Hubble Space TelescopeAdvanced 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 starin the globular cluster Omega Centauri (NGC 5139). The star was found as partof our wide-field imaging study of Omega Cen using the Advanced Camera forSurveys (ACS) on Hubble Space Telescope. Its magnitude and color (R_625 = 25.2,B_435 - R_625 = 1.5) place it more than 1.5 magnitudes to the blue side of themain sequence. Through an H-alpha filter it is ~ 1.3 magnitudes brighter thancluster stars of comparable M_625 magnitude. The blue color and H-alpha excesssuggest the presence of an accretion disk, implying that the neutron star isaccreting from a binary companion and is thus a quiescent low-mass X-raybinary. If the companion is a main-sequence star, then the faint absolutemagnitude (M_625 ~ 11.6) constrains it to be of very low mass (M <~ 0.14Msolar). The faintness of the disk (M_435 ~ 13) suggests a very low rate ofaccretion onto the neutron star. We also detect 13 probable white dwarfs andthree possible BY Draconis stars in the 20" x 20" region analyzed here,suggesting that a large number of white dwarfs and active binaries will beobservable in the full ACS study.Comment: Accepted for Publication in the Astrophysical Journal. 5 pages, 4 figures, 1 table, uses emulateapj.sty. Figures 1 and 3 at reduced resolution. New version contains revised magnitude calibration