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REVIEW 3 major objections 5 minor 54 references

The Advanced Virgo+ detector's strain data from the O4 observing run are calibrated with residual bias below about 1 percent in amplitude and 20 milliradians in phase most of the time, and with frequency-dependent uncertainties of 2–3 perce

Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →

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T0 review · deepseek-v4-flash

2026-08-01 12:54 UTC pith:QAFOOB5I

load-bearing objection A transparent, well-executed O4 calibration paper; the uncertainty claims are credible, with two disclosed soft spots — the 50 Hz phase uncertainty understatement and the unverified Ncal absolute scale — neither of which sinks the central result. the 3 major comments →

arxiv 2607.19273 v1 pith:QAFOOB5I submitted 2026-07-21 gr-qc

Calibration of the AdvancedVirgo+ Gravitational Wave Detector and Reconstruction of the Detector Strain h(t) during the Observing Run O4

Virgo Collaboration This is my paper
classification gr-qc PACS 04.80.Nn
keywords gravitational wavesdetector calibrationstrain reconstructionAdvanced Virgo+Newtonian calibratorphoton calibratoruncertainty estimationO4 observing run
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper establishes that Virgo's reconstructed gravitational-wave strain for the 2024–2025 O4 observing periods is accurately calibrated: after removing a known frequency-dependent bias, the residual bias stays below about 1 percent in amplitude and 20 milliradians in phase most of the time, and the quoted systematic uncertainties are 2–3 percent in amplitude and below 30 milliradians in phase in the 10–2000 Hz band, apart from larger values near 50 and 150 Hz. It introduces two firsts: the Newtonian calibrator—rotating masses that exert a calculable gravitational pull—is used as the absolute length reference, and the measured strain bias is corrected online so the low-latency data stream is unbiased. The paper concludes that the Virgo calibration uncertainties did not affect the estimation of source parameters for the roughly 173 events detected while Virgo was observing. If right, the publicly released strain data can be used directly for distance measurements, cosmology, and tests of gravity without extra calibration-systematic penalties.

Core claim

The central claim is that the Analysis-Ready strain h(t) delivered for O4b and O4c is an unbiased estimate of the true gravitational-wave strain at the level of roughly 1 percent in modulus and 20 milliradians in phase (most of the time), with frequency-dependent uncertainties of about 2–3 percent in amplitude and below 30 milliradians in phase over 10–2000 Hz, except near 50 Hz and 150 Hz. The calibration chain rests on a Newtonian calibrator that sets the absolute length scale, with the photon calibrators re-scaled to match it—a factor of 1.09 applied to the NE photon calibrator. The photon calibrators then transfer the calibration to the mirror actuators, reaching 0.77 percent modulus pre

What carries the argument

The key mechanism is a calibration-transfer chain. A Newtonian calibrator—rotating masses near a mirror that exert a calculable gravitational force—sets the absolute length scale; photon calibrators, which push mirrors with laser radiation pressure, transfer that scale up to roughly 2 kHz; and a set of permanently injected sine-wave lines continuously measures the ratio of reconstructed to injected strain, h_unbias/h_inj, to quantify residual bias and uncertainty. The ratio-based monitoring detects relative errors but inherits any common-scale error from the absolute reference.

Load-bearing premise

The absolute length scale of Virgo's O4 strain data rests on the Newtonian calibrator's force model; if that model (rotor masses, geometry, or the aluminum-to-PVC rotor change) is off by more than its claimed ~0.12%, the whole strain scale—and every distance inferred from it—shifts without being caught by the paper's ratio-based monitors, because those monitors compare against the same absolute scale.

What would settle it

Compare a loud gravitational-wave event's luminosity distance estimated with Virgo's O4 strain against the distance inferred by an independently calibrated detector in the same network; a common shift larger than the combined quoted uncertainties would reveal an unaccounted absolute-scale error in the Newtonian-calibrator chain. A second, more direct check: install a third independently calibrated Newtonian rotor pair at 36 Hz and test whether the NE/WE calibration constants (currently 1.09 and 1.00) hold.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

