Gravitational microlensing¹ is a powerful technique for measuring the mass of isolated and faint or non-luminous objects in the Milky Way^2,3. In most cases, however, additional observations to the photometric light curve are required to measure accurately the mass of the microlens. Long-baseline optical/infrared interferometry provides a new and efficient way to deliver such independent constraints^4,5,6,7, as demonstrated recently by first interferometric observations in microlensing event TCP J05074264+2447555 (‘Kojima-1’)⁸. Here we report real-time observations of gravitationally lensed arcs in rotation around a microlens, Gaia19bld⁹, made with the PIONIER instrument¹⁰ at the Very Large Telescope Interferometer. Our data allowed us to determine the angular separation and length of the arcs, as well as their rotation rate. Combining these measurements with ground-based photometric data enabled the determination of the microlens mass, M = 1.147 ± 0.029 M_⊙, to a very high accuracy. We anticipate interferometric microlensing to play an important future role in the mass and distance determination of isolated stellar-mass black holes^11,12,13 in the Galaxy, which cannot be addressed by any other technique.

引力微透镜¹是一种强大的技术，用于测量银河系中孤立、微弱或不发光物体的质量^2,3。然而，在大多数情况下，需要对光度曲线进行额外的观察才能准确测量微透镜的质量。正如最近在微透镜事件 TCP J05074264+2447555 ('Kojima-1') ⁸中的第一次干涉观测所证明的那样，长基线光学/红外干涉测量提供了一种新的和有效的方法来提供这种独立的约束^4,5,6,7。在这里，我们报告了使用 PIONIER 仪器^{10制作的围绕微透镜 Gaia19bld 9}旋转的引力透镜弧的实时观察在超大望远镜干涉仪上。我们的数据使我们能够确定弧的角间距和长度，以及它们的旋转速率。将这些测量结果与基于地面的光度数据相结合，可以非常准确地确定微透镜质量_M = 1.147  ± 0.029  M⊙ 。我们预计干涉微透镜将在未来确定银河系中孤立恒星质量黑洞^11、12、13的质量和距离方面发挥重要作用，这是任何其他技术都无法解决的。