The hot and cool component of the symbiotic nova SMC 3. A supersoft X-ray variable and a small-amplitude red variable

The ∼6 year supersoft X-ray lightcurve of the symbiotic nova SMC 3 (=RX J0048.4-7332) in the Small Magellanic Cloud is derived from archival ROSAT PSPC and HRI data. It shows one deep X-ray eclipse during which the count rate decreased by a factor of ≈gt80. In MACHO B-band data sinusoidal variation is found with a quasi-periodicity of ∼4 years. The minimum of the B-band flux occurs during the X-ray eclipse. In OGLE II I-band observations performed after the ROSAT observations we detect 110±2 day oscillations which we interpret as pulsations of the M0 giant star in the symbiotic system. The observed duration of the supersoft X-ray eclipse of ∼0.4-1.8 years is explained by the occultation of the white dwarf by the giant companion with an orbital period of ∼(4.0-4.8) years and a strong wind blown from its surface with a mass loss rate of ∼(2.6-8.2)× 10 ^-7 M_⊙ yr^-1, assuming that ∼(1-3.5)% of the ionized phase is neutral (e.g. due to dust) and assuming a terminal velocity of ∼30 km s^-1. The ∼4 year quasi-periodicity found in the optical is explained as the binary orbital period of the system. It is less likely that it reflects the activity (or mass-loss) time scale of the red giant star. A ∼(700-800) day quasi-periodicity found in the OGLE II and MACHO data is explained as the first harmonic of a binary orbital cycle. SMC 3 therefore may be classified as a small-amplitude red variable star (SARV). The hot star most likely is in a state of steady nuclear burning with an accretion rate somewhat below the upper critical value of ∼10^-7 M_⊙ yr^-1.

Based on archival data from the ROSAT, OGLE-II and MACHO project.