Predictions for the Very Early Afterglow and the Optical Flash

According to the internal-external shocks model for $\gamma $-ray bursts(GRBs), the GRB is produced by internal shocks within a relativistic flow whilethe afterglow is produced by external shocks with the ISM. We explore the earlyafterglow emission. For short GRBs the peak of the afterglow will be delayed,typically, by few dozens of seconds after the burst. For long GRBs the earlyafterglow emission will overlap the GRB signal. We calculate the expectedspectrum and the light curves of the early afterglow in the optical, X-ray and$\gamma $-ray bands. These characteristics provide a way to discriminatebetween late internal shocks emission (part of the GRB) and the early afterglowsignal. If such a delayed emission, with the characteristics of the earlyafterglow, will be detected it can be used both to prove the internal shockscenario as producing the GRB, as well as to measure the initial Lorentz factorof the relativistic flow. The reverse shock, at its peak, contains energy whichis comparable to that of the GRB itself, but has a much lower temperature thanthat of the forward shock so it radiates at considerably lower frequencies. Thereverse shock dominates the early optical emission, and an optical flashbrighter than 15th magnitude, is expected together with the forward shock peakat x-rays or $\gamma$-rays. If this optical flash is not observed, stronglimitations can be put on the baryonic contents of the relativistic shellderiving the GRBs, leading to a magnetically dominated energy density.Comment: 23 pages including 4 figure