Impact of thermal pions on the bulk viscosity of nuclear matter

Weak interactions in the dense nuclear matter in neutron stars give rise to a bulk viscosity which leads to damping of the density oscillations that naturally occur when two neutron stars merge. If nuclear matter consists of just neutrons, protons, and electrons, the Urca processes are the relevent equilibration process, and it has been shown that they can dissipate significant amounts of energy on millisecond timescales. It is expected that in dense matter at the high temperatures encountered in neutron star mergers, a thermal population of pions will be present. If so, then both the Urca processes and various pion decay and production processes give rise to bulk viscosity as they attempt to restore beta equilibrium. We find that the bulk viscosity in matter containing a thermal population of pions is enhanced by up to an order of magnitude compared to the pionless case.