Inflationary non-Gaussianity from thermal fluctuations

We calculate the contribution of fluctuations with a thermal origin to the inflationary non-Gaussianity. We find that even a small component of radiation can lead to a large non-Gaussianity. We show that this thermal non-Gaussianity always has positive f(NL). We illustrate our result in the chain inflation model and the very weakly dissipative warm inflation model. We show that f(NL) similar to O(1) is general in such models. If we allow a modified equation of state, or some decoupling effects, a large thermal non-Gaussianity of order f(NL) > 5 or even f(NL) similar to 100 can be produced. We also show that the power spectrum of chain inflation should have a thermal origin. In the appendix, we give a clarification on the different conventions used in the literature related to the calculation of f(NL).