Outburst behaviour of accretion discs in close binary systems

I present the first two- and three-dimensional simulations of accretion disc-driven outbursts in cataclysmic variables and X-ray transients. A Smoothed Particle Hydrodynamics scheme is used to model the accretion disc, which is subject to the full tidal potential of a binary system. The scheme, which includes a time-varying viscosity, is applied to cataclysmic variables and X-ray transients with differing mass ratios and orbital periods.;Simulations of the dwarf nova SS Cygni, with mass ratio q = 0.6, produce coherent outbursts and a wide range of other observationally testable results. When applied to the tidally unstable system Z Chamaeleontis (q = 0.15), the scheme reproduces superoutbursts and superhumps. The tidal and thermal instabilities are shown to be entirely decoupled: a superoutburst is simply a normal outburst in the presence of a 3:1 eccentric inner Lindblad resonance.;The model is refined for X-ray transients to include a simple treatment of irradiation of the disc by the X-rays generated at the primary. In short periods systems with correspondingly small discs, if q is small tidal effects can influence the outburst behaviour. If a small region at the outer edge of the disc remains shielded from the X-rays, tidal forces can drive this gas inwards through the disc, producing the rebrightening that has been observed in several X-ray light curves of soft X-ray transient outbursts.;At longer periods, the disc cannot ever entirely be heated by the X-rays and more complex outburst behaviour is possible. Simulations show that this behaviour is determined by competition between the variable size of the irradiated region and the thermal instability operating in the cool region.