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10 Yr Transformation of the Obscuring Wind in NGC 5548

Mehdipour, Missagh; Kriss, Gerard A.; Costantini, Elisa; Gu, Liyi; Kaastra, Jelle S.; Landt, Hermine; Mao, Junjie

10 Yr Transformation of the Obscuring Wind in NGC 5548 Thumbnail


Missagh Mehdipour

Gerard A. Kriss

Elisa Costantini

Liyi Gu

Jelle S. Kaastra

Junjie Mao


A decade ago, the archetypal Seyfert-1 galaxy NGC 5548 was discovered to have undergone major spectral changes. The soft X-ray flux had dropped by a factor of 30 while new broad and blueshifted UV absorption lines appeared. This was explained by the emergence of a new obscuring wind from the accretion disk. Here we report on the striking long-term variability of the obscuring disk wind in NGC 5548 including new observations taken in 2021–2022 with the Swift Observatory and the Hubble Space Telescope's Cosmic Origins Spectrograph. The X-ray spectral hardening as a result of obscuration has declined over the years, reaching its lowest in 2022, at which point we find the broad C iv UV absorption line to have nearly vanished. The associated narrow low-ionization UV absorption lines, produced previously when shielded from the X-rays, are also remarkably diminished in 2022. We find a highly significant correlation between the variabilities of the X-ray hardening and the equivalent width of the broad C iv absorption line, demonstrating that X-ray obscuration is inherently linked to disk winds. We derive for the first time a relation between the X-ray and UV covering fractions of the obscuring wind using its long-term evolution. The diminished X-ray obscuration and UV absorption are likely caused by an increasingly intermittent supply of outflowing streams from the accretion disk. This results in growing gaps and interstices in the clumpy disk wind, thereby reducing its covering fractions.


Mehdipour, M., Kriss, G. A., Costantini, E., Gu, L., Kaastra, J. S., Landt, H., & Mao, J. (2022). 10 Yr Transformation of the Obscuring Wind in NGC 5548. Astrophysical Journal Letters, 934(2),

Journal Article Type Article
Acceptance Date Jul 19, 2022
Online Publication Date Jul 29, 2022
Publication Date 2022
Deposit Date Sep 5, 2022
Publicly Available Date Sep 5, 2022
Journal The Astrophysical Journal Letters
Print ISSN 2041-8205
Electronic ISSN 2041-8213
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 934
Issue 2
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Published Journal Article (727 Kb)

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Copyright Statement
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

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