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REVIEW 3 major objections 4 minor 1 references

Beyond Sgr A* and M87*: Sub-Microarcsecond Black Hole Shadow Detection via Lunar-based Extremely Long Baseline Interferometry

T0 review · 3 major / 4 minor · reviewed 2026-08-03 · deepseek-v4-flash

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Pith's one-line read A lunar-based radio telescope paired with the Earth-based Event Horizon Telescope at 230 GHz could detect the black hole shadow in six galaxies beyond M87* and Sgr A*, using the first visibility null of a ring model as the signature.

desk verdict Useful geometric criterion (linear u,v coverage from near-coplanar sources enables first-null sampling), but the detectability claims rest on an optimistic single-ring assumption and the arXiv abstract contradicts the body on antenna sizes and photon-ring yields. read the letter →

arxiv 2601.02812 v2 pith:ZEV2ZWMZ submitted 2026-01-06 astro-ph.GA astro-ph.IM

Shan-Shan Zhao , Ru-Sen Lu , Lei Liu , Zhiqiang Shen , Yosuke Mizuno This is my paper · ORCID
classification astro-ph.GAastro-ph.IM
keywords blackholeshadowsverylongbaselineinterferometrylunar-observatorysupermassiveholesvisibilitynullsphotonrings230GHzastronomyspaceVLBI
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 aims to show that a radio telescope on the Moon, observing at 230 GHz together with the Event Horizon Telescope on Earth, could detect the shadow of a supermassive black hole not just in M87 and the Galactic center but in at least six other galaxies. The argument rests on a simple geometric model of the shadow as a uniform bright ring: such a ring produces a characteristic first null in the visibility amplitude, and the paper identifies six candidate galaxies whose predicted null falls within the projected Moon-Earth baseline range and whose secondary peak rises above the array's sensitivity threshold. If correct, this would expand black hole shadow observations from two objects to a small sample spanning a range of masses and environments, sharpening tests of strong-field gravity. The paper also explores photon-ring detection, finding that with additional space telescopes filling the baseline gaps, the n=1 photon ring could be resolved for four sources, but only Sgr A* is bright enough for a 100 m lunar antenna.

What carries the argument

The load-bearing object is the geometric ring model of the black hole shadow, defined as a uniform brightness annulus with outer diameter θ = 2√27 GM/(Dc²) and fractional width 0.4. Its visibility amplitude—an analytic expression involving Bessel functions (Eq. 3)—oscillates with baseline length, with a first null whose baseline position encodes θ and a secondary peak whose amplitude scales with the 230 GHz flux. The paper couples this model to simulated (u,v) coverage for a lunar-based telescope at the antipode (Site S1). Sources whose sky positions lie near the Moon's orbital plane produce nearly linear (u,v) tracks, sweeping projected baselines from sub-Earth-diameter out to the Moon–Eart

What would settle it

A decisive test would be a 230 GHz VLBI observation of M104 with baselines from ~1 to ~300 Gλ. The model predicts a first visibility null near 15.8 Gλ and a secondary peak of ~55 mJy. If the visibility amplitude is observed to be monotonic (no null) or the secondary peak is absent above the array sensitivity, the uniform-ring assumption is falsified for M104; since the same model underlies the detectability of all six candidates, the list would collapse to those with independent justification. More broadly, a detection of the null at a baseline different from the predicted value by more than t

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Extended reading notes

Core claim

The central claim is that a lunar-based VLBI station at the Moon's antipode, combined with the ground-based EHT at 230 GHz, can resolve black hole shadows for six specific supermassive black holes beyond M87* and Sgr A*: M104, NGC 524, PGC 049940, NGC 5077, NGC 5252, and NGC 1052. The shadow signature used is the first visibility null produced by a uniform ring model of diameter θ = 2√27 GM/(Dc²) and width 0.4θ. For these six sources, the simulated projected Moon-Earth baselines span the null location, and the secondary peak of the ring visibility exceeds the sensitivity threshold set by the lunar telescope's aperture (5–100 m depending on the source). The paper further claims that if space

Load-bearing premise

The calculation assumes that the measured 230 GHz flux of each galaxy is emitted by a single uniform ring of diameter 2√27 GM/(Dc²) and width 0.4 times that diameter, with no significant unresolved core, jet, or extended emission adding to the flux; if that assumption fails, the first null depth and secondary peak amplitude drop, and some of the six candidates could fall below the detection threshold.

