Application of hydrostatic local thermodynamic equilibrium atmosphere models to interpretations of supersoft X-ray source spectra

Abstract

Supersoft X-ray sources (SSSs) are accreting white dwarfs (WDs) with stable or recurrent thermonuclear burning on their surfaces. High-resolution X-ray spectra of such objects are rather complex, often consist of several components, and are difficult to interpret accurately. The main emission source is the hot surface of the WD and the emergent radiation can potentially be described by hot WD model atmospheres. We present a new set of such model atmosphere spectra computed in the effective temperature range from 100 kK to 1000 kK, for eight values of surface gravity and three different chemical compositions. These compositions correspond to the solar one as well as to the Large and Small Magellanic Clouds, with decreased heavy element abundances, at one-half and one-tenth of the solar value. The presented model grid covers a broad range of physical parameters and, thus, it can be applied to a wide range of objects. It is also publicly available in XSPEC format. As an illustration, we applied it here for the interpretation of Chandra and XMM grating spectra of two classical SSSs, namely, CAL 83 (RX J0543.5–6823) and RX J0513.9–6951. The obtained effective temperatures and surface gravities of Teff ≈ 560 kK, log g ≈ 8.6–8.7, and Teff ≈ 630 kK, log g ≈ 8.5–8.6, respectively, are in a good agreement with previous estimations for both sources. The derived WD mass estimations are within 1.1–1.4 M⊙ for CAL 83 and 1.15–1.4 M⊙ for RX J0513.9–6951. The mass of the WD in CAL 83 is consistent with the mass predicted from the respective model of recurrent thermonuclear burning.