It is generally accepted that the afterglow is produced when the
relativistic ejecta is slowed down by the surrounding matter
[236].
The afterglow begins at Rext where
most of the energy of the ejecta is transferred to the shocked
external medium. For a long burst this takes place while the burst
is still going on (see Sari
[363] and
Section VID). Initially the process might be
radiative,
namely a significant fraction of the kinetic energy is dissipated
and the radiation process effects the hydrodynamics of the shock.
I discuss this phase in Section VIIC. Later the
radiation processes become less efficient and an adiabatic phase
begins during which the radiation losses are minor and do not
influence the hydrodynamics. The hydrodynamic evolution at this
stage is adiabatic. If the ejecta is in the form of a jet with an
opening angle then a
jet transition will take place when
reaches
-1. A
transition into the Newtonian regime takes place when
- 1
0.5. I begin the
discussion of the afterglow with the hydrodynamics of the
adiabatic phase and with the resulting synchrotron light curve. I
continue with a discussion of the possible early radiative
evolution. Then I turn to the jet break and to the Newtonian
transition. I continue with various complications and variations
on these themes.