  We study the moduli-space scattering of a two-charge supertube in the
background of a rotating BPS D1-D5-P black hole in 4+1 dimensions, extending
the static analysis of Bena and Kraus (hep-th/0402144). While the magnetic
forces associated with this motion change the details considerably, the final
conclusion is similar to that of the static analysis: we find that one can
bring the supertube to the horizon, so that the BMPV black hole and the
supertube merge. However, our analysis shows that this can occur even at
significantly larger values of the angular momentum than was indicated by the
static analysis. For a range of parameters, conservation laws and the area
theorem forbid the result of the merger from being any single known object:
neither near-extremal black holes nor non-supersymmetric black rings are
allowed. Such results suggest that the merger triggers an instability of the
rotating D1-D5-P black hole, perhaps leading to bifurcation into a pair of
black objects.
