Spiral structure in the accretion disc of the binary IP Pegasi

We have found the first convincing evidence for spiral structure in the accretion disc of a close binary. The eclipsing dwarf nova binary IP Peg, observed during the end phase of a rise to outburst, shows strong Palmer and helium emission lines in its spectra, with asymmetric double-peaked velocity profiles produced in the accretion disc around the white dwarf. To reveal the two-armed spiral on the accretion disc, we de-project the: observed emission-line profiles on to a Doppler coordinate frame, a technique known as Doppler tomography. The two-armed spiral structure that we see in the Doppler tomograms is expected to form when the disc becomes sufficiently large in outburst that the tides induced by the secondary star can excite waves in the outer disc. Such spiral waves have been predicted in studies of tidal interaction on discs and are fundamental to an understanding of the angular momentum budget of accretion discs.