# Timing Properties of Shocked Accretion Flows around Neutron Stars -- II. Viscous Disks and Boundary Layers

## TL;DR

This study uses simulations to explore how viscosity affects accretion flow structures around neutron stars, revealing the formation of boundary layers and disks, and their connection to matter ejection.

## Contribution

It presents the first self-consistent simulation of transitions in sub-Keplerian flows around neutron stars, including boundary layer and disk formation influenced by viscosity.

## Key findings

- Formation of a single NBOL with small viscosity.
- Development of RAKED with increased viscosity.
- Presence of CENBOL similar to black hole systems.

## Abstract

We use Smoothed Particle Hydrodynamics to study viscous accretion flows around a weakly magnetic neutron star. We show the formation of multiple ``boundary" layers in presence of both cooling and viscosity. We find that with the introduction of a small viscosity in a sub-Keplerian flow, much like the wind accretion in HMXBs such as Cir X-1, only a single Normal Boundary Layer (NBOL) forms to adjust the rotational velocity component. With the increase of viscosity, the region extends radially and beyond some critical value, a RAdiative KEplerian Disk/layer (RAKED) forms between the sub-Keplerian flow and the NBOL. When viscosity is increased further only NBOL and RAKED remain. In all such cases, the CENtrifugal pressure dominated BOundary Layer (CENBOL) is formed, away from the star, as in the case of black holes. This is the first self-consistent study where such a transition from sub-Keplerian flows has been reported for neutron stars. We also identify the connection between accretion and ejection of matter, following the Two-Component Advective Flow for black holes, for neutron stars. The results are crucial in the understanding of the formation of disks, boundary layers and outflows in wind dominated neutron star systems.