Fast Ejecta as a Potential Way to Distinguish Black Holes from Neutron Stars in High-mass Gravitational-wave Events

High-mass gravitational-wave events in the neutron-star mass range, such as GW190425, have recently started to be detected by the LIGO/Virgo detectors. If the masses of the two binary components fall in the neutron-star mass range, such a system is typically classified as a binary neutron-star system, although the detected gravitational-wave signal may be too noisy to clearly establish a neutron-star nature of the high-mass component in the binary and rule out a black hole–neutron star system for such an event. We show that high-mass binary neutron-star mergers with a very massive neutron-star primary close to the maximum-mass limit, m₁ ≳ 2.2 M_⊙, produce fast dynamical mass ejecta from the spin-up of the primary star at merger. By simulating the merger of black hole–neutron star systems of exactly the same masses and spins, we show that these fast ejecta are entirely absent if the primary is instead a black hole. In addition, we find that both systems leave almost identical amounts of baryon mass behind, which is not immediately accreted by the black hole. This implies that both systems will likely have comparable electromagnetic afterglow emission stemming from the remnant disk. Hence, fast ejecta at merger have the potential to distinguish neutron stars from black holes in high-mass mergers, although these ejecta may be challenging to detect observationally.