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 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 10 8 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 β > ∼ 1 MeV at orbital frequencies ν orb > ∼ 200 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 ν orb > ∼ 200 Hz. The spectrum of emitted ν e andν e , including radiation transfer effects, is presented for a range of orbital separations.

Beta Equilibrium under Neutron Star Merger Conditions

“…Some works do the full phase space integration, but use nonrelativistic approximations for the matrix element and nucleon dispersion relations [5,[19][20][21]. The vast majority of calculations use nonrelativistic approximations of the matrix element and the nucleon dispersion relations, together with the Fermi surface approximation [22][23][24][25][26][27][28][29][30][31][32][33][34]. All of these calculations are approximations of the full phase space integration using the fully relativistic formalism.…”

Excitation of f -modes during mergers of spinning binary neutron star

“…For f-modes, the most significant dissipation comes from the GW radiation of the mode itself, with a damping timescale of ∼0.03 s[83], which is much longer than the mode period in the corotating frame. Shear and bulk viscosity due to electron scattering[36], as well as Urca reactions[84], have even more negligible effects on the dynamics. Therefore, we also assume that the background star's spin is unaffected by the tidal interaction (see also Ref [85]…”

Thermal Effects on Tidal Deformability of a Coalescing Binary Neutron Star System

“…Contrariwise, there are some theoretical statements or conjectures that the temperature of the star might even be a few MeV [6][7][8]. Following the very recent study of Arras and Weinberg [9] tides transfer mechanical energy and angular momentum to the star at the expense of the orbit, and then friction within the star converts the mechanical energy into heat. During the inspiral, these effects are potentially detectable as a deviation of the orbital decay rate from the general relativistic point-mass result, or as an electromagnetic precursor if heating ejects the outer layers of the star.…”

“…During the inspiral, these effects are potentially detectable as a deviation of the orbital decay rate from the general relativistic point-mass result, or as an electromagnetic precursor if heating ejects the outer layers of the star. Different treatments have been used to estimate the transfer of energy and the size of the tidal friction, leading to different conclusions about the importance of the pre-merger tidal effects [9].…”

“…[10][11][12][13][14][15]. Recently, Arras and Weinberg [9] studied the impact of Urca reactions driven by tidally induced fluid motion during binary neutron star inspiral [9].…”