On accretion discs formed in MHD simulations of black hole–neutron star mergers with accurate microphysics

Abstract

\n Remnant accretion discs formed in compact object mergers are an important ingredient in the understanding of electromagnetic afterglows of multimessenger gravitational-wave events. Due to magnetically and neutrino-driven winds, a significant fraction of the disc mass will eventually become unbound and undergo r-process nucleosynthesis. While this process has been studied in some detail, previous studies have typically used approximate initial conditions for the accretion discs, or started from purely hydrodynamical simulations. In this work, we analyse the properties of accretion discs formed from near equal-mass black hole–neutron star mergers simulated in general-relativistic magnetohydrodynamics in dynamical spacetimes with an accurate microphysical description. The post-merger systems were evolved until $120\\, {\\rm ms}$ for different finite-temperature equations of state and black hole spins. We present a detailed analysis of the fluid properties and of the magnetic-field topology. In particular, we provide analytic fits of the magnetic-field strength and specific entropy as a function of the rest-mass density, which can be used for the construction of equilibrium disc models. Finally, we evolve one of the systems for a total of $350\\, \\rm ms$ after merger and study the prospect for eventual jet launching. While our simulations do not reach this stage, we find clear evidence of continued funnel magnetization and clearing, a prerequisite for any jet-launching mechanism.