2018ApJ...863...58X

We present numerical simulations of energetic flows propagating through the debris cloud of a binary neutron star (BNS) merger. Starting from the scale of the central engine, we use a moving-mesh hydrodynamics code to simulate the complete dynamical evolution of the relativistic jets produced. We compute synchrotron emission directly from the simulations and present multiband light curves of the early (subday) through late (weeks to years) afterglow stages. Our work systematically compares two distinct models for the central engine, referred to as the narrow- and wide-engine scenarios, respectively associated with a successful structured jet and quasi-isotropic explosion. Both engine models naturally evolve angular and radial structures through hydrodynamical interaction with the merger debris cloud. They both also result in a relativistic blast wave capable of producing the observed multiband afterglow data. However, we find that the narrow- and wide-engine scenarios might be differentiated by a new emission component that we refer to as a merger flash. This component is a consequence of applying the synchrotron radiation model to the shocked optically thin merger cloud. Such modeling is appropriate if injection of nonthermal electrons is sustained in the breakout relativistic shell, for example by internal shocks or magnetic reconnection. The rapidly declining signature may be detectable for future BNS mergers during the first minutes to the day following the gravitational wave chirp. Furthermore, its nondetection for the GRB170817A event may disfavor the wide, quasi-isotropic explosion model.