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

Abstract

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 threedimensional, general-relativistic magnetohydrodynamic (GRMHD) simulation of such a disk which is evolved for long enough (∼ 9 s, or ∼ 6 × 10 5 r g /c) to achieve completion of 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 , or 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 ∼ 0.1M can account for the red kilonova from GW170817 but underpredicts the blue component. About ∼ 10 −3 M 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 ∼ 1 − 10 to (over)produce the non-thermal emission from GW1708107.

Short GRB 160821B: A Reverse Shock, a Refreshed Shock, and a Well-sampled Kilonova

“…The best fitting model is one represented by a dynamic ejecta mass of ∼ 0.001 M and a secular ejecta mass ∼ 0.01 M . The secular ejecta mass, required for the early blue excess, is consistent with the expectation of the mass-loss from a torus surrounding a massive neutron star (Fujibayashi et al 2018;Fernández et al 2019). However, the best-fit model from our parameter sample under-predicts the observed g-band emission at ∼ 2 and ∼ 4 days post-burst, this is likely due to the finite parameter spacing of the kilonova model samples.…”

Spiral-wave Wind for the Blue Kilonova

“…While alternative interpretations have been proposed, they are either disfavored by current simulations and observations (e.g. jets) [40,42], or require the presence of large-scale strong magnetic fields which might not be formed in the postmerger [35,43,44,75]. We identified a robust dynamical mechanism for mass ejection that might explain observations without requiring any fine-tuning.…”

Unequal mass binary neutron star simulations with neutrino transport: Ejecta and neutrino emission

“…other forms of ejecta that become more important over longer evolution time scales. After merger, magneticallydriven [82,91] and neutrino-driven [92] winds as well as outflows due to viscous angular momentum transport and α-particle recombination [93] should combine to unbind a large fraction of the remnant disk (∼ 40% for disks around black holes in [82]). Based on these results for the long-term evolution of post-merger disk, we can provide order-of-magnitude estimates of the evolution of the remnant produced at the end of our simulations.…”