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

TL;DR

This study uses high-quality NuSTAR X-ray data and advanced modeling to analyze the physical and geometrical properties of AGN tori, revealing their inhomogeneity and covering factors in an unbiased sample of obscured AGN.

Contribution

It introduces a new broadband spectral analysis method with the borus02 model to characterize AGN tori and compares properties between Compton thin and thick AGN in an unbiased sample.

Findings

AGN tori have similar average column densities (~1.4×10^{24} cm^{-2}) regardless of Compton thickness.

The obscuring torus medium is significantly inhomogeneous, with differences between line-of-sight and average column densities.

The average torus covering factor is 0.67, indicating about 33% of AGN are unobscured.

Abstract

The obscuration observed in active galactic nuclei (AGN) is mainly caused by dust and 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 this work, we perform a broadband X-ray spectral analysis of a large, unbiased sample of obscured AGN (with line-of-sight column density 23$\le$log(NH)$\le$24) in the nearby universe which has high-quality archival NuSTAR data. The source spectra are analyzed using the recently developed borus02 model, which enables us to accurately characterize the physical and geometrical properties of AGN obscuring tori. We also compare our results obtained from the unbiased Compton thin AGN with those of Compton-thick AGN. We find that Compton thin and Compton-thick AGN may possess similar tori, whose average column density is Compton thick (N$\rm _{H,tor,ave}$ $\sim$1.4$\times$10$^{24}$ 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 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 AGN is $\sim$33%. We develop a new method to measure 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.