Obscured and Compton-thick AGN in NuSTAR hard X-ray surveys

This Thesis focuses on NuSTAR hard X-ray surveys of Active Galactic Nuclei (AGN). One of the major goals of the NuSTAR mission is to study with unprecedented detail the obscured and heavily obscured (Compton-thick, CT) populations of AGN, significantly contributing to the diffuse cosmic X-ray background (CXB),and major actors within the evolutionary cycle of galaxies. In the first part, a survey of a small sample (∼ a dozen) of local (z < 0.03) heavily obscured AGN, selected by their water megamaser emission at ∼22.23 GHz, is presented. Thanks to the NuSTAR hard X-ray spectral coverage (extending between ∼3−80 keV), a robust estimate of the CT fraction is obtained. Building up on this result and combining X-ray and radio measurements, a toy model of the dusty megamaser disk within the classical dusty torus is proposed. A spin-off of the megamaser project is related to a well-known local obscured AGN (Mrk1210) and devoted to studying its long-term (∼17 yrs) X-ray variability with NuSTAR, from which some constraints on the AGN environment can be drawn. Finally, the search and study of CT AGN is pushed to higher redshifts, fully exploiting the NuSTAR X-ray capabilities in terms of sensitivity. The hunt for faint and distant heavily obscured AGN is performed exploiting the combined NuSTAR, XMM-Newton and Chandra coverages of the UKIDSS-UDS field. The deep NuSTAR survey strategy is presented, along with the modeling of the NuSTAR background in order to optimize the detection of faint sources. A broadband X-ray spectroscopic analysis of all the detected sources is performed, and combined with the standard hardness ratio (HR) diagnostic, in order to select all the possible CT candidates. Such CT candidates are then analyzed again with appropriate X-ray spectral models specifically developed to deal with CT absorbers.