paper

X-ray and UV observations of the dwarf nova VW Hyi in quiescence

arXiv:astro-ph/0309418 · doi:10.1111/j.1365-2966.2003.07199.x

Abstract

We present an analysis of X-ray and ultra-violet data of the dwarf nova VW Hyi that were obtained with XMM-Newton during the quiescent state. The X-ray spectrum indicates the presence of an optically thin plasma in the boundary layer that cools as it settles onto the white dwarf. The plasma has a continuous temperature distribution that is well described by a power-law or a cooling flow model with a maximum temperature of 6-8 keV. We estimate from the X-ray spectrum a boundary layer luminosity of 8*10^30 erg/s, which is only 20 per cent of the disk luminosity. The rate of accretion onto the white dwarf is 5*10^-12 solar masses per year, about half of the rate in the disk. From the high-resolution X-ray spectra, we estimate that the X-ray emitting part of the boundary layer is rotating with a velocity of 540 km/s, which is close the rotation velocity of the white dwarf but significantly smaller than the Keplerian velocity. We detect a 60-s quasi-periodic oscillation of the X-ray flux that is likely due to the rotation of the boundary layer. The X-ray and the ultra-violet flux show strong variability on a time scale of ~1500 s. We find that the variability in the two bands is correlated and that the X-ray fluctuations are delayed by ~100 s. The correlation indicates that the variable ultra-violet flux is emitted near the transition region between the disk and the boundary layer and that accretion rate fluctuations in this region are propagated to the X-ray emitting part of the boundary layer within ~100 s. An orbital modulation of the X-ray flux suggests that the inner accretion disk is tilted with respect to the orbital plane. The elemental abundances in the boundary layer are close to their solar values.

arXiv commentAccepted for publication in MNRAS, 12 pages, 6 figures

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