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The polarisation behaviour of OJ 287 viewed through radio, millimetre and optical observations between 2015 and 2017

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

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Pith's one-line read A single jet component, moving at 0.12 milliarcseconds per year, links the December 2015 disk-impact flare of OJ 287, a 210-degree optical polarisation rotation, and the February 2017 very-high-energy gamma-ray flare into one timeline.

desk verdict A solid multiwavelength dataset that resolves the 210-degree optical rotation, but the connecting timeline leans on a knot speed that is one assumed co-location epoch away from being a measurement. read the letter →

arxiv 2501.08652 v1 pith:3TNAHHOK submitted 2025-01-15 astro-ph.HE

J. Jormanainen , T. Hovatta , E. Lindfors , A. Berdyugin , W. Chamani , V. Fallah Ramazani , H. Jermak , S. G. Jorstad
show 8 more authors
A. Lähteenmäki C. McCall K. Nilsson P. Smith I. A. Steele J. Tammi M. Tornikoski F. Wierda
This is my paper · ORCID
classification astro-ph.HE
keywords OJ287blazarpolarisationEVPArotationvery-long-baselineinterferometryvery-high-energygammarayssupermassiveblackholebinaryrelativisticjet
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

OJ 287 is a blazar long suspected of hosting a supermassive black hole binary, and this paper tries to tie together three major events in its 2015–2017 observing season. The paper argues that a single moving jet knot, K, was ejected from the core region near the December 2015 'general relativistic' flare predicted by the binary model, that the same knot produced a fast 210-degree rotation of the optical polarisation angle, and that its later passage through a quasi-stationary shock feature S1 triggered the first very-high-energy gamma-ray flare of OJ 287 in February 2017. The argument hinges on a revised speed for K of 0.12 milliarcseconds per year, slower than previously estimated, which places K inside the 86 GHz core at the time of the 2015 flare and still interacting with S1 during the 2017 gamma-ray flare. The paper also shows that millimetre-wave polarisation follows the centimetre-wave behaviour except for a ~100-degree offset while K crosses S1, indicating that the millimetre bands resolve jet substructure that the centimetre bands blend together. If correct, the picture unifies a disk-impact flare, a jet-launch event, and a high-energy outburst in one source, strengthening the case that the binary's orbit drives OJ 287's multiwavelength behaviour.

What carries the argument

The central object is the moving knot K and the two quasi-stationary features S1 and S2 resolved in 86 GHz VLBI images: K is a bright emission component travelling down the relativistic jet, while S1 and S2 are standing shocks whose brightening marks K's passage. The argument is carried by a speed estimate: using the epoch when S1 dominated the image (MJD 57661) as the time K was at $0.1\,\mathrm{mas}$, and the first resolved detection of K at $0.16\,\mathrm{mas}$ on MJD 57843, the paper derives $0.12\,\mathrm{mas\,yr^{-1}}$. Backward and forward extrapolation of this speed is what connects the core-crossing time (around MJD 57357) to the GR flare and the S1-crossing time to the VHE flare. A secondary mechanism is the correction of the 180-degree EVPA ambiguity with the Kiehlmann et al. (2016) scheme, which lets the paper compare optical, millimetre, and centimetre polarisation rotations on a common curve.

What would settle it

Take the 86 GHz image from any epoch between MJD 57661 and MJD 57843 and measure where K actually is: if K is resolved as a separate component at a distance that, when joined to the core at MJD 57357, requires a speed other than 0.12 mas/yr, or if K is not inside S1 at MJD 57661, the claimed timeline fails.

Watch

Extended reading notes

Core claim

The paper's central claim is that the moving component K in OJ 287's jet moves at $0.12\,\mathrm{mas\,yr^{-1}}$, not the $0.38\,\mathrm{mas\,yr^{-1}}$ derived in the earlier study. This slower speed follows from placing K, already blended within the quasi-stationary feature S1, at $0.1\,\mathrm{mas}$ from the core on MJD 57661, when S1 dominated the 86 GHz image. Extrapolating backward at constant speed puts K inside the 86 GHz core around MJD 57357, within about a week of the start of the December 2015 GR-model flare and the onset of the $\sim 210^\circ$ optical EVPA rotation. Extrapolating forward, K is still crossing S1 during the February 1–5, 2017 VHE flare, making the passage a plausible trigger for that flare. The paper further claims that the millimetre-band EVPA follows the centimetre bands' slow rotation but departs by about $100^\circ$ while K is inside S1, because the optically thin millimetre and optical bands trace substructures that remain unresolved in the centimetre bands.

