The X-ray emissivity of low-density stellar populations

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 10 4 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.

Properties of Cataclysmic Variables in Globular Clusters

“…In this way, Cheng et al (2018a [104]) have shown that dynamical interactions are less dominant than previously believed, and that the primordial formation channel has a substantial contribution. That result was also supported more recently by Heinke et al (2020 [94]), though these authors also discuss some caveats in the work by Cheng et al (2018a [104]) which could have influenced their conclusions. Another interesting and striking result by Cheng et al (2018a [104]) is that, unlike what has been previously thought, the faint X-ray populations are under-abundant in GCs with respect to the Solar neighbourhood and Local Group dwarf elliptical galaxies.…”

“…A large fraction of 'exotic' sources in GCs are X-ray emitters (see Heinke et al 2020 [94] PoS(GOLDEN2019)013…”

“…We would like to mention that, apart from these sub-populations comprising faint X-ray sources, in many cases there is contamination from foreground/background objects (e.g. Heinke et al 2020 [94], and references…”

“…On the other hand, Cheng et al (2018a [104]) used a simple method for studying the GC X-ray population, which is based on simply extracting all X-ray flux within the cluster half-mass radius. In comparison with identifying individual X-ray sources, this is a much simpler method (see Heinke et al 2020 [94], for a discussion regarding caveats in the approach by Cheng et al 2018a [104]). One problem, for example, is that when used on distant clusters or clusters with very few X-ray sources, the approach by Cheng et al (2018a [104]) will be much less precise than for closer clusters.…”

A Deep Search for Faint Chandra X-ray Sources, Radio Sources, and Optical Counterparts in NGC 6752

“…However, lower-density clusters are theoretically expected to have more primordially-formed CVs than dynamically formed CVs (Davies 1997;Belloni et al 2019), and indeed there is strong observational support for this (Kong et al 2006;Haggard et al 2009;Cheng et al 2018;Belloni et al 2019). The strength of the correlation between stellar encounter rate and X-ray sources (which varies depending on the sample) has been recently discussed by Cheng et al (2018) and Heinke et al (2020).…”