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D. Haggard, J.L. Dorfman, A.M. Cool (San Francisco State University), J. Anderson (Rice University), C.D. Bailyn (Yale University), P.D. Edmonds, J.E. Grindlay (Harvard-Smithsonian Center for Astrophysics)
We present initial results of a wide-field imaging study of the globular cluster Omega Cen (NGC 5139) using the Advanced Camera for Surveys (ACS). We have obtained a mosaic of 3x3 pointings of the cluster using the HST/ACS Wide Field Camera covering approximately 10' x 10', roughly out to the cluster's half-mass radius. Using F435W (B435), F625W (R625) and F658N (H-alpha) filters, we are searching for optical counterparts of Chandra X-ray sources and studying the cluster's stellar populations. Here we report the discovery of an optical counterpart to the X-ray source identified by Rutledge et al. (2002) as a possible quiescent neutron star on the basis of its X-ray spectrum. The star's magnitude and color (R625 = 24.4, B435-R625 = 1.5) place it more than 1.5 magnitudes to the blue side of the main sequence. Through the H-alpha filter it is about 1.3 magnitudes brighter than cluster stars of comparable R625 magnitude. The blue color and H-alpha excess suggest the presence of an accretion disk, implying that the neutron star is a member of a quiescent low-mass X-ray binary. The object's faint absolute magnitude (M625 ~ 10.6, M435 ~ 11.8) implies that the system contains an unusually weak disk and that the companion, if it is a main-sequence star, is of very low mass (< 0.16 solar masses). We also identify ~ 10 probable white dwarfs and three possible BY Draconis stars in a 20'' x 20'' region, suggesting that a large number of white dwarfs and active binaries will be observable in the full ACS study.
This work is supported by NASA grant GO-9442 from the Space Telescope Science Institute.
The author(s) of this abstract have provided an email address for comments about the abstract: dhaggard@astro.washington.edu
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.