Does NGC 6397 contain an intermediate-mass black hole or a more diffuse
  inner subcluster?
  We analyze proper motions from the Hubble Space Telescope (HST) and the
second Gaia data release along with line-of-sight velocities from the MUSE
spectrograph to detect imprints of an intermediate-mass black hole (IMBH) in
the center of the nearby, core-collapsed, globular cluster NGC 6397. For this,
we use the new MAMPOSSt-PM Bayesian mass-modeling code, along with updated
estimates of the surface density profile of NGC 6397. We consider different
priors on velocity anisotropy and on the size of the central mass, and we also
separate the stars into components of different mean mass to allow for mass
segregation. The velocity ellipsoid is very isotropic throughout the cluster,
as expected in post-core collapsed clusters subject to as strong a Galactic
tidal field as NGC 6397. There is strong evidence for a central dark component
of 0.8 to 2% of the total mass of the cluster. However, we find robust evidence
disfavoring a central IMBH in NGC 6397, preferring instead a diffuse dark inner
subcluster of unresolved objects with a total mass of 1000 to 2000 solar
masses, half of which is concentrated within 6 arcsec (2% of the stellar
effective radius). These results require the combination of HST and Gaia data:
HST for the inner diagnostics and Gaia for the outer surface density and
velocity anisotropy profiles. The small effective radius of the diffuse dark
component suggests that it is composed of compact stars (white dwarfs and
neutron stars) and stellar-mass black holes, whose inner locations are caused
by dynamical friction given their high progenitor masses. We show that
stellar-mass black holes should dominate the mass of this diffuse dark
component, unless more than 25 per cent escape from the cluster. Their mergers
in the cores of core-collapsed globular clusters could be an important source
of the gravitational wave events detected by LIGO.
