Two SMC Symbiotic Stars Undergoing Steady Hydrogen Burning

Two symbiotic stars in the Small Magellanic Cloud (SMC), Lin 358 and SMC 3, have been supersoft X-ray sources (SSSs) for more than 10 years. We fit atmospheric and nebular models to their X-ray, optical, and UV spectra obtained at different epochs. The X-ray spectra are extremely soft and appear to be emitted by the white dwarf atmosphere, not by the nebula, as in some other symbiotics. We find that the white dwarf of SMC 3, the hottest of the two sources, had an approximately constant effective temperature T_eff~=500,000 K in 1993-1994, 2003, and 2006, without indications of a decrease in 12 years. The bolometric luminosity of this system in 2003 March was more than an order of magnitude lower than 3 years later; however, the time of the observation is consistent with a partial eclipse of the white dwarf, previously found in ROSAT and optical observations. The red giant wind must be asymmetric or very clumpy in SMC 3, because the filling factor of the nebula around the source is not higher than 0.1. The compact object in Lin 358 has been at T_eff>=180,000 K since 1993, and there is some evidence of a moderate increase. Atmospheric fits for both objects are obtained with logg=9, which is appropriate for white dwarf masses >1.18 M_solar. No nova-like outbursts of these systems have been recorded in the last 50 years, despite continuous optical monitoring of the SMC, and there are no indications of cooling of the white dwarf, expected after a thermonuclear flash. We suggest therefore that in both systems hydrogen is burning steadily in a shell on the WD at the rate ~=10^-7 M_solar yr^-1, sufficiently high to inhibit nova-type mass loss as required for Type Ia supernovae progenitors.