Mullard Space Science Laboratory

S. B. Potter
A multiwavelength study of the accretion region in magnetic cataclysmic variables

1998 (supervisor: M. S. Cropper)

This thesis is a study of the accretion regions in magnetic cataclysmic variables (MCVs).

After an overview of our current understanding of accretion regions in MCVs, I describe a model I have developed which models the polarized cyclotron emission from MCVs. I then use this model to show how different types of cyclotron light curves arise for various shaped accretion regions.

I then use this model on the MCV PQ GEM. PQ Gem is an asynchronous intermediate polar (a subclass of the MCVs) which was discovered to have polarized emission on the spin period of the white dwarf. It was therefore an ideal candidate to observe and model. I found that the polarized light curves could be modelled by emission from two extended emission regions located on opposite hemispheres of the white dwarf. The model light curves could explain the observations as a combination of self occultation and absorption of the emission regions. The model is also more self consistent than other attempts in that it models the multi colour observations of PQ Gem simultaneously and is in agreement with X-ray observations. I also give an estimate for the magnetic field strength of the white dwarf in this system.

Several authors over the past decade have modelled cyclotron emission from MCVs using their own models. However model fits have until now been obtained by a trial and error method. I obtained the results for PQ Gem by a similar method. Therefore I have developed an optimisation technique that is more analytical and objective that previous methods. This was achieved with the combination of a genetic algorithm and a more traditional line minimisation routine. I described the technique in detail and rigorously test it on simulated light curves.

Having developed the optimisation technique, I then applied it to real data in order to predict the shape and size of their emission regions. The results are compared to previous models and preconceptions on the shape of emission regions. Also, by making assumptions on the nature of the magnetic field, I then trace the magnetic field lines that feed the emission regions to the orbital plane in order to locate the threading regions in these systems.

Finally, I give a summary of this thesis and discuss possible work for the future.


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