X-ray observations of the eclipsing nova-like variable UX UMa

ROSAT observations of the eclipsing nova-like variable UX UMa are presented. 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 ∼0.3, where this is defined as the number of counts above 0.5 keV divided by the number of counts below 0.5 keV in the PSPC bandpass. This hardness ratio is extremely low, even compared to other nova-like variables. The X-ray luminosity, Lx∼1030 erg s−1, is a factor of 10 lower than other, lower inclination, nova-like variables, and much lower than the luminosity expected from a boundary layer. UX UMa was marginally detected in the extreme-ultraviolet (EUV) at the 3 σ level. For the wide field camera (WFC) with the S1A filter its count rate is 3±1×10−3 count s−1. Phase-folded light curves show no evidence for the presence 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 σ 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 come from a corona or wind above the accretion disc or the secondary star, but probably from a combination of these.