Editorial extensions

If this is right

  • If the six candidate shadows are detected, the sample of resolved black hole shadows would grow from two to eight, spanning roughly 10^8 to 10^9 M⊙ and including both radio-loud and radio-quiet nuclei.
  • The first-null criterion offers a quantitative, pre-observable ranking of targets that can be used to decide where to build a lunar radio telescope and how large an antenna is needed.
  • For M104—the brightest and most favorable target—a 5 m lunar antenna paired with ALMA would suffice, making it a realistic first experiment rather than a distant concept.
  • The photon-ring analysis indicates that sensitivity, not resolution, is the main bottleneck for testing strong-field gravity, pointing future designs toward larger apertures, wider bandwidths, and longer coherent integration times.

Reading between the lines

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

  • The six candidates have predicted shadow diameters from ~1 to ~10 µas, so detecting their visibility nulls would provide an independent, model-light estimate of M/D that could be compared with dynamical black hole masses, testing the M–σ relation and distance scale.
  • The null-and-secondary-peak test is generic: any future space VLBI configuration that sweeps a comparable baseline range could reuse these exact criteria, so the paper effectively outlines a template for sub-microarcsecond shadow searches beyond Earth.
  • If the first null is not found where predicted for a bright source like M104, that would itself be an informative result—it would mean the 230 GHz emission is not dominated by a shadow ring, perhaps due to jet or accretion-disk structure, and would call into question the simple ring assumption for all six sources.
  • A further, testable extension would be to apply the same ring model and detectability criteria to the larger catalog of hundreds of potentially resolvable black holes at 1 µas resolution, ranking them by expected null location and secondary-peak flux to guide target selection for lunar VLBI.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

3 major / 4 minor

Summary. The paper proposes a lunar-based 230 GHz VLBI telescope operating jointly with the EHT to detect black hole shadows beyond M87* and Sgr A*. Using a uniform geometric ring model for the source structure, the authors simulate Moon–Earth (u,v) coverage with the public OmniUV toolkit, apply two criteria (first visibility null within the projected baseline range, secondary peak above a 7σ sensitivity threshold), and identify six SMBHs—M104, NGC 524, PGC 049940, NGC 5077, NGC 5252, and NGC 1052—as shadow-detectable candidates with lunar-based antennas of 5–100 m. They also assess photon-ring detectability for 31 candidates. The paper explicitly acknowledges the idealized ring model and the one-dimensional (u,v) sampling but does not quantitatively address how unresolved core/jet emission or two-component morphologies would affect the null-filling and the candidate list.

Significance. If the ring-purity assumption is accepted, the paper provides a concrete, falsifiable target list and a quantitative sensitivity framework for a lunar-Earth interferometer, expanding the sample of shadow-detectable SMBHs from two to eight and specifying required antenna sizes. The use of a public simulation toolkit, explicit parameter tables, and uncertainty estimates for the six candidates are strengths. However, the central claim is conditional on the unverified assumption that essentially all measured 230 GHz compact flux is emitted by the shadow ring; for faint candidates even a small unresolved core would fill the primary null. The paper is therefore best read as a feasibility study, not a robust detection prediction, until the two-component analysis is supplied.

major comments (3)
  1. [§2.2, Eqs. (3)–(4); §5.1; §5.2] The detectability calculation implicitly assumes that the entire measured 230 GHz flux F230 (Table 1) is emitted by a single uniform ring of width 0.4θ. In reality, an unresolved core or jet of flux S adds coherently to the visibility: V(u) = S + V_ring(u). At the first null, V_ring = 0, so the null has amplitude S rather than zero. For NGC 5252 (F230 ≈ 10.5 mJy, secondary peak 3.11 mJy), S above ~1.4 mJy (~13% of total) would push the null above the 100-m telescope 7σ floor; for NGC 524 (F230 ≈ 18.2 mJy) the corresponding fraction is ~8%. The paper itself concedes in §6 the degeneracy with a pair of compact Gaussian components, but no quantitative two-component model is presented. This is load-bearing for the six-candidate list, especially NGC 524 and NGC 5252.
  2. [Abstract vs §5.2, §6] The arXiv abstract states: 'PGC 049940 requires 10 m; NGC 524 requires 20 m; and NGC 5252 requires 40 m.' The body (§5.2 and Fig. 6) gives PGC 049940 a secondary peak of 12.03 mJy, which is above the 20-m threshold (7.14 mJy) but below the 10-m threshold (14.35 mJy), so it requires at least 20 m; NGC 524 has 5.08 mJy, requiring 100 m (threshold 1.40 mJy), not 20 m; NGC 5252 has 3.11 mJy, requiring 100 m. The conclusion also says all six are detectable with a 100-m telescope. This is a direct inconsistency in the primary quantitative claims and must be corrected in the abstract and throughout.
  3. [§5.4 and Abstract] The full-text abstract states that photon-ring detection for Sgr A*, M87*, NGC 1600, and M31 'is possible if space telescopes fill the baseline coverage gaps and sensitivity requirements are met.' §5.4 concludes that after applying the sensitivity criterion, only Sgr A* remains detectable with a 100-m telescope; M87*, NGC 1600, and M31 satisfy the resolution criterion but are not bright enough. The arXiv abstract's claims about the n=2 photon ring for Sgr A* and M87* with a 10-m telescope and 12 candidates for the n=1 photon ring are not supported by the body. These numbers should be reconciled or removed.
minor comments (4)
  1. [§2.2, Eq. (4)] Equation (4) appears to have a typo: a2 is printed as 'd w / 2' but should be (d + w)/2, based on the standard ring visibility formula and the definition of a1 as (d − w)/2.
  2. [Abstract] The first abstract gives a resolution of '~0.7 μas', while the full-text abstract and Eq. (2) give ~0.85 μas. Please align the numbers.
  3. [Table 1] The row for M87* is garbled: '1.265±7 16.8±0.8 42±3<204, 5, 6' does not clearly separate flux, mass, distance, θ, and w. The same applies to some other rows where the reference numbers run into the data columns.
  4. [§3.1] In the text describing site S3, 'NGC 3899' is mentioned, but the candidate list contains NGC 3998; likely a typographical error.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the detectability ranking follows from external mass/distance/flux inputs and a stated geometric visibility model, not from fitted outputs.

