High-speed circular polarimetry of AM Herculis

Abstract

The magnetic variable AM Her shows synchronous 3.09 hr variations in its optical flux and polarization, its spectral features, and its two-component X-ray flux. Similar optical behavior is seen in AN UMa and VV Pup. While most theories for these systems assume that the accreting, magnetic white dwarf is rotating with the orbital period, a ''fast rotator'' model has been suggested by Fabian et al. Their model predicts strong modulation of the optical circular polarization with a period of approx.1 minute. We have measured the circular polarization in Am Her for 6 hours with 2 s resolution and obtain an upper limit of 0.2% on the semiamplitude of any steady oscillation in the polarization with a period less than 2 minutes. Such a low limit essentially eliminates fast-rotator models for AM Her. However, the circular polarization is found to show strong flickering with time scales greater than 10 s. This is correlated with the varying optical flux, proving that the optical flickering occurs near the surface of the white dwarf where the field is strong enough for optical cyclotron emission. The lack of significant flickering with time scales less than 10 s and a 30 s delay between flickers inmore » the flux and in polarization suggest that the optical emission region is more extended than ''thin shock'' models predict.« less