NGC 6791 Chandra X-Ray Point Source Catalog

NGC 6791 Chandra X-Ray Point Source Catalog NGC6791CXO ivo://nasa.heasarc/ngc6791cxo NASA/GSFC HEASARC van den Berg et al. 2026-10-09 NASA/GSFC HEASARC help desk heasarc-vo at athena.gsfc.nasa.gov Survey Source This table contains some of the results from the first X-ray study of NGC 6791, one of the oldest open clusters known (8 Gyr). This Chandra observation was aimed at uncovering the population of close interacting binaries down to an X-ray luminosity (LX) of ~1 x 1030 erg/s (0.3-7 keV). The authors detect 86 sources within 8 arcminutes of the cluster center, including 59 inside the half-mass radius of 4.42 arcminutes. centered on 19h 20m 53s, +37o 46' 18" (J2000.0). They identify 20 sources with proper-motion cluster members, which are a mix of cataclysmic variables (CVs), active binaries (ABs), and binaries containing sub-subgiants. With follow-up optical spectroscopy, the authors confirm the nature of one CV. They also discover one new, X-ray variable candidate CV with Balmer and He II emission lines in its optical spectrum; this is the first X-ray-selected CV in an open cluster. The number of CVs per unit mass is consistent with the field, suggesting that the 3-4 CVs observed in NGC 6791 are primordial. The authors compare the X-ray properties of NGC 6791 with those of a few old open clusters (NGC 6819, M67) and globular clusters (47 Tuc, NGC 6397). It is puzzling that the number of ABs brighter than 1 x 1030 erg/s normalized by cluster mass is lower in NGC 6791 than in M 67 by a factor ~3-7. CVs, ABs, and sub-subgiants brighter than 1 x 1030 erg/s are under-represented per unit mass in the globular clusters compared to the oldest open clusters, and this accounts for the lower total X-ray luminosity per unit mass of the former. This indicates that the net effect of dynamical encounters may be the destruction of even some of the hardest (i.e., X-ray-emitting) binaries. The authors observed NGC 6791 with the Advanced CCD Imaging Spectrometer (ACIS) on Chandra from 2004 July 1 20:51 UTC until July 2 10:49 UTC for a total exposure time of 48.2ks (ObsID 4510). They obtained low-resolution spectra of candidate optical counterparts to guide the classification of the X-ray sources. A total of 16 candidate counterparts brighter than V ~18.3 were observed with the FAST long-slit spectrograph on the 1.5m Tillinghast telescope on Mt. Hopkins on nine nights between 2005 June 7 to September 2 (coverage from 3480 to 7400 Angstrom and a 3 Angstrom resolution). Candidate optical counterparts fainter than V ~17 were observed with the fiber-fed multi-object spectrograph Hectospec on the 6.5m Multi-Mirror Telescope. A total of 16 candidate counterparts were observed on the nights of 2005 May 13 and July 4-6 (spectra that cover 3700 to 9150 Angstrom with a 6-Angstrom resolution). The authors performed source detections in broad (0.3-7.0 keV), soft (0.3-2.0 keV) and hard (2.0-7.0 keV) energy bands, also used in their Chandra study of M 67 (van den Berg et al. 2004, A&A, 418. 509), so as to facilitate comparison. The CIAO detection routine wavdetect was run for scales of 1.0 to 11.3 pixels, in steps increasing by a factor of sqrt(2), with the larger scales appropriate for large off-axis angles where the point-spread function (PSF) becomes significantly broader. The authors computed exposure maps for the response at 1 keV to account for spatial variations of the sensitivity. The wavdetect detection threshold was set to 10-6, from which the authors expect two spurious detections per detection scale (so 16 spurious detections in total) in the area that they consider here. Combination of the broad, soft, and hard-band source lists results in a master catalog of 86 distinct sources within 8 arcmin of the cluster center, of which 59 lie inside the half-mass radius rh. To investigate the validity of the sources, the authors also ran wavdetect with a threshold of 10-7 or an expected number of spurious sources of 1.6. The 14 sources not detected in this run are marked with a value of the source_flag parameter of 'T' in this table (replacing the '*' symbol used in the original table). This table was created by the HEASARC in August 2015 based on the <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/ApJ/770/98">CDS Catalog J/ApJ/770/98</a> files table1.dat and table2.dat. This is a service provided by NASA HEASARC . 