The collimation of relativistic jets in post-neutron star binary merger simulations

TL;DR

This study uses advanced simulations to analyze how relativistic jets are collimated in neutron star merger events, revealing the importance of a low-density polar cavity and ejecta in shaping narrow, energetic jets consistent with observed short gamma-ray bursts.

Contribution

It demonstrates that a polar cavity formed during merger is essential for jet breakout and collimation, providing new insights into jet formation in neutron star mergers.

Findings

Polar cavity enables jet breakout and collimation.

Jets are narrowly collimated to 4-7 degrees.

Jet energies align with observed short gamma-ray bursts.

Abstract

The gravitational waves from the binary neutron star merger GW170817 were accompanied by a multi-wavelength electromagnetic counterpart, which confirms the association of the merger with a short gamma-ray burst (sGRB). The afterglow observations implied that the event was accompanied by a narrow, $\sim 5~$deg, and powerful, $\sim 10^{50}$ erg, jet. We study the propagation of a Poynting flux-dominated jet within the merger ejecta (kinematic, neutrino-driven and MRI turbulence-driven) of a neutrino-radiation-GR-MHD simulation of two coalescing neutron stars. We find that the presence of a post-merger low-density/low-pressure polar cavity, that arose due to angular momentum conservation, is crucial to let the jet break out. At the same time the ejecta collimates the jet to a narrow opening angle. The collimated jet has a narrow opening angle of $\sim 4$-$7$ deg and an energy of $10^{49}$-$10^{50}~$erg, in line with the observations of GW170817 and other sGRBs.