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In this paper, we show that a rigidly rotating string can extract therotational energy from a rotating black hole. We consider Nambu-Goto stringsstationary with respect to a corotating Killing vector with an uniform angularvelocity $\omega$ in the Kerr spacetime. We show that a necessary condition ofthe energy-extraction process is that an effective horizon on the string worldsheet, which corresponds to the inner light surface, is inside the ergosphereof the Kerr black hole and the angular velocity $\omega$ is less than that ofthe black hole $\Omega_\mathrm{h}$. Furthermore, we discuss globalconfigurations of such strings in both of a slow-rotation limit and theextremal Kerr case.

Energy extraction from Kerr black holes by rigidly rotating strings