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The tidal disruption of a star around a supermassive black hole (SMBH) offersa unique opportunity to study accretion onto a SMBH on a human-timescale. Wepresent results from our 1000+ days NICER, Swift and Chandra monitoringcampaign of AT 2019avd, a nuclear transient with TDE-like properties. Ourprimary finding is that approximately 225 days following the peak of X-rayemission, there is a rapid drop in luminosity exceeding two orders ofmagnitude. This X-ray drop-off is accompanied by X-ray spectral hardening,followed by a 740-day plateau phase. During this phase, the spectral indexdecreases from 6.2+-1.1 to 2.3+-0.4, while the disk temperature remainsconstant. Additionally, we detect pronounced X-ray variability, with an averagefractional root mean squared amplitude of 47%, manifesting over timescales of afew dozen minutes. We propose that this phenomenon may be attributed tointervening clumpy outflows. The overall properties of AT 2019avd suggest thatthe accretion disk evolves from a super-Eddington to a sub-Eddington luminositystate, possibly associated with a compact jet. This evolution follows a patternin the hardness-intensity diagram similar to that observed in stellar-massblack holes, supporting the mass invariance of accretion-ejection processesaround black holes.

Rapid dimming followed by a state transition: a study of the highly variable nuclear transient AT 2019avd over 1000+ days