OSCILLATING APPARENT HORIZONS IN NUMERICALLY GENERATED SPACETIMES

We investigate the evolution of the apparent horizon in three families of numerically generated spacetimes: the 'black hole plus Brill wave' spacetimes of Bernstein et al., the non-time symmetric generalisation of this by Brandt, and the Misner two black hole spacetime. Various measures of the curvature and shape of the horizon are shown as a function of coordinate time at infinity and it is found that the horizon oscillates at the lowest quasinormal mode frequency of the hole. In addition, in the spacetimes with angular momentum the total angular momentum of the final hole can be read off from the oscillations of the horizon directly without having to extract it from the gravitational radiation emitted by the hole.