Nearby Heavily Obscured AGN in the First Decade of NuSTAR

Despite the long history of studies of active galactic nuclei (AGN), the basic nature of obscuring tori hiding the central engine of obscured AGN, remains unclear. NuSTAR enabled studies of nearby AGN to extend into the hard X-ray band with unprecedented angular resolution and sensitivity. Broadband X-ray spectroscopy gives us unique insight into the anatomy of obscured AGN as hard X-rays penetrate even heavy obscuration, enabling us to study both the X-ray source and the properties of the obscuring matter. Much has been learned from hundreds of observations of nearby AGN taken over the first 10 years of NuSTAR operation, many of which are heavily obscured up to, and well into, the Compton-thick regime. I will present results from phenomenological spectral analyses relevant for synthesis models of the cosmic X-ray background and constraints on the structure of the obscuring torus — its covering factor and globally averaged column density — based on a large sample of nearby obscured AGN observed with NuSTAR. I will then show how results based on single-epoch NuSTAR observations of highly obscured AGN can be significantly enhanced by self-consistent broadband X-ray spectral modeling over multiple epochs. Further constraints would be enabled by sensitive broadband X-ray spectroscopy with an observatory such as the Probe-class concept HEX-P. Complementing these X-ray studies, multi-wavelength data with much higher angular resolution will be essential in helping to advance our understand the complexity of the structure known under the deceptively simple name of the obscuring torus.