Spiral structure in IP Pegasi: How persistent is it?

Abstract

We present spectroscopy of the dwarf nova IP Pegasi taken during two consecutive nights, 5 and 6 d after the start of an outburst. Even this late in the outburst, Doppler maps show marked spiral structure in the accretion disc, at least as strongly as seen earlier in other outbursts of IP Peg. The spiral shocks are present on both nights with no diminution in strength from one night to the next. The light curves of the lines show an offset to earlier phases, with the mid-eclipse of the emission lines displaced to phases between -0.015 ± 0.001 and -0.045 ±0.009. This cannot be explained by the presence of the accretion shocks. As well as the fixed spiral pattern, the disc shows strong flaring in the Balmer and He II λ4686-Å lines. Irradiation-induced emission is seen from the companion star in the Balmer, He I, He II, Mg II, C II, and other lines. The emission is located near the poles of the companion star, suggesting that the accretion disc shields the companion star substantially and thus has an effective H/R of order 0.2 at extreme-ultraviolet (EUV) wavelengths. The Balmer emission is distinctly broader than the other lines, consistent with non-Doppler broadening.