Increasing activity in T CrB suggests nova eruption is impending

TL;DR

This study reanalyzes archival data to suggest that the recurrent nova T CrB is currently in an accretion high state, increasing the likelihood of an impending nova eruption around 2026 due to episodic mass accumulation on the white dwarf.

Contribution

It proposes that nova eruptions in T CrB are triggered during accretion high states caused by disk instabilities, explaining the timing of past eruptions and predicting a future one.

Findings

T CrB's 1946 eruption occurred during an accretion high state.

Current high state suggests an eruption may occur around 2026.

Accretion disk instabilities likely set the timing of eruptions.

Abstract

Estimates of the accretion rate in symbiotic recurrent novae (RNe) often fall short of theoretical expectations by orders of magnitude. This apparent discrepancy can be resolved if the accumulation of mass by the white dwarf (WD) is highly sporadic, and most observations are performed during low states. Here we use a reanalysis of archival data from the Digital Access to a Sky Century @Harvard (DASCH) survey to argue that the most recent nova eruption in symbiotic RN T CrB, in 1946, occurred during -- and was therefore triggered by -- a transient accretion high state. Based on similarities in the optical light curve around 1946 and the time of the prior eruption, in 1866, we suggest that the WD in T CrB accumulates most of the fuel needed to ignite the thermonuclear runaways (TNRs) during accretion high states. A natural origin for such states is dwarf-nova like accretion-disk instabilities, which are expected in the presumably large disks in symbiotic binaries. The timing of the TNRs in symbiotic RNe could thus be set by the stability properties of their accretion disks. T CrB is in the midst of an accretion high state like the ones we posit led to the past two nova eruptions. Combined with the approach of the time at which a TNR would be expected based on the 80-year interval between the prior two novae ($2026 \pm$3), the current accretion high state increases the likelihood of a TNR occurring in T CrB in the next few years.