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

Abstract

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 RS CVn sources have been explored by polarimetric observations of very long baseline interferometry (VLBI). To expand the effort, we have analyzed the existing Very Long Baseline Array (VLBA) astrometric data for the RS CVn binary FF Ursae Majoris (FF UMa). In the 5GHz 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 (10^7 K) and the moderate fractional circular polarization (10%-30%) over about three years 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 anti-correlation with fractional circular polarization. A mean centroid offset of 13.4 +/- 3.1 solar radii 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.

Figures (6)

• Figure 1: Images of the target source FF UMa in all epochs. Each panel covers a field of view of $30 \times 30~\mathrm{mas}^2$. The color bar on the right side of each panel is in units of mJy/beam$^{-1}$. In each panel, the upper subpanel shows the Stokes $I$ image and its $+10\sigma$ (white solid), $+3\sigma$ (white dashed), and $-3\sigma$ (red dashed) contours. The lower subpanel shows the Stokes $V$ image, overlaid with the Stokes $I$$+10\sigma$ (black solid) contour. The synthesized beam is shown in the lower left corner.
• Figure 2: Top: orbital phase coverage of the multi-epoch VLBA observations of FF UMa. The observing date and epoch number are labeled next to each point. Bottom: total intensity $S_\mathrm{I}$ (top), circularly polarized flux density $S_\mathrm{V}$ (middle), and fractional circular polarization $f_\mathrm{c}$ (bottom) vs. observing time (MJD). Different marker shapes indicate different observing epochs. Green symbols denote quiescent states, while orange symbols mark flaring epochs.
• Figure 3: Positional offsets between the Stokes $I$ and $V$ centroids of FF UMa. Different symbols denoted different observing epochs. Epoch 6, which corresponded to a flaring state, was shown in orange, while the remaining epochs in quiescent states were shown in green. In both panels, the Stokes $I$ position was taken as the coordinate origin and was not explicitly plotted. Left: Offsets plotted in the sky plane. The SNR of the positional offset for each epoch was indicated in the legend. The gray dashed ellipses denote the synthesized beams with the minimum (Epoch 3) and maximum (Epoch 2) areas among all epochs. Right: Offsets plotted in the binary reference frame. Red and blue circles indicated the positions of the primary and secondary stars, respectively; their sizes did not represent the actual physical scales of the stars, $0^\circ$ corresponded to the direction from the primary star toward the secondary star.
• Figure 4: Left: Relationship between radio luminosity and orbital period for RS CVn-type binary systems. The gray squares represent data from Huang2025, including 42 RS CVn-type radio stars (FF UMa included), observed at frequencies between 144 and 3000 MHz. The shape of each symbol indicates the observing frequency: circles denote 5 GHz and triangles denote 8.4 GHz. Colors denote the sources: red for FF UMa, green for HR 1099, blue for UX Ari, and orange for HR 5110. The black dashed line shows the best-fitting relation obtained in this work. Right: Relationship between the scaled radio luminosity and fractional circular polarization. The symbol colors and shapes are the same as in the left panel. The scaling factor applied to the radio luminosity of each source is indicated in the legend. The black dashed line represents the best-fitting relation derived in this work.
• Figure 5: Calibrator J0921$+$6215 in epoch 2. Left: Stokes $I$ image with its $+10\sigma$ (white solid), $+3\sigma$ (white dashed), and $-3\sigma$ (red dashed) contours. Right: Stokes $V$ image overlaid with the Stokes $I$$+10\sigma$ (black solid) contour.