Cyclotron Radiation From Magnetic Cataclysmic Variables (Polarization, Plasmas, Magnetized, Stars, Herculis, Puppis).

Abstract

The absorption coefficient for the ordinary and extraordinary modes of wave propagation are calculated for cyclotron radiation from hot magnetized plasmas (kT < 50 keV). Two relativistic methods are used to calculate the absorption coefficients: the dielectric formulation and the single particle formulation. A nonrelativistic approximation which includes the effects of inverse-bremsstrahlung and Thomson scattering (collisions) is also made. The equations of radiative transfer for a homogeneous plasma, with large Faraday rotation, are solved, and simple analytic expressions for the Stokes parameters Q and V are derived in terms of the optical depths in both modes. The results are applied to the accretion columns of AM Herculis binaries. The inclusion of collisional effects in the nonrelativistic approximation reduces the amount of fractionally polarized light to levels which agree better with the observations. For small viewing angles with the magnetic field, the circular polarization does not approach 100% as is observed in the relativistic calculations without the effects of collisions. The polarization approaches a value much less than 100%. This result may provide a qualitative explanation of the standstill which observed in some AM Herculis binaries. Comparisons of theoretical and observational circular polarization curves for AM Herculis give surprisingly good agreement for a magnetic field of 2.7 x 10('7) gauss, temperature of 0.2 keV, and plasma slab thickness of 2.6 x 10('8) cm. The detection of cyclotron lines in the optical spectrum is limited to a small parameter space in magnetic field ((2-10) x 10('7) gauss), plasma temperature (< 15 keV), and direction of the accretion column (nearly perpendicular to the line of sight for extended periods of time; (TURN)10 minutes). Theoretical spectra confirm the conclusion by Wickramasinghe and Meggitt (1982) that the broad lines in VV Puppis are due to cyclotron emission, but dispute their conclusion that the addition of an unpolarized component of radiation in the blue and UV spectrum is required. A best fit to the data of VV Puppis yields a polar magnetic field of 3.15 x 10('7) gauss, a postshock temperature of 8.7 keV, and a dimensionless plasma parameter of (TURN)10('6).