Long-term GRMHD simulations of neutron star merger accretion discs: implications for electromagnetic counterparts

We investigate the long-term evolution of black hole accretion disks formed in neutron star mergers. These disks expel matter that contributes to an $r$-process kilonova, and can produce relativistic jets powering short gamma-ray bursts. Here we report the results of a three-dimensional, general-relativistic magnetohydrodynamic (GRMHD) simulation of such a disk which is evolved for long enough ($\sim 9$s, or $\sim 6\times 10^5 r_{\rm g}/c$) to achieve convergence in mass ejection far from the disk. Our model starts with a poloidal field, and fully resolves the most unstable mode of the magnetorotational instability. We parameterize the dominant microphysics and neutrino cooling effects, and compare with axisymmetric hydrodynamic models with shear viscosity. The GRMHD model ejects mass in two ways: a prompt MHD-mediated outflow and a late-time, thermally-driven wind once the disk becomes advective. The total amount of unbound mass ejected ($0.013M_\odot$, or $\simeq 40\%$ of the initial torus mass) is twice as much as in hydrodynamic models, with higher average velocity ($0.1c$) and a broad electron fraction distribution with a lower average value ($0.16$). Scaling the ejected fractions to a disk mass of $\sim 0.1M_\odot$ can account for the red kilonova from GW170817 but underpredicts the blue component. About $\sim 10^{-3}M_\odot$ of material should undergo neutron freezout and could produce a bright kilonova precursor in the first few hours after the merger. With our idealized initial magnetic field configuration, we obtain a robust jet and sufficient ejecta with Lorentz factor $\sim 1-10$ to (over)produce the non-thermal emission from GW1708107.