Explaining changing-look AGN with state transition triggered by rapid
  mass accretion rate drop
  We model the broadband (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 $\sim 8$ years. The brightest spectrum, with Eddington ratio
$L/L_{\rm Edd}\sim 0.08$, 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 $L/L_{\rm Edd} \sim 0.006$, 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 $L/L_{\rm Edd} \sim
0.02$, 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. Nonetheless, 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 timescales even further. We suggest that all
changing-look AGNs are similarly associated with the state transition at
$L/L_{\rm Edd} \sim$ a few percent.
