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

TL;DR

This paper details the calibration process of the Virgo interferometer for the O4b run, focusing on precision calibration methods, bias and uncertainty estimation, and the resulting accuracy of the reconstructed gravitational wave strain h(t).

Contribution

It introduces a new frequency-dependent bias and uncertainty computation method for real-time unbiasing of the h(t) strain during gravitational wave detection.

Findings

Calibration precision achieved: 0.48% on mirror displacement

Strain reconstruction precision: 2% in modulus, 30 mrad in phase

Effective online bias correction improves strain accuracy

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.