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