Localizing narrow Fe Kα emission within bright AGN^⋆

¹ Instituto de Astrofísica and Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
e-mail: carolina.p.andonie@durham.ac.uk
² Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago, Chile
³ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁴ Space Science Institute, 4750 Walnut Street, Suite, 205, Boulder, CO 80301, USA
⁵ Instituto de Física y Astronomía, Universidad de Valparaíso, Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile
⁶ Department of Astronomy and Astrophysics 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA
⁷ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
⁸ Department of Physics, 104 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
⁹ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
¹⁰ Department of Physics and Astronomy, University of Southern California, Los Angeles, CA 90089, USA
¹¹ Department of Astronomy, Columbia University, New York, NY 10027, USA
¹² Eureka Scientific, 2452 Delmer Street Suite 100, Oakland, CA 94602-3017, USA
¹³ Núcleo de Astronomía de la Facultad de Ingeniería, Universidad Diego Portales, Av. Ejéercito Libertador 441, Santiago 22, Chile
¹⁴ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
¹⁵ George Mason University, Department of Physics & Astronomy, MS 3F3, 4400 University Drive, Fairfax, VA 22030, USA

Received: 18 October 2021
Accepted: 12 April 2022

Abstract

Context. The 6.4 keV Fe Kα emission line is a ubiquitous feature in X-ray spectra of active galactic nuclei (AGN), and its properties track the interaction between the variable primary X-ray continuum and the surrounding structure from which it arises.

Aims. We clarify the nature and origin of the narrow Fe Kα emission using X-ray spectral, timing, and imaging constraints, plus possible correlations to AGN and host galaxy properties, for 38 bright nearby AGN (z < 0.5) from the Burst Alert Telescope AGN Spectroscopic Survey.

Methods. Modeling Chandra and XMM-Newton spectra, we computed line full-width half-maxima (FWHMs) and constructed Fe Kα line and 2–10 keV continuum light curves. The FWHM provides one estimate of the Fe Kα emitting region size, R_FeKα, assuming virial motion. A second estimate comes from comparing the degree of correlation between the variability of the continuum and line-only light curves, compared to simulated light curves. Finally, we extracted Chandra radial profiles to place upper limits on R_FeKα.

Results. For 90% (21/24) of AGN with FWHM measurements, R_FeKα is smaller than the fiducial dust sublimation radius, R_sub. From timing analysis, 37 and 18 AGN show significant continuum and Fe Kα variability, respectively. Despite a wide range of variability properties, the constraints on the Fe Kα photon reprocessor size independently confirm that R_FeKα is smaller than R_sub in 83% of AGN. Finally, the imaging analysis yields loose upper limits for all but two sources; notably, the Circinus Galaxy and NGC 1068 show significant but subdominant extended Fe Kα emission out to ∼100 and ∼800 pc, respectively.

Conclusions. Based on independent constraints, we conclude that the majority of the narrow Fe Kα emission in typical AGN predominantly arises from regions smaller than and presumably inside R_sub, and thus it is associated either with the outer broad line region or outer accretion disk. However, the large diversity of continuum and narrow Fe Kα variability properties are not easily accommodated by a universal scenario.