Localizing narrow Fe K$\alpha$ emission within bright AGN

TL;DR

This study investigates the origin and size of narrow Fe Kα emission in bright AGN using spectral, timing, and imaging data, revealing it mostly arises from regions smaller than the dust sublimation radius, likely the outer broad line region or accretion disk.

Contribution

It provides the first comprehensive analysis combining spectral, timing, and imaging constraints to localize Fe Kα emission regions in AGN, confirming they are generally within the dust sublimation radius.

Findings

Most Fe Kα emission regions are smaller than the dust sublimation radius.

The Fe Kα emitting region is often inside the broad line region or accretion disk.

Extended Fe Kα emission is significant only in a few sources like Circinus and NGC 1068.

Abstract

The 6.4 keV Fe Ka emission line is a ubiquitous feature in X-ray spectra of AGN, and its properties track the interaction between the variable primary X-ray continuum and the surrounding structure from which it arises. We clarify the nature and origin of the narrow Fe Ka 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 BAT AGN Spectroscopic Survey. Modeling Chandra and XMM-Newton spectra, we computed line full-width half-maxima (FWHMs) and constructed Fe Ka line and 2-10 keV continuum light curves. The FWHM provides one estimate of the Fe Ka emitting region size, RFeKa, 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 RFeKa. We found that for 90% (21/24) of AGN with FWHM measurements, RFeKa is smaller than the fiducial dust sublimation radius, Rsub. Despite a wide range of variability properties, the constraints on the Fe Ka photon reprocessor size independently confirm that RFeKa is smaller than Rsub 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 Ka emission out to $\sim$100 and $\sim$800 pc, respectively. Based on independent constraints, we conclude that the majority of the narrow Fe Ka emission in typical AGN predominantly arises from regions smaller than and presumably inside Rsub, 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 Ka variability properties are not easily accommodated by a universal scenario.