- ID:
- ivo://nasa.heasarc/chansexsi
- Title:
- Chandra Serendipitous Extragalactic X-Ray Source ID (SEXSI) Catalog
- Short Name:
- Chan/Seren/ID
- Date:
- 02 May 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- The Serendipitous Extragalactic X-Ray Source Identification (SEXSI) program is designed to extend greatly the sample of identified extragalactic hard X-ray (2 - 10 keV) sources at intermediate fluxes (~10<sup>-13</sup> to 10<sup>-15</sup> erg/cm<sup>2</sup>/s). SEXSI, which studies sources selected from more than 2 deg<sup>2</sup>, provides an essential complement to the Chandra Deep Fields, which reach depths of 5 x 10<sup>-16</sup> erg/cm<sup>2</sup>/s (2 - 10 keV) but over a total area of less than 0.2 deg<sup>2</sup>. In their published paper, the authors describe the characteristics of the survey and their X-ray data analysis methodology. They present the cumulative flux distribution for the X-ray sample of 1034 hard sources and discuss the distribution of spectral hardness ratios. Their log N -log S in this intermediate flux range connects to those found in the Deep Fields, and by combining the data sets, they constrain the hard X-ray population over the flux range in which the differential number counts change slope and from which the bulk of the 2 - 10 keV X-ray background arises. They further investigate the log N - log S distribution separately for soft and hard sources in the sample, finding that while a clear change in slope is seen for the softer sample, the hardest sources are well described by a single power law down to the faintest fluxes, consistent with the notion that they lie at lower average redshift. In the SEXSI program, fields were selected with high Galactic latitude (|b| > 20 degrees) and with declinations accessible to the optical facilities available to the authors (declination > -20 degrees). They used observations taken with Chandra's Advanced Camera for Imaging Spectroscopy (ACIS I- and S-modes; Bautz et al., 1998, Proc. SPIE, 3444, 210) only (for sensitivity in the hard band). All the fields presented in this paper have data that are available in the Chandra public archive. This table was created by the HEASARC in June 2004 based on <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/ApJ/596/944/table4">CDS Catalog J/ApJ/596/944/table4</a>.dat which is a representation of Table 4 from the published version. Note that it does not include the Soft-Band-Only Source Catalog (Table 6 in the published version of the paper). This is a service provided by NASA HEASARC .
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- ID:
- ivo://nasa.heasarc/chansexoid
- Title:
- Chandra Serendipitous Extragalactic X-Ray Source ID (SEXSI) Optical Follow-Up
- Short Name:
- Chan/Seren/Opt
- Date:
- 02 May 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- The Serendipitous Extragalactic X-ray Source Identification (SEXSI) Program is designed to expand significantly the sample of identified extragalactic hard X-ray sources at intermediate fluxes, 10<sup>-15</sup> ergs/cm<sup>2</sup>/s < 2-10 keV Flux <~ 10<sup>-13</sup> ergs/cm<sup>2</sup>/s. SEXSI, which includes sources derived from more than 2 square degrees of Chandra images, provides the largest hard X-ray-selected sample yet studied, offering an essential complement to the Chandra Deep Fields (total area of 0.2 square degrees). In Eckart et al. (2005, Paper II) R-band optical imaging of the SEXSI fields from the Palomar P60 and P200, the MDM 2.4m and 1.3m, and the Keck I telescopes is described. The authors have identified counterparts or derived flux limits for nearly 1000 hard X-ray sources. Using the optical images, they have derived accurate source positions. They have investigated correlations between optical and X-ray flux, and optical flux and X-ray hardness ratio. They have also studied the density of optical sources surrounding X-ray counterparts, as well as the properties of optically faint, hard X-ray sources. In Eckart et al. (2006, Paper III) optical spectra of 477 counterparts are presented. These spectra reach to R-band magnitudes of <~24 and have produced identifications and redshifts for 438 hard X-ray sources. Typical completeness levels in the 27 Chandra fields studied are 40-70%. The vast majority of the 2-10 keV selected sample are AGNs with redshifts between 0.1 and 3; the highest redshift source lies at z = 4.33. This table which combines data presented in Eckart et al. (2005, 2006) has links to the list of SEXSI X-ray sources (the HEASARC Browse table CHANSEXSI: see Paper I = Harrison et al. 2003, ApJ, 596, 944). This table was originally created by the HEASARC in June 2005 based on the CDS version of Table 3 from Eckart et al. (2005: CDS table J/ApJS/156/35/table3.dat). It was updated in August 2006 to include information from Table 2 of Eckart et al. (2006: the electronic version available at the electronic ApJ web site). This is a service provided by NASA HEASARC .
