Non-Gaussianity from axionic curvaton

We study non-Gaussianity of density perturbations generated by an axionic curvaton, focusing on the case that the curvaton sits near the hilltop of the potential during inflation. Such hilltop curvatons can generate a red-tilted density perturbation spectrum without invoking large-field inflation. We show that, even when the curvaton dominates the Universe, the non-Gaussianity parameter f(NL) is positive and mildly increases towards the hilltop of the curvaton potential, and that f(NL) = O(10) is a general and robust prediction of such hilltop axionic curvatons. In particular, we find that the non-Gaussianity parameter is bounded as f(NL) less than or similar to 30 - 40 for a range of the scalar spectral index, n(s) = 0.94 - 0.99, and that f(NL) = 20 - 40 is realized for the curvaton mass m(sigma) = 10 - 10(6) GeV and the decay constant f = 10(12) - 10(17) GeV. One of the plausible candidates for the axionic curvaton is an imaginary component of a modulus field with mass of order 10 - 100 TeV and decay constant of 10(16-17) GeV. We also discuss extreme cases where the curvaton drives a second inflation and find that f(NL) is typically smaller compared to non-inflating cases.