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 (less than or similar to 1 yr to a few decades). While previous studies have found Balmer-line (broad H alpha and/or H beta) 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 alpha and H beta 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 angstrom to being consistent with zero), together with little optical continuum variation (Delta Vmax-min = 0.17 +/- 0.05 mag) coevally over similar to 10 yr, rules out dust extinction or a tidal disruption event. Combined with previously known H beta 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.