AT2018fyk: Candidate Tidal Disruption Event by a (Super)Massive Black Hole Binary

The tidal disruption event (TDE) AT2018fyk has unusual X-ray, UV, and optical light curves that decay over the first ∼600 days, rebrighten, and decay again around 1200 days. We explain this behavior as a one-off TDE associated with a massive black hole (BH) binary. The sharp drop-offs from t ^‑5/3 power laws at around 600 days naturally arise when one BH interrupts the debris fallback onto the other BH. The BH mass M _• derived from fitting X-ray spectra with a slim disk accretion model and, independently, from fitting the early UV/optical light curves, is smaller by 2 orders of magnitude than predicted from the M _•–σ _* host galaxy relation, suggesting that the debris is accreted onto the secondary, with the fallback cut off by the primary. Furthermore, if the rebrightening were associated with the primary, it should occur around 5000 days, not the observed 1200 days. The secondary's mass and dimensionless spin is M•,s=2.7‑1.5+0.5×105M⊙ and a _•,s > 0.3 (X-ray spectral fitting), while the primary's mass is M _•,p = 10^7.7±0.4 M _⊙ (M _•–σ _* relation). An intermediate mass BH secondary is consistent with the observed UV/optical light-curve decay, i.e., the secondary's outer accretion disk is too faint to produce a detectable emission floor. The time of the first accretion cutoff constrains the binary separation to be (6.7 ± 1.2) × 10^‑3 pc. X-ray spectral fitting and timing analyses indicate that the hard X-rays arise from a corona above the secondary's disk. The early UV/optical emission, suggesting a super-Eddington phase for the secondary, possibly originates from shocks arising from debris circularization.