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Abstract: We propose a physical framework for interpreting the characteristicfrequencies seen in the broad band power spectra from black hole and neutronstar binaries. We use the truncated disc-hot inner flow geometry, and assumethat the hot flow is generically turbulent. Each radius in the hot flowproduces fluctuations, and we further assume that these are damped on theviscous frequency. Integrating over radii gives broad band continuum noisepower between low and high frequency breaks which are set by the viscoustimescale at the outer and inner edge of the hot flow, respectively.Lense-Thirring vertical precession of the entire hot flow superimposes thelow frequency QPO on this continuum power.We test this model on the power spectra seen in the neutron star systemsatolls as these have the key advantage that the upper kHz QPO most likelyindependently tracks the truncation radius. These show that this model can givea consistent solution, with the truncation radius decreasing from 20-8 Rg whilethe inner radius of the flow remains approximately constant at ~4.5 Rg i.e. 9.2km. We use this very constrained geometry to predict the low frequency QPO fromLense-Thirring precession of the entire hot flow from r o to r i. The simplestassumption of a constant surface density in the hot flow matches the observedQPO frequency to within 25 per cent. This match can be made even better byconsidering that the surface density should become increasingly centrallyconcentrated as the flow collapses into an optically thick boundary layerduring the spectral transition. The success of the model opens up the way touse the broad band power spectra as a diagnostic of accretion flows in stronggravity.



Autor: Adam Ingram, Chris Done

Fuente: https://arxiv.org/



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