The Role of Magnetic Field Geometry in the Evolution of Neutron Star Merger Accretion Discs

Abstract: 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 $E_\mathrm{jet} \sim (4{-}30)\times10^{50}$ erg, isotropic equivalent energies $E_\mathrm{iso}\sim(4{-}20)\times10^{52}$ erg, opening angles $\theta_\mathrm{jet}\sim6{-}13^\circ$, and durations $t_j\lesssim1$ s. Accompanying the production of jets is the ejection of $f_\mathrm{ej}\sim30{-}40\%$ 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 $E_\mathrm{jet}\simeq2\times10^{48}\,\mathrm{erg}$, $E_\mathrm{iso}\sim10^{51}$ erg, $\theta_\mathrm{jet}\sim3.5{-}5^\circ$, and $t_j\sim0.1$ s, is followed by $\gtrsim4$ s of striped jet activity with $f_\mathrm{ej}\simeq27\%$. The dissipation of such stripes could power the short 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 under-predict 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.