Heavily Obscured Active Galactic Nuclei in the \u3ci\u3eNuSTAR\u3c/i\u3e Era

Active galactic nuclei (AGN) are among the brightest objects in the sky. The enormous intrinsic emission of the AGN is produced by the gas accreted onto the central supermassive black hole (SMBH). Most of the AGN intrinsic emission are obscured by the material in a toroidal structure (or the torus) in AGN. This obscuring torus has been resolved in recent observations. However, many of its properties are still unknown, e.g., the torus covering factor and the torus density. My dissertation is focused on studying the AGN torus properties in X-rays. Heavily obscured AGN (NH ≥10^{23} cm^{−2}) are the best sample to study the AGN properties due to their clear signature of the torus emission in the X-ray spectrum. Thanks to the launch of NuSTAR (focus at 3-79 keV energy range) in 2012, which has the best sensitivity at 10 keV, it provides an unprecedented opportunity to study heavily obscured AGN in the nearby Universe. The most obscured sources are the Compton-thick (CT-) AGN (NH ≥ 10^{24} cm^{-2}), which are predicted to be ~30-50% of all AGN. However, CT-AGN have never been detected in large numbers (only 5%-11% of all AGN are CT-AGN in observations). In Section 2 and Section 3, I present the studies of two CT-AGN candidates in the nearby Universe using deep NuSTAR and XMM-Newton observations. Although large efforts have been made in searching for CT-AGN, the number of the discovered CT-AGN is still limited and the detection of CT-AGN is biased against. Therefore, we cannot perform a statistical study of the torus properties using the CT-AGN sample. Heavily obscured Compton thin AGN (23≤ log(NH) ≤24) are the best objects to probe the properties of the obscuring torus due to their large number of detections. In Section 4, I present a systematic study of the properties of the obscuring tori using an unbiased sample of heavily obscured Compton thin AGN with NuSTAR observations. We found that, on average, the sources in the sample have CT tori with large covering factors. Our results also show that the tori are significantly inhomogeneous. The inclination angle of AGN can be derived from the AGN torus or from analyzing the narrow line region (NLR) of AGN. In Section 5, I present a systematic analysis on the AGN inclination. We found that the inclinations of AGN measured using the torus are not always consistent with those measured using the NLR. We found that the X-ray spectral modeling is not sensitive to the inclination angle. However, one should always leave the inclination angle free to vary when analyzing the X-ray spectra of AGN. X-ray surveys are the most efficient methods to identify and trace the AGN population. In Section 6, I present the NuSTAR extragalactic surveys on the North Ecliptic Pole (NEP) Time-Domain Field. The survey covers a ~0.16 deg^2 area with a total exposure of 681 ks performed in three epochs. The NEP survey is one of the most sensitive survey among the NuSTAR extragalactic surveys so far. A total of 33 sources were detected. Variability is the general property of AGN. However, the variability of AGN has never been statistically studied in hard X-rays. The NuSTAR NEP survey was designed to have variability as its prime focus. 64% of the sources which were observed in multi-epochs show flux variabilities. The dissertation is organized as follows: In section 1, I introduce the history, structure, X-ray spectrum, and X-ray studies of AGN. In section 2-6, I present the papers that have been published or will be published as part of my Ph.D. work. In section 7, I make a conclusion of the above works and present an outlook on the future opportunities