Recurring tidal disruption events a decade apart in IRAS F01004-2237

¹ Department of Physics, Anhui Normal University, Wuhu, Anhui 241002, China
² CAS Key Laboratory for Researches in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, Anhui 230026, China
³ School of Astronomy and Space Sciences, University of Science and Technology of China, Hefei 230026, China
⁴ Center for Astrophysics, Guangzhou University, Guangzhou 510006, China
⁵ Department of Astronomy, School of Physics, Peking University, 5 Yiheyuan Road, Haidian District, Beijing 100871, China
⁶ Kavli Institute of Astronomy and Astrophysics, Peking University, 5 Yiheyuan Road, Haidian District, Beijing 100871, China
⁷ National Astronomical Observatories, Chinese Academy of Science, 20A Datun Road, Chaoyang District, Beijing 100101, China
⁸ South African Astronomical Observatory, P.O. Box 9, Observatory, 7935 Cape Town, South Africa
⁹ Astrophysics Science Division, NASA Goddard Space Flight Center, MC 661, Greenbelt, MD 20771, USA
¹⁰ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Rm. N204, Tucson, AZ 85721-0065, USA
¹¹ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
¹² Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
¹³ Key Laboratory for the Structure and Evolution of Celestial Objects, Yunnan Observatories, Kunming 650011, China

^⋆ Corresponding authors; sunluming@ahnu.edu.cn; jnac@ustc.edu.cn

Received: 26 September 2024
Accepted: 23 October 2024

Abstract

Context. In theory, recurring tidal disruption events (TDEs) may occur when a close stellar binary encounters a supermassive black hole, if one star is captured and undergoes repeating partial TDEs, or if both stars are tidally disrupted (double TDEs). In addition, independent TDEs may be observed over decades in some special galaxies where the TDE rate is extremely high. Exploring the diversity of recurring TDEs and probing their natures with rich observational data helps us to understand these mechanisms.

Aims. We report the discovery of a second optical flare that occurred in September 2021 in IRAS F01004-2237, where a first flare that occurred in 2010 had already been reported. We also present a detailed analysis of multi-band data. We aim to understand the nature of the flare and explore the possible causes of the recurring flares.

Methods. We describe our analysis of the position of the flare, the multi-band light curves (LCs), the optical and ultraviolet (UV) spectra, and the X-ray LC and spectra.

Results. The position of the flare coincides with the galaxy centre with a precision of 650 pc. The flare peaks in ∼50 days with an absolute magnitude of ∼ − 21 and fades in two years, roughly following L ∝ t^−5/3. It maintains a nearly constant blackbody temperature of ∼22 000 K in later stages. Its optical and UV spectra show hydrogen and helium broad emission lines with full width at half maxima of 7000–21 000 km s⁻¹ and a He II/Hα ratio of 0.3–2.3. It shows weak X-ray emission relative to UV emission, with X-ray flares lasting for < 2 − 3 weeks, during which the spectrum is soft with a power-law index of Γ = 4.4_−1.3^+1.4. These characters are consistent with a TDE, ruling out the possibilities of a supernova or an active galactic nucleus flare. With a TDE model, we infer a peak UV luminosity of 3.3 ± 0.2 × 10⁴⁴ erg s⁻¹ and an energy budget of 4.5 ± 0.2 × 10⁵¹ erg.

Conclusions. A TDE caused the flare that occurred in 2021. The two optical flares separated by 10.3 ± 0.3 years can be interpreted as repeating partial TDEs, double TDEs, or two independent TDEs. Although no definitive conclusion can be drawn, the partial TDEs interpretation predicts a third flare around 2033, and the independent TDEs interpretation predicts a high TDE rate of ≳10⁻² yr⁻¹ in F01004-2237, both of which can be tested by future observations.