The Ubiquity and Magnitude of Large FeKα Equivalent Widths in AGN Extended Regions

Abstract

Narrow Fe Kα fluorescent emission lines arising at ∼kpc-scale separations from the nucleus have only been detected in a few AGN. The detections require that the extended line emission be spatially resolved and sufficiently bright. Compared to narrow Fe Kα lines arising closer to the nucleus, they have much lower fluxes but show substantially larger equivalent widths, EWFₑₖα. We show that, in the optically-thin limit, a purely analytical argument naturally predicts large, EWFₑₖα∼1 keV, values for such lines, regardless of the details of equivalent hydrogen column density, Nₕ, or reprocessor geometry. Monte Carlo simulations corroborate this result and show that the simple analytic EWFₑₖα prescription holds up to higher Nₕ approaching the Compton-thick regime. We compare to Chandra observations from the literature and discuss that our results are consistent with the large EWFₑₖα values reported for local AGN, for which the line is detected in extended, up to ∼kpc-scale, regions. We argue that large EWFₑₖα from kpc-scale regions in AGN should be ubiquitous, because they do not depend on the absolute luminosity of the central X-ray source, and are measured only against the scattered continuum. We predict values to be of the order of ∼1 keV or larger, even for covering factors ≪1, and for arbitrarily small column densities. We propose that the large-scale molecular material that is now routinely being detected with the Atacama Large Millimeter/Submillimeter Array (ALMA) may act as an extended X-ray scattering reprocessor giving rise to ∼kpc-scale Fe Kα emission.