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REVIEW 2 major objections 5 minor 22 references

Observations of radio sources near the Sun

T0 review · 2 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read A dedicated VLBI session observing radio sources 1.15°–2.6° from the Sun measures the PPN parameter $\gamma$ with a formal uncertainty below $1\times10^{-4}$, about 0.01 percent, matching or beating the formal accuracy of global…

desk verdict Plausible single-session VLBI gamma measurement with formal errors near 1e-4, but the coronal cancellation assumption is the untested load-bearing part. read the letter →

arxiv 1908.00973 v1 pith:VTPPTLKW submitted 2019-08-02 astro-ph.IM

Oleg Titov , Sebastien Lambert , Benedikt Soja , Fengchun Shu , Alexei Melnikov , Jamie McCallum , Lucia McCallum , Matthias Schartner
show 10 more authors
Aletha de Witt Dmitrii Ivanov Andrei Mikhailov Sang Oh Yi Wen Chen Bo Xia Masafumi Ishigaki Sergei Gulyaev Tim Natusch Stuart Weston
This is my paper · ORCID
classification astro-ph.IM
keywords VLBIgeneralrelativityPPNparametergammasolargravitationaldeflectionShapirodelaycoronageodeticradioastrometry
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

Geodetic very long baseline interferometry (VLBI) can test general relativity by timing how much radio waves slow down as they pass the Sun's gravitational field. This paper reports that two specially scheduled 17-hour sessions, tracking reference radio sources at angular distances of 1.15° to 2.6° from the Sun, determine the PPN parameter $\gamma$ — which equals 1 in general relativity — with a single-session formal uncertainty near $1\times10^{-4}$, below 0.01 percent. That precision matches or beats the formal error of global VLBI solutions built from all available observations at arbitrary elongation, and both independent analyses give values consistent with $\gamma=1$. The result matters because it shows that solar-corona plasma does not necessarily destroy close-to-the-Sun astrometry: standard dual-frequency calibration removes the quiet corona's dispersive delay, so dedicated close-approach sessions are a viable route toward challenging the Cassini bound of about $2\times10^{-5}$.

What carries the argument

The load-bearing object is the VLBI group-delay observable with a PPN-delay model: the extra travel time of a radio signal passing near the Sun is proportional to $(1+\gamma)/2$ times the Shapiro delay, so fitting $\gamma$ to delays recorded while the source is close to the limb measures the deflection. The analysis uses dual-frequency S/X-band calibration to remove the dispersive delay of the solar corona and ionosphere, estimates troposphere and clock parameters as nuisances, and restricts attention to compact sources whose structure delay is below about 2 ps. The 1.15°–2.6° geometry makes the gravitational delay large, while the 17-hour session and quiet-corona ray paths keep random coronal scatter small enough for the formal error to reach $\sim 10^{-4}$.

What would settle it

Split the AUA020 session into independent time blocks, or rerun the same two-source schedule when the solar corona is active rather than quiet, and compare the fitted $\gamma$ values; if the estimates differ by substantially more than their formal errors, the cancellation assumption fails and the stated $\sim 10^{-4}$ accuracy is not the real uncertainty.

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Extended reading notes

Core claim

On the paper's own terms, the central claim is that a single geodetic VLBI session can measure gravitational light deflection near the Sun with better formal accuracy than the global solution using all available VLBI data. Tracking the compact ICRF sources 0229+131 and 0235+164 at elongations of 1.15°–2.6° and fitting the PPN parameter $\gamma$ to the 17 hours of group delays yields $\sigma_\gamma$ between $0.9\times10^{-4}$ and $4\times10^{-4}$ depending on the source subset; the best solution with one station downweighted gives $\gamma-1 = (0.91 \pm 0.94)\times10^{-4}$, and the independent software solution gives $(-0.22 \pm 1.10)\times10^{-4}$. Both are consistent with $\gamma=1$. The paper interprets these results as demonstrating that coronal plasma perturbations are random rather than systematic for ray paths in quiet coronal regions, so they cancel over the session after dual-frequency calibration. A second session, AOV022, had statistics two to three times worse, attributed to source structure rather than the corona.

Load-bearing premise

The load-bearing assumption is that coronal and small-scale plasma perturbations produce only random scatter that cancels over the 17-hour session, so the formal $\sim 1\times10^{-4}$ uncertainty is the real accuracy; the paper states this explicitly and notes it held because the ray paths were in quiet coronal regions.

Editorial extensions

If this is right

  • Single dedicated sessions near the Sun can constrain $\gamma$ as tightly as multi-decade global VLBI solutions, making general-relativity tests schedulable instead of requiring years of accumulated data.
  • The closer source (0235+164) gives roughly a factor of two smaller formal error than the farther source (0229+131), confirming that low-elongation observations carry most of the sensitivity.
  • Standard dual-frequency calibration can handle the quiet solar corona, so future sessions near solar minimum can push to even smaller elongations and higher signal-to-noise.
  • Source structure, not the corona, limited the second session, so compactness screening of target sources is a prerequisite for further improvement.
  • Accumulating several such sessions should move the VLBI $\gamma$ uncertainty toward the Cassini limit of about $2\times10^{-5}$.

