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

Context. 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. Aims. 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. Methods. 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. Results. The X-ray spectrum can be described by thermal bremsstrahlung of temperature $kT_1=6.89 \pm 0.23$ keV plus a black-body component (upper limit) with temperature $kT_2=30_{-6}^{+8}$ eV. Neglecting the black-body, the bolometric absorption corrected flux is $F^{\rm{Bol}}=(6.5\pm 0.1)\,\times\,10^{-12}$ erg s-1 cm-2, which, for a distance of HT Cas of 131 pc, corresponds to a bolometric luminosity of $(1.33\pm 0.02)\,\times\,10^{31}$ erg s-1. In a standard accretion scenario where $L_{\rm BL}\simeq 0.125 L_{\rm acc}$ assuming $\Omega _{\rm WD} \simeq 0.5 \Omega_{\rm K}(R_{\rm WD})$, the amount of matter accreting onto the central white dwarf is found to be $1.7\,\times\, 10 ^{-11}~M_{\odot}$ yr-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 $\simeq$$1.5 \sigma$) 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.