Modulating the experimental signature of a stochastic gravitational wave background

Detecting a stationary, stochastic gravitational wave signal is complicated by the impossibility of observing the detector noise independently of the signal. One consequence is that we require at least two detectors to observe the signal, which will be apparent in the cross-correlation of the detector outputs. A corollary is that there remains a systematic error, associated with the possible presence of correlated instrumental noise, in any observation aimed at estimating or limiting a stochastic gravitational wave signal. Here we describe a method of identifying this systematic error by varying the orientation of one of the detectors, leading to separate and independent modulations of the signal and noise contribution to the cross-correlation. Our method can be applied to measurements of a stochastic gravitational wave background by the ALLEGRO-LIGO Livingston Observatory detector pair. We explore—in the context of this detector pair—how this new measurement technique is insensitive to a cross-correlated detector noise component that can confound a conventional measurement.