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ONCCXOXRAY - Orion Nebula Cluster Chandra HRC X-Ray Point Source Catalog


In the first of two companion papers on the Orion Nebula Cluster (ONC), the authors presented their analysis of a 63 ks Chandra HRC-I observation that yielded 742 X-ray detections within the 30' x 30' field of view. To facilitate their interpretation of the X-ray image, they compiled a multiwavelength catalog of nearly 2900 known objects in the region by combining 17 different catalogs from the recent literature. They defined two reference groups: an infrared sample, containing all objects detected in the K band, and an optical sample comprising low-extinction, well-characterized ONC members. They showed for both samples that field object contamination is generally low. Their X-ray sources are primarily low-mass ONC members. The detection rate for optical sample stars increases monotonically with stellar mass from zero at the brown dwarf limit to ~100% for the most massive stars but shows a pronounced dip between 2 and 10 M_sun. They determined L_X and L_X/L_bol or all stars in their optical sample and utilized this information in their companion paper to study correlations between X-ray activity and other stellar parameters.

In particular, the authors assembled an extensive catalog of known X-ray/optical/IR and radio objects that fell within the HRC FOV. In addition to their list of HRC sources and the Chandra source lists of Garmire et al. (2000, CDS Cat. <J/AJ/120/1426>) and Schulz et al. (2001, CDS Cat. <J/ApJ/549/441>), they considered 14 catalogs from recent publications, producing a database of nearly 2900 distinct objects reported in at least one of the studies considered. A full list of references is given in the first column of Table 2 of the reference paper, along with a concise classification of the work and the referenced table number(s) from the original work.

The HRC on board the Chandra X-Ray Observatory (Weisskopf et al., 2002PASP..114....1W) observed the ONC for 63.2ks on 2000 February 4. The pointing (R.A. = 5h 35m 17s, DE=-5{deg} 23' 16" (J2000.0)) was chosen to place the Trapezium region and the bright O star Theta^1 Ori C in the center of the field of view (FOV). A good fraction of the ONC region was included in the 30' x 30' HRC FOV. This HEASARC table lists the properties of the 742 X-ray sources detected in this observation as presented in the full version of Table 1 of the reference paper.

Catalog Bibcode



Chandra X-ray observation of the Orion Nebula Cluster.
I. Detection, identification, and determination of X-ray luminosities.
    Flaccomio E., Damiani F., Micela G., Sciortino S., Harnden F.R.Jr,
    Murray S.S., Wolk S.J.
   <Astrophys. J. 582, 382 (2003)>


This table was created by the HEASARC in March 2007 based on the CDS table J/ApJ/582/382, file table1.dat.

HEASARC Implementation

The table of the stars in the ONC Optical Sample (Table 4 of the reference paper) is available as the HEASARC Browse table ONCCXOOPT. The HEASARC has created links connecting entries in the present table to those in this related table, based on having a positional coincidence within 5 arcseconds. This compromise value may be too large for positions near the center of the field (and hence produce multiple matches) but too small for positions near the edge of the field (and hence miss some matches): users can use the cross-correlation functionality in Browse to vary the matching radius between the two tables.


A running number for the X-ray source in order of increasing J2000.0 Right Ascension within the field. Note that this is not the same as the source numbering scheme used for the stars in the optical sample.

The name of the X-ray source using the '[FDM2003] X' prefix recommended by the CDS Dictionary of Nomenclature of Celestial Objects for the X-ray sources listed in this table combined with the X-ray source number.

The Right Ascension of the X-ray source in the selected equinox. This was given in J2000.0 coordinates to a precision of 0.01 seconds of time in the original table.

The Declination of the X-ray source in the selected equinox. This was given in J2000.0 coordinates to a precision of 0.01 arcseconds in the original table.

The Galactic Longitude of the X-ray source.

The Galactic Latitude of the X-ray source.

The statistical uncertainty in the X-ray position, in arcseconds.

The signal-to-noise ratio of the detected X-ray source.The authors chose to accept detections with SNR > 4.74, corresponding to 10 expected spurious detections throughout the FOV, instead of the more customary one, because: (1) 10 is still a small fraction of the total number of detected sources; (2) by lowering the threshold from 5.25-sigma (corresponding to one spurious source) to 4.74, they gained ~100 good detections (after a "hand filtering" of the detection list), of which only nine are expected to be spurious; and (3) Chandra's superb spatial resolution ensures that the ~10 spurious sources are extremely unlikely to be identified with other wavelength counterparts.

The net number of background and vignetting-corrected X-ray source counts.

The uncertainty in the net source counts.

The effective exposure time, in seconds, at the position of the X-ray source. This quantity, describing the spatially varying sensitivity of the HRC plus Chandra mirror system, is derived from an exposure map calculated with CIAO for an incident energy of 2.0 keV, i.e., the approximate temperature of the ONC X-ray sources (see Section 5.1 of the reference paper). This choice of temperature is not critical, however, because the normalized effective area at any given point on the detector depends only marginally on energy: for 0.5 keV < kT < 3.5 keV, the values of effective exposure times at any given location on the detector vary at most at the ~4% level.

Contact Person

Questions regarding the ONCCXOXRAY database table can be addressed to the HEASARC User Hotline.
If you have any problems, please consult the help page or mail ledas-help@star.le.ac.uk