PREPRINT
098D3FBD-B805-4723-B120-DA887FD05A8F

# Improving LIGO calibration accuracy by using time-dependent filters to compensate for temporal variations

The response of the Advanced LIGO interferometers is known to vary with time [arXiv:1608.05134]. Accurate calibration of the interferometers must therefore track and compensate for temporal variations in calibration model parameters. These variations were tracked during the first three Advanced LIGO observing runs, and compensation for some of them has been implemented in the calibration procedure. During the second observing run, multiplicative corrections to the interferometer response were applied while producing calibrated strain data both in real-time and in high-latency. In a high-latency calibration produced after the second observing run and during the entirety of the third observing run, a correction involving updating filters was applied to the calibration -- the time dependence of the coupled cavity pole frequency ${f}_{\mathrm{c}\mathrm{c}}$. Methods have now been developed to compensate for variations in the interferometer response requiring time-dependent filters, including variable zeros, poles, gains, and time delays. Compensation for well-modeled time dependence of the interferometer response has helped to reduce systematic errors in the calibration to $<2$% in magnitude and $<{2}^{\circ }$ in phase across LIGO's most sensitive frequency band of 20 - 2000 Hz [arXiv:2005.02531, arXiv:2107.00129]. Additionally, such compensation was shown to reduce uncertainty and bias in the sky localization for a simulated binary neutron star merger.