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Changing-look active galactic nuclei

Nature Astronomy volume 7, pages 1282–1294 (2023)Cite this article

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Abstract

Active galactic nuclei (AGNs) are known to show flux variability over all observable timescales and across the entire electromagnetic spectrum. Over the past decade, a growing number of sources have been observed to show dramatic flux and spectral changes, in both the X-ray and the optical/ultraviolet regimes. Such events, commonly described as ‘changing-look AGNs’, can be divided into two well-defined classes. Changing-obscuration objects show strong variability of the line-of-sight column density, mostly associated with clouds or outflows eclipsing the central engine of the AGN. Changing-state AGNs are instead objects in which the continuum emission and broad emission lines appear or disappear, and are typically triggered by strong changes in the accretion rate of the supermassive black hole. Here we review our current understanding of these two classes of changing-look AGNs, and discuss open questions and future prospects.

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Fig. 1: Spectral transitions in different types of CL-AGNs.
Fig. 2: X-ray spectra and NH variations of changing-obscuration events.
Fig. 3: The cometary-shaped absorbers in NGC 1365.
Fig. 4: Light curves and spectra of CS-AGNs showing an emergent BLR.
Fig. 5: X-ray spectra and light curve of CS-AGNs.

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Acknowledgements

C.R. acknowledges support from the Fondecyt Regular (grant 1230345) and ANID BASAL (project FB210003). B.T. acknowledges support from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation programme (grant agreement 950533) and from the Israel Science Foundation, (grant 1849/19). We thank G. Zeltyn for his help with Fig. 1.

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  1. Instituto de Estudios Astrofísicos, Universidad Diego Portales, Santiago, Chile

    Claudio Ricci

  2. Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, China

    Claudio Ricci

  3. School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel

    Benny Trakhtenbrot

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C.R. and B.T. devised, together, the concept and structure of the Review.

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Ricci, C., Trakhtenbrot, B. Changing-look active galactic nuclei. Nat Astron 7, 1282–1294 (2023). https://doi.org/10.1038/s41550-023-02108-4

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