Letter to the Editor

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

¹ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
e-mail: liuzhu@mpe.mpg.de
² Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
³ International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987 Perth, WA 6845, Australia
⁴ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 21 November 2023
Accepted: 28 February 2024

Abstract

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.