Direct photon emission from hadronic sources: Hydrodynamics vs. Transport theory

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 3-dimensional fluiddynamic calculation. The effects of viscosity, present in the transport model but neglected in ideal fluid-dynamics, are examined. We study the contribution of different production channels and non-thermal collisions to the spectrum of direct photons. Detailed comparison to the measurements by the WA~98-collaboration are undertaken.