A Hard X-ray View of the Distant Active Galactic Nucleus Population with NuSTAR

Abstract

Active galactic nuclei (AGNs), the sites of mass accretion onto supermassive black holes, have been hosted by most galaxies at some point in their lifetime. X-rays are a direct and efficient means of identifying AGNs and measuring their intrinsic properties reliably. A recent breakthrough in this regard is the Nuclear Spectroscopic Telescope Array (NuSTAR), the first space satellite observatory with the ability to focus high-energy (i.e., ``hard''; >10 keV) X-ray photons. In this thesis I use NuSTAR to study the distant hard X-ray emitting AGN population, with a view to improving the cosmic census of AGNs as well as understanding their demography and evolution. In addition to these broad goals, a more specific focus is to identify elusive Compton-thick (CT) AGNs, which may represent an important phase of hidden black hole growth. Two overall approaches are taken: (1) optically selected Type 2 quasars suspected to be CT (i.e., candidate CTQSO2s) are deliberately targetted with NuSTAR; and (2) a large and unbiased serendipitous survey of ~500 X-ray sources is performed using almost all of the science data taken with the NuSTAR observatory over a 40-month period. For both of these complementary samples, the broad-band X-ray and multiwavelength properties are studied. For the candidate CTQSO2s, the addition of >10 keV NuSTAR data provides an improvement compared to constraints with Chandra and XMM alone (i.e., with the most sensitive observatories at <10 keV), generally allowing significantly higher column densities (Nh) and intrinsic AGN luminosities (Lx) to be constrained, and providing strong evidence for CT absorption in some cases. Implications for the Nh distribution of Type 2 quasars are discussed. For the NuSTAR serendipitous survey, an extensive ground-based followup program has been undertaken, which was crucial to obtain spectroscopic redshifts and classifications for the bulk of the sample. The serendipitous survey AGNs cover a redshift range of z=0.002 to 3.4 (median of <z>=0.56) and a hard X-ray luminosity range of log(L_10-40keV [erg/s]) ~ 39 to 46. Singling out the most extreme likely-CT sources in the serendipitous survey gives an insight into the prevalence of such extreme systems within the general AGN population.