A Deep X-ray Survey of the Globular Cluster Omega Centauri

Abstract

We identify 233 X-ray sources, of which 95 are new, in a 222 ks exposure of Omega Centauri with the Chandra X-ray Observatory's ACIS-I detector. The limiting unabsorbed flux in the core is $f_x$ (0.5$-$6.0 keV) $\simeq$ 3 $\times$ 10${}^{-16}$ erg s${}^{-1}$ cm${}^{-2}$ ($L_X$ $\simeq$ 1 $\times$ 10${}^{30}$ erg s${}^{-1}$ at 5.2 kpc). We estimate that ~$60\pm 20$ of these are cluster members, of which ~30 lie within the core ($r_c$ $=$ 155 arcsec), and another ~30 between 1$-$2 core radii. We identify four new optical counterparts, for a total of 45 likely identifications. Probable cluster members include 18 cataclysmic variables (CVs) and CV candidates, one quiescent low-mass X-ray binary, four variable stars, and five stars that are either associated with w Cen's anomalous red giant branch, or are sub-subgiants. We estimate that the cluster contains $40\pm 10$ CVs with $L_X$ $>$ 10${}^{31}$ erg s${}^{-1}$, confirming that CVs are underabundant in w Cen relative to the field. Intrinsic absorption is required to fit X-ray spectra of six of the nine brightest CVs, suggesting magnetic CVs, or high-inclination systems. Though no radio millisecond pulsars (MSPs) are currently known in w Cen, more than 30 unidentified sources have luminosities and X-ray colours like those of MSPs found in other globular clusters; these could be responsible for the Fermi-detected gamma-ray emission from the cluster. Finally, we identify a CH star as the counterpart to the second-brightest X-ray source in the cluster and argue that it is a symbiotic star. This is the first such giant/white dwarf binary to be identified in a globular cluster.

References in corpus (20)

• The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. I. Overview of the Project and Detection of Multiple Stellar Populations
• X-ray Emissions from Accreting White Dwarfs: a Review
• N-body modeling of globular clusters: Masses, mass-to-light ratios and intermediate-mass black holes
• Improved Mass and Radius Constraints for Quiescent Neutron Stars in Omega Cen and NGC 6397
• Formation and evolution of compact binaries in globular clusters: I.Binaries with white dwarfs
• Long-term observations of the pulsars in 47 Tucanae - II. Proper motions, accelerations and jerks
• A New Catalog of Globular Clusters in the Milky Way
• Giants in the globular cluster omega Centauri: dust production, mass loss and distance
• On the Origin of Sub-subgiant Stars. I. Demographics
• Identification of Faint Chandra X-ray Sources in the Core-Collapsed Globular Cluster NGC 6397: Evidence for a Bimodal Cataclysmic Variable Population
• A spectral atlas of post-main-sequence stars in omega Centauri: kinematics, evolution, enrichment and interstellar medium
• New Cataclysmic Variables and other Exotic Binaries in the Globular Cluster 47 Tucanae
• A Deep Wide-Field Variable Star Catalog of Omega Centauri
• On the Origin of Sub-Subgiant Stars II: Binary Mass Transfer, Envelope Stripping, and Magnetic Activity
• MOCCA-SURVEY database I. Accreting white dwarf binary systems in globular clusters -- I. cataclysmic variables -- present-day population
• The State-of-the-Art HST Astro-Photometric Analysis of the core of $\omega$ Centauri. II. Differential-Reddening Map
• Dynamical Formation of Cataclysmic Variables in Globular Clusters
• On the Origin of Sub-subgiant Stars. III. Formation Frequencies
• Identification of Faint Chandra X-ray Sources in the Core-Collapsed Globular Cluster NGC 6752
• ChaMPlane Discovery of Candidate Symbiotic Binaries in Baade's and Stanek's Windows

Cited by in corpus (27)

• MOCCA-SURVEY database I. Accreting white dwarf binary systems in globular clusters -- IV. cataclysmic variables -- properties of bright and faint populations
• Detection of a $\sim$100,000 M${}_{\odot }$ black hole in M31's most massive globular cluster: A tidally stripped nucleus
• The MAVERIC Survey: Chandra/ACIS Catalog of Faint X-ray sources in 38 Galactic globular clusters
• Evidence for mass-dependent peculiar velocities in compact object binaries: Towards better constraints on natal kicks
• A Census of X-ray Millisecond Pulsars in Globular Clusters
• Discovery of Millisecond Pulsars in the Globular Cluster Omega Centauri
• MeerKAT discovery of 13 new pulsars in Omega Centauri
• On the absence of symbiotic stars in globular clusters
• The Galactic bulge millisecond pulsars shining in X rays: A gamma-ray perspective
• A Deep Search for Faint Chandra X-ray Sources, Radio Sources, and Optical Counterparts in NGC 6752
• New constraints on the central mass contents of Omega Centauri from combined stellar kinematics and pulsar timing
• On the origin of the gamma-ray emission from Omega Centauri: Milisecond pulsars and dark matter annihilation
• Multi-Epoch X-ray Imaging of Globular Cluster M62 with Chandra
• Gamma-ray spectral properties of the Galactic globular clusters: constraint on the numbers of millisecond pulsars
• A comparison of millisecond pulsar populations between globular clusters and the Galactic field
• Timing of Pulsars in the Globular Cluster Omega Centauri
• Constraints on annihilating dark matter in the Omega Centauri cluster
• Outliers: multicolour photometry guiding the search for evolved binary systems in the globular cluster 47 Tucanae
• Exotica in the Globular Cluster M4, Studied with Chandra, HST, and the VLA
• Exploring the Mass Segregation Effect of X-ray Sources in Globular Clusters. IV. Evidence of Black Hole Burning in $\omega$ Centauri
• What Could be the Observational Signature of Dark Matter in Globular Clusters?
• Dark matter annihilation in $\omega$ Centauri: Astrophysical implications derived from the MWA radio data
• A Search for Annihilating Dark Matter in 47 Tucanae and Omega Centauri
• Mining the time axis with TRON. I. Millisecond pulsars in Omega Centauri, Terzan 5 and 47 Tucanae detected through MeerKAT interferometric imaging
• Reevaluating the Origin of Detectable Cataclysmic Variables in Globular Clusters: Testing the Importance of Dynamics
• Exploration of faint X-ray and radio sources in the massive globular cluster M14: A UV-bright counterpart to Nova Ophiuchus 1938
• Are most Cataclysmic Variables in Globular Clusters dynamically formed?