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REVIEW 4 major objections 5 minor 51 references

Variations of absolute source positions determined from quad-band VLBI observations

T0 review · 4 major / 5 minor · reviewed 2026-08-10 · deepseek-v4-flash

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Pith's one-line read The apparent position of a quasar in quad-band VLBI is set by unresolved jet structure inside the beam, which moves 15% of sources significantly away from their legacy S/X positions and makes a dedicated quad-band catalog mandatory.

desk verdict First dedicated quad-band VGOS position catalog and time-series analysis with convincing source-structure case studies; the headline 15%/6% offset statistics rest on a hand-drawn declination-dependent error floor that needs to be made reproducible before those numbers are quoted. read the letter →

arxiv 2501.18276 v1 pith:D3O7GFR5 submitted 2025-01-30 astro-ph.GA

Ming Hui Xu , Patrick Charlot This is my paper · ORCID
classification astro-ph.GA
keywords celestialreferenceframeVLBIVGOSAGNsourcestructurequad-bandastrometryICRF3positionstabilityradio-opticaltie
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

Quasars observed by very long baseline interferometry (VLBI) anchor the celestial reference frame used for astronomy and geodesy, but this study shows that a quasar's measured position is not a stable point: it is the brightness centroid of the radio emission inside the interferometer beam, and that centroid moves as jet components brighten and fade. Analyzing 177 sessions of the new quad-band VGOS system (simultaneous 3.3, 5.5, 6.6, and 10.5 GHz) from 2017 to 2024, the authors find that for sources above $20^\circ$ declination this unresolved 'invisible' structure is the dominant cause of position instability, while for southern sources the sparse network's poor sensitivity to declination dominates. Comparing their 377-source quad-band catalog with the legacy S/X (2.2/8.6 GHz) positions of the ICRF3, they find 15% of sources offset by more than $3\sigma$ and 6% offset by more than 0.8 milli-arcseconds; the extreme case, 0723$-$008, is offset by 4.73 mas, with the quad-band position tracking a bright jet knot rather than the core. The operational conclusion is that quad-band observations must be processed against a quad-band catalog, because treating the offsets as noise to be absorbed by inflated uncertainties would hide real physics.

What carries the argument

The argument is carried by per-source position time series obtained from 190 dedicated global solutions of 177 quad-band sessions, in which a stable datum of 46 ICRF3 sources holds the frame fixed while each source's coordinates are estimated session by session. On top of these time series, the concept of source structure from Porcas (2010) does the explanatory work: emission inside the beam ('invisible' structure) displaces the measured position to its brightness centroid, while emission outside the beam ('visible' structure) barely moves the position but creates closure delays. A 7-point first-order polynomial smoothing filter separates the systematic structure-driven wander from white noise, and the lower envelope of the smoothed residuals defines a declination-dependent error floor — about 40 $\mu$as in the north, growing to 0.32 mas in RA$^*$ and 0.47 mas in Dec below $-45^\circ$ — which is used to inflate the catalog uncertainties. The significance statistic is the normalized arc length $X_\rho = \rho/\sigma_\rho$ between quad-band and S/X positions, with $\sigma_\rho$ obtained by projecting the two-coordinate uncertainties onto the offset direction; the authors test the inflation by checking that the $X_\rho$ distribution approaches the Rayleigh distribution expected for pure noise.

What would settle it

Recompute the quad-band catalog with the 177 sessions split into two disjoint halves (or with the 13 stations split into two sub-networks) and apply the same Table 1 error floor to each half. If the same sources appear with >3 sigma offsets to S/X in both halves, the offsets are stable, structure-driven signals; if the set of 'significant' sources changes between halves, the error floor understates the noise and the quoted fractions are artifacts. A second, time-domain check: monitor 0723-008 and 2229+695, since the framework predicts that when the currently brightest jet component fades below the core, the measured quad-band position will jump back toward the core position.

