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D. Steeghs, K. Horne (U. St.Andrews, UK), R. Stehle (U. Leicester, UK), E. Harlaftis (Astronomical Inst., Obs. of Athens, Greece)
Motivated by the repeated detection of spiral arms in the accretion disc of the binary IP Pegasi during several outbursts, we compare the current state of observations with numerical simulations of the accretion flow in close binary discs. The properties of the spiral arms, and their imprint on observational data, is explored as a function of local physical parameters such as the local viscocity, disc temperature and binary mass ratio. The observed disc images of IP Pegasi can indeed be understood as prominent spiral arms in the outer parts of a hot and extended accretion disc. Similar behaviour is expected in other dwarf novae in outburst and in nova-like variables which both harbour large, hot discs. During quiescence, however, the tightly wound tidal waves leave little imprint in the emission lines. We also show that the vertical structure introduced by such tidal waves can have a dramatic impact by shielding large portions of the accretion disc when observed at high orbital inclinations. The search is now on to establish similar disc structure in other accreting binaries using indirect imaging techniques.
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