  The mechanism responsible for IMF emission in central $^{58}$Ni + $^{197}$Au
reactions at 30 and 45 MeV/nucleon is investigated by looking at the thermal
bremsstrahlung photon production. An IMF - photon anticorrelation signal is
observed, for central collisions, at 45 MeV/nucleon with IMF velocity around
the center of mass value. This observation is proposed as an evidence for
prompt nuclear fragmentation events.