  • The publicly released O4 strain data can be used in gravitational-wave analyses with per-frequency calibration uncertainties already incorporated, so no extra systematic penalty is needed.
  • Because the online strain series is corrected for bias in near real time, low-latency searches using the 10-second-delayed data stream do not lose detection efficiency to calibration bias.
  • Source parameter estimation for the O4 events—distances, masses, sky localization—is not degraded by Virgo calibration uncertainties, according to the paper.
  • Frequency-dependent bias and uncertainty channels stored alongside the strain time series allow analysts to apply calibration errors bin by bin rather than as a single global number.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The paper leaves implicit that the Newtonian calibrator's absolute scale is the only anchor for all absolute distances from Virgo O4 events; if that scale carries an unmodeled offset, it propagates uniformly into luminosity distances and Hubble-constant measurements without appearing in the internal consistency checks.
  • A direct extension would be to monitor the Ncal-vs-Pcal ratio over the full run and to use a third independent calibrator at a different frequency; such a cross-check could validate the 1.09/1.00 factors outside the 36 Hz band.
  • The paper notes that the quoted phase uncertainty near 50 Hz is underestimated by about an order of magnitude; whether that is harmless depends entirely on analyses continuing to veto those bands, which could be verified by re-running parameter estimation with a correctly inflated uncertainty.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

3 major / 5 minor

Summary. This paper reports the calibration of the Advanced Virgo+ detector and the reconstruction of the calibrated strain h(t) during the O4b and O4c observing periods. The calibration uses the Newtonian calibrator (Ncal) as the primary length reference for the first time, with the Photon calibrators (Pcal) rescaled accordingly and then used to calibrate the mirror actuators. The paper describes the online and offline ('AnalysisReady') h(t) production, including online bias correction, permanent calibration-line monitoring, broadband injection checks, and frequency-dependent uncertainty estimation. The claims are that the residual bias is mostly below ~1% in modulus and 20 mrad in phase, that the amplitude/phase uncertainties are about 2–3% and below 30 mrad in 10–2000 Hz (except near 50 Hz and 150 Hz), and that Virgo calibration uncertainties did not affect the O4 source-parameter estimation.

Significance. If the claims are correct, this is an important calibration paper: it underpins the public GWOSC O4 Virgo strain data used for parameter estimation, cosmology, and tests of gravity. The paper's strengths include a very long continuous run (O4b+O4c), a clear breakdown of actuator uncertainties (Tables 2–3), permanent in-run monitoring with many cross-checks (Ncal vs Pcal vs mirror actuators; Fig. 2), broadband injection validation (Fig. 17), and the first online bias correction for Virgo. The machine-readable-like frequency-dependent bias and uncertainty channels are a useful product. However, two load-bearing issues need to be addressed: (1) the absolute strain scale is anchored to the Ncal without an independent validation that would catch a common multiplicative scale error in the ratio-based checks; and (2) the paper itself admits that the phase uncertainty around 50 Hz is underestimated in the delivered data product. These issues directly affect the strength of the central claims.

