Below the observed continuum flux is dominated by radiation from the white dwarf. Shortly before and after geometric eclipse by the M-giant, the white dwarf continuum is strongly attenuated. The spectral characteristic of the flux reduction indicates Rayleigh scattering by neutral hydrogen in the M-giant wind and line blanketing by Fe II absorption lines (Dumm et al. 1999; see also Vogel 1991; Shore & Aufdenberg 1993). In addition to the attenuation close to the photospheric eclipse by the M-giant, RW Hya shows this phenomenon also at , when the binary system is approximately at quadrature. In Fig. 1 we compare the IUE spectrum taken at this phase with the unaffected spectrum at phase obtained with HST. The strength of the attenuation requires a column density of neutral hydrogen of .
The flux variation , where Rayleigh scattering is the dominant attenuation source, is shown in Fig. 2. At phases 0.77 and 0.81 we still detect weak attenuation but the spectrum taken at is not affected. The flux reduction at quadrature lasts thus for , and is well isolated from the normal photospheric eclipse by the M-giant. The orbit of RW Hya is known accurately, we therefore exclude significant errors in the phases.
In Sect. 5 we shall model this observational finding with an accretion wake trailing the white dwarf. A necessary requirement for such a model to be appropriate is that the white dwarf radiation field cannot prevent accretion, and that the white dwarf has no significant wind.
© European Southern Observatory (ESO) 2000
Online publication: February 25, 2000