Four binary microlenses with directly measured masses

¹ Department of Physics, Chungbuk National University, Cheongju 28644, Republic of Korea
² Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
³ Korea Astronomy and Space Science Institute, Daejon 34055, Republic of Korea
⁴ Institute of Natural and Mathematical Science, Massey University, Auckland 0745, New Zealand
⁵ University of Canterbury, Department of Physics and Astronomy, Private Bag 4800, Christchurch 8020, New Zealand
⁶ Department of Astronomy, Ohio State University, 140 West 18th Ave., Columbus, OH 43210, USA
⁷ University of Science and Technology, Daejeon 34113, Republic of Korea
⁸ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
¹⁰ Department of Astronomy and Tsinghua Centre for Astrophysics, Tsinghua University, Beijing 100084, China
¹¹ School of Space Research, Kyung Hee University, Yongin, Kyeonggi 17104, Republic of Korea
¹² Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
¹³ Villanova University, Department of Astrophysics and Planetary Sciences, 800 Lancaster Ave., Villanova, PA 19085, USA
¹⁴ Institute for Space-Earth Environmental Research, Nagoya University, Nagoya 464-8601, Japan
¹⁵ Code 667, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁶ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
¹⁷ Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
¹⁸ Instituto de Astrofísica de Canarias, Vía Láctea s/n, E-38205 La Laguna, Tenerife, Spain
¹⁹ Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
²⁰ Department of Physics, The Catholic University of America, Washington, DC 20064, USA
²¹ Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
²² Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
²³ Department of Physics, University of Auckland, Private Bag 92019, Auckland, New Zealand
²⁴ University of Canterbury Mt. John Observatory, P.O. Box 56 Lake Tekapo 8770, New Zealand

^⋆ Corresponding author: leecu@kasi.re.kr

Received: 18 April 2025
Accepted: 13 August 2025

Abstract

Aims. We investigated binary-lens events from the 2022–2024 microlensing surveys, aiming to identify events suitable for lens mass measurements. We focused on two key light curve features: distinct caustic spikes with resolved crossings for measuring the angular Einstein radius (θ_E), and long durations enabling microlens-parallax (π_E) measurements. Four events met these criteria: KMT-2022-BLG-1479, KMT-2023-BLG-0932, OGLE-2024-BLG-0142, and KMT-2024-BLG-1309.

Methods. We estimated the angular Einstein radius by combining the normalized source radius measured by modeling the resolved caustic spikes with the angular source radius derived from the source color and magnitude. Additionally, we determined the microlens parallax through light curve modeling, taking higher-order effects caused by the orbital motions of Earth and the binary lens into consideration.

Results. With measurements of the event timescale, angular Einstein radius, and microlens parallax, we uniquely determined the mass and distance of the lens. For the events KMT-2022-BLG-1479, KMT-2023-BLG-0932, and KMT-2024-BLG-1309, both components of the binary lens have masses lower than that of the Sun, consistent with M-type dwarfs, which are the most common type of lenses in Galactic microlensing events. These lenses are relatively nearby, with distances of ≲2.5 kpc, indicating their location within the Galactic disk. In contrast, for OGLE-2024-BLG-0142, the primary lens component has a mass similar to that of the Sun, while the companion lens component has about half the mass of the primary. This lens system is situated at a greater distance, roughly 4.5 kpc.