On the jet-ejecta interaction in 3D GRMHD simulations of binary neutron star merger aftermath

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