Eclipse maps of spiral shocks in the accretion disc of IP Pegasi in outburst

Eclipse light curves of the dwarf nova IP Peg during the 1996 November outburst are analysed with eclipse mapping techniques to constrain the location and investigate the spatial structure of the spiral shocks observed in the Doppler tomograms of Harlaftis et al. Eclipse maps in the blue continuum and in the Ciii+Niii λ4650 emission line show two asymmetric arcs of ~90° in azimuth, extending from the intermediate to the outer disc regions R~=(0.2-0.6)R_L1, where R_L1 is the distance from the disc centre to the inner Lagrangian point], which are interpreted as being the spiral shocks seen in the Doppler tomograms. The Heii λ4686 eclipse map also shows two asymmetric arcs diluted by a central brightness source. The central source probably corresponds to the low-velocity component seen in the Doppler tomogram, and is understood in terms of gas outflow in a wind emanating from the inner parts of the disc. We estimate that the spirals contribute about 16 and 30per cent of the total line flux, respectively, for the Heii and Ciii+Niii lines. Comparison of the Doppler and eclipse maps reveals that the Keplerian velocities derived from the radial position of the shocks are systematically larger than those inferred from the Doppler tomography, indicating that the gas in the spiral shocks has sub-Keplerian velocities. We undertake simulations with the aim of investigating the effect of artefacts on the image reconstruction of the spiral structures.