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Detection of an unidentified soft X-ray emission feature in NGC 5548

Gu, Liyi; Mao, Junjie; Kaastra, Jelle S.; Mehdipour, Missagh; Pinto, Ciro; Grafton-Waters, Sam; Bianchi, Stefano; Landt, Hermine; Branduardi-Raymont, Graziella; Costantini, Elisa; Ebrero, Jacobo; Petrucci, Pierre-Olivier; Behar, Ehud; di Gesu, Laura; De Marco, Barbara; Matt, Giorgio; Mitchell, Jake A.J.; Peretz, Uria; Ursini, Francesco; Ward, Martin

Detection of an unidentified soft X-ray emission feature in NGC 5548 Thumbnail


Liyi Gu

Junjie Mao

Jelle S. Kaastra

Missagh Mehdipour

Ciro Pinto

Sam Grafton-Waters

Stefano Bianchi

Graziella Branduardi-Raymont

Elisa Costantini

Jacobo Ebrero

Pierre-Olivier Petrucci

Ehud Behar

Laura di Gesu

Barbara De Marco

Giorgio Matt

Uria Peretz

Francesco Ursini


Context. NGC 5548 is an X-ray bright Seyfert 1 active galaxy. It exhibits a variety of spectroscopic features in the soft X-ray band, in particular including the absorption by the active galactic nucleus (AGN) outflows of a broad range of ionization states, with column densities up to 1027 m−2, and having speeds up to several thousand kilometers per second. The known emission features are in broad agreement with photoionized X-ray narrow and broad emission line models. Aims. We report on an X-ray spectroscopic study using 1.1 Ms XMM-Newton and 0.9 Ms Chandra grating observations of NGC 5548 spanning two decades. The aim is to search and characterize any potential spectroscopic features in addition to the known primary spectral components that are already modeled in high precision. Methods. For each observation, we modeled the data using a global fit including an intrinsic spectral energy distribution of the AGNs and the known distant X-ray absorbers and emitters. We utilized as much knowledge from previous studies as possible. The fit residuals were stacked and scanned for possible secondary features. Results. We detect a weak unidentified excess emission feature at ∼18.4 Å (18.1 Å in the restframe). The feature is seen at > 5σ statistical significance taking the look-elsewhere effect into account. No known instrumental issues, atomic transitions, or astrophysical effects can explain this excess. The observed intensity of the possible feature seems to anticorrelate in time with the hardness ratio of the source. However, even though the variability might not be intrinsic, it might be caused by the time-variable obscuration by the outflows. An intriguing possibility is the line emission from charge exchange between a partially ionized outflow and a neutral layer in the same outflow, or in the close environment. Other possibilities, such as emission from a highly ionized component with high outflowing speed, cannot be fully ruled out.


Gu, L., Mao, J., Kaastra, J. S., Mehdipour, M., Pinto, C., Grafton-Waters, S., …Ward, M. (2022). Detection of an unidentified soft X-ray emission feature in NGC 5548. Astronomy & Astrophysics, 665, Article A93.

Journal Article Type Article
Acceptance Date Jul 16, 2022
Online Publication Date Sep 15, 2022
Publication Date 2022
Deposit Date Nov 17, 2022
Publicly Available Date Nov 17, 2022
Journal Astronomy & Astrophysics
Print ISSN 0004-6361
Electronic ISSN 1432-0746
Publisher EDP Sciences
Peer Reviewed Peer Reviewed
Volume 665
Article Number A93
Public URL


Published Journal Article (Advance online version) (1.1 Mb)

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Copyright Statement
Advance online version Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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