**5.2. Models for The Energy Conversion**

Within the baryonic model the energy transport is in the from of the kinetic energy of a shell of relativistic particles with a width . The kinetic energy is converted to "thermal" energy of relativistic particles via shocks. These particles then release this energy and produce the observed radiation. There are two modes of energy conversion (i) External shocks, which are due to interaction with an external medium like the ISM. (ii) Internal shocks that arise due to shocks within the flow when fast moving particles catch up with slower ones. Similar division to external and internal energy conversion occurs within other models for the energy flow.

External shocks arise from the interaction of the shell with external
matter. The typical length scale is the Sedov length,
*l* (*E* /
*n*_{ism} *m*_{p}
*c*^{2})^{1/3}. The rest mass energy within a
sphere of radius *l*, equals the energy of the shell. Typically
*l* ~ 10^{18} cm. As we see later (see
section 8.7.1)
relativistic external shocks (with a Newtonian reverse shock) convert
a significant fraction of their kinetic energy at
*R*_{} = *l* /
^{2/3}
10^{15} -
10^{16} cm, where the external mass encountered equals
^{-1}
of the shell's mass. Relativistic shocks (with a relativistic reverse
shock) convert their energy at
*R*_{} = *l*^{3/4}
^{1/4}
10^{16} cm,
where the shock crosses the shell.

Internal shocks occur when one shell overtakes another. If the initial
separation between the shells is
and both move with a
Lorentz factor
with a difference of order
these
shocks take place at:
^{2}.
A typical value is 10^{12} - 10^{14} cm.