NGC 6231 Chandra X-Ray Point Source Catalog 2

NGC 6231 Chandra X-Ray Point Source Catalog 2 NGC6231CX2 ivo://nasa.heasarc/ngc6231cx2 NASA/GSFC HEASARC Kuhn et al. 2026-10-09 NASA/GSFC HEASARC help desk heasarc-vo at athena.gsfc.nasa.gov Survey Source NGC 6231 is a young cluster (age ~2-7 Myr) dominating the Sco OB1 association (distance ~1.59 kpc) with ~100 O and B stars and a large pre-main-sequence stellar population. The authors combine a reanalysis of archival Chandra X-ray data with multi-epoch near-infrared (NIR) photometry from the VISTA Variables in the Via Lactea (VVV) survey and published optical catalogs to obtain a catalog of 2148 probable cluster members. This catalog is 70% larger than previous censuses of probable cluster members in NGC 6231. It includes many low-mass stars detected in the NIR but not in the optical and some B stars without previously noted X-ray counterparts. In addition, the authors identify 295 NIR variables, about half of which are expected to be pre-main-sequence stars. With the more complete sample, they estimate a total population in the Chandra field of 5700-7500 cluster members down to 0.08 Msun (assuming a universal initial mass function) with a completeness limit at 0.5 Msun. A decrease in stellar X-ray luminosities is noted relative to other younger clusters. However, within the cluster, there is little variation in the distribution of X-ray luminosities for ages less than 5 Myr. The X-ray spectral hardness for B stars may be useful for distinguishing between early-B stars with X-rays generated in stellar winds and B-star systems with X-rays from a pre-main-sequence companion (>35% of B stars). A small fraction of catalog members have unusually high X-ray median energies or reddened NIR colors, which might be explained by absorption from thick or edge-on disks or being background field stars. This work makes use of some basic cluster properties available from the literature. Summaries of older studies are provided by Sana et al. (2006, J/A+A/454/1047), available in <a href="/W3Browse/xmm-newton/ngc6231xmm.html">NGC6231XMM</a>, and Reipurth (2008hsf2.book.....R). Expanded catalogs of cluster members have been provided by Sung et al. (2013, J/AJ/145/37) and Damiani et al. 2016, J/A+A/596/A82 (DMS2016), available at <a href="/W3Browse/chandra/ngc6231cxo.html">NGC6231CXO</a>. Chandra X-ray observations were made using the imaging array on the Advanced CCD Imaging Spectrometer (ACIS-I; Garmire et al. 2003SPIE.4851...28G). This instrument is an array of four CCD detectors that subtends 17'x17'. The target was observed in 2005 July (Sequence 200307; PI: S. Murray) in two observations (ObsID 5372 and 6291), and the data were retrieved from the Chandra Data Archive. The NIR ZYJHKs data were obtained from the VVV survey (Minniti et al. 2010NewA...15..433M; Saito et al. 2012, Cat. II/337). VVV is a multi-epoch NIR survey that covers both the Galactic bulge and an adjacent Galactic disk region and was carried out using the 4.1 m VISTA telescope on Cerro Paranal. The VVV data were taken with the VISTA Infrared CAMera (VIRCAM; Dalton et al. 2006SPIE.6269E..0XD), a 4x4 array of Raytheon VIRGO 2048x2048 20 micron pixel detectors with a pixel scale of 0.34". In addition to the VVV photometry, public optical or infrared catalogs are available from surveys and publications. We have included VPHAS+ photometry (Drew et al. 2014, J/MNRAS/440/2036), UBVRI (Johnson-Cousins system) and H-alpha photometry from Sung et al. (2013, J/AJ/145/37), and Spitzer/IRAC photometry from the GLIMPSE survey (Benjamin et al. 2003, Cat. II/293). This table was created by the HEASARC in October 2020 based upon the <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/AJ/154/87">CDS Catalog J/AJ/154/87</a> file table1.dat, table3.dat, and table5.dat. This is a service provided by NASA HEASARC . 