Coherent Network Detection of Gravitational Waves: The Redundancy Veto

A network of gravitational wave detectors is called redundant if, given the direction to a source, the strain induced by a gravitational wave in one or more of the detectors can be fully expressed in terms of the strain induced in others in the network. Because gravitational waves have only two polarizations, any network of three or more differently oriented interferometers with similar observing bands is redundant. For LISA, this ``null'' output is known as the Sagnac mode, and its use in discriminating between detector noise and a cosmological gravitational wave background is well understood [1][2]. But the usefulness of the null veto for ground-based detector networks has been ignored until now. We show that it should make it possible to discriminate in a model-independent way between real gravitational waves and accidentally coincident non-Gaussian noise ``events'' in redundant networks of two or more broadband detectors (e.g, three LIGO detectors and GEO). It has been shown that with three detectors, the null output can be used to locate the direction to the source, and then two other linear combinations of detector outputs give the optimal ``coherent'' reconstruction of the two polarization components of the signal. We discuss briefly the implementation of such a detection strategy in realistic networks.