Bulk viscosity in neutrino-transparent nuclear matter

In nuclear matter, bulk viscosity arises from a phase lag between an imposed density oscillation and beta reequilibration of the particle content of the nuclear matter, which proceeds via the Urca process. When bulk viscosity is sufficiently strong, which happens when the reequilibration rate nearly matches the frequency of the density oscillation, it can noticeably dampen the oscillation. Consequently, it might be necessary to consider bulk viscosity in neutron star merger simulations, depending on the temperature where bulk viscosity attains its maximum value. Traditionally, the bulk viscosity calculation assumes that only particles near their Fermi surfaces participate in the Urca process (termed the "Fermi surface approximation"). We recalculate the bulk viscosity, computing the Urca rates with a complete phase space integral instead of using the Fermi surface approximation, and find that the temperature at which bulk viscosity is important changes by about 1 MeV compared to the results using the Fermi-surface approximation.