2017AJ....154...87K https://heasarc.gsfc.nasa.gov/W3Browse/all/ngc6231cx2.html Catalog Research served-by HEASARC TAP https://heasarc.gsfc.nasa.gov/xamin/vo/cone?showoffsets&table=ngc6231cx2& GET text/xml 180 99999 true 253.487316 -42.005491 0.0166666666666667 https://heasarc.gsfc.nasa.gov/xamin/vo/tap https://heasarc.gsfc.nasa.gov/cgi-bin/W3Browse/getvotable.pl?name=ngc6231cx2 GET text/xml https://heasarc.gsfc.nasa.gov/cgi-bin/W3Browse/w3query.pl?tablehead=name=heasarc_ngc6231cx2&Action=More+Options&Action=Parameter+Search&ConeAdd=1 CHANDRA 0.0166666666666667 X-ray public ngc6231cx2 NGC 6231 Chandra X-Ray Point Source Catalog 2 name X-Ray Source Designation (CXOU JHHMMSS.ss+DDMMSS.s) meta.id;meta.main char nullable ra Right Ascension degree pos.eq.ra;meta.main double nullable dec Declination degree pos.eq.dec;meta.main double nullable fb_flux Absorption-Corrected 0.5-8 keV X-Ray Flux erg/s/cm^2 phot.flux;em.X-ray double nullable fb_flux_stat_err 1-Sigma Statistical Uncertainty in Absorption-Corrected 0.5-8 keV X-Ray Flux erg/s/cm^2 stat.error;phot.flux;em.X-ray double nullable fb_log_lum Logarithm of the X-Ray (0.5-8 keV) Luminosity (in erg/s) log(erg/s) phys.luminosity;em.X-ray double nullable fb_log_lum_error 1-Sigma Statistical Uncertainty on logLX-t (in erg/s) log(erg/s) stat.error;phys.luminosity;em.X-ray double nullable sb_log_lum Logarithm of the X-Ray (0.5-2 keV) Luminosity (in erg/s) log(erg/s) phys.luminosity;em.X-ray.soft double nullable sb_log_lum_error 1-Sigma Statistical Uncertainty on logLX-s (in erg/s) log(erg/s) stat.error;phys.luminosity;em.X-ray double nullable spect_type Spectral Type Tabulated by Sana et al. (2006, J/MNRAS/372/661) src.spType char nullable age Age Estimated from the V Versus V-I Diagram Myr time.age double nullable twomass_hmag 2MASS H-Band Magnitude mag phot.mag;em.IR.H double nullable sb_bck_counts Observed 0.5-2 keV Counts in Merged Background Regions ct instr.background;em.X-ray.soft short nullable irac_5p8_um_mag Spitzer/IRAC 5.8-um Band Magnitude mag phot.mag;em.IR.4-8um double nullable apec_kt_error Uncertainty in kT Plasma Temperature keV stat.error;phys.temperature double nullable vmag_error 1-Sigma Uncertainty in Vmag mag stat.error;phot.mag;em.opt.V double nullable bvf99_number OB Star Name in the Baume et al. (1999, J/A+AS/137/233) Catalog meta.id.cross short nullable vvv_jmag VVV J-Band Magnitude mag phot.mag;em.IR.J double nullable ks_mag_absorption Estimated Absorption in the K_s-Band (A_K_) mag phys.absorption double nullable r2_mag VPHAS r2-Band Magnitude mag phot.mag;em.opt.R double nullable afterglow_fraction Suspected afterglow Fraction stat.fit.param double nullable twomass_flags 2MASS JHK Photometric Quality Flag meta.code.qual;phot char nullable cpd_number OB Star Name in the CPD Catalog (Gill & Kapteyn 1897AnCap...4....1G) meta.id.cross short nullable ks_mag Merged 2MASS and VVV K_s-Band Photometry in the 2MASS System mag phot.mag;em.IR.K double nullable twomass_name Source Designation in the 2MASS Catalog (HHMMSSss+DDMMSSs) meta.id char nullable fb_flux_obs 