  We solve the Riemann problem for the deceleration of an arbitrarily
magnetized relativistic flow injected into a static unmagnetized medium in one
dimension. We find that for the same initial Lorentz factor, the reverse shock
becomes progressively weaker with increasing magnetization \sigma (the
Poynting-to kinetic energy flux ratio), and the shock becomes a rarefaction
wave when \sigma exceeds a critical value, \sigma_c, defined by the balance
between the magnetic pressure in the flow and the thermal pressure in the
forward shock. In the rarefaction wave regime, we find that the rarefied region
is accelerated to a Lorentz factor that is significantly larger than the
initial value. This acceleration mechanism is due to the strong magnetic
pressure in the flow. We discuss the implications of these results for models
of gamma-ray bursts and active galactic nuclei.
