Possible cataclysmic variable in the core of the globular cluster 47 Tucanae

Abstract

SEVERAL globular clusters have low-luminosity X-ray sources at their cores^1-3, 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.