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

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: T_eff = 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.