Reading between the lines

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

  • A direct extension of the paper's cancellation assumption: split the AUA020 session into two or three independent time blocks and compare the fitted $\gamma$ values; if they scatter by more than the formal errors, the reported $\sim 10^{-4}$ accuracy is optimistic.
  • The gravitational deflection grows toward the solar limb, so scheduling even closer than the current 1.15° minimum (possible near solar minimum with strong, compact sources) could improve precision faster than adding more sessions, an inference the paper leaves implicit.
  • The same close-approach geometry could in principle be used to constrain other solar-system relativistic effects, such as the solar quadrupole moment or the PPN parameter $\beta$, because their delay signatures have different dependence on impact parameter; the paper does not discuss this.
  • The 2.7$\sigma$ deviation in one single-source solution hints at a source- or model-dependent systematic, so future sessions should include a third compact source at similar elongation to distinguish a modeling artifact from real physics.
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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

2 major / 5 minor

Summary. The paper reports two dedicated geodetic VLBI sessions, AUA020 (May 2017) and AOV022 (May 2018), that observed two ICRF radio sources at solar elongations between 1 and 3 degrees. Using two independent analysis packages (OCCAM and Calc/Solve), the authors estimate the PPN parameter gamma from single-session group delays. For AUA020, formal errors reach 0.94e-4 (OCCAM with Sejong downweighted) and 1.10e-4 (Calc/Solve), which the paper presents as comparable to or better than formal errors from global VLBI solutions. AOV022 produced degraded statistics attributed to source structure. The authors conclude that small-scale coronal and ionospheric perturbations cancel over the 17-hour session and that future experiments near solar minimum could challenge the Cassini bound on gamma.

Significance. If the formal errors are accepted as realistic uncertainties, this is an important proof-of-concept: a single 17-hour VLBI session near the Sun constrains gamma at the 1e-4 level, roughly two orders of magnitude better than typical per-session estimates and competitive with global VLBI solutions. The paper's strengths include the dual-software analysis with two independent packages, explicit reporting of formal errors and postfit rms in Table 1, an honest acknowledgment of the unexplained 2.7-sigma Calc/Solve result for 0229+131, and a clear statement of the degraded AOV022 statistics. However, the headline precision is a formal covariance statement, and the manuscript does not yet establish it as a realistic total uncertainty because the cancellation of coronal plasma perturbations is assumed rather than tested. The central values are consistent with general relativity, but the paper is best read as a demonstration of attainable formal precision rather than as a completed measurement of gamma.

major comments (2)
  1. [Section 4] The paper's central claim of ~1e-4 precision depends on the assumption that small-scale coronal perturbations 'cancel out over the period of observations (17 hours with observations angularly close to the Sun).' This is the step that converts a formal covariance error into a scientific uncertainty, and it is not tested. The very next sentence concedes that the ray paths 'happened to be in quiet regions,' a post hoc justification. With only two sources and a single session, a slowly varying or common-mode coronal contribution would not cancel in the assumed way, and the two targets are close enough in the sky that their ray paths may share structure. Please provide a quantitative test, such as estimating an additional coronal electron-density parameter, correlating results with a solar-activity index, or adding a conservative systematic term to the error budget, before the headline precision is presented as a measurement uncertainty.
  2. [Table 1 and Section 3] The Calc/Solve solution using only 0229+131 gives gamma-1 = (-6.84 +/- 2.53) x 10^-4, a 2.7-sigma deviation, while the OCCAM solution for the same source with Sejong downweighted gives (0.32 +/- 2.83) x 10^-4. The text states the origin is unclear and suggests differences in estimation method or troposphere/clock handling. This unresolved discrepancy is evidence that the quoted formal errors may not capture some systematic effect, and it sits in the same paragraph as the statement 'No large systematics are detected.' The anomaly should be modeled or propagated into the uncertainty, or at minimum explicitly listed as a limitation on the sub-10^-4 precision claim.
minor comments (5)
  1. [General] There are several typographical errors: 'publsihed' in Section 2, 'responce' in Section 4, 'Auckand' in the affiliation block, and 'abd' in the author list. These should be corrected.
  2. [Section 3] The AOV022 results are described only qualitatively as '2-3 times worse' than AUA020. A table or at least quoted values for the formal errors and postfit rms would allow the reader to assess the degradation quantitatively.
  3. [Table 1] In the OCCAM rows, the entries labeled 'Both' are identical to 'All scans' for both station-weighting cases. Please clarify whether 'Both' includes only the two target sources and why it coincides exactly with the all-scan solution.
  4. [Section 3] The comparison with 'global solutions' should state the exact published value of the global-solution formal error being cited; the current text references Lambert and Le Poncin-Lafitte (2009, 2011) but does not reproduce their numerical uncertainties, making the abstract's 'better than' claim hard to verify.
  5. [Section 4] The sentence 'It was a general misconception in the past that the effect of the plasma of the solar corona completely disturbs the interferometric responce for light rays passing within several degrees from the Sun' would benefit from a citation or a more hedged phrasing, as it appears to summarize a broad historical claim without a specific reference.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the PPN gamma is estimated from VLBI group delays with independent software packages and compared with external benchmarks; self-citations are methodological, not load-bearing.

