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

Journal Article · 29 October 2018 · Monthly Notices of the Royal Astronomical Society

DOI: https://doi.org/10.1093/mnras/sty2932 · OSTI ID:1543975

Fernández, Rodrigo ^[1]; Tchekhovskoy, Alexander ^[2]; Quataert, Eliot ^[3]; Foucart, Francois ^[4]; Kasen, Daniel ^[5]

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1. Univ. of Alberta, Edmonton, AB (Canada). Dept. of Physics
2. Northwestern Univ., Evanston, IL (United States). Center for Interdisciplinary Exploration & Research in Astrophysics (CIERA), Physics & Astronomy
3. Univ. of California, Berkeley, CA (United States). Dept. of Astronomy & Theoretical Astrophysics Center, and Dept. of Physics
4. Univ. of New Hampshire, Durham, NH (United States). Dept. of Physics and Astronomy
5. Univ. of California, Berkeley, CA (United States). Dept. of Astronomy & Theoretical Astrophysics Center, and Dept. of Physics; Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Nuclear Science Division

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 (~9s, or ~6×10⁵r$$_g$$/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 ~10-3M_⊙ 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.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States); Univ. of California, Oakland, CA (United States)
• Sponsoring Organization: Gordon and Betty Moore Foundation (GBMF); National Aeronautics and Space Administration (NASA); Natural Sciences and Engineering Research Council (NSERC) of Canada; USDOE
• Contributing Organization: National Energy Research Scientific Computing Center (NERSC)
• Grant/Contract Number: AC02-05CH11231; SC0017616; SC0018297
• OSTI ID: 1543975
• Journal Information: Monthly Notices of the Royal Astronomical Society, Journal Name: Monthly Notices of the Royal Astronomical Society Journal Issue: 3 Vol. 482; ISSN 0035-8711
• Publisher: Royal Astronomical SocietyCopyright Statement
• Country of Publication: United States
• Language: English