{"id":"a75ea8e0-94b1-49c6-b677-44b505f679c8","arxiv_id":"2607.26872","paper_version":1,"verdict":"ACCEPT","confidence":"HIGH","novelty_score":4.0,"correctness_risk":"low","formal_verification":"none","parameter_count":4,"one_line_summary":"Virgo completed dual recycling for O4 but was limited by nearly unstable cavities and unidentified excess noise, yielding 68.9% duty cycle and 53 Mpc BNS range after SR misalignment.","lead":"Advanced Virgo ran in O4 with a newly installed signal-recycling mirror, achieving 68.9% duty cycle and a 53 Mpc binary-neutron-star range. The paper documents why nearly unstable dual recycling forced lower power and intentional signal-recycling misalignment, and what that means for O4 data and the next upgrade.","discovery_kind":"extension","skeptic_critique":{"model":"grok-4.5","headline":"No significant objection identified","rationale":"The Reader correctly treats this as an authoritative collaboration instrumentation paper whose deliverable is the O4 configuration and measured performance, not a first-principles identification of the excess noise. The weakest-assumption flag on the HOM hypothesis is accurate and already caveated by the authors, but it is not load-bearing for the headline metrics. Duty cycle and BNS range are empirical; the mitigation strategy (power cut + ~2 µrad SR misalignment) is what was actually run, regardless of whether HOMs are the ultimate carrier. No adjustment to ACCEPT is warranted.","tokens_in":39790,"tokens_out":393,"duration_ms":7151,"concrete_test":"Recompute the O4b+O4c science-time fraction and the median of the online 84 s BNS-range time series strictly from the public LVK segment lists / range channels cited in §8; confirm they reproduce 68.9% and 53 Mpc to the quoted precision.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper’s central claim is an instrument-status report of measured O4 performance (68.9% science duty cycle, 53 Mpc median BNS range) under dual recycling with intentional SR misalignment and reduced input power. Those headline numbers are direct operational observables, independent of the still-unidentified physical origin of the 50–200 Hz excess noise. Section 5.2–5.3 presents a consistent HOM-carrier hypothesis (DARM-offset scaling, optical-response shape under misalignment, HOM redistribution after the WE swap) and correctly states that the noise source coupling through HOMs “remains unidentified.” That scientific loose end does not undercut the reported duty-cycle or range figures, nor the description of the configuration actually used. No internal inconsistency or unsupported leap is required for the strongest claim to hold.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.5","summary":"This manuscript reports the configuration, commissioning challenges, upgrades, and observational performance of Advanced Virgo during the LIGO–Virgo–KAGRA O4 run (10 April 2024 – 18 November 2025). After installing the signal-recycling mirror and completing the dual-recycled Fabry–Perot Michelson design, the detector was operated with reduced input power (~17 W) and intentional SR misalignment (~2 µrad) to mitigate a dominant broadband excess noise in the 50–200 Hz band associated with carrier higher-order modes resonant in the nearly unstable SRC. Headline results are a 68.9% science duty cycle and an angle-averaged median BNS range of 53 Mpc. The paper documents lock acquisition with ALS, thermal compensation of the central interferometer, automatic alignment (including double-dither SR control), laser-noise coupling control, scattered-light and environmental-noise work, and several successful subsystem upgrades (high-finesse OMC, squeezing path, IMC payload, Newtonian calibrator, electronics) whose net sensitivity impact was limited by the dual-recycling challenges.","tokens_in":39980,"tokens_out":1404,"duration_ms":38412,"significance":"As the definitive instrument-status account of Virgo in O4, the paper is of clear and lasting value to the gravitational-wave community. It supplies the operational configuration actually used for astrophysical analyses (misaligned SR, reduced power, bandwidth ~190 Hz), quantitative duty-cycle and range statistics, a simplified but useful noise budget, measured back-scatter fractions, a squeezing loss budget, and a candid diagnosis of nearly unstable recycling cavities. The consistency tests linking excess noise to SRC-resonant carrier HOMs (DARM-offset scaling, optical-response shape under misalignment, HOM redistribution after the WE swap) are carefully framed, and the residual unidentified physical origin is stated explicitly rather than over-claimed. The documented path to stable recycling cavities for O5 is a concrete outcome of the commissioning experience. Strengths include direct operational observables, cross-reference to the companion optical-parameter paper, and transparent tables (duty cycle, f_sc, squeezing losses).","major_comments":[{"comment":"§5.2–5.3 and Fig. 10: The case that excess noise is carried by SRC-resonant carrier HOMs is consistent and well argued (linear scaling with DARM offset, shape change under SR misalignment matching optical response plus HOM gain, direction change after WE swap). The manuscript correctly states that the physical origin of the noise that couples through HOMs “remains unidentified.” For the status claim this is acceptable, but the paper would be stronger if §5.3 briefly quantified residual alternatives still allowed (e.g., upper bounds on