The fingerprint of binary intermediate mass black holes in globular clusters: supra-thermal stars and angular momentum alignment

We propose a new approach to unveil the presence of binary intermediate mass black holes (IMBHs) in the core of globular clusters (GCs). We study the signature that a binary IMBH imprints on the velocity and on the angular momentum of cluster stars, simulating 3-body encounters between a star and a binary IMBH. We find that the binary IMBH generates a family of few hundreds of stars (∼ 100-300) which remain bound to the cluster and have velocity significantly higher than the dispersion velocity. For this reason we term them ”supra-thermal” stars. We also notice that, after the interaction, a considerable fraction (55-70%) of stars tend to align their orbital angular momentum with the angular momentum of the binary IMBH, introducing an anisotropy in the angular momentum distribution of cluster stars. This phenomenon strongly depends on the reduced mass and on the orbital separation of the binary IMBH. We simulate the dynamical evolution of these supra-thermal stars before thermalization, and find that these stars tend to cluster at a distance of few core radii from the GC center. We, further, provide an estimate of the number of supra-thermal stars which can be detected through spectroscopic or photometric measurements. We conclude that the detection of these supra-thermal stars, even if quite difficult with present instrumentation, can provide an indication of the existence of a binary IMBH in the GC core. We shortly discuss angular momentum alignment and dynamical heating by a supermassive black hole binary on stars in a galactic nucleus.