WISE View of Changing-look Active Galactic Nuclei: Evidence for a Transitional Stage of AGNs

The discovery of changing-look active galactic nuclei (CLAGNs) with a significant change in 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 Wide-field Infrared Survey Explorer archive data. We find that Eddington-scaled mid-IR luminosities of both optical and X-ray CLAGNs stay just between those of low-luminosity AGNs and luminous QSOs. The average Eddington-scaled mid-IR luminosities for optical and X-ray CLAGNs are similar to 0.4% and similar to 0.5%, respectively, which roughly correspond to the bolometric luminosity of transition between a radiatively inefficient accretion flow and a Shakura-Sunyaev disk. 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.