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We present optical spectroscopic and Swift UVOT/XRT observations of the X-rayand UV/optical bright tidal disruption event (TDE) AT 2018fyk/ASASSN-18uldiscovered by ASAS-SN. The Swift lightcurve is atypical for a TDE, entering aplateau after $\sim$40 days of decline from peak. After 80 days the UV/opticallightcurve breaks again to decline further, while the X-ray emission becomesbrighter and harder. In addition to broad H, He and potentially O/Fe lines,narrow emission lines emerge in the optical spectra during the plateau phase.We identify both high ionisation (O III) and low ionisation (Fe II) lines,which are visible for $\sim$45 days. We similarly identify Fe II lines inoptical spectra of ASASSN-15oi 330 d after discovery, indicating that a classof Fe-rich TDEs exists. The spectral similarity between AT 2018fyk, narrow-lineSeyfert 1 galaxies and some extreme coronal line emitters suggests that TDEsare capable of creating similar physical conditions in the nuclei of galaxies.The Fe II lines can be associated with the formation of a compact accretiondisk, as the emergence of low ionisation emission lines requires opticallythick, high density gas. Taken together with the plateau in X-ray andUV/optical luminosity this indicates that emission from the central source isefficiently reprocessed into UV/optical wavelengths. Such a two-componentlightcurve is very similar to that seen in the TDE candidate ASASSN-15lh, andis a natural consequence of a highly relativistic orbital pericenter.

Evidence for rapid disk formation and reprocessing in the X-ray bright tidal disruption event AT 2018fyk