0.5-8 keV X-Ray Flux erg/s/cm^2 phot.flux;em.X-ray double nullable ub_color_error 1-Sigma Uncertainty in U-B mag stat.error;phot.color;em.opt.U;em.opt.B double nullable sb_prob_no_src p-Value for No-Source Hypothesis in the Soft (0.5-2 keV) Energy Band stat.fit.goodness double nullable off_axis_min Smallest Off-Axis Angle for Merged Observations arcmin instr.offset;stat.min double nullable twomass_ks_mag 2MASS K_s-Band Magnitude mag phot.mag;em.IR.K double nullable sbl98_number OB Star Name in the Sung et al. (1998, J/AJ/115/734) Catalog meta.id.cross short nullable hb_flux_stat_err 1-Sigma Statistical Uncertainty in Absorption-Corrected 2-8 keV X-Ray Flux erg/s/cm^2 stat.error;phot.flux;em.X-ray.hard double nullable fb_prob_no_src p-Value for No-Source Hypothesis in the Total (0.5-8 keV) Energy Band stat.fit.goodness double nullable vvv_ks_mag_flag Cambridge Astronomical Survey Unit (CASU) Flag for VVV K_s-Band Magnitude meta.code short nullable hmag Merged 2MASS and VVV H-Band Photometry in the 2MASS System mag phot.mag;em.IR.H double nullable umag VPHAS u-Band Magnitude mag phot.mag;em.opt.U double nullable irac_8_um_mag Spitzer/IRAC 8.0-um Band Magnitude mag phot.mag;em.IR.8-15um double nullable hb_counts_neg_err 1-Sigma Lower Bound on Nct-h ct stat.error;phot.count;em.X-ray.hard double nullable jmag Merged 2MASS and VVV J-Band Photometry in the 2MASS System mag phot.mag;em.IR.J double nullable vi_color_error 1-Sigma Uncertainty in V-I mag stat.error;phot.color;em.opt.V;em.opt.I double nullable fb_flux_sys_err 1-Sigma Systematic Uncertainty in Absorption-Corrected 0.5-8 keV X-Ray Flux erg/s/cm^2 stat.error.sys;phot.flux;em.X-ray double nullable ref_spect_type Reference(s) for Spectral Type Given by Sana et al. (2006, J/MNRAS/372/661) meta.ref;src.spType char nullable sb_log_photon_flux Logarithm of the Incident 0.5-2 keV Photon Flux log(photon/cm^2/s) phot.flux;em.X-ray.soft double nullable vvv_hmag_error 1-Sigma Uncertainty in HmagV mag stat.error;phot.mag;em.IR.H double nullable glimpse_name Source Designation in the GLIMPSE Catalog (SSTGLMA GLLL.llll+BB.bbbb) meta.id char nullable fb_counts_pos_err 1-Sigma Upper Bound on Nct-t ct stat.error;phot.count;em.X-ray double nullable fb_bck_counts Observed 0.5-8 keV Counts in Merged Background Regions ct instr.background;em.X-ray short nullable hb_median_energy Median 2-8 keV Energy, Observed Spectrum keV em.energy double nullable sb_median_energy Median 0.5-2 keV Energy, Observed Spectrum keV em.energy double nullable log_mass_6p4myr Estimated Mass for an Age of 6.4 Myr Solar_Mass phys.mass double nullable vvv_zmag_error 1-Sigma Uncertainty in ZmagV mag stat.error;phot.mag;em.opt.I double nullable sb_counts_pos_err 1-Sigma Upper Bound on Nct-s ct stat.error;phot.count;em.X-ray.soft double nullable alt_name IAU Source Designation (CXOVVV JHHMMSS.ss+DDMMSS.s) meta.id char nullable vvv_ymag_error 1-Sigma Uncertainty in YmagV mag stat.error;phot.mag;em.IR.K double nullable vi_color V-I Color mag phot.color;em.opt.V;em.opt.I double nullable twomass_ks_mag_error 1-Sigma Uncertainty in 2MASS K_s-Band Magnitude mag stat.error;phot.mag;em.IR.K double nullable rmag VPHAS r-Band Magnitude mag phot.mag;em.opt.R double nullable log_nh Logarithm of the Hydrogen Column Density log(cm^-2) phys.columnDensity double nullable twomass_photo_flags Source of JHK Default Magnitude meta.bib char nullable r2_mag_error 1-Sigma Uncertainty in r2_mag mag stat.error;phot.mag;em.opt.R double