NuSTAR Observations of 52 Compton-thick Active Galactic Nuclei Selected by the Swift/BAT All-sky Hard X-Ray Survey

TL;DR

This study systematically analyzes the X-ray spectra of 52 candidate Compton-thick AGNs using multiple observatories, revealing differences in torus properties and suggesting intrinsic structural variations between obscured and less obscured AGNs.

Contribution

First broadband X-ray spectral analysis of 52 CTAGN candidates using the XClumpy torus model, providing new insights into their obscuration properties and torus structure.

Findings

28 out of 52 are confirmed as Compton-thick AGNs

The average CTAGN torus covering factor is approximately 36%

Differences in torus structure suggest intrinsic variations between CTAGN and less obscured AGN

Abstract

We present the systematic broadband X-ray spectral analysis of 52 Compton-thick ($24 \leq \log N_{\mathrm{H}}^{\mathrm{LOS}}/\mathrm{cm}^{-2}$) active galactic nucleus (CTAGN) candidates selected by the Swift/BAT all-sky hard X-ray survey observed with Chandra, XMM--Newton, Swift/XRT, Suzaku, and NuSTAR. The XMM--Newton data of 10 objects and the NuSTAR data of 15 objects are published for the first time. We use an X-ray spectral model from a clumpy torus (XClumpy) to determine the torus properties. As a result, the hydrogen column density along the line of sight $N_{\mathrm{H}}^{\mathrm{LOS}}$ obtained from the XClumpy model indicate that 24 objects are Compton-thin AGNs and 28 objects are Compton-thick AGNs in the 90\% confidence interval. The main reason is the difference in the torus model applied. The hydrogen column density along the equatorial direction $N_{\mathrm{H}}^{\mathrm{Equ}}$ of CTAGNs inferred from the XClumpy model is larger than that of less obscured AGNs. The Compton-thin torus covering factor $C_{22}$ obtained from the XClumpy model is consistent with that of Ricci et al. (2017) in the low Eddington ratio ($\log R_{\mathrm{Edd}} \leq -1.0$), whereas $C_{22}$ inferred from the XClumpy model is larger than that of Ricci et al. (2017) in the high Eddington ratio ($-1.0 \leq \log R_{\mathrm{Edd}}$). The average value of the Compton-thick torus covering factor $C_{24}$ obtained from the XClumpy model is $36_{-4}^{+4}$\%. This value is larger than that of Ricci et al. (2015) ($C_{24} \simeq 27_{-4}^{+4}$\%) based on the assumption that all AGNs have intrinsically the same torus structure. These results suggest that the structure of CTAGN may be intrinsically different from that of less obscured AGN.