Gravitational waves from the 3D collapse of a neutron star to a Kerr black hole

We present the gravitational waveforms emitted when a neutron star, modeled as an initially uniformly rotating polytrope, collapses to a Kerr black hole. We investigate the gravitational collapse by carefully studying not only the dynamics of the matter, but also that of the trapped surfaces, of both the apparent and event horizons formed during the collapse. The use of these surfaces, together with the dynamical horizon framework, allows for a precise measurement of the black hole mass and spin, as well as of the energy lost to gravitational waves. The ability to extract the waveforms depends on having computational domains with boundaries in the wave-zone and on extending the simulation past the formation of the singularity. Both requirements are met by using fixed mesh-refinement techniques and by excising a region of the computational domain which includes the singularity and is within the apparent horizon.