  Coalescing massive black hole binaries are produced by the mergers of
galaxies. The final stages of the black hole coalescence produce strong
gravitational radiation that can be detected by the space-borne LISA. In cases
where the black hole merger takes place in the presence of gas and magnetic
fields, various types of electromagnetic signals may also be produced. Modeling
such electromagnetic counterparts of the final merger requires evolving the
behavior of both gas and fields in the strong-field regions around the black
holes. We have taken a step towards solving this problem by mapping the flow of
pressureless matter in the dynamic, 3-D general relativistic spacetime around
the merging black holes. We find qualitative differences in collision and
outflow speeds, including a signature of the merger when the net angular
momentum of the matter is low, between the results from single and binary black
holes, and between nonrotating and rotating holes in binaries. If future
magnetohydrodynamic results confirm these differences, it may allow assessment
of the properties of the binaries as well as yielding an identifiable
electromagnetic counterpart to the attendant gravitational wave signal.
