17081

We propose Hubble imaging of the black hole candidate microlens system MOA-2019-BLG-284 in order to measure the astrometric microlensing signal and determine the lens system mass. Microlensing is the only method that can detect black holes that are isolated or in stable comma non-accreting binary systems comma so routine detection of black hole astrometric microlensing will provide a significant advance in our understanding of the formation of stellar mass black holes. Our proposed observations will determine if the MOA-2019-BLG-284 lens system is a rare stable binary system with a black hole primary or an isolated stellar mass black hole. This observing program is designed to address possible systematic errors in the astrometric measurements. We will also use the Hubble data to correct systematic errors in the ground-based photometry that determines the microlensing parallax. Since both astrometric microlensing and microlensing parallx measurements are needed to determine black hole masses comma an understanding of systematic errors in both quantaties is needed for reliable mass measurements. The novel systematic error correction methods that we propose may be used to increase the sensitivity of future black hole mass measurements from astrometric and photometric microlensing.