  We discuss the reference time t_0 of afterglow light curves in the context of
the standard internal-external shock model. The decay index of early afterglow
is very sensitive to the reference time one chooses. In order to understand the
nature of early afterglow, it is essential to take a correct reference time.
Our simple analytic model provides a framework to understand special
relativistic effects involved in early afterglow phase. We evaluate light
curves of reverse shock emission as well as those of forward shock emission,
based on full hydrodynamic calculations. We show that the reference time does
not shift significantly even in the thick shell case. For external shock
emission components, measuring times from the beginning of the prompt emission
is a good approximation and it does not cause an early steep decay. In the thin
shell case, the energy transfer time from fireball ejecta to ambient medium
typically extends to thousands of seconds. This might be related to the shallow
decay phases observed in early X-ray afterglow at least for some bursts.

Astrophysics