{"id":"c200ace3-120b-4a7b-ab56-364295ab613f","arxiv_id":"2509.08951","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"Cir X-1 is the first neutron star X-ray binary known to have large, moving relativistic jets, whose direction changed by at least 110 degrees over 25 years.","lead":"Radio images of the neutron star binary Cir X-1 show a bent, S-shaped jet about a parsec long, with some parts still moving at roughly a fifth of the speed of light. Over 25 years the jet's direction swung by at least 110 degrees, the largest such swing seen in an X-ray binary, probably from precession.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The >=110 deg jet-axis swing may partly be an artifact of comparing PAs at different angular scales/radii along the curved S-shape; the new multi-epoch core-PA data alone span only ~45 deg.","rationale":"The observational results -- the S-shaped morphology, the proper motions (~0.19c) of components between 2023 and 2025, and the variability of the arcsecond-scale core emission -- are robust and independently support that Cir X-1 has expanding jets on parsec scales. The load-bearing step for the precession claim is the compilation of a >=110 deg change in the jet launch axis from observations at different angular scales. The reader's weakest assumption correctly identifies this as the critical point, and the manuscript's own caveat that arcsecond core PAs are not necessarily the instantaneous launch axis reinforces the concern. If the historical PAs are effectively measuring the tangent of a static curved jet at different radii, the large swing is an artifact, and the precession constraints (P>10 yr, psi>33 deg) would not follow. The proposed test would settle this by checking whether the S-shape alone contains the full PA range. The paper's additional caveats in Section 4 (degeneracies in the ballistic model, possible mode switching, non-steady precession) further justify a conditional verdict. I therefore see no reason to change the reader's CONDITIONAL verdict; the concern is the same one the reader flagged, and the recommended action is to strengthen the evidence for a common PA definition or retarded-time correction before accepting the strong precession interpretation.","tokens_in":15773,"tokens_out":7737,"duration_ms":97122,"concrete_test":"Using the 2023 S2 image (Figure 1), trace the ridge line of the S-shaped emission and measure the local position angle as a function of angular radius from the core. If the S-shape itself spans >=110 deg in local PA between the inner (r~1 arcsec) and outer (r~10 arcsec) regions, then a >=110 deg PA range can be generated from a single epoch and does not by itself prove a time-varying launch axis; the precession interpretation would then rest only on the ~45 deg core-PA change seen across the 2018-2025 MeerKAT epochs. If the S-shape spans <110 deg, the older multi-scale measurements are needed, and their scale-dependent interpretation must be validated.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central evidence for a changing jet launch axis is the >=110 deg spread in position angles compiled in Section 3 from measurements at 20 mas (Miller-Jones et al. 2012), ~2-10 arcsec (Tudose et al. 2008; Coriat et al. 2019), and the new arcsecond core-emission PAs (2018-2025). The authors acknowledge that arcsecond core PAs are not necessarily the instantaneous launch axis, but may trace the launch direction at a retarded time or be affected by local curvature. The S-shape is a curved trajectory, so the PA measured at angular radius r is the tangent of the trajectory at that radius, corresponding to the launch direction at t - r/v_app. Different observations have different resolutions, sensitivities, and core-subtraction schemes, so they may effectively sample different radii along the S-shape. If so, the PA spread partially (or entirely) reflects the curvature of a single S-shaped jet rather than a secular change of the launch axis. The new multi-epoch core-PA measurements (2018: -9 deg, 2023: 13 deg, 2024: 0-36 deg, 2025: 28 deg) are at roughly the same angular scale and do show a change, but the total range is ~45 deg, not 110 deg. The 110 deg claim depends on including the older, larger-scale measurements. Unless it is shown that all compiled PAs refer to the same physical quantity (the launch axis at a common retarded epoch), the 'extreme swing' -- and hence the precession constraints (P>10 yr, psi>33 deg) -- are not secure.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","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.","tokens_in":16190,"tokens_out":6774,"duration_ms":80903,"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":[{"comment":"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","section":"Section 4 and Table A1"},{"comment":"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.","section":"Section 3, proper-motion measurement"},{"comment":"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","section":"Section 3, proper-motion measurement"}],"minor_comments":[{"comment":"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.","section":"Abstract and Section 3"},{"comment":"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).","section":"Equation (1)"},{"comment":"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.","section":"Figures 2 and 3"}],"recommendation":"major_revision","confidential_remarks":"The paper contains a genuinely interesting new dataset and the proper-motion analysis is a solid step forward. My main concern is whether the headline '>=110 deg jet-axis swing' can be supported by the heterogeneous PA compilation as presented. If the authors can provide a retarded-epoch correction or otherwise demonstrate that the PA spread is not dominated by curvature/scale effects, the paper would be a strong MNRAS Letter; otherwise the claims should be moderated to what the homogeneous New data alone support. I would not require new observations, but the existing data need to be re-analyzed or the conclusions re-scaled accordingly."