AR Ursae Majoris: The First High-Field Magnetic Cataclysmic Variable

We identify the luminous soft X-ray source AR UMa as a magnetic cataclysmic variable containing a white dwarf with the highest field yet detected in an accreting binary. lUE and optical spectroscopy, optical photometry, and circular polarimetry and spectropolarimetry define remarkably distinct accretion states of this binary. Circular polarization is nearly absent in the high state, but the low state exhibits values which vary between 2% and 5% on the orbital period of 1.932 hr. The UV continuum contains a broad absorption feature near 1300 Å, while optical spectropolarimetry during the low state reveals a number of strongly polarized dips. These are interpreted as Zeeman components of hydrogen Lyα and another atmospheric species, possibly He I, in a photo spheric magnetic field of ∼230 MG.

The radial velocity curve of the low-state optical emission lines shares the period of the optical photometry and polarimetry and is phased appropriately for an origin on the irradiated secondary star. While the high state exhibits prominent UV line emission typical of the magnetic variables, the strength of the UV continuum does not vary appreciably with a change in accretion state. This, combined with the high soft X-ray luminosity and lack of circular polarization, indicates that accretion occurs largely in the form of dense filaments which avoid a standoff shock and thermalize their kinetic energy below the white dwarf photo sphere. We suggest that these phenomena may play a role in the apparent lack of high-field systems with easily detectable circular polarization during high-accretion states.