HST/ACS Imaging of Omega Centauri: Optical Counterparts of Chandra X-Ray
  Sources
  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 {\omega} 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 {\omega} Cen, the largest
number yet known in any globular cluster.
