Model Atmosphere Spectrum Fit to the Soft X-Ray Outburst Spectrum of SS Cyg

V F Suleimanov,R Ya Zhuchkov,C W Mauche,K Werner

doi:10.14311/app.2015.02.0143

Abstract

The X-ray spectrum of SS Cyg in outburst has a very soft component that can be interpreted as the fast-rotating optically thick boundary layer on the white dwarf surface. This component was carefully investigated by Mauche (2004) using the Chandra LETG spectrum of this object in outburst. The spectrum shows broad ( ≈5 °A) spectral features that have been interpreted as a large number of absorption lines on a blackbody continuum with a temperature of ≈250 kK. Because the spectrum resembles the photospheric spectra of super-soft X-ray sources, we tried to fit it with high gravity hot LTE stellar model atmospheres with solar chemical composition, specially computed for this purpose. We obtained a reasonably good fit to the 60–125 °A spectrum with the following parameters: Teff = 190 kK, log g = 6.2, and NH = 8 · 1019 cm−2, although at shorter wavelengths the observed spectrum has a much higher flux. The reasons for this are discussed. The hypothesis of a fast rotating boundary layer is supported by the derived low surface gravity.

Highlights

SS Cyg is one of the brightest cataclysmic variables (CVs), one of the best-studied dwarf nova stars (Warner 1995), and was the first CV discovered in X-ray radiation (Rappaport et al 1974)
It is commonly accepted that the X-ray radiation of non-magnetic CVs arises in the boundary layer (BL) between the white dwarf (WD) and the accretion disc (Pringle & Savonije 1979; Tylenda 1981; Patterson & Raymond 1985a, b; Kley 1991), which are optically thick at high accretion rates (M > 1016 g s−1) and optically thin at lower accretion rates
This spectrum looks like the photospheric spectra of super-soft X-ray sources (Lanz et al 2005; Rauch et al 2010; van Rossum 2012), so it probably could be described using the spectra of hot stellar model atmospheres

Summary

Introduction

SS Cyg is one of the brightest cataclysmic variables (CVs), one of the best-studied dwarf nova stars (Warner 1995), and was the first CV discovered in X-ray radiation (Rappaport et al 1974). The soft X-ray spectrum of SS Cyg in outburst was carefully investigated by Mauche (2004) using a highresolution spectrum obtained with the Chandra LETG He phenomenologically described the observed 40–130 ̊A spectrum by a blackbody with temperature T ≈ 250 kK and numerous broad absorption features of ions of cosmically abundant O, Ne, Mg, Si, S, and Fe. The BL luminosity and WD spin were evaluated in this work. For the considered model atmospheres, a radiation pressure force grad due to spectral lines becomes larger than the surface gravity at the upper atmospheric layers (see Fig. 1, bottom panel). The positions of the computed models on the Teff –log g plane are shown

Results

Discussion and Conclusion

DISCUSSION