# Timing Properties of Shocked Accretion Flows around Neutron Stars in Presence of Cooling

## TL;DR

This study uses numerical simulations to analyze shock formation and oscillations in accretion flows onto neutron stars, revealing how boundary layers and angular momentum influence observed X-ray variability.

## Contribution

First robust SPH simulation of accretion flows onto neutron stars focusing on boundary layers and shock oscillations without magnetic fields.

## Key findings

- Two shocks form in the flow with cooling and low viscosity.
- Shock oscillations depend on angular momentum, with radial or vertical dominance.
- Simulations reproduce observed QPOs and wind features in X-ray binaries.

## Abstract

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 (or, NBOL) right on the star surface in addition to the centrifugal pressure supported boundary layer (or, 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 more sub-Keplerian component as compared to the Keplerian disc component. We show that our simulation of flows with a 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 low angular momentum flows, the radial oscillation appears to be preferred. For flows with higher angular momentum, the vertical oscillation appears to become dominant. In all the cases, asymmetries w.r.t. the Z=0 plane are seen and instabilities set in due to the interaction of 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.