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P. Uttley (NASA-GSFC)
Several recent discoveries, notably the linear 'rms-flux relation' (e.g. Uttley 2004) have shown that the aperiodic X-ray variability of both black hole and neutron star X-ray binary systems is non-linear: variations on different time-scales are multiplied together, rather than added, to produce the observed variability. This observational fact can be understood if the aperiodic variability is driven by variations in the accretion flow, which couple together as they propagate towards the X-ray emitting region. Such a picture also lends itself to the interpretation of quasi-periodic oscillations, which may correspond to certain transition regions in the accretion flow. Here we use the methods of the rms-flux relation, and the bicoherence (which quantifies coupling between variations on different time-scales), to examine how QPOs in various X-ray binaries couple to other QPOs, and how they relate to the aperiodic variability. We demonstrate that QPOs do not in general follow the linear rms-flux relations which are universally observed in the aperiodic variability, but show a variety of correlations dependent on the QPO frequency. Despite this difference between QPOs and aperiodic variations, we show that the QPOs are in fact coupled to the aperiodic variability, and are not simply independent of it. Our results demonstrate a new set of diagnostics which can be used to test models for the origins of various QPOs in XRBs.
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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.