full rationale

The derivation chain is self-contained and non-circular. θ is computed from published MBH and DBH via Eq. (1); the visibility null position and secondary-peak amplitude are then obtained from the standard ring visibility formula, Eq. (3) (Kamruddin & Dexter 2013), with V0 set to the measured F230. No parameter is fitted to the visibility predictions: the ring width is an explicitly stated 40% of θ, quoted from EHT measurements of Sgr A*/M87*, and the photon-ring flux fraction (20%) and width (5%) are likewise stated assumptions from external estimates. The six candidates are selected by applying two declared operational criteria (first null inside the simulated Moon–Earth baseline range; secondary peak above the tabulated 7σ sensitivity), i.e., a decision rule, not a fitted result. The (u,v) coverage itself is generated with OmniUV (Liu et al. 2022), a public code; the co-author self-citation is peripheral and checkable, and Yan et al. (2024) supplies only an M104 viewing angle, not a load-bearing conclusion. Section 5.3 propagates alternative masses, distances, and flux values, and Section 6 explicitly concedes the idealized ring model and possible morphological degeneracies; these are assumption/robustness limitations, not circularities. There is no step where a fitted parameter is renamed a prediction or where an input is defined in terms of the output.

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

The central feasibility claim rests on five modeling assumptions: the uniform-ring morphology, the null-as-shadow indicator, the Schwarzschild size formula, the sensitivity parameters, and the photon-ring model. These are reasonable first-order approximations but are not derived from first principles for each target.

free parameters (4)
  • Ring width fraction (w/θ) = 0.4
    Adopted from EHT Sgr A*/M87* approximate ring widths; sets null depth and secondary-peak amplitude in the sensitivity criterion (§2.2, §5.1).
  • Photon ring width fraction (wp/θ) = 0.05
    Assumed thin photon ring (Broderick et al. 2022) used in the §5.4 resolution criterion.
  • Photon ring flux fraction (Fp,0/F230) = 0.2
    Assumed from Johnson et al. (2024) to estimate resolved photon-ring flux in Fig. 9; authors call this an upper limit.
  • Sensitivity model parameters = Tsys=100 K, aperture eff.=0.5, Δν=8 GHz, Δt=10 s, 7σ
    Chosen system parameters and detection threshold that determine whether each source's secondary peak is detectable (Table 3, Eq. 6).
assumptions (5)
  • domain assumption All measured 230 GHz compact flux is contained in the uniform ring used for visibility modeling.
    Secondary peak flux is proportional to this flux; if a jet/core carries part of it, fainter candidates like NGC 5252 may not be detectable (§2.2, §5.1).
  • domain assumption The first visibility null is a sufficient indicator of a black hole shadow.
    Authors adopt null detection as the primary shadow signature; they acknowledge in §6 that two compact Gaussians could mimic a ring along the sparse one-dimensional (u,v) track.
  • domain assumption Schwarzschild shadow diameter θ=2√27 GM/(D c²) gives the ring diameter for all candidates.
    Neglects spin, inclination, and accretion-flow details; EHT experience shows ~10% deviations, which could shift first-null locations (Eq. 1).
  • domain assumption Photonic-ring emission is a thin ring with 5% width and 20% of compact flux.
    Used only in §5.4; the authors explicitly describe these estimates as optimistic upper limits.
  • standard math JPL DE421 ephemerides and IERS 2010 conventions give accurate station trajectories for (u,v) simulation.
    Tooling assumption inside OmniUV (§4); unverified here but standard practice.

