  Hard-photon (E$_{\gamma} >$ 30 MeV) emission issuing from proton-neutron
bremsstrahlung collisions is investigated in four different heavy-ion reactions
at intermediate bombarding energies ($^{36}$Ar+$^{197}$Au, $^{107}$Ag,
$^{58}$Ni, $^{12}$C at 60{\it A} MeV) coupling the TAPS photon spectrometer
with two charged-particle multidetectors covering more than 80% of the solid
angle. The hard-photon spectra of the three heavier targets result from the
combination of two distinct exponential distributions with different slope
parameters, a result which deviates from the behaviour expected for hard-photon
production just in first-chance proton-neutron collisions. The thermal origin
of the steeper bremsstrahlung component is confirmed by the characteristics of
its slope and angular distribution. Such thermal hard-photons convey
undisturbed information of the thermodynamical state of hot and excited nuclear
systems undergoing multifragmentation.
