The narrow Fe Kα line and the molecular torus in active galactic nuclei: an IR/X-ray view^⋆

¹ Department of Astronomy, University of Geneva, ch. d’Ecogia 16, 1290 Versoix, Switzerland
² Department of Astronomy, Kyoto University, Oiwake-cho, Sakyo-ku, 606-8502 Kyoto, Japan
e-mail: ricci@kusastro.kyoto-u.ac.jp
³ Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
⁴ Astronomical Observatory, Volgina 7, 11060 Belgrade, Serbia
⁵ Isaac Newton Institute of Chile, Yugoslavia Branch, Volgina 7, 11060 Belgrade, Serbia
⁶ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281-S9, 9000 Gent, Belgium
⁷ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Correo Central, Santiago, Chile
⁸ Department of Physics, Ehime University, 790-8577 Matsuyama, Japan

Received: 18 September 2013
Accepted: 24 March 2014

Abstract

The narrow component of the iron Kα is an almost ubiquitous feature in the X-ray spectra of active galactic nuclei (AGN) and is believed to originate in neutral material, possibly located in the molecular torus. This would imply a tight connection between the Fe Kα equivalent width (EW) and the physical properties of the torus. In a recent work we have shown that the decrease in the covering factor of the torus with the luminosity, as expected by luminosity-dependent unification models, would be able to explain the decrease in Fe Kα EW with the luminosity (i.e. the X-ray Baldwin effect). Recent developments in the study of the mid-IR (MIR) spectrum of AGN allow important parameters of the torus to be deduced, such as its covering factor (f_obs) and equatorial column density (N_H^T), by applying clumpy torus models. Using XMM-Newton/EPIC observations of a sample of 24 type-I AGN, we investigate the relation between the physical parameters of the torus obtained by recent MIR works and the properties of the Fe Kα line. We correct the values of the Fe Kα EW by taking the inclination angle, the photon index, the equatorial column density, and half-opening angle of the torus into account using a physical torus model of X-ray reprocessed radiation. We find that the relation between Fe Kα EW and f_obs shows a slope that is consistent with the expected value, albeit with low statistical significance. A trend that is consistent with the theoretical prediction is also found when comparing the Fe Kα EW to N_H^T. Our work seems to confirm that the bulk of the narrow Fe Kα line is produced by the same material responsible for the MIR emission.