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Localizing narrow Fe Kα emission within bright AGN

Andonie, Carolina; Bauer, Franz E.; Carraro, Rosamaria; Arévalo, Patricia; Alexander, David M.; Brandt, William N.; Buchner, Johannes; He, Adam; Koss, Michael J.; Ricci, Claudio; Salinas, Vicente; Solimano, Manuel; Tortosa, Alessia; Treister, Ezequiel

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Franz E. Bauer

Rosamaria Carraro

Patricia Arévalo

William N. Brandt

Johannes Buchner

Adam He

Michael J. Koss

Claudio Ricci

Vicente Salinas

Manuel Solimano

Alessia Tortosa

Ezequiel Treister


Context. The 6.4 keV Fe Kα 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 Kα 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 Kα line and 2–10 keV continuum light curves. The FWHM provides one estimate of the Fe Kα emitting region size, RFeKα, 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 RFeKα. Results. For 90% (21/24) of AGN with FWHM measurements, RFeKα is smaller than the fiducial dust sublimation radius, Rsub. From timing analysis, 37 and 18 AGN show significant continuum and Fe Kα variability, respectively. Despite a wide range of variability properties, the constraints on the Fe Kα photon reprocessor size independently confirm that RFeKα 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 Kα emission out to ∼100 and ∼800 pc, respectively. Conclusions. Based on independent constraints, we conclude that the majority of the narrow Fe Kα 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 Kα variability properties are not easily accommodated by a universal scenario.


Andonie, C., Bauer, F. E., Carraro, R., Arévalo, P., Alexander, D. M., Brandt, W. N., …Treister, E. (2022). Localizing narrow Fe Kα emission within bright AGN. Astronomy & Astrophysics, 664,

Journal Article Type Article
Acceptance Date Apr 12, 2022
Online Publication Date Aug 4, 2022
Publication Date 2022
Deposit Date Sep 15, 2022
Publicly Available Date Sep 15, 2022
Journal Astronomy & Astrophysics
Print ISSN 0004-6361
Electronic ISSN 1432-0746
Publisher EDP Sciences
Peer Reviewed Peer Reviewed
Volume 664
Public URL


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