Start Presentation: Johannes Buchner, Antonis Georgakakis, Andrea Merloni, Kirpal Nandra

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Author: Johannes Buchner

Homepage: http://astrost.at/istics/

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Johannes Buchner, Antonis Georgakakis, Andrea Merloni, Kirpal Nandra

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Johannes Buchner, Antonis Georgakakis, Andrea Merloni, Kirpal Nandra Ongoing research deals with the problem that the reflection hump and Fe-line is pronounced to various degrees. Introducing additional reflection (see Figure 1, right column) is very strongly preferred by model selection, however there are sources which show a weaker Fe-line than the torus model predicts, potentially indicative of a different geometry, ionization state or metallicity. Bayesian spectral characterisation of X-ray selected AGN in the CDF-S Figure 2. Most observed spectra, like the strongly obscured one shown here, can be described by a powerlaw source embedded in a torus of constant density (“torus”, green area) and a “scattering” powerlaw (red area). The model with background (blue) is in agreement with the observed counts (green points). Figure 1. Various geometries considered. The left first four cases represent zero, small, medium and large covering fractions respectively. Additional components, such as Thomson-scattering off ionized clouds (soft scattering) and reflection are considered (see text). (1) extract the most information from typically low signal-to-noise spectra of z~1 AGN, which represent the majority of black hole growth in the Universe. (2) compare various non-nested models using Bayesian model selection (see Figure 1, left column) corresponding to different geometries. Active Galactic Nuclei (AGN) show obscuration due to cold, neutral material in the line of sight. With a Bayesian analysis method, X-ray selected AGN in the Chandra Deep Field South (CDF-S) were analysed using a range of X-ray models. Using Bayesian model selection, we find that the obscurer has to have a large covering geometry with strong Compton reflection. The sphere or torus geometry with a scattering component (see Figure 2) rule out all other geometries with very strong evidence (Bayes factor > 30). We find: AGN are thought to intrinsically emit a Comptonized spectrum in X-rays which can be approximated by a powerlaw. The variety of observed spectra is attributed largely to viewing angles (unification scheme). Interaction processes such as photoelectric absorption, Fe fluorescence and Compton-scattering determine the spectral shape in dependence of the obscurer geometry and density. Using a Bayesian approach, we analyse X-ray spectra of 413 X-ray detected AGN in the CDF-S. This method allows us to