Astron. Astrophys. Suppl. Ser. 138, 537-538

The early afterglow

R. Sari¹ and T. Piran ²

1 - Theoretical Astrophysics, California Institute of Technology, Pasadena, CA 91125, U.S.A.
2 - Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel , and Department of Physics, Columbia University, New York, NY 10027, U.S.A.

Received December 29; accepted December 29, 1998

Abstract:

We calculate the expected spectrum and light curves of the early afterglow. For short GRBs the peak of the afterglow will be delayed, typically, by few dozens of seconds after the burst. The X-ray and $\gamma$-ray characteristics of this delayed emission provide a way to discriminate between late internal shocks emission (part of the GRB) and the early afterglow signal. Detection of this delayed emission will prove the internal shock scenario as producing the GRB, and will pinpoint the initial Lorentz factor $\gamma_0$. In the optical band, the dominant emission arises from the reverse shock. This shock, carries a comparable amount of energy to the forward shock. It radiates this energy at much lower frequencies, producing a short optical flash of 15th magnitude or brighter.