X-ray and optical studies of intermediate polars

Intermediate polars form a small (~12 members) subclass of the cataclysmic variables, The latter comprise a red dwarf secondary and a more massive accreting white dwarf, but in intermediate polars the white dwarf is deduced to have a magnetic field of strength ~1 MG, from the pulsed emission which these stars exhibit. These systems are a relatively new discovery (less than two decades old), and therefore provide an active domain for current research. The present work concerns the accretion geometry in intermediate polars, an area of considerable debate. After an introduction to the field (chapter 1), I present, in chapter 2, an analysis of the X-ray orbital modulations seen in EXOSAT timeseries from intermediate polars. These data have substantial implications for the accretion geometry in these systems. The bulk of this thesis, however, is concerned with the systems BG Canis Minoris and GK Persei. Chapter 3 presents optical CCD photometry of BG CMi, and I refine the 913 s pulsational ephemeris using timings both new and previously published. In chapters 4 and 5, I present spin phase-resolved optical spectroscopy on these respective systems. A similar analysis of other systems has shown that this is a powerful method for probing the geometry and optical emission properties of the accretion flow in the vicinity of the white dwarf. This is of particular interest in BG CMi, where both the presence of an accretion disc and the spin period of the white dwarf are currently debated. In GK Per the existence of a disc is essentially incontestable, so the data presented in chapter 5 offer an excellent opportunity to investigate the curtain model which has been proposed to explain the spin modulation in intermediate polars. Finally, I detail, in an appendix, the various techniques which I have employed for the reduction and analysis of the data contained in the previous chapters.