Simulating bulk viscosity in neutron stars II: evolution in spherical symmetry

Emergence of microphysical viscosity in binary neutron star post-merger dynamics

“…This appearance of dissipation also leads to the production of entropy, increasing it on average by ∆s ∼ 0.1k B /baryon compared to a merger without Urca processes (see also [129]). This opens up the interesting prospect of studying these late-time effects in a complementary way using general-relativistic viscous hydrodynamics [130][131][132][133][134][135][136][137][138]. Ideally, one would like to identify a signature for the appearance of dissipative re-equilibration in the gravitational wave signal emitted after the collision.…”

“…In this assumption, the nuclear matter is treated as strongly degenerate [75,140]. The net rate of direct Urca is given by (see also [124,138])…”

Relativistic liquids: GENERIC or EIT?

“…• we have further corroborated the idea that the Israel-Stewart theory represents a universality class of hydrodynamic theories. In fact, the linearised Israel-Stewart theory (or one of its particular realisations) is recovered whenever we take the linear limit of a large class of nonlinear models, including divergence-type theories [1, 81], Carter's theory [18,82], relativistic fluids with chemical or nuclear reactions [17,20,21], and now also the GENERIC theory. • as we can see from equations ( 44) and ( 46), the linearised GENERIC theory is just Maxwell's model for viscoelasticity.…”

“…The interpretation that we adopt here is that Israel-Stewart should be regarded as a quasihydrodynamic theory [10,11]. thermodynamics (EIT) has been rediscovered [12][13][14][15]: the Israel-Stewart field equations constitute a very common (but not universal [11,16]) kind of quasi-equilibrium evolution 4 , which is observed in many different substances [10,15], including superfluids [18], fluid-radiation systems [19] and neutron star matter undergoing nuclear reactions [20,21]. Therefore, we may regard the Israel-Stewart theory as a 'universality class', which encompasses materials having very different microscopic properties, but the same non-equilibrium behaviour at macroscopic scales [22].…”

Modeling Neutron Star Matter in the Age of Multimessenger Astrophysics

“…To meet the challenges posed by the interpretation of upcoming data, theoretical models must be capable of providing a consistent description of both equilibrium and dynamical properties of neutron star matter in the temperature regime corresponding to T = m π -m π ≈ 140 MeV being the pion mass -where nucleons are believed to be the dominant degrees of freedom. In addition to the EOS, these properties include the transport coefficients describing matter viscosity (Benhar & Valli 2007;Camelio et al 2022aCamelio et al , 2022b and heat transfer (Benhar et al 2010), and the neutrino mean free path in the nuclear medium (Lovato et al 2013(Lovato et al , 2014, which play a critical role in the evolution of proto-neutron stars (Camelio et al 2017), as well as in the post-merger phase of neutron star coalescence (Figura et al 2020(Figura et al , 2021. Owing to the complexity and nonperturbative nature of nuclear forces, however, the achievement of the above goal involves nontrivial conceptual and computational issues.…”