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REVIEW 3 major objections 3 minor 104 references

Relativistic precessing jets powered by an accreting neutron star

T0 review · 3 major / 3 minor · reviewed 2026-08-04 · deepseek-v4-flash

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Pith's one-line read MeerKAT images show the neutron star binary Cir X-1 has propagating parsec-scale jets whose axis swings by at least 110 degrees, indicating precession.

desk verdict Strong observational case for moving parsec-scale jets from the NSXB Cir X-1, but the headline >=110 deg axis swing leans on mixing PAs from different angular scales and epochs; the precession period/opening-angle constraints are conditional, and the paper says so itself. read the letter →

arxiv 2509.08951 v1 pith:MWX3PVIY submitted 2025-09-10 astro-ph.HE

F. J. Cowie , R. P. Fender , I. Heywood , A. K. Hughes , K. Savard , P. A. Woudt , F. Carotenuto , A. J. Cooper
show 4 more authors
J. van den Eijnden K. V. S. Gasealahwe S. E. Motta P. Saikia
This is my paper
classification astro-ph.HE
keywords CircinusX-1neutronstarX-raybinaryrelativisticjetsjetprecessionparsec-scaleMeerKATradioobservationsmorphology
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 uses five epochs of MeerKAT radio imaging of the neutron star X-ray binary Cir X-1 to reveal a curved S-shaped radio morphology extending about one parsec from the core. Comparing images taken years apart, the authors find that components of this S-shape move outward at an apparent speed of about 0.19c, proving they are genuinely propagating relativistic jets rather than static interstellar structure. The position angle of the jet's core emission changes by roughly 5 degrees per year, and combining with archival measurements spanning 25 years gives a total swing of at least 110 degrees. The authors argue that the smooth, symmetric bending and the axis drift are best explained by a precessing launch direction, making Cir X-1 the first neutron star X-ray binary with moving parsec-scale jets and the largest jet-axis swing seen in any X-ray binary. If the precession is steady, the data imply a period longer than 10 years and a cone half-opening angle greater than 33 degrees.

What carries the argument

The load-bearing object is the S-shaped radio morphology itself, read as a two-dimensional time-lapse record of the jet launch axis: the bend encodes the history of launch directions, and measured proper motions of the jet components convert that morphology into a dynamical measurement. The quantitative constraints come from the geometry of a steadily precessing jet cone: the observed minimum projected position-angle range (≥110 degrees), combined with the VLBI-derived requirement that the instantaneous jet axis reaches an inclination >75 degrees at some epoch, yields a half-opening angle ψ > 33 degrees, while the mean axis rotation of ~5 degrees per year with no observed turnaround sets the

What would settle it

A multi-epoch VLBI campaign at milliarcsecond resolution, taken simultaneously with arcsecond-scale images over two to three years: if the mas-scale jet axis and the arcsecond core-emission axis disagree by more than the ~10-degree uncertainties, or if the arcsecond axis stops rotating while the mas axis continues, the single-launch-axis assumption behind the 110-degree swing and the precession bounds fails.

Watch

Extended reading notes

Core claim

The central discovery is that the radio jets of Cir X-1 remain collimated and moving at mildly relativistic speed out to a projected distance of ~1 pc, the first time such large-scale propagating jets have been seen from a confirmed neutron star X-ray binary. The S-shaped morphology is shown to be jet-associated through flux-density and positional changes between 2023 and 2025, with proper motions of ~3.5 mas/day corresponding to ~0.19c at 9.4 kpc. The paper further establishes that the jet launch axis on the sky has swept through at least ~110 degrees over the past two decades, with the arcsecond-scale core-emission position angle rotating from about -10 degrees to at least 36 degrees since

Load-bearing premise

The argument depends on treating position angles measured on different angular scales—milliarcsecond VLBI, arcsecond core emission, and ~10-arcsecond jet scales—as snapshots of a single continuously changing jet launch axis; if those angles instead trace bent, separate, or scale-dependent jet components, the 110-degree swing and the precession constraints do not follow.

