Speaker
Description
The vertical (Lense–Thirring) precession of the innermost accretion flows has been suggested as the cause of a particular type of low-frequency quasi-periodic variability (LF QPOs) observed in accreting neutron stars (NSs). In this context, LF QPOs have been discussed as a sensitive potential indicator of NS rotational properties and their equation of state, since the precession frequency vanishes for a non-rotating star. However, no observational evidence showing a clear trend of increasing QPO frequencies with increasing NS spin across a group of sources was found, which may falsify the model. We examined the expected precession frequencies within the innermost accretion region in detail. Taking the effects of stellar oblateness into account, we found that the precession frequency only increases within a short interval of increasing spin; then it drops and only increases again for even higher spins. We conclude that very different groups of accreting NSs — slow and fast rotators — can display the same precession frequencies. Therefore, the lack of observational evidence for a correlation between QPO frequencies and NS spin does not contradict their precession origin.