Modelling the delayed shock-breakout emission following jet-launching binary neutron star mergers via relativistic MHD simulations

Abstract

In binary neutron star (BNS) mergers launching a relativistic jet, an electromagnetic (EM) signal is produced when the jet-driven shock breaks out of the merger ejecta. The observed time delay of this shock-breakout (SBO) emission with respect to the gravitational-wave (GW) signal from the merger provides a powerful probe of the physical conditions governing jet launching and early-time jet propagation. Considering different models of jet propagation in realistic post-merger environments, we investigate the SBO emission and corresponding GW-EM delay that would be observed depending on the viewing angle and the assumed ejecta opacity. We perform relativistic MHD simulations of jets propagating through a post-merger environment directly imported from the outcome of a previous BNS merger simulation. We also introduce a specific procedure to faithfully reconstruct the early dynamical ejecta up to their natural front. The evolution is followed in 3D up to 0.6 s and then continued imposing axisymmetry and higher resolution. Varying jet launching time and luminosity, we identify three representative models spanning regimes from early breakout to extended jet choking. For each case, we track the jet-driven forward shock up to the photosphere and compute the angle-dependent bolometric SBO luminosity, assuming full conversion of the thermal energy within the shocked material into radiation, and taking into account non-radial photon propagation, relativistic Doppler shifts, and light-travel-time effects. We consider two opacity values spanning a factor of 10. We find that the GW-EM delay depends weakly on both viewing angle and ejecta opacity, making it a robust diagnostic for constraining models. Comparing with GRB 170817A, the model resulting in a substantially choked jet provides the most plausible peak bolometric luminosity and the closest match to the observed GW-EM delay and signal duration.