major comments (3)
  1. [§3.2, Tables 2–3, §7.2–7.4, §9] The absolute strain scale is now set by the Ncal, but the uncertainty budget and the validation chain are not independent of that choice. §3.2 states that Ncal was used as the main length reference, that the NE Pcal was corrected by a factor 1.09 (WE: 1.00), and that Pcal uncertainties were retained because Ncal stability 'was not yet assessed.' However, after this rescaling, Tables 2–3 still list 'Pcal calibration 0.5%' as if Pcal were the absolute reference. More importantly, all in-run bias/uncertainty estimates (§7.2–7.4) are based on h_unbias/h_inj, where h_inj is computed from the NE actuator model that inherits the Ncal scale via the corrected Pcal. A multiplicative error in the Ncal force model (e.g., rotor masses/geometry or the aluminum-to-PVC change) cancels in every one of these ratios. The Ncal-vs-Pcal comparison in Fig. 2 constrains only relative drift, and the 'confirmatio
  2. [§7.4, Fig. 17(b), Fig. 19] The paper explicitly acknowledges that the phase uncertainty around 50 Hz is underestimated: §7.4 says 'there is a known inconsistency in this estimate' and that the phase uncertainty should be of order 500 mrad, not the 50 mrad quoted in the text and presumably in the AR frame files. Because the AR files are the public data product and the paper's purpose is to provide reliable frequency-dependent uncertainties, this is not merely cosmetic: a user who does not excise the 49–51 Hz band will severely underestimate the phase error. Please correct the AR frame uncertainty vectors (or clearly flag the band as invalid), and update the abstract/text so that the 50 Hz phase uncertainty is not misleadingly quoted as ~50 mrad.
  3. [§9] The sentence 'For O4, the estimation of the source parameters was not affected by the Virgo calibration uncertainties' is a strong causal claim, but this paper does not report any injection/recovery or parameter-estimation study that varies the calibration within the quoted uncertainties and demonstrates no impact. If this conclusion is based on external GWTC analyses (e.g., refs [47], [54]), please cite the specific studies; otherwise soften the claim to something like 'within the quoted uncertainties, the LVK analyses did not identify a measurable impact.' As written, the statement overreaches what the calibration validation in §7 can establish, especially given the 50 Hz uncertainty issue in the previous comment.
minor comments (5)
  1. [§3.1] The sentence 'In order to frequency band where the interferometer is most sensitive, the SR mirror was added...' appears to be missing a word; likely 'In order to broaden the frequency band where the interferometer is most sensitive...'.
  2. [§3.2] The text gives Ncal uncertainties of 0.17% [24] before the run and 0.12% [25] after the rotor change, but it is not immediately clear whether 0.17% refers to the pre-PVC configuration or to a different estimate. Please clarify the chronology and which uncertainty applies to which Ncal configuration.
  3. [Fig. 2 caption] The caption mentions a 6-mm displacement of the WE mirror that introduced a 0.4% bias in the WE Pcal until May 2025. Please state whether these periods are flagged in data quality or corrected in the AR frames, so that users know whether the 0.4% excursion affects the delivered h(t).
  4. [Eq. (3)] The factor TF_true/TF_pole is explained in the text but not defined in the equation itself. A brief definition or a pointer to §7.1 would help readers who skip the prose.
  5. [References] Several key technical references (e.g., [15], [21], [22]) are listed as 'in preparation' or 'preprint.' For a calibration paper, this is acceptable, but please ensure the versions are public or provide technical notes in the TDS at the time of publication.

Circularity Check

0 steps flagged

No significant circularity; absolute scale is set by external Newtonian/photon calibrators and the ratio-based monitoring is a disclosed self-consistency check.

full rationale

The absolute length scale is anchored externally: the Newtonian calibrator force is computed from rotor masses and geometry via Newtonian gravity, and the photon calibrator applies a calculable radiation pressure; neither depends on the reconstructed strain h(t) (§3.2, §5). The 1.09 NE-Pcal correction is an empirical adjustment of the Pcal to match the Ncal reference, not a fit to h(t). The h_unbias/h_inj monitoring (§7.1, §7.4) is explicitly a relative check: h_inj is built from the same actuator models used in the reconstruction, so a common multiplicative scale error would cancel in the ratio. The paper does not use this ratio alone for the absolute scale; it adds the NE actuator calibration uncertainty (0.77%, traced through the Pcal/Ncal chain) in quadrature. The only by-construction element is that the residual bias measured with NE injections is flat after the bias correction is derived from those same injections; the paper states this openly (§7.3.1) and corroborates with WE injections and broadband injections, which are not forced to unity. The admitted underestimation of the 50 Hz/150 Hz phase uncertainty (§7.4, Fig. 17) and the unassessed Ncal stability (§3.2) are limitations of the uncertainty budget, not circular definitions. The central claim therefore rests on external physical calibrators plus honestly propagated uncertainties; I find no significant circularity.

Axiom & Free-Parameter Ledger

8 free parameters · 8 axioms · 0 invented entities

The central claim rests on standard physics (Newtonian gravity for the Ncal reference force, radiation pressure for the Pcal, the long-wavelength approximation in Eq. 1) and on modeling choices: the stationarity of the interferometer response between reference and target measurements (Eq. 2), the single-pole optical-response approximation with downstream antenna-pattern compensation (Eq. 3), the 10 µs end-mirror delay, and the 8th-order Butterworth sensing model. No invented entities. Free parameters are the fitted actuator models (Table 1), the Ncal-derived Pcal correction factors (1.09/1.00), the tracked optical gain and cavity pole, the interpolated bias-correction function, and the noise-subtraction transfer functions — all disclosed as calibrated or fitted quantities.