- ID:
- ivo://nasa.heasarc/csc
- Title:
- Chandra Source Catalog, v2.1.1
- Short Name:
- CSC
- Date:
- 02 May 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- The Chandra Source Catalog (CSC) is the definitive catalog of X-ray sources detected by the Chandra X-ray Observatory. By combining Chandra's sub-arcsecond on-axis spatial resolution and low instrumental background with consistent data processing, the CSC delivers a wide variety of uniformly calibrated properties and science ready data products for detected sources over four decades of flux. The Chandra Source Catalog version 2.1 (CSC 2.1) was released on April 2nd 2024; the current minor release is version 2.1.1, updated on 2024 October 18. (Refer to the <a href="https://cxc.cfa.harvard.edu/csc/versions.html">version history</a> for details.) This release of the catalog includes measured properties for 407,806 unique compact and extended X-ray sources in the sky, allowing statistical analysis of large samples, as well as individual source studies in the "Master Sources" table, provided herein. The extracted properties are provided for 1,304,376 individual observation detections, identified in Chandra ACIS and HRC-I imaging observations released publicly through the end of 2021, at the <a href="https://cxc.harvard.edu/csc/about.html">Chandra X-ray Center</a>. CSC 2.1.1 includes -- as an "alpha" release -- photometric properties for 1,717 highly extended (> ~30") sources, together with surface brightness polygons for several contour levels. The sensitivity limit for compact sources in CSC 2.1.1 is ~5 net counts, achieved by using a two-stage approach that involves co-adding multiple observations of the same field prior to source detection, and then using an optimized source detection method. For each X-ray detection and source, the catalog provides a detailed set of more than 100 tabulated positional, spatial, photometric, spectral, and temporal properties (each with associated lower and upper confidence intervals and measured in multiple energy bands). The catalog Bayesian aperture photometry code produces robust photometric probability density functions (PDFs), even in crowded fields and for low count detections. Releases beyond 2 use Bayesian Blocks analysis to identify multiple observations of the same source that have similar photometric properties, and these are analyzed simultaneously to improve S/N. The energy bands used to derive many of the CSC properties are defined in Table 4 of the reference paper: ultrasoft (u: 0.2-0.5 keV), soft (s: 0.5-1.2 keV), medium (m: 1.2-2.0 keV), hard (h: 2.0-7.0) and broad (b: 0.5-7.0 keV) for the ACIS energy bands, and wide (w: 0.1-10.0 keV) for the HRC energy band. The energy bands are chosen to optimize the detectability of X-ray sources while simultaneously maximizing the discrimination between different spectral shapes on X-ray color-color diagrams. Numerous source-specific catalog properties are evaluated within defined apertures. The authors define the "PSF 90% ECF aperture" for each source to be the ellipse that encloses 90% of the total counts in a model PSF centered on the source position. Because the size of the PSF is energy-dependent, the dimensions of the PSF 90% ECF aperture vary with energy band. They define the "source region aperture" for each source to be equal to the corresponding 3-sigma source region ellipse included in the merged source list, scaled by a factor of 1.5. Like the PSF 90% ECF aperture, the source region aperture is also centered on the source position, but the dimensions of the aperture are independent of energy band. This database table was last updated by the HEASARC in March 2025 and is based on a download of the online version of the Chandra Source Catalog, v2.1.1, at the CXC using the CLI. Refer to <a href="http://cxc.harvard.edu/csc/">http://cxc.harvard.edu/csc/</a> for details. This is a service provided by NASA HEASARC .
