Atmospheric emission from hydrogen-shell-burning white dwarfs as supersoft X-ray sources

The radiation spectra of supersoft X-ray sources based on a model for hydrogen burning on the white dwarf surface are investigated by solving hydrostatic equilibrium, radiative equilibrium, statistical equilibrium and radiative transfer self-consistently for various sets of mass, radius and luminosity. It is found that the radiation spectrum shows many bound–free emission/absorption features and greatly deviates from the blackbody spectrum at the effective temperature. By the effect of incoherent Compton scattering, the bound–free emission/absorption features do not appear in strong emission/absorption edges, as predicted by coherent models without Compton scattering, but appear as weak humps and relatively shallow absorption edges. The difference between the incoherent model and the coherent model is prominent for L 0.5 L _Edd. A calculated spectrum is fitted to the ASCA observations of RX J0925.7?4758. It is found that the entire spectrum of RX J0925.7?4758 cannot be reproduced by model atmospheres with any parameter sets. This suggests that the observed spectrum consists of two or more components. In this case, the atmospheric component is explained by the emission from a white dwarf near the Chandrasekhar limit (～1.4 M_⊙) with L ～0.2 L _Edd at a distance of 16–20 kpc.