Jet propagation in neutron star mergers and GW170817

The gravitational wave event from the binary neutron star (BNS) merger GW170817 and the following multimessenger observations present strong evidence for (i) merger ejecta expanding with substantial velocities and (ii) a relativistic jet that 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 are 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,L-0 similar to (3 x 10(49))-(2.5 x 10(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 short gamma-ray burst (sGRB) sGRB 170817A is similar to typical sGRBs if observed on-axis.