Scale-dependent gravitational waves from a rolling axion
  We consider a model in which a pseudo-scalar field $\sigma$ rolls for some
e-folds during inflation, sourcing one helicity of a gauge field. These fields
are only gravitationally coupled to the inflaton, and therefore produce scalar
and tensor primordial perturbations only through gravitational interactions.
These sourced signals are localized on modes that exit the horizon while the
roll of $\sigma$ is significant. We focus our study on cases in which the model
can simultaneously produce (i) a large gravitational wave signal, resulting in
observable B-modes of the CMB polarizations, and (ii) sufficiently small scalar
perturbations, so to be in agreement with the current limits from temperature
anisotropies. Different choice of parameters can instead lead to a localized
and visible departure from gaussianity in the scalar sector, either at CMB or
LSS scales.
