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CR261CXO - Collinder 261 Chandra X-Ray Point Source Catalog

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Overview

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 LX ~ 4 x 1029 erg s-1 (0.3-7 keV) for stars in the cluster. The authors detected 107 sources within the cluster half-mass radius rh, and they estimate that among the sources with LX >~ 1030 erg s-1, 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 Msun, 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 arcminute2) and therefore the authors placed the center of the cluster (J2000.0 RA = 12h 38m 06.0s, Dec = -68o 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 10-7. 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 10-6, 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 = 10-6, 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 10-6 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.


Catalog Bibcode

2017ApJ...837..130V

References

A Chandra X-ray census of the interacting binaries in old open clusters -
Collinder 261
     Vats S., van den Berg M.
    <Astrophys. J. 837, 130 (2017)>
    =2017ApJ...837..130V      (SIMBAD/NED BibCode)

Provenance

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.

Parameters

Source_Number
A unique sequential identification number (CX) for each Chandra X-ray source in the catalog, sorted in order of decreasing X-ray counts in the broad energy band (0.3 - 7 keV), i.e., the lower the source number, the more counts.

Source_Flag
This source flag is set to 'd' to indicate that the source was detected by wavdetect using a detection threshold sigthresh of 10-6, but not with a sigthresh value of 10-7. The positions of 7 of these extra 38 sources thus flagged 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 (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 (flagged by 'd') detections.

Name
The name of the X-ray source using the 'CXOU' prefix (for Chandra X-ray Observatory Unregistered source) and the J2000.0 coordinates of the source in the standard "JHHMMSS.s+DDMMSS" format.

RA
The Right Ascension of the X-ray source in the selected equinox as found by wavdetect. This was given in J2000 decimal degree coordinates to a precision of 10-6 degrees in the original table.

Dec
The Declination of the X-ray source in the selected equinox as found by wavdetect. This was given in J2000 decimal degree coordinates to a precision of 10-6 degrees 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 95%-confidence radius of the X-ray source position found by wavdetect, in arcseconds.

Radial_Distance
The angular offset of the X-ray source from the center of the cluster, in arcminutes. The authors used their newly derived value of the cluster center (J2000.0 RA = 12h 38m 07.1s, Dec = -68o 23' 33") for this purpose rather than the value of RA = 12h 38m 06.0s, Dec = -68o 22' 01" according to Kharchenko et al. (2013). The two positions differ by about 1.5 arcminutes.

FB_Counts
The net extracted counts in the X-ray source in the broad (0.3-7) keV band.

FB_Counts_Error
The 1-sigma uncertainty in the net extracted counts in the X-ray source in the broad (0.3-7 keV) band.

SB_Counts_Limit
This flag parameter is set to '<' if the X-ray source was not detected in the soft (0.3-2.0 keV) energy band, in which case the quoted soft band counts correspond to the 1-sigma upper limit. 5 X-ray sources (numbers 109, 111, 131, 141 and 150) were so flagged.

SB_Counts
The net extracted counts in the X-ray source in the soft (0.3-2.0 keV) band.

SB_Counts_Error
The 1-sigma uncertainty in the net extracted counts in the X-ray source in the soft (0.3-2.0 keV) band.

Flux
The unabsorbed broad (0.3-7 keV) band X-ray flux of the Chandra source, assuming a 2 keV MeKaL model and a neutral hydrogen column NH of 1.9 x 1021 cm-2 (the value for Cr 261), in erg s-1 cm-2. The MeKaL model describes the emission from a hot, diffuse gas or optically thin plasma, as is appropriate for active binaries. The X-ray sensitivity limit, as defined by the unabsorbed flux of the faintest detection, was found to be ~ 6 x 10-16 erg s-1 cm-2 for this 2 keV MeKaL model and the assumed cluster NH, which corresponds to an X-ray luminosity of LX ~ 4 x 1029 erg s-1 (0.3-7 keV) at the adopted cluster distance of 2.5 kpc.

Median_Energy
The median energy E50 of the photons in the Chandra source, in keV.

Median_Energy_Error
The 1-sigma uncertainty in the median energy E50 of the photons in the Chandra source, in keV.

Optical_Match
This flag parameter is set to 'Y' or 'N' to indicate the presence or absence of at least one optical counterpart match to the X-ray source, respectively. Information about these optical counterparts is given in the HEASARC table CR261OID (based on Table of the reference paper) to which this current table has links.


Contact Person

Questions regarding the CR261CXO database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 15-Jun-2017