J/A&A/... AGN Torus model comparison of AGN in the CDFS (Buchner+, 2014)
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X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue.
J. Buchner, A. Georgakakis, K. Nandra, L. Hsu, C. Rangel, M. Brightman, A. Merloni, M. Salvato, J. Donley, D. Kocevski
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ADC_Keywords: X-ray sources; Surveys; Active gal. nuclei, Redshifts ;Magnitudes
Keywords: cosmology: observations - diffuse radiation - galaxies: active -
surveys - X-rays: galaxies
Mission_Name: Chandra
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, N_{H}, 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 2-10 keV band and the column density, N_{H}, 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-sigma 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 $N_H$ posterior in bans. As a reference, the narrowing of a Gaussian from prior to posterior by a factor of 2 corresponds to 0.13 ban, and thus values higher than that correspond to significant discriminatory information in the data.
Annotations:
S when f_scat>3%, s when f_scat>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 > 10^24^ cm^-2^,
Compton-thin (O) if 10^22^ cm^-2^ < N_H < 10^24^ 10^22^ cm^-2^,
Unobscured (U, N_H < 10^22^ cm^-2^, each with $\geq 50\%$ probability.
Byte-by-byte Description of file: table_trs.dat
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Bytes Format Units Label Explanations
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1- 3 I3 XID 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)
18- 19 I2 deg DEd Degree of Declination (J2000)
21- 22 I2 arcmin DEm Arcminute of Declination (J2000)
24- 28 F5.2 arcsec DEs Arcsecond of Declination (J2000)
30- 34 I5 ct counts Full-band counts at 0.5-8keV
36- 40 F5.3 --- z Redshift (z, median)
42- 46 F5.3 --- z_lo z (lower 1-sigma equivalent quantile)
48- 52 F5.3 --- z_hi z (upper 1-sigma equivalent quantile)
54- 58 F5.2 --- L Logarithm of intrinsic luminosity in the 2-10 keV restframe band (L, median)
60- 63 F4.2 --- L_lo L (lower 1-sigma equivalent quantile)
65- 68 F4.2 --- L_hi L (upper 1-sigma equivalent quantile)
70- 74 F5.2 --- NH Logarithm of neutral hydrogen equivalent column density in cm^-2 (NH, median)
76- 79 F4.2 --- NH_lo NH (lower 1-sigma equivalent quantile)
81- 84 F4.2 --- NH_hi NH (upper 1-sigma equivalent quantile)
86- 89 F4.2 --- Gamma Intrinsic photon index (Gamma, median)
91- 94 F4.2 --- Gamma_lo Gamma (lower 1-sigma equivalent quantile)
96- 99 F4.2 --- Gamma_hi Gamma (upper 1-sigma equivalent quantile)
101-104 F4.2 % f_scat Scattering fraction (f_scat)
106-109 F4.2 % f_scat_lo f_scat (lower 1-sigma equivalent quantile)
111-114 F4.2 % f_scat_hi f_scat (upper 1-sigma equivalent quantile)
116-119 F4.2 --- R Reflection component, relative normalisation (R, median)
121-124 F4.2 --- R_lo R (lower 1-sigma equivalent quantile)
126-129 F4.2 --- R_hi R (upper 1-sigma equivalent quantile)
131-134 F4.2 ban KL_NH Information gain for column density parameter (KL)
136-145 A10 --- Notes Notes (1)