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T. Belloni (Osservatorio Astronomico di Brera, Merate, Italy)
Recently, a number of authors have pointed out that there are correlations linking the characteristic frequencies of power density spectra of BH and NS systems. However, the actual shapes of the noise components in different classes of systems seem to be rather different. Here I discuss the modeling of Power Density Spectra of BHCs and NS LMXBs. I show that their main features can be described as the sum of a number of components, whose parameters are different for different classes of sources. With this decomposition, no scale-invariant power law component is needed in the PSDs of BHCs. Remarkably, the model adopted is the same for both classes of sources, the differences being only in the values of the parameters.
The characteristic frequencies associated to this model follow global correlations spanning a wide range, from below the Hz up to the kHz region. I present and discuss the current status of these correlations, originally presented by Belloni & Hasinger (1990, A&A 227, L33), Wijnands & van der Klis (1999, ApJ, 514, 939) and Psaltis, Belloni & van der Klis (1999, ApJ, 520, 262) including recent results from different authors. In particular, I discuss the evidence for a correlation linking the upper kHz QPOs with high-frequency features in BHCs and low-luminosity NS systems. The picture that results from this analysis points towards a common physical origin of the observed variability, regardless of the nature of the compact object in the system. I discuss these results in terms of current theoretical models for aperiodic variability of X-ray binaries.
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