Neutron Star QPOs from Oscillating, Precessing Hot, Thick Flow

Abstract

Across black hole (BH) and neutron star (NS) low-mass X-ray binaries (LMXBs), there appears to be some correlation between certain high- and low-frequency quasi-periodic oscillations (QPOs). In a previous paper, we showed that for BH LMXBs, this could be explained by the simultaneous oscillation and precession of a hot, thick, torus-like corona. In the current work, we extend this idea to NS LMXBs by associating the horizontal branch oscillations (HBO) with precession and the upper-kiloHertz (ukHz) QPO with vertical epicyclic motion. For the Atoll source 4U 1608-52, the model can match many distinct, simultaneous observations of the HBO and ukHz QPO by varying the inner and outer radius of the torus, while maintaining fixed values for the mass (M_{NS}) and spin (a_*) of the neutron star. The best fit values are M_{NS} = 1.38 \pm 0.03 M_\odot and a_* = 0.325 \pm 0.005. By combining these constraints with the measured spin frequency, we are able to obtain an estimate for the moment of inertia of I_{NS} = 1.40 \pm 0.02 \times 10^{45} g cm^2, which places constraints on the equation of state. The model is unable to fit the lower-kHz QPO, but evidence suggests that QPO may be associated with the boundary layer between the accretion flow and the neutron star surface, which is not treated in this work.