Jet Propagation in Neutron Star Mergers and GW170817

TL;DR

This paper develops an analytic model and conducts numerical simulations of jet propagation in neutron star merger ejecta, providing new insights into jet breakout times, engine luminosity, and implications for observed gamma-ray bursts.

Contribution

It introduces a novel analytic model accounting for ejecta expansion and validates it with extensive simulations, applying it to GW170817 to constrain engine properties.

Findings

Analytic model matches simulation results across parameters.

Constraints on engine luminosity for GW170817's jet.

Delay time between merger and engine activation less than 1.3 seconds.

Abstract

The gravitational wave event from the binary neutron star (BNS) merger GW170817 and the following multi-messenger observations present strong evidence for i) merger ejecta expanding with substantial velocities and ii) a relativistic jet which had to propagate through the merger ejecta. The ejecta's expansion velocity is not negligible for the jet head motion, which is a fundamental difference from the other systems like collapsars and active galactic nuclei. Here we present an analytic model of the jet propagation in an expanding medium. In particular, we notice a new term in the expression of the breakout time and velocity. In parallel, we perform a series of over a hundred 2D numerical simulations of jet propagation. The BNS merger ejecta is prepared based on numerical relativity simulations of a BNS merger with the highest-resolution to date. We show that our analytic results agree with numerical simulations over a wide parameter space. Then we apply our analytic model to GW170817, and obtain two solid constraints on: i) the central engine luminosity as $L_{iso,0} \sim 3\times10^{49}-2.5\times10^{52}$ erg s$^{-1}$, and on ii) the delay time between the merger and engine activation $t_0-t_m < 1.3$ s. The engine power implies that the apparently-faint \textit{short} gamma-ray burst (\textit{s}GRB) \textit{s}GRB 170817A is similar to typical \textit{s}GRBs if observed on-axis.