A {\it Chandra} Survey of Milky Way Globular Clusters I: Emissivity and Abundance of Weak X-ray Sources

TL;DR

This study uses Chandra X-ray data to analyze weak X-ray sources in 69 Milky Way globular clusters, revealing a lower abundance than in the field and suggesting primordial binaries play a significant role in X-ray source formation.

Contribution

It provides the first comprehensive X-ray survey of nearly half of the Milky Way globular clusters, quantifying source abundance and revisiting formation mechanisms.

Findings

Weak X-ray sources are less abundant in GCs than in the field.

X-ray luminosity correlates with cluster mass and encounter rate.

Primordial binaries significantly contribute to X-ray source populations.

Abstract

Based on archival {\it Chandra} data, we have carried out an X-ray survey of 69, or nearly half the known population of, Milky Way globular clusters (GCs), focusing on weak X-ray sources, mainly cataclysmic variables (CVs) and coronally active binaries (ABs). Using the cumulative X-ray luminosity per unit stellar mass (i.e., X-ray emissivity) as a proxy of the source abundance, we demonstrate a paucity (lower by $41\%\pm27\%$ on average) of weak X-ray sources in most GCs relative to the field, which is represented by the Solar neighborhood and Local Group dwarf elliptical galaxies. We also revisit the mutual correlations among the cumulative X-ray luminosity ($L_X$), cluster mass ($M$) and stellar encounter rate ($\Gamma$), finding $L_{X}\propto M^{0.74\pm 0.13}$, $L_{X}\propto\Gamma^{0.67\pm0.07}$ and $\Gamma \propto M^{1.28 \pm 0.17}$. The three quantities can further be expressed as $L_{X} \propto M^{0.64\pm0.12}\ \Gamma^{0.19 \pm0.07}$, which indicates that the dynamical formation of CVs and ABs through stellar encounters in GCs is less dominant than previously suggested, and that the primordial formation channel has a substantial contribution. Taking these aspects together, we suggest that a large fraction of primordial, soft binaries have been disrupted in binary-single or binary-binary stellar interactions before they can otherwise evolve into X-ray-emitting close binaries, whereas the same interactions also have led to the formation of new close binaries. No significant correlations between $L_{X}/L_{K}$ and cluster properties, including dynamical age, metallicity and structural parameters, are found.