Determining the covering factor of Compton-thick active galactic nuclei with NuSTAR

Abstract

The covering factor of Compton-thick obscuring material associated with the torus in active galactic nuclei (AGN) is at present best understood through the fraction of sources exhibiting Compton-thick absorption along the line of sight ($N_H>1.5\times 10^{24}$ cm${}^{-2}$) in the X-ray band, which reveals the average covering factor. Determining this Compton-thick fraction is difficult however, due to the extreme obscuration. With its spectral coverage at hard X-rays ($>$10 keV), NuSTAR is sensitive to the AGN covering factor since Compton scattering of X-rays off optically thick material dominates at these energies. We present a spectral analysis of 10 AGN observed with NuSTAR where the obscuring medium is optically thick to Compton scattering, so called Compton-thick (CT) AGN. We use the torus models of Brightman & Nandra which predict the X-ray spectrum from reprocessing in a torus and include the torus opening angle as a free parameter and aim to determine the covering factor of the Compton-thick gas in these sources individually. Across the sample we find mild to heavy Compton-thick columns, with $N_H$ measured from $10^{24}-10^{26}$ cm${}^{-2}$, and a wide range of covering factors, where individual measurements range from 0.2-0.9. We find that the covering factor, $f_c$, is a strongly decreasing function of the intrinsic 2-10 keV luminosity, $L_X$, where $f_c=(-0.41\pm 0.13)$log${}_{10}$($L_X$/erg s${}^{-1}$)$+18.31\pm 5.33$, across more than two orders of magnitude in $L_X$ (10${}^{41.5}-10^{44}$ erg s${}^{-1}$). The covering factors measured here agree well with the obscured fraction as a function of $L_X$ as determined by studies of local AGN with $L_X>10^{42.5}$ erg s${}^{-1}$.

Accepted for publication in ApJ