# Discovery of A Mg II Changing-look AGN and its Implications for a Unification Sequence of Changing-look AGNs

## TL;DR

This paper reports the discovery of the first unambiguous Mg II changing-look AGN, SDSS J152533.60+292012.1, revealing new insights into the temporal evolution and unification of changing-look AGNs through systematic spectral analysis.

## Contribution

It presents the first confirmed Mg II changing-look AGN and constructs a CL sequence incorporating Mg II and Hβ AGNs, advancing understanding of AGN variability.

## Key findings

- Discovered the first unambiguous Mg II CL AGN turning off within 286 days.
- Constructed a CL sequence combining Mg II and Hβ AGNs based on spectral evolution.
- Identified two candidate Mg II turn-on CL AGNs and a large sample of Mg II emitters.

## Abstract

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.