Speaker
Description
Quasi-periodic oscillations (QPOs) in the X-ray variability of accreting neutron stars provide key insights into accretion in strong gravity and the equation of state of dense matter (EoS). We discuss the possible precession origin of twin-peak QPOs, focusing particularly on two specific models. The first is the relativistic precession model, which links QPO frequencies to the frequencies of general relativistic test particle motion near the innermost stable circular orbit. The second model makes the same QPO frequency assignment but considers the precession of a critically thick fluid accretion flow rather than particle precession. Despite being based on different physical assumptions, both models share the same set of governing equations, which are expressed in opposite limits. Consequently, they have an equal number of free parameters, which facilitates comparison with observations. The fluid precession model clearly fits the data better than the geodesic model and implies lower NS masses. Using the CompOSE library, we demonstrate that, in contrast to the geodesic model, the fluid model is well compatible with a wide range of over 130 NS EoS. However, this range narrows if low-frequency QPOs are interpreted as vertical precession of the flow.