Ubiquity and magnitude of large FeK α equivalent widths in AGN extended regions
Abstract
Narrow Fe K α fluorescent emission lines arising at ∼kpc -scale separations from the nucleus have only been detected in a few active galactic nuclei (AGN). The detections require that the extended line emission be spatially resolved and sufficiently bright. Compared to narrow Fe K α lines arising closer to the nucleus, they have much lower fluxes but show substantially larger equivalent widths, EWFeK α . We show that, in the optically thin limit, a purely analytical argument naturally predicts large, EWFeK α ∼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 EWFeK α 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 EWFeK α values reported for local AGN, for which the line is detected in extended, up to ∼kpc -scale , regions. We argue that large EWFeK α 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 K α emission.
- Publication:
-
Physical Review D
- Pub Date:
- December 2023
- DOI:
- 10.1103/PhysRevD.108.123037
- arXiv:
- arXiv:2312.00124
- Bibcode:
- 2023PhRvD.108l3037T
- Keywords:
-
- Astrophysics - High Energy Astrophysical Phenomena;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Accepted for publication in Physical Review D. 9 pages, 6 figures