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NGC6530CXO - NGC 6530 Chandra X-Ray Point Source Catalog

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Overview

In a deep 60 ks Chandra ACIS-I X-ray observation of the very young (~ 1.5 - 2.0 Myr) cluster NGC 6530 on 2001 Jun 18-19, the authors have detected 884 X-ray point sources and argue that a very large fraction of them (90%-95%) must be pre-main-sequence (PMS) cluster members, mostly low-mass stars. This is a significant enlargement of the known NGC 6530 stellar population with respect to previous optical studies, including H-alpha surveys. They identify 220 X-ray sources with catalogued stars down to V = 17, while most unidentified sources have fainter counterparts. Moreover, they find an infrared counterpart in the 2MASS (CDS. No. <II/246>) Catalog for 731 X-ray sources. The optically identified cluster X-ray sources are found in a band in the H-R diagram above the main sequence, in the locus of 0.5 - 1.5 Myr PMS stars, with masses down to 0.5 - 1.5 solar masses (M_sun).

The pointing direction for the Chandra observation was the NGC 6530 cluster center at RA = 18^h 04^m 24.38^s, Dec = -24^o 21' 05.8" (J2000.0). The PWDetect algorithm found 884 X-ray point sources in the ACIS-I image above a detection significance threshold chosen to ensure only 1 spurious detection on the average. The Sung et al. (2000, AJ, 120, 333; CDS Cat. <J/AJ/120/333>) = SCB Catalog of optical objects against which the X-ray point source list was compared doed not cover the easternmost 2.25' of the ACIS FOV (RAs later than 18^h 04^m 52^s), notice, which comprises about 13% of the ACIS FOV. There are 46 detected X-ray sources (5.2% of the total) in the area not covered by the SCB Catalog. A matching distance of 4 times the X-ray error radius or 2.0" (whichever is greater) was used to identify optical counterparts to the X-ray sources, after a systematic shift between the X-ray and optical positions of -0.4" and 1.84" in RA and declination, respectively, was applied. The authors estimate that as many as 28 of their 220 optical identifications may be spurious, preferentially those in the outer parts of the FOV where the positional uncertainties are larger.

There are 8792 'good' 2MASS sources in the ACIS FOV. A matching distance of 4 times the X-ray error radius or 1.5" (whichever is greater) was used to identify 2MASS counterparts to the X-ray sources, after systematic corrections of 0.3" and 1.75" in RA and declination, respectively, were applied to the 'raw' X-ray positions. There are 13 cases where there are two possible IR counterparts to a single X-ray source, and 2 cases where there are three possible IR Counterparts to a single X-ray source. (Notice that, in such cases, this table contains multiple entries, one for each counterpart, and hence there are 901 entries compared to 884 X-ray sources.) The authors conclude that the plausible number of spurious X-ray-2MASS identifications is between 30 and 50. Overall, there remain 146 X-ray sources with no optical or IR identification.


Catalog Bibcode

2004ApJ...608..781D

References

A deep Chandra X-ray observation of the rich young cluster NGC 6530.
I. The X-ray source catalog and the cluster population.
    Damiani F., Flaccomio E., Micela G., Sciortino S., Harnden F.R.Jr,
    Murray S.S.
   <Astrophys. J., 608, 781-796 (2004)>
   =2004ApJ...608..781D

Provenance

This table was created by the HEASARC in December 2006 based on CDS table J/ApJ/608/781, the file table1.dat.

Parameters

Source_Number
A running sequential identification number in order of increasing J2000 Right Ascension. This was used by the HEASARC to construct a name for the X-ray source, using the prefix '[DFM2004 ' recommended by the Dictionary of Nomenclature of Celestial Objects.

Name
The name of the X-ray source constructed using the prefix '[DFM2004 ' recommended by the Dictionary of Nomenclature of Celestial Objects and the running sequential identification number (source_number).

RA
The Right Ascension of the X-ray source in the selected equinox. This was given in J2000 coordinates to a precision of 0.01 seconds of time in the originating table. The quoted value includes a systematic correction of 0.3" based on matching the X-ray and 2MASS positions.

Dec
The Declination of the X-ray source in the selected equinox. This was given in J2000 coordinates to a precision of 0.1 arcseconds in the originating table. The quoted value includes a systematic correction of 1.75" based on matching the X-ray and 2MASS positions.

LII
The Galactic Longitude of the X-ray source.

BII
The Galactic Latitude of the X-ray source.

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

Count_Rate
The count rate of the X-ray source in the energy range from 0.3 - 8.0 keV, in counts per second, as measured using PWDetect, a wavelet-based source detection algorithm.

Count_Rate_Error
The uncertainty in the count rate of the X-ray source, in counts per second.

Ks_Probability
The probability according to the Kolmogorov-Smirnov Test that the X-Ray source was constant during the exposure, in percent.

Hardness_Ratio_1
The first spectral hardness ratio, (H-S)/(H+S), of the X-ray source, where H is the count rate in the 0.75 - 1.5 keV energy band, and S is the count rate in the 0.3 - 0.75 keV band.

Hardness_Ratio_2
The second spectral hardness ratio, (H-S)/(H+S), of the X-ray source, where H is the count rate in the 1.5 - 8.0 keV energy band, and S is the count rate in the 0.75 - 1.5 keV band.

Scb_Number
The number NNNN of the optical counterpart to the X-ray source in the Sung et al. (2000, AJ, 120, 333; CDS Cat. <J/AJ/120/333>) catalog, otherwise the SCB star number. These objects can be located in the Simbad database using the full name of 'Cl* NGC 6530 SCB NNNN'.

Xray_Opt_Offset
The offset between the X-ray and SCB optical positions, in arcseconds.

Vmag
The V-band magnitude of the optical counterpart to the X-ray source as given in the SCB Catalog (Sung et al. (2000, AJ, 120, 333; CDS Cat. <J/AJ/120/333>).

VI_Color
The (V-I) color of the optical counterpart to the X-ray source as given in the SCB Catalog (Sung et al. (2000, AJ, 120, 333; CDS Cat. <J/AJ/120/333>).

BV_Color
The (B-V) Color of the optical counterpart to the X-ray source as given in the SCB Catalog (Sung et al. (2000, AJ, 120, 333; CDS Cat. <J/AJ/120/333>).

Twomass_Name
The name of the infrared counterpart to the X-ray source found in the 2MASS ( CDS Cat. <II/246>) Catalog.

Xray_IR_Offset
The offset between the X-ray and 2MASS infrared positions, in arcseconds.

Jmag
The J-band magnitude of the 2MASS near-IR counterpart to the X-ray source.

Hmag
The H-band magnitude of the 2MASS near-IR counterpart to the X-ray source.

Kmag
The K-band magnitude of the 2MASS near-IR counterpart to the X-ray source.


Contact Person

Questions regarding the NGC6530CXO database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 8-Dec-2006