Spectral mapping of the spiral structures in IP Pegasi on the decline from an outburst

We report eclipse mapping of time-resolved spectroscopy of the dwarf nova IP Pegasi on the late decline from the 1993 May outburst. The continuum light curves exhibit an asymmetric 'V' shape with broad bulges and results in eclipse maps with two asymmetric arcs extended both in radius [R similar or equal to(0.2-0.6)R (L1), where R (L1) is the distance from the disc centre to the inner Lagrangian point] and in azimuth (by similar or equal to90degrees), interpreted as a two-armed spiral shock. The spiral arms are thus still visible some 8 d after the onset of the outburst. Their fractional contribution to the continuum emission, 12 per cent of the total light, is similar to that measured close to outburst maximum, whereas their orientation is rotated by 58degrees with respect to the spirals seen in the eclipse map at outburst maximum. The radial temperature distribution computed from the spiral-free disc regions is flat, with temperatures of similar to5000 K at all disc radii. Velocity-resolved light curves across the Halpha and the He i lines show the classical rotational disturbance, with the blue side of the line being eclipsed earlier than the red side. The differences between the Halpha and the He i maps are significant. The spiral arms are clearly seen in the He i maps, with the receding arm being stronger on the red side, while the approaching arm is stronger on the blue side of the line. The analysis of the Halpha maps suggests that this emission arises mainly from a large and vertically extended region that we interpret as an outflowing (and spiralling) disc wind. The Halpha emission-line surface brightness is flat in the inner disc regions (I (nu) proportional toR (-0.3) for R <0.3R (L1) ) but decreases sharply with radius in the outer disc (I (nu) proportional toR (-2) for R >0.3R (L1) ). The spectrum of the uneclipsed light is dominated by a strong, blueshifted and narrow Halpha emission line superimposed on a red continuum and can be understood as a combination of emission from an M5V secondary star plus optically thin emission from the outer parts of the vertically extended disc wind. The inner disc regions show an emission-line spectrum with a strong [equivalent width EW=(100+/-2) Angstrom] and broad (full-width zero intensitysimilar or equal to3000 km s(-1) ) Halpha component superimposed on a flat continuum. This is in marked contrast with the results from the spectral mapping of nova-like variables of comparable inclination and mass ratio and suggests that intrinsically different physical conditions hold in the inner disc regions of outbursting dwarf novae and nova-like systems.