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 m easures of the curvature and shape of the horizon are shown as a funct ion of coordinate time at infinity and it is found that the horizon os cillates at the lowest quasinormal mode frequency of the hole. In addi tion, in the spacetimes with angular momentum the total angular moment um of the final hole can be read off from the oscillations of the hori zon directly without having to extract it from the gravitational radia tion emitted by the hole.