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

Abstract

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, $L_{2-10keV}/L_{\mathrm{Edd}}$, follow negative and positive correlations when $L_{2-10keV}/L_{\mathrm{Edd}}$ 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 $\Gamma -L_{2-10keV}/L_{\mathrm{Edd}}$ correlation respectively. Our result suggest that the change of the accretion mode may be the physical reason for the CL AGNs.

16 pages, 3 figures and one table, submitted to ApJ, and comments are welcomed

References in corpus (1)

• X-ray spectral shape variations in changing-look Seyfert galaxy SDSSJ155258+273728

Cited by in corpus (3)

• Changing-look active galactic nuclei: close binaries of supermassive black holes in action
• The Eddington ratio-dependent changing look events in NGC 2992
• The effects of large-scale magnetic fields on the model for repeating changing-look AGNs