Black Holes in Massive Dynamical Chern-Simons gravity: scalar hair and quasibound states at decoupling

Black holes have a unique sensitivity to the presence of ultralight matter fields or modifications of the underlying theory of gravity. In the present paper we combine both features by studying an ultralight, dynamical scalar field that is nonminimally coupled to the gravitational Chern-Simons term. In particular, we numerically simulate the evolution of such a scalar field around a rotating black hole in the decoupling approximation and find a new kind of massive scalar hair anchored around the black hole. In the proximity of the black hole, the scalar exhibits the typical dipolar structure of hairy solutions in (massless) dynamical Chern-Simons gravity. At larger distances, the field transitions to an oscillating scalar cloud that is induced by the mass term. Finally, we complement the time-domain results with a spectral analysis of the scalar field characteristic frequencies.