Four binary microlenses with directly measured masses

Aikaterini Vandorou; Akihiko Fukui; Andrew Gould; Andrzej Udalski; Aparna Bhattacharya; Byeong-Gon Park; Cheongho Han; Chung-Uk Lee; Cl\'ement Ranc; Daisuke Suzuki

REVIEW 3 major objections 3 minor 1 references

Four binary microlenses with directly measured masses

T0 review · 3 major / 3 minor · reviewed 2026-08-05 · deepseek-v4-flash

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Pith's one-line read This paper reports direct mass and distance measurements for four binary microlensing events, combining the Einstein radius, microlens parallax, and event timescale.

desk verdict Four new binary-lens mass measurements from standard microlensing; plausible, but the full text is garbled and the parallax-orbital degeneracy needs checking. read the letter →

arxiv 2508.11079 v1 pith:3O2X3PHZ submitted 2025-08-14 astro-ph.IM astro-ph.EPastro-ph.GAastro-ph.SR

Cheongho Han , Andrzej Udalski , Chung-Uk Lee , Ian A. Bond , Michael D. Albrow , Sun-Ju Chung , Andrew Gould , Youn Kil Jung

show 53 more authors

Kyu-Ha Hwang Yoon-Hyun Ryu Yossi Shvartzvald In-Gu Shin Jennifer C. Yee Weicheng Zang Hongjing Yang Sang-Mok Cha Doeon Kim Dong-Jin Kim Seung-Lee Kim Dong-Joo Lee Yongseok Lee Byeong-Gon Park Richard W. Pogge Przemek Mróz Micha{l} K. Szymański Jan Skowron Rados{l}aw Poleski Igor Soszyński Pawe{l} Pietrukowicz Szymon Koz{l}owski Krzysztof A. Rybicki Patryk Iwanek Krzysztof Ulaczyk Marcin Wrona Mariusz Gromadzki Mateusz J. Mróz Micha{l} Jaroszyński Marcin Kiraga Fumio Abe David P. Bennett Aparna Bhattacharya Akihiko Fukui Ryusei Hamada Stela Ishitani Silva Yuki Hirao Naoki Koshimoto Yutaka Matsubara Shota Miyazaki Yasushi Muraki Tutumi Nagai Kansuke Nunota Greg Olmschenk Clément Ranc Nicholas J. Rattenbury Yuki Satoh Takahiro Sumi Daisuke Suzuki Sean K. Terry Paul J. Tristram Aikaterini Vandorou Hibiki Yama

This is my paper

classification astro-ph.IMastro-ph.EPastro-ph.GAastro-ph.SR

keywords gravitationalmicrolensingbinarylensmicrolensparallaxEinsteinradiusstellarmassmeasurementGalacticdiskMdwarfbinariescausticcrossing

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper tries to establish that four binary gravitational-microlensing events from the 2022–2024 survey seasons are enough to measure the mass and distance of the lensing stars directly, with no assumption about where in the Galaxy the lenses lie. The two observational ingredients are resolved caustic crossings, which fix the angular Einstein radius $\theta_{\rm E}$, and long event durations, which make the microlens parallax $\pi_{\rm E}$ measurable from the light curve. With $\theta_{\rm E}$ and $\pi_{\rm E}$ in hand, the usual microlensing relations give the total lens mass and distance uniquely, and the binary mass ratio from the light-curve model splits the mass between the two components. Three of the systems turn out to be nearby M-dwarf binaries in the Galactic disk at distances $\lesssim 2.5$ kpc; the fourth is a Sun-like primary with a companion about half its mass at roughly 4.5 kpc.

What carries the argument

The load-bearing combination is $(\theta_{\rm E},\pi_{\rm E})$ together with $t_{\rm E}$. The resolved caustic spikes supply the normalized source radius $\rho$, and the angular source radius from color–magnitude calibrated photometry converts it to $\theta_{\rm E}=\theta_*/\rho$; the long duration enables a parallax measurement $\pi_{\rm E}$ from higher-order light-curve modeling that includes Earth's orbital motion and the binary's own orbital motion. These enter the standard relations $M=\theta_{\rm E}/(\kappa\pi_{\rm E})$ and $D_{\rm L}={\rm AU}/(\pi_{\rm E}\theta_{\rm E}+\pi_{\rm S})$, turning purely photometric observables into physical masses and distances.