- ID:
- ivo://nasa.heasarc/chantypgpr
- Title:
- Chandra Typical Galactic Plane Region Point Source Catalog
- Short Name:
- CHANTYPGPR
- Date:
- 02 May 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- Using the Chandra Advanced CCD Imaging Spectrometer Imaging array (ACIS-I), the authors carried out a deep hard X-ray observation of the Galactic plane region at a location (l, b) ~ (28.5 degrees, 0.0 degrees), where no discrete X-ray source had been reported previously. They detected 274 new point X-ray sources (4-sigma or greater confidence in any of the 3 energy bands 0.5 - 3.0 keV, 3.0 - 8.0 keV or 0.5 - 8.0 keV), as well as strong Galactic diffuse emission within two partially overlapping ACIS-I fields (~ 250 square arcminutes in total). The point-source sensitivity was ~ 3 x 10<sup>-15</sup> ergs/s/cm<sup>2</sup> in the 2 - 10 keV band and ~ 2 x 10<sup>-16</sup> erg/s/cm<sup>2</sup> in the 0.5 - 2 keV band The sum of all the detected point-source fluxes accounts for only ~ 10% of the total X-ray flux in the field of view. Only 26 point sources were detected in both the soft and hard bands, indicating that there are two distinct classes of X-ray source distinguished by their spectral hardness ratios. The surface number density of the hard sources is only slightly higher than that measured in high Galactic latitude regions, indicating that the majority of the hard sources are background AGNs. Following up the Chandra observation, the authors performed a near-infrared (NIR) survey with SofI at ESO/NTT. Almost all the soft X-ray sources have been identified in the NIR, and their spectral types are consistent with main-sequence stars, suggesting that most of them are nearby X-ray-active stars. On the other hand, only 22% of the hard sources had near-IR counterparts, which are presumably Galactic. From X-ray and near-IR spectral study, they are most likely to be quiescent cataclysmic variables. This Browse table was created by the HEASARC in December 2006 based on <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/ApJ/635/214/">CDS Catalog J/ApJ/635/214/</a>, the file table1.dat. This is a service provided by NASA HEASARC .
- ID:
- ivo://nasa.heasarc/cxoxassist
- Title:
- Chandra XAssist Source List
- Short Name:
- Chan/XAssist
- Date:
- 02 May 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- This database table contains the latest Chandra XAssist source list. XAssist is a NASA-funded project for the automation of X-ray astrophysics, with emphasis on galaxies. It is capable of data reprocessing, source detection, and preliminary spatial, temporal, and spectral analysis for all sources with sufficient counts. The bulk of the system is written in Python, which in turn drives underlying software, e.g., CIAO for Chandra data. Pipelines running on Chandra observations of galaxies have generated the source list which comprises this HEASARC table. The pipeline also includes fields requested by users for various projects, most notably observations of a sample of quasars and several deep field observations have been processed. Note that the pipline processing is completely automated; therefore, users should visually inspect the results of any queries. This table was first created by the HEASARC in May 2005 based on the Chandra source list available at the XAssist website. In September 2010, the HEASARC switched over to using the "pipeline4" Chandra source list. It is updated on a weekly basis when and if the XAssist source list file at <a href="https://asd.gsfc.nasa.gov/xassist/pipeline4/chandra/master_srclist.csv">https://asd.gsfc.nasa.gov/xassist/pipeline4/chandra/master_srclist.csv</a> is updated. This is a service provided by NASA HEASARC .
