Rapid evolution of the recurrence time in the repeating partial tidal disruption event eRASSt J045650.3−203750

In this letter, we present the results from subsequent X-ray and UV observations of the nuclear transient eRASSt J045650.3−203750 (hereafter, J0456−20). We detected five repeating X-ray and UV flares from J0456−20, marking it as one of the most promising repeating partial tidal disruption event (pTDE) candidates. More importantly, we also found rapid changes in the recurrence time, T_recur, of the X-ray flares by modelling the long-term X-ray light curve of J0456−20. We found that T_recur first decreased rapidly from about 300 days to around 230 days. It continued to decrease to around 190 days with an indication of a constant T_recur, as evidenced by the latest three cycles. Our hydrodynamic simulations suggest that, in the repeating pTDE scenario, such a rapid evolution of T_recur could be reproduced if the original star is a 1 M_⊙ main sequence star near the terminal age, losing nearly 80-90% of its mass during the initial encounter with a supermassive black hole (SMBH) of a mass around 10⁵ M_⊙. The inferred mass loss of 0.8-0.9 M_⊙ is higher than the estimated value of around 0.13 M_⊙ drawn from observations, which could be explained if the radiation efficiency is low (i.e. ≪0.1). Our results indicate that repeating pTDEs could be effective tools for exploring the dynamics around SMBHs beyond our own Galaxy.