Hubble Space Telescope advanced camera for surveys imaging of ω 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 omega Centauri (NGC 5139). The star was found as part of our wide-field imaging study of omega Cen using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. Its magnitude and color (R-625 = 25.2, B-435-R-625 = 1.5) place it more than 1.5 mag to the blue side of the main sequence. Through an Halpha filter it is similar to3 mag brighter than cluster stars of comparable R-625 magnitude. The blue color and Halpha 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 (M-625 similar or equal to 11.6) constrains it to be of very low mass (M less than or similar to 0.14 M-circle dot). The faintness of the disk (M-435 similar to 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" x 20" region analyzed here, suggesting that a large number of white dwarfs and active binaries will be observable in the full ACS study.