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NGC6357CXO - NGC 6357 Chandra X-Ray Point Source Catalog



This contains some of the results from the first high spatial resolution X-ray study of the massive star-forming region NGC 6357, which were obtained in a 38 ks Chandra/ACIS observation. Inside the brightest constituent of this large H II region complex is the massive open cluster Pismis 24. It contains two of the brightest and bluest stars known, yet remains poorly studied; only a handful of optically bright stellar members have been identified. The authors have investigated the cluster extent and initial mass function and detected ~800 X-ray sources with a limiting sensitivity of ~ 10^30 ergs s^-1: this provides the first reliable probe of the rich intermediate-mass and low-mass population of this massive cluster, increasing the number of known members from optical studies by a factor of ~ 50. The high-luminosity end (log L[2-8 keV] >= 30.3 ergs s^-1) of the observed X-ray luminosity function in NGC 6357 is clearly consistent with a power-law relation as seen in the Orion Nebula Cluster and Cepheus B, yielding the first estimate of NGC 6357's total cluster population, a few times the known Orion population. The long-standing L_X ~ 10^-7 L_(bol) correlation for O stars is confirmed. Twenty-four candidate O stars and one possible new obscured massive YSO or Wolf-Rayet star are presented. Many cluster members are estimated to be intermediate-mass stars from available infrared photometry (assuming an age of ~ 1 Myr), but only a few exhibit K-band excess. The authors report the first detection of X-ray emission from an evaporating gaseous globule at the tip of a molecular pillar; this source is likely a B0-B2 protostar.

NGC 6357 was observed on 2004 July 9 with the Imaging Array of the Advanced CCD Imaging Spectrometer (ACIS-I) on board Chandra. Four front-illuminated (FI) CCDs form the ACIS-I, which covers a field of view (FOV) of ~ 17 by 17 arcminutes. The observation was made in the standard Timed Exposure, Very Faint mode, with 3.2 s integration time and 5 pixel by 5 pixel event islands. The total exposure time was 38 ks and the satellite roll angle was 289 degrees. The aim point was centered on the O3 If star Pis 24-1, the heart of the OB association Pismis 24. The Chandra observation ID is 4477.

Data reduction started with filtering the Level 1 event list processed by the Chandra X-ray Center pipeline to recover an improved Level 2 event list. To improve absolute astrometry, X-ray positions of ACIS-I sources were obtained by running the wavdetect wavelet-based source detection algorithm within the Chandra Interactive Analysis of Observations (CIAO) package on the original Level 2 event list, using only the central 8 by 8 arcminutes of the field. The resulting X-ray sources were matched to the 2MASS point source catalog. The authors calculated the position offsets between 277 X-ray sources and their NIR counterparts and applied an offset of +0.02" in right ascension (R.A.) and -0.33" in declination (decl.) to the X-ray coordinates.

From an initial list of 910 potential X-ray sources, the authors rejected sources with a P_B > 1% likelihood of being a background fluctuation. The trimmed source list includes 779 sources, with full-band (0.5 - 8.0 keV) net (background-subtracted) counts ranging from 1.7 to 1837 counts. The 779 valid sources were purposely divided by the authors into two lists: the 665 sources with P_B < 0.1% make up the primary source list of highly reliable sources (Table 1 in the reference paper; sources with source_type = 'M' in this table), and the remaining 114 sources with P_B >= 0.1% likelihood of being spurious background fluctuations were listed as tentative sources in Table 2 of the reference paper (source_type = 'T' in this table). The authors believe that most of these tentative sources are likely real detections.

Catalog Bibcode



An X-ray Census of Young Stars in the Massive Southern Star-Forming
Complex NGC 6357
     Wang J., Townsley L.K., Feigelson E.D., Getman K.V., Broos P.S.,
     Garmire G.P., Tsujimoto M.
    <Astrophys. J. Suppl., 168, 100-127 (2007)>


This table was created by the HEASARC in October 2007 based on the merger of the electronic versions of Tables 1 (Main Source Catalog) and 2 (Tentative Sources which were obtained from the ApJ website. To help distinguish from which original table entries in this Browse table come from, the HEASARC has created a parameter called source_type which is set to 'M' for sources from Table 1 and to 'T' for sources from Table 2.


A running source number in order of increasing J2000.0 Right Ascension which uniquely identifies an X-ray source.

To help distinguish from which original table entries come from, the HEASARC created this parameter: it is set to 'M' for sources from Table 1 of the reference paper and to 'T' for sources from Table 2.

An alternative name for the X-ray source using the '[WTF2007]' prefix (for Wang, Townsley, and Feigelson, 2007) and the source number, as recommended by the CDS Dictionary of Astronomical Nomenclature of Celestial Objects.

The standard Chandra X-ray source designation, using the standard 'CXOU J' prefix for Chandra X-ray sources and the truncated J2000.0 coordinates of the source (

The Right Ascension of the X-ray source in the selected equinox. This was given in J2000.0 equatorial decimal degree coordinates to a precision of 1 microdegree (0.0036") in the original reference. The original positions of the X-ray sources were adjusted to the 2MASS reference frame as described in the Overview (above).

The Declination of the X-ray source in the selected equinox. This was given in J2000.0 equatorial decimal degree coordinates to a precision of 1 microdegree (0.0036") in the original reference. The original positions of the X-ray sources were adjusted to the 2MASS reference frame as described in the Overview (above).

The Galactic Longitude of the X-ray source.

The Galactic Latitude of the X-ray source.

The positional error of the X-ray source, in arcseconds.

The off-axis angle of the X-ray source, in arcminutes.

The net (background-subtracted) full-band (0.5-8 keV) extracted counts in the X-ray source.

The 1-sigma equivalent uncertainty in the full-band source counts, based on Poisson statistics.

The estimated number of background counts in the X-ray source extraction region.

The net (background-subtracted) hard-band (2-8 keV) extracted counts in the X-ray source.

The fraction of the PSF used for source extraction. Note that a reduced PSF fraction (significantly below 90%) indicates that the source is in a crowded region.

The photometric significance of the X-ray source.

This limit parameter is set to '<' if the corresponding background probability (log_bck_prob) is an upper limit.

The logarithm of the probability P_B that the extracted full-band counts for the X-ray source are solely due to a spurious background fluctuation.

This parameter contains the following values which flag various source anomalies:

  g = fractional time that source was on a detector (FRACEXPO from mkarf) is <0.9
  e = source on field edge;
  p = source piled up;
  s = source on readout streak.

This parameter contains the following values which characterize the source constancy or variability (no test was performed for sources with fewer than 4 total full-band counts, and no value is reported for sources in chip gaps or on field edges):

      a = no evidence for variability;
      b = possibly variable;
      c = definitely variable.

The effective exposure time, in seconds, defined as the amount of exposure time needed for the source to accumulate the current number of counts if it had been placed at the Chandra aimpoint.

The median energy of the X-ray source photons, in keV, in the full spectral band after background subtraction.

Contact Person

Questions regarding the NGC6357CXO database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 5-Nov-2007