Cocoon shock breakout emission from binary neutron star mergers

Shock breakout emission is likely the first electromagnetic signal from a wide variety of astrophysical explosive phenomena, including supernovae and neutron star (NS) mergers; as exemplified by GRB 170817A, this signal can be the dominant component in low-luminosity short $\ensuremath{\gamma}$-ray bursts. In this work, we investigate the cocoon shock breakout emission in NS mergers and how its signal depends on the outermost layers of the ejecta profile, which we derive from general-relativistic radiation hydrodynamic simulations. To explore the influence of the outermost layers of the ejecta on the breakout emission, we model the ejecta profile as either having a sharp cutoff or an extended smooth tail. We find that the shock breakout emission is strongly influenced by the shape of the outermost layers of the ejecta, with breakouts from extended density profiles yielding emission consistent with the observed properties of GRB 170817A; on the contrary, breakouts from ejecta with a sharp cutoff tend to overestimate the radiated energy. Using a Bayesian analysis, we estimate the best-fit parameters for the central engine, considering both accreting black hole (BH) and magnetized NS scenarios. Our findings indicate a slight preference for scenarios in which the remnant suffers an early collapse to a BH. Our work probes the nature of neutron star mergers and highlights the importance of carefully treating the shape of the ejecta outermost layers in modeling early electromagnetic counterparts from these events.