Modeling general-relativistic plasmas with collisionless moments and dissipative two-fluid magnetohydrodynamics

Abstract

Relativistic plasmas are central to the study of black hole accretion, jet physics, neutron star mergers, and compact object magnetospheres. Despite the need to accurately capture the dynamics of these plasmas and the implications for relativistic transients, their fluid modeling is typically done using a number of (overly) simplifying assumptions, which do not hold in general. This is especially true when the mean free path in the plasma is large compared to the system size, and kinetic effects start to become important. Going beyond common approaches used in the literature, we describe a fully relativistic covariant 14-moment based two-fluid system appropriate for the study of electron-ion or electron-positron plasmas. This generalized Israel-Stewart-like system of equations of motion is obtained directly from the relativistic Boltzmann-Vlasov equation. Crucially, this new formulation can account for non-ideal effects, such as anisotropic pressures and heat fluxes, not present in previous formulations of two-fluid magnetohydrodynamics. We show that a relativistic two-fluid plasma can be recast as a single fluid coupled to electromagnetic fields with (potentially large) out-of-equilibrium corrections. In particular, we keep all electron degrees of freedom, which provide self-consistent evolution equations for electron temperature and momentum. The out-ofequilibrium corrections take the form of a collisional 14-moment closure previously described in the context of viscous single fluids. The equations outlined in this paper are able to capture the full two-fluid character of collisionless plasmas found in black hole accretion and flaring processes around compact objects, as well Braginskii-like two-fluid magnetohydrodynamics applicable to weakly collisional plasmas inside accretion disks. This new formulation will be instrumental in the construction of a large class of next-generation simulations of relativistic transient phenomena produced around black holes and neutron stars.

Electromagnetic precursor flares from the late inspiral of neutron star binaries

“…Meaningful simulations require the use of more accurate Ohm's law closures for the electric field [122]. Future work using novel approaches to relativistic two-fluid systems will be required to improve the global modeling of these transient phenomena [107].…”

“…Yet, we can still provide a meaningful upper bound on the energy available for such processes by computing the amount of energy, which is (numerically) dissipated in the calculations. Future work will be necessary to augment force-free-type simulations with mean-ingful closure prescriptions for electromagnetic energy dissipation [107].…”

“…In this specific model, A30, locally up to 100-times larger than in the orbital current sheet. We emphasize that in the absence of a more realistic Ohm's law prescription [107] the precise numbers should be considered approximate.…”

New developments in relativistic magnetohydrodynamics

“…Recently, a variant of this method was developed and widely adopted to derive the causal second-order hydrodynamics [59,60,165,[190][191][192][193][196][197][198][199][200][201][202][203]. The following discussions closely follow Refs.…”