We present 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 .
Our observation with the Chandra X-ray Observatory is aimed at uncovering the close interacting binaries in Cr 261 , and reaches a limiting X-ray luminosity of L _ { X } \approx 4 \times 10 ^ { 29 } erg s ^ { -1 } ( 0.3–7 keV ) for stars in the cluster .
We detect 107 sources within the cluster half-mass radius r _ { h } , and we estimate that among the sources with L _ { X } \gtrsim 10 ^ { 30 } erg s ^ { -1 } , \sim 26 are associated with the cluster .
We 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 colour-magnitude diagram .
Based on a deep optical source catalogue of the field , we estimate that Cr 261 has an approximate mass of 6500 M _ { \odot } , 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 neighbourhood ) 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 .