full rationale

This is a measurement paper. The PPN parameter gamma is estimated by fitting a standard VLBI delay model to observed group delays from a dedicated session, using two independent analysis packages (OCCAM and Calc/Solve). The reported uncertainties are formal covariance outputs, and the central comparison is to external results: the Cassini bound of Bertotti et al. (2003) and earlier global VLBI estimates of Lambert and Le Poncin-Laffite (2009, 2011). No step defines gamma in terms of the fitted result itself, and no 'prediction' is constructed from a parameter fitted to the same target quantity. Self-citations, such as Titov et al. (2018) for scheduling and analysis design and Titov (2000) for the least-squares collocation method, are methodological references rather than sources of the extracted gamma value; they do not import an unverified uniqueness theorem or ansatz that forces the result. The assumption in Section 4 that coronal small-scale perturbations 'cancel out over the period of observations' is an untested systematic-error assumption that could make the formal errors unrealistically small, and the unexplained 2.7-sigma Calc/Solve value for 0229+131 indicates possible underestimated systematics. However, this concerns the realism of the uncertainty, not circularity of the derivation: the measured gamma is not equivalent by construction to its input, and the paper is self-contained against external benchmarks. Therefore no circular step is exhibited.

Assumptions & free parameters 1 free parameters · 4 assumptions · 0 invented entities

The central claim depends on the standard VLBI delay model and on the assumption that nuisance effects (corona, troposphere, source structure) are either calibrated or random. No new entities are postulated. The only fitted quantity of interest is the PPN parameter itself, whose formal uncertainty is the subject of the claim.

free parameters (1)
  • PPN parameter gamma = (0.91 ± 0.94) × 10^-4 for OCCAM with Sejong downweighted; (-0.22 ± 1.10) × 10^-4 for Calc/Solve
    The PPN parameter gamma is the target of the experiment, estimated by least-squares fit to the group delays. The central claim concerns the formal error of this estimate, so the fitted value and its uncertainty are the result itself, not an external input.
assumptions (4)
  • domain assumption Standard PPN formalism and Shapiro delay model describe the gravitational deflection of radio waves.
    The analysis assumes the parameterized post-Newtonian deflection model with gamma as the free parameter, as introduced in Section 1 and used in the Section 3 fits.
  • domain assumption Dual-frequency calibration removes dispersive corona and ionosphere delays.
    Section 4 asserts that the standard dual-frequency calibration handles coronal charged-particle noise 'by the same way as for the ionosphere around the Earth,' a key assumption that the coronal plasma is dispersive.
  • domain assumption Coronal perturbations are random and cancel over the 17-hour session.
    Section 4: 'Since these perturbations do not systematically affect the observations, we assume that they cancel out over the period of observations.' This is what justifies treating formal errors as realistic.
  • domain assumption Source structure delays are small or correctable for the target sources.
    Section 2 cites structure indices below 2 ps for AUA020, but Section 3 attributes the worse AOV022 statistics to 'severe source structure delay effect,' showing the assumption is not always safe.

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Pith. "Pith review of Observations of radio sources near the Sun." pith.science (2026). https://pith.science/paper/VTPPTLKW

@misc{pith2026190800973,
  author       = {Pith},
  title        = {Pith review of: Observations of radio sources near the Sun},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/VTPPTLKW}},
  note         = {Machine review of arXiv:1908.00973}
}
abstract

Geodetic Very Long Baseline Interferometry (VLBI) data are capable of measuring the light deflection caused by the gravitational field of the Sun and large planets with high accuracy. The parameter $\gamma$ of the parametrized Post-Newtonian (PPN) formalism estimated using observations of reference radio sources near the Sun should be equal to unity in the general relativity. We have run several VLBI experiments tracking reference radio sources from 1 to 3 degrees from the Sun. The best formal accuracy of the parameter $\gamma$ achieved in the single-session mode is less than 0.01 percent, or better than the formal accuracy obtained with a global solution included all available observations at arbitrary elongation from the Sun. We are planning more experiments starting from 2020 using better observing conditions near the minimum of the Solar activity cycle.

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Reference graph

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