Load-bearing premise

The load-bearing premise is that on MJD 57661, when the quasi-stationary feature S1 dominated the 86 GHz image, the moving knot K was already blended inside S1 at about 0.1 mas from the core, and that K then moved at a constant 0.12 mas/yr with no acceleration; if either gives way, the reconstructed timeline collapses.

Editorial extensions

If this is right

  • If K really follows this timeline, the December 2015 GR flare, the 210-degree optical rotation, and the February 2017 VHE flare are different manifestations of a single disturbance moving down the jet.
  • The passage of K through S1 becomes the leading explanation for the February 2017 gamma-ray flare, consistent with the SED modelling of that flare.
  • The ~100-degree cm-to-mm EVPA difference during the S1 episode implies that millimetre and optical polarimetry can locate jet substructures that are still blended in centimetre observations.
  • The slower speed aligns K with the MOJAVE components at 0.2 and 0.4 mas having speeds of 0.13 and 0.16 mas/yr, so K should become visible at lower frequencies over later years.
  • The near-simultaneity of the GR impact and the knot's appearance in the core suggests that the binary interaction enhances activity at the jet base, even if the ejected matter itself needs roughly 100 days to propagate.

Reading between the lines

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

  • Editorial extension: if the 0.12 mas/yr speed holds, a continuous 86 GHz movie of OJ 287 from 2015 to 2018 should show K emerging from the core and crossing S2 at a predictable time, a check that can be done with archival GMVA data.
  • Beyond the paper: the same method of anchoring a knot's position to a brightened quasi-stationary feature could be applied to other blazars with multi-epoch 86 GHz imaging, but it inherits the same blending assumption.
  • A further testable prediction: if the disk-impact picture is right, other GR-model flares of OJ 287 should show a comparable optical EVPA rotation, and the rotation amplitude should correlate with the thermal flare's brightness.
  • The paper implicitly assumes K's speed is constant between the core, S1, and S2; if knots in OJ 287 accelerate or decelerate on 0.1–0.2 mas scales, the timeline could shift by months, which future phased-ALMA 86 GHz observations could test.
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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 / 3 minor

Summary. The paper combines optical photometry/polarimetry from several programs with centimetre and millimetre radio polarimetry (including AMAPOLA) and previously published 86 GHz GMVA images to reinterpret the 2015-2017 activity of OJ 287. Its main new quantitative result is a revised speed of 0.12 mas/yr for the moving component K, obtained by assuming K was blended inside the quasi-stationary feature S1 at MJD 57661 rather than using Lico et al.'s starting point at the VHE flare epoch. With this speed the authors backward-extrapolate K's position to the 86 GHz core around MJD 57357, which they associate with the December 2015 GR-model optical flare, and they argue that K was still interacting with S1 during the February 2017 VHE flare. The paper also reports a fast ~210-degree optical EVPA rotation with improved cadence, slow EVPA rotations in the mm bands consistent with cm-band trends, and an mm-versus-cm EVPA difference during the S1 passage that they attribute to mm bands tracing substructures not resolved in cm bands.

Significance. If the speed estimate is correct, the paper provides a coherent multiwavelength timeline linking a predicted binary-disk impact flare, an optical EVPA rotation, a new VLBI component, and the first VHE detection of OJ 287. The improved optical cadence is a genuine observational contribution, as is the inclusion of mm-band polarimetry that bridges the cm and optical regimes. The agreement with the 15 GHz MOJAVE component speeds is a non-fitted external check. However, the central speed is not directly measured: it is built on the assumed co-location of K with S1 at one epoch, and no uncertainty is propagated through any of the subsequent timing claims. The paper therefore presents a plausible and interesting scenario rather than a secure measurement, and the quantitative claims need to be reworked or substantially caveated.

