SIMBAD references

In Unification Models, Active Galactic Nuclei (AGN) are believed to be surrounded by an axisymmetric structure of dust and gas, which greatly influences their observed properties according to the direction from which they are observed. The main aim of this work is to constrain the properties of this obscuring material using X-ray observations. The distribution of column densities observed by Chandra in the Chandra Deep Field South is used to determine geometrical constraints for already proposed torus models. It is found that the best torus model is given by a classical ``donut shape'' with an exponential angular dependency of the density profile. The opening angle is strongly constrained by the observed column densities. Other proposed torus models are clearly rejected by the X-ray observations.