Influence of impact parameter on thermal description of relativistic heavyion collisions at (1-2)A GeV

Attention is drawn to the role played by the size of the system in the ther modynamic analysis of particle yields in relativistic heavy ion collisions at SIS energies. This manifests itself in the nonlinear dependence of K+ an d K- yields in AA collisions at (1-2)A GeV on the number of participants. I t is shown that this dependence can be quantitatively well described in ter ms of a thermal model with a canonical strangeness conservation. The measur ed particle multiplicity ratios (pi(+)/p, pi(-)/pi(+), d/p, K+/pi(+), and K +/K- but not eta/pi(0)) in central Au-Au and Ni-Ni collisions at (0.8-2.0)A CeV are also explained in the context of a thermal model with a common fre eze-out temperature and chemical potential. Including the concept of collec tive flow a consistent picture of particle energy distributions is derived with the flow velocity being strongly impact-parameter dependent. [S0556-28 13(99)01003-1].