any residual displacement-like or RF-sideband contribution after the tests listed) so that future analyses know what has been closed versus left open.","section":"§5.2–5.3"},{"comment":"§5.1 / Fig. 9: The excess-noise term is introduced as an empirical power law (−0.6 ± 0.1, or “1/f^{0.67}” in the caption text) multiplied by the detector optical response. Clarify in one place whether the quoted slope is fit in strain or in optical power at the dark port, and whether the same index is used for both the aligned and misaligned projections in Fig. 10; a one-sentence statement of the fit band and degrees of freedom would make the simplified budget fully reproducible from the text.","section":"§5.1, Fig. 9"}],"minor_comments":[{"comment":"Abstract and §8: Duty cycle 68.9% and median BNS range 53 Mpc are clear; consider adding the O4b vs O4c_2 split (71.8% / 62.3%) in the abstract or early in §8 so readers see the post-break hardware impact without opening Table 3.","section":"Abstract, §8"},{"comment":"Fig. 9 caption vs body: body uses −0.6 ± 0.1; caption writes 1/f^{0.67}. Harmonize the exponent notation.","section":"Fig. 9"},{"comment":"Table 1: The factor-~7 rise in SDB1 f_sc from pre-O4 to O4 is attributed to a July 2024 waveplate intervention and/or SR misalignment; a short note on whether f_sc was re-measured after subsequent bench work would help.","section":"§6.1.2, Table 1"},{"comment":"§7.2 / Table 2: Squeezing efficiency (41 ± 4)% in the aligned high-frequency band matches the loss budget well; the statement that no useful trade-off exists with SR misalignment is important—consider a single sentence pointing to the BNS-range optimum (~55 Mpc) versus high-frequency shot-noise cost so non-squeezing readers see the global figure of merit.","section":"§7.2"},{"comment":"Many supporting results are cited as “in preparation” (TCS [10], alignment [28], injection [34], calibration [70], etc.). Where a public arXiv or TDS number already exists, adding it would improve citability; otherwise leave as is.","section":"References"},{"comment":"Appendix A: Eq. (4) in the main text and Eq. (17) in the appendix are the same content; a forward reference in §2 to Appendix A is present—ensure symbol r_SRC vs r_SR is consistent throughout.","section":"§2, Appendix A"},{"comment":"Typographical: “inteferometer” (§6.1), “Superattanuator” (§7.6), “guarantees” style inconsistencies in author list footnotes; standard copy-edit pass.","section":null}],"recommendation":"minor_revision","confidential_remarks":"Solid instrument paper; the unidentified excess-noise microphysics is a scientific loose end but is handled honestly and does not undermine the performance claims. Fit for a instrumentation/status article in CQG or equivalent. No novelty or citation-pattern concerns. I would not block on the two major points if the authors prefer only light text additions."},"author_rebuttal":null,"desk_editor":{"model":"grok-4.5","letter":"This is the authoritative O4 Virgo status paper. Headline numbers are direct: 68.9% science duty cycle and 53 Mpc median BNS range over April 2024–November 2025, after finishing the design dual-recycled layout with the SR mirror. Those figures, plus the network coincidence fractions, are what anyone analyzing O4 data or planning O5 needs.\n\nWhat is new is the operational reality of nearly-unstable recycling cavities (g ≈ 0.999988). They document the ALS/CARM-offset lock path, TCS strategy with CH/DAS/CHRoCC, double-dither SR alignment that deliberately holds ~2 µrad misalignment to cut bandwidth from ~430 Hz to ~190 Hz, measured back-scatter fractions after baffling, the high-finesse OMC swap, squeezing loss budget (SRC dominates at 20–25%), Newtonian calibrator as absolute reference, and the WE mirror replacement that shifted the optimal misalignment direction. The simplified noise budget (Fig. 9), HWS curvature fits, and Table 1/2 numbers are concrete and cross-checked against the companion optical-parameter paper.\n\nThe soft spot is real but contained. Section 5 shows the 50–200 Hz excess noise scales with DARM offset and optical response under SR misalignment, and tracks HOM redistribution after the WE swap; they correctly state the physical source that couples through carrier HOMs “remains unidentified.” That is an honest loose end, not a circular claim. It does not invalidate the duty-cycle or range figures, which are external observables. Laser intensity/frequency, polarization, and OMC length were checked and excluded. Free parameters (power ~17 W, misalignment angle/direction, MICH setpoint) are operational choices, not fitted to force the conclusion.\n\nMath and citations look standard for a collaboration instrument paper; self-cites are to prior Virgo work and the optical characterization. No load-bearing contradiction.