Searching for intermediate-mass black holes in globular clusters with gravitational microlensing

TL;DR

This paper explores the use of gravitational microlensing to detect intermediate-mass black holes in globular clusters, highlighting promising candidates like M 22 and estimating detection probabilities with current and future data.

Contribution

It demonstrates the potential of astrometric microlensing as a method to identify IMBHs in globular clusters, providing specific detection probability estimates for different black hole masses.

Findings

M 22 has the highest chance of IMBH detection via microlensing.

Detection probability for a $10^5M_\odot$ IMBH in M 22 is 86% over 20 years.

Future observations will increase detection chances with extended baselines.

Abstract

We discuss the potential of the gravitational microlensing method as a unique tool to detect unambiguous signals caused by intermediate-mass black holes in globular clusters. We select clusters near the line of sight to the Galactic Bulge and the Small Magellanic Cloud, estimate the density of background stars for each of them, and carry out simulations in order to estimate the probabilities of detecting the astrometric signatures caused by black hole lensing. We find that for several clusters, the probability of detecting such an event is significant with available archival data from the Hubble Space Telescope. Specifically, we find that M 22 is the cluster with the best chances of yielding an IMBH detection via astrometric microlensing. If M 22 hosts an IMBH of mass $10^5M_\odot$, then the probability that at least one star will yield a detectable signal over an observational baseline of 20 years is $\sim 86\%$, while the probability of a null result is around $14\%$. For an IMBH of mass $10^6M_\odot$, the detection probability rises to $>99\%$. Future observing facilities will also extend the available time baseline, improving the chance of detections for the clusters we consider.