Watch

Extended reading notes

Core claim

On the paper's own terms, the discovery is that the apparent position of an AGN in quad-band VLBI is governed by the structure of the source on scales smaller than the beam — the 'invisible' structure — whose brightness centroid shifts as jet components are ejected, travel outward, and fade, dragging the measured position along by $0.2$–$1.0$ mas even though the structure never appears in images. The 'visible' structure on larger scales, by contrast, changes positions by only tens of micro-arcseconds but produces closure delays that inflate the delay residuals of geodetic solutions. This single mechanism organizes the three observed behaviors — stable positions near the $0.02$–$0.04$ mas noise level (1803+784), continuous milli-arcsecond drift (2229+695, moving 1.7 mas eastward as its brightest component is a jet knot, not the core), and back-and-forth wander along the jet as components cross the beam boundary (3C418) — and it explains why quad-band, S/X, and Gaia optical positions can point at different physical features of the same object. The authors show this explicitly for 0723$-$008, where the quad-band position lies on a jet component 4.7 mas from the optical core, and conclude that a dedicated quad-band catalog is mandatory and that inflating uncertainties to force agreement with S/X would be the wrong response.

Load-bearing premise

The load-bearing premise is that the declination-dependent error floor of Table 1 — read by eye as the lower envelope of the smoothed residual scatter, with no algorithm or uncertainty stated — truly represents the noise floor of quad-band positions; if that floor is drawn too low, the inflated uncertainties are too small and the 15% and 6% offset statistics lose their significance.

Editorial extensions

If this is right

  • A quad-band source catalog must be used to analyze quad-band VLBI observations; continuing to use ICRF3 S/X positions as a priori injects systematic errors for the roughly 15% of sources whose positions differ between the bands.
  • For northern ($\delta > 20^\circ$) core-dominated sources, the accuracy of VLBI positions is limited by in-beam source structure at the $0.1$–$0.2$ mas level, so no amount of additional observing time reduces this floor.
  • Radio–optical position offsets measured against Gaia are substantially caused by in-beam structure pulling the radio position along the jet, so the offsets cannot be removed by improving random errors alone.
  • For jet-dominated sources such as 2229+695, 0642+449, and 0723$-$008, a catalog position averaged over years does not represent the source's position at any single epoch, since positions can drift by more than a milli-arcsecond.
  • Southern sources cannot yet reach the quad-band noise floor because the VGOS network lacks long north–south baselines; the declination-dependent error floor quantifies how much the network must be extended.

Reading between the lines

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

  • Editorial extension: because the 'invisible' scale is set by the beam ($0.4$–$2.0$ mas across the four VGOS bands), the framework predicts a frequency dependence — the fraction and size of structure-induced offsets should shrink at higher observing frequencies; comparing K/Ka-band or single-band VGOS positions against the same S/X catalog would test this scaling.
  • Editorial extension: phase-referencing astrometry that uses AGN calibrators inherits the calibrator's in-beam structure offset, so the optical–radio frame tie via VGOS-era phase referencing will remain limited by this effect unless calibrators are screened for position stability and jet geometry.
  • Editorial extension: the back-and-forth model makes a concrete prediction — when a bright jet component crosses from inside to outside the beam, the position shifts back toward the core; a re-analysis of the existing time series could detect these crossing events directly and measure their amplitude against the jet's position angle.
  • Editorial extension: the paper attributes the three-month-timescale systematic wander to source structure by ruling out tropospheric effects, but a cleaner test would compare the wander direction with the jet position angle measured from the closure-only images for the full sample; a correlation between wander direction and jet angle would confirm the mechanism across all 96 sources rather than th
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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

4 major / 5 minor

Summary. The paper analyzes 177 quad-band (3.3/5.5/6.6/10.5 GHz) VLBI sessions from the IVS VGOS program and derives source position time series from dedicated global solutions. The authors identify three types of position variability (stable, continuous motion, back-and-forth motion), attribute the variability to source structure, and introduce a declination-dependent 'error floor' obtained as the lower envelope of smoothed wrms residuals. They inflate quad-band position uncertainties with this floor and compare the resulting positions with ICRF3 S/X positions, reporting that 15% of sources show >3σ offsets and 6% show offsets larger than 0.8 mas. The paper also presents a 377-source quad-band position catalog and argues that a dedicated quad-band catalog is mandatory for processing VGOS observations.

