An unusual brightening of the eclipsing binary star AKO 9 in the globular cluster 47 Tucanae observed with the Hubble Space Telescope

Fifteen sequential images of the core of the globular cluster 47 Tucanae taken in ultraviolet light with the Hubble Space Telescope have revealed one star that increased in brightness by more than 2 mag in less than an hour. By the end of our observations, this star was the brightest object in the core of the cluster in our bandpass. Auriere et al. first cataloged this star as a blue object (AKO 9), considering it as a potential visible counterpart of the then still single X-ray source. Edmonds et al. found it to be an eclipsing binary, located in the color-magnitude diagram in the vicinity of the main-sequence turnoff. Possible causes for such a brightening are (1) a very large flare on a magnetically active star (RS CVn), (2) an increase in the brightness due to an accretion disk instability in a cataclysmic variable or (3) in a soft X-ray transient, or (4) a nova. There are arguments against every one of these possibilities, and more observations will be needed to understand this system.