Scale dependence of fNL in N-flation

Adopting the horizon-crossing approximation, we derive the spectral index of fNL in general N-flation model. Axion N-flation model is taken as a typical model for generating a large fNL which characterizes the size of local form bispectrum. We find that its tilt nfNL is negligibly small when all inflatons have the same potential, but a negative detectable nfNL can be achieved in the axion N-flation with different decay constants for different inflatons. The measurement of nfNL can be used to support or falsify the axion N-flation in the near future