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We propose to acquire the first deep X-ray observations of a fast and blueoptical transient (FBOT) at very late times. FBOTs are a new class of transientswith luminosities and time scales that challenge traditional SN models. As aresult the intrinsic nature of their energy source is unknown. Motivated by ourrecent finding of persistent luminous X-ray emission at the location of thenearest FBOT AT2018cow we propose deep XMM observations of this source at$delta t$$>$$5$ yrs. By mapping the long-term evolution and spectrum of thisX-ray source these observations will shed light on the connection of FBOTs withmanifestations of accreting BHs. By exploring a new part of the phase spacethis proposal opens a new window of investigation on this new class of engine-driven transients. [truncated! Please see actual data for full text]