Moving black holes: Energy extraction, absorption cross section, and the ring of fire

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Maths. Dept. Room Sousa Pinto
Rodrigo Vicente - CENTRA IST
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 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 $\sim GM/c^2$.