  The origin of bimodal behavior in the residue distribution experimentally
measured in heavy ion reactions is reexamined using Boltzmann-Uehling-Uhlenbeck
simulations. We suggest that, depending on the incident energy and impact
parameter of the reaction, both entrance channel and exit channel effects can
be at the origin of the observed behavior. Specifically, fluctuations in the
reaction mechanism induced by fluctuations in the collision rate, as well as
thermal bimodality directly linked to the nuclear liquid-gas phase transition
are observed in our simulations. Both phenomenologies were previously proposed
in the literature, but presented as incompatible and contradictory
interpretations of the experimental measurements. These results indicate that
heavy ion collisions at intermediate energies can be viewed as a powerful tool
to study both bifurcations induced by out-of-equilibrium critical phenomena, as
well as finite size precursors of thermal phase transitions.
