Jet–environment interaction after delayed collapse in binary neutron star mergers

Binary neutron star (BNS) mergers are among the most powerful engines in the universe, capable of producing both gravitational waves and short gamma-ray bursts (SGRBs). In this talk, I will present general relativistic magnetohydrodynamic (GRMHD) simulations that, for the first time, self-consistently capture the full sequence from merger to jet launching for the delayed collapse scenario. In our models, the merger forms a metastable massive neutron star (MNS) that eventually collapses into a black hole (BH). We explore different MNS lifetimes of about 25 ms and 50 ms—long enough to allow the emergence of magnetically driven polar outflows prior to collapse. When the BH forms, the resulting jet interacts dynamically with this earlier ejecta, producing shock heating and possible observable electromagnetic signatures. Using an unprecedentedly low-numerical-density floor that scales as r^-6, we are able to follow the jet propagation out to distances of about 1e4 km, revealing how the surrounding environment—shaped by the MNS lifetime—strongly influences jet collimation, the terminal Lorentz factor, and Poynting-flux luminosity. For comparison, I will also discuss a non-collapsing scenario, in which the long-lived MNS drives a dense, slow outflow that effectively chokes any potential relativistic jet, reinforcing the need for a BH central engine. These results provide a unified picture for the delayed collapse scenario, linking merger dynamics, outflows and jet formation, and potential impact on observable transients—an essential step toward end-to-end modeling of short gamma-ray burst progenitors in the multi-messenger era.