THE DISCOVERY OF AN X-RAY ECLIPSE DURING A LOW STATE OF THE DWARF NOVA HT CASSIOPEIAE

X-ray and optical observations of the dwarf nova I-IT Gas, some simultaneous, are presented. The X-ray light curve shows that the X-ray source is eclipsed and comes from the near vicinity of the white dwarf. It must therefore be a boundary layer. The X-ray flux (0.1-1.2 keV) is similar to 5x10(-13) erg s(-1). The spectrum is consistent with an absorbed thermal bremsstrahlung model with temperature kT similar to 2.4 keV and hydrogen column density n(H) similar to 1.8x10(20) cm(-2). The low luminosity in the boundary layer, L similar to 5x10(30) erg s(-1), may be caused by having a low mass-transfer rate on to the white dwarf, of similar to 1x10(-12) M. yr(-1), or perhaps the presence of a rapidly rotating white dwarf. The optical observations show that HT Cas has low states where the accretion rate in the disc is dramatically reduced from the usual quiescent level. The drop from a normal to a low state can occur in less than two weeks. The measured white dwarf fluxes in the low state show that the white dwarf has a temperature of similar to 13 200 K, cooler than the temperature determined from the normal state of 18 700 K. The cooling of the white dwarf must occur in less than two weeks. The ROSAT observations were obtained during a low state.