  The tidal disruption event (TDE) AT2018dyk/ASASSN-18UL showed a rapid dimming
event 500 days after discovery, followed by a re-brightening roughly 700 days
later. It has been hypothesized that this behavior results from a repeating
partial TDE (rpTDE), such that prompt dimmings/shutoffs are coincident with the
return of the star to pericenter and rebrightenings generated by the renewed
supply of tidally stripped debris. This model predicted that the emission
should shut off again around August of 2023. We report AT2018fyk's continued
X-ray and UV monitoring, which shows an X-ray (UV) drop in flux by a factor of
10 (5) over a span of two months, starting 14 Aug 2023. This sudden change can
be interpreted as the second emission shutoff, which 1) strengthens the rpTDE
scenario for AT2018fyk, 2) allows us to constrain the orbital period to a more
precise value of 1306$\pm$47 days, and 3) establishes that X-ray and UV/optical
emission track the fallback rate onto this SMBH -- an often-made assumption
that otherwise lacks observational verification -- and therefore the UV/optical
lightcurve is powered predominantly by processes tied to X-rays. The second
cutoff implies that another rebrightening should happen between May-Aug 2025,
and if the star survived the second encounter, a third shutoff is predicted to
occur between Jan-July 2027. Finally, low-level accretion from the less bound
debris tail (which is completely unbound/does not contribute to accretion in a
non-repeating TDE) can result in a faint X-ray plateau that could be detectable
until the next rebrightening.
