  It is shown that azimuthal asymmetry in transverse momentum distribution of
particles observed in non-central heavy ion collisions can originate from
pre-equilibrium rescattering. The phenomenon is studied using computer
simulation of expanding pion gas created in Pb+Pb 160 GeV/n collisions. Results
obtained are discussed in comparison with experimental results of NA49
collaboration. Theoretical understanding of mechanism generating the asymmetry
is presented. Conclusion about non-equilibrium properties of the phenomenon
studied is drawn.
