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 ( u kHz ) QPO with vertical epicyclic motion .
For the Atoll source 4U 1608-52 , the model can match many distinct , simultaneous observations of the HBO and u kHz QPO by varying the inner and outer radius of the torus , while maintaining fixed values for the mass ( M _ { \mathrm { NS } } ) and spin ( a _ { * } ) of the neutron star .
The best fit values are M _ { \mathrm { 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 _ { \mathrm { 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 .