On the Jet–Ejecta Interaction in 3D GRMHD Simulations of a Binary Neutron Star Merger Aftermath

Short $\gamma$-ray burst (sGRB) jets form in the aftermath of a neutron star merger, drill through disk winds and dynamical ejecta, extend over four to five orders magnitude distance before breaking out ejecta. We present first 3D general-relativistic magnetohydrodynamic sGRB simulations span this enormous scale separation. They feature three possible outcomes: jet+cocoon, cocoon, neither. Typical break ejecta if (i) bound ejecta's isotropic equivalent mass along pole at time BH formation is $ \lesssim10^{-4}~{\rm M_{\odot}} $, setting limit on delay between merger formation, otherwise, perish inside leave jet-inflated cocoon power low-luminosity sGRB; (ii) post-merger remnant contains strong large-scale vertical magnetic field, $\gtrsim10^{15}$ G; (iii) are weak ($\lesssim10^{50}$ erg), must be small ($\lesssim10^{-2}~{\rm M_{\odot}}$). Generally, jet structure shaped by early interaction with rather than As long as our they retain significant magnetization ($\lesssim1$), suggesting that reconnection fundamental property emission. The angular outflow energy after breakout consistently features flat core followed steep power-law distribution (slope $\gtrsim3$), similar hydrodynamic jets. In cocoon-only outcome, broadens flattens it $\sim1.5$).