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Description
The authors analyzed a ~ 70 ks Chandra Advanced CCD Imaging Spectrometer (ACIS) exposure of the globular cluster Omega Cen (NGC 5139). The ~ 17' x 17' field of view fully encompasses three cluster core radii and almost twice the half-mass radius. They detected 180 sources to a limiting flux of ~ 4.3 x10 <sup>-16</sup> erg/cm<sup>2</sup>/s (L<sub>x</sub> = 1.2 x 10<sup>30</sup> erg/s at the 4.9 kpc distance to the cluster). After accounting for the number of active galactic nuclei and possible foreground stars among the detected X-ray sources, they estimate that 45-70 of the sources are cluster members. Four of the X-ray sources have previously been identified as accreting compact binaries in the cluster - three cataclysmic variables (CVs) and one quiescent neutron star. Correlating the Chandra positions with known variable stars yields 8 matches, of which 5 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, the authors 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. The authors place a 2-sigma upper limit of L<sub>x</sub> < 3 x 10<sup>30</sup> erg/s on the integrated luminosity of any additional faint, unresolved population of sources in the core of the cluster. In their paper, they explore the significance of these findings in the context of primordial versus dynamical channels for CV formation. They note that the number of CVs per unit mass in Omega 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. The authors obtained 2 exposures of Omega Cen using the imaging array of the Advanced CCD Imaging Spectrometer (ACIS-I) on 2000 January 24 - 25, in "very faint" (VF) mode. The total exposure time was 72.4 ks. The authors determined source counts using 95% encircled energy radii as determined from model PSFs, derived using the CIAO tool mkpsf at an intermediate energy of ~ 1.5 keV (the PSF shape being somewhat energy dependent). Counts were extracted in three bands: "soft" (0.5 - 1.5 keV), "medium" (0.5 - 4.5 keV), and "hard" (1.5 - 6.0 keV). The authors determined the background to subtract from each source by dividing the image into 1 arcminute-wide annuli centered on the aim point in chip 3 (the innermost "annulus" being a circle of radius 1.5 arcminutes). Background values adopted for sources in a given annulus were averages determined from several source-free regions within that annulus, after verifying that the background levels were azimuthally symmetric. For 12 sources ( source_numbers 11b, 12b, 13e, 22c, 32c, 41b, 41c, 84a, 84b, 84c, 93a, and 93b) that fell in the chip gaps or near the outer edge of a chip, background regions were chosen specifically to reflect these conditions. Local background determinations were also made for a small number of sources to the west of the cluster center that lie on or near a large diffuse X-ray source ~7 arcminutes west of the cluster center (see below). Following background subtraction, the authors applied aperture corrections and also corrected for reduced effective exposure times off-axis and in the chip gaps using the exposure map. This table was created by the HEASARC in June 2011 based on the electronic versions of Table 1 from the reference paper which was obtained from the CDS (their catalog J/ApJ/697/224 file table1.dat). This is a service provided by NASA HEASARC .
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