  A model is developed for the confinement and collimation of a baryon poor
outflow by its surrounding medium. Both, confinement by kinetic pressure of a
static corona, and confinement by the ram pressure of a supersonic wind
emanating from a disk surrounding the inner source are considered. Solutions
are presented for the structure of the shocked layers of a deflected baryon
poor jet (BPJ) and exterior wind. The dependence of the opening angle of the
BPJ on the parameters of the confining medium are carefully examined. It is
found that the BPJ shock may either converge to the symmetry axis or diverge
away from it, depending on the opening angle of the BPJ injection cone. In the
latter case the inner flow exhibits a non-uniform structure, consisting of an
ultra-relativistic core containing the unshocked BPJ enveloped by the slower,
shocked BPJ layer. The implications of our results to the prompt GRB emission
are briefly discussed.

Astrophysics