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From the Virgo interferometer calibration to the bias and uncertainty of the h(t) detector strain during the O4 run

Cervane Grimaud, Florian Aubin, Benoît Mours, Thierry Pradier, Loïc Rolland, Monica Seglar-Arroyo, Hans Van Haevermaet, Pierre Van Hove, Didier Verkindt

TL;DR

The paper addresses precise calibration of the Virgo interferometer to reliably reconstruct the $h(t)$ gravitational-wave strain during the O4 run. It combines Newtonian and Photon Calibrators to establish and transfer mirror-displacement references, achieving sub-percent accuracy in the 10 Hz–2 kHz band and enabling online unbiasing of $h(t)$ via a dedicated, frequency-dependent bias and uncertainty framework. The core contribution is a novel method that uses weekly and permanent line injections to compute, online, a bias and an uncertainty that are propagated into the reconstructed strain, yielding improved, frequency-dependent confidence in the $h(t)$ signal. The practical impact is a latency-controlled, online-unbiased $h(t)$ stream with quantifiable, frequency-dependent uncertainties, supporting LVK low-latency analyses and monthly offline updates to track and limit reconstruction biases over time.

Abstract

Since the first gravitational wave detection in 2015, ground-based interferometer sensitivities have significantly improved, requiring highly precise calibration to ensure accurate reconstruction of the h(t) strain signal. In this talk we will outline the Virgo interferometer calibration steps performed in preparation of the O4b run started in April 2024. We will first describe the Photon Calibrator power devices intercalibration allowing for a 0.48% precision on mirror displacement. Before explaining how the Photon Calibrator is used to calibrate every Virgo mirror actuators. We will also discuss the monitoring of the h(t) strain reconstruction during the run showing that, on the 10 Hz to 2 kHz band, the reconstructed strain achieves a precision of 2% in modulus and 30 mrad in phase. Special emphasis will be given on the newly developed frequency-dependent bias and uncertainty computation method and the resulting online unbiasing of the h(t) strain.

From the Virgo interferometer calibration to the bias and uncertainty of the h(t) detector strain during the O4 run

TL;DR

The paper addresses precise calibration of the Virgo interferometer to reliably reconstruct the gravitational-wave strain during the O4 run. It combines Newtonian and Photon Calibrators to establish and transfer mirror-displacement references, achieving sub-percent accuracy in the 10 Hz–2 kHz band and enabling online unbiasing of via a dedicated, frequency-dependent bias and uncertainty framework. The core contribution is a novel method that uses weekly and permanent line injections to compute, online, a bias and an uncertainty that are propagated into the reconstructed strain, yielding improved, frequency-dependent confidence in the signal. The practical impact is a latency-controlled, online-unbiased stream with quantifiable, frequency-dependent uncertainties, supporting LVK low-latency analyses and monthly offline updates to track and limit reconstruction biases over time.

Abstract

Since the first gravitational wave detection in 2015, ground-based interferometer sensitivities have significantly improved, requiring highly precise calibration to ensure accurate reconstruction of the h(t) strain signal. In this talk we will outline the Virgo interferometer calibration steps performed in preparation of the O4b run started in April 2024. We will first describe the Photon Calibrator power devices intercalibration allowing for a 0.48% precision on mirror displacement. Before explaining how the Photon Calibrator is used to calibrate every Virgo mirror actuators. We will also discuss the monitoring of the h(t) strain reconstruction during the run showing that, on the 10 Hz to 2 kHz band, the reconstructed strain achieves a precision of 2% in modulus and 30 mrad in phase. Special emphasis will be given on the newly developed frequency-dependent bias and uncertainty computation method and the resulting online unbiasing of the h(t) strain.

Paper Structure

This paper contains 4 sections, 2 equations, 2 figures.

Table of Contents

  1. Virgo calibration
  2. h(t) detector strain reconstruction principle
  3. h(t) reconstruction bias and uncertainty computation method
  4. Bias and uncertainty results during the O4 run

Figures (2)

  • Figure 1: Comparison between the $h_{raw}/h_{inj}$ measurements performed using an average of the weekly lines data from the 28th of January to the 25th of February 2025 and the $h_{unbias}/h_{inj}$ measurements performed with the same average period.
  • Figure 2: Result of the reconstruction residual bias and uncertainty for the period 2024-05-14 to 2024-06-18. The residual bias is computed using the weekly lines and the uncertainty is computed using the permanent lines.