Gravitational modulated reheating and non-Gaussianity in Supergravity R^2 inflation

Reheating after R^2 inflation proceeds through gravitational particle production of conformally noninvariant fields. We argue that the nonvanishing expectation value of flat directions generic in supersymmetric theories break conformal invariance of the fields coupled to them in a position-dependent manner due to quantum fluctuations. As a result modulated reheating can occur after the supergravity R^2-inflation. The resultant curvature fluctuation is a mixture of the one produced during inflation and that produced by modulated reheating. The spectral index takes a value between n_s=0.960 and 0.983, the nonlinearity parameter of the local-type non-Gaussianity can be f_nl\sim \pm 10, and the tensor-to-scalar ratio is r \le 4\times 10^{-3}.