Gravitational Microlensing Rates in Milky Way Globular Clusters

TL;DR

This study estimates microlensing event rates in Milky Way globular clusters to detect dark compact objects, highlighting the potential of upcoming surveys like Rubin Observatory for such detections.

Contribution

It provides the first detailed estimates of microlensing rates for various compact objects in GCs using advanced models, emphasizing the role of initial conditions and cluster properties.

Findings

White dwarfs dominate microlensing rates due to their abundance.

Higher initial densities and core-collapse clusters have increased microlensing likelihood.

M22 and 47 Tuc are promising targets for future microlensing surveys.

Abstract

Many recent observational and theoretical studies suggest that globular clusters (GCs) host compact object populations large enough to play dominant roles in their overall dynamical evolution. Yet direct detection, particularly of black holes and neutron stars, remains rare and limited to special cases, such as when these objects reside in close binaries with bright companions. Here we examine the potential of microlensing detections to further constrain these dark populations. Based on state-of-the-art GC models from the CMC Cluster Catalog, we estimate the microlensing event rates for black holes, neutron stars, white dwarfs, and, for comparison, also for M dwarfs in Milky Way GCs, as well as the effects of different initial conditions on these rates. Among compact objects, we find that white dwarfs dominate the microlensing rates, simply because they largely dominate by numbers. We show that microlensing detections are in general more likely in GCs with higher initial densities, especially in clusters that undergo core collapse. We also estimate microlensing rates in the specific cases of M22 and 47 Tuc using our best-fitting models for these GCs. Because their positions on the sky lie near the rich stellar backgrounds of the Galactic bulge and the Small Magellanic Cloud, respectively, these clusters are among the Galactic GCs best-suited for dedicated microlensing surveys. The upcoming 10-year survey with the Rubin Observatory may be ideal for detecting lensing events in GCs.