free parameters (8)
  • NE mirror actuator model (gain, pole, zero, delay, Table 1) = G=0.434 µm/V; f_p=105.5 Hz; f_z=110.1/6648 Hz; delay −157.6 µs
    Fitted to pre-O4b calibration measurements (Aug 2023-Apr 2024) and used in h(t) reconstruction.
  • WE mirror actuator model (until May 2025) = G=0.409 µm/V; f_p=193.5 Hz; f_z=197.7/5269 Hz; delay −155.3 µs
    Fitted to pre-O4b measurements; used until the WE mirror was replaced.
  • WE mirror actuator model (from June 2025) = G=0.426 µm/V; f_p=137.2 Hz; f_z=139.4/8805.6 Hz; delay −163.4 µs
    Refit after WE mirror replacement during the 2025 commissioning break.
  • NE/WE Pcal correction factors = 1.09 (NE), 1.00 (WE)
    Set so the Pcals match the Ncal reference (§3.2); the 1.09 is a 9% adjustment of the established NE Pcal scale.
  • Optical gain (time-varying) = ~3.2×10^9 W/m (1.8×10^9 during Oct 2025 anomaly)
    Tracked every 4 s from calibration lines (§6.2); a fitted parameter of the optical response model.
  • Double cavity pole frequency (time-varying) = ~180 Hz (195 Hz before 24 Jun 2024)
    Tracked every 4 s; replaces the nominal 55 Hz (no-SR) cavity pole (§6.2).
  • Bias correction function B_raw = interpolated at 0.125 Hz resolution from 27-line injections (up to 4% modulus, 50 mrad phase)
    Fitted correction applied online to remove the raw strain bias (§7.2); updated only a few times during O4.
  • Noise-subtraction transfer functions = two witness channels (LSC MICH 8-200 Hz, LSC SRCL 8-40 Hz), updated every 240 s
    Fitted transfer functions used to subtract correlated noise from h_raw (§6.3).
axioms (8)
  • standard math Long-wavelength approximation: h(t) = ΔL(t)/L0 with L0 = 3 km
    §1 Eq. (1); standard for GW interferometry, the detector strain is the differential arm length over arm length.
  • domain assumption Newtonian calibrator produces a known force from rotating masses (Newtonian gravity)
    §3.2 and refs. [24-26]; the absolute length reference of the O4 calibration.
  • domain assumption Photon calibrator applies known radiation pressure from a modulated laser
    §3.2 and refs. [21,33]; anchor for the 10 Hz-2 kHz chain.
  • domain assumption Interferometer optical response R is identical during reference and target measurement datasets (minutes apart)
    §5 Eq. (2): 'It assumes that the response R of the interferometer is the same during the two datasets.' Verified by repeated measurements but load-bearing.
  • ad hoc to paper Optical response is a single-pole cavity approximation; the resulting bias is compensated later in the data analysis chain via the antenna pattern
    §7.1 Eq. (3) and ref. [43]; the model uses TF_true/TF_pole to inject the same pole bias into h_inj, assuming downstream analyses exactly cancel it.
  • domain assumption End-mirror actuation has an additional 10 µs delay relative to the response to a passing GW
    §7.1 Eq. (3), from refs. [19,43].
  • domain assumption Photodiode sensing chain modeled as 8th-order Butterworth at 10 kHz plus pure delay, accurate to 0.01%/0.4 µs
    §4; verified by LED/PPS timing measurements within 1 µs.
  • ad hoc to paper Line-based bias/uncertainty monitoring at 11 frequencies is interpolable to the full band
    §7.2-7.4; interpolation at 0.125 Hz resolution; the 50 Hz discrepancy shows this interpolation misses narrow-band structure — the paper compensates with broadband injections and enlarged 50/150 Hz uncertainties.

pith-pipeline@v1.3.0-alltime-deepseek · 31316 in / 26900 out tokens · 214895 ms · 2026-08-01T12:54:29.219113+00:00 · methodology

0 comments
read the original abstract

From 10 April 2024 15:00 UTC to 18 November 2025 16:00 UTC, the AdvancedVirgo+ gravitational wave detector participated in the LIGO-Virgo-KAGRA O4 observing run, started on 24 May 2023 15:00 UTC. Around 173 transient gravitational wave (GW) sources, all corresponding to coalescences of binary compact objects involving black holes and neutron stars, were detected online during the two run periods O4b and O4c when Virgo was taking data in the detector network. Despite its sensitivity being limited around 55 Mpc, the inclusion of Virgo into the network allowed to improve the accuracy of the source parameter estimation, in particular the sky localisation of the detected events. This article describes the AdvancedVirgo+ detector calibration and the reconstruction of the detector strain h(t) during O4, as well as the estimation of the associated frequency-dependent uncertainties. The detector calibration is based on auxiliary actuators, Newtonian Calibrators and Photon Calibrators, described in other publications. The h(t) reconstruction, including linear noise subtraction, was processed online with a latency of about 10 s. The so-called AnalysisReady strain data were then produced offline. Most of the time, the strain time series was a copy of the online time series, but with updated frequency-dependent uncertainties, around 2-3% in amplitude and below 30 mrad in phase in the 10-2000 Hz frequency band, with the exception of larger uncertainties around 50 Hz and 150 Hz. The AnalysisReady strain data and associated uncertainties have been used for the offline LIGO-Virgo-KAGRA data analysis and are also the data made publicly available through the Gravitational Wave Open Science Center (GWOSC).

