GRB Fireball Properties Derived from Early Afterglow Observations

Author: Richard Harrison

Abstract: The early optical afterglow emission for gamma-ray burst (GRB) events contain precious information on the ejecta from the central engine. The relative strength of the forward and reverse shock emission in early afterglow indicates the ratio of magnetic energy densities in the two shock regions (magnetization). We numerically show that the standard method underestimates the fireball magnetization by a factor of 10-100. We apply our new method to early afterglow observations to provide the magnetization estimates for several events. Furthermore the standard reverse shock model assumes a uniform fireball shell collides into the ambient medium. However, even after the internal shock phase, the fireball ejecta is expected to have significant density structures. Although the decay index of the reverse shock emission is rather insensitive to the structure after the shock crossing time, we show that the rising index depends on the dimensionless xi parameter (thin-thick shell regimes) as well as the structure of the shell. We discuss implications to early afterglow modeling and possible correlations between prompt gamma-rays and early afterglow radiation.