Significance. If the results hold, the paper is a valuable step toward establishing a VGOS celestial reference frame. The analysis is unusually thorough in its handling of session-wise editing, global solutions, and datum definition, and the combination of astrometric time series with closure-only imaging for sources such as 2229+695 and 3C418 is a clear strength. The authors are also careful to distinguish 'visible' and 'invisible' source structure and to connect these concepts to astrometric position offsets. The release of the quad-band catalog and the machine-readable Table 3 is a useful community resource. However, the headline quantitative claims (15% and 6% offsets) rest on an error floor that is specified only as a hand-drawn lower envelope, with no algorithm, uncertainty, or robustness test; this is the main weakness. The qualitative conclusion that source structure affects quad-band positions is well supported by the case studies, but the statistical fractions are not yet anchored to a reproducible error-floor definition.

major comments (4)
  1. [Section 4, Table 1] The declination-dependent error floor is defined as the lower envelope of the smoothed wrms residuals shown as the grey curve in Fig. 7, but no algorithm is given for constructing this envelope and no uncertainty is attached to it. Because Table 1 is used in Section 5 to inflate the uncertainties that enter Eq. (2), the reported 15% and 6% offset fractions depend directly on this envelope. Please specify the envelope construction (e.g., binned quantile, curve fit, or explicit interpolation rule) and quantify its sensitivity to the 7-point smoothing window, the declination bin size, and the subset of most stable sources.
  2. [Section 5, Fig. 9] The Rayleigh comparison in Fig. 9 is presented as a consistency check, but it is not an independent validation: the error floor and the normalized arc-length distribution are derived from the same quad-band position residuals. An error floor chosen as the lower envelope will, by construction, make the inflated uncertainties smaller for the most stable sources and therefore influence the tail of the X_rho distribution. Please provide an external or hold-out validation, for example by estimating the floor from a subset of sessions and testing on a disjoint subset, or by comparing the inflated uncertainties against independent astrometric results such as those of Petrov (2024).
  3. [Section 5, Table 2, source 0723-008] The largest offset in the sample, 4.73 +/- 0.29 mas for 0723-008, is derived from a single quad-band session (the text states this is the only VGOS session in which the source was observed). This single-epoch position is then included in the 6% statistic for offsets larger than 0.8 mas. A single-session measurement is more vulnerable to unmodeled session-specific errors than the multi-epoch averaged positions used for most other sources. Please either repeat the offset statistics excluding single-session sources or provide a robustness test showing that the 6% result is unchanged.
  4. [Section 5, N_sessions scaling] The inflation procedure scales the error floor by the square root of the number of sessions, with sessions counted only when the source has more than 500 good observations and with partial credit for sparser sessions. The authors state that the 500 threshold is 'not critical' between 100 and 1000, but no test is shown. Since Eq. (2) depends on sigma_rho, which includes this scaling, a sensitivity analysis over the session-count threshold and over the treatment of fractional sessions is needed to establish that the 15% and 6% percentages are robust rather than artifacts of the weighting choice.
minor comments (5)
  1. [Section 3.1] In the paragraph discussing 1803+784, the text refers to '1804+784' when discussing extended structure; this appears to be a typo for 1803+784.
  2. [Section 4, Table 1] The text says the error floor is interpolated from Table 1, but does not specify the interpolation scheme (linear, nearest-bin, or spline) between the 15-degree declination bins; please state the scheme.
  3. [Figure 7] The grey curve is described only in the caption as the lower edge of the red-dot distribution; please add a legend or textual description of how the grey curve was traced, since it is central to the error-floor definition.
  4. [Abstract and Section 7] The terms 'invisible structure' and 'in-beam structure' are used interchangeably in the abstract and in Section 7; define them at first use in both places and keep the terminology consistent throughout.
  5. [Section 5] The sentence comparing the present results with 'Figs. 8 and 9 in Petrov (2024)' would be clearer if it stated that the comparison refers to the distribution of position offsets at different frequencies, not to the full contents of those figures.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the 15%/6% offset statistics use a data-derived error floor and a Rayleigh consistency check, but the floor is not fitted to the offset distribution and the central structure claims rest on independent imaging.

