Compton-thick AGNs in the NuSTAR Era. III. A Systematic Study of the Torus Covering Factor

Abstract

We present the analysis of a sample of 35 candidate Compton-thick active galactic nuclei (AGNs) selected in the nearby universe (average redshift 〈z〉 ~ 0.03) with the Swift-BAT 100-month survey. All sources have available NuSTAR data, thus allowing us to constrain with unprecedented quality important spectral parameters such as the obscuring torus line-of-sight column density (N_(H,z)), the average torus column density (N H,tor), and the torus covering factor (f_c ). We compare the best-fit results obtained with the widely used MYTorus (Murphy & Yaqoob 2009) model with those of the recently published borus02 model (Baloković et al. 2018) used in the same geometrical configuration of MYTorus (i.e., with f_c = 0.5). We find a remarkable agreement between the two, although with increasing dispersion in N_(H,z) moving toward higher column densities. We then use borus02 to measure f_c . High-f c sources have, on average, smaller offset between N_(H,z) and N_(H,tor) than low-f_c ones. Therefore, low f c values can be linked to a "patchy torus" scenario, where the AGN is seen through an overdense region in the torus, while high-f c objects are more likely to be obscured by a more uniform gas distribution. Finally, we find potential evidence of an inverse trend between f c and the AGN 2–10 keV luminosity, i.e., sources with higher f c values have on average lower luminosities.