2019ApJ...873..119B

We carry out the first robust numerical simulation of accretion flows on a weakly magnetized neutron star using smoothed particle hydrodynamics (SPH). We follow the two-component advective flow (TCAF) paradigm for black holes and focus only on the advective component for the case of a neutron star. This low-viscosity sub-Keplerian flow will create a Normal Boundary Layer (NBOL) right on the star surface in addition to the CENtrifugal pressure supported BOundary Layer (CENBOL) present in a black hole accretion. These density jumps could give rise to standing or oscillating shock fronts. During a hard spectral state, the incoming flow has a negligible viscosity, causing a larger sub-Keplerian component as compared to the Keplerian disk component. We show that our simulation of flows with cooling and a negligible viscosity produces precisely two shocks and strong supersonic winds from these boundary layers. We find that the specific angular momentum of matter dictates the locations and the nature of oscillations of these shocks. For flows with low angular momentum, the radial oscillation appears to be preferred. For flows with higher angular momentum, the vertical oscillation appears to become dominant. In all cases, asymmetries with respect to the Z = 0 plane are seen, and instabilities set in due to the interaction of the inflow and outgoing strong winds. Our results capture both the low- and high-frequency quasi-periodic oscillations without invoking magnetic fields or any precession mechanism. Most importantly, these solutions directly corroborate observed features of wind-dominated high-mass X-ray binaries, such as Cir X-1.