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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 .
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