1997A&A...322..591H

Hot optically thick plasmas in the temperature range 10⁵<T_eff<10⁶K emit soft X-rays (∼0.1-1keV). The spectra are in zero order often approximated with blackbodies. Considering LTE-atmospheres as a better first order approximation Heise et al. show that these atmospheres are more efficient X-ray emitters than blackbodies. In this paper we study the assumption of Local Thermodynamic Equilibrium (LTE) and present calculations of unblanketed, hot, high-gravity Non-LTE (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 (0.1-2.0keV). The gravitational fields of g=10^7.5-10¹⁰cm/s² are applicable to massive white dwarfs. It appears that NLTE spectra are comparable to LTE spectra if logg≥9, i.e. for the most massive white dwarfs (M≥0.6M_☉). At these gravities the density is sufficiently high to assume that collisional ionizations dominate and that LTE determines the degree of ionizations and the atomic population levels. We show fits of H, He, C, N, O, Ne NLTE model atmospheres to observed ROSAT spectra of Supersoft Sources (SSS) in the Galaxy and the LMC. The resulting effective temperatures strongly depend on the assumed model parameters, such as the gravity and the metallicities. With the present X-ray spectral resolution, temperature, gravity and abundances cannot be determined independently.