Detection of a Quiescent Low Mass X-Ray Binary Companion in 47 Tucanae

Abstract

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 HST 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 MS 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.