Evidence for rapid disk formation and reprocessing in the X-ray bright
  tidal disruption event AT 2018fyk
  We present optical spectroscopic and Swift UVOT/XRT observations of the X-ray
and UV/optical bright tidal disruption event (TDE) AT 2018fyk/ASASSN-18ul
discovered by ASAS-SN. The Swift lightcurve is atypical for a TDE, entering a
plateau after $\sim$40 days of decline from peak. After 80 days the UV/optical
lightcurve breaks again to decline further, while the X-ray emission becomes
brighter 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 in
optical spectra of ASASSN-15oi 330 d after discovery, indicating that a class
of Fe-rich TDEs exists. The spectral similarity between AT 2018fyk, narrow-line
Seyfert 1 galaxies and some extreme coronal line emitters suggests that TDEs
are capable of creating similar physical conditions in the nuclei of galaxies.
The Fe II lines can be associated with the formation of a compact accretion
disk, as the emergence of low ionisation emission lines requires optically
thick, high density gas. Taken together with the plateau in X-ray and
UV/optical luminosity this indicates that emission from the central source is
efficiently reprocessed into UV/optical wavelengths. Such a two-component
lightcurve is very similar to that seen in the TDE candidate ASASSN-15lh, and
is a natural consequence of a highly relativistic orbital pericenter.
