Calibration of the Advanced LIGO detectors for the discovery of the binary black-hole merger GW150914

TL;DR

This paper details the calibration process of Advanced LIGO detectors, ensuring accurate gravitational-wave strain measurement crucial for detecting and analyzing events like GW150914, with uncertainties below 10%.

Contribution

It introduces a comprehensive calibration method for LIGO detectors, quantifying uncertainties and systematic errors for gravitational-wave strain estimation.

Findings

Calibration uncertainty less than 10% in magnitude

Phase uncertainty less than 10 degrees

Validated calibration with 38 days of detector data

Abstract

In Advanced LIGO, detection and astrophysical source parameter estimation of the binary black hole merger GW150914 requires a calibrated estimate of the gravitational-wave strain sensed by the detectors. Producing an estimate from each detector's differential arm length control loop readout signals requires applying time domain filters, which are designed from a frequency domain model of the detector's gravitational-wave response. The gravitational-wave response model is determined by the detector's opto-mechanical response and the properties of its feedback control system. The measurements used to validate the model and characterize its uncertainty are derived primarily from a dedicated photon radiation pressure actuator, with cross-checks provided by optical and radio frequency references. We describe how the gravitational-wave readout signal is calibrated into equivalent gravitational-wave-induced strain and how the statistical uncertainties and systematic errors are assessed. Detector data collected over 38 calendar days, from September 12 to October 20, 2015, contain the event GW150914 and approximately 16 of coincident data used to estimate the event false alarm probability. The calibration uncertainty is less than 10% in magnitude and 10 degrees in phase across the relevant frequency band 20 Hz to 1 kHz.