Impact of viscosity on the perturbations of neutron stars

Viscous dissipation may play an important role in the late inspiral, merger, and ringdown of neutron star binaries. As a step towards determining how important these effects may be, we compute the quasinormal modes and tidal Love numbers of compact stars that are made up of a ``BDNK'' fluid. The BDNK (Bemfica, Disconzi, Noronha, and Kovtun) fluid model is a relativistic generalization of Navier-Stokes theory that is hyperbolic, has stable equilibrium solutions, and incorporates all possible leading-order dissipative effects (bulk viscosity, shear viscosity, and heat conduction), which arguably makes it the most physically realistic model of relativistic viscous fluids currently available. Making use of a covariant framework to consider spherical perturbations, we demonstrate how viscous effects can couple axial (odd-parity) fluid and gravitational perturbations, and discuss how sensitive our results are on different equations of state.