SIMBAD references

Changing-look active galactic nuclei (CLAGNs) are a unique population of AGNs that exhibit the appearance (turn-on) or disappearance (turn-off) of broad emission lines. This study aims to explore the intrinsic mechanisms of CLAGNs by investigating their photometric variability using data from the Zwicky Transient Facility (ZTF), which has provided high-cadence observations over the past 6 yr. By visual inspections, we construct a sample of 152 CLAGNs from the literature, all of which show spectral transitions and large optical variability in their ZTF light curves. By analyzing 90 of these CLAGNs and the control samples of Type 1 AGNs, Type 2 AGNs, and extremely variable quasars (EVQs), matched in redshift (0.2<z < 0.8) and supermassive black hole mass, we compare the color variability, structure function (SF), and variability metric σ_QSO, which quantifies how closely the light curves resemble a damped random walk model. We find that while CLAGNs and EVQs differ from typical Type 1 and Type 2 AGNs in bolometric luminosity and Eddington ratio, the on- and off-state CLAGNs share similar variability patterns with the overall CLAGN population, and distinct from EVQs and Type 1 and Type 2 AGNs. This suggests that “on” and “off” CLAGNs are not simply equivalent to Type 1 and Type 2 AGNs, respectively. Instead of undergoing genuine transitions between two AGN types, CLAGNs may inhabit a critical state where moderate fluctuations in the accretion rate lead to temporary spectral changes.