On the Origin of Quasi-Periodic Oscillations and Broad-band Noise in   Accreting Neutron Stars and Black Holes
  Accreting neutron stars and black holes share a number of rapid variability characteristics, such as quasi-periodic oscillations and broad-band noise. The frequencies of these features were recently shown to be comparable to fundamental general relativistic frequencies of perturbed test-particle orbits near the compact objects. In this paper we propose a dynamical model for producing modulations in the inner disk properties at these characteristic frequencies. We postulate that a transition radius exists in the accretion disk close to the compact object and investigate its response to a broad spectrum of density fluctuations. We find that the transition radius acts as a low band-pass filter with a broadband response that is constant at frequencies lower than the inverse radial drift timescale and decreases above it as the inverse of the frequency. Moreover, the response shows strong resonances at frequencies nearly equal to the orbital, periastron-precession, and nodal-precession frequencies, thereby accounting for the observed power-density spectra of accreting compact objects and the correlations between their characteristic frequencies. 
Dimitrios Psaltis (Harvard-Smithsonian CfA), Colin Norman (JHU)