Discriminating a gravitational-wave background from instrumental noise using time-delay interferometry

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Abstract
The first in-flight Doppler tracking measurements performed by LISA will be used to assess its detector performance. The existence of a strong stochastic background of gravitational radiation from many close binary systems in our Galaxy will prevent us from identifying the interferometer noise level in the frequency band 0.1-8 mHz. This in turn will prevent us from making an unambiguous detection of the background itself. By implementing the proposed multiple Doppler readouts, it is possible to generate several observables that are independent of laser phase fluctuations, and at the same time have different couplings to gravitational waves and to the various LISA instrumental noises. Comparison, for example, of the Michelson interferometer observable with the fully symmetric Sagnac data-type allows discrimination between a confusion-limited gravitational-wave background and instrumental noise.