The hot and cool component of the symbiotic nova SMC 3

Abstract

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 $\ga$80. 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 $110\pm2$ 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 ~$(2.6{-}8.2)\times 10 ^{-7} {M}_{\odot} {\rm yr^{-1}}$, assuming that ~$(1{-}3.5)\%$ of the ionized phase is neutral (e.g. due to dust) and assuming a terminal velocity of ~$30 {\rm km} {\rm s^{-1}}$. 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 ~$10^{-7} {M}_{\odot} {\rm yr}^{-1}$.