Hubble Space Telescope 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 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. Based on observations with the NASA/ESA Hubble Space Telescope obtained at STScI, which is operated by Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.