Explaining changing-look AGN with state transition triggered by rapid mass accretion rate drop

We model the broad-band (optical/UV and X-ray) continuum spectrum of the 'changing-look' active galactic nucleus (AGN) Mrk 1018, as it fades from Seyfert 1 to 1.9 in similar to 8 years. The brightest spectrum, with Eddington ratio L/L-Edd similar to 0.08 has a typical type 1 AGN continuum, with a strong 'soft X-ray excess' spanning between the UV and soft X-rays. The dimmest spectrum, at L/L-Edd similar to 0.006, is very different in shape as well as luminosity, with the soft excess dropping by much more than the hard X-rays. The soft X-ray excess produces most of the ionizing photons, so its dramatic drop leads to the disappearance of the broad-line region, driving the 'changing-look' phenomena. This spectral hardening appears similar to the soft-to-hard state transition in black hole binaries at L/L-Edd similar to 0.02, where the inner disc evaporates into an advection dominated accretion flow, while the overall drop in luminosity appears consistent with the hydrogen ionization disc instability. None the less, both processes happen much faster in Mrk 1018 than predicted by disc theory. We critically examine scaling from Galactic binary systems and show that a major difference is that radiation pressure should be much more important in AGNs, so that the sound speed is much faster than expected from the gas temperature. Including magnetic pressure to stabilize the disc shortens the time-scales even further. We suggest that all changing-look AGNs are similarly associated with the state transition at L/L-Edd similar to a few per cent.