  The kinetics of a periodically driven nonlinear oscillator, bistable in a
nearly resonant field, has been investigated theoretically and through analogue
experiments. An activation dependence of the probabilities of fluctuational
transitions between the coexisting attractors has been observed, and the
activation energies of the transitions have been calculated and measured for a
wide range of parameters. The position of the kinetic phase transition (KPT),
at which the populations of the attractors are equal, has been established. A
range of critical phenomena is shown to arise in the vicinity of the KPT
including, in particular, the appearance of a supernarrow peak in the spectral
density of the fluctuations, and the occurrence of high-frequency stochastic
resonance (HFSR). The experimental measurements of the transition
probabilities, the KPT line, the multipeaked spectral densities, the strength
of the supernarrow spectral peak, and of the HFSR effect are shown to be in
good agreement with the theoretical predictions.
