POSSIBLE CATACLYSMIC VARIABLE IN THE CORE OF THE GLOBULAR-CLUSTER 47-TUCANAE

SEVERAL globular clusters have low-luminosity X-ray sources at their cores1-3, of which X0021.8-7221 in 47 Tucanae (NGC104) is one of the brightest. The nature of these sources is a mystery, and attempts to detect optical counterparts have been frustrated by the overcrowding of stars in cluster cores. The resolution of ground-based observations is insufficient to pick out the faint blue objects that are likely to be the optical counterparts to the X-ray sources, but the Faint Object Camera (FOC) on the Hubble Space Telescope has proved effective in identifying blue objects in dense clusters such as 47 Tuc4. Thus encouraged to use the FOC to search for the optical counterpart to X0021.8-7221, we have discovered a faint, variable and very blue object located in the error circle5 for X0021.8-7221 from the Einstein satellite High-Resolution Imager. The object has the spectral characteristics of a cataclysmic variable, and the high inferred X-ray to optical brightness ratio suggests that it is a magnetic cataclysmic variable, one of the candidates that has been suggested as the X-ray source in globular clusters.