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

The discovery of changing-look active galactic nuclei (CL AGNs), with appearance and disappearance of broad emission lines and/or with strong variation of line-of-sight column density within a few years, challenges the AGN unification model. We explore the physical mechanisms based on the X-ray spectral evolution for a sample of 15 CL AGNs. We find that the X-ray photon index, $\mathrm{\Gamma }$, and Eddington-scaled X-ray luminosity, $L_{\rm 2-10 keV}/L_{\rm Edd}$,follownegativeandpositivecorrelationswhen$L_{\rm 2-10 keV}/L_{\rm Edd}$islowerandhigherthanacriticalvalueof$\sim 10^{-3}$. This different X-ray spectral evolution is roughly consistent with the prediction of the accretion-mode transition (e.g., clumpy cold gas or cold disk to advection dominated accretion flow, or vice visa). With quasi-simultaneous X-ray and optical spectrum observations within one year, we find that the CL AGNs observed with and without broad emission lines stay in the positive and negative part of the $\mathrm{\Gamma }-L_{2-10\mathrm{k}\mathrm{e}\mathrm{V}}\mathrm{/}{L}_{\mathrm{E}\mathrm{d}\mathrm{d}}$ correlation respectively. Our result suggest that the change of the accretion mode may be the physical reason for the CL AGNs.