Path to Black Hole Astrometric Microlensing from 10-Year KMTNet Database

Abstract

I present a Fisher analysis of the astrometric signature of black holes (BHs) that may lie in the 10-year KMTNet microlensing database, motivated by the recent success of the pilot study by Segev et al. (2026), which reduced the scatter at $I=18$ to $σ\sim 10\,$mas. Seeing-limited ground-based observations still suffer from a great deal of systematics, but under the assumption that these systematics can be controlled, I find that with this nominal precision, the Einstein radius can be measured with errors $σ(θ_{\rm E})\sim 0.5\,$mas in the most intensively monitored fields ($\sim 12\,$deg$^2$), and hence to twice that value in an additional $\sim 28\,$deg$^2$ area. This precision hardly depends on whether the peak of the event is during the observing season or not, as long as the impact parameter is within the broad range that is accessible to photometric recognition of the event, i.e., $u_0\lesssim 1.5$. However, longer events do have somewhat better precision. The analysis presented here ignores blending, but this is a good first approximation for most sources $I\lesssim 18$, which lie near or on the giant branch. I outline a practical program, combining photometric and astrometric searches, aimed at identifying isolated BHs in the KMTNet database and measuring their masses and distances.