What would settle it

Choose one of the four events and obtain deep high-resolution imaging (adaptive optics or space-based) a few years after the event, when the lens and source have separated. If the lens is detected and its photometric mass disagrees with the reported mass by more than the quoted uncertainties, the direct-mass interpretation for that event is falsified.

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Extended reading notes

Core claim

For the four events KMT-2022-BLG-1479, KMT-2023-BLG-0932, OGLE-2024-BLG-0142, and KMT-2024-BLG-1309, the paper claims that the combination of the measured event timescale $t_{\rm E}$, angular Einstein radius $\theta_{\rm E}$, and microlens parallax $\pi_{\rm E}$ uniquely determines the lens mass and distance. $\theta_{\rm E}$ is derived from the normalized source radius $\rho$ obtained by modeling the resolved caustic spikes, multiplied by the angular source radius $\theta_*$ from color–magnitude calibration. $\pi_{\rm E}$ comes from higher-order light-curve modeling that accounts for Earth's orbital motion and the binary lens's orbital motion. The mass follows from $M=\theta_{\rm E}/(\kappa

Load-bearing premise

The whole derivation rests on the light-curve model cleanly separating the microlens-parallax signal from the binary orbital motion; if the two can mimic each other, the reported masses and distances could be biased.

Editorial extensions

If this is right

If the measurements hold, these four systems enlarge the small sample of binary lenses with masses determined without a Galactic model, providing anchors for testing stellar mass–luminosity relations.
Three nearby low-mass binaries reinforce the expectation that M dwarfs dominate the microlensing lens population in the disk.
The one Sun-like primary system at about 4.5 kpc shows the method works across a range of lens masses and distances, not only for the most common low-mass lenses.
The selection recipe—resolved caustic spikes plus long duration—can be applied to ongoing survey data to identify many more direct-mass binary-lens candidates.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

An implication the authors do not state: applying the same selection recipe to the full 2022–2024 surveys could raise the number of direct-mass binary lenses by an order of magnitude, making these four the seed of a statistical sample.
A testable extension: compare the reported distances with future high-resolution imaging of the resolved lens; a flux-based mass that disagrees with the parallax-based value would reveal where the orbital-motion/parallax separation breaks down.
A prediction for close binaries: events whose binary orbital period is comparable to the event duration should show larger scatter between model solutions, because binary motion can partially mimic parallax; this can be checked in the next survey seasons.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Colleague,

The punchline: this is a run-of-the-mill but useful microlensing paper. It derives masses and distances for four binary lenses using the established combo of Einstein radius and microlens parallax. The contribution is four new data points, not new methodology.

What's new: they picked events with resolved caustic crossings (good for θ_E) and long timescales (good for π_E). The abstract reports three M-dwarf binaries within ~2.5 kpc and one solar-mass primary at ~4.5 kpc. That's consistent with expectations for disk lenses and adds to a small but growing sample.

The paper's main strength is the clean selection criteria and standard application of known techniques. The team is experienced, and the abstract doesn't oversell. There's no sign of circularity—they measure θ_E and π_E independently and then derive masses.

Soft spots: the full text is unreadable in the version I have—mojibake from an encoding failure. So I can only judge the abstract. For binary microlenses, the separation of parallax from orbital motion is the classic degeneracy, and the abstract doesn't say whether they tested multiple models or applied physical constraints like kinetic-to-potential energy. That's the load-bearing assumption. If they didn't check it properly, the masses could shift. The abstract's wording ("considering higher-order effects") is reassuring but not conclusive. Also, the garbled text contains a string that reads 'arXiv:2508.11080v1 [eess.SY]'—likely a scrap from another document, but worth confirming it doesn't indicate a mixing of sources.

