Extreme color–magnitude variability: Connection to changing-look active galactic nuclei

Context. Changing-look active galactic nuclei (CL-AGNs) challenge the unified model of AGNs and offer key insights into the physics of the accretion processes of super-massive black holes. While systematic spectroscopic comparisons have successfully identified large samples of CL-AGNs, photometric selection based on variability features provides an efficient alternative. Aims. We used the color–magnitude (CM) variability pattern method to continue our identification of the CL transition in AGNs. The resulting CL-AGN sample can help improve understanding of this phenomenon. Methods. The CM variability pattern method utilizes the slope ( k ) of the CM variations to identify strong “bluer-when-brighter” behavior, while the variation amplitudes in optical and mid-infrared (MIR) bands are also considered. The candidates selected from the Type-2 AGNs given in the Sloan Digital Sky Survey catalog were spectroscopically observed by us using the 3.6-m Devasthal Optical Telescope and the 2-m Himalayan Chandra Telescope. Results. We successfully confirm seven turn-on CL-AGNs among 12 candidates in this work. Compared with both the general AGN populations and the spectroscopically identified CL-AGN sample, these CL-AGNs show larger optical and MIR variations and k values. The extreme CM variabilities of these sources (with optical magnitude changes > 0.9) occurred recently. For four sources, flare-like brightening episodes were temporally associated with turn-on transitions within 3–7 years, suggesting that these flares may trace short-timescale accretion enhancement, central brightening, and broad-line region re-illumination. In addition, the estimated Eddington ratios for the confirmed CL-AGNs appear to cluster around a critical value of λ Edd  ∼ 0.01. Conclusions. The extreme CM variability serves as a highly efficient criterion for finding CL-AGNs. The properties of the CL-AGNs thus found suggest that they may represent AGNs at a pivotal state, which likely occurs in CL transitions due to enhanced accretion activity, while the cause of the accretion activity, which was determined to have a timescale of several years in this work, remains to be investigated further.