Abstract

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.