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Description
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 (L<sub>X</sub>) of ~1 x 10<sup>30</sup> 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 19<sup>h</sup> 20<sup>m</sup> 53<sup>s</sup>, +37<sup>o</sup> 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 10<sup>30</sup> 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 10<sup>30</sup> 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<sup>-6</sup>, 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 r<sub>h</sub>. To investigate the validity of the sources, the authors also ran wavdetect with a threshold of 10<sup>-7</sup> 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 .
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