Inactive Longitude and Superflare in the Active Single-lined Pre-main-sequence Binary V2279 Cyg

Abstract Young, solar-like stars in the pre-main-sequence (PMS) stage exhibit vigorous magnetic activity that significantly influences their circumstellar environments and the processes of planetary formation and evolution. In binary systems, tidal forces and magnetic interactions can further shape the magnetic geometry. We report a longitudinal preference of starspots, chromospheric activities, and flares in the active single-lined spectroscopic PMS binary system V2279 Cyg, based on long-term photometric observations from Kepler and TESS alongside spectroscopic data from LAMOST. The system is classified as a weak-line T Tauri binary, with component masses estimated at 0.86 M ⊙ and 0.27 M ⊙ . V2279 Cyg’s nearly circular orbit is synchronized with its 4.126 day rotational period. Observations reveal large starspot regions clustered near the far-side hemisphere. Spectroscopic data show strong, double-peak H α emission, the strength of which is highly correlated with starspot distribution, indicating the presence of an active longitude on the primary star. We also mapped the prominence structure corotating with the primary star, suggesting a dense structure close to the near-side hemisphere. Furthermore, we identify an inactive longitude of flares during the 4 yr Kepler observations, where the frequency of flare activity is significantly reduced after the superior conjunction, marking the first such identification in active binary systems. Additionally, a white light superflare, releasing energy of 2.5 × 10 37 erg, was detected in TESS observations. These findings provide valuable insights into the magnetic field geometry and dynamo processes in PMS binaries, underscoring the critical role of tidal interactions in shaping magnetic activities.