A Chandra Study of the Galactic Globular Cluster Omega Centauri

We analyze a ~70 ks Chandra Advanced CCD Imaging Spectrometer exposure of the globular cluster ω Cen (NGC 5139). The ~17' × 17' field of view fully encompasses three core radii and almost twice the half-mass radius. We detect 180 sources to a limiting flux of ~4.3 × 10^-16 erg cm^-2 s^-1 (L_x = 1.2 × 10³⁰ erg s^-1 at 4.9 kpc). After accounting for the number of active galactic nuclei and possible foreground stars, we estimate that 45-70 of the sources are cluster members. Four of the X-ray sources have previously been identified as compact accreting binaries in the cluster—three cataclysmic variables (CVs) and one quiescent neutron star. Correlating the Chandra positions with known variable stars yields eight matches, of which five are probable cluster members that are likely to be binary stars with active coronae. Extrapolating these optical identifications to the remaining unidentified X-ray source population, we estimate that 20-35 of the sources are CVs and a similar number are active binaries. This likely represents most of the CVs in the cluster, but only a small fraction of all the active binaries. We place a 2σ upper limit of L_x < 3 × 10³⁰ erg s^-1 on the integrated luminosity of any additional faint, unresolved population of sources in the core. We explore the significance of these findings in the context of primordial versus dynamical channels for CV formation. The number of CVs per unit mass in ω Cen is at least 2-3 times lower than in the field, suggesting that primordial binaries that would otherwise lead to CVs are being destroyed in the cluster environment.