Tidal torques, disc radius variations, and instabilities in dwarf novae and soft X-ray transients

Abstract

The study of outer disc radius variations in close binary systems is important for understanding the structure and evolution of accretion discs. These variations are predicted by models of both quasi steady and time-dependent discs, and these predictions can be confronted with observations. We consider theoretical and observational consequences of such variations in cataclysmic variables and low mass X-ray binaries. We find that the action of tidal torques, that determine the outer radius at which the disc is truncated, must be important also well inside the tidal radius. We conclude that it is doubtful that the tidal-thermal instability is responsible for the superoutburst/superhump phenomena in dwarf novae, and confirm that it cannot be the reason for the outbursts of soft X-ray transients. It is likely that tidal torques play a role during superoutbursts of very-low mass-ratio systems but they cannot be the main and only cause of superhumps.