Hubble Space Telescope detection of a quiescent low-mass X-ray binary companion in 47 Tucanae

We present the results of a search for optical counterparts to the two quiescent low-mass X-ray binaries (X5 and X7) in the globular cluster 47 Tucanae, using high-quality Chandra and Hubble Space Telescope images. A faint blue (V = 21.7; U-V = 0.9) star within 0″.03 of the eclipsing system X5 shows variability on both short and long timescales and is the counterpart of the X-ray source. The colors and variability of this object are consistent with the combination of light from an accretion disk and a red main-sequence star (possibly somewhat larger than a normal main-sequence star with similar luminosity). No evidence is found for a star showing either variability or unusual colors near the position of X7, but a probable chance superposition of a star with V = 20.25 limits the depth of our search.