New polarimetric observations and a two pole model for the cyclotron emission from AM Herculis

We present high signal-to-noise ratio phase dependent linear polarization and intensity data on AM Herculis which show the presence of structured linear pulses in the optical and near IR regions. The data have been analysed to construct a detailed model which reproduces closely the observed pulse structure and polarization angle variations and clearly demonstrates the presence of two interfering cyclotron emission regions located close to the foot points of a closed field line in an offset dipole field distribution. Both emission regions are linearly extended on the white dwarf surface. The main region has a high-density edge at magnetic colatitude θ=16° and extends up to θ ∼ 8° at almost constant magnetic longitude. The secondary region has a similar angular extent in θ but a larger width in magnetic longitude ψ (ΔΨ ∼20-30°). The linear bright phase which, in our model corresponds to the phases of visibility of the secondary region, coincides with the X-ray bright phase of the anomalous state of AM Herculis. We show that the positions of the emission regions and their relative contributions to the total intensity change with time and that the linear pulses in different wavebands originate from different regions of structured shocks. The coupling region in the orbital plane extends from 18 R_wd to 7 R_wd with the distance of closest approach being about a third of the value that is calculated from standard theory for the magnetospheric radius.