Coupling between QPOs and broadband noise components in GRS 1915+105

TL;DR

This paper uses bispectrum analysis to reveal the coupling between quasi-periodic oscillations and broadband noise in GRS 1915+105, providing insights beyond traditional power spectrum analysis.

Contribution

It demonstrates that bispectrum analysis can distinguish between models and shows coupling between QPOs and noise, which is not evident from power spectra alone.

Findings

Bispectrum reveals differences in QPO and noise coupling.

QPOs are coupled to broadband noise components.

Bispectrum analysis distinguishes models with similar power spectra.

Abstract

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.