A physical interpretation of the variability power spectral components in accreting black holes and neutron stars

We propose a physical framework for interpreting the broad band power spectra from black hole and neutron star binaries. We use the truncated disc/hot inner flow geometry, and assume that the hot flow is generically turbulent. Each radius in the hot flow produces fluctuations, and we further assume that these are damped on the viscous frequency. Integrating over radii gives broad band continuum noise power between low and high frequency breaks which are set by the viscous timescale at the outer and inner edge of the hot flow, respectively. Lense-Thirring (vertical) precession of the entire hot flow superimposes the low frequency QPO on this continuum power. We test this model on the power spectra seen in the neutron star systems (atolls) as these have the key advantage that the (upper) kHz QPO most likely independently tracks the truncation radius. These show that this model can give a consistent solution, with the truncation radius decreasing from 20 − 8Rg while the inner radius of the flow remains remarkably constant at 6 Rg. This very constrained geometry still does not quite completely determine the Lense-Thirring precession frequency as this also depends on the radial distribution of mass in the hot flow. Nonetheless, we show that this can be consistent with the low frequency QPO, showing that the broad band power spectra can be used as a diagnostic of accretion flows in strong gravity.