Linear polarization in AM Herculis objects

The three known AM Herculis--like objects, AM Her, AN UMa, and VV Pup, show a temporal pulse of linear polarization which is synchronous with the periodic variations in their circular polarization, optical flux, and spectral features. The three are thought to be close binary systems with material accreting onto a highly magnetic white dwarf. We suggest that the observed linearly polarized light is created in the accretion column near one of the white dwarf's obliquely oriented magnetic poles. The light is emitted perpendicular to the magnetic field in a fanlike beam. The strong angular dependence may be the result of the combined effects of high harmonic cyclotron emission and Faraday depolarization. For axially symmetric accretion about the magnetic pole, the phase-locked rotation of the white dwarf will bring the fan into view twice per period. In this idealized case, polarization data for both pulses would yield a detailed picture of the accretion geometry. Broad asymmetric flow into the pole region may dilute or obscure the fan and thus allow only one pulse to be seen.