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REVIEW 2 major objections 5 minor 128 references

KMT-2025-BLG-2093 has θ_E = 13.1 ± 2.8 μas, placing it as only the second isolated microlens inside the Einstein Desert between free-floating planets and brown dwarfs or stars.

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KMT-2025-BLG-2093 has θ_E = 13.1 ± 2.8 μas, the second isolated microlens in the Einstein Desert, with unresolved host status and implications for Roman/Earth 2.0 FFP searches.

T0 review reviewed 2026-07-12 challenge →

load-bearing objection Second Einstein-Desert FSPL event with a clean light curve and honest caveats; the color inference is the only real soft spot and is already quantified. the 2 major comments →

arxiv 2606.27725 v2 pith:Q2HDTSBV submitted 2026-06-26 astro-ph.EP astro-ph.SR

KMT-2025-BLG-2093: Free-Floating Planet Candidate Near the Shore of the Einstein Desert

Yoon-Hyun Ryu , Andrew Gould , Kyu-Ha Hwang , Qiyue Qian , Michael D. Albrow , Sun-Ju Chung , Cheongho Han , Youn Kil Jung
show 11 more authors
Zhixing Li Shude Mao In-Gu Shin Yossi Shvartzvald Hongjing Yang Jennifer C. Yee Weicheng Zang Dong-Jin Kim Chung-Uk Lee Byeong-Gon Park Richard W. Pogge
This is my paper
classification astro-ph.EP astro-ph.SR
keywords gravitational microlensingfree-floating planetsEinstein Desertangular Einstein radiusfinite-source effectsKMTNet
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

The reading

Microlensing surveys have found a clear gap in angular Einstein radius, the Einstein Desert (roughly 9–25 μas), that separates free-floating planet candidates from brown dwarfs and stars. This paper analyzes KMT-2025-BLG-2093, a short finite-source point-lens event whose measured θ_E = 13.1 ± 2.8 μas puts it inside that desert, only the second such isolated object published. Because high extinction blocked a direct color measurement, the source radius (and therefore θ_E) is inferred from the source’s dereddened magnitude and an assumed color for a bulge sub-giant or base-of-giant-branch star. A binary-lens search finds a possible host signature, but the cumulative χ² is dominated by one data set and is judged non-compelling; late-time imaging is required to settle the host question. The event is an outlier in peak magnification, source faintness, and relative proper motion; those traits illustrate the selection biases that will dominate future high-cadence, high-sensitivity surveys of free-floating planets.

Core claim

KMT-2025-BLG-2093 is a well-fit finite-source point-lens event with θ_E = 13.1 ± 2.8 μas, making it only the second published isolated microlens that lies inside the Einstein Desert (9 μas < θ_E < 25 μas) between free-floating planets on one side and brown dwarfs and stars on the other. Its physical nature remains unresolved.

What carries the argument

The angular Einstein radius θ_E = θ_*/ρ, obtained by measuring the normalized source radius ρ from finite-source light-curve effects and estimating the source angular radius θ_* from the source’s dereddened magnitude and an inferred color. This single quantity places the lens inside the Einstein Desert and supplies the proper-motion measurement μ_rel = θ_*/t_*.

Load-bearing premise

The source color, and therefore its angular size, is not measured but is assigned from its absolute magnitude alone, because extinction made the V-band light curve unusable.

What would settle it

Late-time high-resolution imaging that resolves a host star at the predicted separation, or a secure space-based microlens-parallax measurement that yields a mass outside the free-floating-planet range.

Watch this falsifier. Get emailed when new claim-graph text bears on it.

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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

2 major / 5 minor

Summary. The paper presents a finite-source point-lens (FSPL) analysis of the short-timescale microlensing event KMT-2025-BLG-2093. From a clean 1L1S fit (Table 1) the authors measure ρ and, after placing the source on an I/K CMD and inferring its dereddened color from absolute magnitude, obtain θ_E = 13.1 ± 2.8 μas. This places the event inside the Einstein Desert (9–25 μas) defined by Gould et al. (2022), making it only the second published isolated microlens in that interval. A 2L1S host search yields a formally significant Δχ^{2} ≈ 51 solution that is correctly judged non-compelling because the signal is dominated by a single observatory/field combination and is consistent with low-level systematics (Figure 3). The discussion situates the event’s high A_max, faint I_s,0 and low μ_rel as selection effects relevant to future FFP surveys (Roman, Earth 2.0).