Editorial extensions

If this is right

  • Cir X-1 becomes the first confirmed neutron star X-ray binary with moving parsec-scale jets, showing that neutron-star accretors can power large-scale relativistic outflows comparable to black-hole X-ray binaries.
  • The ~110 degree position-angle range is the largest jet-axis swing seen in any X-ray binary, making precession the most plausible explanation and providing a new benchmark for jet-launch variability.
  • If steady precession is the correct model, the lower bounds P > 10 yr and ψ > 33 degrees place Cir X-1 in a parameter space very different from SS 433 (162-day period, 20-degree half-angle), implying a much slower, wider precession.
  • The long-period constraint rules out Lense-Thirring precession of the accretion disc as the driver, since that mechanism produces periods of days or shorter, leaving tidal precession by a low-mass companion as a viable alternative.
  • The detection of mildly relativistic motion out to ~1 pc implies that the jet power of Cir X-1 is comparable to the most powerful black-hole X-ray binary jets, provided the ambient medium is similar to those systems.

Reading between the lines

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

  • Editorial inference: The system's extreme youth (its supernova remnant is less than ~4600 years old) suggests the jet axis may be changing chaotically rather than on a fixed precession cone; if so, future position-angle measurements would show irregular rather than smooth rotation, distinguishing a young disturbed jet from a steady precessing one.
  • Editorial inference: The 110-degree swing is assembled from measurements made on different angular scales at different epochs; a direct test would be to observe the core position angle simultaneously at milliarcsecond and arcsecond scales, verifying that both track the same launch axis rather than independent jet components or bends.
  • Editorial inference: The interpretation predicts that the arcsecond core position angle will continue to rotate at roughly 5 degrees per year and that an inflexion point in the S-shape may eventually appear; detecting that inflexion would pin down the precession period and cone angle rather than just the lower bounds derived here.
  • Editorial inference: The diffuse, roughly E-W oriented emission perpendicular to the current jet axis could be analogous to the 'wings' seen in precessing radio-galaxy jets; if future observations show it to be moving or varying, it would strengthen the scale-invariance picture of precessing jets down to neutron-star binaries.
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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. This Letter reports MeerKAT L- and S-band observations of the neutron star X-ray binary Cir X-1. The images reveal a curved, roughly symmetric S-shaped radio morphology on ~20 arcsec (~1 pc) scales. By comparing 2023 and 2025 S-band images, the authors measure proper motions of the northern and southern S-shaped components of ~3.5 mas/day, corresponding to an apparent velocity of ~0.19c at the assumed 9.4 kpc distance, and interpret this as evidence that the jets are still propagating at parsec scales. The paper further compiles position-angle measurements from the new data and from the literature (Tudose et al. 2008; Miller-Jones et al. 2012; Coriat et al. 2019) and claims that the jet launch axis has varied by at least 110 deg over 25 years. Under the assumption of steady precession, the authors derive lower limits on the precession period and half-opening angle of >10 yr and >33 deg, respectively. The paper explicitly cautions that the ballistic precession fit is degenerate and not uniquely constraining, and discusses alternative explanations.

Significance. If the central claims hold, this would be the first confirmed neutron star X-ray binary with dynamically detected parsec-scale jets and the largest jet-axis swing observed in any X-ray binary, placing Cir X-1 in a parameter region distinct from SS 433. The paper is careful in several respects: it uses two independent methods to measure proper motions, adopts conservative astrometric systematic uncertainties, tests for artifacts from core subtraction, and openly acknowledges the degeneracies of the ballistic precession model. However, the headline claim of a >=110 deg jet-axis variation rests on combining position angles measured at very different angular scales and thus possibly at different physical locations along a curved jet. This issue is load-bearing for the extreme-swing and precession-constraint conclusions, and it needs to be addressed before the claims can be accepted at face value.

