X-ray sources in Galactic globular clusters and old open clusters

Abstract

The features and make up of the population of X-ray sources in Galactic star clusters reflect the properties of the underlying stellar environment. Cluster age, mass, stellar encounter rate, binary frequency, metallicity, and maybe other properties as well, determine to what extent we can expect a contribution to the cluster X-ray emission from low-mass X-ray binaries, millisecond pulsars, cataclysmic variables, and magnetically active binaries. Sensitive X-ray observations with XMM-Newton and certainly Chandra have yielded new insights into the nature of individual sources and the effects of dynamical encounters. They have also provided a new perspective on the collective X-ray properties of clusters, in which the X-ray emissivities of globular clusters and old open clusters can be compared to each other and to those of other environments. I will review our current understanding of cluster X-ray sources, focusing on star clusters older than about 1 Gyr, illustrated with recent results.

Properties of Cataclysmic Variables in Globular Clusters

“…Verbunt & Hut (1987 [105]) showed that bright X-ray sources in GCs seem consistent with being formed through dynamical interactions. There are currently 21 bright X-ray sources in 15 GCs, being eight persistent and 13 transients (van den Berg 2020 [68]), and they are still consistent with being dynamically formed, especially considering their mass density, which is much higher than in the Galactic disc. Pooley et al (2003 [106]) analysed Chandra observations of 12 GCs and found that the number of X-ray sources correlates with Γ.…”

“…Finally, they could also be detected as X-ray sources, which is generated in the boundary layer (in case of DNe) or in the post-shock region (in case of magnetic CVs) (see e.g. van den Berg 2020 [68] and references therein).…”

“…Concerning the results from the fitting scheme, we show in the remaining columns the radius within which the stellar groups have been considered, the number of systems in each stellar group involved in the fitting scheme, the mass ratio with respect to the MS stars close to the turn-off point (MSTO), and the average mass of each stellar group. and references therein, and also van den Berg 2020 [68], for a recent review on X-ray sources in Galactic GCs and old open clusters) and they can be separated between bright X-ray sources (L X 10 36 erg s −1 ) and faint X-ray sources (L X 10 34 erg s −1 ). The population of bright Xray sources are dominated by low-mass X-ray binaries, which are either black holes or neutron stars accreting from MS stars at very high rates (e.g.…”