Accretion-Mode Transition: The Driver Behind Spectral Changes in Changing-Look AGNs

The physical origin of optical changing-look AGNs (CLAGNs), characterized by the appearance or disappearance of broad emission lines, is thought to be mainly driven by the variation of the black-hole (BH) accretion rate. In this work, we explore this issue based on a sample of {224} CLAGNs with UV-to-optical continua, where the UV radiation is more sensitive to the accretion state near the BH horizon. We find that the luminosity correlation of $L_{3000}$--$L_{5100}$ at 3000$\rm \AA$ and 5100$\rm \AA$ becomes steeper at low luminosities (e.g., $L_{3000}\lesssim10^{44}\rm erg/s$), where the sources with high luminosities are roughly consistent with the prediction of a standard accretion disk. At lower luminosities, the observations are more consistent with the prediction of a truncated disk. The whole sample has a median bolometric Eddington ratio of $\sim$2.2\%, which is consistent with the critical value for state transition in X-ray binaries. Such transitions can significantly alter the UV-to-optical continuum, largely due to variations in the truncation radius, even when the change in the overall accretion rate is minimal. The deficit of ionization photons resulting from an increase in the truncation radius will lead to the weakening or disappearance of broad lines, which triggers the AGN changing-look.