Simulations of spiral structure in the accretion disc of IP Pegasi during outburst

We consider the implications of the detection of spiral structure in the accretion disc of the binary IP Pegasi. We use numerical simulations of the development of a disc outburst to construct predicted Doppler tomograms, which are found to be in close agreement with the observations if the spiral pattern arises as a transient feature when the disc expands viscously at the start of the outburst. The good agreement of such viscous disc simulations with the data is consistent with models in which most of the angular momentum transport in the disc originates in internal stresses rather than globally excited waves or shocks. Future detailed observations of the development of transient spiral features offer the potential to measure the dependence of the disc viscosity on the local physical conditions in the disc.