New generation mechanisms for X-ray quasi-periodic oscillations in accreting neutron stars

Abstract

We propose three mechanisms for the generation of quasi-periodic oscillations (QPOs) in X-ray binaries. Two of them are based on an analogy with nonlinear oscillations of gaseous cavities in a fluid. The first mechanism, called magnetocavitation, implies that X-ray QPOs are produced by radial oscillations of the neutron-star magnetosphere interacting with accreted plasma. The photon-cavitation mechanism is considered when studying X-ray QPOs in neutron stars with critical (Eddington) luminosities. In this case, X-ray QPOs are generated by radial oscillations of photon cavities in the fully ionized hydrogen plasma that settles in the accretion column of a compact object. The mechanism according to which X-ray QPOs result from nonlinear oscillations of current sheets originating in accretion disks is suggested to explain QPOs in X-ray binaries with black holes and in cataclysmic variables. The calculated values of basic physical parameters of QPOs, such as the characteristic frequency, the dependences of QPO frequency and amplitude on X-ray flux, photon energy, and QPO lag time between photons at different energies are in good agreement with observational data.