Cataclysmic variables in the extreme ultraviolet.
thesisposted on 19.11.2015, 09:18 by Peter James. Wheatley
The UK ROSAT Wide Field Camera was used to carry out the first all-sky survey at extreme- ultraviolet wavelengths (EUV). In this thesis, an optimised method for the extraction of survey light curves is developed and applied to an all-sky sample of eighty-one cataclysmic variables. Twenty-two sources were detected with high confidence. The sample of detected systems is dominated by AM Hers, which are found to be exceptionally strong EUV sources. Six of these are new systems, identified through optical follow up of survey sources, and these EUV-selected systems are found to have the most extreme EUV/optical flux ratios. Orbital periods are detected in most AM Hers, and the implications of the new orbital-period distribution of AM Hers are discussed. Phase-folded light curves of a flux limited sample are presented and spectral constraints are applied. Three AM Hers show evidence of spectral modulation with phase. Three intermediate polars are detected, though a distinct EUV emission component is required only in the case of RE 07514+14. Upper limits to other systems show that RE 0751 is unusually bright within the bandpass of the WFC. Limits are placed on temperatures and absorption columns of similar components in other systems, but the WFC does not provide useful limits on the luminosities of such components. The energy balance of intermediate polars remains an open question. Six non-magnetic cataclysmic variables were detected, four of which are dwarf novae caught in their outburst state. These include VW Hyi and SS Cyg, which are discussed in the context of extensive multi-wavelength coverage; incorporating optical, ultraviolet and X-ray observations. SS Cyg was detected as a strong EUV source during decline, which was found to be faster than the optical decline. VW Hyi was observed throughout an optical outburst, with but no enhancement of EUV flux. This suggests the EUV flux detected by EXOSAT must lie softwards of ~ 70 eV. WFC limits show that the boundary layer of VW Hyi during outburst must be less luminous than the disk. In VW Hyi and SS Cyg the hard X-ray flux is suppressed during outburst, and remains low until the very end of optical decline. This is taken as evidence that the hard X-ray flux is regulated by the boundary layer.