full rationale

The paper's derivation chain is self-contained. The position time series come from independent pSolve global solutions of 177 IVS quad-band sessions, and the central claims about source structure are supported by closure-only images, MOJAVE data, and Gaia positions for 2229+695, 3C418, and 0723-008. The error floor in Table 1 is derived as the lower envelope of the wrms residuals in Fig. 7 and is then used to inflate quad-band position uncertainties in Section 5. Although this floor is estimated from the same time series, it is not fitted to the quad-band versus S/X arc-length distribution: the paper explicitly distances itself from Petrov (2024)'s approach of inflating uncertainties until the Rayleigh distribution is matched, and it states that one should not expect an exact Rayleigh match unless source structure is modeled. Thus the normalized arc lengths in Eq. (2) are not forced by construction to produce the reported 15% and 6% fractions; those fractions are computed after a data-derived, declination-dependent uncertainty inflation, which is a calibration choice rather than a circular reduction. The Rayleigh comparison in Fig. 9 is a consistency check, not an independent validation, but the paper does not claim it as a validation of the floor. The visible/invisible structure terminology is attributed to Porcas (2010), an external source, and the same-author citations (e.g., Xu et al. 2021a, 2022, 2023) provide supporting context but are not load-bearing for the main conclusions. Concerns about the hand-drawn nature and missing uncertainty of the error-floor envelope are legitimate robustness and correctness risks, not circularity.

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

The main quantitative results depend on the fitted error floor and the smoothing filter, plus the domain assumption that observed variations on ~3-month timescales are source-structure driven. No new physical entities are introduced; the visible/invisible structure distinction is adopted from Porcas (2010).

free parameters (5)
  • Declination-dependent error floor = RA*: 0.32 mas at -45 deg to 0.03 mas at 90 deg; Dec: 0.47 mas at -45 deg to 0.04 mas at 90 deg
    From Table 1, derived as the lower envelope of wrms residuals; used to inflate quad-band position uncertainties.
  • Smoothing filter window = 7 points, first-order polynomial
    Chosen in Section 4 to model systematic variations over about 3 months; affects the wrms residuals underlying the error floor.
  • Minimum observation threshold per session = 500 group delays
    Used in Section 5 to count sessions when scaling the error floor by 1/sqrt(N); authors state the exact value between 100 and 1000 is not critical.
  • Declination bin size for error floor tabulation = 15 degrees
    Table 1 is tabulated at 15-degree intervals; affects interpolation of the error floor.
  • Datum source selection thresholds = RA* standard deviation < 0.1 mas, Dec < 0.2 mas
    Used in Section 6 to choose 46 sources that define the celestial reference frame datum; affects all derived positions.
assumptions (4)
  • domain assumption IERS 2010 delay model and Galactic aberration correction are appropriate for quad-band VLBI data analysis
    Used for all solutions in Section 2; standard in geodetic VLBI.
  • domain assumption Source structure can be divided into visible (larger than beam) and invisible (within beam) components with distinct astrometric effects
    Following Porcas (2010), this split is used in Sections 3.1, 3.3, and 7 to interpret position variations and delays.
  • domain assumption Systematic position variations with timescales around 3 months are caused by source structure rather than tropospheric effects
    Section 4 asserts that tropospheric effects are very unlikely to induce such variations, but does not provide a demonstration.
  • ad hoc to paper The lower envelope of the wrms residuals versus declination represents the true precision floor of quad-band positions
    This defines the error floor in Table 1; no algorithm or uncertainty is given for the envelope.

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Cite this review

Pith. "Pith review of Variations of absolute source positions determined from quad-band VLBI observations." pith.science (2026). https://pith.science/paper/D3O7GFR5

@misc{pith2026250118276,
  author       = {Pith},
  title        = {Pith review of: Variations of absolute source positions determined from quad-band VLBI observations},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/D3O7GFR5}},
  note         = {Machine review of arXiv:2501.18276}
}
read the original abstract

Active Galactic Nuclei (AGNs) observed with the technique of very long baseline interferometry (VLBI) are used as fiducial references on the sky to precisely measure the shape and orientation of the Earth. Their positions form a celestial reference frame that plays an important role in both astronomy and geodesy. This study investigates the accuracy and stability of the positions of the AGNs that are measured by simultaneous VLBI observations at 3.3, 5.5, 6.6, and 10.5 GHz. Based on position time series from dedicated geodetic solutions, we characterize the observed source position variations and identify the possible factors causing such variations. We find that the primary contributor is source structure for sources above 20-degree declination while the sensitivity of the observations to the declination coordinate predominates for sources below 20-degree declination. The position time series are further explored to derive more realistic uncertainties for the quad-band positions. Significant position offsets with respect to the positions at 2.2/8.6 GHz are found for 15% of the sources. For 6% of the sources, the offsets are larger than 0.8 milli-arcseconds. Source structure may be divided into two parts: the invisible structure (within the beam size) and the visible structure (on larger scales). The latter causes closure delays enlarging post-fit delay residuals in geodetic solutions whereas the former causes source position changes. Such position changes will contribute significantly to the offsets between radio and optical positions. Overall, this work highlights the necessity to have a specific quad-band catalog for processing operational quad-band observations.