nullable ssb2013_halpha_mag_error 1-Sigma Uncertainty in Halpha_MagSSB mag stat.error;phot.flux;em.line.Halpha double nullable irac_8_um_mag_error 1-Sigma Uncertainty in 8.0mag mag stat.error;phot.mag;em.IR.8-15um double nullable twomass_conf_flags Flags Indicate Artifact Contamination, Confusion meta.code char nullable irac_3p6_um_mag Spitzer/IRAC 3.6-um Band Magnitude mag phot.mag;em.IR.3-4um double nullable fb_counts_neg_err 1-Sigma Lower Bound on Nct-t ct stat.error;phot.count;em.X-ray double nullable psf_fraction Average PSF Fraction (at 1.5 keV) for Merged Observations instr.det.psf double nullable yso_classii_flag Flag Indicates Classification as a Class II YSO src.class short nullable hb_counts Net 2-8 keV Counts in Merged apertures ct phot.count;em.X-ray.hard double nullable fb_median_energy Median 0.5-8 keV Energy, Observed Spectrum keV em.energy double nullable fb_log_flux_obs Logarithm of the Observed X-Ray Flux log(erg/s/cm^2) phot.flux;em.X-ray double nullable bii Galactic Latitude degree pos.galactic.lat double nullable vphas_id Source Identifier in the VPHAS+ Catalog (Drew et al. 2014, J/MNRAS/440/2036) meta.id char nullable gmag_error 1-Sigma Uncertainty in gmag mag stat.error;phot.mag;em.opt.B double nullable vvv_ymag VVV Y-Band Magnitude mag phot.mag;em.IR.J double nullable sb_total_counts Observed 0.5-2 keV Counts in Merged apertures ct phot.count;em.X-ray.soft short nullable off_axis_max Largest Off-Axis Angle for Merged Observations arcmin instr.offset;stat.max double nullable sb_counts_neg_err 1-Sigma Lower Bound on Nct-s ct stat.error;phot.count;em.X-ray.soft double nullable irac_5p8_um_mag_flag GLIMPSE Quality Flag for 5.8mag meta.code.qual;phot.mag;em.IR.4-8um char nullable ir_excess_flag Flag Indicates of Infrared Excess phot.color.excess short nullable twomass_fit_flags JHK Components Fit to Source meta.code char nullable detect_flag Detection Flag meta.code.error short nullable hb_log_photon_flux Logarithm of the Incident 2-8 keV Photon Flux log(photon/cm^2/s) phot.flux;em.X-ray.hard double nullable hb_prob_no_src p-Value for No-Source Hypothesis in the Hard (2-8 keV) Energy Band stat.fit.goodness double nullable halpha_mag VPHAS H-Alpha Band Magnitude mag phot.flux;em.line.Halpha double nullable dms2016_number Source Number in the DMS2016 Catalog (Damiani et al. 2016, J/A+A/596/A82) meta.id.cross short nullable hb_flux_sys_err 1-Sigma Systematic Uncertainty in Absorption-Corrected 2-8 keV X-Ray Flux erg/s/cm^2 stat.error.sys;phot.flux;em.X-ray.hard double nullable aperture_area Average Aperture Area for Merged Observations (in units of (0.492 arcsec)^2) phys.area;instr double nullable num_obs_extracted Total Number of Observations Extracted meta.number short nullable vmag V-Band Magnitude in the CIT System mag phot.mag;em.opt.V double nullable fit_dof Degrees of Freedom in Fit stat.fit.dof short nullable error_radius 1-Sigma Position Error Circle arcsec stat.error;pos double nullable twomass_a_flag Flag [0] Indicates Source Is Not Associated with an Asteroid or Comet meta.code short nullable fb_arf_value Mean 0.5-8 keV ARF Value cm^2.ct/ph phys.area;instr double nullable hb_bck_counts Observed 2-8 keV Counts in Merged Background Regions ct instr.background;em.X-ray.hard short nullable irac_3p6_um_mag_error 1-Sigma Uncertainty in 3.6mag mag stat.error;phot.mag;em.IR.3-4um double nullable hb_arf_value Mean 2-8 keV ARF Value cm^2.ct/ph phys.area;instr double nullable apec_log_nh Hydrogen