Circular polarization in the radio emission of RS canum venaticorum binaries

A puzzling feature of the polarization of the radio emission of RS CVn binary systems is the observed reversal in the sense of polarization between high and low frequencies. This reversal has been assumed to be a property of the quiescent emission of these systems, which is widely agreed to be gyrosynchrotron emission. However, the reversal has proved difficult to explain in terms of plausible gyrosynchrotron models, and we demonstrate that its location with respect to the spectral peak is not consistent with such models. Instead we propose that a form of coherent radio emission is common at low frequencies in these systems and present several examples in which such a component is clearly present. This coherent component is highly circularly polarized in the sense opposite to that of the higher frequency gyrosynchrotron emission. At low freqeuncies the gyrosynchrotron component is also present and may be stronger than the coherent component, but is probably weakly polarized so that the polarization of the integrated emission is dominated by the coherent component. Based on the sense of polarization of the coherent component, we suggest that it is probably a form of plasma emission. It can show both rapid variability as well as slower modulation similar to that of the gyrosynchrotron component and has a bandwidth of at least 80 MHz. We also point out that the observed fact that the degree of polarization of the quiescent emission at high frequencies tends to increase with frequency is also inconsistent with gyrosynchrotron models.