Evidence of Long-lived Powerful Gyrosynchrotron Radio Emission in the Close Binary FF UMa

Journal article, 2026

RS Canum Venaticorum (RS CVn) close binaries, characterized by tidal locking, rapid rotations, and strong magnetic fields, are ideal laboratories for high-resolution radio observations to probe emission processes, magnetic field configurations, and interaction activity. Despite their importance, only a few RSCVn sources have been explored by polarimetric observations of very long baseline interferometry. To expand the effort, we have analyzed the existing Very Long Baseline Array (VLBA) astrometric data for the RS CVn binary FF Ursae Majoris. In the 5 GHz VLBA experiments conducted between 2021 and 2024, both total intensity and circularly polarized emission were clearly detected at six of seven epochs. The consistently high brightness temperatures (>= 107 K) and the moderate fractional circular polarization (10%-30%) over about 3 yr indicate that the radio emission is mainly produced by gyrosynchrotron radiation from mildly relativistic electrons in the highly ordered magnetic field. The radio luminosities are also comparable to those of previously studied powerful RS CVn binaries and show a significant anticorrelation with fractional circular polarization. A mean centroid offset of 13.4 +/- 3.1 R circle dot between the Stokes I and V emission was found across multiple epochs, indicating a possible additional contribution from the secondary star via a magnetically active corona, a giant magnetic loop, or significant interaction activity with the primary star in the quiescent state.