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Cite this review

Pith. "Pith review of Beyond Sgr A* and M87*: Sub-Microarcsecond Black Hole Shadow Detection via Lunar-based Extremely Long Baseline Interferometry." pith.science (2026). https://pith.science/paper/ZEV2ZWMZ

@misc{pith2026260102812,
  author       = {Pith},
  title        = {Pith review of: Beyond Sgr A* and M87*: Sub-Microarcsecond Black Hole Shadow Detection via Lunar-based Extremely Long Baseline Interferometry},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/ZEV2ZWMZ}},
  note         = {Machine review of arXiv:2601.02812}
}
read the original abstract

The 1.3 mm ground-based very long baseline interferometry (VLBI) array Event Horizon Telescope (EHT), is limited by Earth's diameter, restricting its black hole shadow imaging to only M87* and Sgr A*. Extending baselines to the Moon would achieve ~0.7 uas angular resolution at 230 GHz, enabling shadow detection for a much larger sample of supermassive black holes (SMBHs). The concept is motivated by space VLBI missions and lunar exploration, including the ongoing Lunar Orbit VLBI EXperiment (LOVEX) aboard QueQiao-2 (Chang'E-7) and the planned International Lunar Research Station (ILRS). We assess shadow detectability for 31 SMBHs with predicted large angular sizes, exploring different telescope location and antenna size. Assuming a telescope at the lunar antipode, we simulate the Moon-Earth (u,v) coverage and show that sources with direction near the Moon's orbital plane yield projected baselines spanning from short to long, enabling sampling of the first visibility null - a key shadow signature. Using a geometric ring model, we identify six shadow-detectable candidates for Moon-Earth VLBI. Among these, M104, NGC 5077, and NGC 1052 are detectable with a 5 m lunar-based telescope; PGC 049940 requires 10 m; NGC 524 requires 20 m; and NGC 5252 requires 40 m. Furthermore, if space telescopes fill the baseline coverage gaps between Moon and Earth, the n=2 photon ring region is detectable for Sgr A*, M87* with a 10 m lunar-based telescope, and 12 candidates are detectable for the n=1 photon ring region using a lunar-based telescope of up to 40 m. These results provide a clear scientific and technical motivation for lunar-based telescopes in future black hole shadow studies.

Figures

Figures reproduced from arXiv: 2601.02812 by the authors.

Figure 1
Figure 1. Ring model images of 29 SMBH candidates ( [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Map of telescope sites on the Moon. MNRAS 000, 1–13 (2024) [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Observable duration for different sites over one lunar sidereal month (27.32 days) The results show a strong latitudinal dependence of the observable duration. Near-equatorial sites (S1, S2) provide an approximately 11-day observable window for most candidates. Site S3 offers longer observable time for northern sky sources (e.g., M81, NGC 3894, NGC 3899, Mrk 501, Cyg A), while Site S4 provides longer dura￾tions for … view at source ↗
Figures from the paper (6 more)
Figure 4
Figure 4. Figure 4: Simulated (u, v) coverage for a Moon-Earth VLBI array, formed by a lunar-based telescope at S1 and the EHT 2025 array. Data points for the Moon-Earth baselines are shown in red, and for the ground-based EHT baselines in blue. The grey circle indicates a baseline of 30 …
Figure 5
Figure 5. Figure 5: Distribution of the minimal projected Moon-Earth baseline on the celestial sphere, with marked positions of SMBH candidates. The sinusoidal pattern reflects the geometric relationship between the source position and the Moon’s orbital path (with a 5.15◦ mean inclinatio…
Figure 6
Figure 6. Figure 6: Left panel: The x-axis represents the first null location, and the y-axis corresponds to the minimal projected Moon-Earth baseline. The dashed grey line denotes the boundary where the two axis are equal. Sources below this line fall within the resolvable region: M104, …
Figure 7
Figure 7. Figure 7: The ring model visibility amplitudes against baseline (black solid lines) for the six best candidates (see Tab. 1 and [PITH_FULL_IMAGE:figures/full_fig_p010_7.png]
Figure 8
Figure 8. Figure 8: Detectability uncertainties for the six best candidates. The x- and y-axes show the first null location and secondary peak flux density of the ring model visibility amplitudes. Colored markers with error bars indicate values and uncertainties derived from different ass…
Figure 9
Figure 9. Figure 9: Photon ring detectability for 31 SMBH candidates. The x-axis shows the resolvability metric wpθbeam, with resolvable region defined by > 1 (right of the red line), where θbeam ∼ 0.85 µas is the Moon–Earth VLBI angular resolution. The y-axis shows the resolved photon ri…

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