Discovery of A Mg II Changing-look AGN and its Implications for a
  Unification Sequence of Changing-look AGNs
  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 ($\lesssim$ 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 AGN from other normal indicators such as broad H$\alpha$ and H$\beta$ or
blue power-law continua) from the Fourteenth Data Release of the Sloan Digital
Sky Survey. We present the discovery of the first unambiguous case of an 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 $\pm$ 26 Angstrom to being consistent with zero), together with
little optical continuum variation ($\Delta V_{\rm max-min}$ $=$ 0.17 $\pm$
0.05 mag) coevally over $\sim$ 10 years, 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. (2019).
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.