\n\nThis is required reading for O4 data users and detector engineers. It deserves a serious referee and should be accepted as the O4 Virgo reference. I would cite the performance numbers and the excess-noise phenomenology.","headline":"Solid O4 instrument paper: first dual-recycled Virgo numbers (68.9% duty cycle, 53 Mpc median BNS range) with a clear commissioning narrative; excess-noise origin still open but does not undercut the reported performance.","tokens_in":43131,"tokens_out":551,"would_cite":true,"duration_ms":11628,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["04.80.Nn","95.55.Ym","07.60.Ly"],"model":"grok-4.5","headline":"Advanced Virgo finished its dual-recycled design for O4, ran at 68.9% duty cycle and 53 Mpc BNS range after taming a mid-band excess noise with power cut and intentional signal-recycling misalignment.","keywords":["Advanced Virgo","O4 observing run","dual recycling","signal recycling","excess noise","higher-order modes","binary neutron star range","duty cycle"],"falsifier":"A direct measurement showing that the excess noise amplitude does not track the optical gain of higher-order modes inside the signal-recycling cavity when that cavity’s alignment or Gouy phase is deliberately varied.","tokens_in":40679,"feed_emoji":"📡","tokens_out":854,"duration_ms":17422,"temperature":0.7,"pith_summary":"This paper reports how Advanced Virgo finally installed its signal-recycling mirror and ran the dual-recycled Fabry–Perot Michelson configuration through the LIGO–Virgo–KAGRA fourth observing run (April 2024–November 2025). The nearly unstable recycling cavities forced a cascade of control, thermal-compensation and alignment work-arounds; a dominant broadband excess noise between 50 and 200 Hz then limited sensitivity. The collaboration mitigated that noise by dropping input power to about 17 W and deliberately misaligning the signal-recycling mirror by roughly 2 µrad, which lowered the detector bandwidth from ~430 Hz to ~190 Hz and raised the binary-neutron-star range from ~40 Mpc to a median 53 Mpc at 68.9 % science duty cycle. The account documents both the successful upgrades (high-finesse output mode cleaner, filter-cavity squeezing source, Newtonian calibrator, scattered-light baffles) and the residual limitations that now motivate replacing the recycling cavities with stable ones.","feed_headline":"Virgo ran dual-recycled O4 at 53 Mpc after SR misalignment","feed_subtitle":"Nearly unstable cavities forced a power cut and 2 µrad tilt that tamed mid-band excess noise","key_machinery":"Nearly unstable dual-recycling cavities (g-factor ~0.999988, round-trip Gouy phase ~0.39°), which co-resonate carrier higher-order modes in the signal-recycling cavity and force the power-reduction plus intentional SR-misalignment mitigation that shaped the final O4 noise curve.","core_discovery":"With the signal-recycling mirror installed, Advanced Virgo operated in its design dual-recycled configuration for O4 and achieved a 68.9 % science duty cycle and a median angle-averaged binary-neutron-star range of 53 Mpc after the dominant mid-band excess noise was reduced by lowering input power and intentionally misaligning the signal-recycling mirror.","pith_inferences":["The same nearly-unstable cavity physics that limited Virgo will reappear in any dual-recycled detector whose recycling Gouy phase is kept this small, so cavity redesign is a network-wide lesson.","Once stable cavities restore the full detector bandwidth, the already-commissioned filter cavity should immediately deliver the missing low-frequency quantum-noise reduction.","The Newtonian calibrator’s sub-percent absolute accuracy is now available as a network reference once the other detectors adopt comparable systems."],"forward_implications":["Stable recycling cavities are now the baseline upgrade path because they would remove the HOM co-resonance that drove both the excess noise and the control offsets.","Frequency-dependent squeezing remains ineffective until the intentional SR misalignment can be removed.","Future runs will need faster lock-acquisition sequences once dither-based set-point loops are no longer required.","Scattered-light and environmental couplings improved by the new baffles and seismometers will become limiting only after the mid-band excess noise is cured."],"fun_headline_variants":["Virgo dual-recycled O4 hits 53 Mpc BNS range at 68.9% duty cycle","SR mirror installed: Virgo completes design config for O4 run","Power cut and 2 µrad SR tilt tame Virgo mid-band noise in O4","Advanced Virgo O4: 53 Mpc range after intentional SR misalignment","Virgo O4 dual-recycled run reaches 68.9% uptime and 53 Mpc"],"cache_read_input_tokens":128,"weakest_assumption_plain":"That the still-unidentified broadband excess noise is carried by carrier higher-order modes resonant in the nearly unstable signal-recycling cavity, so that misaligning that mirror correctly suppresses it.","fun_headline_variants_meta":{"raw":{"variants":["Virgo dual-recycled O4 hits 53 Mpc BNS range at 68.9% duty cycle","SR mirror installed: Virgo completes design config for O4 run","Power cut and 2 µrad SR tilt tame Virgo mid-band noise in O4","Advanced Virgo O4: 53 Mpc range after intentional SR misalignment","Virgo O4 dual-recycled run reaches 68.9% uptime and 53 Mpc"]},"model":"grok-4.5","effort":"low","cost_usd":0.003379,"raw_usage":{"total_tokens":1077,"prompt_tokens":662,"num_sources_used":0,"completion_tokens":122,"cost_in_usd_ticks":33788000,"prompt_tokens_details":{"text_tokens":662,"audio_tokens":0,"image_tokens":0,"cached_tokens":256},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":293,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":662,"tokens_out":122,"duration_ms":8706,"temperature":1.0,"reasoning_tokens":293,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-07-30T18:30:49.644746+00:00","model_set":{"reader":"grok-4.5"},"falsifier":"A direct measurement showing that the excess noise amplitude does not track the optical gain of higher-order modes inside the signal-recycling cavity when that cavity’s alignment or Gouy phase is deliberately varied.","supporting_citations":[],"review_version":1}