Speaker
Description
Low-frequency quasi-periodic oscillations (LF QPOs) in neutron star (NS) X-ray binaries are commonly interpreted as manifestations of Lense–Thirring (LT) precession of the inner accretion flow. We utilise the Hartle–Thorne spacetimes to test this hypothesis, employing test particle motion as well as models of geometrically thick accretion tori. Our focus is on the atoll source 4U 1608–52, for which both LF QPO frequencies and the NS spin are well constrained by observations. By comparing the frequencies predicted by LT precession models with those permitted by realistic NS models, we find that parameter combinations capable of reproducing the observed LF QPOs are incompatible with realistic NS configurations. In particular, such configurations imply stellar radii that exceed the innermost stable circular orbit, thereby limiting the maximum achievable precession frequency. Therefore, we conclude that identifying LF QPOs directly with the fundamental LT precession frequency is not viable. Instead, our results suggest that the observed variability may be associated with harmonics of the LT precession frequency, which is consistent with some earlier suggestions.