New polarimetric observations and a 2 pole model for the cyclotron emission from AM Herculis

Abstract

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 is presented. The data have been analyzed 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. Th main region has a high-density edge at magnetic colatitude theta = 16 deg and extends up to theta to about 8 deg at almost constant magnetic longitude. The secondary region has a similar angular extent in theta but a larger width in magnetic longitude psi (Delta psi about 20-30 deg). The linear bright phase which, in the 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. It is shown 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.