The unluckiest star: A spectroscopically confirmed repeated partial tidal disruption event AT 2022dbl

TL;DR

This paper reports the first spectroscopic confirmation of a repeated partial tidal disruption event (pTDE) caused by a star repeatedly passing close to a supermassive black hole, with two observed flares showing consistent spectral features.

Contribution

It provides the first robust spectroscopic evidence for a repeated pTDE, confirming the scenario of a star undergoing multiple partial disruptions.

Findings

Two optical/UV flares observed in 2022 and 2024 with similar spectral features.

Spectroscopic evidence rules out multiple-star TDEs, confirming a single star repeatedly disrupted.

The second flare's properties are consistent with a partial disruption, supporting the pTDE model.

Abstract

The unluckiest star orbits a supermassive black hole elliptically. Every time it reaches the pericenter, it shallowly enters the tidal radius and gets partially tidal disrupted, producing a series of flares. Confirmation of a repeated partial tidal disruption event (pTDE) requires not only evidence to rule out other types of transients, but also proof that only one star is involved, as TDEs from multiple stars can also produce similar flares. In this letter, we report the discovery of a repeated pTDE, AT 2022dbl. In a quiescent galaxy at $z=0.0284$, two separate optical/UV flares have been observed in 2022 and 2024, with no bright X-ray, radio or mid-infrared counterparts. Compared to the first flare, the second flare has a similar blackbody temperature of ~26,000 K, slightly lower peak luminosity, and slower rise and fall phases. Compared to the ZTF TDEs, their blackbody parameters and light curve shapes are all similar. The spectra taken during the second flare show a steeper continuum than the late-time spectra of the previous flare, consistent with a newly risen flare. More importantly, the possibility of two independent TDEs can be largely ruled out because the optical spectra taken around the peak of the two flares exhibit highly similar broad Balmer, N III and possible He II emission lines, especially the extreme ~4100{\AA} emission lines. This represents the first robust spectroscopic evidence for a repeated pTDE, which can soon be verified by observing the third flare, given its short orbital period.