Resolving X-ray Obscuration Biases with Isotropic AGN Selection - the NuLANDS Legacy Survey
Astronomical Institute Of The Czech Academy Of Sciences, Prague
University of Southampton, Southampton
Most mass is accreted onto supermassive black holes behind thick columns of gas and dust. An accurate assessment of the fraction of heavily obscured, "Compton-thick" AGN in the local Universe provides important insights into the composition and structure of the circum-nuclear AGN obscurer, as well as its connection with the evolution of supermassive black holes and their surrounding host galaxies across cosmic time. However, current estimates of the Compton-thick fraction vary dramatically between ~20-70%, and it remains unclear whether this large range is driven by selection effects, inadequate sample sizes, luminosity/Eddington rate dependencies, statistical issues associated with fitting low signal-to-noise X-ray spectra or something else entirely. The main handicap of previous works has been the inability to effectively select objects that are *representative* in terms of sampling N(H) parameter space, i.e. are unbiased even by Compton-thick obscuration. To investigate such issues, we present NuLANDS - a large far-infrared legacy survey with the X-ray satellites NuSTAR, XMM-Newton and Swift (~2 Ms of total new time) aimed at constructing an unbiased census of AGN obscuration in the local Universe. The infrared selection using AGN-like colours guarantees that we are not affected by line-of-sight X-ray obscuration biases, even into the log N(H)/cm-2 > 25 regime. In this talk, I will first report on multiple new Compton-thick AGN identified with our novel fitting approach, combining Nested Sampling with a large library of different geometrical models for the AGN obscurer. By fitting from the global multi-dimensional prior parameter space, Nested Sampling robustly estimates parameter uncertainties without requiring parameter tuning. Our results ultimately show that hard X-ray selection alone remains biased against the most heavily obscured AGN, and I will highlight the importance of multi-wavelength selection in completing the local AGN census with future next generation instruments. NuLANDS thus marks a major step in completing this census, and will provide vital boundary conditions for determining the composition of the Cosmic X-ray Background, as well as geometrical insights into the densest regions of the AGN torus.