Variability of soft X-ray emission of EX Hydrae observed with Einstein Observatory

The cataclysmic variable EX Hydrae has been observed in X-rays with Einstein Observatory. A 67 min periodic X-ray modulation has been found in the energy range. 1 – 2 keV, whereas the source is approximately constant above 2 keV. The modulation is approx. in phase with the stable 47 min modulation in the optical brightness. The X-ray spectrum changes slightly with the 67 min phase but is consistent with a two component model with constant temperature only changing in relative intensity. Quasi-simultaneous optical observations were also obtained and used to extend the time base of the optical modulation. The result is indicative of a decrease in the 67 min period on a time scale of 3 10⁶ yr. The implications for the hypothesis of a non-uniform rotating white dwarf as the origin of the 67 min optical and X-ray modulation are discussed.