Hubble Space Telescope Advanced Camera for Surveys Imaging of ω Centauri: Optical Counterpart for the Quiescent Low-Mass X-Ray Binary

Abstract

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 (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_solar). 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^''×20^'' region analyzed here, suggesting that a large number of white dwarfs and active binaries will be observable in the full ACS study.

Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.