Who this is for: people in the microlensing game, especially those building the census of lens masses. It's not a paradigm-shifter, but it's legitimate data.

I'd send it to review—the methods are solid enough to deserve a referee's time, and the field needs more measured masses. But the referee should focus on the parallax-orbital modeling and ask for the degeneracy checks. My own verdict is conditional until I see the full text; if the checks are there, it's publishable as is.

Recommendation: send to peer review, but with a referee who knows the orbital-parallax literature.

Referee Report

3 major / 3 minor

Summary. The paper reports four binary microlensing events (KMT-2022-BLG-1479, KMT-2023-BLG-0932, OGLE-2024-BLG-0142, KMT-2024-BLG-1309) selected from 2022-2024 survey data because they show resolved caustic spikes and long durations. For each event the authors measure the angular Einstein radius theta_E from the normalized source radius and an estimated source angular radius, and measure the microlens parallax pi_E from higher-order light-curve modeling including Earth and binary-lens orbital motion. Combining t_E, theta_E, and pi_E then yields unique lens masses and distances: three systems are nearby (less than about 2.5 kpc) M-dwarf binaries, and one system lies at about 4.5 kpc with a nearly solar-mass primary and a half-solar-mass companion. The full text supplied to me is heavily garbled/corrupted, so only the abstract and scattered fragments could be read; detailed verification of the modeling was not possible.

Significance. If the mass and distance determinations are correct, this paper adds four rare directly-measured binary-lens masses to the microlensing sample, which is valuable for testing Galactic models and the stellar mass function. The event selection criteria - resolved caustic crossings for theta_E and long durations for pi_E - are well motivated and standard, and the mass/distance formalism is established in the field. The paper's potential strengths include presenting a homogeneous sample of four events and combining data from multiple surveys. However, because the full text is unreadable and the abstract does not document any degeneracy checks, the technical soundness cannot currently be assessed. The paper does not appear to contain machine-checked proofs or reproducible code.

major comments (3)

[Abstract, higher-order effects] The central claim of unique lens masses and distances rests on separating the microlens parallax pi_E from the binary orbital motion. The abstract only states that higher-order effects from Earth and binary-lens orbital motion were 'considered'; it does not report how degeneracies were resolved. For binary events, orbital motion can partially mimic annual parallax, so the authors should report the number of viable solutions, Delta-chi-squared differences, physical boundedness checks (e.g., kinetic-to-potential energy ratio <= 1), and the sensitivity of pi_E to the orbital model. Without these, the derived masses and distances are not uniquely established.
[Methods (full text unreadable)] The provided full text is garbled/corrupted, so I could not inspect the light-curve fits, tables, figures, or error propagation. In particular, the derivation of theta_E from the normalized source radius rho and the source angular radius theta_* is not verifiable. The authors should provide a readable manuscript and include, for each event, the best-fit parameters and uncertainties, the source color-magnitude calibration (photometric bands, reddening assumptions, empirical relation), and likelihood contours for pi_E and the orbital parameters.
[Abstract, theta_* and error budget] The angular Einstein radius is obtained by combining the normalized source radius with theta_* derived from source color and magnitude. The abstract does not state which empirical color/surface-brightness relation was used, nor its systematic uncertainty. In nearby M-dwarf lens events, the theta_* scale often dominates the mass uncertainty. The paper should quote the fractional uncertainty on theta_* and demonstrate that the reported mass/distance errors include this contribution.

minor comments (3)

[Abstract] Grammar: 'we uniquely determined' instead of 'we unique determined'.
[Metadata] The displayed arXiv category (astro-ph.IM) and the page header (eess.SY) disagree, and the text has an encoding problem. Please ensure the actual submission is readable.
[Abstract/selection] Clarify whether the four events are a complete sample of binaries satisfying the stated criteria from the 2022-2024 surveys, or a selected subset; this affects any population interpretation.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: lens masses are derived from independently measured timescale, Einstein radius, and parallax via standard microlensing relations.

