Transport and dissipation in neutron star mergers

Mark Alford (Washington University St. Louis)

Neutron star mergers will provide a unique opportunity for us to gather observational information, especially via gravitational waves, about the behavior of nuclear matter at high density and moderate temperature. Numerical simulations of the merger are an essential tool for exploiting this opportunity. However, up to now such simulations have not included the effects of transport or dissipation. 

To evaluate the likely importance of such effects, we obtained estimates of the equilibration time associated with thermal diffusion, shear viscosity, and bulk viscosity. We find that thermal diffusion and shear viscosity could play a significant role in mergers if neutrinos are trapped and there are gradients on a short distance scale of order 0.1 km. We find that bulk viscosity will be important if direct Urca processes remain suppressed.

Our conclusion is that observations of neutron star mergers have the potential to yield valuable information about previously unmeasured transport and dissipative properties of nuclear matter, and it is important to consider such properties in numerical simulations.