- ID:
- ivo://nasa.heasarc/changalxrb
- Title:
- Chandra X-Ray Binary Catalog of SINGS Galaxies
- Short Name:
- CHANGALXRB
- Date:
- 02 May 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- The authors of this catalog presented new Chandra constraints on the X-ray luminosity functions (XLFs) of X-ray binary (XRB) populations, as well as their scaling relations, for a sample of 38 nearby galaxies (D = 3.4-29 Mpc). The galaxy sample is drawn primarily from the Spitzer Infrared Nearby Galaxies Survey (SINGS) and contains a wealth of Chandra (5.8 Ms total) and multiwavelength data, allowing for star formation rates (SFRs) and stellar masses (M<sub>*</sub>) to be measured on subgalactic scales. The authors divided the 2478 X-ray-detected sources into 21 subsamples in bins of specific SFR (sSFR=SFR/M<sub>*</sub>) and constructed XLFs. To model the XLF dependence on sSFR, they fitted a global XLF model, containing contributions from high-mass XRBs (HMXBs), low-mass XRBs (LMXBs), and background sources from the cosmic X-ray background that respectively scale with SFR, M<sub>*</sub>, and sky area. They found an HMXB XLF that is more complex in shape than previously reported and an LMXB XLF that likely varies with sSFR, potentially due to an age dependence. When applying the global model to XLF data for each individual galaxy, the authors discovered a few galaxy XLFs that significantly deviated from their model beyond statistical scatter. Most notably, relatively low-metallicity galaxies have an excess of HMXBs above ~10<sup>38</sup>erg/s, and elliptical galaxies that have relatively rich populations of globular clusters (GCs) show excesses of LMXBs compared to the global model. Additional modeling of how the XRB XLF depends on stellar age, metallicity, and GC specific frequency is required to sufficiently characterize the XLFs of galaxies. In this work, the authors utilized 5.8 Ms of Chandra ACIS data, combined with UV-to-IR observations, for 38 nearby (D < ~30 Mpc) Spitzer Infrared Nearby Galaxies Survey (SINGS; Kennicutt+ <a href="https://ui.adsabs.harvard.edu/abs/2003PASP..115..928K">2003PASP..115..928K</a>) galaxies to revisit scaling relations of the HMXB and LMXB X-ray luminosity functions (XLFs) with SFR and M<sub>*</sub>, respectively. This table contains the X-ray properties for 4442 X-ray point sources, including those with L<sub>X</sub> < 10<sup>35</sup>erg/s, which were excluded from the XLF analysis. This table was created by the HEASARC in April 2023 based upon the <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/ApJS/243/3">CDS Catalog J/ApJS/243/3</a> file table7.dat. This is a service provided by NASA HEASARC .
- ID:
- ivo://nasa.heasarc/chicagocxo
- Title:
- ChIcAGO Survey Chandra X-Ray Source Catalog
- Short Name:
- CHICAGOCXO
- Date:
- 02 May 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- This table contains results from the 'Chasing the Identification of ASCA Galactic Objects' (ChIcAGO) survey, which is designed to identify the unknown X-ray sources discovered during the ASCA Galactic Plane Survey (AGPS). Little is known about most of the AGPS sources, especially those that emit primarily in hard X-rays (2-10 keV) within the X-ray flux range from ~ 10<sup>-13</sup> to 10<sup>-11</sup> erg cm<sup>-2</sup> s<sup>-1</sup>. In ChIcAGO, the sub-arcsecond localization capabilities of Chandra have been combined with a detailed multi-wavelength follow-up program, with the ultimate goal of classifying the > 100 unidentified sources in the AGPS. Overall to date, 93 unidentified AGPS sources have been observed with Chandra as part of the ChIcAGO survey. A total of 253 X-ray point sources have been detected in these Chandra observations within 3 arcminutes of the original ASCA positions. The authors have identified infrared and optical counterparts to the majority of these sources, using both new observations and catalogs from existing Galactic plane surveys. X-ray and infrared population statistics for the X-ray point sources detected in the Chandra observations reveal that the primary populations of Galactic plane X-ray sources that emit in the X-ray flux range from ~ 10<sup>-13</sup> to 10<sup>-11</sup> erg cm<sup>-2</sup> s<sup>-1</sup> are active stellar coronae, massive stars with strong stellar winds that are possibly in colliding wind binaries, X-ray binaries, and magnetars. There is also another primary population that is still unidentified but, on the basis of its X-ray and infrared properties, likely comprises partly Galactic sources and partly active galactic nuclei. A total of 93 AGPS sources have been observed with Chandra as part of the ChIcAGO survey, of which 84 were imaged with ACIS-S and 9 were imaged with HRC-I. The ChIcAGO Chandra observations took place over a 3.5 yr period, from 2007 January to 2010 July. The Chandra exposure times ranged from ~ 1 to 10 ks. All the details of these Chandra observations are listed in Table 1 of the reference paper. The initial automated analysis of these Chandra observations was conducted using the ChIcAGO Multi-wavelength Analysis Pipeline (MAP), described in Section 2.2 of the reference paper. ChIcAGO MAP takes the ACIS-S or HRC-I Chandra observation of an AGPS source field and detects and analyzes all point sources within 3 arcminutes, equivalent to the largest likely position error, for the original AGPS source positions supplied by Sugizaki et al. (2001, ApJS, 134, 77). The authors then performed a more detailed X-ray analysis and counterpart study for those 74 sources with > 20 X-ray counts, as such sources are approximately within the original AGPS sources X-ray flux range (see Sections 3.2 and 3.3 of the reference paper). Infrared and optical follow-up were primarily performed on those ChIcAGO sources having > 20 X-ray counts. In order to determine which optical and infrared sources are counterparts to ChIcAGO sources, the authors used a technique similar to that described by Zhao et al. (2005, ApJS, 161, 429), using their Equation (11). If the separation between a ChIcAGO source's wavdetect position and its possible counterpart is less than the quadratic sum of their 3-sigma positional errors and the 3-sigma Chandra pointing error, then the X-ray and optical (or infrared) sources are likely to be associated. The 1-sigma positional errors for all sources in the 2MASS PSC and GLIMPSE catalogs are 0.1 arcseconds and 0.3 arcseconds, respectively. USNO B has an astrometric accuracy of < 0.25 arcseconds. The authors have assumed that the error distributions of the Chandra observations, Chandra pointing, and USNO B Catalog are all Gaussian for the purposes of identifying possible counterparts to the ChIcAGO sources. This table was created by the HEASARC in June 2014 based on electronic versions of Tables 1, 2 and 12 from the reference paper which were obtained from the ApJS website. This is a service provided by NASA HEASARC .
