The X-ray eclipse of the dwarf nova HT CAS observed by the XMM-Newton satellite: spectral and timing analysis

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A cataclysmic variable is a binary system consisting of a white dwarf that accretes material from a secondary object via the Roche-lobe mechanism. In the case of long enough observation, a detailed temporal analysis can be performed, allowing the physical properties of the binary system to be determined. We present an XMM-Newton observation of the dwarf nova HT Cas acquired to resolve the binary system eclipses and constrain the origin of the X-rays observed. We also compare our results with previous ROSAT and ASCA data. After the spectral analysis of the three EPIC camera signals, the observed X-ray light curve was studied with well known techniques and the eclipse contact points obtained. The X-ray spectrum can be described by thermal bremsstrahlung of temperature kT₁=6.89 ± 0.23 keV plus a black-body component (upper limit) with temperature kT₂=30₋₆⁺⁸ eV. Neglecting the black-body, the bolometric absorption corrected flux is FBol=(6.5± 0.1)×10⁻¹² erg s⁻¹ cm⁻², which, for a distance of HT Cas of 131 pc, corresponds to a bolometric luminosity of (1.33± 0.02)×10³¹ erg s⁻¹. The study of the eclipse in the EPIC light curve permits us to constrain the size and location of the X-ray emitting region, which turns out to be close to the white dwarf radius. We measure an X-ray eclipse somewhat smaller (but only at a level of ≃ 1.5 σ) than the corresponding optical one. If this is the case, we have possibly identified the signature of either high latitude emission or a layer of X-ray emitting material partially obscured by an accretion disk.