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CYGOB2CXO - Cygnus OB2 Association Chandra X-Ray Point Source Catalog

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Overview

The aim of this study is to identify the so far unknown low mass stellar population of the ~2 Myr old Cygnus OB2 star forming region, and to investigate the X-ray and near-IR stellar properties of its members. The authors analyzed a 97.7ks Chandra ACIS-I observation pointed at the core of the Cygnus OB2 region. Sources were detected using the PWDETECT code and were positionally correlated with optical and near-IR catalogs from the literature. Source events were extracted with the ACIS EXTRACT package. X-ray variability was characterized through the Kolmogorov-Smirnov test and spectra were fitted using absorbed APEC thermal plasma models. The authors detected 1003 X-ray sources. Of these, 775 have near-IR counterparts and are expected to be almost all associated with Cygnus OB2 members. From near-IR color-color and color-magnitude diagrams, they estimate a typical absorption toward Cygnus OB2 of A_V ~ 7.0 mag. Although the region is young, very few stars (~ 4.4%) show disk-induced excesses in the near-IR. X-ray variability is detected in ~ 13% of the sources, but this fraction increases, up to 50%, with increasing source statistics. Flares account for at least 60% of the variability. Despite being generally bright, all but 2 of the 26 detected O-type and early B-type stars are not significantly variable. Typical X-ray spectral parameters are log N_H ~ 22.25 (cm^-2) and kT ~ 1.35 keV with 1-sigma dispersion of 0.2 dex and 0.4 keV, respectively. Variable and flaring sources have harder spectra with median kT = 3.3 and 3.8 keV, respectively. OB stars are typically softer (kT ~ 0.75 keV). X-ray luminosities range between 10^30 and 10^31 erg s^-1 for intermediate-mass and low-mass stars, and between 2.5 x 10^30 and 6.3 x 10^33 erg s^-1 for OB stars.

Cygnus OB2 was observed with the ACIS detector onboard the Chandra X-ray Observatory (CXO) on 2004 January 16 (Obs.Id. 4511; PI: E. Flaccomio). The ACIS-I 17' x 17' field of view is covered by 4 chips each with 1024 x 1024 pixels (scale 0.49 arcseconds per pixel). The observation was pointed towards J2000.0 (RA,Dec) = (20 33 12.2, +41 15 00.7). An SNR threshold of 4.5 sigma was chosen which resulted in an initial source list of 1054 sources, 51 of which were subsequently rejected as either instrumental artifacts or multiple detections of the same source with different spatial scales. An additional 10 of the 1003 X-ray sources in the present table are likely spurious statistical fluctuations rather than real sources.


Catalog Bibcode

2007A&A...464..211A

References

Unveiling the Cygnus OB2 stellar population with Chandra.
    Albacete Colombo J.F., Flaccomio E., Micela G., Sciortino S., Damiani F.
   <Astron. Astrophys., 464, 211-227 (2007)>
   =2007A&A...464..211A

Provenance

This table was created by the HEASARC in March 2008 based on CDS catalog J/A+A/464/211 files table1.dat, table2.dat and table3.dat.

Parameters

Source_Number
A running number for the X-ray source in order of increasing J2000.0 Right Ascension.

Alt_Name
An alternative X-ray source designation as recommended by the CDS Dictionary of Nomenclature of Celestial Objects, using the '[AFM2007]' prefix (for Albacete Colombo, Flaccomio, Micela 2007) followed by the source number.

Name
The recommended X-ray source designation using the 'CXOCyg' prefix as registered with the CDS Dictionary of Nomenclature of Celestial Objects, and the J2000.o position, e.g., 'CXOCyg JHHMMSS.ss+DDMMSS.s'.

RA
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.

Dec
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.

LII
The Galactic Longitude of the X-ray source.

BII
The Galactic Latitude of the X-ray source.

Error_Radius
The uncertainty in the position of the X-ray source, in arcseconds.

Off_Axis
The distance of the X-ray source from the Chandra optical axis, in arcminutes.

SNR
The statistical significance (number of sigma) of the X-ray source detection.

Extraction_Area
The extraction area used for the particular X-ray source, in pixels.

PSF_Fraction
The PSF fraction of the X-ray source within the extraction box area, assuming a photon energy E = 1.49 keV, in percent.

Counts
The background-corrected extracted X-ray source counts in the 0.5 - 8.0 keV energy band.

Exposure
The effective exposure time for the X-ray source, in seconds.

Count_Rate
The total (0.5 - 8.0 keV) band count rate of the X-ray source, in ct/s, calculated as the ratio of the source photons in this band (corrected for the PSF fraction outside the extraction box) to the exposure time.

SB_Count_Rate
The soft (0.5 - 1.7 keV) band count rate of the X-ray source, in ct/s, calculated as the ratio of the source photons in this band (corrected for the PSF fraction outside the extraction box) to the exposure time.

MB_Count_Rate
The medium (1.7 - 2.8 keV) band count rate of the X-ray source, in ct/s, calculated as the ratio of the source photons in this band (corrected for the PSF fraction outside the extraction box) to the exposure time.

