Bulk viscosity of hot neutron-star matter and the maximum rotation rates of neutron stars

The bulk viscosity of neutron-star matter, arising from the time lag in achieving beta equilibrium as the density is changed, is calculated. In the model used in standard cooling calculations, it is found, for the case of normal neutron matter, that the bulk viscosity goes as the sixth power of the temperature (as compared with a $T^{\mathrm{-}2}$ dependence for the shear viscosity), and that at temperatures above ${10}^9$ K the bulk viscosity may dominate the dissipation term which regulates the gravitational-wave instability of rapidly rotating neutron stars. This raises the possibility that in the first years of a neutron-star’s life the star could become unstable as the bulk viscosity decreases through cooling, with potentially observable consequences.