  General Relativity coupled to a self-interacting scalar field in three
dimensions is shown to admit exact analytic soliton solutions, such that the
metric and the scalar field are regular everywhere. Since the scalar field
acquires slow fall-off at infinity, the soliton describes an asymptotically AdS
spacetime in a relaxed sense as compared with the one of Brown and Henneaux.
Nevertheless, the asymptotic symmetry group remains to be the conformal group,
and the algebra of the canonical generators possesses the standard central
extension. For this class of asymptotic behavior, the theory also admits hairy
black holes which raises some puzzles concerning an holographic derivation of
their entropy \`a la Strominger. Since the soliton is devoid of integration
constants, it has a fixed (negative) mass, and it can be naturally regarded as
the ground state of the "hairy sector", for which the scalar field is switched
on. This assumption allows to exactly reproduce the semiclassical hairy black
hole entropy from the asymptotic growth of the number of states by means of
Cardy formula. Particularly useful is expressing the asymptotic growth of the
number of states only in terms of the spectrum of the Virasoro operators
without making any explicit reference to the central charges.
