Determination of the covering factor of Compton thick AGN with NuSTAR and its appliction to X-ray survey data
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>1.5x1024 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 from optically thick material dominates at these energies. We present a spectral analysis of 10 AGN observed with NuSTAR where the obscuringmedium 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 , across more than two orders of magnitude in LX. We also discuss results where this analysis has been applied to X-ray survey data of higher redshift AGN in order to examine how the covering factor changes with redshift.
- Publication:
-
IAU General Assembly
- Pub Date:
- August 2015
- Bibcode:
- 2015IAUGA..2255331B