Searching for Changing-look AGN Candidates through Optical and Mid-infrared Variability

Abstract Changing-look (CL) active galactic nuclei (AGNs) are an extremely rare type of AGNs, characterized by the appearance (turn-on) or disappearance (turn-off) of broad emission lines within months to years. We construct a sample of 211 CL AGN candidates by systematically searching the Sloan Digital Sky Survey spectra classified as GALAXY , but with the Zwicky Transient Facility and Near-Earth Object Wide-Field Infrared Explorer mid-infrared light curves showing significant variability, at the redshift range from 0.1 to 0.75. To examine the success rate of this method, we cross-match the candidate sample with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope and the Dark Energy Spectroscopic Instrument for repeat spectra, finding that the success rate of this method is 50%. By incorporating previously reported CL AGNs, a total of 48 CL AGNs have been identified within our sample, including two recurrent CL AGNs newly confirmed in this work. Our results suggest that the typical rest-frame timescale of CL behavior is from 1.7 to 18.1 yr, which is inconsistent with the obscuration model, and the Eddington ratios of CL AGNs are about 0.01 in the bright state. Additionally, our CL AGN sample follows the M BH – σ ⋆ relation of quiescent galaxies. In the future, we expect to detect more galaxies turning into Type 1 AGNs in this CL AGN candidate sample.