Horocycle averages on closed manifolds and transfer operators

We study semigroups of weighted transfer operators for Anosov flows of regularity C^r, r>1, on compact manifolds without boundary. We construct an anisotropic Banach space on which the resolvent of the generator is quasi-compact and where the upper bound on the essential spectral radius depends continuously on the regularity. We apply this result to the ergodic average of the horocycle flow for C^3 contact Anosov flows in dimension three.