Weak-noise limit of stochastic resonance

The autocorrelation function of a periodically driven bistable system is considered in the limit of weak noise. We predict a broad window in the Fourier spectrum with a universal (power law) decay of the spectral density, accompanied by dips of finite width at the second and fourth harmonics of the driving frequency. For higher driving frequencies the dips are replaced by peaks of finite though extremely narrow width. Analytical results are confirmed by numerical studies and are used to analyze previous experimental data.