Column Density log(cm^-2) phys.columnDensity double nullable bck_scaling Scaling of the Background Extraction (Broos et al. 2010ApJ...714.1582B, Sec 5.4) instr.background double nullable bv_color_error 1-Sigma Uncertainty in B-V mag stat.error;phot.color;em.opt.B;em.opt.V double nullable vvv_hmag_flag CASU Flag for HmagV meta.code short nullable lii Galactic Longitude degree pos.galactic.lon double nullable sb_arf_value Mean 0.5-2 keV ARF Value cm^2.ct/ph phys.area;instr double nullable ssb2013_number Source Designation in the Sung et al. (2013, J/AJ/145/37) Catalog meta.id int nullable nominal_exposure Total Exposure Time in Merged Observations s time.duration;obs.exposure double nullable vvv_ymagv_flag CASU Flag for YmagV meta.code short nullable vvv_hmag VVV H-Band Magnitude mag phot.mag;em.IR.H double nullable prob_no_src_min Smallest of No-t, No-s and No-h stat.fit.goodness double nullable umag_error 1-Sigma Uncertainty in umag mag stat.error;phot.mag;em.opt.U double nullable vvv_ks_mag_error 1-Sigma Uncertainty in VVV K_s-Band Magnitude mag stat.error;phot.mag;em.IR.K double nullable ks_prob_constant_limit Limit Flag [<>] for X-Ray Variability Probability meta.code;stat.probability char nullable single_ks_prob Smallest p-Value for the Kolmogorov-Smirnov Statistic within a Single Obs. stat.fit.goodness double nullable irac_4p5_um_mag_error 1-Sigma Uncertainty in 4.5mag mag stat.error;phot.mag;em.IR.4-8um double nullable frac_expo_discard Fraction of Exposure discarded in merge time.duration;obs.exposure;arith.ratio double nullable yso_classi_flag Flag Indicates Classification as a Class I YSO src.class short nullable log_nh_stat_err 1-Sigma Statistical Uncertainty on Log_NH log(cm^-2) stat.error;phys.columnDensity double nullable vvv_name Source Designation in the Catalog of K_s Variables in VVV (HHMMSS.ss+DDMMSS.s) meta.id char nullable class Browse Object Classification src.class short nullable log_lum_6p4myr Estimated Luminosity for an Age of 6.4 Myr Solar_Luminosity phys.mass double nullable twomass_jmag_error 1-Sigma Uncertainty in 2MASS J-Band Magnitude mag stat.error;phot.mag;em.IR.J double nullable merged_ks_prob Smallest p-Value for the Kolmogorov-Smirnov Statistic over Merged Obs. stat.fit.goodness double nullable halpha_mag_error 1-Sigma Uncertainty in Halpha_Mag mag stat.error;phot.flux;em.line.Halpha double nullable bv_color B-V Color mag phot.color;em.opt.B;em.opt.V double nullable log_mass_3p2myr Estimated Mass for an Age of 3.2 Myr Solar_Mass phys.mass double nullable hb_total_counts Observed 2-8 keV Counts in Merged apertures ct phot.count;em.X-ray.hard short nullable vvv_zmag VVV Z-Band Magnitude mag phot.mag;em.opt.I double nullable vvv_ra Right Ascension of VVV Source degree pos.eq.ra double nullable fb_log_photon_flux Logarithm of the Incident 0.5-8 keV Photon Flux log(photon/cm^2/s) phot.flux;em.X-ray double nullable vvv_jmag_flag CASU Flag for JmagV meta.code short nullable apec_kt kT Plasma Temperature keV phys.temperature double nullable alt_name_2 Massive Star Designation meta.id char nullable vvv_ks_mag VVV K_s-Band Magnitude mag phot.mag;em.IR.K double nullable irac_5p8_um_mag_error 1-Sigma Uncertainty in 5.8mag mag stat.error;phot.mag;em.IR.4-8um double nullable dms2016_flag Membership Classification in the DMS2016 Catalog src.class short nullable apec_log_nh_error Uncertainty in Hydrogen Column Density log(cm^-2) stat.error;phys.columnDensity double nullable irac_4p5_um_mag_flag