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Title:
Hubble Space Telescope Advanced Camera for Surveys Imaging of ω Centauri: Optical Counterpart for the Quiescent Low-Mass X-Ray Binary
Authors:
Haggard, Daryl; Cool, Adrienne M.; Anderson, Jay; Edmonds, Peter D.; Callanan, Paul J.; Heinke, Craig O.; Grindlay, Jonathan E.; Bailyn, Charles D.
Affiliation:
AA(Department of Physics and Astronomy, San Francisco State University, 1600 Holloway Avenue, San Francisco, CA 94618; .; Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195; .), AB(Department of Physics and Astronomy, San Francisco State University, 1600 Holloway Avenue, San Francisco, CA 94618; .), AC(Department of Physics and Astronomy, Rice University, 6100 Main Street, Houston, TX 77005; .), AD(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; , , .), AE(Department of Physics, University College Cork, Ireland; .), AF(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; , , .), AG(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; , , .), AH(Department of Astronomy, Yale University, New Haven, CT 06520; .)
Publication:
The Astrophysical Journal, Volume 613, Issue 1, pp. 512-516. (ApJ Homepage)
Publication Date:
09/2004
Origin:
UCP
ApJ Keywords:
Galaxy: Globular Clusters: Individual: NGC Number: NGC 5139, Stars: Neutron, Techniques: Photometric, Stars: White Dwarfs, X-Rays: Binaries
DOI:
10.1086/421549
Bibliographic Code:
2004ApJ...613..512H

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 Msolar). 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.

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