Spiral shocks in the accretion disc of IP Peg during outburst maximum

In response to our recent discovery of spiral arms in the accretion disc of IP Peg during rise to outburst, we have obtained time-resolved spectrophotometry of IP Peg during outburst maximum. In particular, indirect imaging of He II 4686, using Doppler tomography, shows a two-arm spiral pattern on the disc image, which confirms repeatability over different outbursts. The jump in He II intensity (a factor of more than 2) and in velocity (similar to 200-300kms(-1)) clarifies the shock nature of the spiral structure. The He rr shocks show an azimuthal extent of similar to 90 degrees, a shallow power-law emissivity similar to V-1, an upper limit of 30 degrees in opening angle, and a flux contribution of 15 per cent of the total disc emission. We discuss the results in view of recent simulations of accretion discs which show that spiral shocks can be raised in the accretion disc by the secondary star.