Polarimetry of magnetic cataclysmic variables
Doctoral Thesis
1985
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University of Cape Town
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Abstract
The design and construction of an astronomical polarimeter is described and an evaluation made of its performance. Extensive observations of cataclysmic variables with emphasis on the AM Her and DQ Her classes are then presented. After consideration of the basic principles involved in the development of an efficient and accurate polarimeter, a design using two super-achromatic retarders (a 1/4 wave and a 1/2 wave) rotating above a fixed analyser was adopted. This permitted simultaneous linear and circular polarisation measurements, or, by rearranging the order of the retarders in the beam, linear polarisation measurements alone, or circular polarisation measurements alone, with enhanced efficiency. The polarimeter was found to have extremely low instrumental polarisations and, because of the superachromatic retarders used, the efficiency correction factors were very close to 1 at all wavelengths. The polarisations are calculated at the telescope and the light curve at a higher time resolution may also be recorded if this is required. Extensive sets of observations using the polarimeter were obtained for six of the ten AM Her variables (or "polars"). EF Eri and El405-451 were observed most. Evidence was found in El405-451 for movement of the apparent location of the accretion region on the primary star and the inclination and magnetic dipole off set from the rotation axis was determined. This allowed a comparison to be made between the competing models for the cyclotron emission, showing that those which take into account the temperature structure of the accretion region provide the best results. Observations and an analysis of the polarisation data from H0139-68, E2003+225, VV Puppis and PG1550+191 are also presented in some detail. A final chapter presents results from observations made to detect a modulation in the polarisation at the rotation period of the primary in the DQ Her variables. Upper limits are set for 4 members of the class and the implications of the results are discussed.
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Bibliography: pages 136-138.
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Reference:
Cropper, M. 1985. Polarimetry of magnetic cataclysmic variables. University of Cape Town.