Abstract: Direct Photons in heavy-ion collisions from microscopic transport theory and fluid dynamics

Direct photon emission in heavy-ion collisions is calculated within the relativistic microscopic transport model UrQMD. We compare the results from the pure transport calculation to a hybrid-model calculation, where the high-density part of the evolution is replaced by an ideal 3D fluiddynamic calculation. The impact of different Equations of State in the latter case is studied. Also, the effects of viscosity, present in the transport model but neglected in ideal fluiddynamics, are examined. We also study the contribution of different production channels and non-thermal collisions to the spectrum of direct photons. Detailed comparison to the measurements by experiments at CERN-SPS (WA98) and BNL-RHIC (PHENIX) are undertaken.