major comments (3)
  1. [Section 4 and Table A1] The claim that the projected jet axis has varied by at least 110 deg combines new arcsecond-scale core PAs (2018: -9 deg, 2023: 13 deg, 2024: ~0-36 deg, 2025: 28 deg) with archival PAs measured on ~20 mas (Miller-Jones et al. 2012), ~2 arcsec (Tudose et al. 2008), and ~2-10 arcsec (Coriat et al. 2019). The paper itself states that arcsecond core PAs are not necessarily the instantaneous launch axis but measure the launch direction at some earlier time. Since the S-shape is a curved trajectory, PAs at different angular radii correspond to different tangent directions and different retarded epochs. No correction or demonstration is provided that all PAs refer to the same physical quantity. The new multi-epoch core-PA data alone span only ~45 deg. The >=110 deg range is therefore not presently established as a secular change of a single jet launch axis; this undermines both the 'extreme swi
  2. [Section 3, proper-motion measurement] The lower limit P > 10 yr is asserted to be 'substantiated by the range of ballistic precession models fitted to the S-shape, with none having periods < 10 years'. However, only one fitted model is shown in Table A1, and Section 3 states that the fitted parameters are degenerate and 'not to be taken as the precession parameters'. No parameter scan, likelihood distribution, or bounds are presented. The period limit is otherwise based on the PA rate and a temporal-sampling argument, both of which inherit the uncertainty from the mixed PA compilation described above. In addition, equation (1) is introduced without derivation; since it is the basis for the half-opening-angle constraint psi > 33 deg, its origin and range of validity need to be shown. The psi constraint also depends on the >=110 deg PA range, so it is only as secure as that range.
  3. [Section 3, proper-motion measurement] The proper-motion measurement compares extended, curved S-shaped components at two epochs using the brightest pixel and Canny edge detection along manually defined axes. The authors adopt the 'usual assumption that the brightest point in the jet is conserved'. Given that the paper argues for deceleration and in situ particle acceleration on parsec scales, the displacement of brightness peaks and edges could reflect pattern motion or brightness redistribution rather than bulk flow of the same fluid elements. The two methods are consistent, which is encouraging, but the analysis does not demonstrate that the same emitting feature is tracked between 2023 and 2025. This is central to the 'first moving large-scale jets from a NSXB' claim. Please add a discussion of how morphological variability or brightness changes could bias the inferred proper motion, and ideally quantify the effect by com
minor comments (3)
  1. [Abstract and Section 3] Typo in the abstract: 'an mildly relativistic' should be 'a mildly relativistic'. Also, the phrase 'Since 2015 the position angle...' in Section 3 is inconsistent with the listed measurements, the earliest of which is 2018; please clarify the reference point or correct the date.
  2. [Equation (1)] Equation (1) uses symbols psi and i without defining them in full (they are defined later in the text, but the inequality would benefit from a brief statement of the geometry and a reference to the specific Hjellming & Johnston equation being used).
  3. [Figures 2 and 3] The white/green contour and beam representations in Figures 2 and 3 are explained, but the dotted lines marking PAs would be easier to interpret if the numeric PA values and their angular uncertainties were printed directly on the figures. The current presentation relies heavily on the captions.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the precession constraints are derived from independent PA measurements and external model equations, not from the degenerate ballistic fit.

full rationale

The paper's derivation chain is self-contained rather than circular. The parsec-scale jets are identified from measured proper motions between 2023 and 2025 epochs, an observational result independent of any precession model. The >=110° launch-axis swing is compiled from published position-angle measurements (Tudose 2008; Miller-Jones 2012; Coriat 2019) plus new core-emission PAs; these are external observational data, not outputs of the present fit. The precession-period lower limit (P>10 yr) is derived from the observed PA evolution rate (~5°/yr) and temporal sampling arguments, and the opening-angle limit (psi>33°) follows from the observed PA range combined with the VLBI flux-asymmetry inequality and the standard Hjellming & Johnston (1981) steady-precession equations. The ballistic precession fit to the S-shape is explicitly labeled degenerate and 'largely unconstraining', and the authors caution that its parameters should not be interpreted as uniquely constraining; the subsequent mention that fitted models all have periods >10 yr is a consistency comment, not the load-bearing derivation. The acknowledged caveat that arcsecond core PAs may trace the launch axis at a retarded time is an interpretive assumption, not a circular reduction. No fitted parameter is renamed as a prediction, no load-bearing claim rests on a self-citation, and no uniqueness theorem is imported from the authors' prior work. The scientific concerns raised about combining PAs at different angular scales are correctness risks, not circularity.