Figures

Figures reproduced from arXiv: 2501.18276 by the authors.

Figure 1
Figure 1. Distribution of positions of source 1803+784 at 118 epochs determined from a global solution of quad-band VLBI observations. The positions are relative to the S/X position of 1803+784 in the ICRF3. Color coding depicts the epoch of each of these 118 VLBI sessions. The standard deviation of the position estimates with respect to the weighted mean is 0.04 mas for both coordinates, as written in the upper-right corner.… view at source ↗
Figure 2
Figure 2. Distribution of the quad-band positions of source 2229+695, with respect to its S/X position in the ICRF3, at 131 epochs. The standard deviation of the positions with respect to the weighted mean value is shown in the upper-right corner. Time [year] 1.5 1.0 0.5 0.0 0.5 1.0 1.5 R elativ e R A * [m as] WRMS residual: 0.05 mas 433 ± 23 as/yr 291 ± 16 as/yr 111 ± 5 as/yr 2018 2019 2020 2021 2022 2023 2024 1.5 1.0 0.5 0.… view at source ↗
Figure 3
Figure 3. Time series of the quad-band positions of source 2229+695. Three linear segments were fitted to the RA∗ coordinate, while one segment was fitted to the Dec coordinate on the entire time span. The fitted segments are shown as red lines. The weighted rms residual with respect to the fitted segments that model the source motion is 0.05 mas for RA∗ and 0.04 mas for declination [PITH_FULL_IMAGE:figures/full_fig_p008_3.png] view at source ↗
Figures from the paper (9 more)
Figure 4
Figure 4. Figure 4: Closure-only images of source 2229+695 at 10.5 GHz for four epochs: 19 February 2019 (first), 10 December 2020 (second), 27 January 2022 (third), and 17 January 2024 (fourth). These images were obtained from quad-band VLBI sessions vt9050, vo0345, vo2027, and vo4017, r…
Figure 5
Figure 5. Figure 5: Illustration of intrinsic position variations for the source 3C418 along its jet. The top panel shows the variation of its quad-band position on the sky, while the bottom panel shows the derived closure-only images at 5.5 GHz (bottom-left) and 10.5 GHz (bottom-right) f…
Figure 6
Figure 6. Figure 6: Variation of the quad-band coordinates of source 0059+581 over time. A smoothing model with one order polynomial and a time window of 7 points is applied to the time series and shown as the red curve. The wrms value of the residuals with respect to this model is report…
Figure 7
Figure 7. Figure 7: Stability of the quad-band source positions as a function of declination for RA∗ (upper panel) and for Dec (lower panel). The standard deviations of the coordinate time series with respect to the weighted means are shown as black dots whereas the wrms values of the coo…
Figure 8
Figure 8. Figure 8: Sensitivity of the existing quad-band VLBI observations to source coordinates as a function of declination. For each source, the median absolute value of the partial derivatives of the group delays with respect to each of the two coordinates, RA∗ and Dec, is calculated…
Figure 9
Figure 9. Figure 9: Distribution of normalized arc lengths between the quad-band and S/X positions. The blue bars correspond to the normalized arc lengths based on the original uncertainties of the quad-band positions whereas the red bars correspond to those derived with inflated uncertai…
Figure 10
Figure 10. Figure 10: Relative positions of source 0723−008 at optical and radio wavelengths with a MOJAVE radio image at 15 GHz in the background. The MOJAVE image selected is from 8 July 2023 and is the closest in time to session vr2302 (carried out on 31 May 2023), the only quad-band se…
Figure 11
Figure 11. Figure 11: Source position offsets between quad-band and S/X band as a function of declination. The error bars are calculated by adding in quadrature the uncertainties of the S/X positions in the ICRF3 to the inflated uncertainties of the quad-band positions. The green triangles…
Figure 12
Figure 12. Figure 12: Sky distribution of 377 radio sources in the quad-band position catalog. The color coding depicts the length of the semi-major axis of the position error ellipse in four predefined ranges, as printed in the bottom-right. 7. DISCUSSION ON THE IMPACT OF SOURCE STRUCTURE…

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Pith tools

Reviewed August 10, 2026 · model on record in the stance chip above.