ROSAT Observations of 3 Cataclysmic Variables

Abstract

Ten cataclysmic variables have been observed with the ROSAT Position-Sensitive Proportional Counter (PSPC). The X-ray spectra can be fitted with optically thin spectra, but several spectra require more than one component. The high emission measures indicate that the emitting plasma is optically thick in strong resonance lines. Variability in the count rate is discovered in seven systems; in the case of VW Hyi this variability is partly orbital. The hardness ratio varies only in IX Vel and GP Com, and is mainly orbital in the latter. The count rate variability in systems that do not show spectral variation can be described with variation of the observable emission measure. In one 9 minute observation the count rate of GP Com recovered from an unusually low initial value, possibly due to varying absorption. During optical decline from a dwarf-nova outburst, the X-ray flux of WX Hyi is observed to be lower than in quiescence between outbursts by a factor approximately 6. The X-ray flux is generally much less than the ultraviolet flux, especially in systems with a high accretion rate.