Far-from-equilibrium bulk-viscous transport coefficients in neutron star mergers

Emergence of Microphysical Bulk Viscosity in Binary Neutron Star Postmerger Dynamics

“…In the light of the relation between chemical reactions and bulk viscous processes discussed in Gavassino & Noronha (2023), one may say that SFHo matter in this regime will be in a resummed (i.e., amplitude-dependent) Navier-Stokes-like limit (Yang et al 2024), allowing us to extract effective bulk viscosities with magnitude ζ eff  10 28 g (cm s) -1 directly from the simulation by computing |ζ eff | ≈ |Π/θ| ≈ |Π/∇ i u i |. We point out that these viscosities can also be calculated precisely in the far-from-equilibrium limit using the same microphysical reactions employed in this work (Yang et al 2024). This appearance of dissipation also leads to the production of entropy, increasing it, on average, by Δs ∼ 0.1k B baryon -1 compared to a merger without Urca processes (see also Most et al 2023).…”

“…We note that, in the far-from-equilibrium regime, Π is not a linear function of δμ, and nonlinear corrections to the equivalence between chemical imbalance and bulk viscosity have to be taken into account(Gavassino & Noronha 2023;Yang et al 2024). …”

“…where z t =  I 0 0 1 0 is the static bulk viscosity coefficient induced by the weak interactions (Sawyer 1989;Sa'd & Schaffner-Bielich 2009;Harris 2020). For periodic density oscillations with frequency ω, δμ and Π also oscillate, and Equation (5) defines an alternating current (AC) bulk viscosity z w Yang et al 2024), which reaches a resonant maximum when ω = 1/τ 0 (see, e.g., Harris 2020). Equation ( 5) is a typical Israel-Stewart-like (Israel & Stewart 1979) equation of motion for the bulk scalar (apart from the source term; Gavassino et al 2021), which is commonly used in the description of the quark-gluon plasma formed in heavy-ion collisions (Romatschke & Romatschke 2019; where ζ 0 and τ 0 are determined by the strong, not the weak, interactions).…”