Speaker
Description
In addition to the Keplerian frequency of orbital motion, the four characteristic oscillatory frequencies of the accretion flow are often discussed in the context of models of quasi-periodic oscillations (QPOs) observed in the neutron star (NS) low-mass X-ray binaries. These are, namely, the vertical and radial epicyclic frequencies, and the frequencies that correspond to the periastron (radial) precession and the Lense-Thirring (vertical) precession of the perturbed orbital motion. We examine the influence of quadrupole moment of a slowly rotating NS on the oscillations of the fluid flow in the innermost region of accretion disks. We apply previously developed analytical methods assuming the background of Hartle-Thorne geometry. We provide explicit formulae for the oscillation and precession frequencies but also their simplified practical versions that allow for an expeditious application of the universal relations determining NS properties. We apply our formulae for precession frequencies to different NS configurations and show that difference of the accretion disk precession frequencies for NSs of the same mass and angular momentum but different oblateness can reach tens of percent. Finally, we briefly discuss the relevance of our findings in the context of QPO models.