  We study the primordial bispectrum of curvature perturbation in the uniform-
density slicing generated by the interaction between the inflaton and isotropic
background gauge fields. We derive the action up to cubic order in perturbation
and take into account all the relevant effects in the leading order of
slow-roll expansion. We first treat the quadratic vertices perturbatively and
confirm the results of past studies, while identifying their regime of
validity. We then extend the analysis to include the effect of the quadratic
vertices to all orders by introducing exact linear mode functions, allowing us
to make accurate predictions long after horizon crossing where the features of
both the power spectrum and the bispectrum are drastically different. It is
shown that the spPPectra become constant and scale-invariant in the limit of
large e-folding. As a result, we are able to impose reliable constraints on the
parameters of our theory using the recent observational data coming from
Planck.
