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

Abstract

In this letter, we present the results from further 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, making it one of the most promising repeating partial tidal disruption event (pTDE) candidates. More importantly, we also found rapid changes in the recurrence time $T_{\text{recur}}$ of the X-ray flares by modelling the long-term X-ray light curve of J0456-20. $T_{\text{recur}}$ 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 $T_{\text{recur}}$ evidenced from the latest three cycles. Our hydrodynamic simulations suggest that, in the repeating pTDE scenario, such rapid evolution of $T_{\text{recur}}$ could be reproduced if the original star is a $1{\mathrm{M}}_{\odot }$ main-sequence star near the terminal age and loses nearly 80-90% of its mass during the initial encounter with a supermassive black hole (SMBH) of mass around $10^5{\mathrm{M}}_{\odot }$. The inferred mass loss of 0.8-0.9 ${\mathrm{M}}_{\odot }$ is higher than the estimated value of around 0.13 ${\mathrm{M}}_{\odot }$ from observation, which could be explained if the radiation efficiency is low (i.e. $\ll 0.1$). Our results indicate that repeating pTDEs could be effective tools to explore the dynamics around supermassive black holes beyond our own Galaxy.

13 pages, 6 figures, accepted for publication in A&A Letter

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