2022A&A...664A..46A

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.