OMEGCENCXO - Omega Centauri Globular Cluster Chandra X-Ray Point Source Catalog

Overview

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 ^-16 erg/cm²/s (L_x = 1.2 x 10³⁰ 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_x < 3 x 10³⁰ 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.

Catalog Bibcode

2009ApJ...697..224H

References

A Chandra study of the Galactic globular cluster omega Centauri.
    Haggard D., Cool A.M., Davies M.B.
   <Astrophys. J., 697, 224-236 (2009)>
   =2009ApJ...697..224H    (SIMBAD/NED BibCode)

Provenance

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).

Parameters

Source_ID
The X-ray source identification number used to refer to individual sources in the reference paper. For ease of locating sources in the X-ray image of the cluster shown in Figures 2 and 3 of the reference paper, the authors named each detected source according to its radial offset from the cluster center and the quadrant in which it falls (see Figure 3 of the reference paper). The first character in the source ID represents the radial offset in arcminutes (rounded to the nearest arcminute, except for sources less than 0.5 arcminutes from the center, which are also assigned a first digit of "1"). The second character (1-4) represents the quadrant in which the source falls (counterclockwise from the northwest quadrant "1"). The last character is a letter (a-l), in the order of azimuthal position (working counterclockwise) within a given annulus and quadrant.

Name
The X-ray source designation registered with the CDS Dictionary of Nomenclature of Celestial Objects, using the prefix of 'CXOHCD' for 'Chandra X-ray Observatory, Haggard, Cool and Davies' and the J2000.0 equatorial coordinates truncated to 0.01 seconds of time and 0.1 arcseconds, e.g., 'CXOHCD J132624.41-472657.8'.

RA
The Right Ascension of the X-ray source in the selected equinox. This was not given explicitly in the reference paper but was constructed by the HEASARC based on the J2000.0-based source name (see above). Uncertainties in these positions will be a combination of the uncertainty in the absolute pointing of the spacecraft (~ 0.6 arcseconds) and the uncertainty in centroiding the counts associated with each source. The latter contribution has been estimated by Feigelson et al. (2002, ApJ, 574, 258) to be ~ 0.25 arcseconds for off-axis angles of <~ 1', 0.5 arcseconds for off-axis angles of ~ 4.5', and ~ 2 - 5 arcseconds toward the edge of the field (off-axis angle ~ 8 - 12 arcminutes).

Dec
The Declination of the X-ray source in the selected equinox. This was not given explicitly in the reference paper but was constructed by the HEASARC based on the J2000.0-based source name (see above). Uncertainties in these positions will be a combination of the uncertainty in the absolute pointing of the spacecraft (~ 0.6 arcseconds) and the uncertainty in centroiding the counts associated with each source. The latter contribution has been estimated by Feigelson et al. (2002, ApJ, 574, 258) to be ~ 0.25 arcseconds for off-axis angles of <~ 1', 0.5 arcseconds for off-axis angles of ~ 4.5', and ~ 2 - 5 arcseconds toward the edge of the field (off-axis angle ~ 8 - 12 arcminutes).

LII
The Galactic Longitude of the X-ray source.

BII
The Galactic Latitude of the X-ray source.

Radial_Distance
The radial offset of the X-ray source from the cluster center, in units of core radii, r_c, where r_c = 155 arcseconds, corresponding to 3.7 pc at the 4.9 kpc distance of the cluster.

MB_Total_Counts
The "medium-band" (0.5 - 4.5 keV) raw X-ray source counts.

MB_Counts
The "medium-band" (0.5 - 4.5 keV) corrected X-ray source counts.

SB_Total_Counts
The "soft-band" (0.5 - 1.5 keV) raw X-ray source counts.

SB_Counts
The "soft-band" (0.5 - 1.5 keV) corrected X-ray source counts.

HB_Total_Counts
The "hard-band" (1.5 - 6.0 keV) raw X-ray source counts

HB_Counts
The "hard-band" (1.5 - 6.0 keV) corrected X-ray source counts.

Cdfs_SB_Total_Counts
The raw X-ray source counts in the Chandra Deep Field South (CDF-S) soft X-ray band (0.5 - 2.0 keV).

Cdfs_SB_Counts
The corrected X-ray source counts in the Chandra Deep Field South (CDF-S) soft X-ray band (0.5 - 2.0 keV).

Hardness_Ratio
The hardness ratio of the X-ray source, defined as the number of soft counts divided by the number of hard counts.

Flux
The source flux in the 0.5 - 2.5 keV ROSAT band, in units of erg/cm²/s. This was derived from the medium-band counts values. The authors used WebPIMMS to estimate that one ACIS-I count per second is equal to an unabsorbed flux of 7.3 x 10^-12 erg/cm²/s in the 0.5 - 2.5 keV band, based on a 1 keV thermal bremsstrahlung spectrum, with an assumed hydrogen column of N_H = 9 x 10²⁰ cm^-2. All but two sources in the final wavdetect list have at least 4 counts. This represents a detection limit of f_x ~ 4.3 x 10^-16 erg/cm²/s (L_x ~ 1.2 x 10³⁰ erg/s at a distance of 4.9 kpc) - about an order of magnitude fainter than the XMM-Newton detection limit (Gendre et al. 2003, A&A, 400, 521) for this cluster (the XMM-Newton source list is available as the HEASARC Browse OMEGCENXMM table, notice).

Alt_Names
Previous X-ray identifications of the X-ray source, if any, using the following reference abbreviations:

  PSPC = Johnston et al. 1994A&A...289..763J (<[JVH94] NGC 5139 NN> in Simbad)
  HRI  = Verbunt & Johnston, 2000A&A...358..910V (<[JVH94] NGC 5139 NNa> in
         Simbad)
  ACIS = Rutledge et al. 2002, CDS Cat. J/ApJ/578/405 (<[RBB2002b] NN> in Simbad)
  XMM  = Gendre et al. 2003, CDS Cat. J/A+A/400/521 (<[GBW2003] NNN> in Simbad)

Optical_ID
Optical cross-identifications of the X-ray source, if any, using the following reference abbreviations:

   HD   = HD Catalog identification: Hog et al. 1998, CDS Cat. I/250; Perryman
          et al. 1997, Cat. I/239;
   CV   = cataclysmic variable: Carson et al. 2000ApJ...532..461C, Haggard et al.
          2002AAS...201.0718H, 2003AAS...203.5203H;
   qNS  = quiescent neutron star: Haggard et al. 2004ApJ...613..512H;
   V,NV = variable star: Kaluzny et al. 2004, CDS Cat. J/A+A/424/1101
          (<Cl* NGC 5139 SAW VNNN> in Simbad)

Contact Person

Questions regarding the OMEGCENCXO database table can be addressed to the HEASARC User Hotline.

A service of the Astrophysics Science Division at NASA/GSFC and the High Energy Astrophysics Division of the Smithsonian Astrophysics Observatory (SAO)

HEASARC Director: Dr. Alan P. Smale
HEASARC Associate Director: Dr. Roger Brissenden
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Web Curator: Meredith Gibb
Last Modified: 14-Jun-2011

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Last Modified: 14-Jun-2011