Four Changing-Look Active Galactic Nuclei Found From Optical Variations

Abstract

We report the finding of four changing-look (CL) active galactic nuclei (AGN). We selected these sources due to their potential as interesting targets when considering their relatively-large optical flux variations and related mid-infrared flux variations. To identify their CL feature, we use archival spectra from the Sloan Digital Sky Survey (SDSS) taken at least 8 years ago as well as spectra taken recently from the Transient Name Server (TNS) and with the 2.4-m LiJiang telescope (LJT). We study the sources' spectral changes by fitting and determining the H$_\alpha$ and H$_\beta$ components and verify their CL behavior. When comparing the TNS and/or LJT spectra to the SDSS ones, all four sources showed the appearance of a broad or a stronger broad H$_\alpha$ component and a relatively weak broad H$_\beta$ component. As two of the four sources are established to have a brighter-and-bluer feature in the photometric data, during the time periods in which the TNS and LJT spectra were taken, this feature likely accompanied the turn-on of the broad components. Thus, we suggest that this brighter-and-bluer feature can be used as a criterion for efficiently finding CL sources among previously spectroscopically classified type 2 AGN, such as from among the sources provided by the SDSS.