Assumptions & free parameters 1 free parameters · 5 assumptions · 0 invented entities

The central claims rest on standard radio-imaging calibration and astrometry, plus a small set of physical assumptions. No new physical entities are introduced. The precession constraints inherit assumptions about distance, jet speed, conserved bright spots, and the applicability of the Hjellming & Johnston geometry.

free parameters (1)
  • Ballistic precession model parameters (P, v_j, psi, chi, i, phi, s_rot) = P=8160 d, v_j=0.7c, psi=35 deg, chi=300 deg, i=37 deg, phi=0.58, s_rot=-1
    Differential evolution fit to the S-shape in Fig. 1; the authors explicitly state these are degenerate and should not be interpreted as constraining (Section 3, Table A1).
assumptions (5)
  • domain assumption Distance to Cir X-1 is ~9.4 kpc (Heinz et al. 2015)
    Used to convert proper motion (3.5 mas/day) to apparent velocity (0.19c) and distances to physical scales (1 pc). Assumed throughout.
  • domain assumption The brightest point in the jet is conserved, so the measured proper motion of the brightest component gives the apparent bulk velocity
    Stated in Section 3: 'making the usual assumption (which appears to hold for SS 433) that the brightest point in the jet is conserved'.
  • domain assumption Jet speed of 0.5c for the VLBI asymmetry constraint
    Section 4 uses i' > 75 deg for a jet speed of 0.5c based on Miller-Jones et al. (2012) VLBI images. This value feeds into the psi > 33 deg constraint.
  • standard math Hjellming & Johnston (1981) steady precession geometry applies, including Eq. (1)
    Used in Section 4 to derive the opening-angle inequality from the observed PA range of 110 deg. Standard model, but the inequality is not derived in the paper.
  • domain assumption The S-shape reflects a changing launch direction rather than jet-medium interactions or instabilities
    Section 4 argues against alternatives (i) and (ii) on qualitative grounds; this is the pivotal interpretation that enables the precession claim.

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Cite this review

Pith. "Pith review of Relativistic precessing jets powered by an accreting neutron star." pith.science (2026). https://pith.science/paper/MWX3PVIY

@misc{pith2026250908951,
  author       = {Pith},
  title        = {Pith review of: Relativistic precessing jets powered by an accreting neutron star},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/MWX3PVIY}},
  note         = {Machine review of arXiv:2509.08951}
}
abstract

Precessing relativistic jets launched by compact objects are rarely directly measured, and present an invaluable opportunity to better understand many features of astrophysical jets. In this Letter we present MeerKAT radio observations of the neutron star X-ray binary system (NSXB) Circinus X-1 (Cir X-1). We observe a curved S-shaped morphology on $\sim 20''$ $(\sim1\:\text{pc})$ scales in the radio emission around Cir X-1. We identify flux density and position changes in the S-shaped emission on year timescales, robustly showing its association with relativistic jets. The jets of Cir X-1 are still propagating with mildly relativistic velocities $\sim1\:\text{pc}$ from the core, the first time such large scale jets have been seen from a NSXB. The position angle of the jet axis is observed to vary on year timescales, over an extreme range of at least $110{\deg}$. The morphology and position angle changes of the jet are best explained by a smoothly changing launch direction, verifying suggestions from previous literature, and indicating that precession of the jets is occurring. Steady precession of the jet is one interpretation of the data, and if occurring, we constrain the precession period and half-opening angle to $>10$ years and $>33{\deg}$ respectively, indicating precession in a different parameter space to similar known objects such as SS~433.

Figures

Figures reproduced from arXiv: 2509.08951 by the authors.

Figure 2
Figure 2. Left: L-band image of the jets of Cir X-1 in 2018 the core (marked by a blue dot) and SE background source have been subtracted. Right: L-band image of the jets of Cir X-1 in 2024 with the same subtractions. The position angle of the core emission, −9 ± 3° E of N in 2018 and ∼ 0° E of N in 2024, is indicated by a dotted line. Between the two images, there is variability in the position angle, morphology and flux den… view at source ↗
Figure 3
Figure 3. S2-band image of the jets of Cir X-1 in May 2025, with known background source and core subtracted. The lower right inset shows the higher resolution S4-band view of the jets close to the subtracted core in October 2024, where a miniature S-shape can be seen. In green are 10𝜎 contours from the observation shown in [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗

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Pith tools

Reviewed August 4, 2026 · model on record in the stance chip above.