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T. Lanz (U. Maryland & GSFC), I. Hubeny (AURA & GSFC)
Supersoft X-Ray Sources are most probably very hot white dwarfs undergoing steady nuclear burning in their envelopes. They are high luminosity (L > 0.1 L\rm Edd), soft (kT\approx 50 eV) low mass binary with a white dwarf primary accreting at a sub-Eddington rate. High-resolution spectra in the soft X-rays are now becoming available with Chandra and XMM-Newton. Photospheric line spectroscopy has the potential to provide accurate diagnostics of the white dwarf properties.
We have therefore embarked on a project aimed at providing new NLTE model atmospheres of supersoft sources. We are building a series of model atmospheres of hot white dwarfs (20\leq kT\leq 100 eV; 8\leq \log g\leq 10) with various surface composition. The models incorporate the most abundant species, H, He, C, N, O, Ne, Mg, Si, S, and Fe, which are allowed to depart from Saha-Boltzmann statitics. Our models include the influence of multi-level model atoms, metal line blanketing, and Auger ionization on the atmospheric structure. A preliminary comparison of models calculated assuming coherent (Thomson) and non-coherent (Compton) scattering is presented.
This work was supported through a NASA ATP grant (NRA 00-01-ATP-153).