  We present a fully microscopics-based calculation of the Casimir effect in a
nonequilibrium system, namely an energy flux driven quantum XX chain. The force
between the walls (transverse-field impurities) is calculated in a
nonequilibrium steady state which is prepared by letting the system evolve from
an initial state with the two halves of the chain prepared at equilibrium at
different temperatures. The steady state emerging in the large-time limit is
homogeneous but carries an energy flux. The Casimir force in this
nonequilibrium state is calculated analytically in the limit when the
transverse fields are small. We find that the the Casimir force range is
reduced compared to the equilibrium case, and suggest that the reason for this
is the reduction of fluctuations in the flux carrying steady state.
