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 star in the globular cluster ω Centauri (NGC 5139). The star was found as part of our wide-field imaging study of ω 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 Hα filter it is ~1.3 mag brighter than cluster stars of comparable R 625 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 ( M 625 ≃ 11.6) constrains it to be of very low mass ( M ≲ 0.14 M ☉ ). The faintness of the disk ( M 435 ~ 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'' × 20'' region analyzed here, suggesting that a large number of white dwarfs and active binaries will be observable in the full ACS study.