The role of magnetic field geometry in the evolution of neutron star merger accretion discs

Neutron star mergers are unique laboratories of accretion, ejection, and r-process nucleosynthesis. We used 3D general relativistic magnetohydrodynamic simulations to study the role of the post-merger magnetic geometry in the evolution of merger remnant discs around stationary Kerr black holes. Our simulations fully capture mass accretion, ejection, and jet production, owing to their exceptionally long duration exceeding 4 s. Poloidal post-merger magnetic field configurations produce jets with energies E_jet ∼ (4-30) × 10⁵⁰ erg, isotropic equivalent energies E_iso ∼ (4-20) × 10⁵² erg, opening angles θ_jet ∼ 6-13^◦, and durations t_j ≲ 1 s. Accompanying the production of jets is the ejection of f_ej ∼ 30-40 per cent of the post-merger disc mass, continuing out to times >1 s. We discover that a more natural, purely toroidal post-merger magnetic field geometry generates large-scale poloidal magnetic flux of alternating polarity and striped jets. The first stripe, of E_jet ≃ 2 × 10⁴⁸ erg, E_iso ∼ 10⁵¹ erg, θ_jet ∼ 3.5-5^◦, and t_j ∼ 0.1 s, is followed by ≳4 s of striped jet activity with f_ej ≃ 27 per cent. The dissipation of such stripes could power the short-duration gamma-ray burst (sGRB) prompt emission. Our simulated jet energies and durations span the range of sGRBs. We find that although the blue kilonova component is initially hidden from view by the red component, it expands faster, outruns the red component, and becomes visible to off-axis observers. In comparison to GW 170817/GRB 170817A, our simulations underpredict the mass of the blue relative to red component by a factor of few. Including the dynamical ejecta and neutrino absorption may reduce this tension.