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 ± 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^{Bol}=(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. In a standard accretion scenario where L_BL≃ 0.125 L_acc assuming Ω _WD ≃ 0.5 Ω_K(R_WD), the amount of matter accreting onto the central white dwarf is found to be 1.7 × 10 ^-11 M_⊙ 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 ≃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.