Six binary brown dwarf candidates identified by microlensing

¹ Department of Physics, Chungbuk National University, Cheongju 28644, Republic of Korea
² Korea Astronomy and Space Science Institute, Daejon 34055, Republic of Korea
³ Institute of Natural and Mathematical Science, Massey University, Auckland 0745, New Zealand
⁴ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
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 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo, Sagamihara, Kanagawa 252-5210, Japan
²³ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
²⁴ Department of Physics, University of Auckland, Private Bag, 92019 Auckland, New Zealand
²⁵ College of Science and Engineering, Kanto Gakuin University, Yokohama, Kanagawa 236-8501, Japan
²⁶ University of Canterbury Mt. John Observatory, P.O. Box 56 Lake Tekapo 8770, New Zealand

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

Received: 21 May 2025
Accepted: 26 October 2025

Abstract

Aims. In single-lens microlensing events, the event timescale (t_E) is typically the only measurable parameter that constrains the lens mass. Since t_E scales with the square root of the lens mass (t_E ∝ M^1/2), a short duration may suggest a low-mass lens, such as a brown dwarf (BD). However, a short t_E can also result from a high relative proper motion between the lens and the source, making it difficult to uniquely identify BD candidates based on timescale alone. In contrast, binary-lens events often allow for the measurement of the angular Einstein radius (θ_E) in addition to t_E. When both t_E and θ_E are small, the likelihood that the lens is of low mass increases significantly. In this study, we analyze microlensing events observed between 2023 and 2024 to identify cases likely caused by binary systems composed of BDs.

Methods. Applying the criteria of well-resolved caustics, short timescales (t_E ≲ 9 days), and small angular Einstein radii (θ_E ≲ 0.17 mas), we identified six candidate binary BD events: MOA-2023-BLG-331, KMT-2023-BLG-2019, KMT-2024-BLG-1005, KMT-2024-BLG-1518, MOA-2024-BLG-181, and KMT-2024-BLG-2486. Analysis of these events leads to models that provide precise estimates for both lensing observables, t_E and θ_E.

Results. We estimated the masses of the binary components through Bayesian analysis, utilizing the constraints from t_E and θ_E. The results show that for the events KMT-2024-BLG-1005, KMT-2024-BLG-1518, MOA-2024-BLG-181, and KMT-2024-BLG-2486, the probability that both binary components lie within the BD mass range exceeds 50%, indicating a high likelihood that the lenses of these events are binary BDs. In contrast, for MOA-2023-BLG-331L and KMT-2023-BLG-2019L, the probabilities that the lower-mass components of the binary lenses lie within the BD mass range exceed 50%, while the probabilities for the heavier components are below 50%, suggesting that these systems are more likely to consist of a low-mass M dwarf and a BD. The BD nature of the binary candidates can ultimately be confirmed by combining the measured lens-source relative proper motions with high-resolution imaging taken at a later time.