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

TL;DR

This work investigates how relativistic jets from binary neutron star mergers propagate through realistic, non-homogeneous environments and how their angular and velocity structure affects afterglow emission. By performing 3D RMHD simulations with varied jet-injection parameters and evolving to the ballistic regime, the authors extract energy and velocity distributions and feed them into a semi-analytic afterglow model that accounts for radial stratification and full 3D geometry. They find that more energetic or earlier-launched jets drill through ejecta more efficiently, while all models develop asymmetries and a complex multi-shock breakout that imprints an early, dimmer peak on the afterglow; velocity stratification broadens the light curves and can produce observable features for off-axis observers. The models remain broadly consistent with GW170817 data, and the analysis highlights the importance of including velocity structure and azimuthal asymmetries to constrain jet-launching conditions and viewing-angle in future BNS 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.

Figures (17)

• Figure 1: Meridional view of total energy density excluding rest-mass contribution ($e_\mathrm{sum}$; left column) and rest-mass density and Lorentz factor (right column), at $275$ ms (bottom), $3$ s, and $9$ s (top) after launch in the fiducial model F. The threshold in $\Gamma$ is 1.5, 1.05 and 1.4 respectively for the three representative times. The top row shows composite panels combining snapshots at $3$ and $9$ s, referring to the former up to $y\!=\!10^6$ km and to the latter above it. See text for the definition of the regions identified by numbers.
• Figure 2: Radially and azimuthally averaged Lorentz factor, and the corresponding $E_\mathrm{iso}$ at $\Theta>10^{\circ}$ (inset), at the jet cap in model b at different evolutionary times. See text for discussion.
• Figure 3: Meridional view of the total (minus rest-mass) energy density at $t-t_\mathrm{j}=3$ s for model $F$. White contours identify shocked surfaces (see text).
• Figure 4: Front-view distributions of the isotropic-equivalent energy of the jet cap for all jet models at $t_\mathrm{end}$. The red line shows the smallest contour that contains 50% of the energy; the white cross indicates the jet's energy barycentre.
• Figure 5: Jet energy distribution in the four velocity-polar angle plane. The dark blue shaded region identifies the interquartile range (25th–75th percentiles) while the range between the 5th and 95th percentiles is shown in light blue.