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Following a tidal disruption event (TDE), the accretion rate can evolve fromquiescent to near-Eddington levels and back over months - years timescales.This provides a unique opportunity to study the formation and evolution of theaccretion flow around supermassive black holes (SMBHs). We present two years ofmulti-wavelength monitoring observations of the TDE AT2018fyk at X-ray, UV,optical and radio wavelengths. We identify three distinct accretion states andtwo state transitions between them. These appear remarkably similar to thebehaviour of stellar-mass black holes in outburst. The X-ray spectralproperties show a transition from a soft (thermal-dominated) to a hard(power-law dominated) spectral state around L$_{\rm bol} \sim $few $ \times10^{-2}$ L$_{\rm Edd}$, and the strengthening of the corona over time$\sim$100--200 days after the UV/optical peak. Contemporaneously, the spectralenergy distribution (in particular, the UV-to-X-ray spectral slope$\alpha_{ox}$) shows a pronounced softening as the outburst progresses. TheX-ray timing properties also show a marked change, initially dominated byvariability at long ($>$day) timescales while a high frequency ($\sim$10$^{-3}$Hz) component emerges after the transition into the hard state. At late times($\sim$500 days after peak), a second accretion state transition occurs, fromthe hard into the quiescent state, as identified by the sudden collapse of thebolometric (X-ray+UV) emission to levels below 10$^{-3.4}$ L$_{\rm Edd}$. Ourfindings illustrate that TDEs can be used to study the scale (in)variance ofaccretion processes in individual SMBHs. Consequently, they provide a newavenue to study accretion states over seven orders of magnitude in black holemass, removing limitations inherent to commonly used ensemble studies.

Rapid accretion state transitions following the tidal disruption event AT2018fyk