Detailed studies of the azimuthal dependence of the mean fragment and flow energies in the $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ and $\mathrm{X}\mathrm{e}+\mathrm{C}\mathrm{s}\mathrm{I}$ systems are reported as a function of incident energy and centrality. Comparisons between data and model calculations show that the flow energy values along different azimuthal directions could be viewed as snapshots of the fireball expansion with different exposure times. For the same number of participating nucleons more transversally elongated participant shapes from the heavier system produce less collective transverse energy. Good agreement with Boltzmann-Uehling-Uhlenbeck calculations is obtained for a soft nuclear equation of state.

• Received 24 February 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.072303