HB_Count_Rate
The hard (2.8 - 8.0 keV) band count rate of the X-ray source, in ct/s, calculated as the ratio of the source photons in this band (corrected for the PSF fraction outside the extraction box) to the exposure time.

Median_Energy
The median photon energy of the X-ray source photons, in keV.

Log_Prob_Constant
The logarithm of the KS-test significance, log(P_KS), that the distrubution of photon arrival times is consistent with that expected for a constant source; values < -4.0 have been truncated to this value. Sources with log(P_KS) < -3.0 can be considered almost definitely variable, with 1 constant source out of the 1003 total sources being likely to be erroneously thus classified, sources with -3.0 < log(P_KS) < -2.0 can be considered as likely variable, with up to 9 constant sources on the average being erroneously thus classified.

Flare_Flag
This flag parameter is set to 'F' to indicate that the X-ray lightcurve has a flare-like feature, i.e., a rapid rise with a slower decay.

TwoMASS_Name
The near-infrared (2MASS) source designation (2MASS JHHMMSSss+DDMMSSs) of the proposed counterpart to the X-ray source.

CXO_2MASS_Offset
The offset between the Chandra X-ray source and its 2MASS counterpart, in arcseconds.

Jmag
The J magnitude of the proposed 2MASS counterpart to the X-ray source.

Jmag_Error
The RMS uncertainty in the 2MASS J magnitude.

Hmag
The H magnitude of the proposed 2MASS counterpart to the X-ray source.

Hmag_Error
The RMS uncertainty in the 2MASS H magnitude.

Ks_Mag
The K_s magnitude of the proposed 2MASS counterpart to the X-ray source.

Ks_Mag_Error
The RMS uncertainty in the 2MASS K_s magnitude.

Jmag_Flag
The 2MASS J-band photometric quality flag, where 'A'. 'B', 'C' and 'D' indicate decreasing quality of measurements and 'U' means that the quoted value is an upper limit. See the 2MASS documantation for further details.

Hmag_Flag
The 2MASS H-band photometric quality flag, where 'A'. 'B', 'C' and 'D' indicate decreasing quality of measurements and 'U' means that the quoted value is an upper limit. See the 2MASS documantation for further details.

Ks_Mag_Flag
The 2MASS K_s-band photometric quality flag, where 'A'. 'B', 'C' and 'D' indicate decreasing quality of measurements and 'U' means that the quoted value is an upper limit. See the 2MASS documantation for further details.

Vmag_Extinction
The absorption A_V in the V band for the 2MASS counterpart to the X-ray source, calculated by measuring the distance of its position in the H-K_s vs. K_s diagram (Fig. 4 of the reference paper) along the reddening vector to the cluster locus for the assumed cluster distance of 1.45 kpc and no extinction.

Vmag_Extinction_Flag
This flag parameter is set to 'U' to indicate that A_V is unconstrained.

Fit_Log_NH
The logarithm of the best-fit hydrogen column density, in atoms cm^-2, obtained by fitting the X-ray source spectrum assuming that the emission is from a thermal plasma in collisional equilibrium (as modeled by the APEC code) behind an absorbing column of gas (the WABS model). Automated one-temperature fits for a plasma with an assumed 0.3 times solar abundance were performed for the 554 (out of 1003) sources with more than 20 counts. (More sophisticated model fits for 26 of the X-ray sources associated with OB stars are not given here but are available in Table 4 of the reference paper). Best-fit values of log N_H < 20.8 cm^-2 were truncated to 20.8 because, in the 0.5 - 8.0 keV energy range, ACIS spectra are insensitive to lower column densities.

Fit_Log_NH_Error
The one-sigma uncertainty in log N_H, in atoms cm^-2.

Fit_APEC_kT
The best-fit plasma temperature, in keV, obtained by fitting the X-ray source spectrum assuming that the emission is from a thermal plasma in collisional equilibrium (as modeled by the APEC code) behind an absorbing column of gas (the WABS model). Automated one-temperature fits for a plasma with an assumed 0.3 times solar abundance were performed for the 554 (out of 1003) sources with more than 20 counts. (More sophisticated model fits for 26 of the X-ray sources associated with OB stars are not given here but are available in Table 4 of the reference paper). Because ACIS spectra are insensitive to the exact values for higher temperature plasmas, best-fit values of kT of > 10.0 keV were truncated to 9.99 kev.

Fit_APEC_kT_Error
The one-sigma uncertainty in the plasma temperature, in keV.

Fit_Log_EM
The logarithm of the best-fit emission measure, in cm^-3, obtained by fitting the X-ray source spectrum assuming that the emission is from a thermal plasma in collisional equilibrium (as modeled by the APEC code) behind an absorbing column of gas (the WABS model). Automated one-temperature fits for a plasma with an assumed 0.3 times solar abundance were performed for the 554 (out of 1003) sources with more than 20 counts. (More sophisticated model fits for 26 of the X-ray sources associated with OB stars are not given here but are available in Table 4 of the reference paper).

Fit_Reduced_Chi2
The reduced chi^2 value for the spectral fit to the X-ray source spectrum.

Fit_DoF
The relative number of degrees of freedom for the spectral fit to the X-ray source spectrum.


Contact Person

Questions regarding the CYGOB2CXO database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 28-Mar-2008