Multiband, Multiepoch Photometry of the Spot-crossing System TOI-3884: Refined System Geometry and Spot Properties

Spot-crossing transits offer a unique opportunity to probe spot properties such as temperature, size, and surface distribution. TOI-3884 is a rare system in which spot-crossing features are persistently observed during every transit. This is due to its unusual configuration: a nearly polar-orbit super-Neptune transits a pole-on mid-M dwarf, repeatedly crossing a polar spot. However, previous studies have reported discrepant values in key system parameters, such as stellar inclination and obliquity. To address this, we conducted multiband, multiepoch transit observations of TOI-3884 b using the MuSCAT instrument series, along with photometric monitoring with the Las Cumbres Observatory 1 m telescopes/Sinistro. We detected time-dependent variations in the spot-crossing signals, indicating that the spot is not exactly on the pole. From the monitoring data, we measured a stellar rotation period of 11.04 3 − 0.053 + 0.054 days with a modulation amplitude of ∼5% in the r band, consistent with the time variability in the spot-crossing features. Our analysis reconciles previous discrepancies and improves the constraints on the parameters of the system geometry ( i ⋆ = 139 . 9 − 2.0 + 1.2 deg and λ = 41 . 0 − 9.0 + 3.7 deg) and those of the spot properties (spot radius of 0.42 5 − 0.011 + 0.018 R ⋆ and a spot–photosphere temperature difference of 20 0 − 9 + 11 K). These results provide a critical context for interpreting upcoming transmission spectroscopy of TOI-3884 b, as well as yielding new insights into the magnetic activity and spin–orbit geometry of M dwarfs.