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HST/ACS Imaging of Omega Centauri: Optical Counterpart for the Quiescent Low-Mass X-Ray Binary

D. Haggard·A. M. Cool·J. Anderson·P. D. Edmonds·P. J. Callanan·C. O. Heinke·J. E. Grindlay·C. D. Bailyn
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Abstract

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 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 the main sequence. Through an H-alpha filter it is ~ 1.3 magnitudes brighter than cluster stars of comparable M_625 magnitude. The blue color and H-alpha 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 Msolar). 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" x 20" region analyzed here, suggesting that a large number of white dwarfs and active binaries will be observable in the full ACS study.

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D. Haggard, A. M. Cool, J. Anderson, P. D. Edmonds, P. J. Callanan, C. O. Heinke, J. E. Grindlay, C. D. Bailyn. 2004-07-02. HST/ACS Imaging of Omega Centauri: Optical Counterpart for the Quiescent Low-Mass X-Ray Binary. https://doi.org/10.1086/421549

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