SIMBAD references

The geometry of the obscuring ``torus'' in AGN is known to be complex in local sources. At z=0.5-3, where most accretion occurs in obscured sources, simple models of line-of-sight (LOS) obscuration are usually assumed. A range of important analyses, such as luminosity functions, accretion density, co-evolution of AGN with host galaxies, rely on results based on these models. We probe the geometry of the torus to find the best model for high-redshift (z>0.5) AGN. 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. We develop a Bayesian framework for model comparison and parameter estimation of X-ray spectra. We also propagate the uncertainty associated with photometric redshifts estimates. Ten physically motivated models of torus geometries are compared using the deepest X-ray field to date, the Chandra Deep Field South (CDFS) of 4Ms exposure time.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 4Ms Chandra Deep Field South. For the ∼350 AGN spectra considered, our analysis identifies four important components to represent the variety of AGN spectra: (1) An intrinsic power law, (2) an 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-Kα line. Simpler models, such as a photo-electrically absorbed power law with a Thomson scattering component, are ruled out with decisive evidence (Z>100). We also note that not considering the Thomson scattering component underestimates the inferred column density N_H. Regarding the geometry of the obscurer, we find strong evidence against both a completely closed, or entirely open, toroidal geometry in favour of an intermediate case.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 associated with Thomson scattering off ionised clouds in the opening angle of the torus, which scatters the unobscured intrinsic radiation past the torus. The additional Compton reflection 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 N_H.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, N_H, of the obscurer.