Event horizon in a compact binary coalescence

Abstract

In this work we introduce a general method, mostly analytical with relatively simple numerics, for obtaining the event horizon in a compact binary coalescence in which one of the components is a supermassive black hole with a mass much greater than its companion's. We not only reproduce previous literature but also go beyond it by not restricting the smaller object to be a black hole. We apply this procedure to study the mergers of the large black hole with: (a) a Schwarzschild black hole, (b) a charged black hole, (c) a charged singularity and (d) several models of neutron stars. For the uncharged black hole, we recover the exact results in the literature using a slightly different method for solving (numerically) the equations. For the charged case, our results agree qualitatively with the results for the event horizon in the neutral case. For the charged singularity, we find that a \hole" (not a black hole) appears in the event horizon, due to the repulsive gravitational effect of the singularity, which we argue is physically pathological. For neutron stars, we discover that the structure of the merger depends on their compactness: above a certain threshold, the supermassive black hole induces the formation of a black hole inside the star, thus making the merger to happen \inside-out" in a phenomenon that we have named \precursory collapse". Our study of the mergers between neutron stars and black holes has given rise to an article that we have submitted for publication