Timing and spectral studies of magnetic cataclysmic variables.

This thesis describes observations of a number of the magnetic cataclysmic variables known as AM Herculis stars (or polars) made in the X-ray and optical wavebands. Chapter 1 provides an introduction to cataclysmic variables (CVs), outlines their different classifications and evolution, and then describes the properties of the AM Her stars in detail. Chapter 2 describes the Ginga and ROSAT X-ray satellites from which much of the data discussed in the remaining chapters were obtained. In chapter 3 the phase resolved Ginga hard X-ray spectrum of AM Her is discussed. We find that simple thermal models do not describe the data well, with the observed iron line emission at lower energies and much stronger than expected from the inferred continuum temperatures. However, if we allow for an extra spectral component due to reflection of X-rays from the white dwarf surface, a consistent physical description of the data can be obtained. The inclusion of the hard reflection spectrum allows a reduction in the required incident continuum temperature, which provides a larger amount of thermal iron line emission at slightly lower energies. Together with the fluorescent line from reflection, this correctly accounts for the observed line energy and strength. The inclusion of the reflection component can also explain the variability of the spectrum and the iron line strength as a function of the white dwarf spin phase. Chapter 4 describes variability studies of AM Her and related systems. To begin with, the results of a search for the expected X-ray counterpart to the optical 1 - 3s quasi-periodic oscillations in the Ginga data of the three systems AM Her, EF Eri, and V834 Cen is presented. No QPO is detected, and the upper limits suggest we are not sensitive enough to the oscillating phenomenon (which is thought to be caused by an oscillating shock). The latter half of this chapter examines the longer timescale variability of AM Her as observed with Ginga, and also reports the results of some simultaneous UBVRI optical coverage. The variability observed in the Ginga band (2 - l0keV) is best described by a shot noise model, which consists of random bursts of emission with an e-folding decay timescale of 70s, allowing the characteristic lengthscale of inhomogeneities in the accreting material to be estimated. The optical red variability is found to correlate well with that observed by Ginga, whereas the optical blue does not. It is argued that multiple emitting blobs, of varying density, can account for the observed correlations. In chapter 5, the results of quasi-simultaneous ROSAT and Ginga observations of the polar QQ Vul are presented. The soft X-ray light curve is complex, at times dominated by a ~ 7.5 min timescale modulation. The detection of a soft X-ray spectral variation, together with soft and hard X-ray light curves which peak at different orbital phases, provides further evidence for the presence of two pole accretion. The data also confirms the soft X-ray excess observed previously in this object. Finally, chapter 6 describes the optical follow up observations of a new source, RE1307+535, which was discovered by the ROSAT WFC all sky survey. The optical photometry of this faint CV show it to be a short period system (Porb = 79.7min), at a distance d ? 750pc. The source is identified as a new polar on the basis of the optical discovery spectrum, and the EUV and optical light curves. The optical spectrum also reveals the presence of a strongly magnetic white dwarf, with B ~ 30-40 MG.