New look at old friends: EVN imaging of prominent radio-loud active galactic nuclei with extremely large radio-optical positional offsets

A. Melnikov; O. Titov; S. Frey; S. Lambert

REVIEW 3 major objections 4 minor 1 cited by

New look at old friends: EVN imaging of prominent radio-loud active galactic nuclei with extremely large radio-optical positional offsets

T0 review · 3 major / 4 minor · reviewed 2026-08-10 · deepseek-v4-flash

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Pith's one-line read A VLBI image confirms the hidden radio core of quasar 3C 287.

desk verdict A solid but preliminary conference report: the new 3C 287 component is a genuine detection, but the 'central engine' association outruns the evidence until astrometric errors and multi-epoch data appear. read the letter →

arxiv 2501.06513 v1 pith:3W3GCATQ submitted 2025-01-11 astro-ph.GA

S. Frey , O. Titov , A. Melnikov , S. Lambert This is my paper · ORCID

classification astro-ph.GA

keywords VLBIactivegalacticnucleiradio-opticaloffsetsGaiaInternationalCelestialReferenceFrame3C287phasereferencingastrometricinstability

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

This paper reports 5-GHz VLBI images, from the European VLBI Network and e-MERLIN, of ten bright radio-loud active galactic nuclei whose radio and optical (Gaia) positions differ by tens to hundreds of milliarcseconds, far beyond measurement errors. For three sources shown as examples, the images reveal what causes the offsets: PKS 1200+045 looks like a compact symmetric object whose optical position marks the hidden central black hole while the radio peak is a lobe hotspot; TXS 1450+641 has a complex, time-variable core–jet; and 3C 287, the key result, shows a compact component roughly 130 mas from its known bright structure that sits very close to the Gaia optical position. The paper argues that this component, never seen before, is the quasar's central engine, confirming the radio 'jump' reported earlier and resolving the apparent offset. If correct, it shows that some large radio–optical offsets are not errors but signposts to the true active nucleus.

What carries the argument

The key tool is phase-referenced very long baseline interferometry at 5 GHz with the European VLBI Network and e-MERLIN, combining antennas separated by a wide range of baseline lengths so that both compact and extended radio structure can be imaged while preserving absolute sky positions relative to nearby ICRF calibrators. The load-bearing object is the newly detected compact component of 3C 287: it is the only part of the radio structure that aligns with the Gaia optical position, and its existence was predicted by the source's astrometric jump. The comparison between the phase-referenced radio images and Gaia DR3 optical positions is what converts a morphological feature into a claim about the location of the central engine.

What would settle it

Multi-frequency, multi-epoch VLBI observations of the compact 3C 287 component would settle the claim: if the component shows a steep radio spectrum or apparent proper motion over 1–2 years, it is a jet feature rather than the stationary core, and the association with the central engine fails. Alternatively, if the component's flux density fades toward the level of the earlier non-detections while the astrometric position jumps back to the southwest, the 'central engine' interpretation would be contradicted.

Watch

Extended reading notes

Core claim

The central discovery is the detection, in a phase-referenced 5-GHz EVN image of the quasar PKS 1328+254 (3C 287), of a compact mas-scale radio component about 130 milliarcseconds from the source's well-known complex bright structure. The component has a present-day flux density of roughly 20 mJy and lies remarkably close to the Gaia DR3 optical position, which the paper takes to mark the accretion disk around the central supermassive black hole. Because this component was previously inferred from an apparent ~130 mas jump in the source's VLBI astrometric position between 2014 and 2017, the new image confirms that the jump was caused by the sudden brightening of a previously unseen part of the source rather than by a measurement artifact. The paper therefore concludes that this compact component is associated with the central engine of the quasar, and it uses the same positional logic to interpret PKS 1200+045 as a compact symmetric object and TXS 1450+641 as a complex core–jet source. This is the first time this component has been seen in any VLBI image.

Load-bearing premise

The argument rests on the assumption that the Gaia optical position marks the accretion disk around the central supermassive black hole, and that the compact radio component in 3C 287 is therefore the central engine because it lies near that position and because the source's astrometric position jumped in that direction; there is no spectral-index, variability, or multi-epoch proper-motion evidence to rule out the component being a jet knot or an unrelated source.

