Effects of polar structure and moving ejecta on the dynamics of SGRB jets

TL;DR

This study uses relativistic hydrodynamic simulations to examine how the structure and expansion of ejecta from neutron star mergers influence the evolution and observable features of short gamma-ray burst jets.

Contribution

It provides new insights into how polar ejecta structure and expansion dynamics affect jet propagation and observable signatures in SGRBs, based on detailed simulations with realistic initial conditions.

Findings

Polar ejecta structure influences early jet evolution.

Expanding ejecta dominate jet behavior at later stages.

Jet energy dissipation remains consistent across models.

Abstract

At least some short gamma-ray bursts (SGRBs) originate from neutron star mergers, systems that release both a relativistic collimated jet and slower, wider ejecta. These jets evolve through a dense, anisotropic, and expanding medium produced during the merger process, resulting in interactions that affect their morphology and observable signatures. We investigate the propagation of SGRB jets through funnel-like structures that can be static or expanding with mildly relativistic speed using 2D axisymmetric relativistic hydrodynamic simulations. Our initial conditions are inspired from radial and angular distributions of density and pressure from general-relativistic magnetohydrodynamic simulations of binary neutron star mergers. We explore different values of the funnel opening angle and density contrast. We find that the polar structure of the ejecta mainly affects the jet evolution in the early stages, whereas the effect of expanding ejecta dominates in later stages. Jets propagating through a low-density polar funnel move initially faster, while the presence of mildly relativistic ejecta maintains the outflow more collimated after the breakout. Despite differences in the external medium, the energy dissipation within the jet and cocoon remains similar across models, while the shocked ambient material shows distinct signatures that could be observationally distinguishable. Our results highlight the importance of the structure and dynamical properties of the ejecta in shaping SGRB jets.