Accretion in the recurrent nova T CrB: Linking the superactive state to the predicted outburst

Abstract

Context. T CrB (NOVA CrB 1946) is a famous recurrent nova with a recurrence timescale of 80 years. Aims. We aim to estimate the colours, luminosity, and mass-accretion rate for T CrB (NOVA CrB 1946) during and after the superactive state. Methods. We performed and analysed UBV photometry of the recurrent nova T CrB. Results. For the hot component of T CrB, we find average dereddened colours of (U − B)0 = −0.70 ± 0.08 and (B − V)0 = 0.23 ± 0.06, which correspond to an effective temperature of 9400 ± 500 K and an optical luminosity of 40 − 110 L⊙ during the superactive state (2016–2022). After the end of the superactive state, the hot component became significantly redder, (U − B)0 ≈ −0.3 and (B − V)0 ≈ 0.6 in August 2023, and its luminosity decreased markedly to 20 − 25 L⊙ in April–May 2023, and to 8 − 9 L⊙ in August 2023. The total mass accreted during the superactive state from 2014 to 2023 is ∼2 × 10−7 M⊙. Conclusions. This is a significant fraction of the mass required to cause a thermonuclear runaway (TNR). Overall our results support a model in which a large accretion disc acts as a reservoir with increased accretion rate onto the central white dwarf during disc high states, ultimately leading to a TNR explosion, which now seems to be imminent.