Editorial extensions

If this is right

If the compact 3C 287 component is the radio counterpart of the quasar's central engine, then the true core is ~130 mas northeast of the bright structure that earlier VLBI studies treated as the core.
The 2014–2017 astrometric 'jump' of 3C 287 is explained as the brightening of this component, meaning VLBI reference-frame positions can shift dramatically when a new part of a source outshines the old one.
Large radio–optical offsets in AGN can indicate that the radio peak is a jet knot or lobe hotspot while the accretion disk, seen by Gaia, is radio-faint; sensitive imaging with short baselines can uncover the missing core.
For PKS 1200+045, the positional coincidence supports its classification as a compact symmetric object with the optical position at the central black hole and the radio peak at a lobe hotspot.

Reading between the lines

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

A natural test outside the paper: multi-epoch, multi-frequency VLBI monitoring of the new 3C 287 component could measure its spectral index and proper motion; a steep spectrum or jet-like motion would mean it is a knot, not the core, and would overturn the identification.
The same phase-referencing strategy applied to the remaining seven targets in this project, and to the wider population of offset AGN, could reveal additional hidden cores and turn some compact-symmetric-object candidates into core–jet sources.
If the 3C 287 component is the core, the jet axis points northeast; the previously studied 'helical jet' structure to the southwest would then be older emission or a lobe, changing the inferred jet geometry and orientation of the quasar.
The Gaia astrometric time series for 3C 287 could be checked for optical variability correlated with the mid-2010s radio outburst; a correlated flare would independently support the association without relying on VLBI spectral information.

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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

The one thing you should know: this is a conference proceedings paper, not a full refereed study, and it reads like one. The genuinely new result is the first VLBI image of the 130-mas-separated compact component in 3C 287, which had been inferred from an astrometric 'jump' in earlier work. That detection looks real — 20 mJy at 5 GHz, seen in a phase-referenced EVN e-MERLIN image, with the Gaia position nearby. If the component is what they think, it matters for ICRF/Gaia frame alignment and for interpreting extreme radio-optical offsets. The imaging itself appears standard and carefully done, and the paper is honest that this is a preliminary report with the full 10-source analysis deferred to a later publication.

What the paper does well: it uses phase referencing to register absolute positions, it compares directly to archival images, and it frames the astrometric instability as a genuine problem for geodetic VLBI. The choice to show three sources as case studies is reasonable for a conference contribution.

Where it wobbles: the central astrophysical claim — that the new 3C 287 component is the quasar's central engine — rests almost entirely on positional coincidence with the Gaia optical position. The paper never states the quantitative separation or its uncertainty, and the phase-reference calibrator is 2.34 degrees away, so residual tropospheric errors could plausibly shift the derived position by several mas. There is no spectral index, no variability history, and no multi-epoch proper motion showing the component is stationary relative to the main structure. As presented, the component could still be a jet knot, a calibration residual, or an unrelated line-of-sight source. The same weak assumption underlies the PKS 1200+045 CSO interpretation: the Gaia position is assumed to mark the accretion disk, but host-galaxy contamination or an astrometric offset in Gaia itself could change the conclusion.

I also note that the paper uses the authors' own earlier astrometric papers to select targets and interpret the 3C 287 jump. That is legitimate — those are prior published results, not circular reasoning — but it does mean the confirmation loop is internal to the same group until independent data check it.

Bottom line: the images are a useful contribution and the 3C 287 detection is worth recording, but the quantitative analysis is not yet at the level of a journal article. If this were submitted as a full paper, I would send it to review but require astrometric error bars, a stated radio-optical offset with uncertainty, and ideally a second-epoch observation before the 'central engine' language is justified. As a conference proceedings, it is what it is: a tantalizing preview. For you personally, if you work on reference frames or AGN jets, bookmark it for the full paper rather than citing this version.

Recommendation: worth a serious referee if expanded into a full paper; for now, treat it as a preliminary but honest report.

Referee Report

3 major / 4 minor

Summary. This conference proceedings paper reports 5-GHz EVN/e-MERLIN phase-referenced VLBI images of three radio-loud AGN selected for extremely large radio-optical positional offsets: PKS 1200+045, TXS 1450+641, and 3C 287. The authors use the images to argue that the large offsets arise from source structure: PKS 1200+045 is interpreted as a candidate compact symmetric object whose Gaia position marks the hidden core, TXS 1450+641 shows a complex time-variable core-jet structure, and 3C 287 shows a new compact component about 130 mas from the known bright structure, close to the Gaia position, which the authors claim is associated with the quasar's central engine. The observations and calibration follow standard VLBI procedures, and the paper states that the full analysis of all 10 targets will appear elsewhere.

