A method for characterization of coherent backgrounds in real time and its application in gravitational wave data analysis

Many experiments, and in particular gravitational wave detectors, produce continuous streams of data whose frequency representations contain discrete, relatively narrowband coherent features at high amplitude. We discuss the application of digital Fourier transforms (DFTs) to characterization of these features, hereafter frequently referred to as lines. Application of DFTs to continuously produced time domain data are achieved through an algorithm hereafter referred to as EFC for efficient time-domain determination of the Fourier coefficients of a data set. We first define EFC and discuss parameters relating to the algorithm that determine its properties and action on the data. In gravitational wave interferometers, these lines are commonly due to parasitic sources of coherent background interference coupling into the instrument. Using GEO 600 data, we next demonstrate that time domain subtraction of lines can proceed without detrimental effects either on features at frequencies separated from that of the subtracted line, or on features at the frequency of the line but having different stationarity properties.