Propagation of a Short GRB Jet in the Ejecta: Jet Launching Delay Time, Jet Structure, and GW170817/GRB 170817A

摘要

We perform a series of relativistic magnetohydrodynamic simulations to investigate how a hot magnetic jet propagates within the dynamical ejecta of a binary neutron star merger, focusing on how the jet structure depends on the delay time of jet launching with respect to the merger time, Δt_(jet). We find that regardless of the jet-launching delay time, a structured jet with an angle-dependent luminosity and Lorentz factor is always formed after the jet breaks out of the ejecta. On the other hand, the jet-launching delay time has an impact on the jet structure. If the jetlaunching delay time is relatively long, e.g., ≥?0.5 s, the line-of-sight material has a dominant contribution from the cocoon. On the other hand, for a relatively short jet-launching delay time, the jet penetrates through the ejecta early on and develops an angular structure afterward. The line-of-sight ejecta is dominated by the structured jet itself. We discuss the case of GW170817/GRB 170817A within the framework of both long and short jet-launching delay time. In the future, more observations of gravitational-wave/gamma-ray burst associations can help to differentiate between these two scenarios.