Cyclotron emission from AM Herculis binaries - A variable magnetic field, variable temperature model

Abstract

This paper examines cyclotron emission from a hot plasma flowing to the surface of a magnetic white dwarf. The two-temperature plasma free-falls along the axis of a dipole magnetic field to the white dwarf surface and cools through bremsstrahlung and optically thick cyclotron emission. It is shown that the ion temperature is constant until the magnetic field is of order of 10 MG. This result is weakly dependent on the accretion rate and the strength of the surface magnetic field. Spectra and polarization fractions are calculated for a variety of model parameters. The spectra have a broad peak at optical wavelengths for surface field strengths above about 20 MG. The emission is highly polarized at all frequencies. Cyclotron harmonic structure is often produced for low surface field strengths. This model produces the observed variation of polarization with orbital phase at both optical and infrared wavelengths. It is argued that the observed magnetic field strengths are a consequence of the plasma cooling in a 30-MG region, and that the surface field strength for some of these systems may actually be larger than the observed values. 26 refs.