Urca reactions during neutron star inspiral

Abstract

We study the impact of nonlinear bulk viscosity due to Urca reactions driven by tidally-induced fluid motion during binary neutron star inspiral. Fluid compression is computed for low radial order oscillation modes through an adiabatic, time-dependent solution of the mode amplitudes. Optically thin neutrino emission and heating rates are then computed from this adiabatic fluid motion. Calculations use direct and modified Urca reactions operating in a $M=1.4\, M_\odot$ neutron star, which is constructed using the Skyrme Rs equation of state. We find that the energy pumped into low order oscillation modes is not efficiently thermalized even by direct Urca reactions, with core temperatures reaching only $T \simeq 10^8\, {\rm K}$ during the inspiral. Although this is an order of magnitude larger than the heating due to shear viscosity considered by previous studies, it reinforces the result that the stars are quite cold at merger. Upon excitation of the lowest order g-mode, the chemical potential imbalance reaches $\beta > 1\, {\rm MeV}$ at orbital frequencies $\nu_{\rm orb} > 200\, {\rm Hz}$, implying significant charged-current optical depths and Fermi blocking. To asses the importance of neutrino degeneracy effects, the neutrino transfer equation is solved in the static approximation for the three-dimensional density distribution, and the reaction rates are then computed including Fermi-blocking. We find that the heating rate is suppressed by factors of a few for $\nu_{\rm orb} > 200\, {\rm Hz}$. The spectrum of emitted $\nu_e$ and $\bar{\nu}_e$, including radiation transfer effects, is presented for a range of orbital separations.