Fitting the Chandra LETG spectrum of SS Cygni in outburst with model atmosphere spectra

TL;DR

This paper models the Chandra LETG spectrum of SS Cyg in outburst using high gravity hot stellar atmospheres, supporting the boundary layer hypothesis and providing parameters that fit the observed spectrum reasonably well.

Contribution

It introduces LTE model atmospheres for fitting the spectrum of SS Cyg's boundary layer, offering new parameters that support the boundary layer's fast rotation.

Findings

Best fit with T_eff=190 kK, log g=6.2, N_H=8x10^{19} cm^{-2}

Spectrum fits well between 60-125 Å but diverges at shorter wavelengths

Low surface gravity supports the fast-rotating boundary layer hypothesis

Abstract

The Chandra / LETG spectrum of SS Cyg in outburst shows broad (\approx 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 (Mauche 2004). It is most probable that this is the spectrum of the fast-rotating optically thick boundary layer on the white dwarf surface. Here we present the results of fitting this spectrum with high gravity hot stellar model atmospheres. An extended set of LTE model atmospheres with solar chemical composition was computed for this purpose. The best fit is obtained with the following parameters: T_eff=190 kK, log g=6.2, and N_H=8 10^{19} cm^{-2}. The spectrum of this model describes the observed spectrum in the 60--125 A range reasonably well, but at shorter wavelengths the observed spectrum has much higher flux. The reasons for this are discussed. The derived low surface gravity supports the hypothesis of the fast rotating boundary layer.