Figures

Figures reproduced from arXiv: 2607.19273 by Virgo Collaboration.

Figure 1
Figure 1. Figure 1: Optical configuration of the Advanced Virgo+ interferometer for the O4 run. The optical configuration of the Advanced Virgo+ detector during O4 is shown in [PITH_FULL_IMAGE:figures/full_fig_p014_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Relative stability of the Ncal and Pcal permanent calibration signals. The center of each bar in these plots is a mean value computed over one day. The vertical width of the bar is the statistical uncertainty for this one-day measurement. On 28 April 2025, to test the impact of the presence of point absorbers (which are small defects or impurities on the mirror that may locally introduce laser beam absorpt… view at source ↗
Figure 3
Figure 3. Figure 3: Modulus and phase of the transfer function between the NE mirror actuator and the NE Pcal measured at the frequency of 98.5 Hz: regular measurements during O4b and O4c, including some post-O4c data. The average values are given with their statistical uncertainties and χ 2/ndf. At this particular frequency, no additional systematic uncertainty is needed [PITH_FULL_IMAGE:figures/full_fig_p021_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Statistical and systematic errors estimated over O4b and O4c for the transfer function from the Pcal to the NE mirror actuator. The errors on the amplitude are expressed in [%] and the errors on the phase are expressed in [mrad]. Left: Statistical errors on the amplitude (red filled squares) of the calibration transfer as a function of frequency and statistical plus systematic errors (red empty circles). R… view at source ↗
Figure 5
Figure 5. Figure 5: Measurements of the modulus and phase of the NE mirror actuator response, averaged over the time period 10 April 2024 (start of O4b) to 18 November 2025 (end of O4c). The superposed red line is the NE actuator response model used for the h(t) reconstruction processing during that same period, that was estimated with pre-O4b data (see [PITH_FULL_IMAGE:figures/full_fig_p022_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Measurement of the modulus and phase of the WE mirror actuator response averaged over the time period 10 April 2024 (start of O4b) to 10 April 2025 (before the WE mirror was replaced). The superposed red line is the WE actuator response model used for the h(t) reconstruction processing during this same period, that was estimated with pre-O4b data (see [PITH_FULL_IMAGE:figures/full_fig_p023_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Principle of the Virgo detector strain reconstruction during the O4 observing run. The first output is the raw channel hraw. A correction for the estimated bias is then applied to obtain the intermediate channel hunbias Calibration lines are subsequently removed and linear noise subtraction is applied to produce the final channel h(t). PDC denotes the dark fringe output power of the interferometer. zCi,m i… view at source ↗
Figure 8
Figure 8. Figure 8: Contributions of the photodiode output signal and of the mirror longitudinal control signals to the raw reconstructed detector strain channel hraw. The pink curve shows the amplitude spectral density of hraw in equivalent watt units (that means after multiplication by the interferometer mean optical response). The black curve shows the amplitude spectral density of the dark fringe signal PDC . The other cu… view at source ↗
Figure 9
Figure 9. Figure 9: Example of an optical response measurement performed during O4b using the NE mirror. The red curve shows a fit based on a model with a high-frequency single pole and a low-frequency double pole accounting the optical-spring. The right panel shows the residual between the measurement and the fit. In this measurement, the effective cavity pole frequency was found around 170 Hz, while the low frequency optica… view at source ↗
Figure 10
Figure 10. Figure 10: Calibration lines visible in the spectrum of in the output dark fringe signal PDC (upper plots) and in the raw reconstructed strain hraw (lower plots). The left plots show the lines around 70 Hz and the right plots shown the lines around 360 Hz. All spectra were computed using data acquired at GPS time 1441932000 (15 September 2025). The calibration lines are strongly suppressed in the reconstructed strai… view at source ↗
Figure 11
