2025NatAs...9..702Z

It is still in dispute whether intermediate-mass black holes (IMBHs) with a mass of ∼10³-10⁵ solar masses ( M _ ⊙ _ ) exist, which are the missing link between stellar-mass black holes (5-50 M _ ⊙ _ ) and supermassive black holes (10⁶-10¹⁰ M _ ⊙ _ ). The bright flares from tidal disruption events (TDEs) provide a new and direct way to probe IMBHs. 3XMM J215022.4-055108 is a unique off-nuclear X-ray transient that can be best explained as the TDE by an IMBH in a massive star cluster, although its mass is not well determined. Here we report the discovery of a transient X-ray quasi-periodicity signal from 3XMM J215022.4-055108 with a period of ∼85 s (at a significance of >3.51 σ ) and fractional root-mean-squared amplitude of ∼10%. Furthermore, the signal is coherent with a quality factor of ∼16. The significance drops to >3.13 σ if considering all light curves with sufficient quality for quasi-periodic oscillation search. Combined with the results from X-ray continuum fittings, the detection of quasi-periodic oscillation allows joint constraints on the black hole mass and dimensionless spin in the range of 9.9 × 10³ to 1.6 × 10⁴ M _ ⊙ _ and 0.26 to 0.36, respectively. This result supports the presence of an IMBH in an off-nuclear massive star cluster and may allow the study of IMBHs through X-ray timing of TDEs.