VLBA imaging of quiescent radio emission from UX Arietis

Using the Very Long Baseline Array (VLBA) and the Very Large Array (VLA), w e 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 resolv ed on linear scales comparable to the projected separation of the binary co mponents, indicating the existence of large magnetic structures. Circular p olarization is detected, and a polarization gradient across the radio sourc e is observed, suggesting the existence of interacting magnetic fields betw een the components or a large magnetic loop anchored to one component. The VLBA lower limit to the brightness temperature (similar to 26 MK) is close to the broad temperature peak in the contemporaneous emission measure distr ibution seen by ASCA; we examine the possibility that the quiescent radio f lux may be gyrosynchrotron emission from a thermal electron energy distribu tion. We find that no simple thermal model can reproduce the observed spect ral index of the quiescent emission.