A Disappearing Act: Constraints From "Missing" Flares of Repeating Partial TDE Candidates

Abstract

Recurrent tidal disruption events (rTDEs) are sources that exhibit multiple TDE-like flares; many are likely powered by the recurring partial disruption of a bound star, in a repeating partial TDE (rpTDE). Two such sources, TDE 2022dbl (ASASSN-22ci) and TDE 2020vdq (ZTF20acaazkt), each exhibited two UV/optical flares and, under the assumption of periodicity, both were expected to exhibit a third flare in early 2026. Neither exhibited such a flare, to limits of $L_{\textrm{UV/optical}} \lesssim 10^{42}$ erg s$^{-1}$, $\sim$30$\times$ fainter than the previous flares. Here, we examine several possible explanations. Observing two independent TDEs from the same galaxy within $\sim$2 yr has a probability of $\lesssim$0.5% for measured average TDE rates and currently expected rate enhancements, unless there is extreme intrinsic dispersion in the rates. Theoretical predictions for a double TDE of both stars in a binary are inconsistent with the observed flares. We therefore conclude that TDE 2022dbl and TDE 2020vdq are rpTDEs. To produce only two observable flares with similar energetics, our semi-analytical modeling strongly favors a main-sequence star promptly placed on a bound orbit with a deep initial tidal encounter at pericenter. These results suggest that the majority of rTDEs with multiple flares over a few-year baseline are likely to be rpTDEs, and that a significant fraction of systems may produce only two observable flares. This has important implications for the use of r(p)TDEs as probes of TDE physics and dynamical processes in the nuclei of other galaxies, in addition to the expected yield from upcoming surveys.