X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue

Abstract

Aims. Active galactic nuclei are known to have complex X-ray spectra that depend on both the properties of the accreting super-massive 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. Methods. 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. Results. 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.Conclusions. 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.

The evolution of the X-ray luminosity functions of unabsorbed and absorbed AGNs out to z∼ 5

“…Ross & Fabian 2005;Brightman & Nandra 2011a). However, Buchner et al (2014) found that more simplistic models such as ours are generally sufficient to reproduce the observed spectral shape of individual, distant AGNs, especially considering our analysis uses broad-band fluxes rather than performing a detailed X-ray spectral analysis. To describe our X-ray spectral model requires three additional parameters-the photon index, Γ, the scattered fraction, fscatt, and the relative normalization of the reflection component, R-which we introduce as "nuisance" parameters (collectively designated by ξ) in our Bayesian analysis.…”

“…Whether the observational evidence supports such changes in the shape of the XLF has been a matter of debate (e.g. A10, Aird et al 2008;Fiore et al 2012;Ueda et al 2014;Buchner et al 2015). Furthermore, a physical interpretation of this complex parametrization is difficult.…”

“…We combine samples selected at both hard and soft X-ray energies and determine the underlying XLF and distribution of NH that adequately describes the observed fluxes in both samples (similar to the approach of Ueda et al 2014, cf. the X-ray spectral analysis used in Buchner et al 2015).…”

The NuSTAR Extragalactic Surveys: X-Ray Spectroscopic Analysis of the Bright Hard-band Selected Sample

“…The spectra and the best-fit models for these sources are reported in Figure 4. This disagreement is not unusual, several works using low-energy data have thus far attempted the spectral identification of distant CT sources in the medium/deep survey fields (e.g., Tozzi et al 2006;Georgantopoulos et al 2007Georgantopoulos et al , 2009Georgantopoulos et al , 2013Comastri et al 2011;Feruglio et al 2011;Brightman & Ueda 2012;Brightman et al 2014;Buchner et al 2014;Lanzuisi et al 2015), and given the range of possible spectral shapes for CT sources and the limited counting statistics (from few tens to ∼100 counts) for these faint sources, especially at the highest energies 5 6 keV  -where the effective area starts to drop, researchers have always struggled to consistently identify CT sources (see, e.g., Castelló-Mor et al 2013). Moreover, their analysis, at least for sources with redshifts 2  (i.e., the great majority of the potential contributors to the CXB, according to populationsynthesis models), have been limited by sampling of the restricted lower-energy part of the reflection component and of the heavily obscured primary emission.…”

7.1 keV sterile neutrino constraints from X-ray observations of 33 clusters of galaxies withChandraACIS

“…As a robustness test we used a Bayesian fitting code (BXA; Buchner et al 2014) with flat priors for the same parameter ranges as our standard fitting method and obtained consistent results. A non-negative prior for the additional Gaussian flux did not improve the constraints.…”