Live to Die Another Day: The Rebrightening of AT 2018fyk as a Repeating Partial Tidal Disruption Event

Abstract

Stars that interact with supermassive black holes (SMBHs) can be either completely or partially destroyed by tides. In a partial tidal disruption event (TDE), the high-density core of the star remains intact, and the low-density outer envelope of the star is stripped and feeds a luminous accretion episode. The TDE AT 2018fyk, with an inferred black hole mass of 107.7±0.4 M ⊙, experienced an extreme dimming event at X-ray (factor of >6000) and UV (factor of ∼15) wavelengths ∼500–600 days after discovery. Here we report on the reemergence of these emission components roughly 1200 days after discovery. We find that the source properties are similar to those of the predimming accretion state, suggesting that the accretion flow was rejuvenated to a similar state. We propose that a repeated partial TDE, where the partially disrupted star is on an ∼1200 day orbit about the SMBH and periodically stripped of mass during each pericenter passage, powers its unique light curve. This scenario provides a plausible explanation for AT 2018fyk’s overall properties, including the rapid dimming event and the rebrightening at late times. We also provide testable predictions for the behavior of the accretion flow in the future; if the second encounter was also a partial disruption, then we predict another strong dimming event around day 1800 (2023 August) and a subsequent rebrightening around day 2400 (2025 March). This source provides strong evidence of the partial disruption of a star by an SMBH.

Unstable Mass Transfer from a Main-sequence Star to a Supermassive Black Hole and Quasiperiodic Eruptions

“…"Stellar EMRIs" occur when stars approach the black hole on a tight inspiraling orbit, such that at the onset of mass transfer the orbit is roughly circular (e  0.5), with hour-day periods, where a small fraction of the stellar mass is stripped at each pericenter passage. We note that "repeating TDEs," producing flares separated by much longer timescales (e.g., Payne et al 2021, with ∼110 days;Liu et al 2023, with ∼200 days;Wevers et al 2023, with ∼10 3 days), may represent an intermediate class of events that, like stellar EMRIs, are long-lived and are not fully destroyed at the onset of mass transfer but, like TDEs, have very eccentric orbits. We briefly address these systems in the discussion section.…”

Partial Tidal Disruptions of Main-sequence Stars by Intermediate-mass Black Holes

“…The partial disruption of a 1 M e star yields a minimum orbital period a few ×10 (10 3 ) yr around a 100 M e (10 4 M e ) IMBH. However, one can still obtain a partially disrupted star on an orbit around a massive black hole as short as a few years through a dynamical exchange of the star that was initially part of a binary system (Cufari et al 2022;Wevers et al 2023). Recently, Wevers et al (2023) proposed that the TDE AT 2018fyk is caused by repeating partial disruption of a star through the Hills mechanism and predicted the orbital period of the captured star around the 10 7.7 M e black hole to be ∼1200 days.…”

X-Ray Unveiling Events in a z ≈ 1.6 Active Galactic Nucleus in the 7 Ms Chandra Deep Field-South

“…It is very unlikely to have two TDEs given the short time interval of ≈2.5 yr and the typical X-ray TDE rate of ≈10 −4 to 10 −5 yr −1 galaxy −1 (e.g., Donley et al 2002;Komossa 2015;Sazonov et al 2021). There have also been suggestions that partial TDEs (e.g., Chen & Shen 2021; Payne et al 2021;Chen et al 2022;Wevers et al 2023) or TDEs in SMBH binaries (e.g., Liu et al 2009;Shu et al 2020) might produce multiple TDE flares. However, there are only a few such candidates, and their X-ray light curves differ from that of XID 403.…”