Interplay of the emission from thermal and direct sources in   relativistic heavy ion collisions
  The separation of the source created in ultrarelativistic heavy-ion collisions into a thermalized dense core and an outer mantle consisting of independent nucleon-nucleon collisions is discussed. Evidence for such a two component picture is found in transverse mass spectra of kaon, protons and antiprotons produced in Au-Au collisions at 200GeV. Estimates of the sizes of the thermal and direct sources are compared to models separating the interaction zone into a core and a corona, according to the density of participants or to the number of collisions. Consequences for the modeling of the dynamics of the small size, thermalized core are described. New initial conditions corresponding to the dense core lead to a stronger azimuthal asymmetry of the hydrodynamically expanding fireball, pressure gradients also increase. 2+1-dimensional hydrodynamic simulations are presented starting from all the matter in the interaction region or from the dense, thermal part of the source. We find faster transverse expansion and stronger elliptic flow for dense core initial conditions. For different impact parameters we find very similar spectra of the thermal part of the source and only adding particles emitted directly from nucleon-nucleon collisions in the corona the experimentally observed softening of the spectra with increasing impact parameter is reproduced. The elliptic flow is stronger for particles emitted from a source separated into a core and a corona. 
Piotr Bozek