Rapid onset of a Comptonisation zone in the repeating tidal disruption event XMMSL2 J140446.9-251135

¹ Telespazio UK for ESA, ESAC, Apartado 78, 28691 Villanueva de la Cañada, Madrid, Spain
² Astrophysics & Space Institute, Schmidt Sciences, New York, NY 10011, USA
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁵ Department of Astronomy and Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA
⁶ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
⁷ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁸ University of Oxford, Dept. of Physics, Denys Wilkinson building, Keble road, OX1 3RU Oxford, UK
⁹ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, ESAC campus, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁰ ESA, ESAC, Apartado 78, 28691 Villanueva de la Cañada, Madrid, Spain
¹¹ SERCO for ESA, ESAC, Apartado 78, 28691 Villanueva de la Cañada, Madrid, Spain
¹² Dept. of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
¹³ Department of Astrophysics/IMAPP, Radboud University P.O. Box 9010 6500 GL Nijmegen, The Netherlands
¹⁴ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100101, China

^⋆ Corresponding author: richard.saxton@ext.esa.int

Received: 19 February 2025
Accepted: 2 October 2025

Abstract

We report here on observations of a tidal disruption event (TDE), XMMSL2 J1404-2511, discovered in an XMM-Newton slew, in a quiescent galaxy at z = 0.043. X-ray monitoring covered the epoch when the accretion disc transitioned from a thermal state, with kT ∼ 80 eV, to a harder state dominated by a warm, optically thick corona. The bulk of the coronal formation took place within 7 days and was coincident with a temporary drop in the emitted radiation by a factor 4. After a plateau phase of ∼100 days, the X-ray flux of XMMSL2 J1404-2511 decayed by a factor 500 within 230 days. We estimate the black hole mass in the galaxy to be M_BH = 4 ± 2 × 10⁶ M_⊙ and the peak X-ray luminosity L_X ∼ 6 × 10⁴³ ergs s⁻¹. The optical/UV light curve is flat over the timescale of the observations with L_opt ∼ 2 × 10⁴¹ ergs s⁻¹. We find that TDEs with coronae are more often found in an X-ray sample than in an optically selected sample. Late-time monitoring of the optical sample is needed to test whether this is an intrinsic property of TDEs or is due to a selection effect. From the fast decay of the X-ray emission we consider that the event was likely due to the partial stripping of an evolved star rather than a full stellar disruption, an idea supported by the detection of two further re-brightening episodes, two and four years after the first event, in the SRG/eROSITA all-sky survey.