X-ray binary accretion states in Active Galactic Nuclei? Sensing the
  accretion disc of supermassive black holes with mid-infrared nebular lines
  Accretion states, which are universally observed in stellar-mass black holes
in X-ray binaries, are also anticipated in active galactic nuclei (AGN). This
is the case at low luminosities, when the jet-corona coupling dominates the
energy output in both populations. Previous attempts to extend this framework
to a wider AGN population have been extremely challenging due to heavy hydrogen
absorption of the accretion disc continuum and starlight contamination from the
host galaxies. We present the luminosity-excitation diagram (LED), based on the
[OIV]$_{25.9\mu m}$ and [NeII]$_{12.8\mu m}$ mid-infrared nebular line fluxes.
This tool enables to probe the accretion disc contribution to the ionising
continuum. When applied to a sample of 167 nearby AGN, the LED recovers the
characteristic q-shaped morphology outlined by individual X-ray binaries during
a typical accretion episode, allowing us to tentatively identify the main
accretion states. The soft state would include broad-line Seyferts and about
half of the Seyfert 2 population, showing highly excited gas and radio-quiet
cores consistent with disc-dominated nuclei. The hard state mostly includes
low-luminosity AGN ($\leq 10^{-3}\, \rm{L_{Edd}}$) characterised by
low-excitation radio-loud nuclei and a negligible disc contribution. The
remaining half of Seyfert 2 nuclei and the bright LINERs show low excitation at
high accretion luminosities, and could be identified with the bright-hard and
intermediate states. Their hosts show ongoing star formation in the central
kiloparsecs. We discuss the above scenario, its potential links with the galaxy
evolution picture and the possible presence of accretion state transitions in
AGN, as suggested by the growing population of changing-look quasars.
