X-ray sources in Galactic old open star clusters

TL;DR

This review summarizes the current understanding of X-ray sources in Galactic old open clusters, highlighting the types of sources present, their abundance, and environmental effects on binary populations.

Contribution

It provides a comparative analysis of X-ray source populations in old open and globular clusters, revealing environmental influences on binary star frequencies.

Findings

ABs do not scale with cluster mass

CVs, ABs, and SSGs are under-abundant in globular clusters

Dense environments suppress some binary populations

Abstract

I review the current status of studies of the X-ray sources in Galactic old open clusters. Cataclysmic variables (CVs), magnetically-active binaries (ABs), and sub-subgiants (SSGs) dominate the X-ray emission of old open clusters. Surprisingly, the number of ABs detected inside the half-mass radius with Lx >~ 1e30 erg/s (0.3-7 keV) does not appear to scale with cluster mass. Comparison of the numbers of CVs, ABs, and SSGs per unit mass in old open and globular clusters shows that each of these classes is under-abundant in globulars. This suggests that dense environments suppress the frequency of even some of the hardest binaries.