2013ApJ...770...98V https://heasarc.gsfc.nasa.gov/W3Browse/all/ngc6791cxo.html Catalog Research served-by HEASARC TAP https://heasarc.gsfc.nasa.gov/xamin/vo/cone?showoffsets&table=ngc6791cxo& GET text/xml 180 99999 true 290.250436 37.714791 0.0166666666666667 https://heasarc.gsfc.nasa.gov/xamin/vo/tap https://heasarc.gsfc.nasa.gov/cgi-bin/W3Browse/getvotable.pl?name=ngc6791cxo GET text/xml https://heasarc.gsfc.nasa.gov/cgi-bin/W3Browse/w3query.pl?tablehead=name=heasarc_ngc6791cxo&Action=More+Options&Action=Parameter+Search&ConeAdd=1 CHANDRA 0.0166666666666667 X-ray public ngc6791cxo NGC 6791 Chandra X-Ray Point Source Catalog source_number Sequential Identification Number for Each Source in the Catalog meta.id;src short nullable name Source Designation (CXOU JHHMMSS.s+DDMMSS) meta.id;meta.main char nullable radial_distance Angular Offset from Cluster Center arcmin pos.angDistance double nullable source_flag Flag [T] Indicates Limit of Detection meta.code;src char nullable ra Right Ascension degree pos.eq.ra;meta.main double nullable dec Declination degree pos.eq.dec;meta.main double nullable error_radius The 95% Positional Uncertainty Radius arcsec stat.error;pos double nullable counts Net 0.3-7 keV Band Counts ct phot;em.X-ray short nullable counts_error Uncertainty in Net 0.3-7 keV Band Counts ct stat.error;phot;em.X-ray short nullable flux Unabsorbed 0.3-7keV Flux erg/s/cm^2 phot.flux;em.X-ray double nullable lx Unabsorbed X-Ray (0.3-7keV) Luminosity erg/s phys.luminosity;em.X-ray double nullable vmag V-Band Magnitude mag phot.mag;em.opt.V double nullable bv_color B-V Color Index mag phot.color;em.opt.B;em.opt.V 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 inside_hmr_flag Flag [YN] Indicates Source Is Located Inside the Half-Mass Radius meta.code char nullable flux_flag Flag [bc] Indicates Unreliable Flux meta.code;phot.flux;em.X-ray char nullable stetson_source_number Source Number of Candidate Optical Counterpart in the S03 Catalog meta.id.cross int nullable opt_var_name Designation of the Associated Optical Variable meta.id.cross char nullable pm_membership_prob Probability for Cluster Membership Based on Proper-Motion Studies percent stat.probability short nullable log_fx_over_fv_limit Limit Flag for Logarithm of FX/FV meta.code;phot.flux;arith.ratio char nullable log_fx_over_fv_flag Flag [:] Indicates Uncertain Logarithm of FX/FV meta.code;phot.flux;arith.ratio char nullable var_period Period of Variability d time.period double nullable cxo_opt_offset Angular Separation Between the X-Ray and Optical Source arcsec pos.angDistance;em.X-ray;em.opt double nullable hst_membership_prob Code for Probable Cluster Membership Based on the HST/ACS Study: meta.code.member char nullable second_ctrpart_flag Flag [a] Indicates This Source Has Another Candidate Counterpart meta.code;src char nullable log_fx_over_fv_error Uncertainty for Logarithm of FX/FV stat.error;phot.flux;arith.ratio double nullable membership_status Group Membership Code: pm=Proper-Motion Cluster Member, ?=Uncertain or No Info, meta.code.member char nullable bii Galactic Latitude degree pos.galactic.lat double nullable vi_color V-I Color Index mag phot.color;em.opt.V;em.opt.I double nullable var_period_flag Flag [s] Indicates Another Period Value Is 23.947 meta.code;time.period char nullable vmag_flag Flag [:b] Indicates Uncertain or Previously Unknown meta.code;phot.mag;em.opt.V char nullable opt_spectrum_notes Spectroscopic Properties of the Candidate Optical Counterpart meta.note char nullable class Browse Object Classification src.class short nullable lii Galactic Longitude degree pos.galactic.lon double nullable source_type Source Classification src.class char nullable median_energy_error Uncertainty in Median Energy keV stat.error;phys.energy double nullable median_energy Median 0.3-7keV Band Energy keV phys.energy double nullable ctrpart_flag Flag [YN] Indicates Optical Counterpart Detected meta.code;src;em.opt char nullable log_fx_over_fv Logarithm of the Ratio of X-Ray (0.3-7keV) to V-Band Fluxes phot.flux;arith.ratio double nullable comments Comments meta.note char nullable bv_color_flag Flag [:] Indicates Uncertain B-V Color Index meta.code;phot.color;em.opt.B;em.opt.V char nullable vi_color_flag Flag [:] Indicates Uncertain V-I Color Index meta.code;phot.color;em.opt.V;em.opt.I char nullable