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Changing-look active galactic nuclei (CLAGNs) show the disappearance andreappearance of broad emission lines in a few years, which challenges theorientation-based AGN unification model. We reduce the X-ray data for fivewell-studied CLAGNs that show a strong change in broad emission lines in thepast several decades. We find that the X-ray photon index, $\Gamma$, and theEddington-scaled X-ray luminosity, $L_{\rm 2-10 keV}/L_{\rm Edd}$, normallyfollow negative and positive correlations when the Eddington ratio is lower andhigher than a critical value of $\sim 10^{-3}$. We find that the CLAGNsobserved with broad H$\beta$ emission lines stay in the positive part of the$\Gamma-L_{\rm 2-10 keV}/L_{\rm Edd}$ correlation, while the broad H$\beta$lines become weak or disappear in the anticorrelation part of the$\Gamma-L_{\rm 2-10 keV}/L_{\rm Edd}$ correlation, which suggests that theevolution of the broad lines should be correlated with the evolution of theunderlying accretion process. We further find that the CLAGNs are consistentwith the other different types of AGNs in the $L_{\rm bol}-L_{\rm bol}/L_{\rmEdd}$ correlation. These results support that the CLAGNs are belong to aspecial stage of AGNs with a bolometric Eddington ratio $\sim$1\%, where thebroad emission lines are easily affected by the strong variation in ionizationluminosity that is caused by the transition of accretion modes.

Coevolution of broad emission lines and X-ray spectrum in changing-look AGNs