Dynamical and Thermodynamical Aspects of Interacting Kerr Black Holes

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We consider the asymptotically flat double-Kerr solution for two equal mass black holes with either the same or opposite angular momentum and with a massless strut between them.

• For fixed angular momentum and mass, the angular velocity of two co-rotating Kerr black holes decreases as they approach one another, from the Kerr value at infinite separation to the value of a single Kerr black hole with twice the mass and the angular momentum at the horizons merging limit.
• We show that the ratio J/M^2 for extremal co-rotating Kerr black holes varies from unity at infinite separation to two at the merging limit.
• These results are interpreted in terms of rotational dragging and compared with the case of counter-rotating Kerr black holes.