AT2021aeuk: A Repeating Partial Tidal Disruption Event Candidate in a Narrow-line Seyfert 1 Galaxy

A black hole (BH) can tear apart a star that ventures within its tidal radius, producing a luminous flare as the stellar debris falls back, known as a tidal disruption event (TDE). While TDEs in quiescent galaxies are relatively well understood, identifying TDEs in active galactic nuclei (AGNs) still remains a significant challenge. We present the discovery of AT2021aeuk, a transient exhibiting dual flares within around 3 yr in a narrow-line Seyfert 1 galaxy. Multiwavelength observations triggered during the second flare in 2023 revealed an extraordinary X-ray V-shaped light curve, strongly anticorrelated with the optical light curve and accompanied by a lag of similar to 40 days. This behavior is inconsistent with both supernova and pure AGN origins. In addition, a new broad component emerges in the Balmer lines during the second flare, showing a clear reverberation signal to the continuum variation. We propose that the dual flare may be linked to a repeating partial TDE (rpTDE), where the second flare results from a collision between the TDE stream and the inner accretion disk, triggering an optical flare while simultaneously partially destroying the X-ray corona. However, other mechanisms, such as a stellar-mass BH merger within an accretion disk, could produce similar phenomena, which we cannot entirely rule out. The Vera C. Rubin Observatory will be a powerful tool for further investigating the nature of such events in the future.