Hot high-gravity NLTE model atmospheres applied to supersoft sources

Abstract

The spectra of optically thick soft X-ray sources, such as Supersoft Sources, are often fitted with blackbodies. Heise et al. 1994 calculated LTE atmospheres and showed that these are more efficient X-ray emitters than blackbodies. In this paper we study the assumption of LTE and present calculations of hot, high-gravity NLTE model atmospheres in hydrostatic and radiative equilibrium. The range of temperatures is chosen such that they emit significantly in the soft X-ray range (0.1–2.0 keV). The gravitational fields of g=10^7.5–10⁹ cm s⁻² are applicable to massive white dwarfs. It appears that NLTE spectra are comparable to LTE spectra if log g≥9.0 i.e. for the most massive white dwarfs (M≥0.6 M_⊙). At these gravities the density is sufficiently high to assume that collisional ionizations dominate and that thermal equilibrium (LTE) determines the degree of ionizations and the atomic population levels.

We show a first fit of H, He, C, N, O, Ne NLTE model atmospheres to the observed ROSAT spectrum of the galactic supersoft source RX J0925.7-4758. The resulting effective temperature strongly depends on the assumed model parameters, such as the gravity and the metallicities. At present X-ray spectral resolution, temperature, gravity and abundances cannot be determined independently.