Significance. If the 3C 287 claim is correct, this would be the first direct imaging of the component responsible for the reported ~130 mas astrometric 'jump' of a reference-frame source, a result of genuine interest for both AGN astrophysics and the stability of the celestial reference frame. The target selection is well motivated by earlier astrometric work, and the images appear sensitive enough to detect the 20 mJy component. However, the main astrophysical conclusion currently rests on an unquantified positional coincidence and lacks supporting physical diagnostics; in its present form the paper is a useful preliminary imaging report rather than a demonstration of the central-engine association.

major comments (3)

[Section 4, Fig. 3] The claim that the new 3C 287 component is 'remarkably close' to the Gaia position and is therefore associated with the central engine is not quantitatively supported. The paper never states the measured radio-Gaia separation or its uncertainty. Phase referencing to J1333+2725 at 2.34 degrees separation can introduce systematic errors of several milliarcseconds or more, and no error budget for the phase-referenced positions is given. Please provide the fitted position of the new component, the Gaia position used, the offset vector, and a total uncertainty that includes both statistical and systematic terms.
[Section 4, Fig. 3] The identification of the new component as the central engine rests solely on positional coincidence with the Gaia source and on the earlier astrometric jump reported by Titov et al. (2022). No independent evidence — spectral index, flux-density variability, proper motion or stationarity over multiple epochs, or brightness temperature — distinguishes a true AGN core from a transient jet knot, a calibration residual, or an unrelated line-of-sight source. The manuscript itself acknowledges that the full analysis of the 10-source sample is being prepared elsewhere, so the conclusion presented here is stronger than the evidence shown. Please either add such diagnostics or explicitly frame the central-engine association as tentative.
[Section 4, Fig. 3] The text says the new image 'confirms' the existence of the component inferred from the apparent astrometric jump, but no quantitative comparison is made between the position of the newly detected component and the position expected from the 2014-2017 jump. At minimum, state the separation between the new component and the main radio structure, the predicted offset from the astrometric analysis, and the uncertainties on both, so that 'confirmation' can be assessed rather than asserted.

minor comments (4)

[Fig. 3 caption] The caption contains a typo: 'the Gaia optical position in the phase-referenced image is close the this component' should read 'close to this component'.
[Figs. 1-3] The Gaia positions are shown only as crosshairs on the images; please add coordinate axes or an inset table giving the exact right ascension and declination of the radio peaks and the Gaia positions, so readers can verify the offsets quantitatively.
[Section 2] The text describes the observations but does not state the on-source integration time per target or the expected phase-referencing astrometric accuracy; a sentence indicating the expected position uncertainty from the calibrator separation and observing duration would help place the results in context.
[Section 3] The interpretation of PKS 1200+045 as a CSO relies on the same assumption that the Gaia position marks the accretion disk around the central black hole; this assumption is standard but should be stated with appropriate caution, and the quantitative offset should be given.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the 3C 287 component claim is an independent imaging detection, not a re-statement of prior astrometry.

full rationale

The paper makes no fitted-parameter prediction and contains no derivation that reduces to its inputs by construction. The chain is observational: sources with large radio–optical offsets were selected from the literature and databases, observed with EVN/e-MERLIN at 5 GHz, phase-referenced to nearby ICRF calibrators, and compared with Gaia DR3 positions. The central 3C 287 claim is that a new VLBI image detects a compact component near the Gaia position, about 130 mas from the known structure, thereby confirming a component previously inferred from an astrometric 'jump' in Titov et al. (2022). This is a genuine independent imaging verification: the 2022 astrometry predicted a new component, and the new image reveals it directly; the detection is not defined in terms of the astrometric jump. The self-citations (Titov & Frey 2020; Frey & Titov 2021; Titov et al. 2022) are used as prior observational evidence and target selection, not as an unverified premise that forces the interpretation; they are legitimate external data. The assumption that the Gaia position marks the accretion disk/central engine is a stated astrophysical interpretation common in the field, not an input fitted to the output. The skeptic's concern that the 3C 287 component identification relies on an unquantified positional coincidence and lacks spectral-index or variability diagnostics is a correctness/robustness issue, not circularity: no quantity is defined in terms of the claim, and no 'prediction' is statistically forced by a fit. Therefore the paper is self-contained against external observation and warrants a circularity score of 0.

