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, $\Gamma$, and Eddington-scaled X-ray luminosity, $L_{\rm 2-10
keV}/L_{\rm Edd}$, follow negative and positive correlations when $L_{\rm 2-10
keV}/L_{\rm Edd}$ is lower and higher than a critical value of $\sim 10^{-3}$.
This different X-ray spectral evolution is roughly consistent with 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 $\Gamma-L_{\rm 2-10 keV}/L_{\rm Edd}$ correlation
respectively. Our result suggest that the change of the accretion mode may be
the physical reason for the CL AGNs.
