Six binary brown dwarf candidates identified by microlensing

TL;DR

This study targets binary brown-dwarf lenses identified through microlensing by selecting events with short $t_{\rm E}$ and small $\theta_{\rm E}$ that exhibit well-resolved caustics. Through binary-lens single-source modeling of six candidates, the authors obtain precise measurements of $t_{\rm E}$ and $\theta_{\rm E}$ and perform Bayesian mass inference to estimate $M_1$ and $M_2$. They find that four events (KMT-2024-BLG-1005, KMT-2024-BLG-1518, MOA-2024-BLG-181, KMT-2024-BLG-2486) have $p_{BD}(M_1)$ and $p_{BD}(M_2) > 0.5$, implying binary brown dwarfs, while MOA-2023-BLG-331L and KMT-2023-BLG-2019L are more consistent with mixed M dwarf + BD systems. The results expand the census of BD binaries, offer constraints on BD formation scenarios, and highlight the role of high-resolution imaging and future space-based parallax in confirming BD nature and refining masses.

Abstract

In this study, we analyze microlensing events from the 2023 and 2024 observing seasons to identify cases likely caused by binary systems composed of BDs. By applying criteria that the binary-lens events exhibit well-resolved caustics, short time scales ($t_{\rm E} \lesssim 9$ days), and have small angular Einstein radii ($θ_{\rm E} \lesssim 0.17$~mas), we identify 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, $t_{\rm E}$ and $θ_{\rm E}$. We estimate the masses of the binary components through Bayesian analysis, utilizing the constraints from $t_{\rm E}$ and $θ_{\rm E}$. 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 brown-dwarf 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.

Figures (9)

• Figure 1: Light curve of the lensing event MOA-2023-BLG-331. The bottom panel displays the full light curve, while the upper panels provide a zoomed-in view around the anomaly features and the residuals from the model. The curve overlaid on the data points represents the best-fit model. The inset in the bottom panel illustrates the lens system configuration, with the red cusped shape representing the caustic, the two blue dots indicating the positions of the lens components (with the larger dot representing the heavier lens component), and the arrowed line depicting the source's trajectory.
• Figure 2: Lensing light curve of KMT-2023-BLG-2019. The notations are the same as those used in Fig. \ref{['fig:one']}. In the bottom panel, the left inset displays the full view of the caustic, while the right inset provides a close-up of the region where the source enters the caustic.
• Figure 3: Lensing light curve and lens-system configuration of KMT-2024-BLG-1005.
• Figure 4: Lensing light curve and lens-system configuration of KMT-2024-BLG-1518.
• Figure 5: Lensing light curve and lens-system configuration of MOA-2024-BLG-181.