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Large Fe Kα Equivalent Widths in AGN Extended Regions: Analytical Basis and Monte Carlo Simulations

Presentation #102.02 in the session AGN Posters.

Published onMay 03, 2024
Large Fe Kα Equivalent Widths in AGN Extended Regions: Analytical Basis and Monte Carlo Simulations

Narrow Fe Kα fluorescent emission lines arising at ~kpc-scale separations from the nucleus have been detected with Chandra in a few AGN where the emission is 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 a purely analytical argument naturally predicts the EWFeKα~1 keV values seen for such lines, regardless of the details of equivalent hydrogen column density, NH, or reprocessor geometry, if optically-thin conditions are assumed. Our Monte Carlo simulations further corroborate this result and show that the simple analytic EWFeKα prescription holds up to higher NH approaching the Compton-thick regime. 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. Large-scale molecular material, now routinely being detected with the Atacama Large Millimeter/Submillimeter Array (ALMA), may act as the extended X-ray scattering reprocessor giving rise to this extended emission.

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