Significance. If the θ_E measurement holds, the paper supplies a rare, well-documented object that sits between the free-floating-planet and brown-dwarf populations. The light-curve reduction, FSPL modeling, and careful treatment of the host-search systematics are of high technical quality and will be useful as a template for the many high-A_max, small-θ_* events expected from Roman. The explicit discussion of how faint sources and low proper motions bias the recovery of θ_E measurements is a concrete contribution to survey design.

major comments (2)
  1. Section 4: the entire θ_E result rests on an inferred (V−I)_0 = 0.95 ± 0.15 assigned solely from M_I,s ≈ 2.37. While the authors already adopt a generous color uncertainty and the 1-σ extremes of θ_E remain inside the Desert, the manuscript should quantify how sensitive the Desert classification is to plausible changes in the color–magnitude prior (e.g., a metal-poor sub-giant sequence or a ±0.3 mag shift in distance modulus). A short Monte-Carlo or grid test would make the claim robust rather than merely plausible.
  2. Section 5 / Figure 3: the dismissal of the Δχ^{2} ≈ 51 host signal is persuasive but still qualitative. The paper should report either (i) the Δχ^{2} obtained when KMTC42 is excluded or (ii) a simple residual-injection test that demonstrates how often comparable Δχ^{2} values arise from pure noise. Without that number the reader cannot judge whether the systematics argument fully closes the host possibility.
minor comments (5)
  1. Table 1: the Is uncertainty is listed as ±0.020 while the text (Section 4) uses ±0.20; the table value appears to be a typographical error.
  2. Figure 4 caption and text: the sample is described as “13 published FSPL FFP events (including one that formally lies in the Einstein Desert)”; a short table or footnote listing the 13 events and their θ_E values would make the comparison fully reproducible.
  3. Equation (3): the propagation statement “taking account of the strong correlation between fs and ρ” is correct but would be clearer if the covariance term or the effective scaling used were written explicitly.
  4. Section 2: the alert time is given as UT 04:04, 18 August 2025 and HJD' = 905.9; a single consistent time system (HJD or UT) would avoid minor confusion.
  5. References: Dong et al. (2026) is cited as the first Einstein-Desert mass measurement; ensure the final published citation details are updated before production.

Circularity Check

0 steps flagged

No significant circularity: θ_E is measured from light-curve ρ plus external color–surface-brightness calibration; Einstein Desert is used only for post-hoc classification.

full rationale

The paper’s central result is a standard FSPL fit (Table 1) yielding ρ and Is, followed by an Is,0 measurement from the red-clump offset on a CMD (Section 4) and an external conversion of an inferred (V−I)0 into θ* via Kervella et al. (2004) and Bessell & Brett (1988). θ_E = θ*/ρ is therefore an ordinary observational product, not forced by any definition internal to the paper. The Einstein Desert interval (9–25 μas) is imported from Gould et al. (2022) solely to classify the measured value; it does not enter the light-curve model or the θ* inference. The 2L1S host search (Section 5) is exploratory, produces only a non-compelling Δχ^{2} that the authors themselves attribute to systematics, and is not used to claim a mass or host. Self-citations supply historical context and the desert boundaries but are not load-bearing for the numerical result. No equation reduces to its own input by construction, no fitted parameter is re-labeled a prediction, and no uniqueness theorem is invoked. The derivation is therefore self-contained against external calibrations and ordinary photometric practice.

