Synchronous Rotation in the (120347) Salacia–Actaea System

We report on roughly 16 yr of photometric monitoring of the trans-Neptunian binary system (120347) Salacia–Actaea, which provides significant evidence that Salacia and Actaea are tidally locked to the mutual orbital period in a fully synchronous configuration. The orbit of Actaea is updated, followed by a Lomb–Scargle periodogram analysis of the ground-based photometry, which reveals a synodic period similar to the orbital period and a peak-to-peak lightcurve amplitude of Δ m = 0.0900 ± 0.0036 mag (1 σ uncertainty). Incorporating archival Hubble Space Telescope photometry that resolves each component, we argue that the periodicity in the unresolved data is driven by a longitudinally varying surface morphology on Salacia, and we derive a sidereal rotation period that is within 1 σ of the mutual orbital period. A rudimentary tidal evolution model is invoked that suggests synchronization occurred within 1.1 Gyr after Actaea was captured/formed.