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Accepted for publication in The Astrophysical Journal. Preprint typeset using L ATEX style emulateapj v. 11/26/03 HST/ACS IMAGING OF OMEGA CENTAURI: OPTICAL COUNTERPART FOR THE QUIESCENT LOW-MASS X-RAY BINARY 1

By Daryl Haggard, Adrienne M. Cool, Jay Anderson, Peter D. Edmonds, Paul J. Callanan, Craig O, Jonathan E. Grindlay and Charles D. Bailyn


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 Hubble Space Telescope. Its magnitude and color (R625 = 25.2, B435 − R625 = 1.5) place it more than 1.5 magnitudes to the blue side of the main sequence. Through an Hα filter it is ∼1.3 magnitudes 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 < ∼ 0.14M⊙). 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

Year: 2004
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