The properties of the AGN torus as revealed from a set of unbiased NuSTAR observations

¹ Department of Physics & Astronomy, Clemson University, Clemson, SC 29634, USA
e-mail: xiuruiz@clemson.edu
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ INAF-Osservatorio Astronomico di Bologna, Via Piero Gobetti, 93/3, 40129 Bologna, Italy

Received: 7 January 2021
Accepted: 30 March 2021

Abstract

The obscuration observed in active galactic nuclei (AGNs) is mainly caused by dust and gas distributed in a torus-like structure surrounding the supermassive black hole. However, the properties of the obscuring torus of an AGN in X-ray have not yet been fully investigated because of a lack of high-quality data and proper models. In this work, we perform a broadband X-ray spectral analysis of a large, unbiased sample of obscured AGNs (with line-of-sight column density 23 ≤ log(N_H) ≤ 24) in the nearby Universe for which high-quality archival NuSTAR data are available. We analyzed the source spectra using the recently developed borus02 model, which enables us to accurately characterize the physical and geometrical properties of AGN-obscuring tori. We compare our results obtained from the unbiased Compton-thin AGNs with those of Compton-thick AGNs. We find that Compton-thin and Compton-thick AGNs may possess similar tori, whose average column density is Compton thick (N_{H, tor, ave} ≈ 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 obscuring torus medium is significantly inhomogeneous, with the torus average column densities being 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 c_f = 0.67, suggesting that the fraction of unobscured AGNs is ∼33%. We developed a new method to measure the intrinsic line-of-sight column density distribution of AGNs in the nearby Universe, and find the results to be in good agreement with constraints from recent population synthesis models.