The Onset of Gamma-Ray Burst Afterglow

We discuss the reference time t0 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 hydrodynamical calculations. We show that the reference time does not shift significantly even in the thick shell case. Measuring times from the beginning of the prompt emission is a good approximation and it does not cause an early steep decay.

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