An Isolated Stellar-mass Black Hole Detected through Astrometric Microlensing*
Authors: Sahu Kailash C. , Anderson Jay , Casertano Stefano , Bond Howard E. , Udalski Andrzej , Dominik Martin , Calamida Annalisa , Bellini Andrea , Brown Thomas M. , Rejkuba Marina , Bajaj Varun , Kains Noé , Ferguson Henry C. , Fryer Chris L. , Yock Philip , Mróz Przemek , Kozłowski Szymon , Pietrukowicz Paweł , Poleski Radek , Skowron Jan , Soszyński Igor , Szymański Michał K. , Ulaczyk Krzysztof , Wyrzykowski Łukasz ,   (OGLE Collaboration), Barry Richard K. , Bennett David P. , Bond Ian A. , Hirao Yuki , Silva Stela Ishitani , Kondo Iona , Koshimoto Naoki , Ranc Clément , Rattenbury Nicholas J. , Sumi Takahiro , Suzuki Daisuke , Tristram Paul J. , Vandorou Aikaterini ,   (MOA Collaboration), Beaulieu Jean-Philippe , Marquette Jean-Baptiste , Cole Andrew , Fouqué Pascal , Hill Kym , Dieters Stefan , Coutures Christian , Dominis-Prester Dijana , Bennett Clara , Bachelet Etienne , Menzies John , Albrow Michael , Pollard Karen ,   (PLANET Collaboration), Gould Andrew , Yee Jennifer C. , Allen William , Almeida Leonardo A. , Christie Grant , Drummond John , Gal-Yam Avishay , Gorbikov Evgeny , Jablonski Francisco , Lee Chung-Uk , Maoz Dan , Manulis Ilan , McCormick Jennie , Natusch Tim , Pogge Richard W. , Shvartzvald Yossi ,   ( μFUN Collaboration), Jørgensen Uffe G. , Alsubai Khalid A. , Andersen Michael I. , Bozza Valerio , Novati Sebastiano Calchi , Burgdorf Martin , Hinse Tobias C. , Hundertmark Markus , Husser Tim-Oliver , Kerins Eamonn , Longa-Peña Penelope , Mancini Luigi , Penny Matthew , Rahvar Sohrab , Ricci Davide , Sajadian Sedighe , Skottfelt Jesper , Snodgrass Colin , Southworth John , Tregloan-Reed Jeremy , Wambsganss Joachim , Wertz Olivier ,   (MiNDSTEp Consortium), Tsapras Yiannis , Street Rachel A. , Bramich D. M. , Horne Keith , Steele Iain A. ,   (RoboNet Collaboration)
Published: July 2022

Abstract
We report the first unambiguous detection and mass measurement of an isolated stellar-mass black hole (BH). We used the Hubble Space Telescope (HST) to carry out precise astrometry of the source star of the long-duration (t
E ≃ 270 days), high-magnification microlensing event MOA-2011-BLG-191/OGLE-2011-BLG-0462 (hereafter designated as MOA-11-191/OGLE-11-462), in the direction of the Galactic bulge. HST imaging, conducted at eight epochs over an interval of 6 yr, reveals a clear relativistic astrometric deflection of the background star’s apparent position. Ground-based photometry of MOA-11-191/OGLE-11-462 shows a parallactic signature of the effect of Earth’s motion on the microlensing light curve. Combining the HST astrometry with the ground-based light curve and the derived parallax, we obtain a lens mass of 7.1 ± 1.3 M
⊙ and a distance of 1.58 ± 0.18 kpc. We show that the lens emits no detectable light, which, along with having a mass higher than is possible for a white dwarf or neutron star, confirms its BH nature. Our analysis also provides an absolute proper motion for the BH. The proper motion is offset from the mean motion of Galactic disk stars at similar distances by an amount corresponding to a transverse space velocity of ∼45 km s−1, suggesting that the BH received a “natal kick” from its supernova explosion. Previous mass determinations for stellar-mass BHs have come from radial velocity measurements of Galactic X-ray binaries and from gravitational radiation emitted by merging BHs in binary systems in external galaxies. Our mass measurement is the first for an isolated stellar-mass BH using any technique.