73 : Photometric redshifts of X-ray sources in CDF-S (Zheng+, 2004)
J/A+A/451/457 : X-ray properties of AGN in CDFS (Tozzi+, 2006)
J/ApJ/640/167 : AGNs and ULIRGs in the CDF-South (Alonso-Herrero+, 2006)
J/A+A/488/73 : Variability-selected AGN in Chandra DFS (Trevese+, 2008)
J/AJ/135/1505 : CDFs AGNs X-ray power-law photon index (Saez+, 2008)
J/ApJ/680/130 : Mid-IR colors of AGNs in the MUSYC ECDF-S (Cardamone+, 2008)
J/A+A/497/81 : Variability-selected AGN in CDFS (Boutsia+, 2009)
J/ApJ/693/1713 : Spectroscopy of X-ray sources in ECDF-S (Treister+, 2009)
J/ApJS/187/560 : Photometric redshifts of the 2Ms CDF-S (Luo+, 2010)
J/ApJS/195/10 : The CDF-S survey: 4Ms source catalogs (Xue+, 2011)
J/A+A/555/A42 : The XMM-CDFS catalogues (Ranalli+, 2013)
Byte-by-byte Description of file: catalog.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- XID [1/570] Source identification
5 I1 h RAh Hour of Right Ascension (J2000)
7- 8 I2 min RAm Minute of Right Ascension (J2000)
10- 14 F5.2 s RAs Second of Right Ascension (J2000)
16 A1 --- DE- Sign of the Declination (J2000)
17- 18 I2 deg DEd Degree of Declination (J2000)
20- 21 I2 arcmin DEm Arcminute of Declination (J2000)
23- 27 F5.2 arcsec DEs Arcsecond of Declination (J2000)
29- 33 I5 ct Ct [27/32100] Full-band (0.5-8ke) photon counts
35- 39 F5.3 --- z [0.1/5.4] Redshift (z, median)
41- 46 F6.3 --- e_z ?=-1 z lower 1-σ equivalent quantile
48- 53 F6.3 --- E_z ?=-1 z upper 1-σ equivalent quantile
55- 59 F5.2 [10-7W] logLX Logarithm of intrinsic luminosity (erg/s) in
the 1-9 keV restframe band (L, median)
61- 64 F4.2 [10-7W] e_logLX logLX lower 1-σ equivalent quantile
66- 69 F4.2 [10-7W] E_logLX logLX upper 1-σ equivalent quantile
71- 75 F5.2 [cm-2] logNH Logarithm of neutral hydrogen equivalent
column density (NH, median)
77- 80 F4.2 [cm-2] e_logNH logNH lower 1-σ equivalent quantile
82- 85 F4.2 [cm-2] E_logNH logNH upper 1-σ equivalent quantile
87- 90 F4.2 --- Gamma Intrinsic photon index (Γ, median)
92- 95 F4.2 --- e_Gamma Gamma lower 1-σ equivalent quantile
97-100 F4.2 --- E_Gamma Gamma upper 1-σ equivalent quantile
102-105 F4.2 % fscat [0/10] Scattering fraction (fscat)
107-110 F4.2 % e_fscat fscat lower 1-σ equivalent quantile
112-115 F4.2 % E_fscat fscat upper 1-σ equivalent quantile
117-120 F4.2 --- R [0/6] Reflection component, relative
normalisation (R, median)
122-125 F4.2 --- e_R R lower 1-σ equivalent quantile
127-130 F4.2 --- E_R R upper 1-σ equivalent quantile
132-135 F4.2 --- KLNH Information gain for column density parameter
(KL) in ban units
137-141 A5 --- Notes Notes (1)
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Note (1): Notes as follows:
S = fscat>3% with ≥90% probability
s = fscat>0.5% with ≥90% probability
R = R>0.3 with ≥90% probability, i.e. strong additional pexmon reflection
CT = Compton-thick if NH>1024cm-2
O = Compton-thin if 1022cm-2<N_H<1024cm-2
U = Unobscured NH<1022cm-2, each with ≥50% probability
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Acknowledgements:
Johannes Buchner, johannes.buchner.acad(at)gmx.com
(End) Patricia Vannier [CDS] 03-Feb-2014