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The discovery of changing-look active galactic nuclei (CL AGNs), withappearance and disappearance of broad emission lines and/or with strongvariation of line-of-sight column density within a few years, challenges theAGN unification model. We explore the physical mechanisms based on the X-rayspectral evolution for a sample of 15 CL AGNs. We find that the X-ray photonindex, $\Gamma$, and Eddington-scaled X-ray luminosity, $L_{\rm 2-10keV}/L_{\rm Edd}$, follow negative and positive correlations when $L_{\rm 2-10keV}/L_{\rm Edd}$ is lower and higher than a critical value of $\sim 10^{-3}$.This different X-ray spectral evolution is roughly consistent with theprediction of the accretion-mode transition (e.g., clumpy cold gas or cold diskto advection dominated accretion flow, or vice visa). With quasi-simultaneousX-ray and optical spectrum observations within one year, we find that the CLAGNs observed with and without broad emission lines stay in the positive andnegative part of the $\Gamma-L_{\rm 2-10 keV}/L_{\rm Edd}$ correlationrespectively. Our result suggest that the change of the accretion mode may bethe physical reason for the CL AGNs.

Evidence for changing-look AGNs is caused by change of accretion mode