Kilohertz QPOs in Neutron Star Binaries modeled as Keplerian Oscillations in a Rotating Frame of Reference

Since the discovery of kHz quasi-periodic oscillations (QPO) in neutron star binaries, the difference between peak frequencies of two modes in the upper part of the spectrum, i.e. ∆ω = ωh − ωk has been studied extensively. The idea that ∆ω is constant and (as a beat frequency) is related to the rotational frequency of the neutron star has been tested previously. The observed decrease of ∆ω when ωh and ωk increase has weakened the beat frequency interpretation. We put forward a different paradigm: a Keplerian oscillator under the influence of the Coriolis force. For such an oscillator, ωh and the assumed Keplerian frequency ωk hold an upper hybrid frequency relation: ω 2 h − ω k = 4Ω, where Ω is the rotational frequency of the star’s magnetosphere near the equatorial plane. For three sources (Sco X-1, 4U 1608-52 and 4U 1702-429), we demonstrate that the solid body rotation Ω = Ω0 = const. is a good first order approximation. Within the second order approximation, the slow variation of Ω as a function of ωK reveals the structure of the magnetospheric differential rotation. For Sco X-1, the QPO have frequencies ∼ 45 and 90 Hz which we interpret as the 1st and 2nd harmonics of the lower branch of the Keplerian oscillations for the rotator with Ω not aligned with the normal of the disk: ωL/2π = (Ω/π)(ωK/ωh) sin δ where δ is the angle between Ω and the vector normal to the disk. Subject headings: accretion, accretion disks — radiation mechanisms: thermal —stars:neutron — X-rays: general