Figure 11. Figure 11: Evolution of the double cavity pole frequency (Hz) and optical gain (W/m) of the average interferometer optical response throughout the O4b and O4c runs, as estimated by the detector strain reconstruction algorithm . Both parameters exhibit good overall stability, although several features can be highlighted. On 24 June 2024, the pole frequency decreased from 195 Hz to 180 Hz while the optical gain increa… view at source ↗
Figure 12
Figure 12. Figure 12: Typical Virgo O4 sensitivity curve before (blue) and after (red) noise subtraction: amplitude spectral densities of hraw(t) and h(t) on 20 April 2024, 11h49m42s UTC (GPS=1397649000). The BNS range associated to this sensitivity curve is 56 Mpc. Two witness channels identified before the O4 observing run were used to subtract the associated noise: • Michelson control noise: The motion of the BS mirror crea… view at source ↗
Figure 13
Figure 13. Figure 13: Evolution of the BNS range improvement due to noise subtraction. This is the difference between the BNS range computed with hraw and the BNS range computed with the final h(t) after noise subtraction. The few time periods where this difference is negative correspond to large glitches in the data which perturbed the transfer function used in the noise subtraction procedure. A protection against such effect… view at source ↗
Figure 14
Figure 14. Figure 14: Top: latency (in s) of the online Virgo detector strain reconstruction over the O4 run. The four horizontal bands are linked to the way the reconstruction algorithm works, taking input data by chunks of 4 seconds. On 28 May 2024, an improvement on the data collection chain allowed to gain one second of latency for the availability of the time series at the input of the reconstruction processing. Bottom: V… view at source ↗
Figure 15
Figure 15. Figure 15: Modulus and phase of the hraw(t) bias estimated from weekly injections between 10 September 2024 and 8 October 2024, before the application of the bias correction [PITH_FULL_IMAGE:figures/full_fig_p035_15.png] view at source ↗
Figure 16
Figure 16. Figure 16: Modulus and phase of the hunbias(t) residual bias estimated from weekly injections between 10 September 2024 and 8 October 2024. a timing difference of approximately 4 µs and indicates a difference in the timing calibration of the actuators, consistent with the 3 µs uncertainty estimated for the mirror actuators in [PITH_FULL_IMAGE:figures/full_fig_p035_16.png] view at source ↗
Figure 17
Figure 17. Figure 17: Broadband measurement of transfer function hunbias/hinj performed using the NE actuator on 14 January 2025. Top: modulus, phase and coherence values of the transfer function. Bottom: zoom around 50 Hz. The FFTs are computed over 10 s time windows and only points with coherence above 0.9 are retained. In practice, the distributions of the hunbias/hinj time series at the 11 calibration frequencies are compu… view at source ↗
Figure 18
Figure 18. Figure 18: Evolution of the residual bias of the detector strain data monitored at 207.5 Hz. The modulus average value remains very close to 1, confirming that the initial raw bias (monitored between -4% and -10% over O4) is properly corrected. The dominant variations are of the order of ±2%. Large excursions are caused by transient glitches whose bandwidth overlaps the 207.5 Hz frequency and which temporarily pertu… view at source ↗
Figure 19
Figure 19. Figure 19: Residual bias and uncertainty computed from hunbias/hinj measurements for the period from 10 September to 8 October 2024. The black curve corresponds to the residual bias of the h(t) time series, while the pink shaded region represents the ±1σ uncertainty around the bias. At the beginning of O4b, and later when computing the h(t) final uncertainties, the uncertainty on the NE actuator was estimated to be … view at source ↗
Figure 20
Figure 20. Figure 20: Residual bias and uncertainties of the Virgo detector strain time series (defined as hrec/hinj ) for the different periods of O4b, as provided in the AR frame files. Figures 20 and 21 summarize the information provided in the AR frame files for the different periods considered (nine periods during O4b and six periods during O4c). Most of the time, the residual bias remained below 1% and 20 mrad, except be… view at source ↗
Figure 21
Figure 21. Figure 21: Residual bias and uncertainties of the Virgo detector strain time series (defined as hrec/hinj ) for the different periods of O4c, as provided in the AR frame files. As stated in the text, the uncertainty on the phase around 50 Hz is underestimated and should be of the order of 500 mrad. 9. Conclusion The Virgo detector participated in the LVK O4 observing run during the O4b and O4c peri￾ods, from 10 Apri… view at source ↗

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