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

Eclipse lightcurves of the dwarf nova IP Peg during the November 1996outburst are analysed with eclipse mapping techniques to constrain the locationand investigate the spatial structure of the spiral shocks observed in theDoppler tomograms (Harlaftis et al. 1999). Eclipse maps in the blue continuumand in the CIII+NIII 4650 emission line show two asymmetric arcs of \sim 90degrees in azimuth and extending from intermediate to the outer disc regions (R\simeq 0.2 - 0.6 R_{L1}, where R_{L1} is the distance from disc centre to theinner Lagrangian point) which are interpreted as being the spiral shocks seenin the Doppler tomograms. The HeII 4686 eclipse map also shows two asymmetricarcs diluted by a central brightness source. The central source probablycorresponds to the low-velocity component seen in the Doppler tomogram and isunderstood in terms of gas outflow in a wind emanating from the inner parts ofthe disc. We estimate that the spirals contribute about 16 and 30 per cent ofthe total line flux, respectively, for the HeII and CIII+NIII lines. Comparisonbetween the Doppler and eclipse maps reveal that the Keplerian velocitiesderived from the radial position of the shocks are systematically larger thanthose inferred from the Doppler tomography indicating that the gas in thespiral shocks has sub-Keplerian velocities. We undertake simulations with theaim to investigate the effect of artifacts on the image reconstruction of thespiral structures