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