Rayleigh-Taylor Gravity Waves and Quasi-periodic Oscillation Phenomena in X-Ray Binaries

Accretion onto compact objects in X-ray binaries (black hole, neutron star [NS], white dwarf) is characterized by nonuniform flow density profiles. Such an effect of heterogeneity in the presence of gravitational forces and pressure gradients causes Rayleigh-Taylor gravity waves. They should be seen as quasi-periodic wave oscillations (QPOs) of the accretion flow in the transition (boundary) layer between the Keplerian disk and the central object. In this paper I show that the main QPO frequency, which is very close to the Keplerian frequency, is split into separate frequencies (hybrid and low branch) under the influence of the gravitational forces in the rotational frame of reference. The observed low and high QPO frequencies are an intrinsic signature of Rossby gravity waves (large-scale fluid motion in the rotational frame of reference). I elaborate the conditions for the density profile for which the gravity-wave oscillations are stable. A comparison of the inferred QPO frequencies with QPO observations is presented. I find that hectohertz frequencies detected from NS binaries can be identified as the low-branch frequencies of the gravity-wave oscillations. I also predict that an observer can see the double NS spin frequency (some sort of the Rossby wave phenomenon) during the NS long (super) burst events when the pressure gradients and buoyant forces are suppressed. The Coriolis force is the only force that acts in the rotational frame of reference, and its presence causes perfect coherent pulsations with a frequency twice that of the NS spin. The QPO observations of NS binaries have established that the high QPO frequencies do not go beyond a certain upper limit. I explain this observational effect as a result of the density profile inversions. In particular, I demonstrate that a particular problem of the gravity waves in the rotational frame of reference in the approximation of very small pressure gradients is reduced to the problem of the classical oscillator in the rotational frame of reference that was previously introduced and applied for the interpretation of the kHz QPO observations in our previous publications. The Rossby type of Rayleigh-Taylor gravity waves must be present, and the related QPOs should be detected in any system where the gravity, buoyancy, and Coriolis force effects cannot be excluded (even in the Earth and solar environments).