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Longitudinal control signals used to keep gravitational wave detectors at astable operating point are often affected by modulations from test massmisalignments leading to an elevated noise floor ranging from 50 to 500 Hz.Nonstationary noise of this kind results in modulation sidebands and increasesthe number of glitches observed in the calibrated strain data. These artifactsultimately affect the data quality and decrease the efficiency of the dataanalysis pipelines looking for astrophysical signals from continuous waves aswell as the transient events. In this work, we develop a scheme to subtract onesuch bilinear noise from the gravitational wave strain data and demonstrate itat the GEO 600 observatory. We estimate the coupling by making use ofnarrow-band signal injections that are already in place for noise projectionpurposes and construct a coherent bilinear signal by a two-stage systemidentification process. We improve upon the existing filter design techniquesby employing a Bayesian adaptive directed search strategy that optimizes acrossthe several key parameters that affect the accuracy of the estimated model. Thescheme takes into account the possible nonstationarities in the coupling byperiodically updating the involved filter coefficients. The resultingpostoffline subtraction leads to a suppression of modulation sidebands aroundthe calibration lines along with a broadband reduction of the midfrequencynoise floor. The observed increase in the astrophysical range and a reductionin the occurrence of nonastrophysical transients suggest that the above methodis a viable data cleaning technique for current and future generationgravitational wave observatories.

Bilinear noise subtraction at the GEO 600 observatory