The Advanced Virgo photon calibrators

As the sensitivities of LIGO, Virgo and KAGRA detectors improve, calibration of the interferometers (ITFs) output is becoming more and more important and may impact scientific results. For the observing run O3, Virgo used for the first time photon calibrators (PCals) to calibrate the ITF, using radiation pressure of a modulated auxiliary laser beam impinging on the Advanced Virgo end mirrors. Those optical devices, also used in LIGO, are now the calibration reference for the global gravitational wave detectors network. The intercalibration of LIGO and Virgo PCals, based on the same absolute reference called the gold standard, has allowed to remove a systematic bias of 3.92% that would have been present in Virgo calibration using the PCal. The uncertainty budget on the PCal-induced displacement of the end mirrors [North end (NE) and West end (WE)] of Advanced Virgo has been estimated to be 1.36% for O3a and 1.40% on NE PCal (resp. 1.74% on WE PCal) for O3b. This uncertainty is the limiting one for the global calibration of Advanced Virgo. It is expected to be reduced below similar to 1% for the next observing runs.

Automated summary

With the increasing sensitivities of LIGO, Virgo, and KAGRA detectors, calibration of interferometers output becomes more important. Virgo used photon calibrators for the first time in observation O3, based on the gold standard to remove systematic bias. The uncertainty of PCal-induced displacement is expected to be further reduced in future observations.