- ID:
- ivo://nasa.heasarc/cr261cxo
- Title:
- Collinder 261 Chandra X-Ray Point Source Catalog
- Short Name:
- CR261CXO
- Date:
- 02 May 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- This table contains some of the results from the first X-ray study of Collinder 261 (Cr 261), which at an age of 7 Gyr is one of the oldest open clusters known in the Galaxy. This observation with the Chandra X-Ray Observatory was aimed at uncovering the close interacting binaries in Cr 261, and reached a limiting X-ray luminosity of L<sub>X</sub> ~ 4 x 10<sup>29</sup> erg s<sup>-1</sup> (0.3-7 keV) for stars in the cluster. The authors detected 107 sources within the cluster half-mass radius r<sub>h</sub>, and they estimate that among the sources with L<sub>X</sub> >~ 10<sup>30</sup> erg s<sup>-1</sup>, about 26 are associated with the cluster. They identify a mix of active binaries and candidate active binaries, candidate cataclysmic variables, and stars that have "straggled" from the main locus of CR 261 in the color-magnitude diagram. Based on a deep optical source catalog of the field, the authors estimate that Cr 261 has an approximate mass of 6500 M<sub>sun</sub>, roughly the same as the old open cluster NGC 6791. The X-ray emissivity of Cr 261 is similar to that of other old open clusters, supporting the trend that they are more luminous in X-rays per unit mass than old populations of higher (globular clusters) and lower (the local neighborhood) stellar density. This implies that the dynamical destruction of binaries in the densest environments is not solely responsible for the observed differences in X-ray emissivity. Cr 261 was observed with the Advanced CCD Imaging Spectrometer (ACIS) on board Chandra starting 2009 November 9 14:50 UTC, for a total exposure time of 53.8 ks (ObsID 11308). The observation was made in Very Faint, Timed exposure mode, with a single frame exposure time of 3.2 s. Kharchenko et al. (2013, A&A, 558, A53) estimate that the radius of Cr 261 is ~ 14.1 arcminutes. This is considerably larger than a single ACIS chip (8 4 x 8 4 arcminute<sup>2</sup>) and therefore the authors placed the center of the cluster (J2000.0 RA = 12<sup>h</sup> 38<sup>m</sup> 06.0<sup>s</sup>, Dec = -68<sup>o</sup> 22' 01" according to Kharchenko et al. 2013) close to the I3 aimpoint so that a larger contiguous part of the cluster could be imaged (see Figure 1 in the reference paper). The CCDs used were I0, I1, I2, and I3 from the ACIS-I array, and S2 and S3 from the ACIS-S array. The authors limited the X-ray analysis to the data from chips I0, I1, I2, and I3. The S2 and S3 chips lie far from the I3 aimpoint, giving rise to large positional errors on any sources detected on them. Such large errors make it hard to identify optical counterparts, and thus to classify the sources. Source detection was done in soft (0.3-2.0 keV), hard (2-7 keV) and broad (0.3-7 keV) energy bands. The CIAO source detection routine wavdetect was run for eight wavelet scales ranging from 1.0 to 11.3 pixels. The wavdetect detection threshold (sigthresh) was set at 10<sup>-7</sup>. The corresponding expected number of spurious detections per wavelet scale is 0.42 for all four ACIS chips combined, or 3.35 in total for all wavelet scales. The authors ran wavdetect for the three different energy bands and then cross-correlated the resulting source lists to obtain a master X-ray source list. They detected 113 distinct X-ray sources. To check if any real sources were missed, they ran wavdetect again with a detection threshold of 10<sup>-6</sup>, which increased the expected total number of spurious detections to 33.5, and found a total of 151 distinct X-ray sources with more than two counts (0.3-7 keV) in this case. The positions of 7 of the extra 38 sources were found to match those of short-period binaries discovered by Mazur et al. (1995, MNRAS, 273, 59; see Section 3.4). Close, interacting binaries are plausible real X-ray sources, and indeed the expected number of chance alignments between the Chandra detections and the binaries in the Mazur catalog is very low, as