Discovery of an Mg ii Changing-look Active Galactic Nucleus and Its Implications for a Unification Sequence of Changing-look Active Galactic Nuclei

Changing look (CL) is a rare phenomenon of active galactic nuclei (AGNs) that exhibit emerging or disappearing broad lines accompanied by continuum variations on astrophysically short timescales ( 1 yr to a few decades). While previous studies have found Balmer-line (broad H and/or Hβ) CL AGNs, the...

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Bibliographic Details
Published in:Astrophysical journal. Letters 2019-10, Vol.883 (2), p.L44
Main Authors: Guo, Hengxiao, Sun, Mouyuan, Liu, Xin, Wang, Tinggui, Kong, Minzhi, Wang, Shu, Sheng, Zhenfeng, He, Zhicheng
Format: Article
Language:English
Subjects:
Active galactic nuclei
Active galaxies
Black hole physics
Disruption
Emitters
H alpha line
Photoionization
Power law
Red shift
Sky surveys (astronomy)
Spectral emittance
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Summary:Changing look (CL) is a rare phenomenon of active galactic nuclei (AGNs) that exhibit emerging or disappearing broad lines accompanied by continuum variations on astrophysically short timescales ( 1 yr to a few decades). While previous studies have found Balmer-line (broad H and/or Hβ) CL AGNs, the broad Mg ii line is persistent even in dim states. No unambiguous Mg ii CL AGN has been reported to date. We perform a systematic search of Mg ii CL AGNs using multi-epoch spectra of a special population of Mg ii-emitters (characterized by strong broad Mg ii emission with little evidence for AGNs from other normal indicators such as broad H and Hβ or blue power-law continua) from the Sloan Digital Sky Survey Data Release 14. We present the discovery of the first unambiguous case of a Mg ii CL AGN, SDSS J152533.60+292012.1 (at redshift z = 0.449), which is turning off within rest-frame 286 days. The dramatic diminishing of Mg ii equivalent width (from 110 26 to being consistent with zero), together with little optical continuum variation (ΔVmax−min = 0.17 0.05 mag) coevally over ∼10 yr, rules out dust extinction or a tidal disruption event. Combined with previously known Hβ CL AGNs, we construct a sequence that represents different temporal stages of CL AGNs. This CL sequence is best explained by the photoionization model of Guo et al. In addition, we present two candidate turn-on Mg ii CL AGNs and a sample of 361 Mg ii-emitters for future Mg ii CL AGN searches.
ISSN:2041-8205
2041-8213
DOI:10.3847/2041-8213/ab4138