A Steady State Model of X-Ray Emitting Coronae around Accreting Non-Magnetic White Dwarfs in Cataclysmic Variables

Abstract

The X-ray emission of non-magnetic cataclysmic variable stars is considered. It is well known that a nearly spherical hot corona is produced around accreting white dwarfs because of strong viscous heating, and of rather inefficient cooling of matter at the boundary layer if the accretion rate is below some critical value. A steady state model of X-ray emitting coronae, into which mass is steadily supplied from the boundary layer, is then constructed and the X-ray spectra are calculated as functions of the mass-supply rate Ṁs. Eclipses of such corona were simulated. It has been found that the thermal energy of the corona is transported downward by conduction to the denser area near to the surface of the white dwarf, where it is emitted as X-ray radiation. Around 5% of the matter supplied by the disk could leave the system as stellar wind. Most of the X-rays are radiated from regions near to the white dwarf surface within this model. Eclipse simulations of this model agree with recent observations of eclipsing system HT Cas.

X‐Ray Observations of the Boundary Layer in Dwarf Novae at Low Accretion Rates

“…the transition region between the disk and the white dwarf, is poorly understood, and theoretical modelling is complicated by the strong shearing and turbulence present in the accretion flow. A number of widely different models have been proposed, including a disk-like boundary layer (Narayan & Popham 1993), a coronal siphon flow (Meyer & Meyer-Hofmeister 1994), a spherical corona (Mahasena & Osaki 1999), and a hot settling flow (Medvedev & Menou 2002).…”

Analysis of the oscillations in HST observations of the quiescent SU UMa type dwarf nova WZ Sagittae

“…Calculations have shown ([.g. (Mahasena & Osaki 1999)] that for this model the X‐rays come from a region extremely close to the surface of the white dwarf and so could still plausibly show an oscillation driven by the white dwarf pulsation.…”

The X-ray Properties of Cataclysmic Variables

“…Truncated accretion disks in Dwarf Novae may also help to explain quiescent brightness levels and also high Ṁ during quiescence in some DNs. Models have been suggested where irradiation by the WD (King 1997), a coronal siphon flow (Meyer & Meyer-Hofmeister 1994, Liu et al 1995, WDs ro-tating near break up velocity (Ponman et al 1995), a spherical corona (Mahasena & Osaki 1999), or a hot settling flow (Medvedev & Menou 2002) that may be responsible for the distuption of the inner disc explaining the lack of X-ray emission and formation of coronal flows.…”