OMEGCENCXO - Omega Centauri Globular Cluster Chandra X-Ray Point Source Catalog
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.
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)
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.
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'.
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).
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).
The Galactic Longitude of the X-ray source.
The Galactic Latitude of the X-ray source.
The radial offset of the X-ray source from the cluster center, in units of core radii, rc, where rc = 155 arcseconds, corresponding to 3.7 pc at the 4.9 kpc distance of the cluster.
The "medium-band" (0.5 - 4.5 keV) raw X-ray source counts.
The "medium-band" (0.5 - 4.5 keV) corrected X-ray source counts.
The "soft-band" (0.5 - 1.5 keV) raw X-ray source counts.
The "soft-band" (0.5 - 1.5 keV) corrected X-ray source counts.
The "hard-band" (1.5 - 6.0 keV) raw X-ray source counts
The "hard-band" (1.5 - 6.0 keV) corrected X-ray source counts.
The raw X-ray source counts in the Chandra Deep Field South (CDF-S) soft X-ray band (0.5 - 2.0 keV).
The corrected X-ray source counts in the Chandra Deep Field South (CDF-S) soft X-ray band (0.5 - 2.0 keV).
The hardness ratio of the X-ray source, defined as the number of soft counts divided by the number of hard counts.
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 NH = 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 fx ~ 4.3 x 10-16 erg/cm2/s (Lx ~ 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).
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 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)