X-RAY OBSERVATIONS OF THE ECLIPSING NOVA-LIKE VARIABLE UX UMA

ROSAT observations of the eclipsing nova-like variable UX UMa are pres ented. X-ray telescope (XRT) observations with the Position Sensitive Proportional Counter (PSPC) show that the X-ray spectrum is extremely soft, with a hardness ratio of similar to 0.3, where this is defined a s the number of counts above 0.5 keV divided by the number of counts b elow 0.5 keV in the PSPC bandpass. This hardness ratio is extremely lo w, even compared to other nova-like variables. The X-ray luminosity, L (x) similar to 10(30) erg s(-1), is a factor of 10 lower than other, l ower inclination, nova-like variables, and much lower than the luminos ity expected from a boundary layer. UX UMa was marginally detected in the extreme-ultraviolet (EUV) at the 3 sigma level. For the wide field camera (WFC) with the S1A filter its count rate is 3 +/- 1 x 10(-3) c ount s(-1). Phase-folded light curves show no evidence for the presenc e of an X-ray eclipse; indeed we can rule out the presence of a total eclipse of a source of X-rays close to the white dwarf at the 2.9 sigm a level. A model where the X-rays from the boundary layer are obscured from view at all orbital phases can explain the low luminosity of the observed X-rays and the lack of an eclipse. The observed X-rays may c ome from a corona or wind above the accretion disc or the secondary st ar, but probably from a combination of these.