Precursory collapse in Neutron Star-Black Hole mergers

We investigate the properties of the event horizon in the merger between a large black hole and a smaller neutron star. We find that, if the star is compact enough, then, in its rest frame a horizon begins to grow inside the star before it merges with the black hole, in a manner analogous to the growth of the event horizon in stellar collapse. We may say that, ahead of its fall into the larger black hole, the neutron star begins to become a black hole itself. We discuss how the phenomenon, even if not directly observable, can be invariantly characterized. We demonstrate it quantitatively by explicitly constructing the merger event horizon in the extreme-mass-ratio limit. We show that the effect is present for realistic neutron star models and admissible values of the compactness.