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Abstract Some electromagnetic outbursts from the nuclei of distant galaxies have been found to repeat on months-to-years timescales, and each of these sources can putatively arise from the accretion flares generated through the repeated tidal stripping of a star on a bound orbit about a supermassive black hole (SMBH), i.e., a repeating partial tidal disruption event (rpTDE). Here, we test the rpTDE model through analytical estimates and hydrodynamical simulations of the interaction between a range of stars, which differ from one another in mass and age, and an SMBH. We show that higher-mass (≳1M_⊙), evolved stars can survive many (≳10−100) encounters with an SMBH while simultaneously losingfew× 0.01M_⊙, resulting in accretion flares that are approximately evenly spaced in time with nearly the same amplitude, quantitatively reproducing ASASSN-14ko. We also show that the energy imparted to the star via tides can lead to a change in its orbital period that is comparable to the observed decay in the recurrence time of ASASSN-14ko’s flares, $\dot{P}\simeq -0.0026$. Contrarily, lower-mass and less-evolved stars lose progressively more mass and produce brighter accretion flares on subsequent encounters for the same pericenter distances, leading to the rapid destruction of the star and cessation of flares. Such systems cannot reproduce ASASSN-14ko-like transients, but are promising candidates for recreating events such as AT2020vdq, which displayed a second and much brighter outburst compared to the first. Our results imply that the lightcurves of repeating transients are tightly coupled with stellar type. 

Abstract The tidal disruption event (TDE) AT2018fyk showed a rapid dimming event 500 days after discovery, followed by a rebrightening 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 2023 August 14. 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 ± 47 days, and (3) establishes that X-ray and UV/optical emission track the fallback rate onto this supermassive black hole—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 2025 May and August, and if the star survived the second encounter, a third shutoff is predicted to occur between 2027 January and July. Finally, low-level accretion from the less-bound debris tail (which is completely unbound/does not contribute to accretion in a nonrepeating TDE) can result in a faint X-ray plateau that could be detectable until the next rebrightening. 

ABSTRACT In a tidal disruption event (TDE), a star is destroyed by the gravitational field of a supermassive black hole (SMBH) to produce a stream of debris, some of which accretes onto the SMBH and creates a luminous flare. The distribution of mass along the stream has a direct impact on the accretion rate, and thus modelling the time-dependent evolution of this distribution provides insight into the relevant physical processes that drive the observable properties of TDEs. Analytic models that only account for the ballistic evolution of the debris do not capture salient and time-dependent features of the mass distribution, suggesting that fluid dynamical effects significantly modify the debris dynamics. Previous investigations have claimed that shocks are primarily responsible for these modifications, but here we show – with high-resolution hydrodynamical simulations – that self-gravity is the dominant physical mechanism responsible for the anomalous (i.e. not predicted by ballistic models) debris stream features and its time dependence. These high-resolution simulations also show that there is a specific length-scale on which self-gravity modifies the debris mass distribution, and as such there is enhanced power in specific Fourier modes. Our results have implications for the stability of the debris stream under the influence of self-gravity, particularly at late times and the corresponding observational signatures of TDEs.