HST observations of IP Pegasi in quiescence: the pre-eclipse spectrum

Abstract

We present time-resolved HST ultraviolet (UV) spectroscopy and ground-based optical photometry of the dwarf nova IP Pegasi in a quiescent state. The observations were obtained prior to an eclipse, when the bright spot caused by the impact of the accretion stream with the edge of the disc dominates the light output. The optical light curve is fairly strongly correlated with the UV spectrophotometric flux curve. An unusual emission-like feature near 1820A in the UV spectrum of IP Peg is likely to be a manifestation of the `Feii curtain'. Composite spectra constructed from the peaks and troughs of flickers in the light curve show substantial differences. The spectrum of the flickers (i.e., peaks minus troughs) is not adequately modelled by a simple blackbody, suggesting that a more sophisticated model is appropriate. We perform a cross- correlation analysis of the variability in spectrophotometric flux curves of the UV continuum and prominent UV emission lines (Cii lambda1335, Siiv lambda1400 and Civ lambda1550). The continuum and lines are not correlated, suggesting that they are produced separately. The Cii and Siiv lines are moderately correlated with each other, but neither line is correlated with Civ, suggesting that the latter forms in a different region from the former. We briefly discuss a qualitative model for the geometry of the emission regions in IP Peg that is consistent with the observed behaviour of the UV lines and continuum.