Multiwavelength study of the pre-eruption dip in the recurrent nova T Coronae Borealis preceding imminent nova eruption

¹ School of Physics and Electrical Engineering, Liupanshui Normal University, Liupanshui Guizhou 553004, China
² Shanghai Astronomical Observatory, Chinese Academy of Sciences 80 Nandan Road Shanghai 200030, China
³ Yunnan Observatories, Chinese Academy of Sciences Kunming 650216, China
⁴ International Centre of Supernovae, Yunnan Key Laboratory Kunming 650216, China
⁵ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences Kunming 650216, China
⁶ Tsung-Dao Lee Institute, Shanghai Jiao Tong University Shanghai 201210, China
⁷ School of Physics and Electronic Science, Qiannan Normal University for Nationalities Duyun 558000, China
⁸ Qiannan Key Laboratory of Radio Astronomy, Guizhou Province Duyun 558000, China
⁹ Ministry of Education Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, School of Physics, Xi’an Jiaotong University 710049 Xi’an, China

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Received: 21 September 2025
Accepted: 19 December 2025

Abstract

We present a multiwavelength study of the symbiotic recurrent nova T Coronae Borealis (T CrB), using the Swift Burst Alert Telescope (BAT)/ X-Ray Telescope (XRT) / UltraViolet Optical Telescope (UVOT) and American Association of Variable Stars Observers (AAVSO) observations from 2005 to 2025. Our analysis spans quiescent, high, and pre-eruption dip states. We find that brightening amplitudes increase toward shorter wavelengths in both the optical and UV bands, while the UV and X-ray fluxes are generally anticorrelated throughout all phases. During the 2023-2024 pre-eruption dip, soft and hard X-rays increased as optical and UV brightness declined, consistent with a transition from an optically thick to an optically thin boundary layer driven by a reduction in the accretion rate. We also report, for the first time, a second, lower-amplitude dip occurring between September 2024 and February 2025 following the primary 2023–2024 pre-eruption dip. The observed variability supports an accretion-variation scenario as a unifying explanation for both the high and dip states and may signal an imminent nova eruption.