Recurrent nova T CrB has just started its Pre-eruption Dip in March/April 2023, so the eruption should occur around 2024.4 +- 0.3

ATel #16107; Bradley E. Schaefer (Louisiana State Univ.), Brian Kloppenborg (AAVSO), Elizabeth O. Waagen (AAVSO), and the AAVSO observers
on 30 Jun 2023; 00:44 UT
Credential Certification: Bradley E. Schaefer (schaefer@lsu.edu)

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T Coronae Borealis (T CrB) is a famous recurrent nova with known eruptions in the years 1217, 1787, 1866, and 1946. Many workers have realized that the rise in brightness from its low state (1954.5 to 2015.0) to its high state (2015.0 to the present) is a precursor and harbinger for an upcoming eruption around 2025.5 +- 1.3 or so (Munari et al. 2016; Schaefer 2023). A distinct and under-appreciated close-up harbinger is the unique and mysterious Pre-eruption Dip (Schaefer 2023). The Dip in 1945--1946 started around 1945.0 (1.1 +- 0.3 years before the 1946 eruption), with the B-band magnitude fading from near 10.5 to 12.0 mag, while the V-band magnitude faded from around 9.8 to 12.3 mag. This fading ended abruptly with the nova eruption.

In anticipation of the start of this Pre-eruption Dip, we have been frequently monitoring the up-to-date light curve as collected into the AAVSO International Database. The AAVSO B and V band light curves from 2021.0 to present, with 2-day binning, for 4330 B-band mags and 12734 V-band mags, all with CCD photometry, are linked below. The normal light curve since 2016 shows the usual ellipsoidal modulation, with a full amplitude of ~0.4 mag for a sinewave at half the orbital period. The light curve shows variations about this average curve on all time scales, with larger variations in the B-band than in the V-band, all arising from ordinary flickering always present since 1867. Starting around 2023.25, T CrB shows a systematic fade from its long-time ellipsoidal variations. This fading is far outside of any historic variations since 2016. The fading in the blue was 0.4 mag in 2023.3 to 0.8 mag in 2023.5. The fading in the V-band was 0.25 mag in 2023.3, and 0.35 mag in 2023.5. The fading in the R and I bands are substantially smaller. This color dependency in the fading is consistent with increasing dust absorption, for a scenario featuring a recent discrete mass ejection in which dust formation occurs (much like for R CrB stars).

So the T CrB Pre-eruption Dip has already started in March/April of this year. If the Dip in 2023 is similar in timing to that in 1945, then the primary eruption should occur roughly 1.1 +- 0.3 years later, or in 2024.4 +- 0.3. This prediction is substantially improved over the prior predictions based only on the 2015 rise to the high-state. Still, possible deviations from the behavior in 1946 could create an early or a late eruption.

This announcement of the start of the Dip and the prediction of the eruption date (2024.4 +- 0.3) will hopefully be of use for researchers for making proposals with a wide variety of telescopes. Further, this serves as advance notice to take all needed pre-eruption baselines, for example obtaining infrared fluxes and background nebulosity images over a wide field for later light echo detections. And it is not too late to try to understand the pre-eruption high-state, with it still being unclear whether the increased luminosity comes from increased accretion or from nuclear burning on the white dwarf. For observations before the upcoming eruption, we particularly point to U-band photometry, UV spectrophotometry, and spectral line profiles, all for measuring the energetic physical mechanism of the Pre-eruption Dip, while long-running infrared photometry might detect dust formation.

T CrB B and V light curves