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The real news here is the proper motion: two independent methods give ~0.19c at ~1 pc from a confirmed neutron star X-ray binary, which makes this the first clear case of large-scale moving jets from an NSXB. The S-shaped morphology is well imaged, the variability on year timescales is convincing, and the astrometry is handled carefully -- the 10% systematic, the source-subtraction tests on background objects, and the consistent offsets between brightest-pixel and edge-detection methods all point to a solid measurement. Credit where due: this is a real discovery, not just a rehash.\n\nThe soft spot is exactly where the stress-test note lands. The >=110 degree position-angle swing is compiled from measurements at 20 mas (Miller-Jones), ~2-10 arcsec (Tudose, Coriat), and the new MeerKAT core PAs. Those sample different radii along a curved jet, so they trace the launch axis at different retarded times -- or worse, they might trace local curvature rather than a changing launch axis. The new core-PA data alone span only ~45 degrees, and the 2018-2025 variation is what is directly observed. The 110 degree number therefore depends on combining non-homogeneous literature PAs. The authors acknowledge that arcsecond PAs are not instantaneous launch directions, but they do not demonstrate that all compiled PAs refer to the same physical quantity. That is a genuine gap for the precession constraints (P>10 yr, psi>33 deg). Eq. (1) is also just dropped in without derivation, which makes the psi bound hard to check. The ballistic precession fit is honestly labelled degenerate and not constraining, so that part is not circular -- good.\n\nThe paper is also unusually candid about alternatives: deceleration, wind redirecting, multiple jet modes, or chaotic axis changes. That honesty makes the central claim -- precession of some kind is the best explanation -- believable, even if the steady-precession parameters are shaky. Data not being public yet is an annoyance, not a flaw.\n\nOverall: the proper-motion and S-shape results deserve a serious referee and will be cited. The 110 degree swing needs to be either bolstered with a same-scale multi-epoch comparison or explicitly downgraded to ~45 degrees directly observed plus a longer-term literature-based spread. I'd send this to review, with the expectation that the precession constraints get softened or better justified.\n\nRecommendation: accept for peer review, but push the authors on the PA-scale mixing and the missing derivation of Eq. (1).","headline":"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.","tokens_in":16738,"tokens_out":1444,"would_cite":true,"duration_ms":19154,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"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.","keywords":["Circinus X-1","neutron star X-ray binary","relativistic jets","jet precession","parsec-scale jets","MeerKAT radio observations","radio morphology"],"falsifier":"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.","tokens_in":1684,"feed_emoji":"📡","tokens_out":2212,"duration_ms":87734,"temperature":0.7,"pith_summary":"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.","feed_headline":"First moving parsec-scale jets from a neutron star binary","feed_subtitle":"Circinus X-1's jet axis sweeps at least 110 degrees, pointing to precession.","key_machinery":"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","core_discovery":"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","pith_inferences":["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."],"forward_implications":["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."],"fun_headline_variants":["Neutron star jets seen moving at parsec scale","Cir X-1 jets sweep 110 degrees, hinting precession","First parsec-scale jets from neutron star binary","Galactic jet precession observed in neutron star binary","Jet axis swings 110 degrees in neutron star system"],"cache_read_input_tokens":17920,"weakest_assumption_plain":"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.","fun_headline_variants_meta":{"raw":{"variants":["Neutron star jets seen moving at parsec scale","Cir X-1 jets sweep 110 degrees, hinting precession","First parsec-scale jets from neutron star binary","Galactic jet precession observed in neutron star binary","Jet axis swings 110 degrees in neutron star system"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000168,"raw_usage":{"total_tokens":1126,"prompt_tokens":798,"completion_tokens":328,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":542,"completion_tokens_details":{"reasoning_tokens":250}},"tokens_in":542,"tokens_out":328,"duration_ms":4391,"temperature":1.0,"reasoning_tokens":250,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-04T19:55:11.939102+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"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.","supporting_citations":[],"review_version":1}