Assumptions & free parameters 0 free parameters · 3 assumptions · 0 invented entities

The paper introduces no new free parameters or invented entities; its burden is carried by domain assumptions about astrometric registration and the physical meaning of Gaia and radio positions.

assumptions (3)

domain assumption Gaia optical position traces the accretion disk emission around the central supermassive black hole, which is often invisible in the radio.
Invoked in Section 3 for PKS 1200+045 and in Section 4 for 3C 287 to associate the optical position with the quasar core. If this association fails for a particular source, the interpretation changes.
domain assumption Phase referencing to a nearby ICRF calibrator preserves the absolute sky position of the target brightness peak.
The astrometric registration of images in Figs. 1-3 relies on this; calibrator separations are 1.28-2.34 degrees and no phase-referencing astrometric uncertainty is quoted.
domain assumption The newly detected compact component in 3C287 is a physical part of the quasar and not an unrelated background source or imaging artifact.
The association is based on positional coincidence with Gaia and the earlier astrometric jump (Titov et al. 2022), not on a measured common origin such as parallax or proper motion.

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Cite this review

Pith. "Pith review of New look at old friends: EVN imaging of prominent radio-loud active galactic nuclei with extremely large radio-optical positional offsets." pith.science (2026). https://pith.science/paper/3W3GCATQ

@misc{pith2026250106513,
  author       = {Pith},
  title        = {Pith review of: New look at old friends: EVN imaging of prominent radio-loud active galactic nuclei with extremely large radio-optical positional offsets},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/3W3GCATQ}},
  note         = {Machine review of arXiv:2501.06513}
}

read the original abstract

When comparing modern fundamental reference frames in the radio (International Celestial Reference Frame) and optical (Gaia), a couple of bright radio reference sources appear to have very large radio-optical offsets, from tens up to hundreds of milliarcseconds (mas). The amount of these positional misalignments exceeds the uncertainty of each individual technique by at least an order of magnitude. In most cases, complex and extended radio structure and its time variability, and thus the difficulty in pinpointing the true location of the central engine, is responsible for the large apparent offsets. Sometimes distant parts of the radio structure are not properly detected due to a lack of shorter interferometer baselines. For our 5-GHz very long baseline interferometry (VLBI) experiment using antennas of the European VLBI Network and the enhanced Multi Element Radio Linked Interferometer Network, we selected 10 bright radio-loud active galactic nuclei with extremely large radio-optical offsets. Sensitive imaging involving a wide range of projected baseline lengths, as well as phase-referencing to nearby sources shed light on the possible causes of positional inconsistencies. Here we show results for 3 selected sources from this project.

Figures

Figures reproduced from arXiv: 2501.06513 by the authors.

**Figure 1.** Naturally-weighted 5-GHz EVN image of PKS 1200+045 from 2022 Mar 9 (ET048A). The crosshairs indicate the Gaia DR3 position which is more than 40 mas away from the radio peak whose position is determined with phase referencing to J1202+0235 (separated by 1.65◦ ). The peak brightness is 156 mJy beam−1 , the lowest contours are drawn at ±1.7 mJy beam−1 (∼ 3.7σ image noise), the positive contour levels increase by a fa… view at source ↗

**Figure 2.** Naturally-weighted 5-GHz EVN image of TXS 1450+641 from 2022 Mar 10 (ET048A). The crosshairs indicate the Gaia DR3 position. The radio peak position is determined with phase referencing to J1441+6318 (separated by 1.28◦ ). The peak brightness is 24.9 mJy beam−1 , the lowest contours are drawn at ±0.4 mJy beam−1 (∼ 3.5σ image noise), the positive contour levels increase by a factor of 2. The half-power width of the c… view at source ↗

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

From compact jets to extended lobes: radio morphologies of distant quasars at z > 4
astro-ph.GA 2026-07 accept novelty 6.0 of 10

Three z>4 quasars are resolved into kiloparsec-scale radio structures: two bent, FR II-like double-lobed sources and one compact one-sided jet.

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Pith tools

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

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& Baan, W

An, T. & Baan, W. A. 2012, ApJ, 760, 77

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