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Published May 20, 2015 | Published + Submitted
Journal Article Open

Determining the Covering Factor of Compton-thick Active Galactic Nuclei with NuSTAR


The covering factor of Compton-thick (CT) obscuring material associated with the torus in active galactic nuclei (AGNs) is at present best understood through the fraction of sources exhibiting CT absorption along the line of sight (N_H > 1.5 × 10^(24) cm^(−2)) in the X-ray band, which reveals the average covering factor. Determining this CT fraction is difficult, however, due to the extreme obscuration. With its spectral coverage at hard X-rays (>10 keV), Nuclear Spectroscopic Telescope Array (NuSTAR) is sensitive to the AGNs covering factor since Compton scattering of X-rays off optically thick material dominates at these energies. We present a spectral analysis of 10 AGNs observed with NuSTAR where the obscuring medium is optically thick to Compton scattering, so-called CT AGNs. We use the torus models of Brightman & Nandra that 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 CT gas in these sources individually. Across the sample we find mild to heavy CT columns, with N_H measured from 10^(24) to 10^(26) cm^(−2), and a wide range of covering factors, where individual measurements range from 0.2 to 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 ± 0.13)log_(10)(L_X/erg s^(−1))+18.31 ± 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 AGNs with L_X > 10^(42.5) erg s^(−1).

Additional Information

© 2015 The American Astronomical Society. Received 2014 November 7; accepted 2015 February 24; published 2015 May 19. This work was supported under NASA Contract No. NNG08FD60C and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software, and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). The work presented here was also based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. This research has also made use of data and software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC and the High Energy Astrophysics Division of the Smithsonian Astrophysical Observatory. Furthermore, this research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. M. Baloković acknowledges support from NASA Headquarters under the NASA Earth and Space Science Fellowship Program, grant NNX14AQ07H. D.R.B. acknowledges support from NSF award AST 1008067; P.G. acknowledges support from STFC (grant reference ST/J003697/1), M.K. acknowledges support from the Swiss National Science Foundation (SNSF) through the Ambizione fellowship grant PZ00P2 154799/1. We also acknowledge support from CONICYT-Chile grants Basal-CATA PFB-06/2007 (FEB), FONDECYT 1141218 (FEB), "EMBIGGEN" Anillo ACT1101 (FEB), Project IC120009 "Millennium Institute of Astrophysics (MAS)" of the Iniciativa Científica Milenio del Ministerio de Economía, Fomento y Turismo (FEB). Facilities: NuSTAR, XMM (EPIC)

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Published - 0004-637X_805_1_41.pdf

Submitted - 1502.07353v1.pdf


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