VLBA Imaging of Quiescent Radio Emission from UX Arietis

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Abstract
Using the Very Long Baseline Array (VLBA) and the Very Large Array (VLA), we have observed the quiescent coronal microwave emission from the active RS CVn system UX Ari. We complement the data with X-ray information obtained from the X-ray satellite ASCA. The 8.4 GHz continuum radio source is resolved on linear scales comparable to the projected separation of the binary components, indicating the existence of large magnetic structures. Circular polarization is detected, and a polarization gradient across the radio source is observed, suggesting the existence of interacting magnetic fields between the components or a large magnetic loop anchored to one component. The VLBA lower limit to the brightness temperature (~26 MK) is close to the broad temperature peak in the contemporaneous emission measure distribution seen by ASCA; we examine the possibility that the quiescent radio flux may be gyrosynchrotron emission from a thermal electron energy distribution. We find that no simple thermal model can reproduce the observed spectral index of the quiescent emission.