Hubble space telescope advanced camera for surveys imaging of w centauri: Optical counterpart for the quiescent low-mass X-ray binary

We report the discovery of an optical counterpart to a quiescent neutron star in the globular cluster w Centauri (NGC 5139). The star was found as part of our wide-field imaging study of w Cen using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. Its magnitude and color (R₆₂₅ = 25.2, B₄₃₅ - R₆₂₅ = 1.5) place it more than 1.5 mag to the blue side of the main sequence. Through an Hα filter it is ∼1.3 mag brighter than cluster stars of comparable R₆₂₅ magnitude. The blue color and Hα excess suggest the presence of an accretion disk, implying that the neutron star is accreting from a binary companion and is thus a quiescent low-mass X-ray binary. If the companion is a main-sequence star, then the faint absolute magnitude (M₆₂₅ ≃ 11.6) constrains it to be of very low mass (M ≲ 0.14 M_⊙). The faintness of the disk (M₄₃₅ ∼ 13) suggests a very low rate of accretion onto the neutron star. We also detect 13 probable white dwarfs and three possible BY Draconis stars in the 20″ × 22″ region analyzed here, suggesting that a large number of white dwarfs and active binaries will be observable in the full ACS study.