  In string theory inspired models of axion-like fields, sub-leading
non-perturbative effects, if sufficiently large, can introduce steep cliffs and
gentle plateaus onto the underlying scalar potential. During inflation, the
motion of a spectator axion $\sigma$ on this potential becomes temporarily
fast, leading to localized amplification of one helicity state of gauge fields.
In this model, the tensor and scalar correlators sourced by the vector fields
exhibit localized peak(s) in momentum space corresponding to the modes that
exit the horizon while the roll of $\sigma$ is fast. Thanks to the
gravitational coupling of gauge fields with the visible sector and the
localized nature of particle production, this model can generate observable
gravitational waves (GWs) at CMB scales while satisfying the current limits on
scalar perturbations. The resulting GW signal breaks parity and exhibit
sizeable non-Gaussianity that can be probed by future CMB B-mode missions.
Depending on the initial conditions and model parameters, the roll of the
spectator axion can also generate an observably large GW signature at
interferometer scales while respecting the bounds on the scalar fluctuations
from primordial black hole limits. In our analysis, we carefully investigate
bounds on the model parameters that arise through back-reaction and
perturbativity considerations to show that these limits are satisfied by the
implementations of the model that generate GW signals at CMB and sub-CMB
scales.
