  The emission pattern of charged pions has been measured in Au+Au collisions
at 1 GeV/nucleon incident energy. In peripheral collisions and at target
rapidities, high-energy pions are emitted preferentially towards the target
spectator matter. In contrast, low-energy pions are emitted predominantly in
the opposite direction. The corresponding azimuthal anisotropy is explained by
the interaction of pions with projectile and target spectator matter. This
interaction with the spectator matter causes an effective shadowing which
varies with time during the reaction. Our observations show that high-energy
pions stem from the early stage of the collision whereas low-energy pions
freeze out later.
