On accretion disks formed in MHD simulations of black hole-neutron star mergers with accurate microphysics

TL;DR

This paper investigates the properties of accretion disks formed in black hole-neutron star mergers using advanced GRMHD simulations with detailed microphysics, providing insights into magnetic fields, entropy, and jet-launching potential.

Contribution

It offers the first detailed analysis of accretion disks from realistic merger simulations with accurate microphysics and provides analytic fits for magnetic and entropy profiles.

Findings

Magnetic field strength and entropy can be modeled as functions of density.

Disks show continued magnetization and funnel clearing, indicating jet-launching potential.

Simulations evolved up to 350 ms post-merger, revealing persistent magnetic structures.

Abstract

Remnant accretion disks formed in compact object mergers are an important ingredient in the understanding of electromagnetic afterglows of multi-messenger gravitational-wave events. Due to magnetically and neutrino driven winds, a significant fraction of the disk 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 disks, or started from purely hydrodynamical simulations. In this work, we analyse the properties of accretion disks 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 disk 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.