major comments (3)
  1. [Sect. 3.2] The new speed of 0.12 mas/yr is not a directly measured VLBI speed: it is obtained by assuming that K was co-located with S1 at S1's average core distance of about 0.1 mas at MJD 57661, when S1 dominated the 86 GHz image. As the manuscript itself states, this co-location is Lico et al.'s interpretation of the S1 brightening, not a separately fitted position for K. Because the 2015 GR-flare timing around MJD 57357 and the VHE-flare interaction both derive from this two-point speed, the central timeline rests entirely on this untested premise. The authors should either obtain a direct K position at MJD 57661 or at additional epochs from the visibility data, or explicitly present the speed as a scenario and quote the uncertainty that follows from S1's size (0.037 +/- 0.021 mas) and the unknown location of K within S1.
  2. [Sect. 3.2, Figs. 1-2] No uncertainty is propagated into the core-crossing time around MJD 57357, although the input position at MJD 57661 is uncertain by at least S1's extent. With the stated S1 size, the corresponding uncertainty in the two-point speed is tens of percent, and the backward extrapolation over about 300 days therefore does not support a precise coincidence with the GR flare at MJD 57361; the agreement is within roughly 12 days only if the zero-distance core boundary and constant motion are assumed. The authors should quote a time range for the core emergence and discuss the effect of a finite 86 GHz core size and possible component acceleration.
  3. [Sect. 3.2 and Sect. 4] The claimed VHE-flare consistency is also tied to the same assumption: with the slower speed, the S1 crossing time is (112 +/- 64) days, while the S1-brightening epoch MJD 57661 and the VHE flare MJD 57785-57789 are 124 days apart. This is only a one-sigma agreement, and the crossing-time uncertainty is dominated by the assumed K-S1 blend rather than by measured component sizes. Moreover, the confirmation by the S2 brightening at MJD 58025 uses the same style of inference (K blended inside S2), so it does not independently verify the speed. Please quantify the probability that the VHE flare coincides with the passage under the proposed speed, or soften the claim.
minor comments (3)
  1. [Sect. 3] There is a typo in 'We also employ the the imaging results of Lico et al. (2022)'; additionally, 'di fferent' appears in place of 'different' in several places (e.g., abstract and introduction).
  2. [Sect. 4] The text says 'a clear ~210 degree rotation at MJD 57570', but the rotation is described elsewhere as starting around MJD 57370 and ending around MJD 57570; please correct the epoch so the discussion is consistent with Sect. 3.1.
  3. [Sect. 3.2] The MOJAVE program is misspelled as 'MOJA VE5' in the sentence citing 15 GHz data; please fix the formatting of the program name.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: the new 0.12 mas/yr speed and its coincidences are derived predictions from explicit prior-work assumptions, not fitted outputs or self-referential redefinitions.

full rationale

The central derivation is the two-point speed estimate for component K in Sect. 3.2. The paper takes the MJD 57661 S1-dominating epoch as the epoch at which K is blended in S1 at ~0.1 mas (from Lico et al. 2022), and the MJD 57843 detection of K at ~0.16 mas, and computes 0.12 mas/yr. This is a transparent arithmetic combination of two published positions/times; it is not an equation in which the result is equivalent to its input. The backward extrapolation to MJD 57357 (time K was inside the 86 GHz core) is a genuine prediction from that speed and the assumed 0.1 mas position, and the claimed coincidence with the December 2015 GR flare is therefore a derived consequence, not a fitted parameter renamed as a prediction. The VHE-flare interaction argument similarly follows from the slower speed and S1 size, and is presented conditionally ('could have been'). The one caveat is that the 57661 co-location is interpretive and is imported from Lico et al. (2022), a paper with overlapping authorship (S. G. Jorstad); however, that citation is to externally published GMVA imaging and their interpretation, it is not a hidden uniqueness theorem, and the paper also cites independent 15 GHz MOJAVE speeds (0.13 and 0.16 mas/yr) as corroboration. The optical 210-degree EVPA rotation and mm/cm EVPA differences are independent data analyses. No circular step of the enumerated kinds is present; the derivation chain is not equivalent to its inputs by construction. Score 1 reflects only the mild reliance on an interpretive prior-work assumption, which is a correctness/robustness concern rather than circularity.

