  We consider the interaction between a plane wave and a (counter-moving) black
hole. We show that energy is transferred from the black hole to the wave,
giving rise to a negative absorption cross-section. Moving black holes absorb
radiation and deposit energy in external radiation. Due to this effect, a black
hole hole of mass M moving at relativistic speeds in a cold medium will appear
to be surrounded by a bright "ring" of diameter 3\sqrt{3}GM/c^2 and thickness
GM/c^2.
