Timing spectroscopy of quasi-periodic oscillations in the low-mass X-ray neutron star binaries

Abstract

The problem of the determination of the physical parameters for celestial bodies has always been of fundamental importance in Astronomy. Study of accreting neutron stars and black holes can shed light on the physics of the strong gravity and dense matter. Millisecond variability related to high compactness of these objects naturally appears in the process of accretion of matter onto a stellarmass compact object. So it is extremely important to learn how to extract information regarding the main physical parameters, particularly, strength and structure of magnetic field, inclination angle between the rotational and the magnetic axis of the star, masses, radii and distances, from millisecond oscillations, i.e. quasi-periodic oscillations (QPOs) frequencies and continuum X-ray spectra. Unfortunately very often the interpretation of the observational results of millisecond variations in X-ray binaries depends on untested, or even unknown properties of spacetime. matter and resonance processes in the presence of strong gravity and magnetic fields. Our objective is to develop models for neutron star (NS) (and for black hole (BH)) and to apply such models to extract direct information regarding the physical parameters from QPOs power-frequencies characteristics. For the NSs we will complete the classification of the QPOs based on the recently developed Two-Oscillator (TO) model (Osherovich V. A. and Titarchuk L. G., Astrophys. J., 522 (1999) L113: Titarchuk L. G. and Osherovich V. A., Astrophys. J., 518 (1999) L95). The new model covers the range of frequencies from 1 Hz to 1200 Hz. It suggests the coexistence of viscous radial oscillations of the innermost part of the rotating disk with the diffusive process which involves also interaction with the normal disk mode (global mode of oscillations of the entire disk). In the outer (Keplerian) part of the disk the Two-Oscillator model interprets the twin peaks kHz QPOs as the Keplerian frequency and the upper hybrid frequency of the Keplerian oscillator under the influence of the Coriolis force in the differentially rotating NS magnetosphere. The developed TO model predicts that a certain combination of the observed kHz QPO frequencies and Horizontal Branch frequencies (HBO) is constant for a particular.