10 - X-rays from cataclysmic variables

Summary

Introduction

Cataclysmic variables (CVs) are a distinct class of interacting binaries, transferring mass from a donor star to a degenerate accretor, a white dwarf (WD). In all observational determinations, and as is required by theory for stable mass transfer, the donor star is of lower mass than the accretor. For comprehensive overviews on the subject of CVs we refer to Hack & La Dous (1993) and Warner (1995).

The majority of CVs have orbital periods, P_orb, between 75 min and 8 h (see Ritter & Kolb 2003) and consist of Roche lobe-filling main sequence donors and WDs. These are WD analogues of the low-mass X-ray binaries (LMXBs; see Chapter 1). In the period range 8 h–3 d the donors must have larger radii than dwarfs in order to fill their Roche lobes and are therefore evolved subgiants. A few CVs are found with P_orb ∼ 200 d, which require giant donors for them to be lobe-filling. The absence of evolved CVs with periods ∼3 to ∼200 d is connected with the dynamical instability that results from an initial donor that had a mass larger than about 67% of that of the WD; such binaries will have experienced rapid mass transfer and shortened their periods during a common envelope phase (e.g., Iben & Livio 1993; see also Chapter 16). Beyond P_orb ∼ 200 d, mass-transferring systems also exist.