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 (NH>1.5×1024 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 NH measured from 1024-1026 cm-2, and a wide range of covering factors, where individual measurements range from 0.2-0.9. We find that the covering factor, fc, is a strongly decreasing function of the intrinsic 2-10 keV luminosity, LX, where fc=(-0.410.13)log10(LX/erg s-1)+18.315.33, across more than two orders of magnitude in LX (1041.5-1044 erg s-1). The covering factors measured here agree well with the obscured fraction as a function of LX as determined by studies of local AGN with LX>1042.5 erg s-1.