  We explore the use of the bispectrum for understanding quasiperiodic
oscillations. The bispectrum is a statistic which probes the relations between
the relative phases of the Fourier spectrum at different frequencies. The use
of the bispectrum allows us to break the degeneracies between different models
for time series which produce identical power spectra. We look at data from
several observations of GRS 1915+105 when the source shows strong
quasi-periodic oscillations and strong broadband noise components in its power
spectrum. We show that, despite strong similarities in the power spectrum, the
bispectra can differ strongly. In all cases, there are frequency ranges where
the bicoherence, a measure of nonlinearity, is strong for frequencies involving
the frequency of the quasi-periodic oscillations, indicating that the
quasi-periodic oscillations are coupled to the noise components, rather than
being generated independently. We compare the bicoherences from the data to
simple models, finding some qualitative similarities.
