Evolution and afterglow emission of gamma-ray burst jets from binary neutron star mergers

Abstract

Relativistic jets launched in binary neutron star (BNS) mergers are widely accepted as the engines powering most of the population of short gamma-ray bursts (GRBs). Understanding their structure and dynamics-particularly during and after breakout from the merger ejecta-is crucial for interpreting GRB afterglows, especially for off-axis observers. Traditional models often assume simple angular or radial jet profiles, potentially missing key features emerging for jets piercing through realistic environments. This work aims to investigate the formation and evolution of the jet structure as it propagates through a non-homogeneous, anisotropic BNS merger environment. We focus on how the interaction with the ambient medium shapes the jet's angular and velocity distributions and assess the impact of this realistic structure on the resulting afterglow light curves. We perform a series of 3D relativistic magnetohydrodynamic simulations of jets launched in post-merger environments, exploring different injection conditions. Simulations are evolved to late times, approaching the ballistic regime, where further dynamical evolution becomes negligible. From the resulting outflows, we extract energy and velocity profiles and compute multi-wavelength afterglow light curves using a semi-analytic model that includes radial stratification and the full 3D jet geometry. More energetic or earlier-launched jets drill more efficiently through the ejecta, but all develop asymmetries that leave clear imprints in the off-axis afterglow light curves. All models exhibit a complex multi-shock breakout structure responsible for an early, dimmer peak in the afterglow. Despite structural differences, all simulated jets are consistent with the observational data of the multi-messenger BNS merger event GW170817.