Microlensing mass measurement from images of rotating gravitational arcs

Gravitational microlensing<SUP>1</SUP> is a powerful technique for measuring the mass of isolated and faint or non-luminous objects in the Milky Way<SUP>2,3</SUP>. 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<SUP>4-7</SUP>, as demonstrated recently by first interferometric observations in microlensing event TCP J05074264+2447555 (`Kojima-1')<SUP>8</SUP>. Here we report real-time observations of gravitationally lensed arcs in rotation around a microlens, Gaia19bld<SUP>9</SUP>, made with the PIONIER instrument<SUP>10</SUP> 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<SUB>⊙</SUB>, 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<SUP>11-13</SUP> in the Galaxy, which cannot be addressed by any other technique.