Extragalactic symbiotic systems --- IV. The supersoft X-ray source SMC 3

Abstract

We present a consistent model for the UV and supersoft X-ray emission from the symbiotic nova SMC3 (=RX J0048.4-7332). Following the present picture of symbiotic stars, the model consists of radiation from a hot star and an emission nebula excited by that star. The observations were compared to theoretical models in which the hot star's emission is calculated with the help of hydrostatic and Wolf-Rayet-type non-LTE model atmospheres. Our analysis clearly shows evidence for mass loss rates of several 10^{-6} Msolar/yr. The minimum effective temperature compatible with both the observed UV and X-ray flux is about 260000K, which is higher than in any other star analyzed with sophisticated NLTE model atmospheres. Since the hydrostatic surface is hidden by the stellar wind no upper limit for the temperature can be determined. However, we were able to determine the total luminosity of a symbiotic nova with reasonable accuracy L_SMC3=10^{4.05\pm 0.05} Lsun. This value is well below the Eddington limit (approx. 50000~Lsun). In order to reproduce the observed energy distribution a carbon-to-helium ratio >2 10^{-4} --- leading to an absorption edge at 0.39\,keV --- is necessary.