discussed in Section 3.5 of the reference paper. It is therefore likely that at least these seven additional sources are real, but given the ~ 34 spurious detections that are expected, the authors do not believe that there are many more real sources among the extra detections. They flagged the sources that are only found for sigthresh = 10<sup>-6</sup>, but kept them in the master source list. This HEASARC table contains the list of 151 X-ray sources found by wavdetect using a detection threshold of 10<sup>-6</sup> from Table 1 of the reference paper. Information about the 135 optical counterparts to these X-ray sources is available in the HEASARC table CR261OID (based on Table 2 of the reference paper) to which this current table has links. This table was created by the HEASARC in June 2017 based upon the machine-readable version of Table 1 from the reference paper, the catalog of Chandra sources in Cr 261, that was obtained from the ApJ web site. This is a service provided by NASA HEASARC .
- ID:
- ivo://nasa.heasarc/cr261oid
- Title:
- Collinder 261 Chandra X-Ray Source Optical Counterparts Catalog
- Short Name:
- CR261OID
- Date:
- 02 May 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- This table contains some of the results from the first X-ray study of Collinder 261 (Cr 261), which at an age of 7 Gyr is one of the oldest open clusters known in the Galaxy. This observation with the Chandra X-Ray Observatory was aimed at uncovering the close interacting binaries in Cr 261, and reached a limiting X-ray luminosity of L<sub>X</sub> ~ 4 x 10<sup>29</sup> erg s<sup>-1</sup> (0.3-7 keV) for stars in the cluster. The authors detected 107 sources within the cluster half-mass radius r<sub>h</sub>, and they estimate that among the sources with L<sub>X</sub> >~ 10<sup>30</sup> erg s<sup>-1</sup>, about 26 are associated with the cluster. They identify a mix of active binaries and candidate active binaries, candidate cataclysmic variables, and stars that have "straggled" from the main locus of CR 261 in the color-magnitude diagram. Based on a deep optical source catalog of the field, the authors estimate that Cr 261 has an approximate mass of 6500 M<sub>sun</sub>, roughly the same as the old open cluster NGC 6791. The X-ray emissivity of Cr 261 is similar to that of other old open clusters, supporting the trend that they are more luminous in X-rays per unit mass than old populations of higher (globular clusters) and lower (the local neighborhood) stellar density. This implies that the dynamical destruction of binaries in the densest environments is not solely responsible for the observed differences in X-ray emissivity. Cr 261 was observed with the Advanced CCD Imaging Spectrometer (ACIS) on board Chandra starting 2009 November 9 14:50 UTC, for a total exposure time of 53.8 ks (ObsID 11308). The observation was made in Very Faint, Timed exposure mode, with a single frame exposure time of 3.2 s. Kharchenko et al. (2013, A&A, 558, A53) estimate that the radius of Cr 261 is ~ 14.1 arcminutes. This is considerably larger than a single ACIS chip (8 4 x 8 4 arcminute<sup>2</sup>) and therefore the authors placed the center of the cluster (J2000.0 RA = 12<sup>h</sup> 38<sup>m</sup> 06.0<sup>s</sup>, Dec = -68<sup>o</sup> 22' 01" according to Kharchenko et al. 2013) close to the I3 aimpoint so that a larger contiguous part of the cluster could be imaged (see Figure 1 in the reference paper). The CCDs used were I0, I1, I2, and I3 from the ACIS-I array, and S2 and S3 from the ACIS-S array. The authors limited the X-ray analysis to the data from chips I0, I1, I2, and I3. The S2 and S3 chips lie far from the I3 aimpoint, giving rise to large positional errors on any sources detected on them. Such large errors make it hard to identify optical counterparts, and thus to classify the sources. The authors retrieved optical images of Cr 261 in the B and V bands from the ESO public archive. These data were taken as part of the ESO Imaging Survey (EIS; program ID 164.O-0561). The observations of Cr 261 were made using the Wide Field Imager (WFI), mounted