Testing colour–magnitude pattern as a method in the search for changing-look AGNs

ABSTRACT We develop a simple method to search for changing-look (CL) active galactic nucleus (AGN) candidates, and conduct a test run. In this method, optical variations of AGNs are monitored and CL-AGNs may appear to have a pattern of being bluer when in brightening flare-like events. Applying this method, previously classified type 2 AGNs that show the bluer-when-brighter (BWB) pattern are selected. Among more than ten thousands type 2 AGNs classified in the Sloan Digital Sky Survey (SDSS), we find 73 candidates with possibly the strongest BWB pattern. We note that 13 of them have previously been reported as CL-AGNs. We have observed nine candidates, and found that five among them showed the CL transition from type 2 to type 1. In addition, we also test extending the selection to previously classified type 1 AGNs in the SDSS by finding sources with a possible redder-when-brighter pattern, but none of the three sources observed by us is found to show the transition from type 1 to type 2. We discuss the variation properties in both the success and failure cases, and plan to observe more candidates selected with the method. From the observational results, a detailed comparison between the CL-AGNs and none CL-AGNs will help quantitatively refine the selection criteria and in turn allow us to configure the general properties of CLAGNs.