Remnant baryon mass in neutron star-black hole mergers: Predictions for binary neutron star mimickers and rapidly spinning black holes

Gravitational-wave (GW) and electromagnetic (EM) signals from the merger of a neutron star (NS) and a black hole (BH) are a highly anticipated discovery. We present a simple formula, validated with 75 simulations, that distinguishes between potential merger outcomes and predicts the baryon mass left outside of the BH after merger. Our formula describes critical unexplored regimes: comparable masses with nonspinning BHs, and higher BH spins, and is essential in assessing whether events such as GW170817 could be NS-BH systems instead of NS-NS mergers.