Coevolution of Broad Emission Lines and X-Ray Spectrum in Changing-look AGNs

Abstract

Changing-look active galactic nuclei (CLAGNs) show the disappearance and reappearance of broad emission lines in a few years, which challenges the orientation-based AGN unification model. We reduce the X-ray data for five well-studied CLAGNs that show a strong change in broad emission lines in the past several decades. We find that the X-ray photon index, Γ, and the Eddington-scaled X-ray luminosity, L 2–10 keV/L Edd, normally follow negative and positive correlations when the Eddington ratio is lower and higher than a critical value of ∼10−3. We find that the CLAGNs observed with broad Hβ emission lines stay in the positive part of the Γ–L 2–10 keV/L Edd correlation, while the broad Hβ lines become weak or disappear in the anticorrelation part of the Γ–L 2–10 keV/L Edd correlation, which suggests that the evolution of the broad lines should be correlated with the evolution of the underlying accretion process. We further find that the CLAGNs are consistent with the other different types of AGNs in the L bol–L bol/L Edd correlation. These results support that the CLAGNs belong to a special stage of AGNs with a bolometric Eddington ratio ∼1%, where the broad emission lines are easily affected by the strong variation in ionization luminosity that is caused by the transition of accretion modes.