X-Ray Observations of HT Cas and UX UMa

Abstract

X-ray and optical observations of the dwarf nova HT Cas were obtained (Wood et al. 1994a). 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 ~ 5×10−13 ergs s−1. The spectrum is consistent with an absorbed thermal bremsstrahling model with temperature, kT ~ 2.4 keV and hydrogen column density, n H ~ 1.8 × 1020 cm−2. The low luminosity in the boundary layer, L ~ 5 × 1030 ergs s−1, may be the result of having a low mass transfer rate onto the white dwarf, of ~ 1 × 10−12 M ⊙ yr −1. The optical observations showed that HT Cas has low states where the accretion rate in the disc is dramatically reduced from the usual quiescent level. The white dwarf fluxes in the low state show that the white dwarf has a temperature of ~ 13200 K, cooler than the temperature determined from the normal state of 18700 K. The cooling of the white dwarf must occur in less than two weeks. The ROSAT observations were obtained during a low state.