Localizing narrow Fe Kα emission within bright AGN

Context. The 6.4 keV Fe K alpha 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 K alpha 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 K alpha line and 2-10 keV continuum light curves. The FWHM provides one estimate of the Fe K alpha emitting region size, R-FeK alpha, 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-FeK alpha. Results. For 90% (21/24) of AGN with FWHM measurements, R-FeK alpha is smaller than the fiducial dust sublimation radius, R-sub. From timing analysis, 37 and 18 AGN show significant continuum and Fe K alpha variability, respectively. Despite a wide range of variability properties, the constraints on the Fe K alpha photon reprocessor size independently confirm that R-FeK alpha 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 K alpha emission out to similar to 100 and similar to 800 pc, respectively. Conclusions. Based on independent constraints, we conclude that the majority of the narrow Fe K alpha 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 K alpha variability properties are not easily accommodated by a universal scenario.

Automated summary

This study clarifies the nature and origin of the narrow Fe K alpha emission in active galactic nuclei (AGN) through X-ray spectral, timing, and imaging analysis. The majority of the narrow Fe K alpha emission is found to originate from the outer broad line region or outer accretion disk.