X-ray Study of Heavily Obscured AGN in the Local Universe

The cosmic X-ray background (CXB), the diffuse X-ray emission observed between 0.5 keV and 300 keV, is thought to be mainly produced by obscured and unobscured active galactic nuclei (AGN). Compton-thick (CT-) AGNs (with absorbing column density NH >10²⁴ cm^-2) are responsible for ~30% of the CXB at its peak and expected to be numerous. However, as of today CT-AGNs have never been detected in large numbers, their observed fraction in the local universe being ~5–10%, significantly below the predictions of different CXB models (~20%–30%). In the first part, I will present a deep observation of two candidate CT-AGNs, selected using an effective technique reported by our group four years ago, using the unprecedented-quality data from NuSTAR and XMM-Newton, which allows us to have a better understanding of the physics of the obscuration process in AGNs.

The obscuration observed in active galactic nuclei (AGN) is mainly caused by dusty gas distributed in a torus-like structure surrounding the supermassive black hole (SMBH). However, properties of the obscuring torus of the AGN in X-ray have not been fully investigated yet due to the lack of high quality data and proper models. In the second part, I will present the results of spectral analysis of a large unbiased sample of obscured AGN in the nearby universe which have high-quality archival NuSTAR data. We find that Compton thin and Compton thick AGN may possess similar tori, whose average column density is Compton thick (~1.4 × 10²⁴ cm⁻²), but they are observed through different (under-dense or over-dense) regions of the tori. We also find that the medium in the obscuring torus is significantly inhomogeneous with the torus average column densities significantly different from their line-of-sight column densities (for most of the sources in the sample). The average torus covering factor of sources in our unbiased sample is 0.67, suggesting that the fraction of unobscured AGN is ~33%. We develop a new method to calculate the intrinsic line-of-sight column density distribution of AGN in the nearby universe, which we find the result is in good agreement with the constraints from recent population synthesis models.

In modeling the X-ray spectra of AGNs, the inclination angle is a parameter that can play an important role in understanding the X-ray spectra of AGNs, but it has never been studied in detail. In the third part, I will present a joint NuSTAR and XMM-newton broadband X-ray study on the role of the inclination in spectral analysis from an [OIII] selected sample of AGNs.