The X-ray Emissivity of Low-Density Stellar Populations
  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 $10^4$
M$_{\odot}$/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.
