  We investigate theoretically the thermal escape behavior of trapped magnetic
fluxons in long annular Josephson junctions in dc magnetic fields, and
perturbed by a probing ac current. The study is motivated by recently published
experimental data that show multi-peaked escape distributions for increasing
bias current in the extreme low temperature regime when the system is perturbed
by an ac current. We demonstrate that the observed behavior of multi-peaked
escape distributions can be reproduced and predicted in the entirely classical,
thermally driven sine-Gordon model, which is widely accepted as accurately
describing the experimental system. We interpret the observed multi-peaked
distributions as being directly induced by dynamical resonances between the
applied ac perturbation and the natural oscillation frequency of a trapped
fluxon.
