Squeezed primordial bispectrum from a general vacuum state

We study the general relation between the power spectrum and the squeezed limit of the bispectrum of the comoving curvature perturbation produced during single-field slow-roll inflation when the initial state is a general vacuum. Assuming the scale invariance of the power spectrum, we derive a formula for the squeezed limit of the bispectrum, represented by the parameter f(NL), which is not slow-roll suppressed and is found to contain a single free parameter for a given amplitude of the power spectrum. Then, we derive the conditions for achieving a scale-invariant f(NL) and discuss a few examples.