AR Ursae Majoris Discovered to Be a Persistent Radio Polar: Results from a VLA Survey of Magnetic Cataclysmic Variables

Abstract

We conducted a VLA survey of nine magnetic cataclysmic variable stars (MCVs) at 8.436 GHz in 2003 October, to constrain models of radio emission. A follow-up study was conducted in 2004 September. We obtained the first radio detections of AR UMa, the MCV with the highest known magnetic field (230 MG), at flux densities of 0.422 ± 0.060 mJy (2003 October 16) and 0.734 ± 0.095 mJy (2004 September 4). We have completed the sample of radio observations of MCVs out to 100 pc. Surprisingly, the only radio-selected CV, FIRST J1023+0038, was not detected. An upper limit of 0.20 mJy (4 σ) was obtained. The original radio MCV, AM Her, was detected with a flux density of 0.586 ± 0.074 mJy (2003 October 17), consistent with previous observations. Typical (4 σ) upper limits of 0.12 mJy were obtained for other targets. The lack of radio emission from isolated magnetic white dwarfs, as well as from nonmagnetic CVs (outside of outburst), places constraints on radio emission mechanisms for CVs. We suggest that accretion disks may preclude radio emission. Also, since we detect radio emission from AM Her in a low state and the pre-CV V471 Tau is a persistent radio source, accretion is not a necessary condition. A secondary with a kilogauss magnetic field might be necessary. We argue that the emission from AR UMa originates near the secondary. AR UMa joins AM Her and AE Aqr as the only confirmed persistent radio MCVs, and we suggest avenues of further study.