2017MNRAS.465.3581V

We analyse all available RXTE data on a sample of 13 low-mass X-ray binaries with known neutron star spin that are not persistent pulsars. We carefully measure the correlations between the centroid frequencies of the quasi-periodic oscillations (QPOs). We compare these correlations to the prediction of the relativistic precession model that, due to frame dragging, a QPO will occur at the Lense-Thirring precession frequency ν_LT of a test-particle orbit whose orbital frequency is the upper kHz QPO frequency ν_u. Contrary to the most prominent previous studies, we find two different oscillations in the range predicted for ν_LT that are simultaneously present over a wide range of ν_u. Additionally, one of the low-frequency noise components evolves into a (third) QPO in the ν_LT range when ν_u exceeds 600 Hz. The frequencies of these QPOs all correlate to ν_u following power laws with indices between 0.4 and 3.3, significantly exceeding the predicted value of 2.0 in 80 per cent of the cases (at 3 to > 20σ). Also, there is no evidence that the neutron star spin frequency affects any of these three QPO frequencies, as would be expected for frame dragging. Finally, the observed QPO frequencies tend to be higher than the ν_LT predicted for reasonable neutron star specific moment of inertia. In the light of recent successes of precession models in black holes, we briefly discuss ways in which such precession can occur in neutron stars at frequencies different from test-particle values and consistent with those observed. A precessing torus geometry and other torques than frame dragging may allow precession to produce the observed frequency correlations, but can only explain one of the three QPOs in the ν_LT range.