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We show that scalar hair can be added to rotating, vacuum black holes (BHs) of general relativity. These hairy black holes (HBHs) clarify a lingering question concerning gravitational solitons: Whether a BH can be added at the centre of a boson star (BS), as it typically can for other solitons. We argue that it can, but only if it is spinning. The existence of such HBHs is related to the Kerr superradiant instability triggered by a massive scalar field. This connection leads to the following conjecture: a (hairless) BH, which is afflicted by the superradiant instability of a given field, must allow hairy generalizations with that field.
Herdeiro et al. (Wed,) studied this question.