# Gravitational waves from remnant massive neutron stars of binary neutron star merger: Viscous hydrodynamics effects

## TL;DR

This study uses viscous hydrodynamics to analyze how gravitational waves emitted by neutron star merger remnants diminish rapidly due to viscosity, highlighting the importance of magnetohydrodynamics in such models.

## Contribution

It introduces a simplified viscous hydrodynamics model to study the evolution of neutron star merger remnants and their gravitational wave signatures.

## Key findings

- Viscosity reduces differential rotation in the remnant within 5 ms.
- Gravitational wave amplitude decays quickly due to viscous effects.
- Magnetohydrodynamics likely plays a key role in the remnant evolution.

## Abstract

Employing a simplified version of the Israel-Stewart formalism of general-relativistic shear-viscous hydrodynamics, we explore the evolution of a remnant massive neutron star of binary neutron star merger and pay special attention to the resulting gravitational waveforms. We find that for the plausible values of the so-called viscous alpha parameter of the order $10^{-2}$, the degree of the differential rotation in the remnant massive neutron star is significantly reduced in the viscous timescale, $\alt 5$\,ms. Associated with this, the degree of non-axisymmetric deformation is also reduced quickly, and as a consequence, the amplitude of quasi-periodic gravitational waves emitted also decays in the viscous timescale. Our results indicate that for modeling the evolution of the merger remnants of binary neutron stars, we would have to take into account magnetohydrodynamics effects, which in nature could provide the viscous effects.