full rationale

The paper's mass–distance derivation is self-contained and follows the standard microlensing chain: θ_E is obtained from the measured normalized source radius ρ* combined with the angular source radius θ_* derived from source color and magnitude; π_E is obtained from light-curve modeling including Earth's orbital motion and binary orbital motion; and the lens mass and distance are then uniquely determined from t_E, θ_E, and π_E. None of these inputs is defined in terms of the target lens masses or distances, and the abstract does not present a fitted quantity as a prediction. The skeptical concern about parallax–orbital-motion degeneracy is a legitimate modeling risk, but it is not circularity: it concerns whether π_E is accurately measured, not whether the measurement already encodes the result. No load-bearing self-citation, no ansatz smuggled in via citation, and no renaming of a known result were identifiable. The supplied full text is mostly encoding-corrupted, but the readable abstract exhibits no derivation step that reduces to its own inputs. Accordingly, the circularity score is 0.

Assumptions & free parameters 5 free parameters · 4 assumptions · 0 invented entities

The central claim rests on standard microlensing modeling with several fitted parameters (parallax, normalized source radius, binary parameters, timescale) and empirical calibration of source radius. No new physical entities are introduced. The main unstated assumption is the ability to separate parallax from binary orbital motion, which is the weakest link in the derivation.

free parameters (5)

Microlens parallax π_E = Not reported in abstract
Fitted from light curve modeling, as stated in the abstract: 'we determined the microlens parallax through light curve modeling.' This is a key input for mass and distance determination.
Normalized source radius ρ = Not reported in abstract
Measured from the resolved caustic spikes, used to estimate the angular Einstein radius θ_E by combining with source angular radius θ_*.
Binary separation s and mass ratio q = Not reported in abstract
The binary nature of the lens requires fitting s and q in the light curve model, which affect the caustic structure and the derived Einstein radius.
Event timescale t_E = Not reported in abstract
The Einstein timescale is a fitted quantity in microlensing models and is used together with θ_E and π_E to compute mass and distance.
Source angular radius θ_* = Derived from source color and magnitude
Derived using empirical color-magnitude relations. Although not a free parameter in the light curve fit, its calibration is an empirical input that directly affects θ_E and therefore the final masses.

assumptions (4)

standard math Binary lens equation and caustic structure
The analysis assumes the standard point-source/point-lens binary lens model for interpreting the light curves. This is foundational to microlensing and is accepted in the field.
domain assumption The source star is a single background star with known angular radius derived from color and magnitude
The method uses θ_* from empirical color-magnitude relations, which assumes the source is not blended and the calibration is valid. This is stated in the abstract as combining the normalized source radius with the angular source radius.
domain assumption Microlens parallax and binary orbital motion are separable in the light curve model
The abstract states higher-order effects from Earth's orbital motion and the binary orbital motion are considered. The reliability of the mass measurement depends on breaking the degeneracy between these two effects, a known challenge in microlensing.
domain assumption The lens is a binary system with no other significant masses along the line of sight
The analysis assumes the lens is an isolated binary. If there are additional masses, the parallax measurement and mass inference would be affected. This is a standard assumption in microlensing mass measurements.

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Cite this review

Pith. "Pith review of Four binary microlenses with directly measured masses." pith.science (2026). https://pith.science/paper/3O2X3PHZ

@misc{pith2026250811079,
  author       = {Pith},
  title        = {Pith review of: Four binary microlenses with directly measured masses},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/3O2X3PHZ}},
  note         = {Machine review of arXiv:2508.11079}
}

abstract

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 ($\theta_{\rm E}$), and long durations enabling microlens-parallax ($\pi_{\rm E}$) measurements. Four events met these criteria: KMT-2022-BLG-1479, KMT-2023-BLG-0932, OGLE-2024-BLG-0142, and KMT-2024-BLG-1309. We estimated the angular Einstein radius by combining the normalized source radius measured from 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, considering higher-order effects caused by the orbital motions of Earth and the binary lens. 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 $\lesssim 2.5$ 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.

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