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

Songpeng Pei, Xiaowan Zhang, Renzhi Su, Yongzhi Cai, Ziwei Ou, Qiang Li, Xiaoqin Ren, Taozhi Yang, Mingyue Li

TL;DR

The paper addresses how accretion-driven variability in the symbiotic recurrent nova T CrB manifests in a long, multi-wavelength dataset spanning quiescence, a prolonged high state, and the 2023–2025 pre-eruption dip. By integrating Swift XRT/BAT/UVOT with AAVSO photometry, the authors show a robust anti-correlation between X-ray and optical/UV emission and identify a wavelength-dependent fading during the pre-eruption dip, accompanied by soft and hard X-ray brightening and a subsequent second dip. They interpret these findings within an accretion-variation framework, linking a transition of the boundary layer from optically thick to thin to reductions in the mass-transfer rate that precede a thermonuclear runaway, consistent with but more complex than the 1946 event. This work provides a comprehensive, modern pre-eruption view that strengthens the case for using multi-wavelength accretion signatures as indicators of imminent nova eruptions in symbiotic recurrent systems, and it establishes a valuable baseline for future monitoring during the next outburst.

Abstract

We present a multi-wavelength study of the symbiotic recurrent nova (RN) T Coronae Borealis (T CrB) using 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 optical and UV bands, while the UV and X-ray fluxes are generally anti-correlated throughout all phases. During the 2023-2024 pre-eruption dip, soft and hard X-rays increased as optical and ultraviolet (UV) brightness declined, consistent with a transition from an optically thick to 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.

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

TL;DR

The paper addresses how accretion-driven variability in the symbiotic recurrent nova T CrB manifests in a long, multi-wavelength dataset spanning quiescence, a prolonged high state, and the 2023–2025 pre-eruption dip. By integrating Swift XRT/BAT/UVOT with AAVSO photometry, the authors show a robust anti-correlation between X-ray and optical/UV emission and identify a wavelength-dependent fading during the pre-eruption dip, accompanied by soft and hard X-ray brightening and a subsequent second dip. They interpret these findings within an accretion-variation framework, linking a transition of the boundary layer from optically thick to thin to reductions in the mass-transfer rate that precede a thermonuclear runaway, consistent with but more complex than the 1946 event. This work provides a comprehensive, modern pre-eruption view that strengthens the case for using multi-wavelength accretion signatures as indicators of imminent nova eruptions in symbiotic recurrent systems, and it establishes a valuable baseline for future monitoring during the next outburst.

Abstract

We present a multi-wavelength study of the symbiotic recurrent nova (RN) T Coronae Borealis (T CrB) using 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 optical and UV bands, while the UV and X-ray fluxes are generally anti-correlated throughout all phases. During the 2023-2024 pre-eruption dip, soft and hard X-rays increased as optical and ultraviolet (UV) brightness declined, consistent with a transition from an optically thick to 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.
Paper Structure (10 sections, 4 figures, 5 tables)

This paper contains 10 sections, 4 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Observation and data reduction
  3. Multi-wavelength variability and timing analysis
  4. Evolution of optical, UV, and X-ray light curves
  5. Characteristics of the high state
  6. Properties of the pre-eruption dip state
  7. Timing analysis
  8. Discussion
  9. Summary and Conclusions
  10. Data availability

Figures (4)

  • Figure 1: From top to bottom: AAVSO light curves of T CrB from early 2005 to early 2025 in the following bands: I (green), R (orange), V (black), B (yellow), and U (red). Subsequent panels display: Swift/UVOT light curves in multiple filters; Swift/XRT 0.3 -- 10 keV light curve (PC mode); Swift/XRT light curves in 3 -- 10 keV (black) and 0.3 -- 3 keV (red); X-ray hardness ratio (3 -- 10 keV / 0.3 -- 3 keV); and the Swift/BAT light curve.
  • Figure 2: From top to bottom: AAVSO light curves of T CrB from late 2021 to early 2025 in the following bands: I (green), R (orange), V (black), B (yellow), and U (red). Subsequent panels display: Swift/UVOT light curves in various filters; the Swift/XRT 0.3 -- 10 keV light curve (PC mode); Swift/XRT light curves in 3 -- 10 keV (black) and 0.3 -- 3 keV (red); the X-ray hardness ratio (3 -- 10 keV / 0.3 -- 3 keV); and the Swift/BAT light curve.
  • Figure 3: Dereddened color evolution of T CrB from early 2005 to early 2025 based on AAVSO photometry. The top panel shows $(B-V)_0$ (red) and the bottom panel shows $(U-B)_0$ (blue). Colors were computed from near-simultaneous ($\pm$0.25 d) AAVSO $UBV$ measurements, adopting $E(B-V)=0.07$2022NewA...9701859N. For clarity, the dereddened color indices were averaged in 1-day bins.
  • Figure 4: AAVSO light curves of T CrB from 2005.78 to early 2025 in the I (green), R (orange), V (black), and B (yellow) bands. The data have been averaged over 227.58-day intervals, corresponding to the orbital period 2025AA...694A..85P.