Supersoft X-ray sources in the Magellanic Clouds

Abstract

The Magellanic Clouds (MCs) have been mapped at soft X-rays (0.1–2.4 keV) with ROSAT during an all-sky survey and numerous pointed observations. A distinct class of supersoft X-ray sources (SSS) was discovered radiating with blackbody temperatures of a few times 10 5 K and with bolometric luminosities ∼ 10 4 times solar. A detailed study of these sources in X-rays, UV and the optical spectral regime has just started. The nature of these sources was in debate with a black hole (Cowley et al. 1990), neutron star (Kylafis & Xilouris 1993) and white dwarf (Van den Heuvel et al. 1992) origin. The identification of SSS in the Small Magellanic Cloud with the hot central star of a planetary nebula (Wang 1991) and the hot component of a symbiotic nova (Kahabka et al. 1994) are in favor for a white dwarf (WD) origin. Van den Heuvel et al. 1992 have shown, that Cal 83 and Cal 87 in the Large Magellanic Cloud are close binaries with a WD accreting at high rates and stable burning hydrogen. Systems of a similar nature may be RXJ0513.9-6951 and 1E0035.4-7230. SSS are observed to be stable burning already for a period of ∼10 years or are of transient nature or even recurrent. Such a behaviour is predicted for accreting and nuclear burning WDs if the accretion rate is within or below a critical range (Fujimoto 1982).