Hydrodynamic Effects on the QPO Frequencies of Accreting Compact Objects
  The variability properties of accreting compact objects predicted by dynamical models are characterized by a number of distinct frequencies that are specific to each model. Because the accretion disks around neutron stars and black holes are hydrodynamic flows, these modulation frequencies cannot be strictly equal to test particle frequencies. I discuss several implications of hydrodynamic corrections to the modulation frequencies predicted by dynamical models. Finally, I show that the recent detection of yet a third kHz QPO in three neutron-star systems favors a previously developed model that attributes the various QPO frequencies to fundamental general relativistic frequencies in the accretion flow. 
Dimitrios Psaltis (MIT)