HEAO-A2 SOFT-X-RAY SURVEY OF CATACLYSMIC VARIABLE-STARS - EX HYDRAE DURING OPTICAL QUIESCENCE

X-ray emission below 2 keV has been detected for the first time from a source whose X-ray variability and position favour identification with EX Hydrae. The soft X-ray intensity was observed to vary by a factor of 3 in 1 hr. A temperature in excess of 1 x 107K was measured for the dwarf nova, which was in an optically quiescent state during the X-ray observation. A thermal bremsstrahlung spectrum with kT = 4.5 keV and a column density of l-7(-no) x 1021 cm-2 gives the best fit to the data, and implies a total X-ray luminosity (E= 0.15-10 keV) of ~2.4xl032 x (7/100pc)2erg/s. Models for the production of X-rays by accretion on to a white dwarf are discussed in light of this new evidence. © Royal Astronomical Society.