Thermal bremsstrahlung probing the thermodynamical state of multifragmenting systems

Inclusive and exclusive hard-photon (E${}_{\gamma }>$ 30 MeV) production in five different heavy-ion reactions (${}^{36}$Ar+${}^{197}$Au, ${}^{107}$Ag, ${}^{58}$Ni, ${}^{12}$C at 60{\it A} MeV and ${}^{129}$Xe+${}^{120}$Sn at 50{\it A} MeV) has been studied coupling the TAPS photon spectrometer with several charged-particle multidetectors covering more than 80% of 4$\pi$. The measured spectra, slope parameters and source velocities as well as their target-dependence, confirm the existence of thermal bremsstrahlung emission from secondary nucleon-nucleon collisions that accounts for roughly 20% of the total hard-photon yield. The thermal slopes are a direct measure of the temperature of the excited nuclear systems produced during the reaction.