EK Tra: a Spectroscopic Twin of VW Hyi

Abstract

Observations of the inter-outburst behaviour of dwarf novae can put constraints on theoretical models for the structure of the accretion disk and the outburst mechanism. The white-dwarf properties, such as temperature, postoutburst cooling timescale, and rotational velocity play a major rôle in the understanding of the processes in the boundary layer. Alas, the white dwarf could be detected directly only in a small number of dwarf novae. We have re-analysed the available IUE observations of the southern dwarf nova EK TrA, testing the suggestion by Verbunt (1987, A&AS 71, 339) that in this system the white dwarf is a significant source of UV radiation during quiescence.We have decomposed the spectrum obtained closest to quiescence (Fig. 1) into accretion disc and white dwarf contribution following the method described by Gansicke & Beuermann (1996, A&A 309, L47): we use the observed spectrum close to outburst maximum to describe the disc contribution and a grid (log g = 8.0 and 11 000K ≤ Twd ≤ 30 000K, solar abundances) of calculated model spectra kindly provided by I. Hubeny for the white dwarf. Following our experience with VW Hyi (Gänsicke & Beuermann, 1996, A&A 309, L47), we allow for Gaussian emission lines of N V λ 1240, Si IV λ 1400 and C IV λ 1549. The observed spectrum was then fitted with the sum of these components using an evolutionary strategy algorithm. The best fit is achieved for a white dwarf of Twd = 16 000 K and the accretion disc contributing 21% and 77% of the integrated SWP flux, respectively (Fig. 1). Assuming an 0.6 M⊙ white dwarf, the scaling factor of the white dwarf model implies a distance of ~ 140 pc, roughly twice that of VW Hyi. However, EK TrA was never observed in a truly quiescent state; at the time of the IUE observations the residual accretion disc or accretion belt still dominates the UV light and the derived white-dwarf parameters remain necessarily uncertain. Comparison with VW Hyi teaches us that the decline of the spectrum towards Lyα should not, at face value, be taken as a direct measure of the white dwarf contribution to the overall flux.