HST/ACS Imaging of Omega Centauri: Optical Counterparts of Chandra X-Ray Sources

Abstract

We present results of a search for optical counterparts of X-ray sources in and toward the globular cluster Omega Centauri (NGC 5139) using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. The ACS data consist of a mosaic of Wide Field Channel (WFC) images obtained using F625W, F435W, and F658N filters; with 9 pointings we cover the central ~10'x10' of the cluster and encompass 109 known Chandra sources. We find promising optical counterparts for 59 of the sources, ~40 of which are likely to be associated with the cluster. These include 27 candidate cataclysmic variables (CVs), 24 of which are reported here for the first time. Fourteen of the CV candidates are very faint, with absolute magnitudes in the range M_625 = 10.4 - 12.6, making them comparable in brightness to field CVs near the period minimum discovered in the SDSS (Gansicke et al. 2009). Additional optical counterparts include three BY Dra candidates, a possible blue straggler, and a previously-reported quiescent low-mass X-ray binary (Haggard et al. 2004). We also identify three foreground stars and 11 probable active galactic nuclei. Finally, we report the discovery of a group of seven stars whose X-ray properties are suggestive of magnetically active binaries, and whose optical counterparts lie on or very near the metal-rich anomalous giant and subgiant branches in ω Cen. If the apparent association between these seven stars and the RGB/SGB-a stars is real, then the frequency of X-ray sources in this metal-rich population is enhanced by a factor of at least five relative to the other giant and subgiant populations in the cluster. If these stars are not members of the metal-rich population, then they bring to 20 the total number of red stragglers (also known as sub-subgiants) that have been identified in ω Cen, the largest number yet known in any globular cluster.

22 pages, 8 figures, 2 tables, accepted to ApJ

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• Chandra studies of the globular cluster 47 Tucanae: A deeper X-ray source catalogue, five new X-ray counterparts to millisecond radio pulsars, and new constraints to r-mode instability window
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• A Deep Chandra X-ray Limit on the Putative IMBH in Omega Centauri
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• A stellar census in globular clusters with MUSE: A spectral catalogue of emission-line sources
• The X-ray Emissivity of Low-Density Stellar Populations
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• A Deep Search for Faint Chandra X-ray Sources, Radio Sources, and Optical Counterparts in NGC 6752
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• Candidate isolated neutron stars in the 4XMM-DR10 catalog of X-ray sources
• Habitability in the Omega Centauri Cluster
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• Globular cluster chemistry in fast rotating dwarf stars belonging to intermediate age open clusters
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• WIYN Open Cluster Study. LXXI. Spectroscopic Membership and Orbits of NGC 6791 Sub-Subgiants
• Reevaluating the Origin of Detectable Cataclysmic Variables in Globular Clusters: Testing the Importance of Dynamics
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