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

Abstract: AGN are known to have complex X-ray spectra that depend on both the properties of the accreting SMBH (e.g. mass, accretion rate) and the distribution of obscuring material in its vicinity ("torus"). Often however, simple and even unphysical models are adopted to represent the X-ray spectra of AGN. 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 X-ray data and photometric redshifts. 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. Despite the use of low-count spectra, our methodology is able to draw strong inferences on the geometry of the torus. For a sample of 350 AGN in the 4 Ms Chandra Deep Field South field, our analysis identifies four components needed to represent the diversity of the observed X-ray spectra: (abridged). Simpler models are ruled out with decisive evidence in favour of a geometrically extended structure with significant Compton scattering. Regarding the geometry of the obscurer, there is strong evidence against both a completely closed or entirely open toroidal geometry, in favour of an intermediate case. 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 with estimated parameters such as the accretion luminosity in the 2-10 keV band and the column density, $N_{H}$, of the obscurer.

• The paper demonstrates that a Bayesian framework rigorously distinguishes among ten physically motivated torus models, challenging simpler approaches.
• The study shows that incorporating components like Thomson scattering and Compton reflection is critical for accurate estimates of AGN luminosity and column density.
• The paper provides decisive evidence favoring an intermediate torus geometry over fully closed or open configurations, delivering a comprehensive AGN catalogue in the CDFS.

X-ray Spectral Modelling of the AGN Obscuring Region in the CDFS: Bayesian Model Selection and Analysis

This paper presents an advanced investigation into the geometry of the obscuring torus in Active Galactic Nuclei (AGN) within the Chandra Deep Field South (CDFS), utilizing a Bayesian framework for model comparison of X-ray spectra. This study aims to refine the understanding of X-ray spectral models, focusing on AGN with high redshift ($z > 0.5$), where most accretion occurs.

The study challenges the prevailing simplistic models often employed to interpret the X-ray spectra of AGN, which can inadequately capture the complexities involved. Such oversimplifications may impede the accurate determination of the AGN luminosity function and accretion history, thereby affecting our comprehension of AGN and host galaxy co-evolution.

Methodology and Data

The authors have implemented a Bayesian framework that allows for rigorous model comparison and parameter estimation, accommodating uncertainties inherent in photometric redshift estimates and the Poisson nature of X-ray data. Using this approach, the paper evaluates ten physically motivated models of torus geometries against the extensive datasets from CDFS with a 4 million-second exposure time.

Results and Analysis

For approximately 350 AGN spectra, the analysis identifies four essential spectral components:

1. Intrinsic Power Law: Consistently present across AGN spectra.
2. Obscurer: This component includes photo-electric absorption, Compton scattering, and Fe-K fluorescence effects.
3. Unabsorbed Power Law: Stemming from Thomson scattering off ionized clouds in the vicinity of AGN.
4. Compton Reflection: Dominated by a pronounced Fe-K line, surpassing expectations from simpler models.

The study provides decisive evidence, quantified by a Bayes factor greater than 100, against simpler models that do not incorporate these complexities. Particularly, models excluding the Thomson scattering component underestimate the column density $N_{H}$, which is critical for understanding the distribution and density of obscuring material.

Further findings reveal robust evidence against both entirely closed and completely open toroidal geometries, with results favoring an intermediate torus configuration. Such a configuration denotes a more nuanced understanding of the obscuring regions around supermassive black holes.

Implications and Future Research

The insights from the Bayesian approach underline the necessity of adopting complex models for accurate AGN characterization. The apparent need for a soft component and additional Compton reflection may imply a nuanced density gradient in the torus or interactions with the accretion disk itself.

This study not only enhances the theoretical comprehension of AGN structures but also advances practical avenues for interpreting future X-ray astronomical data. By issuing a comprehensive catalogue of AGN in the CDFS, including parameters such as accretion luminosity and column density, the paper facilitates further empirical inquiry.

In conclusion, this research sets a benchmark for robust AGN spectral modeling, demonstrating significant potential for expanding the understanding of the cosmic evolution of accreting supermassive black holes. Future investigations may build on these findings by incorporating additional data and exploring alternative spectral models, promising further advancements in both theoretical frameworks and observational techniques.