WISE view of changing-look AGNs: evidence for a transitional stage of AGNs
The discovery of changing-look active galactic nuclei (CLAGNs) with the significant change of optical broad emission lines (optical CLAGNs) and/or strong variation of line-of-sight column densities (X-ray CLAGNs) challenges the orientation-based AGN unification model. We explore mid-infrared (mid-IR) properties for a sample of 57 optical CLAGNs and 11 X-ray CLAGNs based on the {\it Wide-field Infrared Survey Explorer} ({\it WISE}) archive data. We find that Eddington-scaled mid-IR luminosities of both optical and X-ray CLAGNs stay just between low-luminosity AGNs (LLAGNs) and luminous QSOs. The average Eddington-scaled mid-IR luminosities for optical and X-ray CLAGNs are $\sim 0.4$\% and $\sim 0.5$\%, respectively, which roughly correspond the bolometric luminosity of transition between a radiatively inefficient accretion flow (RIAF) and Shakura-Sunyaev disk (SSD). We estimate the time lags of the variation in the mid-IR behind that in the optical band for 13 CLAGNs with strong mid-IR variability, where the tight correlation between the time lag and the bolometric luminosity ($\tau - L$) for CLAGNs roughly follows that found in the luminous QSOs.