on the 2.2 m MPG/ESO telescope at La Silla, Chile. After correcting the X-ray source positions for the (almost negligible) boresight correction (0.06 =/- 0.07 arcseconds in RA and 0.09 +/- 0.08 arcseconds in Dec), the authors matched their X-ray source list with the entire optical source list, using 95% match radii. For 89 unique X-ray sources, they found 124 optical matches; of the latter, 104 are present in both the V and B images, while for 20 there is only a V or B detection. The authors also inspected the area around each X-ray source in the WFI images by eye, and discovered that five more X-ray sources have candidate optical counterparts that are saturated and therefore missing from their optical catalog. Finally, they added to the list of candidate counterparts six optical sources that lay just outside the 95% match radius, but inside the 3-sigma radius. In total, 98 of the 151 unique X-ray sources were thus matched to one or more optical sources. This HEASARC table contains the list of the 135 optical counterparts to 98 of the 151 X-ray sources from Table 2 of the reference paper. Information about the 151 X-ray sources is available in the HEASARC table CR261CXO (based on Table 1 of the reference paper) to which this current table has links. This table was created by the HEASARC in June 2017 primarily based upon the machine-readable version of Table 2 from the reference paper, the catalog of optical counterparts to Chandra sources in Cr 261, that was obtained from the ApJ web site. The information on the X-ray source positions was taken from the machine-readable version of Table 1 from the reference paper that was also obtained from the ApJ web site. This is a service provided by NASA HEASARC .
- ID:
- ivo://nasa.heasarc/cygob2cxo2
- Title:
- Cygnus OB2 Association Chandra X-Ray Point Source Catalog 2
- Short Name:
- CYGOB2CXO2
- Date:
- 02 May 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- This table contains a catalog of 1696 X-ray sources detected in the massive star-forming region (SFR) Cygnus OB2 and extracted from two archival Chandra observations of the center of the region. A deep source extraction routine, exploiting the low background rates of Chandra observations was employed to maximize the number of sources extracted. Observations at other wavelengths were used to identify low count-rate sources and remove likely spurious sources. Monte Carlo simulations were also used to assess the authenticity of these sources. X-ray spectra were fitted with thermal plasma models to characterize the objects and X-ray light curves were analyzed to determine their variability. The authors used a Bayesian technique to identify optical or near-IR counterparts for 1501 (89%) of our sources, using deep observations from the INT Photometric H-alpha Survey, the Two Micron All Sky Survey (2MASS), and the UKIRT Infrared Deep Sky Survey-Galactic Plane Survey. 755 (45%) of these objects have six-band r', H-alpha, i', J, H, and K optical and near-IR photometry. From an analysis of the Poisson false-source probabilities for each source they estimate that their X-ray catalog includes <1% of false sources, and an even lower fraction when only sources with optical or near-IR associations are considered. A Monte Carlo simulation of the Bayesian matching scheme allows this method to be compared to more simplified matching techniques and enables the various sources of error to be quantified. The catalog of 1696 objects presented here includes X-ray broad-band fluxes, spectral model fits, and optical and near-IR photometry in what is one of the largest X-ray catalogs of a single SFR to date. The high number of stellar X-ray sources detected from relatively shallow observations confirms the status and importance of Cygnus OB2 as one of our Galaxy's most massive SFRs. This table was created by the HEASARC in October 2009 based on electronic versions of Tables 2, 3 and 4 from the reference paper which were obtained from the CDS (their catalog J/ApJS/184/84 files table2.dat, table3.dat and table4.dat). This is a service provided by NASA HEASARC .
