Characterisation of the Components in Cataclysmic Variables

Abstract

This thesis presents new and archive X-ray, optical and ultraviolet observations of cataclysmic variables, and discusses the results obtained in the context of the relationship between the physical and spectral components visible. It concentrates on the eclipsing dwarf nova OY Carinae, which has been observed both in superoutburst and quiescence. Optical 'wide B' band light curves were obtained at the end of the 1994 superoutburst and on the decline. Eclipse mapping of these light curves reveals an accretion disc with a considerable physical flare (~ 10'). These are the first maps of a disc in the superoutburst state that clearly show such flaring. Contemporaneous X-ray observations were obtained with the ROSAT HRI. No eclipse of the X-ray flux was detected, a similar result to that obtained from the EXOSAT observations of the 1985 superoutburst by Naylor et al. (1988), supporting the case for the existence of a coronal component source for the X-rays in high accretion rate systems. Later ROSAT observations of OY Car in quiescence are presented. A 1994 PSPC observation allows constraints to be placed on the quiescent X-ray spectrum. OY Car returns similar values for the temperature of the emitting gas and the emission measure as its fellow eclipsing systems. It is difficult, however, to reconcile the column density inferred from the X-ray observation with that found from HST ultraviolet observations. A column density of nH = 10 22 cm 2 , found in the 'iron curtain' study by Horne et al. (1994), is not compatible with the X-ray spectrum. New optical photometry, obtained during quiescence in 1998, is used to update the orbital ephemeris of the system. A further ROSAT HRI light curve, obtained in quiescence with good phase coverage, is presented. It displays an eclipse of the X-ray flux, detected at the 13o level, which is coincident with the optical eclipse of the primary and suggests that the region of X-ray emission is comparable in size to the white dwarf. This confirms that the boundary layer region is visible in OY Car in quiescence, and, in common with similar systems, implies that the boundary layer is the source of the X-ray flux during periods of low mass accretion. The general picture of the X-ray emission from CVs, which has been built up from observations of different objects using different satellites, has thus been confirmed for the first time for the same object using the same satellite. The fact that the quiescent boundary layer can be seen, and that the soft X-ray flux is not extinguished, together suggest that the 'iron curtain' may be variable and connected to the accretion. The thesis also explores for the first time the effect a 'warm absorber'would have on the X-ray spectrum and the deduced column densities of high inclination CVs in quiescence. The application of synthetic spectral analyses to ultraviolet observations of CVs is extensively reviewed, with particular emphasis on the spectral components observed. White dwarf model atmospheres and synthetic spectra are generated using TLUSTY and SYNSPEC, and those IUE spectra of cataclysmic variables where the white dwarf can be seen are modelled using a X2 fitting routine. White dwarf synthetic spectra veiled by an 'iron curtain' are also calculated and applied to JUE archive observations of OY Car and similar systems. The resulting independently estimated white dwarf temperatures are compared with published values and with temperatures obtained from the application of different model atmosphere codes. The column density found by Home et al. (1994) is confirmed. IUE archive observations of U Gem show a similar cooling time (~ 30 days) to more recent HST observations. It is shown that the major source of systematic error in estimating the white dwarf temperature from ultraviolet observations is uncertainty in the masses of the white dwarfs. The final Chapter shows how scheduled (simultaneous) HST and ASCA observations of OY Car in quiescence will be used to place further constraints on the temperature of the emitting gas and the column density, and how the techniques developed for the ultraviolet modelling can be applied to the HST data. Future (accepted) XMM observations of UX UMa1, to constrain spectral components in the high mass accretion rate regime, are also discussed