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Free keywords:
General Relativity and Quantum Cosmology, gr-qc, Astrophysics, Instrumentation and Methods for Astrophysics, astro-ph.IM, Physics, Instrumentation and Detectors, physics.ins-det
Abstract:
We develop a scheme to subtract off bilinear noise from the gravitational
wave strain data and demonstrate it at the GEO 600 observatory. Modulations
caused by test mass misalignments on longitudinal control signals are observed
to have a broadband effect on the mid-frequency detector sensitivity ranging
from 50 Hz to 500 Hz. We estimate this bilinear coupling by making use of
narrow-band signal injections that are already in place for noise projection
purposes. A coherent bilinear signal is constructed by a two-stage system
identification process where the involved couplings are approximated in terms
of stable rational functions. The time-domain filtering efficiency is observed
to depend upon the system identification process especially when the involved
transfer functions cover a large dynamic range and have multiple resonant
features. We improve upon the existing filter design techniques by employing a
Bayesian adaptive directed search strategy that optimizes across the several
key parameters that affect the accuracy of the estimated model. The resulting
post-offline subtraction leads to a suppression of modulation side-bands around
the calibration lines along with a broadband reduction of the mid-frequency
noise floor. The filter coefficients are updated periodically to account for
any non-stationarities that can arise within the coupling. The observed
increase in the astrophysical range and a reduction in the occurrence of
non-astrophysical transients suggest that the above method is a viable data
cleaning technique for current and future gravitational wave observatories.
