The X-ray emissivity of low-density stellar populations
van den Berg, M.; Heinke, C. O.; Cohn, H. N.; Lugger, P. M.; Ivanova, N.; Sivakoff, G. R.; Rosolowsky, E. W.; Sarazin, C. L.; Kong, A. K. H.; Shaw, A. W.; Ruiter, A. J.; Chomiuk, L.; de Boer, T.; Crothers, S.; Nelson, L.; Ivanov, M. G.; Koch, E. W.; Andrews, R.; Leigh, N. W. C.; Parr, C. J.
Canada, United States, United Kingdom, Taiwan, Chile, Australia
Abstract
The dynamical production of low-mass X-ray binaries and brighter cataclysmic variables (CVs) in dense globular clusters is well-established. We investigate how the X-ray emissivity of fainter X-ray binaries (principally CVs and coronally active binaries) varies between different environments. We compile calculations (largely from the literature) of the X-ray emissivity of old stellar populations, including open and globular clusters and several galaxies. We investigate three literature claims of unusual X-ray sources in low-density stellar populations. We show that a suggested quiescent neutron star in the open cluster NGC 6819 is a foreground M dwarf. We show that the suggested diffuse X-ray emission from an old nova shell in the globular cluster NGC 6366 is actually a background galaxy cluster. And we show that a suggested population of quiescent X-ray binaries in the Sculptor Dwarf Galaxy is mostly (perhaps entirely) background galaxies. We find that above densities of 104 M⊙ pc-3, the X-ray emissivity of globular clusters increases, due to dynamical production of X-ray emitting systems. Below this density, globular clusters have lower X-ray emissivity than the other populations, and we do not see a strong dependence of X-ray emissivity due to density effects. We find significant correlations between X-ray emissivity and binary fraction, metallicity, and density. Sampling these fits via bootstrap techniques gives less significant correlations, but confirms the effect of metallicity on low-density populations, and that of density on the full globular cluster sample.