Lack of Significant Orbital-Phase Locking in the Active Phases of the Recurrent Nova T CrB

Abstract

T Coronae Borealis (T CrB) is a symbiotic recurrent nova (RN) that exhibits both nova eruptions and long-term active phases resembling superoutbursts and normal outbursts. Motivated by proposed connections between these events and the binary orbit, we test whether the onset, maximum, or termination of the active phases is locked to orbital phase. We use long-term optical $B$- and $V$-band light curves from the American Association of Variable Stars Observers (AAVSO) International Database and historical photometry from the literature. We measure the onset, maximum, and termination times of superoutbursts and normal outbursts and convert these times to orbital phase. We test the resulting circular distributions with Kuiper and Watson statistics. We find no statistically significant orbital-phase locking. The onset phases and maxima are consistent with a uniform phase distribution. The smallest probabilities occur for the 13 measurable termination phases ($p_{\rm MC}=0.043$ for the Kuiper statistic and $p_{\rm MC}=0.048$ for the Watson statistic), but this result is only marginal in an uncorrected $p<0.05$ sense, far from a $3σ$ detection, and insufficient to establish robust phase locking. The four historical nova eruptions likewise do not provide robust evidence for a unique ignition phase once the small sample size, historical date uncertainties, and long-term period changes are considered. The two known secondary eruptions occurred at similar phases, but two events are insufficient to establish an orbital-geometry connection. Overall, the active phases of T CrB appear to be governed primarily by accretion-disk physics rather than by a fixed binary phase.