GLIMPSE Quality Flag for 4.5mag meta.code.qual;phot.mag;em.IR.4-8um char nullable gmag VPHAS g-Band Magnitude mag phot.mag;em.opt.B double nullable vvv_dec Declination of VVV Source degree pos.eq.dec double nullable hb_flux Absorption-Corrected 2-8 keV X-Ray Flux erg/s/cm^2 phot.flux;em.X-ray.hard double nullable ks_excess_flag Flag Indicates K_s-Band Excess phot.color.excess short nullable log_nh_sys_err 1-Sigma Systematic Uncertainty on Log_NH log(cm^-2) stat.error.sys;phys.columnDensity double nullable frac_exposure Fraction of Nominal Exposure That Source Was Observed time.duration;obs.exposure double nullable ub_color U-B Color mag phot.color;em.opt.U;em.opt.B double nullable irac_4p5_um_mag Spitzer/IRAC 4.5-um Band Magnitude mag phot.mag;em.IR.4-8um double nullable fb_total_counts Observed 0.5-8 keV Counts in Merged apertures ct phot.count;em.X-ray short nullable sb_counts Net 0.5-2 keV Counts in Merged apertures ct phot.count;em.X-ray.soft double nullable num_obs_merged Number of Observations Merged to Estimate Photometry Properties meta.number short nullable twomass_hmag_error 1-Sigma Uncertainty in 2MASS H-Band Magnitude mag stat.error;phot.mag;em.IR.H double nullable ssb2013_halpha_mag SSB H-Alpha Magnitude mag phot.flux;em.line.Halpha double nullable vvv_jmag_error 1-Sigma Uncertainty in JmagV mag stat.error;phot.mag;em.IR.J double nullable hd_number OB Star Name in the HD Catalog (Cannon 1936AnHar.100....1C) meta.id.cross int nullable pos_algorithm Algorithm Used to Estimate Position (Broos et al. 2010ApJ...714.1582B, Sec. 7.1) meta.id;stat.fit char nullable irac_3p6_um_mag_flag GLIMPSE Quality Flag for 3.6mag meta.code.qual;phot.mag;em.IR.3-4um int nullable twomass_extend_flag Flag [0] Indicates Source Is Not Extended Contamination meta.code short nullable fb_counts Net 0.5-8 keV Counts in Merged apertures ct phot.count;em.X-ray double nullable imag VPHAS i-Band Magnitude mag phot.mag;em.opt.I double nullable off_axis Average Off-Axis Angle Theta for Merged Observations arcmin instr.offset;stat.mean double nullable segg_number OB Star Name in the Seggewiss (1968ZA.....68..142S) Catalog meta.id.cross short nullable chi_squared Chi^2 Statistic stat.fit.chi2 double nullable hb_counts_pos_err 1-Sigma Upper Bound on Nct-h ct stat.error;phot.count;em.X-ray.hard double nullable fb_log_flux_obs_error Uncertainty in Logarithm of the Observed X-Ray Flux log(erg/s/cm^2) stat.error;phot.flux;em.X-ray double nullable twomass_jmag 2MASS J-Band Magnitude mag phot.mag;em.IR.J double nullable vvv_zmag_flag CASU Flag for ZmagV meta.code short nullable fb_log_flux Logarithm of the Absorption-Corrected X-Ray Flux log(erg/s/cm^2) phot.flux;em.X-ray double nullable ks_prob_constant X-Ray Variability Probability P_KS_ percent stat.probability double nullable rmag_error 1-Sigma Uncertainty in rmag mag stat.error;phot.mag;em.opt.R double nullable imag_error 1-Sigma Uncertainty in imag mag stat.error;phot.mag;em.opt.I double nullable hb_flux_obs 2-8 keV X-Ray Flux erg/s/cm^2 phot.flux;em.X-ray.hard double nullable irac_8_um_mag_flag GLIMPSE Quality Flag for 8.0mag meta.code.qual;phot.mag;em.IR.8-15um char nullable log_lum_3p2myr Estimated Luminosity for an Age of 3.2 Myr Solar_Luminosity phys.mass double nullable "__x_ra_dec" System unit vector column double nullable "__y_ra_dec" System unit vector column double nullable "__z_ra_dec" System unit vector column double nullable "__row" System index column char nullable