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We analyze proper motions from the Hubble Space Telescope (HST) and thesecond Gaia data release along with line-of-sight velocities from the MUSEspectrograph to detect imprints of an intermediate-mass black hole (IMBH) inthe center of the nearby, core-collapsed, globular cluster NGC 6397. For this,we use the new MAMPOSSt-PM Bayesian mass-modeling code, along with updatedestimates of the surface density profile of NGC 6397. We consider differentpriors on velocity anisotropy and on the size of the central mass, and we alsoseparate the stars into components of different mean mass to allow for masssegregation. The velocity ellipsoid is very isotropic throughout the cluster,as expected in post-core collapsed clusters subject to as strong a Galactictidal field as NGC 6397. There is strong evidence for a central dark componentof 0.8 to 2% of the total mass of the cluster. However, we find robust evidencedisfavoring a central IMBH in NGC 6397, preferring instead a diffuse dark innersubcluster of unresolved objects with a total mass of 1000 to 2000 solarmasses, half of which is concentrated within 6 arcsec (2% of the stellareffective radius). These results require the combination of HST and Gaia data:HST for the inner diagnostics and Gaia for the outer surface density andvelocity anisotropy profiles. The small effective radius of the diffuse darkcomponent suggests that it is composed of compact stars (white dwarfs andneutron stars) and stellar-mass black holes, whose inner locations are causedby dynamical friction given their high progenitor masses. We show thatstellar-mass black holes should dominate the mass of this diffuse darkcomponent, unless more than 25 per cent escape from the cluster. Their mergersin the cores of core-collapsed globular clusters could be an important sourceof the gravitational wave events detected by LIGO.

Does NGC 6397 contain an intermediate-mass black hole or a more diffuse inner subcluster?