A Reverse-Shock Model for the Early Afterglow of GRB 050525A

The prompt localization of GRB 050525A by Swift allowed rapid follow-up of the afterglow. The observations revealed that the optical afterglow had a major rebrightening starting at ~0.01 days and ending at ~0.03 days, which was followed by an initial power-law decay. Here we show that this early emission feature can be interpreted as reverse-shock emission superposed by forward-shock emission in an interstellar medium environment. By fitting the observed data, we further constrain some parameters of the standard fireball-shock model: the initial Lorentz factor of the ejecta (gamma)0 > 120, the magnetic energy fraction (epsioln)B > 4 * 10^-6, and the medium density n < 2-cm^-3. These limits are consistent with those from other very early optical afterglows observed so far. In principle, a wind environment for GRB 050525A is disfavored.