Axiom & Free-Parameter Ledger

3 free parameters · 4 axioms · 0 invented entities

The central θ_E measurement rests on standard microlensing geometry, published red-clump calibrations, and an ad-hoc color assignment required by missing V-band data. No new physical entities are postulated; free parameters are the usual light-curve fit parameters plus the color prior.

free parameters (3)
  • 1L1S light-curve parameters (t0, u0, tE, ρ, Is) = tE = 1.36 ± 0.18 d, ρ = 0.136 ± 0.021, Is = 21.44 ± 0.20
    Fitted directly to the TLC photometry (Table 1); ρ and Is are strongly correlated and drive the final θ_E uncertainty.
  • source color prior (V−I)0 = 0.95 ± 0.15
    Assigned by hand from the inferred absolute magnitude because no V-band light curve exists; directly sets θ_* and therefore θ_E.
  • 2L1S host parameters (s, q, α) = s ≈ 6.1, q ≈ 275, α ≈ 162°
    Fitted in the host search; the resulting Δχ² is later discounted as likely systematics, so they do not enter the main claim.
axioms (4)
  • standard math Standard single-lens finite-source magnification formula and the relation θ_E = θ_*/ρ
    Used throughout Sections 3–4; taken as given from Gould (1994) and subsequent literature.
  • domain assumption Red-clump dereddened magnitude and color on this line of sight (I_cl,0 = 14.39, (V−I)_cl,0 = 1.06)
    Taken from Nataf et al. (2013) and Bensby et al. (2013) to convert observed Is into Is,0 (Section 4).
  • domain assumption Color–surface-brightness relation of Kervella et al. (2004) after Bessell & Brett (1988) color transformation
    Converts the inferred (V−K)0 into θ_* (Section 4).
  • ad hoc to paper Source lies within ±0.2 mag of the mean bulge distance modulus
    Explicitly adopted to enlarge the MI,s error bar before assigning the color prior (Section 4).

reviewed 2026-07-12 · how reviews work

0 comments
Cite this review

Pith. "Pith review of KMT-2025-BLG-2093: Free-Floating Planet Candidate Near the Shore of the Einstein Desert." pith.science (2026). https://pith.science/paper/Q2HDTSBV

@misc{pith2026260627725,
  author       = {Pith},
  title        = {Pith review of: KMT-2025-BLG-2093: Free-Floating Planet Candidate Near the Shore of the Einstein Desert},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/Q2HDTSBV}},
  note         = {Machine review of arXiv:2606.27725}
}
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read the original abstract

We analyze KMT-2025-BLG-2093, with angular Einstein radius $\theta_{\rm E}=13.1\pm 2.8\,\mu{\rm as}$, which makes it the second isolated microlens that lies in the ``Einstein Desert'' ($9\,\mu{\rm as}<\theta_{\rm E}<25\,\mu{\rm as}$) between free-floating planets (FFPs) on one side and brown dwarfs and stars on the other. We discuss how its characteristics may give clues to future exploration of FFPs, especially in the era of satellite missions that have a major FFP focus, including Earth 2.0 and Roman.

Figures

Figures reproduced from arXiv: 2606.27725 by Andrew Gould, Byeong-Gon Park, Cheongho Han, Chung-Uk Lee, Dong-Jin Kim, Hongjing Yang, In-Gu Shin, Jennifer C. Yee, Kyu-Ha Hwang, Michael D. Albrow, Qiyue Qian, Richard W. Pogge, Shude Mao, Sun-Ju Chung, Weicheng Zang, Yoon-Hyun Ryu, Yossi Shvartzvald, Youn Kil Jung, Zhixing Li.

Figure 1
Figure 1. Figure 1: The parameters of this fit (t0, u0, tE, ρ, Is) are given in [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 1
Figure 1. Figure 1: — Light curve of KMT-2025-BLG-2093, which is well-fit by the FS [PITH_FULL_IMAGE:figures/full_fig_p013_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: — Color-magnitude diagram (CMD) of field stars in a 100 [PITH_FULL_IMAGE:figures/full_fig_p014_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: — ∆χ 2 = χ 2 (1L1S) − χ 2 (2L1S) test of the putative host of FFP candidate KMT￾2025-BLG-2093. The upper panel shows the difference in predicted flux between the two solutions ∆F. Because of severe extinction, this peaks at ∆F = 0.0034, corresponding to I = 18 − 2.5 log ∆F = 24.2, well below the noise level of the data, so we do not display these. The lower panel shows the contributions of the individual d… view at source ↗
Figure 4
Figure 4. Figure 4: — KMT-2025-BLG-2093 (red) is compared to 12 published FSPL [PITH_FULL_IMAGE:figures/full_fig_p016_4.png] view at source ↗

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This paper was first reviewed by grok-4.5 on July 12, 2026.