Resolving the AGN Torus Using Broadband X-ray and VLBI Observations

As a result of NuSTAR and Swift observations of over a hundred obscured AGN in the local universe, recent studies placed interesting constraints on some basic geometrical parameters of the obscuring torus directly from the X-ray band. In particular, the covering factor was found to peak around the typical Seyfert X-ray luminosity and decrease toward both lower and higher luminosities. Constraints on the covering factor can be significantly improved over single-epoch spectral analyses using a self-consistent multi-epoch approach, which we will demonstrate on the nearby low-luminosity AGN NGC 1052. Both the spectral fitting and line-of-sight column density variability statistics lead to the picture of a nearly uniform Compton-thin torus with a large covering factor in this AGN. This is consistent with recent findings for other low-luminosity AGN. However, such a conclusion is model-dependent and appears to be in conflict with subparsec-scale measurements of the free-free absorption profile based on VLBI observations with milliarcsecond angular resolution. We will show how these spatially resolved measurements can inform building of new X-ray spectral models for spatially unresolved signatures of the obscuring torus in the X-ray band. With these new models, combining the two fundamentally different and complementary types of observations could provide vital clues for interpretation of high-resolution observations (spectral or spatial) probing the structure of the torus at these and other wavelengths in the near future.