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

The central claim rests on a small number of domain assumptions: the EVPA unwrapping rule, the co-location of K with S1 at MJD 57661, constant speed, negligible cm-mm Faraday rotation, and the accuracy of the GR flare times. There are no fitted free parameters and no invented physical entities; the paper's contribution is a re-analysis of existing and newly collected data.

assumptions (5)
  • domain assumption EVPA unwrapping via Eq. (1) recovers the true polarisation angle evolution when the true change between consecutive points is less than 90 degrees.
    Used for all optical and radio EVPA in Sect. 2. The authors note in Sect. 3.1 that faster rotations can be misinterpreted; the 210 degree optical rotation is claimed to be secure because cadence was <1 day.
  • domain assumption K was co-located with quasi-stationary feature S1 at MJD 57661 when S1 dominated the 86 GHz image.
    Section 3.2: the new speed places K at S1's distance of ~0.1 mas at that epoch. This is inherited from Lico et al. (2022) and is the load-bearing premise for the speed.
  • domain assumption K moved at constant speed between the core, S1, and S2, with no acceleration over 2015-2017.
    Implicit in computing 0.12 mas/yr from two epochs and extrapolating back to the core at MJD 57357 in Sect. 3.2.
  • domain assumption Faraday rotation between the cm and mm bands is small enough that a ~100 degree EVPA difference at MJD 57570-57730 is dominated by jet substructure rather than propagation effects.
    Section 4: the cm-band variation is attributed to rotation measures 80-330 rad/m2, while the cm-mm difference is said to be too large for Faraday rotation.
  • domain assumption The December 2015 flare times from the GR binary model (Valtonen et al. 2016) are accurate.
    Used in Sect. 3 to align the optical flare, EVPA rotation, and K core emergence; the coincidence would be weaker if the flare timing were off.

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Pith. "Pith review of The polarisation behaviour of OJ 287 viewed through radio, millimetre and optical observations between 2015 and 2017." pith.science (2026). https://pith.science/paper/3TNAHHOK

@misc{pith2026250108652,
  author       = {Pith},
  title        = {Pith review of: The polarisation behaviour of OJ 287 viewed through radio, millimetre and optical observations between 2015 and 2017},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/3TNAHHOK}},
  note         = {Machine review of arXiv:2501.08652}
}
read the original abstract

OJ 287 is a bright blazar with century-long observations, and one of the strongest candidates to host a supermassive black hole binary. Its polarisation behaviour between 2015 and 2017 (MJD 57300-58000) contains several interesting events that we re-contextualise in this study. We collected optical photometric and polarimetric data from several telescopes and obtained high-cadence light curves from this period. In the radio band, we collected mm-wavelength polarisation data from the AMAPOLA program. We combined these with existing multifrequency polarimetric radio results and the results of very-long-baseline-interferometry imaging with the Global mm-VLBI Array at 86 GHz. In December 2015, an optical flare was seen according to the general relativistic binary black hole model. We suggest that the overall activity near the accretion disk and the jet base during this time may be connected to the onset of a new moving component K seen in the jet in March 2017. With the additional optical data, we find a fast polarisation angle rotation of 210 degrees coinciding with the December 2015 flare, hinting at a possible link between these events. Based on the 86-GHz images, we calculated a new speed of 0.12 mas/yr for K, which places it inside the core at the time of the 2015 flare. This speed also supports the scenario where the passage of K through the quasi-stationary feature S1 could have been the trigger for the very-high-energy gamma-ray flare of OJ 287 seen in February 2017. With the mm-polarisation data, we established that these bands follow the cm-band data but show a difference during the time of K passing through S1. This indicates that the mm-bands trace the substructures of the jet still unresolved in the cm-bands.

Figures

Figures reproduced from arXiv: 2501.08652 by the authors.

Figure 1
Figure 1. Light curves displaying the multifrequency radio flux, PD and EVPA evolution. Starting from MJD 57586, the EVPA data at 4.85 GHz has been shifted by 180◦ for better comparison with the higher frequencies. The grey data points in the bottom panel show the evolution of the optical data (shown in [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Light curves displaying the R-band flux, and combined R-, V- and g*-band PD and EVPA evolution. The inset shows a zoom-in of the ∼ 210◦ rotation seen in the optical data. The grey data points in the bottom panel show the evolution of the radio band (as shown in [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Top panel: light curves of 37, 97.5, and 343.4 GHz bands from the period MJD 57300-57550. To construct the spectra, the data was rebinned with a weekly cadence and averaged. Highlighted data points show the selected averaged bins of each light curve. The red dotted line shows the time when K was within the 86 GHz core. The two blue dashed lines show the times of the 2015 GR flares. Bottom panel: time evolution of th… view at source ↗

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Pith tools

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