Discs in cataclysmic variables and X-ray binaries

Summary

Abstract I review recent progress in the study of accretion discs in cataclysmic variables (CVs) and X-ray binaries. Observations of CVs, especially eclipse mapping, give detailed agreement with steady-state disc theory. Coronae and winds are probably universal features of discs in such systems. Our present ignorance of the disc viscosity is the main barrier to progress in understanding time dependence and stability properties. Non-axisymmetric structure is particularly prominent in observations of low-mass X-ray binaries. This may be caused by the interaction of the mass transfer stream from the companion star with the disc.

Introduction.

Cataclysmic variables (CVs) are close binary systems, having periods of a few hours, in which a white dwarf accretes material from a main-sequence companion which fills the Roche lobe. If the white dwarf is replaced by a neutron star or black hole we have a low-mass X-ray binary (LMXB).

The formation of an accretion disc lying in the orbital plane is very likely under these circumstances since the accretion stream from the companion is highly supersonic and follows an essentially ballistic trajectory; its closest approach to the accreting object is a few ×10⁹ cm, larger than the radius of any likely accreting object. The resulting self-collisions of the stream imply dissipation. As this can remove energy much more effectively than angular momentum the matter arranges itself into a collection of orbits of lowest energy for fixed angular momentum, i.e.