Localizing Narrow Fe K Alpha Emission Within Bright Agn

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