  Relativistic jets associated with long/soft gamma-ray bursts are formed and
initially propagate in the interior of the progenitor star. Because of the
subsequent loss of their external pressure support after they cross the stellar
surface, these flows can be modeled as moving around a corner. A strong
steady-state rarefaction wave is formed, and the sideways expansion is
accompanied by a rarefaction acceleration. We investigate the efficiency and
the general characteristics of this mechanism by integrating the steady-state,
special relativistic, magnetohydrodynamic equations, using a special set of
partial exact solutions in planar geometry (r self-similar with respect to the
"corner"). We also derive analytical approximate scalings in the
ultrarelativistic cold/magnetized, and hydrodynamic limits. The mechanism is
more effective in magnetized than in purely hydrodynamic flows. It
substantially increases the Lorentz factor without much affecting the opening
of the jet; the resulting values of their product can be much grater than
unity, allowing for possible breaks in the afterglow light curves. These
findings are similar to the ones from numerical simulations of axisymmetric
jets by Komissarov et al and Tchekhovskoy et al, although in our approach we
describe the rarefaction as a steady-state simple wave and self-consistently
calculate the opening of the jet that corresponds to zero external pressure.
